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Review of the solar flux distribution in concentrated solar power: Non-uniform features, challenges, and solutions
Accepted Manuscript Review of the solar flux distribution in concentrated solar power: non-uniform features, challenges, and solutions Ya-Ling He, Kun Wang, Yu Qiu, Bao-Cun Du, Qi Liang, Shen Du PII: DOI: Reference:
S1359-4311(18)36076-9 https://doi.org/10.1016/j.applthermaleng.2018.12.006 ATE 13013
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
3 October 2018 3 December 2018 3 December 2018
Please cite this article as: Y-L. He, K. Wang, Y. Qiu, B-C. Du, Q. Liang, S. Du, Review of the solar flux distribution in concentrated solar power: non-uniform features, challenges, and solutions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.006
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Review of the solar flux distribution in concentrated solar power: non-uniform features, challenges, and solutions Ya-Ling He*, Kun Wang, Yu Qiu, Bao-Cun Du, Qi Liang, Shen Du Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China *Corresponding author.
E-mail address: [email protected]
Abstract: Concentrated solar flux distribution in the concentrated solar power (CSP) systems is extremely non-uniform, which can lead to high local temperature and large temperature gradient in solar receivers that will cause great challenges for the safety and efficient operation of the system. This paper introduces the non-uniform flux features in four CSP technologies including the parabolic-trough collector, the linear Fresnel collector, the solar power tower, and the parabolic-dish collector. Challenges including degeneration of the materials, thermal stress and deformation, and overburning are summarized. The corresponding solutions proposed to tackle these challenges are emphatically reviewed, and a recommendation for the optimization of the solar collector is provided from this review, which is that the solar flux distribution and the heat transfer ability of the heat transfer fluid (HTF) should match with each other as well as possible. From this point of view, the existing solutions are classified into two groups. One is optimizing the heat transfer ability of the HTF to match with the flux distribution, which is called the passive approach. The other is homogenizing the flux distribution to match with the identical heat transfer ability of the HTF, which is called the active approach. This review can help to have a better understanding of the non-uniform solar flux features in CSPs, and provide guidance for solving the corresponding challenges. Keywords: Concentrated solar power; Solar concentrator; Solar receiver; Non-uniform solar flux; Non-uniform temperature; Challenges and solutions 1 Introduction Environmental issues introduced by the utilization of fossil fuels have imposed negative effects 1
on the society and the economic development. For solving the problems, utilization of solar energy is a highly competitive approach[1]. Among different solar technologies, the concentrated solar power (CSP) technology is a promising option. In recent years, it has become a development tendency to promote this technology towards a low-cost and large-scale utilization [2]. A typical CSP plant mainly consist of a solar concentrator, a receiver, a thermal storage system, a heat exchange network, and a power block. In the plant, the solar radiation is firstly concentrated, and then transformed into thermal energy that is taken away by the heat transfer fluid (HTF) in the receiver. Then, the thermal energy can be stored in the thermal storage unit or utilized to produce high-temperature steam or gas in heat exchangers. Finally, the steam or gas will drive a power cycle for electricity generation. The practical CSP technologies can be classified into the following four types: parabolic-trough collector (PTC), linear Fresnel collector (LFC), solar power tower (SPT), and parabolic-dish collector (PDC). The solar concentrator and the receiver are key components serving as the energy source of the whole CSP system. In the absorber of the CSP receiver, the concentrated solar flux distribution is extremely non-uniform, which can cause challenges to the safe and efficient power generation. Therefore, many studies have been carried out to reveal the non-uniform flux, and lots of solutions have been proposed to solve the corresponding challenges. In this paper, the concentrators and the receivers in the four CSP technologies are briefly introduced. The characteristics of non-uniform solar fluxes in the systems are summarized. The challenges caused by the non-uniform fluxes are also discussed. Furthermore, a comprehensive review of the corresponding solutions proposed to tackle these challenges is presented. Finally, a summary of the key solutions is presented, and some suggestions are provided. It is to be noted that the non-uniform flux means the non-uniformity of the solar flux in or on the solar absorber in the current review. However, the structures of the absorbers in different receivers are significantly different. As a result, the non-uniformity has different meanings for different absorbers. In the linear evacuated receivers of PTC and LFC, the non-uniformity means non-uniformity in circumferential direction. For the absorbers using tube panels in SPT, LFC and PDC, the 2
non-uniformity means non-uniformity among different regions of the absorber panels. For the porous volumetric absorber in SPT and PDC, the non-uniformity means the non-uniformity in the whole volumetric absorber. Before the detailed review, some performance parameters of the solar collectors are defined as follows. The receiver optical efficiency (ηR,opt) is defined as the ratio of the radiant solar power absorbed by the absorber (Qabs) and the incident power on the receiver aperture (Qape) as shown in Eq.(1). The receiver thermal efficiency (ηR,th) is defined as the ratio of the power transferred to the HTF (QHTF) and Qabs as shown in Eq.(1). The receiver efficiency (ηR) is defined as ratio of QHTF and the Qape as shown in Eq.(2). The collector optical efficiency (ηC,opt) is defined as the ratio of the Qabs and the available radiant solar power that can be accepted by the collector (QC) as shown in Eq.(3). The collector efficiency (ηC) is defined as the ratio of the QHTF and QC as shown in Eq.(4).
R,opt Qabs Qape , R,th QHTF Qabs
(1)
R QHTF Qape
(2)
C,opt Qabs QC , QC DNI AC
(3)
C QHTF QC
(4)
where DNI indicates the direct normal irradiation; AC is the solar thermal collector net collection area. 2. Parabolic-Trough Collector 2.1. Description of PTC The parabolic-trough collector (PTC) which is the most mature practical CSP technology mainly consists of a linear receiver and a parabolic trough concentrator, as shown in Fig. 1. The receiver is generally installed along the focal line of the concentrator, where the evacuated tube 3
receiver is mostly employed. A typical evacuated tube is mainly composed of a glass cover and a steel absorber tube plated with a selective coating on its outer surface. The absorber tube and the glass cover are sealed by metal bellows to form a vacuum annular gap, where the getter is generally placed to maintain the vacuum (<0.013 Pa) to reduce the heat loss. In addition, heat pipes[3] and cavity receivers have also been proposed for PTC but are currently barely used[4]. As a result, this review would focus on the widely used evacuated tube. The typical PTCs using evacuated tubes are summarized in Table 1. HTF
Sun
Glass cover
Rays
Evacuated tube
Concentrator
Absorber
Fig. 1 Schematic diagram of parabolic-trough collector[5] (License Number 4476230830026). Table 1 Parameters of the typical parabolic-trough collector[6-9] Type
LS-1
LS-2
LS-3
Eurotrough
DS-1
Collector width / m
2.55
5
5.76
5.76
5
Collector length / m
6.3
8
12
12
8
Focal length / m
0.94
1.49
1.71
1.71
1.49
Absorber outer diameter / m
0.04
0.07
0.07
0.07
0.07
Geometric concentrating ratioa
20
23
26
26
23
Solar reflectance of concentrator
0.93
0.94
0.94
0.95
0.95
Solar absorptance of coating
0.94
0.94
0.96
0.96
0.96
Radiant emissivity of coating at 400 oC
0.3
0.2
0.1
0.1
0.1
4
a
Solar transmittance of glass cover
0.95
0.95
0.965
0.965
0.965
Theoretical peak collector optical efficiency
0.734
0.737
0.772
0.80
0.80
Geometric concentrating ratio is the ratio between the width of collector aperture and the perimeter of the
absorber.
Solar energy is concentrated onto the absorber and converted into thermal energy that is taken away by the heat transfer fluid (HTF), as shown in Fig. 1. Heat transfer oil or water/steam is widely used as the HTF. For example, in SEGS I-SEGS VIII plants built by LUZ company, the heat transfer oil is used, where the maximum temperature is 390 oC
[10]
. While in SEGS IX, the
water/steam is used, and the maximum temperature is increased to 402 oC. In addition, some other fluids such as CO2[11], helium[12], air[12] and molten salt[13] are also proposed as the HTFs in PTCs. 2.2. Non-uniform solar flux distribution in PTC The solar energy is concentrated by the concentrator and mainly focused at the bottom of the absorber tube. The solar flux distribution in the PTC is extremely non-uniform, which has been revealed by many studies. In experimental aspects, Riffelmann et al.[14], Lüpfert et al.[15] and Schiricke et al.[16] proposed two methods, namely PARAbolic Trough Flux SCANner (PARASCAN) and Camera Target Method (CTM), to measure the solar flux distribution on the PTC receiver. In the PARASCAN measurement, glass fiber sensors with high resolution are used to record the solar flux distribution along the circumferential direction. While in the CTM measurement, a diffuse reflecting Lambertian target is held perpendicularly to the focal line surrounding the absorber, and a calibrated camera are used to take pictures for the target. The paths of the reflected rays on the receiver tube and the detected local optical errors can be visualized with the resulting pictures. Fig. 2 shows the measured solar flux around the receiver of Eurotrough using both the PARASCAN[16] and CTM[14], where the non-uniform characteristic is clearly observed.
5
(a) Measured by PARASCAN (b) Measured by CTM Fig. 2 Solar flux distribution on Eurotrough receiver with PTC[14-16] (License Number 4476241391268)
In simulation aspects, there are mainly three methods for the calculation of solar flux distributions including cone optics method, semi-finite integration formulation, and ray tracing method. The above methods have been widely used to reveal the non-uniformity of the solar flux distribution on the absorber of PTC. Jeter[17] developed semi-finite integration formulation model to calculate the solar flux distribution in the PTC, and analyzed the effects of structure size on the non-uniformity of the flux. Daly[18] revealed the non-uniform flux distribution by using the reversed ray tracing method. Roberto[19] applied ray tracing method to simulate the solar flux distribution on the receiver surfaces, and further analyze the effects of the tracking and slope errors on the flux distribution. Men et al.[20] also obtained the solar flux distribution on the absorber by using Monte Carlo ray tracing (MCRT) method. He et al. [21] also developed an optical model based on MCRT method to calculate the solar flux distribution on the absorber surface as shown in Fig. 3. It can be observed that the flux distribution can be divided into four regions[21]. Section 1 is the shadow region, where the solar rays are shadowed by the receiver. Region 2 is the flux increasing region, where more rays are reflected to the receiver, so the flux increases steady. Region 3 is the flux decreasing region, where the flux reduces rapidly with the increasing angle. Region 4 is the direct irradiation region, where the absorber only receives direct solar rays, and the flux is in very low. In addition Khanna et al.[22] and Christian et al.[23] simulated the solar flux distribution on the absorber surface of a bent receiver by convolution method and ray tracing method, respectively. They found that the solar flux distribution can be more non-uniform for the bent absorber, as shown in Fig. 4.
6
60
2
1
2
3
Solar flux ql/ kW·m-2
4
40 30
y /m
Solar flux ql / kW·m-2
50
3
4
20 10 0 -180
-120
-60
0
60
120
Circumferential angle around tube /
180 o
x /m
z /m
55 50 45 40 35 30 25 20 15 10 5 0
(a) Circumferential distribution (b) 3D solar distribution [21, 24] Fig. 3 Solar flux distribution on the absorber surface of PTC (License Number 4476251364787).
In summary, from both the experimental measurements and the simulation studies, it can be observed clearly that the solar radiation is mainly concentrated on the bottom of the receiver tube, and the flux distribution is extremely non-uniform.
Fig. 4 Solar flux distribution for the bent receiver tube with PTC[22] (License Number 4476761022123)
2.3. Challenges and corresponding solutions Due to the limited thermal conductivity of the receiver tube and the heat convection ability within the tube, the non-uniform flux distribution will inevitably cause non-uniform temperature field in the tube wall. He and Cheng et al. [21, 25, 26] proposed a MCRT and Finite Volume Method (FVM) coupled simulation method to simulate the conjugated heat transfer within the receiver and 7
obtain the temperature distribution, where the non-uniform flux on the absorber surface obtained using MCRT is treated as a boundary condition in FVM. Fig. 5 shows the temperature distribution on the absorber outer wall. It can be observed clearly that the temperature distribution is extremely non-uniform, and great temperature gradient along the circumferential direction is found. In particular, if the carbon dioxide, the water/steam, the air, or the helium is adopted as the HTF, the non-uniform temperature distribution caused by the non-uniform flux can be more distinct due to their lower thermal capacity[11, 27]. The non-uniform flux results in high local temperature and large temperature gradient, which can cause great challenges for the safety and the efficiency of the receiver. The challenges can be summarized as follows. Firstly, the large circumferential temperature gradient can result in the deflection of the absorber tube. The deflection makes the absorber tube deviate from the focus line of the concentrator, thus increasing the optical loss. The absorber tube can even touch the glass envelope and rupture it, causing vacuum loss[27-30]. Secondly, the selective coating tends to degrade when the local temperature is too high, which limits the maximum operation temperature. Thirdly, Hydrogen formation and permeation occurs as a result of the thermal degradation of the HTF such as oil in the high-temperature environment[31], which will increase the heat loss. 680
Temperature / K
670 665 660
y /m
Temperature / K
675
655 650 645 -180
z /m
-120 -60 0 60 120 o Circumferential angle around tube /
180
(a) Circumferential distribution (z=5 m)
x /m
(b) 3D distribution
Fig. 5 Non-uniform temperature distribution of the receiver tube wall
[24, 25]
( License Number 4476460198306)
To avoid the above problems, it is helpful to flatten the temperature distribution by reducing the peak temperature and decreasing the temperature gradient. Fig. 6 summarizes the solutions to flatten the temperature distribution. The solutions can be approximately classified into three groups as follows. 8
Nonuniform solar flux
Cause
Limited convective heat transfer coefficient Limited conductivity
Nonuniform temp.
High local temp.
Large temp. gradient
Solutions Fins
Solution-1
Solution-2
Enhancing convective heat transfer
Improving tube conductivity
Spiral vortex generator
Porous insert
longitudinal vortex generator
Copper tube]
Bimetal tube
Twisted tape
Nano-fluids
Aluminium Tube et al.
Variable focus Secondary reflector in PTC with secondary reflector annular space parabolic trough
Solution-3
Homogenizing solar flux distribuion
Fig. 6 Techniques for solving the non-uniform temperature in PTC receiver.
2.3.1
Heat transfer enhancement and optimization
The first group of solutions is enhancing the convective heat transfer within the absorber tube. Numerous techniques for the convective heat transfer enhancement have been proposed for PTC receiver. Aldali et al.[29] and Munoz et al.[30] recommended the incorporation of internal helical fins within the tube to enhance the convection heat transfer. Massidda et al.[32] enhanced the convection heat transfer by increasing the surface roughness and adopting the helical swirl generators within the absorber tube. Deluss[11] also adopted two approaches including increasing the surface roughness and placing fins inside the absorber tube to flatten the temperature distribution. Aggrey et al.[33] recommended to insert wall detached twisted tape for the heat transfer enhancement in the absorber. These studies found that the peak temperature and temperature gradient can be reduced. Wang et al.[34, 35] designed an asymmetric outward convex corrugated tube receiver(ACPTR), and they found that the maximum enhancement of overall heat transfer performance factor can achieve 148% as shown in Fig. 7, and the maximum restrain of von-Mises thermal strain is 26.8%.
9
2.4 factor (Nu/Nus)3/(f/fs)
Overall heat transfer performance
2.6
2.2 ACPTR
2.0 1.8
p=Corrugation pitch D=diameter
1.6
1.4 1.2 1.0 14
28
42
56 Re / ×103
70
84
Fig. 7 Nusselt number(Nu) in the tube at different Reynolds number(Re)
98 [34, 35]
(License Number
4476491200760).
In these studies, the heat transfer enhancement techniques are implemented on the whole inner wall of the absorber. However, the use of enhancements at the upper part of the absorber helps little to the heat transfer because this part just accepts a little radiation, which would result in unnecessary pressure drop. As a result, to better match the heat transfer enhancement with the non-uniform flux distribution, Cheng and He et al. [36] proposed a novel receiver enhanced by longitudinal vortexes. In this receiver, longitudinal vortex generators are only employed at the absorber bottom which accepts the main concentrated solar radiation. They found that this receiver has a good comprehensive heat transfer performance, and the maximum wall temperature and temperature gradient can be greatly reduced as shown in Fig. 8. More recently, unilateral pin fins designed by Gong et al.[37] and unilateral straight fins optimized by Bellos et al.
[38]
have also been
employed in PTC receivers.
(a) Smooth absorber (b) novel absorber with unilateral generators Fig. 8 Comparison of temperature distributions on the absorber tube wall[36] (License Number 4476760270084). 10
Inserting porous medium into the absorber is also an effective technique for convective heat transfer enhancement. Reddy et al.[39-42] and Mwesigye et al.[43] carried out the thermal analysis of the receiver with porous inserts. Their results show that the porous inserts could enhance the heat transfer and consequently flatten the temperature. Moreover, a few studies have also considering the match between the non-uniform solar flux distribution and heat transfer enhancement. Wang et al.[44] studied the effects of the height (h) of two porous inserts in Fig. 9 (a) and (b). They found that the optimum thermo-hydraulic performance is obtained when h=0.25D in Fig. 9 (b), where the Nusselt number (Nu) increases about 5-10 times with the increase of 10-20 times in friction factor (f). Furthermore, Zheng et al.[45, 46] proposed a novel optimization method coupling the genetic algorithm (GA) and computational fluid dynamics to optimize the configurations of porous insert within the absorber. In the optimization, the performance evaluation criteria (PEC)[46] was treated as the objective function. Fig. 9(c) shows the optimal configuration of the porous insert. Fig. 10 shows the comparison of the axis velocity field distribution of the three inserts shown in Fig. 9. It can be observed that the optimal porous configuration can reshape the flow field to make more HTF flow through the bottom region. Moreover, the optimal configuration can improve the effective thermal conductivity at the bottom region. Therefore, the heat transfer ability of the GA configuration can better match the non-uniform flux. Fig. 11 shows the collector efficiency and the maximum temperature difference on tube outer wall. It can be seen that the optimal configuration can yield the largest collector efficiency and the lowest temperature difference with a wide range of Reynolds numbers.
D
Porous
h h (a) Reference-I
(b) Reference-II
(c) GA
Fig. 9 Two reference configurations and the optimal configuration of the porous insert [46] (License Number 4476760759145).
11
250
66
200 tem. diff
efficiency
Clear tube Fully filling Reference-I Reference-II GA
64
62
150
100
60
58 0.0
50
2.0x10
5
4.0x10
5
6.0x10
5
8.0x10
5
0 6 1.0x10
Max temp. difference of tube outer wall / K
Collector efficiency / %
68
Re
Fig. 10 Comparison of the axis velocity field distribution[46] (License
Number 4476760759145).
2.3.2
Fig. 11 Collector efficiency and maximum temperature difference on tube outer wall [46] (License Number
4476760759145).
Improvement of thermal conductivity of the tube
The second group of solutions is improving the thermal conductivity of the absorber tube. The simulation results from Delussu[11] and Aldai et al.[29] used the copper absorber tube with high thermla conductivity to flatten the tempearture distribuiton. However, the mechanical strength of copper is too low compared with that of steel. Almanza et al.[27] carried out experiments and pointed out that bending of absorber tube could almost be eliminated by replacing a steel tube with a copper tube due to the smaller circumferential temperature differences resulting from the high thermal conductivity of copper. In view of this, Flores et al.[47] recommended to use a bimetallic copper-steel absorber to reduce the deflection of the tube benefited from both the high thermal conductivity of the copper and the mechanical resistance of the steel. 2.3.3
Optimization of the collector to homogenize solar flux
Compared with the above two groups, homogenizing the flux distribution seems to be a more direct approach for flattening the temperature distribution. Several studies proposed some novel concentrators for homogenizing the flux. Tsai et al.[48] proposed an variable focus parabolic trough (VFPT) concentrator in which the focal length varied as a function of the displacement of the reflector relative to the receiver. Fig. 12 shows the compration of the solar flux distributions along the circumferential direction among the VFPT, the conventional parabolic trough (CPT), the cylindrical trough (CT), and the ideal trough. It can be observed clearly that the uniformity of the flux distribution was improved significantly. Moreover, Gee et al.[49] placed a secondary reflector within the annular space to reflect more solar rays onto the top of the abosrber, as shown in Fig. 13. In addition, Wang et al.[50] proposed to make the following improvements based on the conventional 12
PTC. Firstly, the absorber tube is moved downward to a suitable position away from the focal line of the PTC. Then, a secondary reflector is added. The secondary reflector and the PTC are arranged in the way shown in Fig. 14, where some solar rays can be reflected by the PTC onto the secondary reflector and then reflected onto the top of the absorber. Fig. 15 shows the the flux and temperature distributions on the absorbers of both the conventional and the novel PTCs. It is seen that the solar flux distribuion can be homogenized significantly, and the flux becomes almost uniform on the absorber in the novel PTC. The tempearture distribution is consequently also flattened, and the circumferential temperature difference was reduced from about 25 K to 3 K at a typical condition.
Fig. 12 Comparison of solar flux distributions amoing
Fig. 13 Ray paths in PTC receiver with a secondary [49]
four PTCs[48] (License Number 4476761195942).
reflector in the annular space
.
Secondary reflctor Absorber tube PTC
Fig. 14 Parabolic-trough collector with secondary reflector[50] (License Number 4476460198306)
13
680
T1, z=1 m
60
T1, z=3 m
675
Conventional PTC
T1, z=5 m T1, z=7 m
TemperatureT / K
ql / kW· m-2
50
40
30
Novel PTC with secondary reflector
20
10
0 -180
670
T2, z=1 m T2, z=3 m
665
T2, z=5 m T2, z=7 m
660 655
T1 for convention PTC T2 for novel PTC
650 -120
-60 0 60 120 o Circumferential angle around tube /
645 -180
180
(a) Comparison of solar flux distribuiotn
-120
-60
0
60
120
Circumferential angle around tube /
o
180
(b) Comparison of temperature distribuiton
Fig. 15 Comparison between conventional PTC and novel PTC [50] ( License Number 4476460198306)
3 Linear Fresnel Collector 3.1 Description of LFC Linear Fresnel Collector (LFC) is a solar collector in which a array of long, narrow, flat or slightly curved mirrors reflects the sun rays onto a fixed linear receiver mounted over a tower above and along the reflectors, as shown in Fig. 16[51]. The linear receiver is usually a single-tube cavity receiver (STCR) or a multiple-tube cavity receiver (MTCR), as shown in Fig. 17. A secondary reflector is also usually used in a STCR for homogenizing the flux in the receiver and increasing the acceptance angle [52]. When the sun shines on the mirror field on sunny days, firstly, the mirrors will track the sun and reflect the sun rays onto the absorber tubes directly for a LFC with a MTCR. However, for a LFC with a STCR, some rays will shine on the tube, and others will be reflected one more time by the secondary reflcetor before shining on the tube. Then, most of the rays will be absorbed by coating on the tube. Finally, the energy will be transferd to the heat transfer fluid in the tube. The LFC technology which has been proposed as an attractive low-cost option for CSP presents important advantages when compared with the PTC technology. Particularly, the use of stationary receiver without rotating joints or high-temperature moving components makes LFCs safer and more cost effective than PTCs. Moreover, LFCs use narrow primary mirrors which need no heavy supporting structures and thus result in lower the construction and operation costs. In addition, LFCs can work in a direct steam generation way with the pressure range of 3.5-11 MPa and the temperature range of 250-450 °C. Thus, it can reduce the size of heat exchangers in the 14
system. As a result, LFCs are being considered as competitors of PTCs in medium-low temperature CSP, and many prototypes and commercial plants have been built up to now, as summarized in Table 2. Table 2 Summary of the LFC plants built in the world [52-56] Capacity
Working
Receiver
Start
/ MWe
parameters
type
Production
Belgium
-
-
Water, steam
STCR
2001
LiddellⅠ
Australia
1.0
6.9 MPa, 285 °C
Water
MTCR
2004
FRESDEMO
Spain
0.8
11 MPa, 450 °C
Water, steam
STCR
2007
Kimberlina plant
USA
5.0
4 MPa,300 °C
Water, steam
MTCR
2008
Puerto Errado 1
Spain
1.4
5.5 MPa, 270 °C
Water
STCR
2009
Puerto Errado 2
Spain
30
5.5 MPa,270 °C
Water
STCR
2012
Liddell Ⅱ
Australia
9.0
5.5 MPa, 270 °C
Water
MTCR
2012
China
1.5
3.5 MPa,>400 °C
Water, steam
Augustin Fresnel 1
France
0.25
10 MPa, 300 °C
Water, steam
STCR
2012
Dhursar
India
125
9 MPa, 390 °C
Water, steam
-
2014
eCare project
Morocco
1.0
280 °C
Water
-
2014
Rende-CSP
Italy
1.0
280 °C
Oil
-
2015
Alba Nova 1
France
12.0
5.5 MPa,300 °C
Water, steam
STCR
2015
Llo plant
France
9.0
7 MPa, 285 °C
Water
-
2015
IRESEN plant
Morocco
1.0
300 °C
Oil
STCR
2016
Dadri ISCC Plant
India
14.0
250 °C
Water
-
2017
China
50.0
-
Water, steam
-
Under const.
China
50.0
-
Molten salt
MTCR
Under const.
Name
Country
Solarmundo
Huaneng Nanshan Power Plant
Zhangjiakou 50MW CSG Fresnel project Dacheng project
HTF
STCR
Receiver
Mirror field
Fig. 16 Schematic diagram of the LFC [51] (License Number 4476210462017) 15
2012
(a)
Insulation
(b)
Absorber
Insulation
Absorber Secondary reflcetor
Glass cover
Glass cover
Fig. 17 Typical receivers in LFCs: (a) Multiple-tube cavity receiver(MTCR); (b) Single-tube cavity receiver(STCR)
3.2 Non-uniform solar flux distribution in LFC The optical performance of a LFC is one of the major points concerned by the researcher and the practitioner, and it mainly determines the total input energy of the system and is of great importance for the performance optimization, system design, and safe operation. Hence, many studies have focused on the optical characterization of LFCs by optical modeling methods and experiments. In the early 1990s, Goswami et al.[57], Mathur et al.[58] and Sootha et al.[59] investigated the flux distribution on four different receivers, including triangular, flat horizontal, flat vertical and tubular receivers, of a LFC. The optical design and performance characteristics of these receivers were presented. At the same time, Feuermann and Gordon[60] evaluated the sensitivity of energy output to typical parameters of a 220 kW LFC with a secondary Compound Parabolic Collector (CPC) by ray tracing technique. They found that the LFC can be cheaper than the corresponding PTC, but the annual collector optical efficiency is about 25% lower. At the turn of the century, two significant breakthroughs were made in LFC technology. One is that a novel design called Compact Linear Fresnel Reflector (CLFR) was proposed by Mills and Morrison[61]. In the CLFR system, two alternative linear receivers on separate towers are offered to each mirror row between the towers as shown in Fig. 18, so it needs less land than PTC to produce a given output. Optical study using a multi-branched ray tracing model shows that a non-uniform characteristic of the flux on the receiver 16
can be observed. The other is the design and construction of the Solarmundo collector with a single-tube cavity receiver (STCR). Häberle et al.[62] investigated its collector optical efficiency (ηC,opt) by 3D ray tracing calculations. It is found that collector can achieve ηC,opt of 61% at normal incidence. receiver
reflector
Fig. 18 Sketch of the Compact Linear Fresnel Reflector (CLFR)
Furthermore, more studies on the solar flux characteristics in the LFC receivers have been conducted in recent years. Qiu and He et al. [63-66] developed a MCRT model and studied the optical characteristics of LFCs using various receivers. Fig. 19 shows the solar flux on the absorber tube in a STCR. It can be seen that the flux is significantly non-uniform with a hot spot at the tube’s bottom which is directly hit by the rays from the mirror field. However, only 23% power shine on upper half of the tube after the concentration of the secondary reflector. Similar non-uniform fluxes have also been observed in Balaji et al’s work[67].
17
Solar flux absorbed by tube / kW·m-2
120
Directly concentrated flux
90
W
θ E
60
30
0 -180
Secondarily concentrated flux
-90 0 90 Angle around absorber tube θ /
180
°
Fig. 19 Flux distribution on the absorber tube surface in the STCR of a LFC at noon[66] (License Number 4476201167570)
The flux distribution in a typical MTCR was also revealed by Qiu and He et al.[66] as shown in Fig. 20. It is seen that flux non-uniformity among the tubes is significant because all mirrors aim at the center of the receiver, where most rays shine on tube 4 and tube 5, while tube 1 and tube 8 are barely utilized. Moreover, Moghimi et al [68, 69] studied the flux distribution on each tube in a MTCR using finite volume method, and the result is shown in Fig. 21. It is seen that the circumferential distribution of the flux on each tube is also non-uniform, and the most power shines on the lower
ql / kW·m -2 10
20
30 40 50
60
70 80
90
-100
N
S Xr / m
0
part of the tube.
-0.3
1 2 3 4 5 6 7 8 Yr / m W E
0.3
Fig. 20 Flux distribution in the MTCR on spring equinox noon with qmax=92 kW·m-2[66] (License Number 4476201167570)
Fig. 21 Flux distribution on a tube in the MTCR of a LFC [69] (License Number 4476210752959)
In addition, Abbas et al.[70-72], Benyakhlef et al.[73] and Bellos et al.[74] investigated the concentration features on flat receivers which can be treated as the aperture of STCR and MTCR in 18
LFCs by the ray tracing method, respectively. It is found that the flux on the flat is also significantly non-uniform with a hot region at the center as shown in Fig. 22.
Fig. 22 Solar flux distribution on the flat receiver in a LFC [74] (License Number 4476211031373)
To sum up, we can see that the solar rays in a LFC are reflected by the mirror field and shine on the receiver from the lower aperture of the receiver, so most energy is concentrated on the lower part of the receiver in both STCR and MTCR. As a result, the fluxes with significant non-uniform characteristics appear in the receivers. 3.3 Challenges and corresponding solutions The non-uniform fluxes in the receivers of Linear Fresnel Collectors (LFCs) will lead to non-uniform temperature characteristics, which is similar to those that happen in PTCs. He et al[63, 64, 75]
developed a MCRT-FVM coupled model based on their work in PTCs, and the heat transfer,
including convection, radiation and conduction, in the receivers were studied under non-uniform fluxes. In this way, the solar-thermal conversion performance of the LFCs and non-uniform temperature characteristics in the receivers were revealed[64,
75]
. Fig. 23 shows the temperature
distributions on the tubes in a STCR and a MTCR. It is seen that large temperature gradients appear in both circumferential and lengthwise directions, and this non-uniform temperature would result in 19
two major negative effects on the system. The first one is that the local high temperature may accelerate the oxidation and degradation of the coating [76]. This is because only very few oxides and carbides can keep stable in air when the temperature is higher than 400 °C
[52]
. Eck et al.[77] also
indicate that the local temperature in the receiver can be larger than 569 °C, which is much higher than the limitation of the coating. The second one is that large temperature gradient would lead to large thermal stress which would result in undesirable distortion and damage of the receiver [76]. E T/K 654 650 646 642 638 634 630 626 622
0.04
zr / m
0.02
0
-0.02 -0.04 0.04
0.02
0
-0.02 -0.04 -8
-6
-4
-2
W
1.0 N S Zr Xr
0
0
yr / m
1
(a) STCR
2
3
4
5
6
7
(b) MTCR
Fig. 23 Non-uniform temperature distributions on the absorber tubes in LFC receivers [64] (License Number 4476211457701)
For solving these negative effects, the heat transfer enhancements for PTCs are also suitable for the LFCs. Besides, some novel designs and optimizations of the collectors have also been proposed for improving the flux uniformity. These solutions include design of the secondary reflector and optimization of aiming strategy, etc. 3.3.1
Design of the secondary reflector
Grena and Tarquini [78] proposed a new double-wing secondary reflector for a LFC, as shown in Fig. 24(a). Their simulation results showed that the flux distribution on the tube was more uniform than that of PTC, as illustrated in Fig. 24(b), and about 37% of the total power shines on the upper half of the tube. However, Hack et al.[79] found that this double-wing design may lead to a 20
8
significant decrease in collector optical efficiency due to the smaller intercept factor compared with that of the ordinary CPC. Prasad et al.
[80]
designed a segmented parabolic secondary reflector that
was made of three parabolic segments, as shown in Fig. 25(a). They found that the new secondary reflector could lead to a reasonable uniform flux under a wide range of the incident angle when it was used in conjunction with the proper aiming strategy, as shown in Fig. 25(b). They also found that the secondary reflector could help to improve the collector optical efficiency slightly under its design condition.
(a) Sketch (b) Relative flux distribution Fig. 24 Sketch of a double-wing secondary reflector and relative flux distribution on the tube [78] (License Number 4476221213103).
(a) Sketch (b) Flux distribution Fig. 25 Sketch of a segmented parabolic concentrator and fluxes on the tube under different incident angles(θi) [80] (License Number 4476230113405).
3.3.2
Optimization of aiming strategy 21
A basic aiming strategy to LFCs is the one-line strategy (S1), as shown in Fig. 26(a), where all mirrors focus on the center line of the aiming plane. Eck et al.[77] indicated that solar flux on the absorber tube can be reduced by defocusing mirror-to-mirror. However, the defocusing can lead to important optical loss if it is not properly designed. Qiu et al.[66] proposed a multi-line aiming strategy (S2) by combining ray tracing and the genetic algorithm (GA), where the optical loss and a non-uniformity index of the flux distribution were used as the objectives. The new strategy is illustrated in Fig. 26(b), where naim aiming lines are uniformly distributed on the aiming plane. The aiming line for each mirror is chosen from the naim lines, and the width of the aiming plane (Waim) in Fig. 26(b) is optimized as well. Waim Aiming plane
Aiming plane
Aiming line 1
All mirrors aim at the center of the plane
2
i
naim Select a line for each mirror
(2) One-line aiming strategy (S1)
(b) GA multi-line aiming strategy (S2)
Fig. 26. Sketches of two aiming strategies for LFCs[66] (License Number 4476201167570).
Application of GA multi-line aiming strategy (S2) indicates that fluxes in both STCR and MTCR can be homogenized efficaciously by S2 in the whole possible time range with a small drop of 0.2-3.8 percentage points in collector optical efficiency compared with those of one-line aiming strategy (S1). In the STCR, 35-40% power can shine on the top half of the tube[66] as shown in Fig. 27. In the MTCR, the maximum flux (ql,max) at typical time is reduced from 92.8 kW·m-2 in Fig. 20 to 38.6 kW·m-2 in Fig. 28[66]. Qiu et al.[66] concluded that the GA multi-line aiming strategy could help to find a compromise between the flux uniformity and collector optical efficiency, and uniform fluxes could be obtained at all possible incident angles. 22
S1: S2:
8.138h, 8.138h,
10.136h, 10.136h,
12h 12h
90 0
ql / kW·m -2 2
6 10 14 18 22 26 30 34 38 42
S Xr / m
W
θ E
N
60
30 -100
local solar flux / kW·m-2
120
0 -180
-90
0
90
angle around the tube θ / °
-0.3
1 2 3 4 5 6 7 8 Yr / m W E
0.3
180
Fig. 27. Flux distributions in STRSC on spring equinox when S1 and S2 are used[66] (License Number 4476201167570)
Fig. 28. Flux in MTCR on spring equinox noon using GA aiming strategy (S2), ql,max= 36 kW·m-2[66] (License Number 4476201167570)
4 Solar Power Tower 4.1 Description of SPT collector The solar collector subsystem in SPT mainly consists of the solar heliostats field, the tower, and the solar receiver[81], as shown in Fig. 29. The solar radiation is concentrated by numerous heliostats and reflected onto the receiver where the heat transfer fluid (HTF) is heated up to a relatively high temperature. The heliostats shown in Fig. 29 are the concentrating components of the plant, and each heliostat consists of mirror panels, frames, tracking and control devices, where toughened glass, rear-silvered and rear-aluminum mirrors are employed [82, 83]. Moreover, the size of the heliostat is usually in a wide range of 1.0-178 m2[84-86]. Heliostat Receiver
Fig. 29 Typical heliostat field and central receiver in a solar power tower plant[87, 88] (License Number 4476231414559, 4476240505245). 23
The receivers used in SPT system can be classified by the operating pressure, the HTFs and the structures. For example, they can be classified into molten salt receivers, air receivers, and water/steam receivers according to various HTFs. Moreover, the receivers can also be divided into tubular receiver as shown in Fig. 30 and volumetric receiver as shown in Fig. 31. Furthermore, the tubular receivers can be further divided into external and cavity structures, and the volumetric receivers can be divided into open-loop types, such as SOLAIR type[89], and closed-loop types, such as REFOS[85] and DIAPR type[90]. In the volumetric receiver, honeycomb channels and foams are usually used as the absorber. Table 3 exhibits different receivers in current SPT plants. In recent years, some new receiver concepts such as falling-particle receiver[91] and falling-film receiver[92] have also been developed, but no plant has been built. Table 3 Summary of typical receivers in SPT plants[81, 84-87, 93-95]. Receiver
Outlet
Start
temperature
Production
Molten salt
565 °C
1995-1999
2.5
Water, steam
440 °C
2009
Spain
19.9
Molten salt
565 °C
2011
Australia
3.0
Water, steam
500 °C
2011
Turkey
1.4
Water, steam
550 °C
2012
Ivanpah
USA
392
Water, steam
565 °C
2014
Crescent Dunes
USA
110
Molten salt
565 °C
2015
Sundrop
Australia
1.5
Water, steam
500 °C
2011
Supcon Delingha
China
10
Molten salt
568 °C
2016
Khi Solar One
South Africa
50
Water, steam
530 °C
2016
SunCan 10MW
China
10
Molten salt
565 °C
2016
Ashalim 1
Israel
121
Water, steam
600 °C
2017
Jemalong
Australia
1.1
sodium
560 °C
2017
NOOR III
Morocco
150
Molten salt
565 °C
2017
SunCan 100MW
China
100
Molten salt
565 °C
2018
Yumen 50MW
China
50
Molten salt
565 °C
2018
Solar One
USA
10
Water, steam
-
1982-1986
Tubular,
THEMIS
France
2.0
Molten salt
450 °C
1983-1986
cavity
MSEE
USA
750 kWe
Molten salt
565 °C
1984-1985
MSS/CTE
USA
5 MWt
Molten salt
565 °C
1987
type
Project
Country
Solar Two
USA
Sierra
USA
Gemasolar Lake Cargelligo Greenway CSP Mersin
Tubular, external
Capacity / MWe
24
HTF
4.5 MPa,
PS10
Spain
10
Water, steam
PS20
Spain
20
Water, steam
Sierra
USA
2.5
Water, steam
ACME Solar
India
2.5
Water, steam
DAHAN
China
1.0
Water, steam
400 °C
2012
TSA
Spain
2.5 MWt
Air
700 °C
1993
Volumetric,
SOLAIR 200
Spain
1.5 MWt
Air
700 °C
2001
open-loop
HiTRec II
Spain
200 kWt
Air
800 °C
2003
SOLAIR 3000
Spain
1.5
Air
800 °C
2006
Consolar/DIAPR
Israel
50 kWe
Air
1200 °C
1992
Volumetric,
CESA-1/REFOS
Spain
350 kWe
Air
800 °C
1996
closed-loop
SOLGATE
Spain
400 kWe
Air
1000 °C
2002
Jülich
Germany
1.5
Air
700 °C
2008
300 °C 4.5 MPa, 300 °C 440 °C 6 MPa, 440 °C
2007 2009 2009 2011
(a) Cavity receiver (b) External receiver [96, 97] Fig. 30 Tubular solar receiver (License Number 4476250147199, 4476250747377). Rays
Wall
Porou s abs orb er
Air
(a) Closed loop [98] (License Number 4476251112251) Fig. 31 Volumetric receiver
25
(b) Open loop
4.2 Non-uniform solar flux distribution in SPT The concentrated solar flux distribution on the receiver influences the receiver performance significantly. So many experimental and simulation studies have been conducted to reveal the solar flux distribution in SPT receiver. Firstly, some experimental measurements have been conducted. For example, Ho et al.[99] presented a photographic flux (PHLUX) method, by which the solar flux distribution in the receiver or target surface could be accurately measured, as shown in Fig. 32. Röger et al.[100] used a similar method to measure the flux at the aperture of a volumetric receiver in Plataforma Solar de Almeria, and the result was validated with a result obtained using moving bar measurement. More measurements for the non-uniform solar flux distribution have also been conducted, which can be acquired from the operation reports of the Solar One, Solar Two[101, 102] and SOLGATE[95] plants.
[99]
Fig. 32 Measured solar flux on the focal plane of the National Solar Thermal Test Facility in Sandia lab
.
Secondly, many simulation codes have been developed to simulate the solar radiation transmisson process in the SPT plant, such as UHC, HFLCAL and DELSOL based on convolution methods, HFLD, MIRVAL, SOLTRACE and SPTOPTIC based on MCRT[96, 103-110]. Yao et al.[111], Besarati et al.[112], He et al.[85], Salomé et al.[113], and Farges et al.[114] investigated the solar flux distributions on the apertures of different plants using these codes. Their results showed that the flux distribution was extremely non-uniform which can be approximated as a Gaussian distribution, 26
as shown in Fig. 33. Moreover, Yu et al.[87, 93], Rinaldi et al.[115], and Wang and He et al.[116] further reveal the non-uniform fluxes on the inner surfaces of the cavity receivers. As an example, the solar flux distribution in PS10 plant is shown in Fig. 34. In addition, Rodríguez-Sánchez et al.[97] and Sanchez Gonzalez et al.[117] gave the non-uniform fluxes on the panels of external receiver as shown in Fig. 35. However, in these studies, the absorber tubes are assumed to be flat panels where the details of the tubes are not considered. In view of this situation, Qiu and He et al
[96]
developed a
software SPTOPTIC based on MCRT, where detailed geometries and optical processes of the SPT were considered. This tool was tested by simulating the DAHAN collector, and detailed solar fluxes on the absorber tubes within the cavity receiver was obtained, as shown in Fig. 36. This software can be employed to study the thermal performance of the receiver.
E
Fig. 34 Typical flux on the inner surfaces of PS10’s cavity receiver [115] (License Number 4476340902945). W
W·m-2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
3
Yr / m
Fig. 33 Typical flux at the focal plane using one-point aiming strategy[112] (License Number 4476330750001).
0
-3
Fig. 35 Typical flux on an [117] external receiver (License Number 4476341328562). W·m -2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
(a) flux on tubes and cavity walls (b) local high flux, highest flux is 514 kW·m-2 Fig. 36 Solar flux on the absorber tubes using one-point aiming strategy at spring equinox noon [96] (License Number 4476250147199).
The reason why the volumetric air receiver is called volumetric is that the solar radiation can 27
penetrate into the “volume” of the absorber through open, permeable cells of material. The flux characteristics of the volumetric receivers have also been studied by He et al.[85, 118], Cheng et al.[119, 120]
, Cui et al.[121], Qiu et al.[122] and Du et al.[98]. It is revealed that the solar flux distributions in the
porous absorbers of both closed-loop and open-loop volumetric receivers are significantly non-uniform, as shown in Fig. 37. It is found that a high-flux region occurs near the center of the closed-loop absorber, which is due to the re-concentration of the secondary concentrator. The peak heat sources in the two absorbers can be larger than 1.1×108 Wm-3 and 2.3×108 Wm-3, respectively. It is also seen that the flux decreases gradually along the incident direction in the absorber because the porous absorber absorbed the incident radiation gradually.
(a) Closed-loop[98] (b) Open-loop[122] Fig. 37 Solar fluxes in the porous absorbers of volumetric receivers(License Number 4476251112251).
In summary, the solar flux distributions on the inner surfaces of the tubular receiver and in the porous volumetric receivers are highly non-uniform. The solar flux is manily distributed at the specific part of the tubular receiver. Particularly, it should be noted that the solar flux is only on the unilateral surface of each single tube. Moreover, for the porous absorber, the peak flux occures at the inlet of the absorber. 4.3 Challenges and the corresponding solutions There is no doubt that the extreme non-uniformity of solar flux distribution will lead to the non-uniform temperature in the receiver. The non-uniform temperature would result in negative effects on the safety and operation of the solar receiver, and local overheating and HTF decomposition and stress fracture may occur[96, 123, 124]. For example, the structure damages of 28
receiver have occurred in the Solar Two plant[101,
102]
. Particularly, comparing with the
parabolic-trough collector and linear Fresnel collector, it will cause much greater challenges for the SPT due to its higher solar flux and operating temperature. In tubular receivers using molten salt as the heat transfer fluid, under the synergistic effect of thermal stress and molten salt corrosion, the strength of the steel tube would be weakened. As a result, the fracture failure of the tube may happen when the thermal stress exceeds the ultimate strength limit after being degraded. Du et al.[125-127] numerically analyzed the influences of the non-uniform flux in a cavity receiver using an optical-thermal-stress coupling model. Fig. 38 shows the solar flux, temperature and thermal stress distributions in the receiver, where extremely non-uniform flux and high thermal stress are observed. Similar results have also been obtained by Montes et al.[128] in a salt cavity receiver and by Yu et al.[129], Xu et al.[130] and Fang et al.[131, 132] in water/vapor cavity receivers.
(a) Solar flux
(b) Temperature
(c) Thermal stress Fig. 38 Typical solar flux, temperature and thermal stress on a cavity receiver at spring equinox noon[127](License Number 4476350714687).
Meanwhile, the thermal performance of the volumetric receiver with complex heat transfer processes have also been studied. He et al.[118, 133] used MCRT and FVM coupled method to study the heat transfer perfornance of a closed-loop porous receiver, where the temperature of the absorber was revealed in Fig. 37(a). Moreover, the temperature in a open-loop receiver shown in 29
Fig. 37(b) has been revealed by Cagnoli et al[134]. It can be observed that the flux is greatly non-uniform from the inlet to the outlet, which could result in overheating and breakage of the absorbers. Similar results have also been obtained by Cheng et al.[119], Capuano et al.[135], and Nakakura et al.[136].
(a) Closed-loop(License Number 4476251112251)
(b) Open-loop porous receiver with channels (License Number 4476351285471) Fig. 39 Temperature distributions in the absorbers of volumetric receivers [98, 134].
In order to avoid the above problems caused by the non-uniform temperature, many solutions have been proposed. They can be divided into three categories including heat transfer enhancement and optimization, optimization of solar absorptance distribution in receiver, and optimization of the aim strategies and the receiver structure. The details are as follows. 4.3.1
Heat transfer enhancement and optimization
The heat transfer enhancements could be used to weaken the non-uniformity of temperature distribution in the solar receiver, thus the negative effects can be weaken. For the tubular receiver, all the enhancing techniques for the heat transfer in tube used in the PTC receiver should still be effective. Moreover, some studies that specially focus on the absorber in SPT have also been conducted. Yang et al.[137] and Lu et al.[138] experimentally studied the heat transfer of molten salt in spiral absorber tube. They found that the Nusselt number of the spiral tube was about 3 times of that of the smooth tube on average as shown in Fig. 40. Meanwhile, the wall temperature of the spiral tube could be about 30 oC smaller than that of the smooth tube under the identical conditions. The heat transfer of molten salt in a transversally corrugated tube has also been studied by Chen et al.[139], and which found that its performance was better than smooth tube. Moreover, similar to that in PTC, porous medium insert has also been considered by Zheng et al[140], 30
and they found that the partly filled tube could achieve better overall performance than the fully filled one. 1400 1200
Nu number
1000
Spiral tube
800 600 400 200
10000
20000
30000 40000 Re number
50000
60000
Fig. 40 Comparison of the Nu numbers for spiral tube and smooth tube[137] (License Number 4476360659740).
In the tubular receiver, two loops arrangement with several connected tube panels in each loop are usually employed. The effects of different fluid flow layouts on the temperature distribution were analyzed by Wang et al.[125], as shown in Fig. 41. It indicates that the average surface temperature of the inwards layout can be greatly lower than that of the outwards layout, which can
Inlet
R ec e ive r he ight
help to reduce the heat loss and high thermal stress area. Wall temp. / 550
Receiver height
500 450 400 350
Out let
W a l l t. e /m p 55 0
50 0 45 0 40 0 35 0
300 Outlet
Receiver width
30 0 Inlet
Outlet
(a) Outwards layout
R e c e i v e r w i d t Ih n l e t
(b) Inwards layout
Fig. 41 Comparison of temperature on the absorber surfaces between two fluid flow layouts[125]
Moreover, a new fluid flow layout was proposed and optimized by Montes et al.[128], as shown in Fig. 42. In this layout, the inwards flow is employed, and different tube diameter and number are used in different panels. As a result, the HTF velocity in the panels near the center that accepts the peak flux would be larger than those near the left or right border. The results indicate that the 31
surface temperature and radiation loss are decreased because the heat transfer ability of fluid matches the flux distribution better. In addition, Rodríguez-Sánchez et al.[141] proposed a Variable Velocity Receiver (VVR) as shown in Fig. 43, which was equipped with valves that allow the division of each panel in two independent panels and could increase the HTF velocity in specific zones of the receiver. The results show that this receiver can avoid tube overheating, allowing more concentrated aiming strategies, which increases the optical efficiency of the heliostat field.
Fig. 42 Optimized fluid flow layout for cavity receiver [128] (License Number 4476370167117)
Fig. 43 Schematic of the flow layout in a variable velocity external receiver[141] (License Number 4476370635883) .
For the closed-loop volumetric receiver, Röger et al.[142] proposed to use air-jet cooling devices to cool the quartz glass. Their experiments found that window temperatures could be kept below 800 °C when the air outlet temperatures is over 1000 °C, which helps to improve the reliability importantly.
[142]
Fig. 44 air-jet cooling devices for the glass window of a closed-loop receiver 32
(License Number
4476371452873).
For the open-loop volumetric air receiver, Nakakura et al.[136] proposed new cut-back inlet designs that removed different amounts of material from the walls at the inlets of square channels. Their numerical results demonstrated that the exit temperature could be increased by 20.0 K, and the pressure drop could be decreased relative to the plane channel receiver. Capuano et al.[135] developed an optimized absorber with spiny inlet. Their results showed that the receiver efficiency of 84% could be achieved at the outlet temperature of around 850 °C, which is improved by 13% compared with the current state-of-the-art mini channel design (Hitrec-II). Moreover, the so-called volumetric effect was observed, since the outlet fluid temperature was higher than the solid inlet temperature, which helped to improve the reliability significantly. Pabst et al.[143] developed and optimized a receiver using cellular metal honeycomb, and three optimal channel designs enhanced by winded pairs of flat and corrugated metal foils were obtained, as shown in Fig. 46. The three designs could achieve similar performance. Fit curves of experimental receiver efficiency data are appearing in Fig. 46, where each curve represents a set of data for the three different channels. It is found that a receiver efficiency of about 83% can be achieved at the outlet temperature of more than 800 °C.
Single unit
Receiver efficiency / %
Receiver efficiency
Outlet temp. 850
New receiver
Fig. 45 Receiver efficiency of the new receiver and that of Hitrec-II receiver[135] (License Number 4476371452873).
4.3.2
Receiver
Three different channels
Fig. 46 Results of the 500 kW receiver efficiency tests of the [143]
new receiver
(License Number 4476380577337).
Optimization of solar absorptance distribution in receiver
The optimization of solar absorptance distribution in the solar receiver is another approach to avoiding the problems from the non-uniform flux and temperature. Wang and He et al.[144], Fang et al. [145], and Tu et al. [132] optimized the absorptance distribution of the coating in the 33
cavity receiver. Especially, the study from Wang and He et al.[144] tried to homogenize the flux distribution while keeping the optical loss as low as possible. Fig. 47 illustrates the optimized solar absorptance and flux distributions in a cavity receiver at spring equinox noon. Compared with the original flux distribution in Fig. 47(b) before the optimization, it is observed in Fig. 47(a) that the optimal absorptance distribution is approximately opposite to this flux distribution. The absorptance is relatively low at the regions where the original flux is relatively high, and vice versa. It is also seen in Fig. 47(c) that the flux distribution become quite uniform, the peak solar flux is significantly reduced with a little increase in reflective loss. For the closed-loop volumetric receiver, Roger et al.
[146]
proposed to use infrared-reflective coating on the quartz
window shown in Fig. 31(a). The results indicates that the glass temperature can be decreased by 65 K.
(a) solar absorptance distribution
(b) Original flux distribution, peak
(c) optimal flux distribution, peak flux= 336 kWm−2,
flux= 467 kWm−2, reflective loss= 3.4%
reflective loss=5.5%
Fig. 47 Optimized absorptance distribution and fluxes using single-point aiming strategy at spring equinox noon[147]
(License Number 4476350714687).
4.3.3
Optimization of aiming strategy and receiver geometry
The single-point aiming strategy for south-facing field and the equatorial aiming strategy for 34
surrounding field are usually adopted in the SPT system for the high interpret efficiency and the low optical loss. However, the solar flux distribution is extremely non-uniform when using such aiming strategy. For homogenizing the flux, multi-point aiming strategy is suggested for the SPT. Sánchez-González and Santana[117] studied the effects of the multi-point strategy on an external receiver for a circular field. Qiu et al
[96]
compared the one-point and multi-point aiming strategies
based on the configurations of DAHAN plant using SPTOPTIC. Du et al
[98]
also designed a
multi-point strategy for a south-facing heliostat field using volumetric receivers. In these studies, the aim point position for each heliostat was designed by estimating the radius of the reflected beam using the method provided by Vant-Hull[148]. A typical solar flux using the multi-point strategy is shown in Fig. 48, which is found to be more uniform compared with that of the one-point strategy as shown in Fig. 36. The highest flux is reduced from 514 kW·m-2 to 460 kW·m-2 with only one percent drop in collector optical efficiency at typical condition. W
E
W·m-2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
Fig. 48 Solar flux in a cavity receiver with multi-point aiming strategy at spring equinox noon [96] (License Number 4476250147199).
Many studies have also been carried out to optimize the multi-point aiming strategy in SPT system. Besarati et al.[112] optimized the flux distribution at the focal plane of a heliostat field using the genetic algorithm. The optimal flux is shown in Fig. 49, and it is seen that the peak solar flux is decreases by 10 times compared with the flux distribution using one-point strategy in Fig. 33. Yu et al.[83] developed a multi-point aiming model using TABU meta-heuristic method for the DAHAN plant, and the solar flux distribution was found to be successfully optimized. Using the non-dominated sorting genetic algorithm, multi-objective optimization of the aiming strategy for the SPT with a cavity receiver was also performed by Wang and He et al
[149]
. Fig. 50 shows the solar
flux distribution on the inner surfaces of a cavity receiver using multi -point aiming strategy. Comparing with the original flux using one-point strategy in Fig. 47(a), it is found that the flux 35
distribution is homogenized by the multi-point aiming strategy obviously, the peak flux can be decreased from 467 kW·m-2 to 280 kW·m-2.
Fig. 49 Optimal solar flux distribution on the cavity aperture using multi-aim strategy[112] (License Number 4476330750001).
Moreover, Sánchez-González et al.[150,
Fig. 50 Solar flux distribution using multi-point aiming strategy[149] (License Number 4476381078603).
151]
presented an aiming model to properly point
heliostats at cylindrical molten salt receivers, and a typical flux and aim points are shown in Fig. 51. Two iterative algorithms (search and fit) were used to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints were translated into allowable flux densities that were handled by the model in their study. Compared with the equatorial aiming, receiver interception is just 0.04-point lower using the proposed strategy, but the peak flux is significantly reduced up to 23%.
Fig. 51 Optimal solar flux distribution on an external cylinder receiver showing aim points(circles)[151] (License Number 4476381453474).
Optimizing the receiver structure is also helpful to improve the uniformity of solar flux 36
distribution. Tu et al [152] found that the solar flux can be more uniform after the optimization of the geometric parameters, as shown in Fig. 52. In addition, Buck et al.[153] proposed a novel dual-layer receiver concept which is designed by installing a tubular evaporator section before a volumetric receiver. Their numerical results indicated that the new concept showed several benefits, especially higher receiver efficiency and lower receiver temperature.
(a) Before optimization (b) After optimization Fig. 52 Comparison of solar flux distributions between cavity receivers with different structures[152] (License Number 4476390466882).
5. Parabolic-Dish Collector 5.1. Description of parabolic-dish collector The Parabolic-Dish Collector (PDC)
[154, 155]
usually consists of a parabolic dish reflector, a
receiver and solar tracing devices as shown in Fig. 53. The receiver is installed at the focus of parabolic dish reflector. During normal operation, firstly, the sunlight is reflected by the reflector to the receiver. Then, the solar energy is absorbed and transformed to thermal energy in the heat transfer fluid through the receiver. Finally, thermal energy is used to generate electricity by heat engine and generator
[156]
. Currently, there are only a few small-scale PDC plants in the world as
shown in Table 4 due to the high cost of investment.
37
Parabolic reflector
Cavity receiver Porous In out
Volumetric receiver
Fig. 53 Schematic diagram of Solar dish system[157, 158] (License Number 4476221004890).
Compared with other CSP systems, the electric generator and receiver are usually coupled with the collector in PDC, which makes it more compact. Now there are two main solar dish systems including solar dish/micro gas turbine system and solar dish/Stirling system. Receiver for dish system is usually a cavity or volumetric receiver as shown in Fig. 53. The cavity receiver has a solar absorbing surface behind the focal plane, so that solar flux on the surface can be smaller than that at the focal plane, and thermal stress can be reduced. Moreover, the heat loss of cavity receiver is also reduced for the using of cavity. The volumetric receiver usually uses a porous media as the absorber, where it is important for enhancing the heat transfer between the absorber and the heat transfer fluid, especially when gaseous fluids that have low heat transfer ability are used. Table 4 Summary of the solar dish plants in the world
[159]
Name
Country
Capacity / MW
Engine
Receiver
Start Production
Current status
Maricopa Solar Project
USA
1.5
Stirling
cavity
2010
Non-operational
Tooele Army Depot
USA
1.5
Stirling
cavity
2013
Non-operational
5.2. Non-uniform solar flux distribution in PDC Similar to the three systems mentioned above, solar flux is extremely non-uniform at the surface of a dish receiver. To gain an insight to the influence of non-uniform solar flux on the performance of a dish receiver, much work has been done on the research of concentration characteristics of a dish collector. 38
Experimentally, it has been proven that solar flux at the entrance of a dish receiver is non-uniform[160-163]. Using Charge Coupled Device (CCD) imaging cameras, Johnston et al.[162], Shuai et al.[164], and Dähler et al.[165] gained non-uniform flux maps at the focal plane of PDCs, and a typical experimental result is illustrated in Fig. 54. Moreover, Jaramillo et al.[163] and Yu et al.[166] studied the distributions of solar flux and temperature at the focal plane. They found the temperature distribution was non-uniform together with non-uniform solar flux.
Fig. 54 Measured solar concentration ratio distribution at focal plane of a PDC[165] (Creative Commons).
Besides experimental study, more researches focused on theoretical revelation and numerical prediction of the non-uniform flux. The early research by Steinfeld et al.[167] used Gaussian distribution to describe the flux distribution at the focal plane. With the development of compute technology, many researchers applied ray tracing method to study the flux characteristics in more complex receivers[168]. Some studies focused on PDCs with cavity receivers. Shuai et al.[161] predicted the optical performance of a PDC for six different cavities, and non-uniform solar fluxes were observed on all cavity walls, as shown in Fig. 55. More similar studies on several receiver geometries have also been conducted by Cui et al.[169, 170], Tao et al.[171], Daabo et al.[172] and Yan et al.[173]. In addition, Dai et al.[174] and Xia et al.[160] numerically investigated concentrating properties of a PDC using a half-ellipse receiver with or without a 3D CPC, and the non-uniform flux in the receiver was also observed. They found that the concentration ratio could be improved by the 3D CPC, but a decrease in collector optical efficiency was also observed.
39
Fig. 55 Solar flux distributions on in different solar receivers of solar dish system[161] (License Number 4476211001312).
For the volumetric receiver, the solar heat source distribution in the porous absorber has also been widely studied [175-177]. Barreto et al.[177] and Chen et al.[176] simulated the radiation transfer in PDCs using porous media receivers which were assumed to be isotropic semitransparent mediums. While Du et al.[178] used X-ray computed tomography technique to reconstruct a realistic porous absorber and obtained the solar energy in the absorber using MCRT, as shown in Fig. 56. These studies found that the heat source is extremely non-uniform, and most power is absorbed at a thin region near the inlet of the absorber.
Fig. 56. Solar energy distribution in a porous volumetric receiver[178] (License Number 4476201217119).
5.3. Challenges and corresponding solutions To reveal the influence of non-uniform solar flux on the performance of the solar dish receiver, Cui and He et al.[170] applied MCRT and FVM coupled method to study the solar-thermal 40
performance of cavity receivers in dish system and analyzed the effect of non-uniform flux on temperature distribution. They found that non-uniform solar flux could result in significant non-uniform temperatures on the receiver walls as shown in Fig. 57, where the lowest temperature of 1296 K and the highest temperature of 1375 K could occur in a single receiver. Moreover, Du et al.[178] studied the pore-scale heat transfer in a porous absorber using MCRT and FVM, and the no-nuniform temperature distributions of the air and solid under the non-uniform solar heat source in Fig. 56 are shown in Fig. 58. In addition, Wang et al.[179, 180] and Chen et al.[175] numerically studied the thermal performance of porous volumetric receivers using the local thermal non-equilibrium model. They also found that the temperature distribution inside the porous absorber was quite non-uniform due to the non-uniform solar heat source.
(a) solar flux distribution (b) temperature distribution Fig. 57 Flux and temperature distribution on the wall of a hemispherical cavity receiver[170] (License Number 4476210516201)..
41
Fig. 58 Temperature distribution in porous volumetric receiver[178] (License Number 4476201217119).
It is known that the non-uniform temperature challenges the safe operation of the system, because it may cause strong thermal stress and serious thermal deformation. Besides, local high temperature may also result in thermal denaturation of heat transfer fluid. Because of larger concentration ratio and higher operation temperature, the issue is more rigorous for solar dish system than that for other concentrating solar power systems. Li et al.[181] pointed out that more uniform temperature distribution in solar dish receivers and higher receiver efficiency were the keys to improve the reliability and economy of solar dish system. To solve these problems introduced by non-uniform flux, several solutions have been proposed and explored. 5.3.1 Optimization and improvement of receiver structure A feasible solution to homogenize the flux in the cavity receiver is the geometric optimization of the receiver. Shuai et al.[182] designed a pear-like cavity receiver based on the concept of equivalent flux, as shown in Fig. 59. It can be seen that the solar flux distribution can be more uniform in the pear-like receiver than that in corresponding hemisphere receiver, as shown in Fig. 42
60. Considering the redistribution effect of the quartz window, a cavity receiver with a plano-convexo quartz window as shown in Fig. 61 was also designed to weaken the non-uniform flux by Shuai et al.[183]. Fig. 62 shows that the receiver with a plano-convexo window can provide a more uniform flux distribution and higher receiver efficiency than the windowless receivers. Moreover, Phase Change Material (PCM) thermal storage technology[171] and heat pipe technology[184] have also been applied to gain more uniform temperature. These technologies have been proven to be effective in reducing the temperature difference and thermal stress of the receiver. For example, Tao and He[171] studied on the coupling heat transfer containing phase change in a cavity receiver. It was found that the temperature gradient could be reduced, especially when the thermal conductivity of PCM is enhanced. In addition, the heat pipe and PCM technologies can also be applied in SPT receivers.
Fig. 59 Sketch of the pear-like receiver for solar dish system[182] (License Number 4476200774743)
Fig. 60 Solar flux distributions of hemisphere receiver and pear-like receiver[182] (License Number 4476200774743)
43
qr / qr,max
x / mm Fig. 61 Sketch of a hemisphere receiver with plano-convex window [183] (License Number 4476200225735)..
Fig. 62 Solar flux distributions in hemisphere receivers with or without window[183] (License Number 4476200225735).
For homogenizing the solar heat source in the volumetric receiver, Chen et al. [185] designed a receiver with double-layer porous media as shown in Fig. 63. It is found that the decreasing-porosity configuration tends to achieve higher air outlet temperature and lower solid inlet temperature, as shown in Fig. 64. Meanwhile, the drawbacks introduced by non-uniform solar heat source can be reduced. Moreover, Du et al.[186] proposed a volumetric receiver with gradually-varied porosity which is optimized using genetic algorithm as shown in Fig. 65(a). They found that the heat source is greatly homogenized by the GA method compared with those of the linear-changed structure with porosity of 0.95-0.65 and uniform structure with porosity of 0.89 that is the average porosity of the optimized solar receiver, as shown in Fig. 65(b). At the same time, the receiver optical efficiency can be improved.
44
Fig. 63 Sketch of double-layer porous receiver[185]
Fig. 64 Temperature in double-layer receiver design[185]
(License Number 4476191488278).
(License Number 4476191488278)..
(a) Porosity distribution in receiver. (b)Solar heat source in receiver Fig. 65 Volumetric receiver with gradually-varied porosity [186] (License number 4476190563569).
5.3.2 Optimization of solar concentrator Another solution for obtaining more uniform flux is the optimization of the reflector. Yan et al.[187] designed a discrete solar dish concentrator (DSDC) for improving the flux uniformity inside cavity receiver as shown in Fig. 66. The DSDC was designed by dividing an ideal parabolic dish into several parts and rotating each part around its one end. It is found that the DSDC can improve the flux uniformity and reduce the peak flux with a drop of several percentage points in collector optical efficiency, as shown in Fig. 67.
45
Fig. 66 Sketch of the discrete solar dish concentrator (DSDC) with a cylinder cavity receiver[187] (License Number 4476181167371).
(a) Ideal parabolic-dish collector (b) DSDC Fig. 67 Flux distributions in the receivers of an ideal parabolic-dish collector and the DSDC[187] (License Number 4476181167371).
6. Summary and conclusion This paper introduces the characteristics of non-uniform solar flux distributions in four different CSPs. The challenges caused by these non-uniform characteristics are summarized. Moreover, the solutions proposed to deal with the challenges are reviewed. It is found that significantly non-uniform solar fluxes occur in different types of receivers. The non-uniform fluxes will inevitably result in the steep temperature gradient and the high local temperature in CSP receivers, which would lead to great challenges for the safe and efficient operation. The high local temperature may introduce the decomposition of the heat transfer fluid and could damage the absorbing coating, which lowers the receiver thermal efficiency. The steep temperature gradient leads to large thermal stress, which results in the deformation and stress failure of the receivers. After the review of the solutions aiming at solving these challenges, a recommendation for the 46
optimization of the solar collector is provided, which is that the solar flux distribution and the heat transfer ability of the HTF should match with each other as well as possible. Because we observe that the major reason for the challenges caused by the non-uniform flux is the mismatch between the identical heat transfer ability of the HTF and the non-uniform solar flux. From this point of view, all the solutions mentioned above can be summarized in two approaches for weakening this mismatch. The first approach is to enhance and optimize the heat transfer ability in the receiver to match with the non-uniform flux distribution, which can be called “passive approach”. The second approach is to improve the uniformity of the flux distribution to match with the heat transfer ability in the receiver, which can be called “active approach”. The exiting solutions divided into the two approaches are summarized in Table 5. This review can help the researchers and practitioners have a better understanding of the non-uniform solar flux features in CSPs, and provide guidance for solving the corresponding challenges. Table 5 Approaches proposed to tackle the challenges caused by non-uniform solar fluxes in CSPs CSPs
PTC
LFC
SPT
PDC
Solution
Approach
Variable focus concentrator for uniform flux
Active
Use of secondary reflector in annular space for uniform flux
Active
Use of secondary reflector above the receiver for uniform flux
Active
Heat transfer enhancement in absorber tube to flatten the temperature
Passive
Use of absorber tube with high thermal conductivity
Passive
Optimize the aiming strategy for uniform flux
Active
Optimize the secondary reflector for uniform flux
Active
Heat transfer enhancements for PTC are also suitable for LFC
Passive
Optimize the aiming strategy for uniform flux
Active
Optimize the solar absorptance distribution in cavity receiver for uniform flux
Active
Optimize the flow path of the HTF in the receiver
Passive
Heat transfer enhancements within the absorber tube
Passive
Use of optical lens at the cavity aperture
Active
Use of heat pipe technology within the receiver
Passive
Optimization of the receiver geometries
Active
Optimization of the concentrator
Active
Use of optical lens at the cavity aperture
Active 47
Utilizations of heat pipe and PCM within the receiver
Passive
Acknowledgements The study is supported by the Key Project of National Natural Science Foundation of China (No.51436007) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No.51721004). Nomenclature AC
collector net collection area
CSP
concentrated solar power
D
diameter / mm, m
DNI
direct normal irradiance / W·m-2
FVM
finite volume method
f
friction factor
GA
genetic algorithm
h
height / mm, m
HTF
heat transfer fluid
LFC
linear Fresnel collector
L
length / m
MCRT
Monte Carlo ray tracing
MTCR
multiple-tube cavity receiver
Nu
Nusselt number
PCM
phase change material
PTC
parabolic-trough collector
PDC
Parabolic-dish collector
PEC
performance evaluation criteria
p
pitch / m
ql
local solar flux / W·m-2
QC
available solar power that can be accepted by collector
Qape
incident power on the receiver aperture
Qabs
radiant solar power absorbed by the absorber
QHTF
power transferred to heat transfer fluid
Re
Reynolds number
SPT
solar power tower
STCR
single-tube cavity receiver
W
width / m
x, y, z
Cartesian coordinates / m 48
Greek symbols ηR,opt
receiver optical efficiency
ηR,th
receiver thermal efficiency
ηR
receiver efficiency
ηC
collector efficiency
ηC,opt
collector optical efficiency
θ
angle on the absorber tube / °
Subscripts abs
absorber parameter
ape
aperture parameter
C
collector parameter
max
maximum value
opt
optical
R
receiver parameter
s
smooth tube
th
thermal parameter
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Highlights Non-uniform characteristics of solar fluxes in CSPs are reviewed Solar flux and fluid heat transfer ability are suggested to match with each other Useful recommendations for the optimization of the solar collector are provided
59
S1359-4311(18)36076-9 https://doi.org/10.1016/j.applthermaleng.2018.12.006 ATE 13013
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
3 October 2018 3 December 2018 3 December 2018
Please cite this article as: Y-L. He, K. Wang, Y. Qiu, B-C. Du, Q. Liang, S. Du, Review of the solar flux distribution in concentrated solar power: non-uniform features, challenges, and solutions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.006
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Review of the solar flux distribution in concentrated solar power: non-uniform features, challenges, and solutions Ya-Ling He*, Kun Wang, Yu Qiu, Bao-Cun Du, Qi Liang, Shen Du Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China *Corresponding author.
E-mail address: [email protected]
Abstract: Concentrated solar flux distribution in the concentrated solar power (CSP) systems is extremely non-uniform, which can lead to high local temperature and large temperature gradient in solar receivers that will cause great challenges for the safety and efficient operation of the system. This paper introduces the non-uniform flux features in four CSP technologies including the parabolic-trough collector, the linear Fresnel collector, the solar power tower, and the parabolic-dish collector. Challenges including degeneration of the materials, thermal stress and deformation, and overburning are summarized. The corresponding solutions proposed to tackle these challenges are emphatically reviewed, and a recommendation for the optimization of the solar collector is provided from this review, which is that the solar flux distribution and the heat transfer ability of the heat transfer fluid (HTF) should match with each other as well as possible. From this point of view, the existing solutions are classified into two groups. One is optimizing the heat transfer ability of the HTF to match with the flux distribution, which is called the passive approach. The other is homogenizing the flux distribution to match with the identical heat transfer ability of the HTF, which is called the active approach. This review can help to have a better understanding of the non-uniform solar flux features in CSPs, and provide guidance for solving the corresponding challenges. Keywords: Concentrated solar power; Solar concentrator; Solar receiver; Non-uniform solar flux; Non-uniform temperature; Challenges and solutions 1 Introduction Environmental issues introduced by the utilization of fossil fuels have imposed negative effects 1
on the society and the economic development. For solving the problems, utilization of solar energy is a highly competitive approach[1]. Among different solar technologies, the concentrated solar power (CSP) technology is a promising option. In recent years, it has become a development tendency to promote this technology towards a low-cost and large-scale utilization [2]. A typical CSP plant mainly consist of a solar concentrator, a receiver, a thermal storage system, a heat exchange network, and a power block. In the plant, the solar radiation is firstly concentrated, and then transformed into thermal energy that is taken away by the heat transfer fluid (HTF) in the receiver. Then, the thermal energy can be stored in the thermal storage unit or utilized to produce high-temperature steam or gas in heat exchangers. Finally, the steam or gas will drive a power cycle for electricity generation. The practical CSP technologies can be classified into the following four types: parabolic-trough collector (PTC), linear Fresnel collector (LFC), solar power tower (SPT), and parabolic-dish collector (PDC). The solar concentrator and the receiver are key components serving as the energy source of the whole CSP system. In the absorber of the CSP receiver, the concentrated solar flux distribution is extremely non-uniform, which can cause challenges to the safe and efficient power generation. Therefore, many studies have been carried out to reveal the non-uniform flux, and lots of solutions have been proposed to solve the corresponding challenges. In this paper, the concentrators and the receivers in the four CSP technologies are briefly introduced. The characteristics of non-uniform solar fluxes in the systems are summarized. The challenges caused by the non-uniform fluxes are also discussed. Furthermore, a comprehensive review of the corresponding solutions proposed to tackle these challenges is presented. Finally, a summary of the key solutions is presented, and some suggestions are provided. It is to be noted that the non-uniform flux means the non-uniformity of the solar flux in or on the solar absorber in the current review. However, the structures of the absorbers in different receivers are significantly different. As a result, the non-uniformity has different meanings for different absorbers. In the linear evacuated receivers of PTC and LFC, the non-uniformity means non-uniformity in circumferential direction. For the absorbers using tube panels in SPT, LFC and PDC, the 2
non-uniformity means non-uniformity among different regions of the absorber panels. For the porous volumetric absorber in SPT and PDC, the non-uniformity means the non-uniformity in the whole volumetric absorber. Before the detailed review, some performance parameters of the solar collectors are defined as follows. The receiver optical efficiency (ηR,opt) is defined as the ratio of the radiant solar power absorbed by the absorber (Qabs) and the incident power on the receiver aperture (Qape) as shown in Eq.(1). The receiver thermal efficiency (ηR,th) is defined as the ratio of the power transferred to the HTF (QHTF) and Qabs as shown in Eq.(1). The receiver efficiency (ηR) is defined as ratio of QHTF and the Qape as shown in Eq.(2). The collector optical efficiency (ηC,opt) is defined as the ratio of the Qabs and the available radiant solar power that can be accepted by the collector (QC) as shown in Eq.(3). The collector efficiency (ηC) is defined as the ratio of the QHTF and QC as shown in Eq.(4).
R,opt Qabs Qape , R,th QHTF Qabs
(1)
R QHTF Qape
(2)
C,opt Qabs QC , QC DNI AC
(3)
C QHTF QC
(4)
where DNI indicates the direct normal irradiation; AC is the solar thermal collector net collection area. 2. Parabolic-Trough Collector 2.1. Description of PTC The parabolic-trough collector (PTC) which is the most mature practical CSP technology mainly consists of a linear receiver and a parabolic trough concentrator, as shown in Fig. 1. The receiver is generally installed along the focal line of the concentrator, where the evacuated tube 3
receiver is mostly employed. A typical evacuated tube is mainly composed of a glass cover and a steel absorber tube plated with a selective coating on its outer surface. The absorber tube and the glass cover are sealed by metal bellows to form a vacuum annular gap, where the getter is generally placed to maintain the vacuum (<0.013 Pa) to reduce the heat loss. In addition, heat pipes[3] and cavity receivers have also been proposed for PTC but are currently barely used[4]. As a result, this review would focus on the widely used evacuated tube. The typical PTCs using evacuated tubes are summarized in Table 1. HTF
Sun
Glass cover
Rays
Evacuated tube
Concentrator
Absorber
Fig. 1 Schematic diagram of parabolic-trough collector[5] (License Number 4476230830026). Table 1 Parameters of the typical parabolic-trough collector[6-9] Type
LS-1
LS-2
LS-3
Eurotrough
DS-1
Collector width / m
2.55
5
5.76
5.76
5
Collector length / m
6.3
8
12
12
8
Focal length / m
0.94
1.49
1.71
1.71
1.49
Absorber outer diameter / m
0.04
0.07
0.07
0.07
0.07
Geometric concentrating ratioa
20
23
26
26
23
Solar reflectance of concentrator
0.93
0.94
0.94
0.95
0.95
Solar absorptance of coating
0.94
0.94
0.96
0.96
0.96
Radiant emissivity of coating at 400 oC
0.3
0.2
0.1
0.1
0.1
4
a
Solar transmittance of glass cover
0.95
0.95
0.965
0.965
0.965
Theoretical peak collector optical efficiency
0.734
0.737
0.772
0.80
0.80
Geometric concentrating ratio is the ratio between the width of collector aperture and the perimeter of the
absorber.
Solar energy is concentrated onto the absorber and converted into thermal energy that is taken away by the heat transfer fluid (HTF), as shown in Fig. 1. Heat transfer oil or water/steam is widely used as the HTF. For example, in SEGS I-SEGS VIII plants built by LUZ company, the heat transfer oil is used, where the maximum temperature is 390 oC
[10]
. While in SEGS IX, the
water/steam is used, and the maximum temperature is increased to 402 oC. In addition, some other fluids such as CO2[11], helium[12], air[12] and molten salt[13] are also proposed as the HTFs in PTCs. 2.2. Non-uniform solar flux distribution in PTC The solar energy is concentrated by the concentrator and mainly focused at the bottom of the absorber tube. The solar flux distribution in the PTC is extremely non-uniform, which has been revealed by many studies. In experimental aspects, Riffelmann et al.[14], Lüpfert et al.[15] and Schiricke et al.[16] proposed two methods, namely PARAbolic Trough Flux SCANner (PARASCAN) and Camera Target Method (CTM), to measure the solar flux distribution on the PTC receiver. In the PARASCAN measurement, glass fiber sensors with high resolution are used to record the solar flux distribution along the circumferential direction. While in the CTM measurement, a diffuse reflecting Lambertian target is held perpendicularly to the focal line surrounding the absorber, and a calibrated camera are used to take pictures for the target. The paths of the reflected rays on the receiver tube and the detected local optical errors can be visualized with the resulting pictures. Fig. 2 shows the measured solar flux around the receiver of Eurotrough using both the PARASCAN[16] and CTM[14], where the non-uniform characteristic is clearly observed.
5
(a) Measured by PARASCAN (b) Measured by CTM Fig. 2 Solar flux distribution on Eurotrough receiver with PTC[14-16] (License Number 4476241391268)
In simulation aspects, there are mainly three methods for the calculation of solar flux distributions including cone optics method, semi-finite integration formulation, and ray tracing method. The above methods have been widely used to reveal the non-uniformity of the solar flux distribution on the absorber of PTC. Jeter[17] developed semi-finite integration formulation model to calculate the solar flux distribution in the PTC, and analyzed the effects of structure size on the non-uniformity of the flux. Daly[18] revealed the non-uniform flux distribution by using the reversed ray tracing method. Roberto[19] applied ray tracing method to simulate the solar flux distribution on the receiver surfaces, and further analyze the effects of the tracking and slope errors on the flux distribution. Men et al.[20] also obtained the solar flux distribution on the absorber by using Monte Carlo ray tracing (MCRT) method. He et al. [21] also developed an optical model based on MCRT method to calculate the solar flux distribution on the absorber surface as shown in Fig. 3. It can be observed that the flux distribution can be divided into four regions[21]. Section 1 is the shadow region, where the solar rays are shadowed by the receiver. Region 2 is the flux increasing region, where more rays are reflected to the receiver, so the flux increases steady. Region 3 is the flux decreasing region, where the flux reduces rapidly with the increasing angle. Region 4 is the direct irradiation region, where the absorber only receives direct solar rays, and the flux is in very low. In addition Khanna et al.[22] and Christian et al.[23] simulated the solar flux distribution on the absorber surface of a bent receiver by convolution method and ray tracing method, respectively. They found that the solar flux distribution can be more non-uniform for the bent absorber, as shown in Fig. 4.
6
60
2
1
2
3
Solar flux ql/ kW·m-2
4
40 30
y /m
Solar flux ql / kW·m-2
50
3
4
20 10 0 -180
-120
-60
0
60
120
Circumferential angle around tube /
180 o
x /m
z /m
55 50 45 40 35 30 25 20 15 10 5 0
(a) Circumferential distribution (b) 3D solar distribution [21, 24] Fig. 3 Solar flux distribution on the absorber surface of PTC (License Number 4476251364787).
In summary, from both the experimental measurements and the simulation studies, it can be observed clearly that the solar radiation is mainly concentrated on the bottom of the receiver tube, and the flux distribution is extremely non-uniform.
Fig. 4 Solar flux distribution for the bent receiver tube with PTC[22] (License Number 4476761022123)
2.3. Challenges and corresponding solutions Due to the limited thermal conductivity of the receiver tube and the heat convection ability within the tube, the non-uniform flux distribution will inevitably cause non-uniform temperature field in the tube wall. He and Cheng et al. [21, 25, 26] proposed a MCRT and Finite Volume Method (FVM) coupled simulation method to simulate the conjugated heat transfer within the receiver and 7
obtain the temperature distribution, where the non-uniform flux on the absorber surface obtained using MCRT is treated as a boundary condition in FVM. Fig. 5 shows the temperature distribution on the absorber outer wall. It can be observed clearly that the temperature distribution is extremely non-uniform, and great temperature gradient along the circumferential direction is found. In particular, if the carbon dioxide, the water/steam, the air, or the helium is adopted as the HTF, the non-uniform temperature distribution caused by the non-uniform flux can be more distinct due to their lower thermal capacity[11, 27]. The non-uniform flux results in high local temperature and large temperature gradient, which can cause great challenges for the safety and the efficiency of the receiver. The challenges can be summarized as follows. Firstly, the large circumferential temperature gradient can result in the deflection of the absorber tube. The deflection makes the absorber tube deviate from the focus line of the concentrator, thus increasing the optical loss. The absorber tube can even touch the glass envelope and rupture it, causing vacuum loss[27-30]. Secondly, the selective coating tends to degrade when the local temperature is too high, which limits the maximum operation temperature. Thirdly, Hydrogen formation and permeation occurs as a result of the thermal degradation of the HTF such as oil in the high-temperature environment[31], which will increase the heat loss. 680
Temperature / K
670 665 660
y /m
Temperature / K
675
655 650 645 -180
z /m
-120 -60 0 60 120 o Circumferential angle around tube /
180
(a) Circumferential distribution (z=5 m)
x /m
(b) 3D distribution
Fig. 5 Non-uniform temperature distribution of the receiver tube wall
[24, 25]
( License Number 4476460198306)
To avoid the above problems, it is helpful to flatten the temperature distribution by reducing the peak temperature and decreasing the temperature gradient. Fig. 6 summarizes the solutions to flatten the temperature distribution. The solutions can be approximately classified into three groups as follows. 8
Nonuniform solar flux
Cause
Limited convective heat transfer coefficient Limited conductivity
Nonuniform temp.
High local temp.
Large temp. gradient
Solutions Fins
Solution-1
Solution-2
Enhancing convective heat transfer
Improving tube conductivity
Spiral vortex generator
Porous insert
longitudinal vortex generator
Copper tube]
Bimetal tube
Twisted tape
Nano-fluids
Aluminium Tube et al.
Variable focus Secondary reflector in PTC with secondary reflector annular space parabolic trough
Solution-3
Homogenizing solar flux distribuion
Fig. 6 Techniques for solving the non-uniform temperature in PTC receiver.
2.3.1
Heat transfer enhancement and optimization
The first group of solutions is enhancing the convective heat transfer within the absorber tube. Numerous techniques for the convective heat transfer enhancement have been proposed for PTC receiver. Aldali et al.[29] and Munoz et al.[30] recommended the incorporation of internal helical fins within the tube to enhance the convection heat transfer. Massidda et al.[32] enhanced the convection heat transfer by increasing the surface roughness and adopting the helical swirl generators within the absorber tube. Deluss[11] also adopted two approaches including increasing the surface roughness and placing fins inside the absorber tube to flatten the temperature distribution. Aggrey et al.[33] recommended to insert wall detached twisted tape for the heat transfer enhancement in the absorber. These studies found that the peak temperature and temperature gradient can be reduced. Wang et al.[34, 35] designed an asymmetric outward convex corrugated tube receiver(ACPTR), and they found that the maximum enhancement of overall heat transfer performance factor can achieve 148% as shown in Fig. 7, and the maximum restrain of von-Mises thermal strain is 26.8%.
9
2.4 factor (Nu/Nus)3/(f/fs)
Overall heat transfer performance
2.6
2.2 ACPTR
2.0 1.8
p=Corrugation pitch D=diameter
1.6
1.4 1.2 1.0 14
28
42
56 Re / ×103
70
84
Fig. 7 Nusselt number(Nu) in the tube at different Reynolds number(Re)
98 [34, 35]
(License Number
4476491200760).
In these studies, the heat transfer enhancement techniques are implemented on the whole inner wall of the absorber. However, the use of enhancements at the upper part of the absorber helps little to the heat transfer because this part just accepts a little radiation, which would result in unnecessary pressure drop. As a result, to better match the heat transfer enhancement with the non-uniform flux distribution, Cheng and He et al. [36] proposed a novel receiver enhanced by longitudinal vortexes. In this receiver, longitudinal vortex generators are only employed at the absorber bottom which accepts the main concentrated solar radiation. They found that this receiver has a good comprehensive heat transfer performance, and the maximum wall temperature and temperature gradient can be greatly reduced as shown in Fig. 8. More recently, unilateral pin fins designed by Gong et al.[37] and unilateral straight fins optimized by Bellos et al.
[38]
have also been
employed in PTC receivers.
(a) Smooth absorber (b) novel absorber with unilateral generators Fig. 8 Comparison of temperature distributions on the absorber tube wall[36] (License Number 4476760270084). 10
Inserting porous medium into the absorber is also an effective technique for convective heat transfer enhancement. Reddy et al.[39-42] and Mwesigye et al.[43] carried out the thermal analysis of the receiver with porous inserts. Their results show that the porous inserts could enhance the heat transfer and consequently flatten the temperature. Moreover, a few studies have also considering the match between the non-uniform solar flux distribution and heat transfer enhancement. Wang et al.[44] studied the effects of the height (h) of two porous inserts in Fig. 9 (a) and (b). They found that the optimum thermo-hydraulic performance is obtained when h=0.25D in Fig. 9 (b), where the Nusselt number (Nu) increases about 5-10 times with the increase of 10-20 times in friction factor (f). Furthermore, Zheng et al.[45, 46] proposed a novel optimization method coupling the genetic algorithm (GA) and computational fluid dynamics to optimize the configurations of porous insert within the absorber. In the optimization, the performance evaluation criteria (PEC)[46] was treated as the objective function. Fig. 9(c) shows the optimal configuration of the porous insert. Fig. 10 shows the comparison of the axis velocity field distribution of the three inserts shown in Fig. 9. It can be observed that the optimal porous configuration can reshape the flow field to make more HTF flow through the bottom region. Moreover, the optimal configuration can improve the effective thermal conductivity at the bottom region. Therefore, the heat transfer ability of the GA configuration can better match the non-uniform flux. Fig. 11 shows the collector efficiency and the maximum temperature difference on tube outer wall. It can be seen that the optimal configuration can yield the largest collector efficiency and the lowest temperature difference with a wide range of Reynolds numbers.
D
Porous
h h (a) Reference-I
(b) Reference-II
(c) GA
Fig. 9 Two reference configurations and the optimal configuration of the porous insert [46] (License Number 4476760759145).
11
250
66
200 tem. diff
efficiency
Clear tube Fully filling Reference-I Reference-II GA
64
62
150
100
60
58 0.0
50
2.0x10
5
4.0x10
5
6.0x10
5
8.0x10
5
0 6 1.0x10
Max temp. difference of tube outer wall / K
Collector efficiency / %
68
Re
Fig. 10 Comparison of the axis velocity field distribution[46] (License
Number 4476760759145).
2.3.2
Fig. 11 Collector efficiency and maximum temperature difference on tube outer wall [46] (License Number
4476760759145).
Improvement of thermal conductivity of the tube
The second group of solutions is improving the thermal conductivity of the absorber tube. The simulation results from Delussu[11] and Aldai et al.[29] used the copper absorber tube with high thermla conductivity to flatten the tempearture distribuiton. However, the mechanical strength of copper is too low compared with that of steel. Almanza et al.[27] carried out experiments and pointed out that bending of absorber tube could almost be eliminated by replacing a steel tube with a copper tube due to the smaller circumferential temperature differences resulting from the high thermal conductivity of copper. In view of this, Flores et al.[47] recommended to use a bimetallic copper-steel absorber to reduce the deflection of the tube benefited from both the high thermal conductivity of the copper and the mechanical resistance of the steel. 2.3.3
Optimization of the collector to homogenize solar flux
Compared with the above two groups, homogenizing the flux distribution seems to be a more direct approach for flattening the temperature distribution. Several studies proposed some novel concentrators for homogenizing the flux. Tsai et al.[48] proposed an variable focus parabolic trough (VFPT) concentrator in which the focal length varied as a function of the displacement of the reflector relative to the receiver. Fig. 12 shows the compration of the solar flux distributions along the circumferential direction among the VFPT, the conventional parabolic trough (CPT), the cylindrical trough (CT), and the ideal trough. It can be observed clearly that the uniformity of the flux distribution was improved significantly. Moreover, Gee et al.[49] placed a secondary reflector within the annular space to reflect more solar rays onto the top of the abosrber, as shown in Fig. 13. In addition, Wang et al.[50] proposed to make the following improvements based on the conventional 12
PTC. Firstly, the absorber tube is moved downward to a suitable position away from the focal line of the PTC. Then, a secondary reflector is added. The secondary reflector and the PTC are arranged in the way shown in Fig. 14, where some solar rays can be reflected by the PTC onto the secondary reflector and then reflected onto the top of the absorber. Fig. 15 shows the the flux and temperature distributions on the absorbers of both the conventional and the novel PTCs. It is seen that the solar flux distribuion can be homogenized significantly, and the flux becomes almost uniform on the absorber in the novel PTC. The tempearture distribution is consequently also flattened, and the circumferential temperature difference was reduced from about 25 K to 3 K at a typical condition.
Fig. 12 Comparison of solar flux distributions amoing
Fig. 13 Ray paths in PTC receiver with a secondary [49]
four PTCs[48] (License Number 4476761195942).
reflector in the annular space
.
Secondary reflctor Absorber tube PTC
Fig. 14 Parabolic-trough collector with secondary reflector[50] (License Number 4476460198306)
13
680
T1, z=1 m
60
T1, z=3 m
675
Conventional PTC
T1, z=5 m T1, z=7 m
TemperatureT / K
ql / kW· m-2
50
40
30
Novel PTC with secondary reflector
20
10
0 -180
670
T2, z=1 m T2, z=3 m
665
T2, z=5 m T2, z=7 m
660 655
T1 for convention PTC T2 for novel PTC
650 -120
-60 0 60 120 o Circumferential angle around tube /
645 -180
180
(a) Comparison of solar flux distribuiotn
-120
-60
0
60
120
Circumferential angle around tube /
o
180
(b) Comparison of temperature distribuiton
Fig. 15 Comparison between conventional PTC and novel PTC [50] ( License Number 4476460198306)
3 Linear Fresnel Collector 3.1 Description of LFC Linear Fresnel Collector (LFC) is a solar collector in which a array of long, narrow, flat or slightly curved mirrors reflects the sun rays onto a fixed linear receiver mounted over a tower above and along the reflectors, as shown in Fig. 16[51]. The linear receiver is usually a single-tube cavity receiver (STCR) or a multiple-tube cavity receiver (MTCR), as shown in Fig. 17. A secondary reflector is also usually used in a STCR for homogenizing the flux in the receiver and increasing the acceptance angle [52]. When the sun shines on the mirror field on sunny days, firstly, the mirrors will track the sun and reflect the sun rays onto the absorber tubes directly for a LFC with a MTCR. However, for a LFC with a STCR, some rays will shine on the tube, and others will be reflected one more time by the secondary reflcetor before shining on the tube. Then, most of the rays will be absorbed by coating on the tube. Finally, the energy will be transferd to the heat transfer fluid in the tube. The LFC technology which has been proposed as an attractive low-cost option for CSP presents important advantages when compared with the PTC technology. Particularly, the use of stationary receiver without rotating joints or high-temperature moving components makes LFCs safer and more cost effective than PTCs. Moreover, LFCs use narrow primary mirrors which need no heavy supporting structures and thus result in lower the construction and operation costs. In addition, LFCs can work in a direct steam generation way with the pressure range of 3.5-11 MPa and the temperature range of 250-450 °C. Thus, it can reduce the size of heat exchangers in the 14
system. As a result, LFCs are being considered as competitors of PTCs in medium-low temperature CSP, and many prototypes and commercial plants have been built up to now, as summarized in Table 2. Table 2 Summary of the LFC plants built in the world [52-56] Capacity
Working
Receiver
Start
/ MWe
parameters
type
Production
Belgium
-
-
Water, steam
STCR
2001
LiddellⅠ
Australia
1.0
6.9 MPa, 285 °C
Water
MTCR
2004
FRESDEMO
Spain
0.8
11 MPa, 450 °C
Water, steam
STCR
2007
Kimberlina plant
USA
5.0
4 MPa,300 °C
Water, steam
MTCR
2008
Puerto Errado 1
Spain
1.4
5.5 MPa, 270 °C
Water
STCR
2009
Puerto Errado 2
Spain
30
5.5 MPa,270 °C
Water
STCR
2012
Liddell Ⅱ
Australia
9.0
5.5 MPa, 270 °C
Water
MTCR
2012
China
1.5
3.5 MPa,>400 °C
Water, steam
Augustin Fresnel 1
France
0.25
10 MPa, 300 °C
Water, steam
STCR
2012
Dhursar
India
125
9 MPa, 390 °C
Water, steam
-
2014
eCare project
Morocco
1.0
280 °C
Water
-
2014
Rende-CSP
Italy
1.0
280 °C
Oil
-
2015
Alba Nova 1
France
12.0
5.5 MPa,300 °C
Water, steam
STCR
2015
Llo plant
France
9.0
7 MPa, 285 °C
Water
-
2015
IRESEN plant
Morocco
1.0
300 °C
Oil
STCR
2016
Dadri ISCC Plant
India
14.0
250 °C
Water
-
2017
China
50.0
-
Water, steam
-
Under const.
China
50.0
-
Molten salt
MTCR
Under const.
Name
Country
Solarmundo
Huaneng Nanshan Power Plant
Zhangjiakou 50MW CSG Fresnel project Dacheng project
HTF
STCR
Receiver
Mirror field
Fig. 16 Schematic diagram of the LFC [51] (License Number 4476210462017) 15
2012
(a)
Insulation
(b)
Absorber
Insulation
Absorber Secondary reflcetor
Glass cover
Glass cover
Fig. 17 Typical receivers in LFCs: (a) Multiple-tube cavity receiver(MTCR); (b) Single-tube cavity receiver(STCR)
3.2 Non-uniform solar flux distribution in LFC The optical performance of a LFC is one of the major points concerned by the researcher and the practitioner, and it mainly determines the total input energy of the system and is of great importance for the performance optimization, system design, and safe operation. Hence, many studies have focused on the optical characterization of LFCs by optical modeling methods and experiments. In the early 1990s, Goswami et al.[57], Mathur et al.[58] and Sootha et al.[59] investigated the flux distribution on four different receivers, including triangular, flat horizontal, flat vertical and tubular receivers, of a LFC. The optical design and performance characteristics of these receivers were presented. At the same time, Feuermann and Gordon[60] evaluated the sensitivity of energy output to typical parameters of a 220 kW LFC with a secondary Compound Parabolic Collector (CPC) by ray tracing technique. They found that the LFC can be cheaper than the corresponding PTC, but the annual collector optical efficiency is about 25% lower. At the turn of the century, two significant breakthroughs were made in LFC technology. One is that a novel design called Compact Linear Fresnel Reflector (CLFR) was proposed by Mills and Morrison[61]. In the CLFR system, two alternative linear receivers on separate towers are offered to each mirror row between the towers as shown in Fig. 18, so it needs less land than PTC to produce a given output. Optical study using a multi-branched ray tracing model shows that a non-uniform characteristic of the flux on the receiver 16
can be observed. The other is the design and construction of the Solarmundo collector with a single-tube cavity receiver (STCR). Häberle et al.[62] investigated its collector optical efficiency (ηC,opt) by 3D ray tracing calculations. It is found that collector can achieve ηC,opt of 61% at normal incidence. receiver
reflector
Fig. 18 Sketch of the Compact Linear Fresnel Reflector (CLFR)
Furthermore, more studies on the solar flux characteristics in the LFC receivers have been conducted in recent years. Qiu and He et al. [63-66] developed a MCRT model and studied the optical characteristics of LFCs using various receivers. Fig. 19 shows the solar flux on the absorber tube in a STCR. It can be seen that the flux is significantly non-uniform with a hot spot at the tube’s bottom which is directly hit by the rays from the mirror field. However, only 23% power shine on upper half of the tube after the concentration of the secondary reflector. Similar non-uniform fluxes have also been observed in Balaji et al’s work[67].
17
Solar flux absorbed by tube / kW·m-2
120
Directly concentrated flux
90
W
θ E
60
30
0 -180
Secondarily concentrated flux
-90 0 90 Angle around absorber tube θ /
180
°
Fig. 19 Flux distribution on the absorber tube surface in the STCR of a LFC at noon[66] (License Number 4476201167570)
The flux distribution in a typical MTCR was also revealed by Qiu and He et al.[66] as shown in Fig. 20. It is seen that flux non-uniformity among the tubes is significant because all mirrors aim at the center of the receiver, where most rays shine on tube 4 and tube 5, while tube 1 and tube 8 are barely utilized. Moreover, Moghimi et al [68, 69] studied the flux distribution on each tube in a MTCR using finite volume method, and the result is shown in Fig. 21. It is seen that the circumferential distribution of the flux on each tube is also non-uniform, and the most power shines on the lower
ql / kW·m -2 10
20
30 40 50
60
70 80
90
-100
N
S Xr / m
0
part of the tube.
-0.3
1 2 3 4 5 6 7 8 Yr / m W E
0.3
Fig. 20 Flux distribution in the MTCR on spring equinox noon with qmax=92 kW·m-2[66] (License Number 4476201167570)
Fig. 21 Flux distribution on a tube in the MTCR of a LFC [69] (License Number 4476210752959)
In addition, Abbas et al.[70-72], Benyakhlef et al.[73] and Bellos et al.[74] investigated the concentration features on flat receivers which can be treated as the aperture of STCR and MTCR in 18
LFCs by the ray tracing method, respectively. It is found that the flux on the flat is also significantly non-uniform with a hot region at the center as shown in Fig. 22.
Fig. 22 Solar flux distribution on the flat receiver in a LFC [74] (License Number 4476211031373)
To sum up, we can see that the solar rays in a LFC are reflected by the mirror field and shine on the receiver from the lower aperture of the receiver, so most energy is concentrated on the lower part of the receiver in both STCR and MTCR. As a result, the fluxes with significant non-uniform characteristics appear in the receivers. 3.3 Challenges and corresponding solutions The non-uniform fluxes in the receivers of Linear Fresnel Collectors (LFCs) will lead to non-uniform temperature characteristics, which is similar to those that happen in PTCs. He et al[63, 64, 75]
developed a MCRT-FVM coupled model based on their work in PTCs, and the heat transfer,
including convection, radiation and conduction, in the receivers were studied under non-uniform fluxes. In this way, the solar-thermal conversion performance of the LFCs and non-uniform temperature characteristics in the receivers were revealed[64,
75]
. Fig. 23 shows the temperature
distributions on the tubes in a STCR and a MTCR. It is seen that large temperature gradients appear in both circumferential and lengthwise directions, and this non-uniform temperature would result in 19
two major negative effects on the system. The first one is that the local high temperature may accelerate the oxidation and degradation of the coating [76]. This is because only very few oxides and carbides can keep stable in air when the temperature is higher than 400 °C
[52]
. Eck et al.[77] also
indicate that the local temperature in the receiver can be larger than 569 °C, which is much higher than the limitation of the coating. The second one is that large temperature gradient would lead to large thermal stress which would result in undesirable distortion and damage of the receiver [76]. E T/K 654 650 646 642 638 634 630 626 622
0.04
zr / m
0.02
0
-0.02 -0.04 0.04
0.02
0
-0.02 -0.04 -8
-6
-4
-2
W
1.0 N S Zr Xr
0
0
yr / m
1
(a) STCR
2
3
4
5
6
7
(b) MTCR
Fig. 23 Non-uniform temperature distributions on the absorber tubes in LFC receivers [64] (License Number 4476211457701)
For solving these negative effects, the heat transfer enhancements for PTCs are also suitable for the LFCs. Besides, some novel designs and optimizations of the collectors have also been proposed for improving the flux uniformity. These solutions include design of the secondary reflector and optimization of aiming strategy, etc. 3.3.1
Design of the secondary reflector
Grena and Tarquini [78] proposed a new double-wing secondary reflector for a LFC, as shown in Fig. 24(a). Their simulation results showed that the flux distribution on the tube was more uniform than that of PTC, as illustrated in Fig. 24(b), and about 37% of the total power shines on the upper half of the tube. However, Hack et al.[79] found that this double-wing design may lead to a 20
8
significant decrease in collector optical efficiency due to the smaller intercept factor compared with that of the ordinary CPC. Prasad et al.
[80]
designed a segmented parabolic secondary reflector that
was made of three parabolic segments, as shown in Fig. 25(a). They found that the new secondary reflector could lead to a reasonable uniform flux under a wide range of the incident angle when it was used in conjunction with the proper aiming strategy, as shown in Fig. 25(b). They also found that the secondary reflector could help to improve the collector optical efficiency slightly under its design condition.
(a) Sketch (b) Relative flux distribution Fig. 24 Sketch of a double-wing secondary reflector and relative flux distribution on the tube [78] (License Number 4476221213103).
(a) Sketch (b) Flux distribution Fig. 25 Sketch of a segmented parabolic concentrator and fluxes on the tube under different incident angles(θi) [80] (License Number 4476230113405).
3.3.2
Optimization of aiming strategy 21
A basic aiming strategy to LFCs is the one-line strategy (S1), as shown in Fig. 26(a), where all mirrors focus on the center line of the aiming plane. Eck et al.[77] indicated that solar flux on the absorber tube can be reduced by defocusing mirror-to-mirror. However, the defocusing can lead to important optical loss if it is not properly designed. Qiu et al.[66] proposed a multi-line aiming strategy (S2) by combining ray tracing and the genetic algorithm (GA), where the optical loss and a non-uniformity index of the flux distribution were used as the objectives. The new strategy is illustrated in Fig. 26(b), where naim aiming lines are uniformly distributed on the aiming plane. The aiming line for each mirror is chosen from the naim lines, and the width of the aiming plane (Waim) in Fig. 26(b) is optimized as well. Waim Aiming plane
Aiming plane
Aiming line 1
All mirrors aim at the center of the plane
2
i
naim Select a line for each mirror
(2) One-line aiming strategy (S1)
(b) GA multi-line aiming strategy (S2)
Fig. 26. Sketches of two aiming strategies for LFCs[66] (License Number 4476201167570).
Application of GA multi-line aiming strategy (S2) indicates that fluxes in both STCR and MTCR can be homogenized efficaciously by S2 in the whole possible time range with a small drop of 0.2-3.8 percentage points in collector optical efficiency compared with those of one-line aiming strategy (S1). In the STCR, 35-40% power can shine on the top half of the tube[66] as shown in Fig. 27. In the MTCR, the maximum flux (ql,max) at typical time is reduced from 92.8 kW·m-2 in Fig. 20 to 38.6 kW·m-2 in Fig. 28[66]. Qiu et al.[66] concluded that the GA multi-line aiming strategy could help to find a compromise between the flux uniformity and collector optical efficiency, and uniform fluxes could be obtained at all possible incident angles. 22
S1: S2:
8.138h, 8.138h,
10.136h, 10.136h,
12h 12h
90 0
ql / kW·m -2 2
6 10 14 18 22 26 30 34 38 42
S Xr / m
W
θ E
N
60
30 -100
local solar flux / kW·m-2
120
0 -180
-90
0
90
angle around the tube θ / °
-0.3
1 2 3 4 5 6 7 8 Yr / m W E
0.3
180
Fig. 27. Flux distributions in STRSC on spring equinox when S1 and S2 are used[66] (License Number 4476201167570)
Fig. 28. Flux in MTCR on spring equinox noon using GA aiming strategy (S2), ql,max= 36 kW·m-2[66] (License Number 4476201167570)
4 Solar Power Tower 4.1 Description of SPT collector The solar collector subsystem in SPT mainly consists of the solar heliostats field, the tower, and the solar receiver[81], as shown in Fig. 29. The solar radiation is concentrated by numerous heliostats and reflected onto the receiver where the heat transfer fluid (HTF) is heated up to a relatively high temperature. The heliostats shown in Fig. 29 are the concentrating components of the plant, and each heliostat consists of mirror panels, frames, tracking and control devices, where toughened glass, rear-silvered and rear-aluminum mirrors are employed [82, 83]. Moreover, the size of the heliostat is usually in a wide range of 1.0-178 m2[84-86]. Heliostat Receiver
Fig. 29 Typical heliostat field and central receiver in a solar power tower plant[87, 88] (License Number 4476231414559, 4476240505245). 23
The receivers used in SPT system can be classified by the operating pressure, the HTFs and the structures. For example, they can be classified into molten salt receivers, air receivers, and water/steam receivers according to various HTFs. Moreover, the receivers can also be divided into tubular receiver as shown in Fig. 30 and volumetric receiver as shown in Fig. 31. Furthermore, the tubular receivers can be further divided into external and cavity structures, and the volumetric receivers can be divided into open-loop types, such as SOLAIR type[89], and closed-loop types, such as REFOS[85] and DIAPR type[90]. In the volumetric receiver, honeycomb channels and foams are usually used as the absorber. Table 3 exhibits different receivers in current SPT plants. In recent years, some new receiver concepts such as falling-particle receiver[91] and falling-film receiver[92] have also been developed, but no plant has been built. Table 3 Summary of typical receivers in SPT plants[81, 84-87, 93-95]. Receiver
Outlet
Start
temperature
Production
Molten salt
565 °C
1995-1999
2.5
Water, steam
440 °C
2009
Spain
19.9
Molten salt
565 °C
2011
Australia
3.0
Water, steam
500 °C
2011
Turkey
1.4
Water, steam
550 °C
2012
Ivanpah
USA
392
Water, steam
565 °C
2014
Crescent Dunes
USA
110
Molten salt
565 °C
2015
Sundrop
Australia
1.5
Water, steam
500 °C
2011
Supcon Delingha
China
10
Molten salt
568 °C
2016
Khi Solar One
South Africa
50
Water, steam
530 °C
2016
SunCan 10MW
China
10
Molten salt
565 °C
2016
Ashalim 1
Israel
121
Water, steam
600 °C
2017
Jemalong
Australia
1.1
sodium
560 °C
2017
NOOR III
Morocco
150
Molten salt
565 °C
2017
SunCan 100MW
China
100
Molten salt
565 °C
2018
Yumen 50MW
China
50
Molten salt
565 °C
2018
Solar One
USA
10
Water, steam
-
1982-1986
Tubular,
THEMIS
France
2.0
Molten salt
450 °C
1983-1986
cavity
MSEE
USA
750 kWe
Molten salt
565 °C
1984-1985
MSS/CTE
USA
5 MWt
Molten salt
565 °C
1987
type
Project
Country
Solar Two
USA
Sierra
USA
Gemasolar Lake Cargelligo Greenway CSP Mersin
Tubular, external
Capacity / MWe
24
HTF
4.5 MPa,
PS10
Spain
10
Water, steam
PS20
Spain
20
Water, steam
Sierra
USA
2.5
Water, steam
ACME Solar
India
2.5
Water, steam
DAHAN
China
1.0
Water, steam
400 °C
2012
TSA
Spain
2.5 MWt
Air
700 °C
1993
Volumetric,
SOLAIR 200
Spain
1.5 MWt
Air
700 °C
2001
open-loop
HiTRec II
Spain
200 kWt
Air
800 °C
2003
SOLAIR 3000
Spain
1.5
Air
800 °C
2006
Consolar/DIAPR
Israel
50 kWe
Air
1200 °C
1992
Volumetric,
CESA-1/REFOS
Spain
350 kWe
Air
800 °C
1996
closed-loop
SOLGATE
Spain
400 kWe
Air
1000 °C
2002
Jülich
Germany
1.5
Air
700 °C
2008
300 °C 4.5 MPa, 300 °C 440 °C 6 MPa, 440 °C
2007 2009 2009 2011
(a) Cavity receiver (b) External receiver [96, 97] Fig. 30 Tubular solar receiver (License Number 4476250147199, 4476250747377). Rays
Wall
Porou s abs orb er
Air
(a) Closed loop [98] (License Number 4476251112251) Fig. 31 Volumetric receiver
25
(b) Open loop
4.2 Non-uniform solar flux distribution in SPT The concentrated solar flux distribution on the receiver influences the receiver performance significantly. So many experimental and simulation studies have been conducted to reveal the solar flux distribution in SPT receiver. Firstly, some experimental measurements have been conducted. For example, Ho et al.[99] presented a photographic flux (PHLUX) method, by which the solar flux distribution in the receiver or target surface could be accurately measured, as shown in Fig. 32. Röger et al.[100] used a similar method to measure the flux at the aperture of a volumetric receiver in Plataforma Solar de Almeria, and the result was validated with a result obtained using moving bar measurement. More measurements for the non-uniform solar flux distribution have also been conducted, which can be acquired from the operation reports of the Solar One, Solar Two[101, 102] and SOLGATE[95] plants.
[99]
Fig. 32 Measured solar flux on the focal plane of the National Solar Thermal Test Facility in Sandia lab
.
Secondly, many simulation codes have been developed to simulate the solar radiation transmisson process in the SPT plant, such as UHC, HFLCAL and DELSOL based on convolution methods, HFLD, MIRVAL, SOLTRACE and SPTOPTIC based on MCRT[96, 103-110]. Yao et al.[111], Besarati et al.[112], He et al.[85], Salomé et al.[113], and Farges et al.[114] investigated the solar flux distributions on the apertures of different plants using these codes. Their results showed that the flux distribution was extremely non-uniform which can be approximated as a Gaussian distribution, 26
as shown in Fig. 33. Moreover, Yu et al.[87, 93], Rinaldi et al.[115], and Wang and He et al.[116] further reveal the non-uniform fluxes on the inner surfaces of the cavity receivers. As an example, the solar flux distribution in PS10 plant is shown in Fig. 34. In addition, Rodríguez-Sánchez et al.[97] and Sanchez Gonzalez et al.[117] gave the non-uniform fluxes on the panels of external receiver as shown in Fig. 35. However, in these studies, the absorber tubes are assumed to be flat panels where the details of the tubes are not considered. In view of this situation, Qiu and He et al
[96]
developed a
software SPTOPTIC based on MCRT, where detailed geometries and optical processes of the SPT were considered. This tool was tested by simulating the DAHAN collector, and detailed solar fluxes on the absorber tubes within the cavity receiver was obtained, as shown in Fig. 36. This software can be employed to study the thermal performance of the receiver.
E
Fig. 34 Typical flux on the inner surfaces of PS10’s cavity receiver [115] (License Number 4476340902945). W
W·m-2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
3
Yr / m
Fig. 33 Typical flux at the focal plane using one-point aiming strategy[112] (License Number 4476330750001).
0
-3
Fig. 35 Typical flux on an [117] external receiver (License Number 4476341328562). W·m -2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
(a) flux on tubes and cavity walls (b) local high flux, highest flux is 514 kW·m-2 Fig. 36 Solar flux on the absorber tubes using one-point aiming strategy at spring equinox noon [96] (License Number 4476250147199).
The reason why the volumetric air receiver is called volumetric is that the solar radiation can 27
penetrate into the “volume” of the absorber through open, permeable cells of material. The flux characteristics of the volumetric receivers have also been studied by He et al.[85, 118], Cheng et al.[119, 120]
, Cui et al.[121], Qiu et al.[122] and Du et al.[98]. It is revealed that the solar flux distributions in the
porous absorbers of both closed-loop and open-loop volumetric receivers are significantly non-uniform, as shown in Fig. 37. It is found that a high-flux region occurs near the center of the closed-loop absorber, which is due to the re-concentration of the secondary concentrator. The peak heat sources in the two absorbers can be larger than 1.1×108 Wm-3 and 2.3×108 Wm-3, respectively. It is also seen that the flux decreases gradually along the incident direction in the absorber because the porous absorber absorbed the incident radiation gradually.
(a) Closed-loop[98] (b) Open-loop[122] Fig. 37 Solar fluxes in the porous absorbers of volumetric receivers(License Number 4476251112251).
In summary, the solar flux distributions on the inner surfaces of the tubular receiver and in the porous volumetric receivers are highly non-uniform. The solar flux is manily distributed at the specific part of the tubular receiver. Particularly, it should be noted that the solar flux is only on the unilateral surface of each single tube. Moreover, for the porous absorber, the peak flux occures at the inlet of the absorber. 4.3 Challenges and the corresponding solutions There is no doubt that the extreme non-uniformity of solar flux distribution will lead to the non-uniform temperature in the receiver. The non-uniform temperature would result in negative effects on the safety and operation of the solar receiver, and local overheating and HTF decomposition and stress fracture may occur[96, 123, 124]. For example, the structure damages of 28
receiver have occurred in the Solar Two plant[101,
102]
. Particularly, comparing with the
parabolic-trough collector and linear Fresnel collector, it will cause much greater challenges for the SPT due to its higher solar flux and operating temperature. In tubular receivers using molten salt as the heat transfer fluid, under the synergistic effect of thermal stress and molten salt corrosion, the strength of the steel tube would be weakened. As a result, the fracture failure of the tube may happen when the thermal stress exceeds the ultimate strength limit after being degraded. Du et al.[125-127] numerically analyzed the influences of the non-uniform flux in a cavity receiver using an optical-thermal-stress coupling model. Fig. 38 shows the solar flux, temperature and thermal stress distributions in the receiver, where extremely non-uniform flux and high thermal stress are observed. Similar results have also been obtained by Montes et al.[128] in a salt cavity receiver and by Yu et al.[129], Xu et al.[130] and Fang et al.[131, 132] in water/vapor cavity receivers.
(a) Solar flux
(b) Temperature
(c) Thermal stress Fig. 38 Typical solar flux, temperature and thermal stress on a cavity receiver at spring equinox noon[127](License Number 4476350714687).
Meanwhile, the thermal performance of the volumetric receiver with complex heat transfer processes have also been studied. He et al.[118, 133] used MCRT and FVM coupled method to study the heat transfer perfornance of a closed-loop porous receiver, where the temperature of the absorber was revealed in Fig. 37(a). Moreover, the temperature in a open-loop receiver shown in 29
Fig. 37(b) has been revealed by Cagnoli et al[134]. It can be observed that the flux is greatly non-uniform from the inlet to the outlet, which could result in overheating and breakage of the absorbers. Similar results have also been obtained by Cheng et al.[119], Capuano et al.[135], and Nakakura et al.[136].
(a) Closed-loop(License Number 4476251112251)
(b) Open-loop porous receiver with channels (License Number 4476351285471) Fig. 39 Temperature distributions in the absorbers of volumetric receivers [98, 134].
In order to avoid the above problems caused by the non-uniform temperature, many solutions have been proposed. They can be divided into three categories including heat transfer enhancement and optimization, optimization of solar absorptance distribution in receiver, and optimization of the aim strategies and the receiver structure. The details are as follows. 4.3.1
Heat transfer enhancement and optimization
The heat transfer enhancements could be used to weaken the non-uniformity of temperature distribution in the solar receiver, thus the negative effects can be weaken. For the tubular receiver, all the enhancing techniques for the heat transfer in tube used in the PTC receiver should still be effective. Moreover, some studies that specially focus on the absorber in SPT have also been conducted. Yang et al.[137] and Lu et al.[138] experimentally studied the heat transfer of molten salt in spiral absorber tube. They found that the Nusselt number of the spiral tube was about 3 times of that of the smooth tube on average as shown in Fig. 40. Meanwhile, the wall temperature of the spiral tube could be about 30 oC smaller than that of the smooth tube under the identical conditions. The heat transfer of molten salt in a transversally corrugated tube has also been studied by Chen et al.[139], and which found that its performance was better than smooth tube. Moreover, similar to that in PTC, porous medium insert has also been considered by Zheng et al[140], 30
and they found that the partly filled tube could achieve better overall performance than the fully filled one. 1400 1200
Nu number
1000
Spiral tube
800 600 400 200
10000
20000
30000 40000 Re number
50000
60000
Fig. 40 Comparison of the Nu numbers for spiral tube and smooth tube[137] (License Number 4476360659740).
In the tubular receiver, two loops arrangement with several connected tube panels in each loop are usually employed. The effects of different fluid flow layouts on the temperature distribution were analyzed by Wang et al.[125], as shown in Fig. 41. It indicates that the average surface temperature of the inwards layout can be greatly lower than that of the outwards layout, which can
Inlet
R ec e ive r he ight
help to reduce the heat loss and high thermal stress area. Wall temp. / 550
Receiver height
500 450 400 350
Out let
W a l l t. e /m p 55 0
50 0 45 0 40 0 35 0
300 Outlet
Receiver width
30 0 Inlet
Outlet
(a) Outwards layout
R e c e i v e r w i d t Ih n l e t
(b) Inwards layout
Fig. 41 Comparison of temperature on the absorber surfaces between two fluid flow layouts[125]
Moreover, a new fluid flow layout was proposed and optimized by Montes et al.[128], as shown in Fig. 42. In this layout, the inwards flow is employed, and different tube diameter and number are used in different panels. As a result, the HTF velocity in the panels near the center that accepts the peak flux would be larger than those near the left or right border. The results indicate that the 31
surface temperature and radiation loss are decreased because the heat transfer ability of fluid matches the flux distribution better. In addition, Rodríguez-Sánchez et al.[141] proposed a Variable Velocity Receiver (VVR) as shown in Fig. 43, which was equipped with valves that allow the division of each panel in two independent panels and could increase the HTF velocity in specific zones of the receiver. The results show that this receiver can avoid tube overheating, allowing more concentrated aiming strategies, which increases the optical efficiency of the heliostat field.
Fig. 42 Optimized fluid flow layout for cavity receiver [128] (License Number 4476370167117)
Fig. 43 Schematic of the flow layout in a variable velocity external receiver[141] (License Number 4476370635883) .
For the closed-loop volumetric receiver, Röger et al.[142] proposed to use air-jet cooling devices to cool the quartz glass. Their experiments found that window temperatures could be kept below 800 °C when the air outlet temperatures is over 1000 °C, which helps to improve the reliability importantly.
[142]
Fig. 44 air-jet cooling devices for the glass window of a closed-loop receiver 32
(License Number
4476371452873).
For the open-loop volumetric air receiver, Nakakura et al.[136] proposed new cut-back inlet designs that removed different amounts of material from the walls at the inlets of square channels. Their numerical results demonstrated that the exit temperature could be increased by 20.0 K, and the pressure drop could be decreased relative to the plane channel receiver. Capuano et al.[135] developed an optimized absorber with spiny inlet. Their results showed that the receiver efficiency of 84% could be achieved at the outlet temperature of around 850 °C, which is improved by 13% compared with the current state-of-the-art mini channel design (Hitrec-II). Moreover, the so-called volumetric effect was observed, since the outlet fluid temperature was higher than the solid inlet temperature, which helped to improve the reliability significantly. Pabst et al.[143] developed and optimized a receiver using cellular metal honeycomb, and three optimal channel designs enhanced by winded pairs of flat and corrugated metal foils were obtained, as shown in Fig. 46. The three designs could achieve similar performance. Fit curves of experimental receiver efficiency data are appearing in Fig. 46, where each curve represents a set of data for the three different channels. It is found that a receiver efficiency of about 83% can be achieved at the outlet temperature of more than 800 °C.
Single unit
Receiver efficiency / %
Receiver efficiency
Outlet temp. 850
New receiver
Fig. 45 Receiver efficiency of the new receiver and that of Hitrec-II receiver[135] (License Number 4476371452873).
4.3.2
Receiver
Three different channels
Fig. 46 Results of the 500 kW receiver efficiency tests of the [143]
new receiver
(License Number 4476380577337).
Optimization of solar absorptance distribution in receiver
The optimization of solar absorptance distribution in the solar receiver is another approach to avoiding the problems from the non-uniform flux and temperature. Wang and He et al.[144], Fang et al. [145], and Tu et al. [132] optimized the absorptance distribution of the coating in the 33
cavity receiver. Especially, the study from Wang and He et al.[144] tried to homogenize the flux distribution while keeping the optical loss as low as possible. Fig. 47 illustrates the optimized solar absorptance and flux distributions in a cavity receiver at spring equinox noon. Compared with the original flux distribution in Fig. 47(b) before the optimization, it is observed in Fig. 47(a) that the optimal absorptance distribution is approximately opposite to this flux distribution. The absorptance is relatively low at the regions where the original flux is relatively high, and vice versa. It is also seen in Fig. 47(c) that the flux distribution become quite uniform, the peak solar flux is significantly reduced with a little increase in reflective loss. For the closed-loop volumetric receiver, Roger et al.
[146]
proposed to use infrared-reflective coating on the quartz
window shown in Fig. 31(a). The results indicates that the glass temperature can be decreased by 65 K.
(a) solar absorptance distribution
(b) Original flux distribution, peak
(c) optimal flux distribution, peak flux= 336 kWm−2,
flux= 467 kWm−2, reflective loss= 3.4%
reflective loss=5.5%
Fig. 47 Optimized absorptance distribution and fluxes using single-point aiming strategy at spring equinox noon[147]
(License Number 4476350714687).
4.3.3
Optimization of aiming strategy and receiver geometry
The single-point aiming strategy for south-facing field and the equatorial aiming strategy for 34
surrounding field are usually adopted in the SPT system for the high interpret efficiency and the low optical loss. However, the solar flux distribution is extremely non-uniform when using such aiming strategy. For homogenizing the flux, multi-point aiming strategy is suggested for the SPT. Sánchez-González and Santana[117] studied the effects of the multi-point strategy on an external receiver for a circular field. Qiu et al
[96]
compared the one-point and multi-point aiming strategies
based on the configurations of DAHAN plant using SPTOPTIC. Du et al
[98]
also designed a
multi-point strategy for a south-facing heliostat field using volumetric receivers. In these studies, the aim point position for each heliostat was designed by estimating the radius of the reflected beam using the method provided by Vant-Hull[148]. A typical solar flux using the multi-point strategy is shown in Fig. 48, which is found to be more uniform compared with that of the one-point strategy as shown in Fig. 36. The highest flux is reduced from 514 kW·m-2 to 460 kW·m-2 with only one percent drop in collector optical efficiency at typical condition. W
E
W·m-2 5.0E5 4.5E5 4.0E5 3.5E5 3.0E5 2.5E5 2.0E5 1.5E5 1.0E5 0.5E5
Fig. 48 Solar flux in a cavity receiver with multi-point aiming strategy at spring equinox noon [96] (License Number 4476250147199).
Many studies have also been carried out to optimize the multi-point aiming strategy in SPT system. Besarati et al.[112] optimized the flux distribution at the focal plane of a heliostat field using the genetic algorithm. The optimal flux is shown in Fig. 49, and it is seen that the peak solar flux is decreases by 10 times compared with the flux distribution using one-point strategy in Fig. 33. Yu et al.[83] developed a multi-point aiming model using TABU meta-heuristic method for the DAHAN plant, and the solar flux distribution was found to be successfully optimized. Using the non-dominated sorting genetic algorithm, multi-objective optimization of the aiming strategy for the SPT with a cavity receiver was also performed by Wang and He et al
[149]
. Fig. 50 shows the solar
flux distribution on the inner surfaces of a cavity receiver using multi -point aiming strategy. Comparing with the original flux using one-point strategy in Fig. 47(a), it is found that the flux 35
distribution is homogenized by the multi-point aiming strategy obviously, the peak flux can be decreased from 467 kW·m-2 to 280 kW·m-2.
Fig. 49 Optimal solar flux distribution on the cavity aperture using multi-aim strategy[112] (License Number 4476330750001).
Moreover, Sánchez-González et al.[150,
Fig. 50 Solar flux distribution using multi-point aiming strategy[149] (License Number 4476381078603).
151]
presented an aiming model to properly point
heliostats at cylindrical molten salt receivers, and a typical flux and aim points are shown in Fig. 51. Two iterative algorithms (search and fit) were used to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints were translated into allowable flux densities that were handled by the model in their study. Compared with the equatorial aiming, receiver interception is just 0.04-point lower using the proposed strategy, but the peak flux is significantly reduced up to 23%.
Fig. 51 Optimal solar flux distribution on an external cylinder receiver showing aim points(circles)[151] (License Number 4476381453474).
Optimizing the receiver structure is also helpful to improve the uniformity of solar flux 36
distribution. Tu et al [152] found that the solar flux can be more uniform after the optimization of the geometric parameters, as shown in Fig. 52. In addition, Buck et al.[153] proposed a novel dual-layer receiver concept which is designed by installing a tubular evaporator section before a volumetric receiver. Their numerical results indicated that the new concept showed several benefits, especially higher receiver efficiency and lower receiver temperature.
(a) Before optimization (b) After optimization Fig. 52 Comparison of solar flux distributions between cavity receivers with different structures[152] (License Number 4476390466882).
5. Parabolic-Dish Collector 5.1. Description of parabolic-dish collector The Parabolic-Dish Collector (PDC)
[154, 155]
usually consists of a parabolic dish reflector, a
receiver and solar tracing devices as shown in Fig. 53. The receiver is installed at the focus of parabolic dish reflector. During normal operation, firstly, the sunlight is reflected by the reflector to the receiver. Then, the solar energy is absorbed and transformed to thermal energy in the heat transfer fluid through the receiver. Finally, thermal energy is used to generate electricity by heat engine and generator
[156]
. Currently, there are only a few small-scale PDC plants in the world as
shown in Table 4 due to the high cost of investment.
37
Parabolic reflector
Cavity receiver Porous In out
Volumetric receiver
Fig. 53 Schematic diagram of Solar dish system[157, 158] (License Number 4476221004890).
Compared with other CSP systems, the electric generator and receiver are usually coupled with the collector in PDC, which makes it more compact. Now there are two main solar dish systems including solar dish/micro gas turbine system and solar dish/Stirling system. Receiver for dish system is usually a cavity or volumetric receiver as shown in Fig. 53. The cavity receiver has a solar absorbing surface behind the focal plane, so that solar flux on the surface can be smaller than that at the focal plane, and thermal stress can be reduced. Moreover, the heat loss of cavity receiver is also reduced for the using of cavity. The volumetric receiver usually uses a porous media as the absorber, where it is important for enhancing the heat transfer between the absorber and the heat transfer fluid, especially when gaseous fluids that have low heat transfer ability are used. Table 4 Summary of the solar dish plants in the world
[159]
Name
Country
Capacity / MW
Engine
Receiver
Start Production
Current status
Maricopa Solar Project
USA
1.5
Stirling
cavity
2010
Non-operational
Tooele Army Depot
USA
1.5
Stirling
cavity
2013
Non-operational
5.2. Non-uniform solar flux distribution in PDC Similar to the three systems mentioned above, solar flux is extremely non-uniform at the surface of a dish receiver. To gain an insight to the influence of non-uniform solar flux on the performance of a dish receiver, much work has been done on the research of concentration characteristics of a dish collector. 38
Experimentally, it has been proven that solar flux at the entrance of a dish receiver is non-uniform[160-163]. Using Charge Coupled Device (CCD) imaging cameras, Johnston et al.[162], Shuai et al.[164], and Dähler et al.[165] gained non-uniform flux maps at the focal plane of PDCs, and a typical experimental result is illustrated in Fig. 54. Moreover, Jaramillo et al.[163] and Yu et al.[166] studied the distributions of solar flux and temperature at the focal plane. They found the temperature distribution was non-uniform together with non-uniform solar flux.
Fig. 54 Measured solar concentration ratio distribution at focal plane of a PDC[165] (Creative Commons).
Besides experimental study, more researches focused on theoretical revelation and numerical prediction of the non-uniform flux. The early research by Steinfeld et al.[167] used Gaussian distribution to describe the flux distribution at the focal plane. With the development of compute technology, many researchers applied ray tracing method to study the flux characteristics in more complex receivers[168]. Some studies focused on PDCs with cavity receivers. Shuai et al.[161] predicted the optical performance of a PDC for six different cavities, and non-uniform solar fluxes were observed on all cavity walls, as shown in Fig. 55. More similar studies on several receiver geometries have also been conducted by Cui et al.[169, 170], Tao et al.[171], Daabo et al.[172] and Yan et al.[173]. In addition, Dai et al.[174] and Xia et al.[160] numerically investigated concentrating properties of a PDC using a half-ellipse receiver with or without a 3D CPC, and the non-uniform flux in the receiver was also observed. They found that the concentration ratio could be improved by the 3D CPC, but a decrease in collector optical efficiency was also observed.
39
Fig. 55 Solar flux distributions on in different solar receivers of solar dish system[161] (License Number 4476211001312).
For the volumetric receiver, the solar heat source distribution in the porous absorber has also been widely studied [175-177]. Barreto et al.[177] and Chen et al.[176] simulated the radiation transfer in PDCs using porous media receivers which were assumed to be isotropic semitransparent mediums. While Du et al.[178] used X-ray computed tomography technique to reconstruct a realistic porous absorber and obtained the solar energy in the absorber using MCRT, as shown in Fig. 56. These studies found that the heat source is extremely non-uniform, and most power is absorbed at a thin region near the inlet of the absorber.
Fig. 56. Solar energy distribution in a porous volumetric receiver[178] (License Number 4476201217119).
5.3. Challenges and corresponding solutions To reveal the influence of non-uniform solar flux on the performance of the solar dish receiver, Cui and He et al.[170] applied MCRT and FVM coupled method to study the solar-thermal 40
performance of cavity receivers in dish system and analyzed the effect of non-uniform flux on temperature distribution. They found that non-uniform solar flux could result in significant non-uniform temperatures on the receiver walls as shown in Fig. 57, where the lowest temperature of 1296 K and the highest temperature of 1375 K could occur in a single receiver. Moreover, Du et al.[178] studied the pore-scale heat transfer in a porous absorber using MCRT and FVM, and the no-nuniform temperature distributions of the air and solid under the non-uniform solar heat source in Fig. 56 are shown in Fig. 58. In addition, Wang et al.[179, 180] and Chen et al.[175] numerically studied the thermal performance of porous volumetric receivers using the local thermal non-equilibrium model. They also found that the temperature distribution inside the porous absorber was quite non-uniform due to the non-uniform solar heat source.
(a) solar flux distribution (b) temperature distribution Fig. 57 Flux and temperature distribution on the wall of a hemispherical cavity receiver[170] (License Number 4476210516201)..
41
Fig. 58 Temperature distribution in porous volumetric receiver[178] (License Number 4476201217119).
It is known that the non-uniform temperature challenges the safe operation of the system, because it may cause strong thermal stress and serious thermal deformation. Besides, local high temperature may also result in thermal denaturation of heat transfer fluid. Because of larger concentration ratio and higher operation temperature, the issue is more rigorous for solar dish system than that for other concentrating solar power systems. Li et al.[181] pointed out that more uniform temperature distribution in solar dish receivers and higher receiver efficiency were the keys to improve the reliability and economy of solar dish system. To solve these problems introduced by non-uniform flux, several solutions have been proposed and explored. 5.3.1 Optimization and improvement of receiver structure A feasible solution to homogenize the flux in the cavity receiver is the geometric optimization of the receiver. Shuai et al.[182] designed a pear-like cavity receiver based on the concept of equivalent flux, as shown in Fig. 59. It can be seen that the solar flux distribution can be more uniform in the pear-like receiver than that in corresponding hemisphere receiver, as shown in Fig. 42
60. Considering the redistribution effect of the quartz window, a cavity receiver with a plano-convexo quartz window as shown in Fig. 61 was also designed to weaken the non-uniform flux by Shuai et al.[183]. Fig. 62 shows that the receiver with a plano-convexo window can provide a more uniform flux distribution and higher receiver efficiency than the windowless receivers. Moreover, Phase Change Material (PCM) thermal storage technology[171] and heat pipe technology[184] have also been applied to gain more uniform temperature. These technologies have been proven to be effective in reducing the temperature difference and thermal stress of the receiver. For example, Tao and He[171] studied on the coupling heat transfer containing phase change in a cavity receiver. It was found that the temperature gradient could be reduced, especially when the thermal conductivity of PCM is enhanced. In addition, the heat pipe and PCM technologies can also be applied in SPT receivers.
Fig. 59 Sketch of the pear-like receiver for solar dish system[182] (License Number 4476200774743)
Fig. 60 Solar flux distributions of hemisphere receiver and pear-like receiver[182] (License Number 4476200774743)
43
qr / qr,max
x / mm Fig. 61 Sketch of a hemisphere receiver with plano-convex window [183] (License Number 4476200225735)..
Fig. 62 Solar flux distributions in hemisphere receivers with or without window[183] (License Number 4476200225735).
For homogenizing the solar heat source in the volumetric receiver, Chen et al. [185] designed a receiver with double-layer porous media as shown in Fig. 63. It is found that the decreasing-porosity configuration tends to achieve higher air outlet temperature and lower solid inlet temperature, as shown in Fig. 64. Meanwhile, the drawbacks introduced by non-uniform solar heat source can be reduced. Moreover, Du et al.[186] proposed a volumetric receiver with gradually-varied porosity which is optimized using genetic algorithm as shown in Fig. 65(a). They found that the heat source is greatly homogenized by the GA method compared with those of the linear-changed structure with porosity of 0.95-0.65 and uniform structure with porosity of 0.89 that is the average porosity of the optimized solar receiver, as shown in Fig. 65(b). At the same time, the receiver optical efficiency can be improved.
44
Fig. 63 Sketch of double-layer porous receiver[185]
Fig. 64 Temperature in double-layer receiver design[185]
(License Number 4476191488278).
(License Number 4476191488278)..
(a) Porosity distribution in receiver. (b)Solar heat source in receiver Fig. 65 Volumetric receiver with gradually-varied porosity [186] (License number 4476190563569).
5.3.2 Optimization of solar concentrator Another solution for obtaining more uniform flux is the optimization of the reflector. Yan et al.[187] designed a discrete solar dish concentrator (DSDC) for improving the flux uniformity inside cavity receiver as shown in Fig. 66. The DSDC was designed by dividing an ideal parabolic dish into several parts and rotating each part around its one end. It is found that the DSDC can improve the flux uniformity and reduce the peak flux with a drop of several percentage points in collector optical efficiency, as shown in Fig. 67.
45
Fig. 66 Sketch of the discrete solar dish concentrator (DSDC) with a cylinder cavity receiver[187] (License Number 4476181167371).
(a) Ideal parabolic-dish collector (b) DSDC Fig. 67 Flux distributions in the receivers of an ideal parabolic-dish collector and the DSDC[187] (License Number 4476181167371).
6. Summary and conclusion This paper introduces the characteristics of non-uniform solar flux distributions in four different CSPs. The challenges caused by these non-uniform characteristics are summarized. Moreover, the solutions proposed to deal with the challenges are reviewed. It is found that significantly non-uniform solar fluxes occur in different types of receivers. The non-uniform fluxes will inevitably result in the steep temperature gradient and the high local temperature in CSP receivers, which would lead to great challenges for the safe and efficient operation. The high local temperature may introduce the decomposition of the heat transfer fluid and could damage the absorbing coating, which lowers the receiver thermal efficiency. The steep temperature gradient leads to large thermal stress, which results in the deformation and stress failure of the receivers. After the review of the solutions aiming at solving these challenges, a recommendation for the 46
optimization of the solar collector is provided, which is that the solar flux distribution and the heat transfer ability of the HTF should match with each other as well as possible. Because we observe that the major reason for the challenges caused by the non-uniform flux is the mismatch between the identical heat transfer ability of the HTF and the non-uniform solar flux. From this point of view, all the solutions mentioned above can be summarized in two approaches for weakening this mismatch. The first approach is to enhance and optimize the heat transfer ability in the receiver to match with the non-uniform flux distribution, which can be called “passive approach”. The second approach is to improve the uniformity of the flux distribution to match with the heat transfer ability in the receiver, which can be called “active approach”. The exiting solutions divided into the two approaches are summarized in Table 5. This review can help the researchers and practitioners have a better understanding of the non-uniform solar flux features in CSPs, and provide guidance for solving the corresponding challenges. Table 5 Approaches proposed to tackle the challenges caused by non-uniform solar fluxes in CSPs CSPs
PTC
LFC
SPT
PDC
Solution
Approach
Variable focus concentrator for uniform flux
Active
Use of secondary reflector in annular space for uniform flux
Active
Use of secondary reflector above the receiver for uniform flux
Active
Heat transfer enhancement in absorber tube to flatten the temperature
Passive
Use of absorber tube with high thermal conductivity
Passive
Optimize the aiming strategy for uniform flux
Active
Optimize the secondary reflector for uniform flux
Active
Heat transfer enhancements for PTC are also suitable for LFC
Passive
Optimize the aiming strategy for uniform flux
Active
Optimize the solar absorptance distribution in cavity receiver for uniform flux
Active
Optimize the flow path of the HTF in the receiver
Passive
Heat transfer enhancements within the absorber tube
Passive
Use of optical lens at the cavity aperture
Active
Use of heat pipe technology within the receiver
Passive
Optimization of the receiver geometries
Active
Optimization of the concentrator
Active
Use of optical lens at the cavity aperture
Active 47
Utilizations of heat pipe and PCM within the receiver
Passive
Acknowledgements The study is supported by the Key Project of National Natural Science Foundation of China (No.51436007) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No.51721004). Nomenclature AC
collector net collection area
CSP
concentrated solar power
D
diameter / mm, m
DNI
direct normal irradiance / W·m-2
FVM
finite volume method
f
friction factor
GA
genetic algorithm
h
height / mm, m
HTF
heat transfer fluid
LFC
linear Fresnel collector
L
length / m
MCRT
Monte Carlo ray tracing
MTCR
multiple-tube cavity receiver
Nu
Nusselt number
PCM
phase change material
PTC
parabolic-trough collector
PDC
Parabolic-dish collector
PEC
performance evaluation criteria
p
pitch / m
ql
local solar flux / W·m-2
QC
available solar power that can be accepted by collector
Qape
incident power on the receiver aperture
Qabs
radiant solar power absorbed by the absorber
QHTF
power transferred to heat transfer fluid
Re
Reynolds number
SPT
solar power tower
STCR
single-tube cavity receiver
W
width / m
x, y, z
Cartesian coordinates / m 48
Greek symbols ηR,opt
receiver optical efficiency
ηR,th
receiver thermal efficiency
ηR
receiver efficiency
ηC
collector efficiency
ηC,opt
collector optical efficiency
θ
angle on the absorber tube / °
Subscripts abs
absorber parameter
ape
aperture parameter
C
collector parameter
max
maximum value
opt
optical
R
receiver parameter
s
smooth tube
th
thermal parameter
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58
Highlights Non-uniform characteristics of solar fluxes in CSPs are reviewed Solar flux and fluid heat transfer ability are suggested to match with each other Useful recommendations for the optimization of the solar collector are provided
59