Performance analysis of conventional and sloped solar chimney power plants in China

Applied Thermal Engineering 50 (2013) 582e592

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Performance analysis of conventional and sloped solar chimney power plants in China Fei Cao a, Liang Zhao a, *, Huashan Li b, c, Liejin Guo a a

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, No. 28, West Xianning Rd, Xi’an 710049, PR China Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, PR China c Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China b

h i g h l i g h t s < The optimum collector angle for maximum power generation is 60
in Lanzhou. < Main parameters influencing performances are the system height and air property. < Ground loss, reflected loss and outlet kinetic loss are the main energy losses. < The sloped styles are suitable for Northwest China. < The conventional styles are suitable for Southeast and East China.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 April 2012 Accepted 21 June 2012 Available online 28 June 2012

The solar chimney power plant (SCPP) has been accepted as one of the most promising approaches for future large-scale solar energy applications. This paper reports on a heat transfer model that is used to compare the performance of a conventional solar chimney power plant (CSCPP) and two sloped solar chimney power plants (SSCPPs) with the collector oriented at 30
and 60
, respectively. The power generation from SCPPs at different latitudes in China is also analyzed. Results indicate that the larger solar collector angle leads to improved performance in winter but results in lower performance in summer. It is found that the optimal collector angle to achieve the maximum power in Lanzhou, China, is around 60
. Main factors that influence the performance of SCPPs also include the system height and the air thermophysical characteristics. The ground energy loss, reflected solar radiation, and kinetic loss at the chimney outlet are the main energy losses in SCPPs. The studies also show SSCPPs are more suitable for high latitude regions in Northwest China, but CSCPPs are suggested to be built in southeastern and eastern parts of China with the combination to the local agriculture. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Conventional solar chimney power plant Sloped solar chimney power plant Solar chimney Solar energy Northwest China

1. Introduction

1.1. Theoretical study of SCPPs

The solar chimney power plant (SCPP) is composed of three components: a solar collector, a chimney situated in the center of the collector and a power conversion unit (PCU). A schematic of the SCPP is shown in Fig. 1. Sunlight transmits through the transparent solar collector cover and heats the ground below. Ambient cold air enters the collectors from the periphery of the collectors and is heated as it flows along the collector toward the center. Due to the pressure created by the density difference between the warm airflow and ambient cold air, the airflow enters the chimney, and with the PCU, the kinetic energy of the airflow is converted into the electric power.

The concept of SCPP was originally proposed in 1903 by Isidoro Cabanyes [1] and then presented in a publication by Günther [2]. A systemic research on the SCPP was first performed by Schlaich. In 1981, with the financial support from the German Ministry of Research and Technology, Schlaich began the construction of a pilot SCPP with the peak power about 50 kW in Manzanares, Spain [3,4]. This is the most systematic study of the solar chimney power technology in practice until now and successfully verifies the feasibility of SCPPs. Since then, extensive research has been carried out on the hugepotential of the SCPP over the world. Haaf et al. carried out a basic research about the energy balance on the ground surface, energy loss in the chimney and turbine of the Manzanares pilot power plant

* Corresponding author. Tel.: þ86 029 82668287; fax: þ86 029 82669033. E-mail addresses: [email protected], [email protected] (L. Zhao). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.06.038

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

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optimal collector slope of the SSCPP [20,21]. The SSCPP is based on the SCPP principle and takes advantage of the local geographical features. So, we could recognize the SSCPP in Fig. 2 as the development of the horizontal solar chimney power plant in Fig. 1, which in this study is called the conventional solar chimney power plant (CSCPP). 1.2. Case study of SCPPs

Fig. 1. Schematic of a solar chimney power plant.

[3]. von Backstrom et al. analyzed the pressure drop in solar chimneys [5], the turbine characteristics [6], and performances of the PCU [7]. Bernardes et al. analyzed some available heat transfer coefficients applicable to SCPP collectors [8], and evaluated some operational control strategies [9]. Pretorius et al. made a sensitivity analysis of the operating and technical specifications of SCPPs [10], and made a critical evaluation on the solar collector and heat storage layer [11]. Zhou et al. analyzed the influence of chimney height on SCPPs [12], and made preliminary research on the special climate around a commercial SCPP [13]. Schlaich et al. reported the design results of commercial SCPPs [14]. Yan et al. investigated the influence of the working fluid and chimney temperature [15]. Pasumarthi and Sherif built a complete mathematical model and analyzed the performances of SCPPs against the experimental research [16]. In 2005, Bilgen and Rheault proposed a sloped solar chimney power plant (SSCPP), whose solar collector is laid along the hillside (see Fig. 2) [17]. They concluded that SSCPP has higher thermal efficiency at high latitudes. This kind of SCPP is suitable for high latitude and mountain areas. Serag-Eldin explored the feasibility of an SCPP with the chimney built over the steep side of the mountain [18]. Zhou and Yang reported a novel SSCPP with floating chimney and predicted its potential in China’s Desert [19]. Wei et al. and Cao et al. analyzed the slope angle effect on receiving insolation and investigated on the

There are some case studies of CSCPPs in literature. Mullet made detailed analysis about the efficiency of SCPP, and concluded the feasibility of building SCPPs in developing countries [22]. Zhou et al. investigated the performances of a 100 MW CSCPP in Qinghai-Tibet Plateau [23]. Nizetic et al. analyzed the feasibility of implementing SCPPs in the Mediterranean region [24]. Larbi et al. made a performance analysis of an SCPP in the southwestern region of Algeria [25]. Ketlogetswe et al. described a systematic experimental study on a mini-solar chimney system in Botswana [26]. Dai et al. explored the feasibility of SCPPs in three regions of Northwest China [27]. However, as for the SSCPP, to our knowledge the only case is that a simulation of SSCPP in Lanzhou, China was carried out by Cao et al. [28]. Because of significant meteorological and geographical differences and local economic differences, case studies of SCPPs for different countries or regions are of high value. The authors’ previous study suggested that the SSCPP had better performances in spring and autumn days, whereas the CSCPP developed superiority in summer days [28]. For Northwest China, where local geographical resources are rich (over 30 mountain chains) and annually solar radiation is also strong (over 5852 MJ/(m2 year)) [29], it is thus of high significance to analyze and compare the performances of CSCPPs and SSCPPs in such areas throughout the year. By using a mathematical model based on the heat transfer, thermodynamics and fluid dynamics theories, a comparative study of the performances of a CSCPP and two SSCPPs in Lanzhou, China is performed. Main tasks of this study include: 1) To build a simplified mathematical model for SCPPs. 2) To compare the performances of the CSCPP and SSCPPs in Lanzhou. 3) To analyze the power generation of CSCPPs and SSCPPs at different latitudes in China.

Fig. 2. Schematic of a sloped solar chimney power plant.

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2. Mathematical model

2.2. Solar collector

2.1. Solar radiation into the collector

The solar collector is made up of three parts: the collector cover, the heat storage layer and the working fluid between them. Ambient air, the working fluid of SCPPs, enters the channel between the collector cover and heat storage layer, and flows into the chimney connected to the outlet of the solar collector. According to Pretorius [11], glass is chosen as the collector cover material because of its high optical and intensity performances. The solar energy received by the collector transmits through the glass cover and is absorbed by the ground. The ground acts as the heat storage layer. Pretorius analyzed various ground types (including sandstone, granite, limestone, sand, wet soil and water), and concluded that stones had better performance than the sand for heat storage [32]. As for Lanzhou, granite is chosen as the ground type in this study. Detailed parameters of solar collector materials are shown in Table 1. There are six basic assumptions used in simulation and they are: (1) steady state conditions, (2) the airflow speed at the collector inlet is ignored, (3) the solar collector is oriented northesouth and facing toward the equator, (4) no evaporation takes place under the collector, (5) the vertical temperature profile of the collector air is constant, and (6) energy loss from the airflow to the ambient air at the collector inlet is ignored. The hot ground transfers heat in the method of convection and radiation to the air above it. The schematic of thermal balance in the collector is shown in Fig. 3. The cold air, with the temperature Ta, enters the collector and is heated by the hot ground. Energy balances in the solar collector are defined as:

Total solar radiation on a horizontal surface Hhor is:

Hhor ¼ Hb þ Hd

(1)

where Hb and Hd are, respectively, the beam and diffuse solar radiation on the horizontal surface. The data of the annually average monthly total and diffuse solar radiation on the horizontal, viz., H hor and H d , are obtained from the National Meteorological Information Centre (NMIC) of China. The received solar radiation Ht on a surface with tilted angle b could be calculated as:

Ht ¼ Ht;b þ Ht;d þ Ht;r

(2)

where Ht,b, Ht,d and Ht,r are the beam, diffuse and reflected solar radiation on the sloped surface, respectively. They could be calculated by the next three equations:

Ht;b ¼ Hb Rb

(3)

Ht;d ¼ Hd Rd

(4)

Ht;r ¼ ref $Hhor Rr

(5)

where ref is the ground reflectance [30]. Rb, Rd and Rr are the coefficients. They could be calculated in the following equations [31]:

Rb ¼


p  uts sinð4  bÞsin d 
180 p us sin 4 sin d cos 4 cos d sin us þ 180

cosð4  bÞcos d sin uts þ

rf vf Af ¼ ro vo Achi

1 þ cos b Rd ¼ 2

(7)

1  cos b Rr ¼ 2

(8)

where 4 is the latitude, us is the sunset angle, uts is the sunset angle toward the sloped surface, and d is the solar declination angle. The total solar radiation on a sloped surface finally could be expressed as:

1 1 Ht ¼ ðHhor  Hd Þ Rb þ Hd ð1 þ cos bÞ þ ref $Hhor ð1  cos bÞ 2 2

1 þ cos b 1  cos b þ Hhor $ref $ar 2 2 (10)

Energy equations for the glass cover


   S1 þ Ut ðTa  Tc Þ þ hr Tp  Tc þ h1 Tf  Tc ¼ 0

(13)

for the ground


     S2 þ Ub Ta  Tp þ h2 Tf  Tp þ hr Tc  Tp ¼ 0

(14)

and for the airflow

Table 1 Configuration sizes and coefficients of the CSCPP and the SSCPP. Style

CSCPP

SSCPP

Solar collector

Collector radius Collector area Collector heighta Collector cover emittance Glass extinction coefficient Glass thickness Refractive index

550 m 950,000 m2 0 0.87 32 m1 5 mm 1.526

e 950,000 m2 0e1378.4 m 0.87 32 m1 5 mm 1.526

Chimney

Height Diameter

547 m 54 m

60 m 54 m

Ground

Material Density Specific heat capacity Thermal conductivity Normal emittance Reflectance

Granite 2640 kg/m3 820 J/(kg K) 1.73 W/(m K) 0.92 0.25

Granite 2640 kg/m3 820 J/(kg K) 1.73 W/(m K) 0.92 0.25

Turbine

Efficiency Inlet loss coefficient

0.8 0.056

0.8 0.056

The monthly average solar radiation transmitted through the glass cover and absorbed by the heat storage layer S2 is:

1 þ cos b S2 ¼ ðHhor  H d Þ Rb ðsaÞb þ Hd ðsaÞd 2 1  cos b þ Hhor $ref $ðsaÞr 2

(12)

(9)

The monthly average solar radiation absorbed by the southfacing sloped solar collector cover S1 is calculated as:

S1 ¼ ðHhor  H d Þ Rb ab þ Hd ad

Continuity equation:

(6)

(11)

where a is the absorptance of the collector cover and (sa) is the transmittanceeabsorptance product of the solar collector. The subscript b, d and r refer to the beam, diffuse and reflected radiation, respectively. Detailed methods for calculating a and (sa) can be found in Ref. [31].

a

Collector heights of SSCPPs depend on the solar collector angles.

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Fig. 3. Thermal balance in the solar collector.



 h1 Acoll Tc  Tf þ h2 Acoll Tp  Tf ¼ Q

(15)

where

Q ¼ ro vo Achi cp ðTo  Ta Þ Tf ¼

To þ Ta 2

(16) (17)

In the above equations, Af is the section area of the solar collector where the air density and airflow speed are rf and vf respectively, Achi is the chimney sectional area, Tc is the collector cover temperature, Tp is the heat storage layer temperature, Tf is the airflow temperature, vo is the airflow speed at the collector outlet, ro is the air density at the collector outlet, To is the airflow temperature at the outlet of the collector, Acoll is the collector area, Q is the energy absorbed by the airflow, ra is the ambient air density, and Ut, hr, h1, Ub and h2 are the heat transfer coefficients. Detailed equations to calculate these heat transfer coefficients could be found in Ref. [16], and the working fluid densities ro and rf are, respectively, determined at To and Tf by the following equation derived from the air properties between 0 and 70
C in [33]:

rðTÞ ¼ 0:000012T 2  0:011167T þ 3:445689

(18)

Energy loss in the solar collector could be divided into three parts, viz. the radiation that is reflected by the collector cover and the ground Sref, the energy loss by the collector cover Qloss,t and the energy loss by the heat storage layer Qloss,b. They could be calculated as: 



Sref ¼ rr Ht þ ref Ht s1

(19)

Qloss;t ¼ Ut ðTc  Ta Þ

(20)

  Qloss;b ¼ Ub Tp  Ta

(21)



where H t is the daily average total solar radiation on tilted surfaces, and rr and s1 are the reflectance and transmittance of the glass cover. Note that the ground reflection from the second time has not been included in the calculation due to its small and negligible influence on the final results. 2.3. Chimney The model described below is based on the following assumptions: (1) Boussinesq approximation is assumed to be valid, (2) energy loss through the chimney wall is ignored, and (3) turbulence after the turbine is ignored. The continuity equation in the solar chimney is:

ro vo ¼ rout vout

(22)

where rout and vout are the air density and airflow speed at the chimney outlet. Noted that rout is assumed to be the density of ambient air (ra) according to the conventional method of SCPPs [3,17]. For a vertical adiabatic chimney, the pressure difference created in the chimney is:

DPchi ¼ ðrout  ro ÞgHchi

assumed

¼

ðra  ro ÞgHchi

(23)

The pressure difference between the inlet and outlet of the solar collector is calculated as:

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F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

2.5. Theoretical solution procedures

gðra  rðzÞÞdz

(24)

inlet

where z denotes the height. According to Haaf et al. [3], Bernardes et al. [8] and Bilgen and Rheault [17], we assume that the air density variation is linear between entrance and exit of the sloped collector. Thus, the air density variation can be calculated as:

rðzÞ ¼ ra þ

ro  ra Hcol

z

(25)

By integrating Eq. (24) between the inlet and the outlet of the sloped solar collector with r(z) from Eq. (25) and assuming z ¼ 0 at the collector inlet, the pressure difference generated in the collector is:

DPcol ¼

ra  ro 2

gHcol

(26)

The total pressure difference yielded due to buoyancy can be expressed as:



DPtot ¼ ðra  ro Þg Hchi þ

Hcol 2

 (27)

where Hchi and Hcol are the chimney and the collector height, respectively. The pressure difference is consumed by four parts, i.e., the friction losses in the collector and the chimney DPf, the kinetic energy losses at the turbine inlet DPin, the kinetic energy losses at the chimney outlet DPout, and the rest is the effective pressure which is used by the turbine to generate electricity DPt.

DPtot ¼ DPf þ DPin þ DPout þ DPt

(28)

where DPf, DPin and DPout could be calculated as:

DPf ¼ f

Lth 1 2 rv D 2 f f 1 2

DPin ¼ g ro v2o DPout

1 ¼ rout n2out 2

(29)

(30)

(31)

Parameters that indicate performances of the SCPP could be confirmed through simultaneous Eqs. (12)e(17), (22), (27), (28) and (32). There are 10 unknown parameters and 10 equations are summarized. So, the equations could be solved through iterative calculations. During the process, an initial guess of the collector cover temperature, airflow temperature, heat storage layer temperature, airflow speed and airflow density is made firstly. Then, an iterative process is initiated, and all the required heat transfer and friction loss coefficients are calculated based on the initially guessed values. Each new assumed result calculated in the SCPPs is then compared with the old corresponding value. Only the difference between any corresponding new and old values is less than the maximal acceptable difference, would the iteration be finally stopped. By this repetitive and iterative process, the temperatures and airflow speed in the collector, the airflow densities in the collector and chimney, the mass flow rate, generated power in the turbine, etc. can be obtained. 3. Results 3.1. Configuration sizes of the CSCPP and SSCPPs in Lanzhou Lanzhou (103.50
E, 36.03
N) is a zonal basin city 1520 m above the sea level, with an area of 13,085.6 km2. It is the capital of Gansu Province and locates in the geographical central of Northwest China. Its annual global solar radiation is more than 5020 MJ/m2, and sunshine duration is over 2600 h per year. Its annually mean temperature is 9.8
C. To carry out the analysis of SCPP performances, we consider three reference 5 MW SCPPs for examples, whose baseline parameters are also given in Table 1 based on Schlaich et al. [14] and Bilgen and Rheault [17]. The main reason for choosing 5 MW SCPPs for this case study is that though many reports suggest that SCPPs with 100 MW power generation or even higher could have higher efficiency [14,23], huge costs of such SCPPs are overbearing to Northwest China, which is the economically under-developed region, and Lorente et al. argued that the principle of “few large and many small” for SCPPs is land-efficient [34]. Besides, only 5 MW typical dimensions of the SSCPP could be found in literature. 3.2. SSCPPs with different solar collector angles Fig. 4 shows the solar radiation on tilted surfaces and power output of SSCPPs with different collector angles. It is found that the

1600

where f is the friction loss coefficient, Lth is the length of the channel, D is the hydraulic diameter and g is the turbine inlet loss coefficient.

Annually solar radiation Annually power output

Maximum solar radiation

200

Maximum power output

1400

2.4. PCU and efficiencies The power generated by the turbine, Pele, is:

Pele ¼ ht DPt vo Achi

(32)

where ht is the turbine efficiency. The collector efficiency can be expressed as:

hcol ¼ Q =ðH t  Acoll Þ

(33)

1200 120 1000

80 800

40

600 20

25

30

35

40

45

50

55

60

65

70

75

Solar collector angle

The system efficiency is:

hsys ¼ Pele =ðH t  Acoll Þ

Solar radiation/ GWh/m

2

160

Annually power output / GWh

DP ¼

outlet Z

(34)

Fig. 4. Solar radiation on different slopes and power generation by SSCPPs with different collector angles.

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

Density Flowrate

C1 C2 C3

-1

50

45

1.30

-3

1.35 40

1.25 35

1.20

Density/ kgm

3

Mass flowrate/ x10 kgs

tilted surface of 30
receives the largest solar radiation. However, the maximum power generation occurs with the collector angle at 60
. This finding agrees well with the result of Li [35] using the criterion of maximum airflow rate to optimize the tilted angle of SSCPPs in Lanzhou. Similarly, a report from Sakonidou et al. showed that to obtain the maximum airflow rate in a solar chimney for ventilation, the collector slope is 20e25
larger than the local latitude [36]. On the other hand, Bilgen and Rheault suggested that to maximize solar radiation received by the collector, the optimum collector slopes are smaller than the latitude by 5e7
[17]. It should be noted that for turbines of the SCPPs, maximum airflow rate is much more essential than the maximum solar radiation. Therefore, in the next part of discussion, we take three SCPPs into comparison: the conventional solar chimney power plant (C1), and the sloped solar chimney power plants with the collector tilted angle of 30
(C2) (following Bilgen and Rheault’s study [17]) and 60
(C3) (following Li’s [35] and Sakonidou et al.’s [36] studies).

587

1.15 Jan

Feb

Mar

Apr

May Jun

Jul

Aug Sep

Oct

Nov Dec

Month in the year Fig. 6. Air densities and air mass flow rates of C1, C2 and C3 throughout the year.

3.3. Performances analysis Fig. 5 shows the ambient air temperature and the airflow temperatures of C1, C2 and C3. It is found that airflow temperatures of C1, C2 and C3 have similar tendency: first rising from January to July then decreasing from July to December. The peak values occur in July and the temperatures are roughly symmetric with the vertical axis in July. Also, it can be seen that the airflow temperature increases of C2 are higher than the other two styles in every month, and temperature increases of C1 are higher than those of C3 only between April and August. The airflow temperature increase is a compound result of the solar irradiation and the system pressure difference: the higher solar radiation would increase the airflow temperature, and the higher system pressure difference would increase airflow speed but decrease the airflow temperature rise. Fig. 6 shows that the air densities in summer are smaller, and their densities match well with the temperatures in Fig. 5, following the principle that the higher temperature results in lower density of the airflow. However, the variations of air mass flow rates of C1, C2 and C3 are different, such as: 1) For C1, the highest air mass flow rate occurs in June (45.87 t/s) and the lowest air mass flow rate locates in November (39.39 t/s); the air mass flow rates in summer are higher than in winter; and the curve is roughly symmetrical with June.

35 30

2) For C2, the highest air mass flow rate locates in March (42.41 t/s) and the lowest air mass flow rate locates in July (38.37 t/s); a small drop first appears in February, and then followed by a long-time gradual drop from March to July; and the tendencies in the rest months are roughly symmetrical with those in the first half year. 3) For C3, the highest air mass flow rate locates in January (50.37 t/s) and the lowest air mass flow rates locates in July (40.78 t/s); the tendencies of C3 are similar to that of C2, but the drop from March to July of C3 is much larger than that of C2. In addition, it is also observed that the highest total air mass flow rate in a year is produced by C3 (396.29 Mt), followed by C1 (382.74 Mt) and then C2 (352.90 Mt). For the turbine generator, air mass flow rate is an important parameter to determine its power output. Fig. 7 shows the input energy and output power of C1, C2 and C3. The input energy is the monthly total solar radiation reaching the solar collector. It is found that the solar radiation on a horizontal surface is roughly symmetrical with July, and the highest solar radiation also appears in July. Tilting the collector would decrease the solar radiation in summer days but increase it in winter days.

C1 Temperature C2 Temperature C3 Temperature

800

Ambient Temperature

700

Radiation Power

C1 C2 C3

Airflow temperature increase

600

15 10

500 400 6 300

5

200

0

100

2

Airflow temperature increase

-5

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

0

0

Airflow temperature increase

Jan -10

4

Dec

Month in the year

Fig. 5. The ambient temperature, the airflow temperature of C1, C2 and C3 throughout the year.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Power generation/ MW

Solar radiation/ MW

20

o

Temperature / C

25

Dec

Month in the year Fig. 7. Monthly total solar radiation received by the solar collector and the power generation of C1, C2 and C3 throughout the year.

588

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

The larger the collector angle is, the more obvious the described increase and decrease tendencies would be created. And the annually total solar radiation of C2 is the largest, up to 5066.82 TJ, then followed by C1 of 4634.87 TJ, and finally C3 of 4529.19 TJ. As for the output power of C1, C2 and C3 throughout the year, it is found that: 1) For C1, the power generation tendency is the same as its input energy; and the highest power is produced in June (5.33 MW) and the lowest in December (2.85 MW). 2) For C2, the power generation is much stable comparing with C1 and C3, with two small peaks in March and September; and the highest power generation occurs in March (3.65 MW) and the lowest in July (3.02 MW). 3) For C3, the power generation is much higher in winter days than that of C1 and C2; the power generation decrease of C3 from March to July is larger than that of C2; and the highest power generation is in January (5.90 MW) and the lowest in July (3.76 MW).

Bilgen and Rheault [17]. The solar collector efficiencies of C1 are symmetrical with July. The solar collector efficiency of C2 is higher than that of C3 in summer days. In addition, C2 has the highest average solar collector efficiency of 56.43%, followed by C3 of 56.11%, and the lowest is C1 of 48.52%. In a word, the tilted collector would enlarge the solar collector efficiency. Besides, the power efficiency in Fig. 8 describes the proportion of generated power to the total solar energy input. Power generation is the ultimate purpose of the SCPPs, thus the power efficiency is of high importance for the SCPP. It is found that throughout the year, the rank of power efficiency is C3, C1 and C2. The power efficiencies of C2 and C3 have the similar tendencies: higher in winter but smaller in summer; the highest power efficiencies both occur in January (0.595% and 0.998% respectively), and the lowest power efficiencies are both in July (0.534% and 0.862% respectively). However, the power efficiency tendency of C1 is opposite to that of C2 and C3. The highest power efficiency of C1 is in June (0.814%), and the lowest in December (0.760%). 4. Discussion

The average power generation of C1, C2 and C3 throughout the year is 4.36 MW, 3.23 MW and 4.78 MW, respectively. The power generation of C1, C2 and C3 in Lanzhou is smaller than the reference SCPP of 5 MW. The differences are caused by the different solar radiation and solar duration between Winnipeg [17], Manzanares [14] and Lanzhou. Note that though the power generation of C2 is smaller than C1, the chimney height of C2 is much smaller than that of C1, and even though chimney height of C3 is smaller than that of C1, power generation of C3 is larger than C1. From the above comparison, it is concluded that: 1) The suggested configuration sizes in literature may not be suitable for other regions. For different regions, specific design work would be required. 2) Inclining the collector could sharply decrease the solar chimney height (from 547 m of C1 to 60 m of C3, as shown in Table 1. The system efficiencies and solar collector efficiencies of C1, C2 and C3 throughout the year are shown in two vertical groups in Fig. 8. The solar collector efficiency describes the ability of solar collectors converting the solar energy into the airflow’s thermal energy. It is found that the solar collector efficiency of C1 is smaller than that of C2 and C3 in each month. The findings agree with

4.1. Energy consumption of SCPPs Energy consumed by the components of SCPPs is shown in Fig. 9, with the total solar radiation utilized by C1, C2 and C3 on its top left. Statistical results indicate that tilting the collector angle to 30
would increase the total solar energy input by 6.14%, but rising the collector angle to 60
, the total solar energy input would decrease by 5.11%. It is also found that: 1) The ground energy loss, the reflected energy, the ground energy storage and the glass cover energy storage are the major energy consumptions for the SCPPs. According to Eq. (19), the reflected energy consists of two parts, viz. the glass cover reflected energy and the ground reflected energy, in which the ground reflected energy holds the major proportion. It is thus found that the ground plays an important role in the energy consumption. Pretorius et al. compared the power outputs of six different ground types: sandstone, granite, limestone, sand, wet soil and water. They found that the SCPPs employing the wet soil and the sand have the lowest and highest power outputs respectively [11], and different materials lead to varying power outputs during the daytime and at night. Pretorius also concluded that increased ground absorptivity holds

Dec Nov Oct Sep Aug

C3 C2 C1

Jul Jun May Apr Mar Feb Jan 1.0

0.8

Power efficiency/ %

0.6

40

45

50

55

Solar collector efficiency/ %

Fig. 8. Power efficiencies and solar collector efficiencies of C1, C2 and C3 throughout the year.

60

Total solar radiation/ TJ

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

5000

5066.82

Energy used by the turbine Energy loss at the chimney exit Friction loss Ground energy loss Glass cover energy loss Reflected Energy Energy storage in the ground Energy storage in the glass

4529.18

4000 3000 2000 1000 0

System styles

4773.91

C1

C2

589

C3

1.67%

C3 16.11%

16.98%

19.61%

2.73%

23.89%

1.58%

17.94%

C2

0.70% 15.92%

28.25%

16.37%

3.49%

23.59%

0.86% 10.81%

C1

1.00% 15.96%

0

18.48%

20

20.78%

40

2.66%

0.46% 12.63%

28.01%

60

80

100

Proportion of the total solar radiation /% Fig. 9. Energy consumption by components of C1, C2 and C3.

positive effects on annual solar chimney power output [32]. Bernardes et al. evaluated the influence of varying the ground heat penetration coefficient from 1000 Ws1/2/(K m2) to 2000 Ws1/2/(K m2) on the performance of SCPPs, and found negligible influence on the power output [37]. Accordingly, it can be deduced that ground types could partly influence the power generation. Though many different ground types exist at locations around the world suitable for the construction of SCPPs, lower heat penetration coefficient materials are suggested to be chosen as the heat storage layer. In addition, better insulation methods could reduce the energy loss to the surroundings [32]. With respect to the glass cover reflected energy and the glass cover energy storage, the quality of glass is of high significance. In this study, the poor glass with the extinct coefficient of 32 m1 is used as the collector cover. Pretorius et al. analyzed the performances of the good glass with extinct coefficient of 4 m1, and indicated that during the summer months the better quality glass could cause the power output a little increase throughout the day, and the overall output during the colder months was also slightly higher [32]. So, the glass with good quality, called the “water white glass”, is a better choice for the solar collector cover. Consequently, the investment of SCPP will increase. 2) The energy loss at the chimney exit is the fifth largest energy loss for C1 and C2 but the third loss for C3, which also holds large proportion to the total energy consumption. Methods for reducing the energy loss at the chimney exit are rarely reported in literature. The airflow at the chimney outlet has high kinetic and gravitational energy. However, the SCPP using air as the working fluid could hardly reuse the energy. Kashiwa et al. proposed a novel “solar cyclone” for extracting freshwater from the atmosphere [38]. Wang et al. proposed the combined solar chimney system for both power generation and seawater desalination [39]. Using water vapor together with air as the working fluid would partially recover the energy loss at the chimney outlet. 3) The glass cover energy loss and friction loss are smaller comparing with others. With the glass cover temperature being the higher, the cover energy loss of C2 is correspondingly higher. The friction loss is the smallest energy loss and of a great relationship with the airflow speed.

4.2. Major factors influencing SSCPP performances In Fig. 10, we compare the performances of nine SSCPPs. Two of them with the solar collector angle of 30
and 60
respectively are chosen as the reference systems. Then we calculate and diagram the relative proportions of other systems to the reference system in Fig. 10(a) and (b) separately. With Fig. 10(a) and (b), it is found that: 1) When the solar collector angle is rising from 15
to 80
, the variations of air densities and airflow temperatures are slight. The maximum air density difference and solar radiation occur at 30
, and the maximum air speed and effective pressure occur at 60
. The height varies from 15
to 60
is significant, whereas a slower height increase is found after 60
. 2) When the solar collector angle decreases from 30
(see Fig. 10 (a)), all parameters decrease except the air density. The decrease of the airflow speed and effective pressure cause the decline of the power output. 3) When the solar collector angle increases from 30
to 60
(see Fig. 10 (a)), the solar radiation and the density differences both decrease, but the air speed, the height and the effective pressure increase. Power generation in this range also increases. 4) When the solar collector angle is larger than 60
(see Fig. 10 (b)), all parameters decrease except the air density and height. However, the power generation decreases in this range. As the inclination of the collector enlarges in this range, two things occur that work in opposite directions with respect to the power generation. A higher inclination results in a lower exposure of the collector surface to solar irradiation and hence yield smaller heat utilization and smaller airflow speed. On the other hand, tilting the collector increases the height as well as pressure difference of the collector and so enhances the airflow. As the influence of collector’s increasing pressure difference is weaker than the decreasing airflow speed, power generation after 60
finally declines. A similar description is also reported by Sakonidou et al. on optimizing tilted angle of a solar chimney for maximum airflow rate [36]. Overall, the main factors that influence SSCPP performances are the air density difference and the height. When the collector

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

a

b

Relative proportions to the reference plant/ % Relative proportions to the reference plant/ %

590

100

o

25

o

25

15

o

30

o

30

o

40

o

40

o

36

o

36

o

50

o

50

o

o

80

o

80

o

70

o

70

60

o

80 60 40 20 0 -20 -40

20

0

-20

-40

-60

15

o

60

o

Collector angle Solar radiation [Ht]

Airflow temperature [(Ta+To)/2]

Density [ρo]

Density difference [ρa-ρo]

Airflow speed [vo]

Height [Hchi+Hcol/2]

Effective pressure [ΔP t ]

Power [Pele]

Fig. 10. Relative proportions of SSCPPs with different collector angles to the reference system: collector angle is 30
, (b) when the reference SSCPP’s collector angle is 60
.

angle is rising from the horizontal, the effect of the height increase plays the leading role. However, when the collector angle is too large (larger than 60
in this study), the main influence would be created by the air density differences. The air density variation with temperature changing is the thermal characteristic of air. In order to enlarge the power generation of the SCPP, a proper working fluid whose density decreases sharply with the temperature rise is suggested to be the working fluid. However, air is the most cost-effective, clean and abundant working fluid for SCPPs. Other working fluids would also increase the complexity of the system [38,39]. Besides, the mass flow rates of SCPPs’ working fluid are huge according to Fig. 6. Utilizing other fluids would be impossible from the techno-economic point of view. The system height has beneficial effects on the power generation. However, the mountain resources, whose surface slopes are 60
for example, are limited. Building SSCPPs along a high slope mountainside would increase the transport and

Performanceangle  Performancereference  100%, (a) when the reference SSCPP’s Performancereference

construction investments. Besides, Zhou et al. reported that the system power generation would be decreased by 13%, if considering the irregularity of the surface [19]. 4.3. Power generation of SCPPs at different latitudes in China The performances of CSCPPs and SSCPPs at different latitudes of China are summarized in Table 2. It can be found that: 1) For every region, there are always a maximum power generation (MPG) angle and a maximum solar radiation (MSR) angle for SSCPPs. For low latitude regions, CSCPPs have higher power generation than SSCPPs with MSR angles; but for high latitudes, SSCPPs with MSR angles have higher power generation. 2) The power generation of SCPPs is not only influenced by the total solar radiation, but also has somehow relationship with the solar duration and the ambient temperature.

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

591

Table 2 Power generations of CSCPPs and SSCPPs at different latitudes of China. City

Local geographical information

CSCPP

SSCPP Maximum solar radiation (MSR)

Guangzhou Wuhan Shanghai Beijing Shenyang Urumchi Harbin

Maximum power generation (MPG)

Latitude /


Solar duration /h

Ambient temperature /K

Radiation on horizontal surfaces /W m2

Power generation /MW

Collector angle /


Radiation on sloped surfaces /W m2

Power generation /MW

Collector angle /


Radiation on sloped surfaces /W m2

Power generation /MW

23.10 30.37 31.10 39.48 41.44 43.47 45.45

1628.0 1835.1 1894.5 2067.8 2468.0 2523.3 2571.2

295.2 289.8 289.3 285.4 281.5 280.1 277.4

679.48 600.04 657.61 514.25 546.64 553.46 538.77

4.98 4.31 4.75 3.61 3.85 3.91 3.77

21 22 26 38 40 36 43

689.81 644.54 686.58 592.84 637.59 623.06 652.70

2.72 2.68 3.29 4.80 4.38 3.97 4.81

61 61 62 68 66 64 67

598.03 560.76 606.41 540.11 593.10 570.82 612.40

5.00 4.77 5.25 5.40 5.44 5.17 5.75

3) MSR angles are nearly 2e8
smaller than the local latitudes. However, the MPG angles are 25e38
higher than the local latitudes. SSCPPs can enlarge the power production and increase the energy conversion efficiency. However, as is shown in Fig. 2, the performance of SSCPPs is closely related to the geographic resources. Fig. 11 shows that most long mountain chains and high insolation areas are distributed in regions with the latitude higher

than 30
N and longitude smaller than 105
E [40,41]. SSCPPs could be an adaptation to the local conditions in this region. For southeast and eastern regions, solar radiation is relatively weak, and mountain chains are discontinuous and short. CSCPP is a more economic and suitable solution, especially in combination with vegetation under the collector roof for possible agricultural purposes. Pretorius reported that though the performances of the SCPP decrease nearly 50% in the vegetable style of SCPP, the advantage of land saving and greenhouse effect for grain and power generation are of

Fig. 11. Solar radiation and mountain chains distribution in China: the radiation is presented by different colors, and the mountain chains are identified by numbers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

592

F. Cao et al. / Applied Thermal Engineering 50 (2013) 582e592

high values [32]. There are only several mountain chains located in Northeast China. Besides, for the Sichuan Basin, the solar radiation is too weak and annually average temperature is high. So, SCPPs may not be suitable to be built in these regions. 5. Conclusions SCPPs are promising for large-scale utilization of solar energy, and extensive research has been carried out to investigate its hugepotential over the world. In this study, we compare the performances, such as the airflow temperature, the mass flow rate, the power generation and the efficiencies, of a CSCPP and two SSCPPs. The energy consumption, the main factors that influence the power generation and the power generation at different latitudes of China are also analyzed. Results show that: 1) CSCPPs, of which the highest power outputs are generated in summer, have symmetrical performances throughout the year. Tilting the solar collector angles from the horizontal would improve SSCPPs’ performances in winter but weaken them in summer. 2) For different regions, configuration sizes of SCPPs are different because of various meteorological and geographic conditions. The SCPP has low power generation efficiency but relatively high thermal efficiency. The ground energy loss, reflected solar radiation and kinetic loss at the chimney outlet are the main energy losses in SCPPs. 3) There are two special collector angles for SSCPPs: the maximum solar radiation (MSR) angle and the maximum power generation (MPG) angle. For Lanzhou (36.03
N), the MSR angle is 6
smaller than the local latitude, and the MPG angle is 24
larger than the latitude. The main reasons causing the angle difference are the system height and the air thermophysical characteristics. For different regions, factors that influence the power generation of SCPPs are the total solar radiation, the solar duration and the ambient air temperature. 4) SSCPPs are suggested to be built in Norwest China, where solar energy and mountain resources are abundant; whereas CSCPPs are more suitable to Southeast and East China, where it could be combined with the local agriculture. Acknowledgements This research was funded by the National Natural Science Foundation of China (Nos.: 50506025, 51121092) and Programme for New Century Excellent Talents in University (No.: NCET-080440). We would also like to thank the National Meteorological Information Centre, China Meteorological Administration for its data support. References [1] J. Lorenzo, Las Chimneas solares: De una propuesta espanola en 1903 a de Manzanares, De Los Archivos Históricos De La Energia Solar, http://www. fotovoltaica.com/chimenea.pdf (accessed 11.11.11). [2] H. Günther, In Hundred Years-future Energy Supply of the World, Franckhsche Verlagshandlung, Stuttgart, 1931. [3] W. Haaf, G. Friedrich, G. Mayr, J. Schlaich, Solar chimneys, part I: principle and construction of the pilot plant in Manzanares, International Journal of Solar Energy 2 (1983) 3e20. [4] W. Haaf, Solar chimneys, part II: preliminary test results from the Manzanares pilot plant, International Journal of Solar Energy 2 (1984) 141e161. [5] T.W. von Backstrom, A. Bernhardt, A.J. Gannon, Pressure drop in solar power plant chimneys, Journal of Solar Energy Engineering 125 (2003) 165e169. [6] T.W. von Backstrom, A.J. Gannon, Solar chimney turbine characteristics, Solar Energy 76 (2004) 235e241. [7] T.P. Fluri, T.W. von Backstrom, Performance analysis of the power conversion unit of a solar chimney power plant, Solar Energy 82 (2008) 999e1008.

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