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An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator
Journal Pre-proof An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator V. Thirunavukkarasu, M. Cheralathan PII:
S0360-5442(19)32330-8
DOI:
https://doi.org/10.1016/j.energy.2019.116635
Reference:
EGY 116635
To appear in:
Energy
Received Date: 30 May 2019 Revised Date:
30 October 2019
Accepted Date: 25 November 2019
Please cite this article as: Thirunavukkarasu V, Cheralathan M, An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator, Energy (2020), doi: https://doi.org/10.1016/j.energy.2019.116635. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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AN EXPERIMENTAL STUDY ON ENERGY AND EXERGY PERFORMANCE OF A SPIRAL TUBE RECEIVER FOR SOLAR PARABOLIC DISH CONCENTRATOR Author: V Thirunavukkarasu Department of Mechanical Engineering, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India
.
Email: [email protected] Corresponding author: M Cheralathan Department of Mechanical Engineering, Faculty of Engineering and Technology SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India.
Email: [email protected]
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AN EXPERIMENTAL STUDY ON ENERGY AND EXERGY PERFORMANCE OF A SPIRAL TUBE RECEIVER FOR SOLAR PARABOLIC DISH CONCENTRATOR.
ABSTRACT In this paper, energy and exergy performance of an external type spiral tube receiver for a solar parabolic dish concentrator is experimentally analyzed and presented. Performance evaluation is done at three different radiation conditions. The receiver is tested in the temperature range of 30oC to 100oC with water as heat transfer fluid and at a flow rate of 1.5 liters per minute. The overall heat loss coefficient of the receiver estimated from the stagnation test is found to be 182 W/m2K. The average thermal and exergy efficiencies of the receiver was determined to be 56.21% and 5.45% respectively under an average beam radiation of 750 W/m2. The variation trend of thermal efficiency of receiver is similar to that of difference between temperature of the heat transfer fluid at receiver inlet and outlet. The results show that this light weight, low cost receiver has potential to be employed with solar parabolic dish concentrator for process heating applications. Keywords: Solar parabolic dish concentrator; Scheffler reflector; spiral tube receiver; energy efficiency ; exergy efficiency; overall heat loss coefficient.
1. Introduction Solar energy is a promising alternative to conventional energy due to its greater global potential. The greatest advantage of solar energy is the minimal usage of traditional and polluting source of energy. Hence the usage of solar energy leads to clean and hygienic environment and huge fuel savings. Recently solar parabolic dish collector is becoming increasingly popular for various process heating applications in the temperature range 80oC to 320oC.
The parabolic dish concentrator works by concentrating the sunlight to the focus of the parabolic reflector and placing the receiver at the focal point. The receiver accumulates the heat, which is then transported to the end use application with the help of a heat transfer fluid. The surface area of receiver is much smaller than that of the dish reflector, thereby higher concentration ratio can be achieved. This concentration allows the increase of energy flux and attainment of higher temperatures at the receiver.
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Solar receiver has a major role in solar thermal energy conversion in parabolic dish concentrator systems. The design of the receiver should be proper so as to receive the concentrated solar radiation and convert it to thermal energy with minimum heat loss. Basically there are two types of solar receivers: external type and cavity type. In the external type receiver, the solar flux is directed on to the front surface of the receiver whereas in the cavity receiver, the radiation is concentrated by the reflector to the receiver aperture and is absorbed by the inside cavity surface. This thermal energy is then absorbed and transported to the application by the heat transfer fluid flowing through the receiver. Different configurations of cavity receivers were proposed and investigated by several researchers for parabolic dish concentrators. Kaushika and Reddy [1] utilized an innovative fuzzy volumetric receiver for a deep parabolic dish. They developed a cheaper steam generating system and achieved an efficiency in the range of 70-80%. A cylindrical solar receiver with a wind skirt was developed by Prakash et al. [2] and carried out studies on convective heat losses by both experimental and numerical analysis. The analysis was also performed to observe the effect of receiver inclinations and inlet temperatures of heat transfer fluid.
Based on the analysis, a
correlation was proposed for obtaining Nusselt number to calculate the heat loss in natural convection mode.
A conical receiver was investigated experimentally and theoretically for a
parabolic concentrator of large rim angle by Hernandez et al [3]. They have reported that the conical receiver is suitable for the deep parabolic dish. Ma [4] developed a receiver in the shape of conical frustum-cylinder and did experimental studies to determine the total and convective losses. The dependence of heat losses on receiver inclination, temperature and aperture size were also studied. Mawire and Simeon [5] used a cavity receiver in the cylindrical shape for an SK-14 domestic dish concentrator and examined its thermal performance. They obtained a peak energy and exergy efficiency of around 45% and 10% respectively for the receiver in a circulation mode of operation.
Reddy et al. [6] did an experimental investigation on the performance of modified cavity receiver for a 20 m2 dish concentrator of fuzzy focal type. Their parabolic dish collector attained an average thermal efficiency of 74% for a flow rate of 250 litres per hour. They reported that the thermal efficiency of collector rises up with increase in beam radiation and also with increase in flow rate. Zhu et al. [7] did an experimental study to examine the efficiencies of a coiled tube receiver for a 57 m2 dish concentrator, with compressed air as the heat transfer fluid. Energy and exergy analysis revealed maximum energy efficiency of around 82% and maximum exergy
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efficiency of around 28% for an open loop test method. A receiver in the form of conical cavity was developed for a 16 m2 Scheffler-type parabolic dish collector by Thirunavukkarasu et al. [8]. They performed experimental energy and exergy analyses with various mass flow rates. The receiver showed maximum energy and exegy efficiency of 66.75% and 10.35% respectively for a heat transfer fluid flow rate of 2.5 liters per minute. Thirunavukkarasu and Cheralathan [9] experimentally studied the effect of aspect ratio of the conical cavity receiver on its thermal performance. Aspect ratio was varied from 0.8 to 1.2 with an increment of 0.2 and the study revealed that the receiver with lowest aspect ratio 0.8 performs better. Le Roux et al. [10] numerically studied the performance of a receiver in the form of rectangular cavity for dish concentrator with air as the heat transfer fluid. Neber and Lee [11] fabricated a low cost – high efficient receiver from silicon carbide for solar thermal dish-Brayton system. The receiver was designed to heat air to a high temperature of 1500 K which improves conversion efficiency by 20% over the power generation by concentrated solar technology based on 1270 K. A thermal model of a tubular cylindrical cavity receiver was developed by Lone et al. [12] to calculate the optimum parameters of the receiver like aperture size, inner tube diameter, and cavity depth and inlet temperature of the working fluid for attaining maximum efficiency.
The comparative study on receiver performance in different cavity shapes has been reported by few researchers. Harris et al. [13] did investigation on the thermal performance of the solar dish concentrators with five different receiver geometry and predicted that cavity geometry and rim angle of the concentrator have greater influence on power profiles of the cavity receiver. Kumar and Reddy [14] compared a cavity, a semi cavity and a modified receiver for a dish concentrator by numerical investigations. They have reported that modified cavity receiver shows better performance because of lower convective losses. Madadi et al. [15] tested both cylindrical and conical cavity receivers, numerically and experimentally and reported that the cylindrical is better than the conical one, in terms of energy and efficiency values. Jilte et al. [16] numerically investigated the forced convective heat loss from cavity of different shapes like conical, cylindrical, cone-cylindrical, hetro-conical and dome cylindrical and reported that conical cavity yields the lowest convective losses among the other cavities. The optical performance of cylindrical, conical and spherical cavity receivers with parabolic dish concentrator was simulated by Daabo et al, [17] and showed that conical cavity receiver has highest optical efficiency. Loni et al. [18] predicted the thermal performance of solar receivers in the cubical and cylindrical cavity shapes by experimental
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method and also by numerical modeling. Their results indicate that cavity receiver in the cubical shape shows higher thermal energy efficiency in comparison to cylindrical shape in the steady-state period.
Few studies are reported on external type receiver for parabolic dish concentrator. Pavlovic et al. [19] developed an external type spiral absorber with corrugated tubes for parabolic dish concentrator and its thermal performance was investigated with water. They have also developed a numerical thermal model for further investigation in various operating conditions and different heat transfer fluids. The examined range of inlet temperature is between 25oC to 350oC. Patil et al. [20] used a water container of 20 liter capacity as a receiver for Scheffler parabolic dish reflector of area 8 m2. An average power of 1.3 kW and efficiency of 21.61% was achieved by them in their experimental performance analysis. The operating temperature is in the range 25oC to 100oC. Qaisrani et al. [21] performed numerical investigations on the heat losses from a rectangular external type receiver under nine wind directions. They developed a correlation to evaluate the convective losses as a function of wind velocity, wind direction, surface area of the receiver and width of the wind blocking wall.
In the solar steam cooking facility at SRM Institute of Science and Technology, Kattankulathur, which is situated at southern part of India, external type receiver in the form of a short cylinder with a flat front surface is being used for 16 m2 Scheffler-type parabolic dish concentrators. The operating temperature of this receiver is approximately in the range 25oC to 180oC. This conventional receiver is heavy and the presence of large metal mass requires longer warm up period. It is observed that large amount of thermal energy is retained by the receiver material during its operation and is not available to the heat transfer fluid. There are two options to address this issue, one is the usage of cavity receiver and the other one is improving the design feature of external type receiver itself. Cavity receivers are best choice for high temperature concentrator systems, where there is a benefit in smaller ratio of aperture to absorbing area. But the supporting structure required for cavity receiver should be larger than that for an external receiver. Apart from this, it must be perfectly insulated to minimize conductive losses through its bigger surface area. This makes a cavity receiver larger, heavier and expensive when compared to external type receiver. Hence in the present work, being in the low and medium temperature range, focus is made on external type receiver.
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The literature study shows that most of the researches were on cavity type receivers and the studies on improvement of external type receiver are very few. This work aims at developing and testing a light weight compact coiled receiver in the spiral form for 16 m2 Scheffler-type parabolic dish concentrator. The contact surface area of receiver per unit volume of HTF is more for spiral tube receiver when compared to the conventional short cylinder receiver, which acts as a mechanism to enhance heat transfer. Energy and exergy efficiencies of receiver was analyzed at actual solar radiation conditions to evaluate its performance. 2. Experimental apparatus and method
2.1. Description of solar receiver The coiled spiral tube solar receiver which is made out of mild steel material is shown in Fig. 1. The focal image size of the reflector is measured to be Ø39 cm ± 1. Also the short cylinder receiver used in the solar steam cooking system for the same 16 m2 Scheffler reflector, has an aperture diameter of Ø40 cm. Hence the receiver for the present study is also constructed with an aperture diameter of Ø40 cm. During fabrication, a dead space of solar energy absorption was induced in the receiver as shown in Fig 1a. This is because the receiver could not be fully covered by the coiled tube till the centre point due to difficulty in coiling at the region of low coiling radius. To avoid the loss of radiation through this dead space, it was covered with a hollow and thick mild steel plate, in such a way that the HTF flows through the hollow plate also to receive the radiation energy before it leaves the receiver, as indicated in the Fig 1b. The back surface of the receiver is thermally insulated with 25 mm thick glass wool of very low thermal conductivity of 0.04 W/mK. An aluminum housing is provided so that insulation is in proper fit. This receiver is similar to the receiver developed by Pavlovic et al. [19]. But they had used corrugated tubes whereas present receiver is made of smooth tube. Moreover the presence of small dead space in their receiver is eliminated in the present design. Receiver design features and the dish specifications are given in Table 1.
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(a)
(b)
(c) Fig.1. Development of spiral tube receiver. a) Tube bending process b) Removal of dead space c) Receiver model with dimensions The receiver is stationary and not connected to the support structure of reflector. This is because the centre of the Scheffler reflector and its focal position are fixed. This is made possible because of the two axes tracking system, which works by clockwork mechanism. When compared to the generic parabolic dish technology, Scheffler type reflector has the advantage of fixed focus which helps in minimizing structural complexity [22].
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Table 1. Concentrator and receiver design parameters Parameters Area of Scheffler dish concentrator (Af) Semi-minor axis of the elliptical frame of the reflector dish (a) Semi-major axis of the elliptical frame of the reflector dish (b) Focal length of the Scheffler reflector (f) Aperture area of the receiver (AR) Concentration ratio on 10/3/2017(Cr) Concentration ratio on 25/3/2017(Cr) Spiral tube outer diameter Spiral tube inner diameter No. of turns of the spiral coil Spiral tube length Inclination angle of receiver at focal point Combined optical efficiency of concentrator system
Values 16 m2 1.9 m 2.65 m 2.5 m 0.1256 m2 92.3 91.5 21.3 mm 14.3 mm 6 5.77 m 13o 71%
2.2 Experimental set up The schematic diagram of the experimental set up developed for performance tests is presented in Fig. 2. This test facility is located at the SRM Institute of Science and Technology, Kattankulathur, near Chennai City; 12°49' latitude and 80° 20' longitude geographical location. Scheffler-type parabolic dish for the present work is made out of multiple pieces of solar grade mirrors of 3 mm thick, 100 mm wide and 180 mm long. Total reflective area of the dish is 16 m2. The east west tracking is done manually by operating the chain linked clockwork mechanism. Fig. 3 shows the photographic view of examined dish collector. Water is circulated as the heat transfer fluid (HTF) from an insulated storage tank of 85 litres capacity to the receiver. For the circulation of heat transfer fluid, a centrifugal pump driven by an electric motor of 1⁄4 hp is used. Water enters the bottom of the receiver, flows from the periphery towards the center and leaves the receiver from the back end. A gate valve is used to control the volume flow rate of HTF through the receiver. A rotameter is used to measure the flow rate of heat transfer fluid. The range of rotameter is 0-500 LPH and the least count is 5 LPH with an accuracy of ± 2%. The pipelines in the HTF circulation are insulated with 50 mm fiber glass wool to prevent heat loss.
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Fig. 2. HTF circulation layout K type thermocouples with ±0.1oC accuracy are used to measure HTF temperature at the receiver inlet and outlet, HTF in the thermo tank, and ambient temperature as well. Temperature of receiver surface is measured for every five minutes using a digital infra-red laser thermometer (DT8811 - MEXTECH make) of range -50 °C to 550 °C with an accuracy of ±0.5 °C. A pyranometer (Dynalab make, Model No. DWR 8101) of sensitivity, accuracy and working range 20 µV/W/m2, ± 3.5% and 0-2000 W/m2 respectively is used for measuring global solar radiation. The diffuse solar radiation is also measured by employing the same kind of pyranometer with a shading ring. The intensity of beam solar radiation is obtained by deducting diffuse radiation from global radiation. A cup type anemometer is used to measure the wind speed (Dynalab make, Model No. DWA 8600) with an accuracy of ±2 %. All the data are recorded using a data logger (Dynalab make, Model No. DWL 1002) which is connected to a computer. Initially a stagnation test is conducted to evaluate the overall heat loss coefficient of the receiver. The condition at which no heat is retrieved by the heat transfer fluid from the receiver is called as stagnation condition. At this condition, the receiver attains it maximum temperature. This upper temperature limit gives the idea of the applications for which the collector can be used.
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Later, experiments are conducted at three different time slots to assess the performance of receiver at different radiation conditions. The first experiment was conducted on 10th March 2017 from 11 AM to 12:20 PM, the time period close to noon. The second experiment was conducted after noon of the same day, from 2 PM to 3:30 PM and the third experiment was conducted on 25th March 2017 from 2:45 PM to 4:35 PM. A flow rate of 1.5 LPM was chosen for the circulation of HTF for all the experiments in a closed loop mode.
Fig. 3 The examined solar dish collector
2.3 Energy analysis The estimation of energy efficiency is presented in this section to analyse the thermal performance of spiral tube receiver used in the Scheffler dish concentrator system. The solar radiation power on the Scheffler reflector can be estimated from the expression: = 1
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where is the intensity of beam radiation in W/m2 and is the effective aperture area of
reflector at any day of the year. The effective aperture area of the Scheffler-reflector is given by [23]: = . cos 43.23 +
2 2
where is the surface area of the elliptical frame of reflector and is given by :
= 3
where a and b are semi-minor axis and semi major axis of the elliptical frame of reflector dish respectively. Solar declination angle ( ) for any day of the year is given by [24]:
= 180/π 0.006918 − 0.399912 cos $ + 0.070257 sin $ − 0.006758 cos 2$ +
0.000907 sin 2$ − 0.002697 cos 3$ + 0.00148 sin 3$ 4
where B is given by $ = ) − 1 . 360/365
(5)
and ) is the day of the year counted from 1st January.
The dish power which is available as input to the receiver is given by:
* = η+, 6
where η-. is the combined optical efficiency based on the reflectivity of dish mirror and
absorptance of the receiver.
Receiver power, which is the useful thermal power output from the receiver can be estimated by [25] / = 012 3+4, − 35 7
where 01 is the mass flow rate of HTF, Cp is the specific heat of HTF, Tin is the temperature of HTF at receiver inlet and Tout is the temperature of HTF at receiver outlet.
On the basis of law of energy conversion, the overall heat loss from the receiver can be obtained as: 6 = * − /
(8)
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where 6 is the overall heat loss from the receiver which consists of convective and radiative heat
losses. Due to the thermal insulation provided on the back surface, conductive heat loss is negligible. The instantaneous thermal energy efficiency of the receiver, which is defined as the ratio of useful power output from the receiver to the radiation energy input to the receiver, is given by η,7,9 =
012 3+4, − 35 9 η+,
The overall heat loss coefficient of the receiver, UL, can be determined by performing the stagnation test as indicated in [6].
The receiver power is given as / = * − 6
(10)
/ = η+, − :6 / 3/ − 3
(11)
By combining equations 1, 6, 7 and 10, QR can also be written as
At stagnation condition,/ = 0, Hence * = 6 , where U< is the overall heat loss coefficient of receiver, which can be obtained as
:6 =
η=>? @A> 9B
@C DC EDA
(12)
2.4 Exergy analysis Energy analysis is on the basis of first law of thermodynamics and does not consider the reductions of energy potential, exergy analysis is based on both first and second law of thermodynamics. The receiver exergy rate is given by [26]: 3+4, J 13 35 where Ta is the ambient temperature. FG/ = 012 H 3+4, − 35 − 3 ln
Solar exergy rate, which is the rate of solar exergy delivered by the sun (source) to the concentrator can be expressed as given by [27]: 1 3 M 43 FG = K1 + − N 14 3 3L 33L
where 3O is the sun’s blackbody temperature whose value is taken as 5762 K [28].
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The dish exergy rate, which is defined as the rate of exergy from the dish concentrator to the receiver, can be expressed as: 1 3 M 43 FG* = η+, K1 + − N 15 3 3L 33L
The exergy efficiency, which is the ratio of receiver exergy rate to the dish exergy rate is
given by: ηPQ =
2.5
FG/ 16 FG*
Uncertainty analysis
An uncertainty analysis is carried out with the dish power, R, receiver power, S , thermal
energy efficiency,η.ℎ,U , exergy efficiency, ηPQ and overall heat loss coefficient, :V using the propagation of error technique explained in [29].
The uncertainty of the dish power is calculated by:
X
X
* * * = W X + δ X 17 δ
While the uncertainty of the receiver power is given by:
/ X / X / X / = W 01 X + 35 X + 3+4, X 18 01 35 3+4,
The uncertainty of thermal energy efficiency can be calculated by: η,7,9
X
η,7,9 X η,7,9 X η,7,9 η,7,9 X η,7,9 X W X X X X = Y Z 01 + Y Z 3+4, + Y Z 35 + Y Z + Y Z δ X 01 3+4, 35 δ (19)
The uncertainty of exergy efficiency is given by:
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] η^_ X
\ ]`1 a
01 X
] η^_ X
+ \]D a 3+4,
X
X
]η^_
+ \ ]D a 35 X +
cd ηPQ = [ X X ] η^_ X ] η ]η \ a 3 X + \ ^_ a X + \ ^_ a δ X =b?
]DA
]9B
(20)
]δc
The uncertainty of overall heat loss coefficient is given by: :6 = W\
]ef X ]9B
]e
X
]e
X
]e
X
a X + \ ]δf a δ X + \]D f a 3/ X + \ ]Df a 3 X c
C
A
(21)
Mass flow rate has an uncertainty of 2% on a relative scale, as described from the accuracy of the rotameter. Therefore, 01 = ±0.0005 kg/s. The uncertainty of all the temperature measurements from the K-type thermocouples is 0.1 K. Thus, 35 = 3+4, = 3 = ±0.1 K. The combined
uncertainty of solar radiation measurements is 3.5 %, as indicated by the secondary standard specification of the pyranometer, and the solar declination angle has an error of 0.035 degrees on any day of the year. Therefore, = ±3.5% of and δ = ±0.035 degrees. The uncertainties of the performance parameters evaluated are listed in Table 2.
Table 2. Uncertainties of performance parameters. Parameters
Uncertainty
Dish power
218.4 W
3.6
Receiver power
70.8 W
2.12
Thermal energy efficiency
2.3%
4.07
Exergy efficiency
0.277 %
4.09
Overall heat loss coefficient 6.58 W/m2K
% Uncertainty
3.54
3. Results and discussion The results of the experiments conducted to predict the performance of the receiver are presented and discussed here in detail. The aperture area of the Scheffler reflector is estimated from the equation (2). Aperture area decreases from 13.457 m2 to 9.0823 m2 between 1st January to 22nd June of the year and again increases from 9.0823 m2 to 13.463 m2 between 23rd June to 31st December. Fig. 4 shows the variation of declination angle and the aperture area of Scheffler - reflector with respect to day of the year.
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Fig. 4 Variation of declination angle and aperture area of Scheffler reflector with respect to day of the year
3.1 Stagnation temperature and overall heat transfer coefficient
The beam radiation (Ib), ambient temperature (Ta) and receiver stagnation temperature (TR) are measured from 9.30 am to 3 pm during the stagnation test. The variations of stagnation temperature, beam radiation and overall heat transfer coefficient during this time period are shown in Fig. 5. During the testing period, the irradiance dropped to very low value at four instances due to the passing clouds. The variation of irradiance has influenced the stagnation temperature, but their variations are not exactly similar. It may be due to the inertia effect of the mild steel material of the receiver. The maximum stagnation temperature recorded is 366oC±0.5oC. Overall heat loss coefficient is at its
maximum of about 440 W/m2K at the beginning of the test because of
preheating of the receiver, and then drops significantly to about 52 W/m2K within 20 minutes of operation. This is followed by a fluctuating trend till the end of the test due to the frequent changes in irradiance level. At the value of irradiance of 118 W/m2 and 600 W/m2, the corresponding value of overall heat transfer coefficient is predicted as 53 W/m2K and 237 W/m2K respectively. The average value of U< for the receiver in a day is observed to be 182 W/m2K.
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Beam radiation
Overall heat transfer coeffecient
Receiver stagnation temperature
500 450
600
Beam radiation (W/m2)
400 500
350 300
400
250 300
200 150
200
100 100 50
Overall heat tranfer coeffecient (W/m2K) Receiver stagnation temperature (oC)
700
0 14:50
14:30
14:10
13:50
13:30
13:10
12:50
12:30
12:10
11:50
11:30
11:10
10:50
10:30
10:10
9:50
9:30
0
Time (hh:mm)
Fig. 5. Variation of the receiver stagnation temperature and overall heat transfer coefficient.
3.2 Temperature and irradiance profile of the receiver Fig. 6a-c shows the variation of solar beam radiation, ambient temperature, HTF outlet and inlet temperatures with respect to time during the first, second and third experiment respectively. . During the first experiment (Fig. 6a) irradiance increased from a minimum value of 690 W/m2 to 757 W/m2 during the first 20 minutes and showed almost constant value during the remaining period. The average value of irradiance recorded was 750 W/m2. In the first ten minutes itself, the outlet temperature attained a temperature of 97oC and then gradually rose to 100oC. The inlet temperature also increased during the test, since the HTF was re-circulated repeatedly between the receiver and the HTF storage tank for conducting the closed loop experiment. Hence this gives a dynamic characterisation method of the receiver. Inlet temperature of HTF increased in a different manner when compared to that of the outlet temperature. It showed only a little increase during the first 40 minutes due to the presence of thermal stratification in the HTF storage tank. Due to continuous mixing of hot HTF from receiver to the cold HTF in the storage tank, the thermal stratification starts to disappear and the temperature of HTF rose to 81oC at a faster rate in the next 20 minutes. After this, the inlet temperature gradually attained a value of 90oC at the end of the test. The receiver temperature is observed to increase from a minimum value of 99oC to 123oC with an
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average value of 108oC. The ambient temperature was almost constant with an average value of 41oC. 900
130
a
800
Temperature (°C)
110 100
700
90
600
80 70
500
60
400
50 300
40 Inlet temperature Outlet temperature Ambient temperature Receiver surface temperature Irradiance
30 20 10
Irradiance (W/m2)
120
200 100
0
0 11:00
11:10
11:20
11:30
11:40
11:50
12:00
12:10
12:20
Time(hh:mm)
130 120
800
b
700
110
600
90
Irradiance (W/m2)
Temperature (°C)
100
500
80 70
400
60 50
300
40 30 20 10 0
Inlet temperature Outlet temperature Ambient temperature Receiver surface temperature Irradiance 02:00 02:10 02:20 02:30 02:40 02:50 03:00 03:10 03:20 03:30 Time (hh:mm)
200 100 0
18
130 120
600
c
110
500
90
400
80 70
300
60 50
200
40 30
Inlet temperature
Outlet temperature
20
Ambient temperature
Receiver surface temperature 100
10
Irradiance
0
Irradiance (W/m2)
Temperature (°C)
100
0 02:45 02:55 03:05 03:15 03:25 03:35 03:45 03:55 04:05 04:15 04:25 04:35 Time(hh:mm)
Fig 6. Temperature and irradiance profiles of the spiral tube receiver for average irradiance of a) 750 W/m2 b) 600 W/m2 c) 380 W/m2 During the second performance test (Fig. 6b), solar irradiance was recorded a peak value of 688 W/m2 at the beginning and later it showed a gradual decrease. The average value of irradiance recorded was 600 W/m2. The outlet temperature presented a different growth rate at different times. It rose progressively from the start of the test till 2.50 pm followed by a low growth gradient till the end of the test. The outlet temperature has attained only 91oC even after 90 minutes of test duration. This variation trend of outlet temperature is because of the declining nature of beam radiation availability. During the first 50 minutes of the test, the inlet temperature rose very slowly, followed by a steep rise during the next 10 minutes and a gradual rise during the next 30 minutes of the test. Thermal stratification was present in the storage tank during the first 50 minutes and disappeared during the later stage of the test. Receiver temperature first increased from 81oC to 98 oC during the first 50 minutes, then decreased to 92 oC at the end. The ambient temperature was almost constant with an average of 39oC during the testing period. The third performance test was conducted to check the performance at low radiation conditions (Fig. 6c). Solar irradiance was recorded a peak value of 519 W/m2 at the beginning and then it decreased towards the end of the test to a lower value of 165 W/m2. An average irradiance value of 380 W/m2 was recorded during the test. The outlet temperature rose comparatively at a slower rate. It took around 110 minutes for the outlet temperature to attain 82oC. Inlet HTF
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temperature showed only a little increase during the first 30 minutes and then rose at an increased rate in the next 20 minutes, followed by a gradual rise to attain a temperature of 71oC at the end of the test. The thermal stratification in the HTF storage tank was observed to be comparatively less during this test because of the low growth rate of HTF temperature at the receiver outlet. Receiver temperature increased from a minimum value of 62oC and recorded a maximum value of 99oC at 4 PM, after which it showed a fluctuating trend. During the last 20 minutes, there was a reduction in the receiver’s surface temperature, the lowest value recorded was 83.5oC. The ambient temperature remained fairly constant with an average of 34oC during the testing period.
3.3 Power profile of the receiver Fig 7a-c depicts the variation of concentrated solar radiation and receiver power with time during the experiment I, II and III respectively. The dish power varies in a similar manner to that of solar beam radiation, showing the direct influence of solar irradiance on it. For all the tests, the receiver power is observed to vary in a similar way to the difference in HTF outlet and inlet temperature. Loni et al. [18] has also made a similar observation in their study on cavity receivers. During the last thirty minutes of second experiment, it can be seen that the receiver power was maintained almost constant even though the dish power declined at a faster rate. This is because the heat supplied from the receiver mild steel material was utilized to deliver the receiver power. The reduction in growth gradient of receiver surface temperature during this period supports this fact. Similarly during the last twenty minutes of the third experiment (Fig. 7c), receiver delivered an almost constant power. The peak and average values of dish power and receiver power are given in the Table 2. An energy and exergy flow diagram based on the results obtained from the first experiment is added in Fig. 8 to indicate various losses of the dish receiver system.
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21 Fig. 7 Power profiles of the receiver for the average irradiance of 750 W/m2 b) 600 W/m2 c) 380 W/m2
Fig. 8 Energy and exergy flow diagram of dish receiver system for average irradiance of 750 W/m2.
3.4 Energy and exergy efficiency profile of the receiver
Fig 9a-c depicts the variation of instantaneous energy and exergy efficiency of the receiver with time during the tests I, II and III respectively. The peak and average energy and exergy efficiency values are given in Table 2. For the first experiment, with average irradiance value of 750 W/m2, the instantaneous energy efficiency curve followed a parallel trend to that of the receiver power, as the solar radiation power concentrated from the dish was almost constant during the testing period. The efficiency first increased to a peak value of 82.6% and then decreased towards the end of the test with a lower value of 16%. The difference between the outlet and inlet temperatures of HTF was relatively large till 11:40 AM which resulted in higher heat gain by the receiver and higher efficiency values. Later the temperature difference started to decrease which resulted in a sharp decrease of efficiency till 12:00 PM and then followed by a gradual decrease. For the second test with average irradiance value of 600 W/m2, the instantaneous energy efficiency curve followed a parallel trend to that of receiver power till 3 PM. Beyond 3 PM, the receiver power was maintained somewhat constant, even though the dish power was declining. Hence the reduction in the values of denominator of the equation (9) for instantaneous efficiency with almost constant value of numerator, resulted in an increase in efficiency values. Similar to the above test results, for the third experiment, the instantaneous energy efficiency curve followed a parallel trend to that of receiver power up to a certain period. From 4.15 PM onwards, as the dish
22
power started to decline drastically, the efficiency curves underwent a steep rise. This is because, as described above, the receiver power was maintained somewhat constant even though there was a decrease in the dish power. 8 7 6 5 4 3 2
Exergy efficiency (%)
a
Energy efficiency
Energy efficiency (%)
100 90 80 70 60 50 40 30 20 10 0
1 0 11:00 11:10 11:20 11:30 11:40 11:50 12:00 12:10 12:20 Time(hh:mm)
8
6
4
2
0 02:00 02:10 02:20 02:30 02:40 02:50 03:00 03:10 03:20 03:30 Time (hh:mm)
Exergy efficiency (%)
b
Energy efficiency Exergy efficiency
Energy efficiency (%)
100 90 80 70 60 50 40 30 20 10 0
100 90 80 70 60 50 40 30 20 10 0
8
c
Energy efficiency Exergy efficiency
7 6 5 4 3 2
Exergy efficiency (%)
Energy efficiency (%)
23
1 0 02:45 03:00 03:15 03:30 03:45 04:00 04:15 04:30 Time(hh:mm)
Fig 9a-c Energy and exergy efficiency profiles of the receiver for the average irradiance of a) 750 W/m2 b) 600 W/m2 c) 380 W/m2
The exergy efficiency values follow a similar pattern to that of the energy efficiency. It is very low when compared to the energy efficiency values, which indicates the low quality of energy being delivered to the receiver at all radiation conditions. A potential reason for the exergy destruction in the solar collector is the high exergy content of the solar input and the temperature difference between inlet and outlet temperatures of HTF [28]. The exergy destruction due to high exergy content of the solar input can be minimized if the receiver surface temperature is maintained at higher values. Moreover, as per the equation (13), receiver exergy rate is greatly influenced by the ratio (3+4, /35 ) because the magnitude of exergy rate is strongly dependent on the exergy loss,
which is expressed as 012 3 ln \
D=b? Dcd
a. Hence to reduce the exergy loss in the receiver, temperature
difference of HTF at receiver outlet and inlet has to be minimized. To increase the receiver surface temperature and also to minimize the ratio (3+4, /35 ), the possible way is to increase the inlet
temperature of HTF.
24
Table 3. Performance parameters of the receiver Performance parameters
Average irradiance (W/m2) 750 600 380
Average receiver surface temperature (TRoC) Peak dish power (kW) Average dish power (kW) Peak receiver power (kW) Average receiver power (kW) Average heat loss from the receiver (kW) Average thermal energy efficiency (%) Peak thermal energy efficiency (%) Average exergy efficiency (%) Peak exergy efficiency (%)
108 6.287 6.128 4.941 3.421 2.707 56.2 82.6 5.5 7.1
88.6 5.666 4.943 3.873 2.28 2.663 44.5 80.0 3.1 6.9
85.8 4.239 3.105 1.758 1.022 2.083 33.8 64.0 3.4 6.9
Rate of rise in HTF temperature in storage tank (oC/min)
0.725
0.488
0.255
The comparison between energy and exergy efficiency values of the previous and current research are summarized in Table 4. The results of experimental analysis in a circulation mode of operation are considered for comparison. Hernandez et al. [3] used a paraboloidal concentrator which is built from an old telecommunication antenna with reflectance of 0.64 to test the developed conical double walled cavity type stainless steel receiver. The receiver received radiation from both inner and outer surfaces. Mawire and Taole [5] did experiments on cylindrical cavity receiver, made of copper coil, with SK-14 parabolic dish concentrator. The exergy efficiency of the present work is less when compared to that of [5] due to the low operating temperature and larger difference between HTF outlet and inlet temperatures. Li et al. [31] used a beam-down solar tower which includes linear Fresnel heliostats and a beam down concentrator to concentrate the solar radiation to their conical spiral tube type receiver. The receiver was made from copper tubes with selective absorption coating with absorptivity of 0.9.
Pavlovic et al. [19] developed a low-cost solar
parabolic dish reflector made of polymethyl methacrylate reflective petals with a silver mirror layer and 0.6 reflectance. The receiver is made out of stainless steel and is a spiral corrugated tube inside an aluminum housing. The downward facing receiver has a drawback of a small dead space, which is being eliminated in the present work by adding a circular hollow plate. The dead space is present at the central region, which is the place where maximum concentration take place. Thirunavukkarasu et al. [8] used a Scheffler-type parabolic dish reflector with a reflectance of 0.8 to
25
test the developed conical cavity spiral tube receiver. The receiver is made from mild steel material. An average thermal energy efficiency of Table 4. Comparison between experimental results of present work to other researchers. Heat transfer fluid Water
Operating temperature range (oC) 30 oC - 90 oC
Cylindrical cavity receiver
Shell S2 Thermic fluid
43 oC - 160 oC
45%
10%
Beam-down solar tower equipped with Fresnel heliostat modules
Cavity receiver with conical spiral tube
Water Therminol VP-1
20oC - 150oC 12oC - 400oC
39 % 60 %
-------
Pavlovic et al., 2017
Paraboloid dish with rim angle 45.6o
Spiral tube receiver
Water Therminol VP-1
30 oC - 85 oC 150oC-160oC
34% ----
2.5% 7.6%
Thirunavukkarasu et al., 2017
Scheffler type parabolic dish concentrator
Conical cavity receiver
Water
30 oC - 100 oC
57%
5.9%
Present work
Scheffler type parabolic dish concentrator
Spiral tube receiver
Water
30 oC - 100 oC
56%
5.5%
Research Hernandez et al., 2012
Parabolic dish type Parabolic concentrator with 90o rim angle
Receiver used Conical cavity double walled receiver
Mawire and Taole, 2014
SK-14 Parabolic dish concentrator
Li et al., 2015
Energy efficiency 32%
Exergy efficiency ---
57.39% and average exergy efficiency of 5.88% were obtained for a flow rate of 2.5 LPM under an average irradiance of 608 W/m2. The efficiency of the present work at average beam radiation of 750 W/m2 is closer to this value. But when the results obtained under an average beam radiation of 600 W/m2 are compared to the author’s previous work [8], the energy efficiency is 12.5% lower and exergy efficiency is 2.78% lower. This may be possibly due to the fact that the chosen mass flow rate for the present work is not adequate and has to be optimized. Based on the comparison with other designs, energy efficiency obtained for the present work is better than the other researchers. The energy and exergy efficiencies of the present work are almost close to that of [8].
26
The possible design enhancements like optimized wall thickness and tube diameter may further improve the system performance. The larger thickness of tube wall increases the thermal mass, which adds inertia effect on the heat transfer. The tube diameter influences the flow characteristics and also the contact area of HTF with tube wall.
4. Conclusion An experimental investigation of the thermal performance of a spiral tube external type receiver for 16 m2 Scheffler type parabolic dish concentrator is carried out at actual solar radiation conditions. The receivers overall heat loss coefficient, thermal energy efficiency and exergy efficiency were evaluated at three different radiation conditions and the following conclusions are made:
The maximum stagnation temperature recorded is 366oC±0.5oC on a bright sunny day, which can be taken as the upper limit of temperature level to which this receiver can be utilized for producing heat. The overall heat loss coefficient of the receiver estimated from the stagnation test is found to be 182 W/m2K.
Beam radiation as well as the difference in temperature of heat transfer fluid between outlet and inlet of receiver greatly influences the receiver output and efficiency values. The average thermal and exergy efficiencies of the receiver was determined to be 56.21% and 5.45% respectively under high beam radiation conditions (750 W/m2).
The receiver developed for the present study acts as a simple test bed and does not have any selective coating, which shows the possibility of further improvement in the performance. Based on the results obtained, it can be concluded that this compact receiver has potential to be used in the applications of temperature up to 100oC. The investigations on the receiver’s performance at temperatures above 100oC is in progress in our laboratory and the results will be presented in near future.
Acknowledgements The authors are grateful to the management of SRM Institute of Science and Technology, Kattankulathur for providing the necessary facilities and continuous encouragement.
References
27
[1] Kaushika ND, Reddy KS. Performance of a low cost solar paraboloidal dish steam generating system. Energ Convers Manag 2000; 41:713–26. [2] Prakash M, Kedare SB, Nayak JK. Investigations on heat losses from a solar cavity receiver. Solar Energy 2009; 83:157–70. doi:10.1016/j.solener.2008.07.011. [3] Hernandez N, Riveros-rosas D, Venegas E, Dorantes RJ, Rojasmorin A, Jaramillo OA, Camilo A, Arancibia-Bulness, Estrada CA. Conical receiver for a paraboloidal concentrator with large rim angle. Sol. Energy 2012; 86:1053–62. doi:10.1016/j.solener.2011.09.008. [4] Ma RY. Wind Effects on Convective Heat Loss from a Cavity Receiver for a Parabolic Concentrating Solar Collector. Sandia National Laboratories Report 1993; SAND92-7293. [5] Mawire A, Taole SH. Experimental energy and exergy performance of a solar receiver for a domestic parabolic dish concentrator for teaching purposes. Energy Sustain Dev 2014; 19:162–9. doi:10.1016/j.esd.2014.01.004. [6] Reddy KS, Kumar S, Veershetty G. Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator. Renew Energy 2015; 74:148–57. doi:10.1016/j.renene.2014.07.058. [7] Zhu J, Wang K, Wu H, Wang D, Du J, Olabi AG. Experimental investigation on the energy and exergy performance of a coiled tube solar receiver. Appl Energy 2015; 156:519–27. doi:10.1016/j.apenergy.2015.07.013. [8] Thirunavukkarasu V, Sornanathan M, Cheralathan M. An experimental study on energy and exergy performance of a cavity receiver for solar parabolic dish concentrator. Int. J. Exergy 2017; 23(2):129 – 148. [9] Thirunavukkarasu V, Cheralathan M. Effect of aspect ratio on thermal performance of cavity receiver for solar parabolic dish concentrator : An experimental study. Renew Energy 2019; 139:573–81. doi:10.1016/j.renene.2019.02.102. [10] Roux WG Le, Bello-ochende T, Meyer JP. The efficiency of an open-cavity tubular solar receiver for a small-scale solar thermal Brayton cycle. Energ Convers Manag 2014; 84:457–70. doi:10.1016/j.enconman.2014.04.048. [11] Neber M, Lee H. Design of a high temperature cavity receiver for residential scale concentrated solar power. Energy 2012; 47:481–7. doi:10.1016/j.energy.2012.09.005. [12] Loni R., Kasaeian A. B, Asli-Ardeh E. A, & Ghobadian B. Optimizing the efficiency of a solar receiver with tubular cylindrical cavity for a solar-powered organic Rankine cycle. Energy 2016; 112: 1259-1272. doi:10.1016/j.energy.2016.06.109
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[13] Harris JA, Lenz TG. Thermal performance of solar concentrator/cavity receiver systems. Sol. Energy 1985; 34: 135-142. [14] Kumar NS, Reddy KS. Comparison of receivers for solar dish collector system. Energ Convers Manag 2008; 49:812–9. doi:10.1016/j.enconman.2007.07.026. [15] Madadi V, Tavakoli T, Rahimi A. First and second thermodynamic law analyses applied to a solar dish collector. Journal of Non-Equilibrium Thermodynamics 2014; 39:183-97. doi: 10.1515/jnet-2014-0023. [16] Jilte RD, Kedare SB, Nayak JK. Investigation on Convective Heat Losses from Solar Cavities under Wind Conditions. Energy Procedia 2014; 57:437–46. doi:10.1016/j.egypro.2014.10.197. [17] Daabo AM, Mahmoud S, Al-dadah RK. The effect of receiver geometry on the optical performance of a small- scale solar cavity receiver for parabolic dish applications. Energy 2016; 114:513–25. doi:10.1016/j.energy.2016.08.025. [18] Loni R, Kasaeian AB, Asli-ardeh EA, Ghobadian B, Gorjian S. Experimental and Numerical Study on Dish Concentrator with Cubical and Cylindrical Cavity Receivers using Thermal Oil. Energy 2018; 154: 168-81 doi:10.1016/j.energy.2018.04.102. [19] Pavlovic S, Bellos E, Le WG, Stefanovic V, Tzivanidis C. Experimental investigation and parametric analysis of a solar thermal dish collector with spiral absorber. Appl Therm Eng 2017;121:126–35. doi:10.1016/j.applthermaleng.2017.04.068. [20] Patil RJ, Gajanan KA, Mahendra PS. Experimental analysis of scheffler reflector water heater. Therm Sci 2011; 15:599–604. doi:10.2298/TSCI100225058P. [21] Qaisrani MA, Fang J, Jin Y, Wan Z, Tu N, Khalid M, Rahman MU, Wei J. Thermal losses evaluation of an external rectangular receiver in a windy environment. Sol. Energy 2019; 184: 281– 291. doi:10.1016/j.solener.2019.03.103. [22] Sardeshpande V, Pillai IR, Effect of micro-level and macro-level factors on adoption potential of solar concentrators for medium temperature thermal applications. Energy Sustain Dev 2012; 16:216–223. doi:10.1016/j.esd.2012.01.001. [23] Anjum M. Design development and modeling of a solar distillation system for the processing of medicinal and aromatic plants. University of Kassel, Germany, Ph. D. Thesis 2010. [24] Duffie JA, Beckman WA. Solar Engineering of Thermal Processes. 4th ed. New York: Wiley; 2013. [25] Kalogirou SA, Solar energy engineering: processes and systems. 2nd ed. USA: Academic Press; 2013.
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[26] MacPhee D, Dincer I. Thermal modeling of a packed bed thermal energy storage system during charging. Appl Therm Eng 2009; 29: 695-705. [27] Petela R. Exergy of undiluted thermal radiation. Sol. Energy 2003; 74: 469-88. [28] Kreith F, Kreider J. Principles of Solar Engineering. New York: Hemisphere-McGraw Hill ; 1978. [29] Moffat RJ. Describing the Uncertainties in Experimental Results. Experimental Thermal and Fluid Science 1988; 1:3–17. doi:10.1016/0894-1777(88)90043-x [30] Acar C, Dincer I. Exergetic performance assessment of an integrated solar energy system. Int. J. Exergy 2016;19: 161-72. [31] Li X, Dai YJ, Wang RZ. Performance investigation on solar thermal conversion of a conical cavity receiver employing a beam-down solar tower concentrator. Sol. Energy 2015;114:134–51. doi:10.1016/j.solener.2015.01.033.
NOMENCLATURE
Effective aperture area of reflector dish (m2)
S
Aperture area of receiver (m2)
h 2
Surface area of the elliptical frame of reflector dish (m2)
Specific heat capacity of heat transfer fluid (J/kgK)
2i
Concentration ratio
FGS
Receiver exergy rate (kW)
Solar beam radiation (W/m2)
R
Concentrated solar radiation power or dish power (kW)
FGR
Concentrated dish exergy rate (kW)
FGj
Solar exergy rate (kW)
01
Mass flow rate of heat transfer fluid (kg/s)
V
Overall rate of heat loss (kW)
j
Solar radiation power on the reflector dish (kW)
S 3
3S
Receiver power (kW)
Ambient temperature (K) Receiver surface temperature (K)
30
3U)
Inlet temperature of heat transfer fluid (K)
3-k.
Outlet temperature of heat transfer fluid (K)
:V
Overall heat loss coefficient (W/m2K)
3O
:V ′
Surface temperature of sun (K)
Overall heat loss factor (W/K)
Semi-minor axis of the elliptical frame of reflector dish (m)
m
Receiver tube length (m)
)
Day of the year
Semi-major axis of the elliptical frame of reflector dish (m)
0
Mass of heat transfer fluid (kg)
Greek symbols
ηnG
Exergy efficiency of the receiver (%)
U
Solar declination angle (degrees)
η.ℎ,
Instantaneous thermal energy efficiency of the receiver (%)
η-.
Combined optical efficiency of reflector dish (%) Inclination angle of receiver (degrees)
Abbreviations o3p
LPM
Heat transfer fluid Litres per minute
Highlights
Spiral tube receiver is developed for solar parabolic dish concentrator.
Experimental performance analysis were carried out at different radiation levels.
The average thermal efficiency of the spiral tube receiver was obtained as 56.2%.
The receiver has potential to be used in the applications of temperature up to 100oC.
DECLARATION OF INTEREST 30/10/2019
To The Chief Editor, Energy Journal
Dear Professor,
Declarations of interest: none We like to declare that there is no conflict of interest in publishing this article titled ‘An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator’. Also we like to state that this research did not receive any specific grant from the funding agencies in the public, commercial or not-for-profit sectors.
Thanking you, Yours sincerely
Dr. M Cheralathan Mr. V Thirunavukkarasu
S0360-5442(19)32330-8
DOI:
https://doi.org/10.1016/j.energy.2019.116635
Reference:
EGY 116635
To appear in:
Energy
Received Date: 30 May 2019 Revised Date:
30 October 2019
Accepted Date: 25 November 2019
Please cite this article as: Thirunavukkarasu V, Cheralathan M, An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator, Energy (2020), doi: https://doi.org/10.1016/j.energy.2019.116635. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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AN EXPERIMENTAL STUDY ON ENERGY AND EXERGY PERFORMANCE OF A SPIRAL TUBE RECEIVER FOR SOLAR PARABOLIC DISH CONCENTRATOR Author: V Thirunavukkarasu Department of Mechanical Engineering, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India
.
Email: [email protected] Corresponding author: M Cheralathan Department of Mechanical Engineering, Faculty of Engineering and Technology SRM Institute of Science and Technology, Kattankulathur, 603203, Tamil Nadu, India.
Email: [email protected]
2
AN EXPERIMENTAL STUDY ON ENERGY AND EXERGY PERFORMANCE OF A SPIRAL TUBE RECEIVER FOR SOLAR PARABOLIC DISH CONCENTRATOR.
ABSTRACT In this paper, energy and exergy performance of an external type spiral tube receiver for a solar parabolic dish concentrator is experimentally analyzed and presented. Performance evaluation is done at three different radiation conditions. The receiver is tested in the temperature range of 30oC to 100oC with water as heat transfer fluid and at a flow rate of 1.5 liters per minute. The overall heat loss coefficient of the receiver estimated from the stagnation test is found to be 182 W/m2K. The average thermal and exergy efficiencies of the receiver was determined to be 56.21% and 5.45% respectively under an average beam radiation of 750 W/m2. The variation trend of thermal efficiency of receiver is similar to that of difference between temperature of the heat transfer fluid at receiver inlet and outlet. The results show that this light weight, low cost receiver has potential to be employed with solar parabolic dish concentrator for process heating applications. Keywords: Solar parabolic dish concentrator; Scheffler reflector; spiral tube receiver; energy efficiency ; exergy efficiency; overall heat loss coefficient.
1. Introduction Solar energy is a promising alternative to conventional energy due to its greater global potential. The greatest advantage of solar energy is the minimal usage of traditional and polluting source of energy. Hence the usage of solar energy leads to clean and hygienic environment and huge fuel savings. Recently solar parabolic dish collector is becoming increasingly popular for various process heating applications in the temperature range 80oC to 320oC.
The parabolic dish concentrator works by concentrating the sunlight to the focus of the parabolic reflector and placing the receiver at the focal point. The receiver accumulates the heat, which is then transported to the end use application with the help of a heat transfer fluid. The surface area of receiver is much smaller than that of the dish reflector, thereby higher concentration ratio can be achieved. This concentration allows the increase of energy flux and attainment of higher temperatures at the receiver.
3
Solar receiver has a major role in solar thermal energy conversion in parabolic dish concentrator systems. The design of the receiver should be proper so as to receive the concentrated solar radiation and convert it to thermal energy with minimum heat loss. Basically there are two types of solar receivers: external type and cavity type. In the external type receiver, the solar flux is directed on to the front surface of the receiver whereas in the cavity receiver, the radiation is concentrated by the reflector to the receiver aperture and is absorbed by the inside cavity surface. This thermal energy is then absorbed and transported to the application by the heat transfer fluid flowing through the receiver. Different configurations of cavity receivers were proposed and investigated by several researchers for parabolic dish concentrators. Kaushika and Reddy [1] utilized an innovative fuzzy volumetric receiver for a deep parabolic dish. They developed a cheaper steam generating system and achieved an efficiency in the range of 70-80%. A cylindrical solar receiver with a wind skirt was developed by Prakash et al. [2] and carried out studies on convective heat losses by both experimental and numerical analysis. The analysis was also performed to observe the effect of receiver inclinations and inlet temperatures of heat transfer fluid.
Based on the analysis, a
correlation was proposed for obtaining Nusselt number to calculate the heat loss in natural convection mode.
A conical receiver was investigated experimentally and theoretically for a
parabolic concentrator of large rim angle by Hernandez et al [3]. They have reported that the conical receiver is suitable for the deep parabolic dish. Ma [4] developed a receiver in the shape of conical frustum-cylinder and did experimental studies to determine the total and convective losses. The dependence of heat losses on receiver inclination, temperature and aperture size were also studied. Mawire and Simeon [5] used a cavity receiver in the cylindrical shape for an SK-14 domestic dish concentrator and examined its thermal performance. They obtained a peak energy and exergy efficiency of around 45% and 10% respectively for the receiver in a circulation mode of operation.
Reddy et al. [6] did an experimental investigation on the performance of modified cavity receiver for a 20 m2 dish concentrator of fuzzy focal type. Their parabolic dish collector attained an average thermal efficiency of 74% for a flow rate of 250 litres per hour. They reported that the thermal efficiency of collector rises up with increase in beam radiation and also with increase in flow rate. Zhu et al. [7] did an experimental study to examine the efficiencies of a coiled tube receiver for a 57 m2 dish concentrator, with compressed air as the heat transfer fluid. Energy and exergy analysis revealed maximum energy efficiency of around 82% and maximum exergy
4
efficiency of around 28% for an open loop test method. A receiver in the form of conical cavity was developed for a 16 m2 Scheffler-type parabolic dish collector by Thirunavukkarasu et al. [8]. They performed experimental energy and exergy analyses with various mass flow rates. The receiver showed maximum energy and exegy efficiency of 66.75% and 10.35% respectively for a heat transfer fluid flow rate of 2.5 liters per minute. Thirunavukkarasu and Cheralathan [9] experimentally studied the effect of aspect ratio of the conical cavity receiver on its thermal performance. Aspect ratio was varied from 0.8 to 1.2 with an increment of 0.2 and the study revealed that the receiver with lowest aspect ratio 0.8 performs better. Le Roux et al. [10] numerically studied the performance of a receiver in the form of rectangular cavity for dish concentrator with air as the heat transfer fluid. Neber and Lee [11] fabricated a low cost – high efficient receiver from silicon carbide for solar thermal dish-Brayton system. The receiver was designed to heat air to a high temperature of 1500 K which improves conversion efficiency by 20% over the power generation by concentrated solar technology based on 1270 K. A thermal model of a tubular cylindrical cavity receiver was developed by Lone et al. [12] to calculate the optimum parameters of the receiver like aperture size, inner tube diameter, and cavity depth and inlet temperature of the working fluid for attaining maximum efficiency.
The comparative study on receiver performance in different cavity shapes has been reported by few researchers. Harris et al. [13] did investigation on the thermal performance of the solar dish concentrators with five different receiver geometry and predicted that cavity geometry and rim angle of the concentrator have greater influence on power profiles of the cavity receiver. Kumar and Reddy [14] compared a cavity, a semi cavity and a modified receiver for a dish concentrator by numerical investigations. They have reported that modified cavity receiver shows better performance because of lower convective losses. Madadi et al. [15] tested both cylindrical and conical cavity receivers, numerically and experimentally and reported that the cylindrical is better than the conical one, in terms of energy and efficiency values. Jilte et al. [16] numerically investigated the forced convective heat loss from cavity of different shapes like conical, cylindrical, cone-cylindrical, hetro-conical and dome cylindrical and reported that conical cavity yields the lowest convective losses among the other cavities. The optical performance of cylindrical, conical and spherical cavity receivers with parabolic dish concentrator was simulated by Daabo et al, [17] and showed that conical cavity receiver has highest optical efficiency. Loni et al. [18] predicted the thermal performance of solar receivers in the cubical and cylindrical cavity shapes by experimental
5
method and also by numerical modeling. Their results indicate that cavity receiver in the cubical shape shows higher thermal energy efficiency in comparison to cylindrical shape in the steady-state period.
Few studies are reported on external type receiver for parabolic dish concentrator. Pavlovic et al. [19] developed an external type spiral absorber with corrugated tubes for parabolic dish concentrator and its thermal performance was investigated with water. They have also developed a numerical thermal model for further investigation in various operating conditions and different heat transfer fluids. The examined range of inlet temperature is between 25oC to 350oC. Patil et al. [20] used a water container of 20 liter capacity as a receiver for Scheffler parabolic dish reflector of area 8 m2. An average power of 1.3 kW and efficiency of 21.61% was achieved by them in their experimental performance analysis. The operating temperature is in the range 25oC to 100oC. Qaisrani et al. [21] performed numerical investigations on the heat losses from a rectangular external type receiver under nine wind directions. They developed a correlation to evaluate the convective losses as a function of wind velocity, wind direction, surface area of the receiver and width of the wind blocking wall.
In the solar steam cooking facility at SRM Institute of Science and Technology, Kattankulathur, which is situated at southern part of India, external type receiver in the form of a short cylinder with a flat front surface is being used for 16 m2 Scheffler-type parabolic dish concentrators. The operating temperature of this receiver is approximately in the range 25oC to 180oC. This conventional receiver is heavy and the presence of large metal mass requires longer warm up period. It is observed that large amount of thermal energy is retained by the receiver material during its operation and is not available to the heat transfer fluid. There are two options to address this issue, one is the usage of cavity receiver and the other one is improving the design feature of external type receiver itself. Cavity receivers are best choice for high temperature concentrator systems, where there is a benefit in smaller ratio of aperture to absorbing area. But the supporting structure required for cavity receiver should be larger than that for an external receiver. Apart from this, it must be perfectly insulated to minimize conductive losses through its bigger surface area. This makes a cavity receiver larger, heavier and expensive when compared to external type receiver. Hence in the present work, being in the low and medium temperature range, focus is made on external type receiver.
6
The literature study shows that most of the researches were on cavity type receivers and the studies on improvement of external type receiver are very few. This work aims at developing and testing a light weight compact coiled receiver in the spiral form for 16 m2 Scheffler-type parabolic dish concentrator. The contact surface area of receiver per unit volume of HTF is more for spiral tube receiver when compared to the conventional short cylinder receiver, which acts as a mechanism to enhance heat transfer. Energy and exergy efficiencies of receiver was analyzed at actual solar radiation conditions to evaluate its performance. 2. Experimental apparatus and method
2.1. Description of solar receiver The coiled spiral tube solar receiver which is made out of mild steel material is shown in Fig. 1. The focal image size of the reflector is measured to be Ø39 cm ± 1. Also the short cylinder receiver used in the solar steam cooking system for the same 16 m2 Scheffler reflector, has an aperture diameter of Ø40 cm. Hence the receiver for the present study is also constructed with an aperture diameter of Ø40 cm. During fabrication, a dead space of solar energy absorption was induced in the receiver as shown in Fig 1a. This is because the receiver could not be fully covered by the coiled tube till the centre point due to difficulty in coiling at the region of low coiling radius. To avoid the loss of radiation through this dead space, it was covered with a hollow and thick mild steel plate, in such a way that the HTF flows through the hollow plate also to receive the radiation energy before it leaves the receiver, as indicated in the Fig 1b. The back surface of the receiver is thermally insulated with 25 mm thick glass wool of very low thermal conductivity of 0.04 W/mK. An aluminum housing is provided so that insulation is in proper fit. This receiver is similar to the receiver developed by Pavlovic et al. [19]. But they had used corrugated tubes whereas present receiver is made of smooth tube. Moreover the presence of small dead space in their receiver is eliminated in the present design. Receiver design features and the dish specifications are given in Table 1.
7
(a)
(b)
(c) Fig.1. Development of spiral tube receiver. a) Tube bending process b) Removal of dead space c) Receiver model with dimensions The receiver is stationary and not connected to the support structure of reflector. This is because the centre of the Scheffler reflector and its focal position are fixed. This is made possible because of the two axes tracking system, which works by clockwork mechanism. When compared to the generic parabolic dish technology, Scheffler type reflector has the advantage of fixed focus which helps in minimizing structural complexity [22].
8
Table 1. Concentrator and receiver design parameters Parameters Area of Scheffler dish concentrator (Af) Semi-minor axis of the elliptical frame of the reflector dish (a) Semi-major axis of the elliptical frame of the reflector dish (b) Focal length of the Scheffler reflector (f) Aperture area of the receiver (AR) Concentration ratio on 10/3/2017(Cr) Concentration ratio on 25/3/2017(Cr) Spiral tube outer diameter Spiral tube inner diameter No. of turns of the spiral coil Spiral tube length Inclination angle of receiver at focal point Combined optical efficiency of concentrator system
Values 16 m2 1.9 m 2.65 m 2.5 m 0.1256 m2 92.3 91.5 21.3 mm 14.3 mm 6 5.77 m 13o 71%
2.2 Experimental set up The schematic diagram of the experimental set up developed for performance tests is presented in Fig. 2. This test facility is located at the SRM Institute of Science and Technology, Kattankulathur, near Chennai City; 12°49' latitude and 80° 20' longitude geographical location. Scheffler-type parabolic dish for the present work is made out of multiple pieces of solar grade mirrors of 3 mm thick, 100 mm wide and 180 mm long. Total reflective area of the dish is 16 m2. The east west tracking is done manually by operating the chain linked clockwork mechanism. Fig. 3 shows the photographic view of examined dish collector. Water is circulated as the heat transfer fluid (HTF) from an insulated storage tank of 85 litres capacity to the receiver. For the circulation of heat transfer fluid, a centrifugal pump driven by an electric motor of 1⁄4 hp is used. Water enters the bottom of the receiver, flows from the periphery towards the center and leaves the receiver from the back end. A gate valve is used to control the volume flow rate of HTF through the receiver. A rotameter is used to measure the flow rate of heat transfer fluid. The range of rotameter is 0-500 LPH and the least count is 5 LPH with an accuracy of ± 2%. The pipelines in the HTF circulation are insulated with 50 mm fiber glass wool to prevent heat loss.
9
Fig. 2. HTF circulation layout K type thermocouples with ±0.1oC accuracy are used to measure HTF temperature at the receiver inlet and outlet, HTF in the thermo tank, and ambient temperature as well. Temperature of receiver surface is measured for every five minutes using a digital infra-red laser thermometer (DT8811 - MEXTECH make) of range -50 °C to 550 °C with an accuracy of ±0.5 °C. A pyranometer (Dynalab make, Model No. DWR 8101) of sensitivity, accuracy and working range 20 µV/W/m2, ± 3.5% and 0-2000 W/m2 respectively is used for measuring global solar radiation. The diffuse solar radiation is also measured by employing the same kind of pyranometer with a shading ring. The intensity of beam solar radiation is obtained by deducting diffuse radiation from global radiation. A cup type anemometer is used to measure the wind speed (Dynalab make, Model No. DWA 8600) with an accuracy of ±2 %. All the data are recorded using a data logger (Dynalab make, Model No. DWL 1002) which is connected to a computer. Initially a stagnation test is conducted to evaluate the overall heat loss coefficient of the receiver. The condition at which no heat is retrieved by the heat transfer fluid from the receiver is called as stagnation condition. At this condition, the receiver attains it maximum temperature. This upper temperature limit gives the idea of the applications for which the collector can be used.
10
Later, experiments are conducted at three different time slots to assess the performance of receiver at different radiation conditions. The first experiment was conducted on 10th March 2017 from 11 AM to 12:20 PM, the time period close to noon. The second experiment was conducted after noon of the same day, from 2 PM to 3:30 PM and the third experiment was conducted on 25th March 2017 from 2:45 PM to 4:35 PM. A flow rate of 1.5 LPM was chosen for the circulation of HTF for all the experiments in a closed loop mode.
Fig. 3 The examined solar dish collector
2.3 Energy analysis The estimation of energy efficiency is presented in this section to analyse the thermal performance of spiral tube receiver used in the Scheffler dish concentrator system. The solar radiation power on the Scheffler reflector can be estimated from the expression: = 1
11
where is the intensity of beam radiation in W/m2 and is the effective aperture area of
reflector at any day of the year. The effective aperture area of the Scheffler-reflector is given by [23]: = . cos 43.23 +
2 2
where is the surface area of the elliptical frame of reflector and is given by :
= 3
where a and b are semi-minor axis and semi major axis of the elliptical frame of reflector dish respectively. Solar declination angle ( ) for any day of the year is given by [24]:
= 180/π 0.006918 − 0.399912 cos $ + 0.070257 sin $ − 0.006758 cos 2$ +
0.000907 sin 2$ − 0.002697 cos 3$ + 0.00148 sin 3$ 4
where B is given by $ = ) − 1 . 360/365
(5)
and ) is the day of the year counted from 1st January.
The dish power which is available as input to the receiver is given by:
* = η+, 6
where η-. is the combined optical efficiency based on the reflectivity of dish mirror and
absorptance of the receiver.
Receiver power, which is the useful thermal power output from the receiver can be estimated by [25] / = 012 3+4, − 35 7
where 01 is the mass flow rate of HTF, Cp is the specific heat of HTF, Tin is the temperature of HTF at receiver inlet and Tout is the temperature of HTF at receiver outlet.
On the basis of law of energy conversion, the overall heat loss from the receiver can be obtained as: 6 = * − /
(8)
12
where 6 is the overall heat loss from the receiver which consists of convective and radiative heat
losses. Due to the thermal insulation provided on the back surface, conductive heat loss is negligible. The instantaneous thermal energy efficiency of the receiver, which is defined as the ratio of useful power output from the receiver to the radiation energy input to the receiver, is given by η,7,9 =
012 3+4, − 35 9 η+,
The overall heat loss coefficient of the receiver, UL, can be determined by performing the stagnation test as indicated in [6].
The receiver power is given as / = * − 6
(10)
/ = η+, − :6 / 3/ − 3
(11)
By combining equations 1, 6, 7 and 10, QR can also be written as
At stagnation condition,/ = 0, Hence * = 6 , where U< is the overall heat loss coefficient of receiver, which can be obtained as
:6 =
η=>? @A> 9B
@C DC EDA
(12)
2.4 Exergy analysis Energy analysis is on the basis of first law of thermodynamics and does not consider the reductions of energy potential, exergy analysis is based on both first and second law of thermodynamics. The receiver exergy rate is given by [26]: 3+4, J 13 35 where Ta is the ambient temperature. FG/ = 012 H 3+4, − 35 − 3 ln
Solar exergy rate, which is the rate of solar exergy delivered by the sun (source) to the concentrator can be expressed as given by [27]: 1 3 M 43 FG = K1 + − N 14 3 3L 33L
where 3O is the sun’s blackbody temperature whose value is taken as 5762 K [28].
13
The dish exergy rate, which is defined as the rate of exergy from the dish concentrator to the receiver, can be expressed as: 1 3 M 43 FG* = η+, K1 + − N 15 3 3L 33L
The exergy efficiency, which is the ratio of receiver exergy rate to the dish exergy rate is
given by: ηPQ =
2.5
FG/ 16 FG*
Uncertainty analysis
An uncertainty analysis is carried out with the dish power, R, receiver power, S , thermal
energy efficiency,η.ℎ,U , exergy efficiency, ηPQ and overall heat loss coefficient, :V using the propagation of error technique explained in [29].
The uncertainty of the dish power is calculated by:
X
X
* * * = W X + δ X 17 δ
While the uncertainty of the receiver power is given by:
/ X / X / X / = W 01 X + 35 X + 3+4, X 18 01 35 3+4,
The uncertainty of thermal energy efficiency can be calculated by: η,7,9
X
η,7,9 X η,7,9 X η,7,9 η,7,9 X η,7,9 X W X X X X = Y Z 01 + Y Z 3+4, + Y Z 35 + Y Z + Y Z δ X 01 3+4, 35 δ (19)
The uncertainty of exergy efficiency is given by:
14
] η^_ X
\ ]`1 a
01 X
] η^_ X
+ \]D a 3+4,
X
X
]η^_
+ \ ]D a 35 X +
cd ηPQ = [ X X ] η^_ X ] η ]η \ a 3 X + \ ^_ a X + \ ^_ a δ X =b?
]DA
]9B
(20)
]δc
The uncertainty of overall heat loss coefficient is given by: :6 = W\
]ef X ]9B
]e
X
]e
X
]e
X
a X + \ ]δf a δ X + \]D f a 3/ X + \ ]Df a 3 X c
C
A
(21)
Mass flow rate has an uncertainty of 2% on a relative scale, as described from the accuracy of the rotameter. Therefore, 01 = ±0.0005 kg/s. The uncertainty of all the temperature measurements from the K-type thermocouples is 0.1 K. Thus, 35 = 3+4, = 3 = ±0.1 K. The combined
uncertainty of solar radiation measurements is 3.5 %, as indicated by the secondary standard specification of the pyranometer, and the solar declination angle has an error of 0.035 degrees on any day of the year. Therefore, = ±3.5% of and δ = ±0.035 degrees. The uncertainties of the performance parameters evaluated are listed in Table 2.
Table 2. Uncertainties of performance parameters. Parameters
Uncertainty
Dish power
218.4 W
3.6
Receiver power
70.8 W
2.12
Thermal energy efficiency
2.3%
4.07
Exergy efficiency
0.277 %
4.09
Overall heat loss coefficient 6.58 W/m2K
% Uncertainty
3.54
3. Results and discussion The results of the experiments conducted to predict the performance of the receiver are presented and discussed here in detail. The aperture area of the Scheffler reflector is estimated from the equation (2). Aperture area decreases from 13.457 m2 to 9.0823 m2 between 1st January to 22nd June of the year and again increases from 9.0823 m2 to 13.463 m2 between 23rd June to 31st December. Fig. 4 shows the variation of declination angle and the aperture area of Scheffler - reflector with respect to day of the year.
15
Fig. 4 Variation of declination angle and aperture area of Scheffler reflector with respect to day of the year
3.1 Stagnation temperature and overall heat transfer coefficient
The beam radiation (Ib), ambient temperature (Ta) and receiver stagnation temperature (TR) are measured from 9.30 am to 3 pm during the stagnation test. The variations of stagnation temperature, beam radiation and overall heat transfer coefficient during this time period are shown in Fig. 5. During the testing period, the irradiance dropped to very low value at four instances due to the passing clouds. The variation of irradiance has influenced the stagnation temperature, but their variations are not exactly similar. It may be due to the inertia effect of the mild steel material of the receiver. The maximum stagnation temperature recorded is 366oC±0.5oC. Overall heat loss coefficient is at its
maximum of about 440 W/m2K at the beginning of the test because of
preheating of the receiver, and then drops significantly to about 52 W/m2K within 20 minutes of operation. This is followed by a fluctuating trend till the end of the test due to the frequent changes in irradiance level. At the value of irradiance of 118 W/m2 and 600 W/m2, the corresponding value of overall heat transfer coefficient is predicted as 53 W/m2K and 237 W/m2K respectively. The average value of U< for the receiver in a day is observed to be 182 W/m2K.
16
Beam radiation
Overall heat transfer coeffecient
Receiver stagnation temperature
500 450
600
Beam radiation (W/m2)
400 500
350 300
400
250 300
200 150
200
100 100 50
Overall heat tranfer coeffecient (W/m2K) Receiver stagnation temperature (oC)
700
0 14:50
14:30
14:10
13:50
13:30
13:10
12:50
12:30
12:10
11:50
11:30
11:10
10:50
10:30
10:10
9:50
9:30
0
Time (hh:mm)
Fig. 5. Variation of the receiver stagnation temperature and overall heat transfer coefficient.
3.2 Temperature and irradiance profile of the receiver Fig. 6a-c shows the variation of solar beam radiation, ambient temperature, HTF outlet and inlet temperatures with respect to time during the first, second and third experiment respectively. . During the first experiment (Fig. 6a) irradiance increased from a minimum value of 690 W/m2 to 757 W/m2 during the first 20 minutes and showed almost constant value during the remaining period. The average value of irradiance recorded was 750 W/m2. In the first ten minutes itself, the outlet temperature attained a temperature of 97oC and then gradually rose to 100oC. The inlet temperature also increased during the test, since the HTF was re-circulated repeatedly between the receiver and the HTF storage tank for conducting the closed loop experiment. Hence this gives a dynamic characterisation method of the receiver. Inlet temperature of HTF increased in a different manner when compared to that of the outlet temperature. It showed only a little increase during the first 40 minutes due to the presence of thermal stratification in the HTF storage tank. Due to continuous mixing of hot HTF from receiver to the cold HTF in the storage tank, the thermal stratification starts to disappear and the temperature of HTF rose to 81oC at a faster rate in the next 20 minutes. After this, the inlet temperature gradually attained a value of 90oC at the end of the test. The receiver temperature is observed to increase from a minimum value of 99oC to 123oC with an
17
average value of 108oC. The ambient temperature was almost constant with an average value of 41oC. 900
130
a
800
Temperature (°C)
110 100
700
90
600
80 70
500
60
400
50 300
40 Inlet temperature Outlet temperature Ambient temperature Receiver surface temperature Irradiance
30 20 10
Irradiance (W/m2)
120
200 100
0
0 11:00
11:10
11:20
11:30
11:40
11:50
12:00
12:10
12:20
Time(hh:mm)
130 120
800
b
700
110
600
90
Irradiance (W/m2)
Temperature (°C)
100
500
80 70
400
60 50
300
40 30 20 10 0
Inlet temperature Outlet temperature Ambient temperature Receiver surface temperature Irradiance 02:00 02:10 02:20 02:30 02:40 02:50 03:00 03:10 03:20 03:30 Time (hh:mm)
200 100 0
18
130 120
600
c
110
500
90
400
80 70
300
60 50
200
40 30
Inlet temperature
Outlet temperature
20
Ambient temperature
Receiver surface temperature 100
10
Irradiance
0
Irradiance (W/m2)
Temperature (°C)
100
0 02:45 02:55 03:05 03:15 03:25 03:35 03:45 03:55 04:05 04:15 04:25 04:35 Time(hh:mm)
Fig 6. Temperature and irradiance profiles of the spiral tube receiver for average irradiance of a) 750 W/m2 b) 600 W/m2 c) 380 W/m2 During the second performance test (Fig. 6b), solar irradiance was recorded a peak value of 688 W/m2 at the beginning and later it showed a gradual decrease. The average value of irradiance recorded was 600 W/m2. The outlet temperature presented a different growth rate at different times. It rose progressively from the start of the test till 2.50 pm followed by a low growth gradient till the end of the test. The outlet temperature has attained only 91oC even after 90 minutes of test duration. This variation trend of outlet temperature is because of the declining nature of beam radiation availability. During the first 50 minutes of the test, the inlet temperature rose very slowly, followed by a steep rise during the next 10 minutes and a gradual rise during the next 30 minutes of the test. Thermal stratification was present in the storage tank during the first 50 minutes and disappeared during the later stage of the test. Receiver temperature first increased from 81oC to 98 oC during the first 50 minutes, then decreased to 92 oC at the end. The ambient temperature was almost constant with an average of 39oC during the testing period. The third performance test was conducted to check the performance at low radiation conditions (Fig. 6c). Solar irradiance was recorded a peak value of 519 W/m2 at the beginning and then it decreased towards the end of the test to a lower value of 165 W/m2. An average irradiance value of 380 W/m2 was recorded during the test. The outlet temperature rose comparatively at a slower rate. It took around 110 minutes for the outlet temperature to attain 82oC. Inlet HTF
19
temperature showed only a little increase during the first 30 minutes and then rose at an increased rate in the next 20 minutes, followed by a gradual rise to attain a temperature of 71oC at the end of the test. The thermal stratification in the HTF storage tank was observed to be comparatively less during this test because of the low growth rate of HTF temperature at the receiver outlet. Receiver temperature increased from a minimum value of 62oC and recorded a maximum value of 99oC at 4 PM, after which it showed a fluctuating trend. During the last 20 minutes, there was a reduction in the receiver’s surface temperature, the lowest value recorded was 83.5oC. The ambient temperature remained fairly constant with an average of 34oC during the testing period.
3.3 Power profile of the receiver Fig 7a-c depicts the variation of concentrated solar radiation and receiver power with time during the experiment I, II and III respectively. The dish power varies in a similar manner to that of solar beam radiation, showing the direct influence of solar irradiance on it. For all the tests, the receiver power is observed to vary in a similar way to the difference in HTF outlet and inlet temperature. Loni et al. [18] has also made a similar observation in their study on cavity receivers. During the last thirty minutes of second experiment, it can be seen that the receiver power was maintained almost constant even though the dish power declined at a faster rate. This is because the heat supplied from the receiver mild steel material was utilized to deliver the receiver power. The reduction in growth gradient of receiver surface temperature during this period supports this fact. Similarly during the last twenty minutes of the third experiment (Fig. 7c), receiver delivered an almost constant power. The peak and average values of dish power and receiver power are given in the Table 2. An energy and exergy flow diagram based on the results obtained from the first experiment is added in Fig. 8 to indicate various losses of the dish receiver system.
20
21 Fig. 7 Power profiles of the receiver for the average irradiance of 750 W/m2 b) 600 W/m2 c) 380 W/m2
Fig. 8 Energy and exergy flow diagram of dish receiver system for average irradiance of 750 W/m2.
3.4 Energy and exergy efficiency profile of the receiver
Fig 9a-c depicts the variation of instantaneous energy and exergy efficiency of the receiver with time during the tests I, II and III respectively. The peak and average energy and exergy efficiency values are given in Table 2. For the first experiment, with average irradiance value of 750 W/m2, the instantaneous energy efficiency curve followed a parallel trend to that of the receiver power, as the solar radiation power concentrated from the dish was almost constant during the testing period. The efficiency first increased to a peak value of 82.6% and then decreased towards the end of the test with a lower value of 16%. The difference between the outlet and inlet temperatures of HTF was relatively large till 11:40 AM which resulted in higher heat gain by the receiver and higher efficiency values. Later the temperature difference started to decrease which resulted in a sharp decrease of efficiency till 12:00 PM and then followed by a gradual decrease. For the second test with average irradiance value of 600 W/m2, the instantaneous energy efficiency curve followed a parallel trend to that of receiver power till 3 PM. Beyond 3 PM, the receiver power was maintained somewhat constant, even though the dish power was declining. Hence the reduction in the values of denominator of the equation (9) for instantaneous efficiency with almost constant value of numerator, resulted in an increase in efficiency values. Similar to the above test results, for the third experiment, the instantaneous energy efficiency curve followed a parallel trend to that of receiver power up to a certain period. From 4.15 PM onwards, as the dish
22
power started to decline drastically, the efficiency curves underwent a steep rise. This is because, as described above, the receiver power was maintained somewhat constant even though there was a decrease in the dish power. 8 7 6 5 4 3 2
Exergy efficiency (%)
a
Energy efficiency
Energy efficiency (%)
100 90 80 70 60 50 40 30 20 10 0
1 0 11:00 11:10 11:20 11:30 11:40 11:50 12:00 12:10 12:20 Time(hh:mm)
8
6
4
2
0 02:00 02:10 02:20 02:30 02:40 02:50 03:00 03:10 03:20 03:30 Time (hh:mm)
Exergy efficiency (%)
b
Energy efficiency Exergy efficiency
Energy efficiency (%)
100 90 80 70 60 50 40 30 20 10 0
100 90 80 70 60 50 40 30 20 10 0
8
c
Energy efficiency Exergy efficiency
7 6 5 4 3 2
Exergy efficiency (%)
Energy efficiency (%)
23
1 0 02:45 03:00 03:15 03:30 03:45 04:00 04:15 04:30 Time(hh:mm)
Fig 9a-c Energy and exergy efficiency profiles of the receiver for the average irradiance of a) 750 W/m2 b) 600 W/m2 c) 380 W/m2
The exergy efficiency values follow a similar pattern to that of the energy efficiency. It is very low when compared to the energy efficiency values, which indicates the low quality of energy being delivered to the receiver at all radiation conditions. A potential reason for the exergy destruction in the solar collector is the high exergy content of the solar input and the temperature difference between inlet and outlet temperatures of HTF [28]. The exergy destruction due to high exergy content of the solar input can be minimized if the receiver surface temperature is maintained at higher values. Moreover, as per the equation (13), receiver exergy rate is greatly influenced by the ratio (3+4, /35 ) because the magnitude of exergy rate is strongly dependent on the exergy loss,
which is expressed as 012 3 ln \
D=b? Dcd
a. Hence to reduce the exergy loss in the receiver, temperature
difference of HTF at receiver outlet and inlet has to be minimized. To increase the receiver surface temperature and also to minimize the ratio (3+4, /35 ), the possible way is to increase the inlet
temperature of HTF.
24
Table 3. Performance parameters of the receiver Performance parameters
Average irradiance (W/m2) 750 600 380
Average receiver surface temperature (TRoC) Peak dish power (kW) Average dish power (kW) Peak receiver power (kW) Average receiver power (kW) Average heat loss from the receiver (kW) Average thermal energy efficiency (%) Peak thermal energy efficiency (%) Average exergy efficiency (%) Peak exergy efficiency (%)
108 6.287 6.128 4.941 3.421 2.707 56.2 82.6 5.5 7.1
88.6 5.666 4.943 3.873 2.28 2.663 44.5 80.0 3.1 6.9
85.8 4.239 3.105 1.758 1.022 2.083 33.8 64.0 3.4 6.9
Rate of rise in HTF temperature in storage tank (oC/min)
0.725
0.488
0.255
The comparison between energy and exergy efficiency values of the previous and current research are summarized in Table 4. The results of experimental analysis in a circulation mode of operation are considered for comparison. Hernandez et al. [3] used a paraboloidal concentrator which is built from an old telecommunication antenna with reflectance of 0.64 to test the developed conical double walled cavity type stainless steel receiver. The receiver received radiation from both inner and outer surfaces. Mawire and Taole [5] did experiments on cylindrical cavity receiver, made of copper coil, with SK-14 parabolic dish concentrator. The exergy efficiency of the present work is less when compared to that of [5] due to the low operating temperature and larger difference between HTF outlet and inlet temperatures. Li et al. [31] used a beam-down solar tower which includes linear Fresnel heliostats and a beam down concentrator to concentrate the solar radiation to their conical spiral tube type receiver. The receiver was made from copper tubes with selective absorption coating with absorptivity of 0.9.
Pavlovic et al. [19] developed a low-cost solar
parabolic dish reflector made of polymethyl methacrylate reflective petals with a silver mirror layer and 0.6 reflectance. The receiver is made out of stainless steel and is a spiral corrugated tube inside an aluminum housing. The downward facing receiver has a drawback of a small dead space, which is being eliminated in the present work by adding a circular hollow plate. The dead space is present at the central region, which is the place where maximum concentration take place. Thirunavukkarasu et al. [8] used a Scheffler-type parabolic dish reflector with a reflectance of 0.8 to
25
test the developed conical cavity spiral tube receiver. The receiver is made from mild steel material. An average thermal energy efficiency of Table 4. Comparison between experimental results of present work to other researchers. Heat transfer fluid Water
Operating temperature range (oC) 30 oC - 90 oC
Cylindrical cavity receiver
Shell S2 Thermic fluid
43 oC - 160 oC
45%
10%
Beam-down solar tower equipped with Fresnel heliostat modules
Cavity receiver with conical spiral tube
Water Therminol VP-1
20oC - 150oC 12oC - 400oC
39 % 60 %
-------
Pavlovic et al., 2017
Paraboloid dish with rim angle 45.6o
Spiral tube receiver
Water Therminol VP-1
30 oC - 85 oC 150oC-160oC
34% ----
2.5% 7.6%
Thirunavukkarasu et al., 2017
Scheffler type parabolic dish concentrator
Conical cavity receiver
Water
30 oC - 100 oC
57%
5.9%
Present work
Scheffler type parabolic dish concentrator
Spiral tube receiver
Water
30 oC - 100 oC
56%
5.5%
Research Hernandez et al., 2012
Parabolic dish type Parabolic concentrator with 90o rim angle
Receiver used Conical cavity double walled receiver
Mawire and Taole, 2014
SK-14 Parabolic dish concentrator
Li et al., 2015
Energy efficiency 32%
Exergy efficiency ---
57.39% and average exergy efficiency of 5.88% were obtained for a flow rate of 2.5 LPM under an average irradiance of 608 W/m2. The efficiency of the present work at average beam radiation of 750 W/m2 is closer to this value. But when the results obtained under an average beam radiation of 600 W/m2 are compared to the author’s previous work [8], the energy efficiency is 12.5% lower and exergy efficiency is 2.78% lower. This may be possibly due to the fact that the chosen mass flow rate for the present work is not adequate and has to be optimized. Based on the comparison with other designs, energy efficiency obtained for the present work is better than the other researchers. The energy and exergy efficiencies of the present work are almost close to that of [8].
26
The possible design enhancements like optimized wall thickness and tube diameter may further improve the system performance. The larger thickness of tube wall increases the thermal mass, which adds inertia effect on the heat transfer. The tube diameter influences the flow characteristics and also the contact area of HTF with tube wall.
4. Conclusion An experimental investigation of the thermal performance of a spiral tube external type receiver for 16 m2 Scheffler type parabolic dish concentrator is carried out at actual solar radiation conditions. The receivers overall heat loss coefficient, thermal energy efficiency and exergy efficiency were evaluated at three different radiation conditions and the following conclusions are made:
The maximum stagnation temperature recorded is 366oC±0.5oC on a bright sunny day, which can be taken as the upper limit of temperature level to which this receiver can be utilized for producing heat. The overall heat loss coefficient of the receiver estimated from the stagnation test is found to be 182 W/m2K.
Beam radiation as well as the difference in temperature of heat transfer fluid between outlet and inlet of receiver greatly influences the receiver output and efficiency values. The average thermal and exergy efficiencies of the receiver was determined to be 56.21% and 5.45% respectively under high beam radiation conditions (750 W/m2).
The receiver developed for the present study acts as a simple test bed and does not have any selective coating, which shows the possibility of further improvement in the performance. Based on the results obtained, it can be concluded that this compact receiver has potential to be used in the applications of temperature up to 100oC. The investigations on the receiver’s performance at temperatures above 100oC is in progress in our laboratory and the results will be presented in near future.
Acknowledgements The authors are grateful to the management of SRM Institute of Science and Technology, Kattankulathur for providing the necessary facilities and continuous encouragement.
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27
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NOMENCLATURE
Effective aperture area of reflector dish (m2)
S
Aperture area of receiver (m2)
h 2
Surface area of the elliptical frame of reflector dish (m2)
Specific heat capacity of heat transfer fluid (J/kgK)
2i
Concentration ratio
FGS
Receiver exergy rate (kW)
Solar beam radiation (W/m2)
R
Concentrated solar radiation power or dish power (kW)
FGR
Concentrated dish exergy rate (kW)
FGj
Solar exergy rate (kW)
01
Mass flow rate of heat transfer fluid (kg/s)
V
Overall rate of heat loss (kW)
j
Solar radiation power on the reflector dish (kW)
S 3
3S
Receiver power (kW)
Ambient temperature (K) Receiver surface temperature (K)
30
3U)
Inlet temperature of heat transfer fluid (K)
3-k.
Outlet temperature of heat transfer fluid (K)
:V
Overall heat loss coefficient (W/m2K)
3O
:V ′
Surface temperature of sun (K)
Overall heat loss factor (W/K)
Semi-minor axis of the elliptical frame of reflector dish (m)
m
Receiver tube length (m)
)
Day of the year
Semi-major axis of the elliptical frame of reflector dish (m)
0
Mass of heat transfer fluid (kg)
Greek symbols
ηnG
Exergy efficiency of the receiver (%)
U
Solar declination angle (degrees)
η.ℎ,
Instantaneous thermal energy efficiency of the receiver (%)
η-.
Combined optical efficiency of reflector dish (%) Inclination angle of receiver (degrees)
Abbreviations o3p
LPM
Heat transfer fluid Litres per minute
Highlights
Spiral tube receiver is developed for solar parabolic dish concentrator.
Experimental performance analysis were carried out at different radiation levels.
The average thermal efficiency of the spiral tube receiver was obtained as 56.2%.
The receiver has potential to be used in the applications of temperature up to 100oC.
DECLARATION OF INTEREST 30/10/2019
To The Chief Editor, Energy Journal
Dear Professor,
Declarations of interest: none We like to declare that there is no conflict of interest in publishing this article titled ‘An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator’. Also we like to state that this research did not receive any specific grant from the funding agencies in the public, commercial or not-for-profit sectors.
Thanking you, Yours sincerely
Dr. M Cheralathan Mr. V Thirunavukkarasu