OPTICAL AND THERMAL ANALYSIS OF DIFFERENT ASYMMETRIC COMPOUND PARABOLIC PHOTOVOLTAIC CONCENTRATORS (ACPPVC) SYSTEMS
FOR BUILDING INTEGRATION
Yupeng Wu
Warwick Institute for Sustainable Energy and Resources School of Engineering, University of Warwick
Coventry, U.K, CV4 7AL
Yupeng.Wu@https://www.doczj.com/doc/2013165861.html,
Philip Eames
Warwick Institute for Sustainable Energy and Resources School of Engineering, University of Warwick
Coventry, U.K, CV4 7AL
P.C.Eames@https://www.doczj.com/doc/2013165861.html,
Mervyn Smyth
Centre for Sustainable Technologies School of Built Environment, University of Ulster Northern Ireland, U.K, BT37 0QB
m.smyth1@https://www.doczj.com/doc/2013165861.html,
Tapas Mallick
Warwick Institute for Sustainable Energy and Resources School of Engineering, University of Warwick
Coventry, U.K, CV4 7AL
T.Mallick@https://www.doczj.com/doc/2013165861.html,
ABSTRACT
Ray-trace techniques have been used to predict the optical performance and angular acceptance of Asymmetric Compound Parabolic Photovoltaic Concentrator (ACPPVC) systems suitable for integration into vertical south facing building fa?ades. Untruncated and truncated ACPPVC systems ACPPVC-50, ACPPVC-55 and ACPPVC-60, which have acceptance angels of 50° and 0°, 55° and 0°, 60° and 0°, respectively, have been simulated. The PV absorber was 0.125m wide in all simulations. Comparisons of angular acceptance between the untruncated and truncated systems are also discussed. I ncreased truncation leads to increased angular acceptance with reduced maximum concentration. From the simulations undertake, the angular acceptance was 100% in the range of incidence angles between the acceptance half angels for all ACPPVC systems. The highest optical efficiency predicted was 88.67% for all ACPPVC systems. The predicted solar flux distributions over the PV surface for the truncated ACPPVC-55 system are presented for selected angles of incidence along with concentration ratio. The temperatures within the truncated ACPPVC-55 system predicted using Computational Fluid Dynamics are presented for various solar incidence angles.
1. INTRODUCTION
To meet the resource and environmental needs of society, renewable energy sources, must play an increasingly important role displacing fossil fuels. Solar energy, one renewable energy source, is both clean and has the potential
to meet all of the world’s energy needs. Photovoltaic systems transform solar energy directly into electrical energy. Currently low efficiencies and high costs prevent their wide scale adoption [1]. Low concentration non-imaging Asymmetric Compound Parabolic Photovoltaic Concentrators (ACPPVC) are suitable for building fa?ade integration. Concentrator systems which increase the solar radiation intensity on the photovoltaic cells may reduce the system cost, if the cost of the concentrator is less than the photovoltaic material displaced [2,3]. For economic reasons, the reflector mirror can be truncated by 50%, fortunately little reduction in
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concentration ratio occurs for this level of truncation [3]. The ACPPVC system design analysed is suitable for integration onto south facing vertical building fa?ades in the U.K. Non-imaging untruncated and truncated ACPPVC systems have been analysed using ray-trace techniques to determine their optical characteristics. Untruncated and truncated ACPPVC systems ACPPVC-50, ACPPVC-55 and
ACPPVC-60 with acceptance angels of 50° and 0°, 55° and 0°, 60° and 0°, respectively. The PV absorber was 0.125m wide in all simulations. The geometrical characteristic of these systems are presented in TABLE 1.
TABLE 1: GEOMETRICAL CHARACTERISTICS OF THE UNTRUNCATED AND TRUNCATED ACPPVC SYSTEM Parameters Untruncated50 Truncated50 Untruncated55 Truncated55 Untruncated60 Truncated60 Acceptance-half angles
50? & 0?
-
55? & 0?
-
60? & 0?
-
Absorber width
125(mm) 125(mm) 125(mm) 125(mm) 125(mm) 125(mm) Aperture width 417.2(mm) 277.9(mm) 383.0(mm) 248.5(mm) 353.7(mm) 223.3(mm) Length of R1 269.5(mm) 269.5(mm) 204.5(mm) 204.5(mm) 155.3(mm) 155.3(mm) Length of R2
664.3(mm)
229.2(mm)
560.4(mm)
169.8(mm)
494.6(mm)
125.9(mm)
Concentration ratio
3.34 2.22 3.06 1.99 2.83 1.79
R1 0 0 0 0 0 0 Truncation
(%)
R2 0 65 0 69.7 0 74.5
2. RAY TRACE ANALYS I S AND OPT I CAL PERFORMANCE FOR THE SELECTED ACPPVC
SYSTEMS
All rays were assumed specular in the ray trace model, the
solar incidence angle (θ) was measured from the horizontal
as illustrated in Fig. 1. The aperture cover was 4mm thick
low iron glass with an extinction. coefficient of 4 m -1.The
reflectance of the reflectors was taken to be 0.98. The
analysis of angular acceptance and optical efficiencies used
10,000 rays incident on the glass aperture cover between 0°
and 90° at 1° intervals. The ray trace analysis allowed both
angular acceptance function and the optical efficiency to be
determined.
Fig. 1: ACPPVC-55 system with acceptance-half angle of
55° and 0°.
From the simulations undertaken, the angular acceptance
was 100% in the range of incidence angles between the acceptance half angels for all ACPPVC systems. The highest predicted optical efficiency for all ACPPVC systems was 88.67%. Due to the angular acceptance and concentration ratio, the truncated ACPPVC-55 system was selected for the further research. A schematic illustration of the cross section of the reflector profiles illustrating the truncation investigated is shown in Fig. 1. Ray trace diagrams for the truncated ACPPVC-55 system for a selection of solar incidence angles are shown in Fig. 2. From Fig. 2 it can be observed that at incidence angles of 15°, 30°, 45° and 50° (from the horizontal), all the incident rays are incident on the PV cells. When the solar incidence
θ=15° θ=30° θ=45° θ=50° Fig. 2: I
llustrative ray trace diagrams for truncated
ACPPVC-55 system, 50 rays are shown for each diagram.
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angle is 45° and 50°, a local high intensity flux can be seen in the middle and lower part of the PV cell. This may lead to an increase in the local temperature of the PV cell, and potentially result in a decrease in the electrical conversion efficiency. Decreasing the solar incidence angle towards the horizontal, more rays are reflected onto the absorber by reflector 1, compared to reflector 2. The angular acceptance and optical efficiency of the truncated ACPPVC-55 system are shown in Fig. 3.
Fig. 3: Angular acceptance and optical efficiency for the
truncated ACPPVC-55 system.
3. DESCRPTION OF THE THERMAL MODEL
The thermophysical behaviour of the truncated ACPPVC-55 system has been evaluated using a validated ‘comprehensive unified’ model for optics and heat transfer in line-axis solar system [4]. The schematic diagram of the truncated ACPPVC-55 system with its boundary condition is shown in Fig. 4. Based on the experimental system, the exterior of the reflectors were set to adiabatic boundaries in the model. For natural convection, the heat transfer coefficient is between 2.8W/m 2K and 5.7W/m 2K [5]. Assuming natural convection occurs at the rear of the aluminium back plate, a value of 5 W/m 2K was imposed at its boundary. For forced convection, assuming a wind velocity of 2 to 3m/s, the heat loss coefficient will be in the range from 8.8W/m 2K to 16.1W/m 2K [5]. Assuming forced convection occurs at the exterior of the aperture cover glass, a value of 12 W/m 2K was imposed for the aperture glass heat loss coefficient. The ray trace technique was used to predict the solar energy flux across the PV cells and is shown in Fig. 5. The incident solar radiation was 1000W/m 2. From Fig. 5, it can be seen that when the
Fig. 4: Schematic diagram of truncated ACPPVC-55
systems showing boundary conditions.
Fig. 5: Energy distributions across the photovoltaic cell of
the truncated ACPPVC-55 System for selected solar incidence angles, the incident solar radiation intensity was 1000W/m 2. solar incidence angle was 15°, two peaks occur in the solar flux on the PV cells, and the PV cell near the upper and lower reflectors has a higher solar flux than that in the central region. This distribution is a combination of the direct radiation from the sun and reflected radiation from the upper and lower reflectors. For the 30° solar incidence angle, the energy distribution has the same characteristics as that for the 15° solar incidence angle, two peaks in solar
Conduction in all boundaries: PV ,
reflector, aperture cover and back plate
Reflector 1 (R1): Specified boundary conditions adiabatic
Reflector 2 (R2): Specified boundary conditions adiabatic
A l u m i n i u m b a c k p l a t e : s p e c i f i e d b o u n d a r y c o n d i t i o n s h c =5W /m 2K 4mm Low glass aperture cover: Specified boundary conditions h c =12W/m 2
K
elec
=15%
Optical
Angular acceptance
0.02
0.040.060.080.10.12
Distance across solar cell (from upper reflector) (m)
A
b s
o r b e d
e n e r g y
(W /
m 2
)
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flux are found on the PV cells. For the 40°, 45° and 50°
solar incidence angles, a single peak solar flux occurs on the PV cell, due to solar radiation being reflected from the lower reflector only. The finite element mesh applied for
this system is shown in Fig. 6(a). The thermophysical properties of the materials used in the simulation are listed in TABLE 2. The ambient temperature was assumed to be 293K.
System dimension (m)
S y s t e m d i m e n s i o n (m )
(a) (b)
(c) (d)
Fig. 6: (a) Finite element mesh employed for the simulation, (b) Preidicted isotherms at 1000W/m 2 radiation and 15° solar
incidence angle, (c) Preidicted isotherms at 1000W/m 2 radiation and 30° solar incidence angle, (d) Preidicted isotherms at 1000W/m 2 radiation and 45° solar incidence angle.
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TABLE 2: THERMOPHYSICAL PROPERTIES ASSUMED
IN THE SIMULATION
Component Material Properties Aperture
Low-iron glass Thermal conductivity: 1.05W/m K
Thickness: 4 mm Reflector
Alanod Miro4270 AG
Thermal conductivity: 238W/m K
Thickness: 0.5 mm Reflectance: 0.98
Absorber
Silicon Solar cells Thermal conductivity:
148 W/m K Thickness: 0.3 mm
Absorptance :1
Back plate Aluminium
Thermal conductivity:
238W/m K
Thickness: 8 mm
4. PRED I CTED THERMOFLU I D BEHA V I OUR FOR THE TRUNCTED ACPPVC-55 SYSTEM Simulations were undertaken for 1000W/m 2 solar radiation at solar incidence angles of 15°, 30° and 45° (from the horizontal). Predicted isotherms at 2°C interval for the truncated ACPPVC-55 system at the selected incidence angles are shown in Fig. 6 (b), (c) and (d), respectively. The predicted average PV surface and aperture glass temperatures are shown in TABLE 3. From TABLE 3, it can be observed that with increasing solar radiation incidence angle, the average PV surface temperature and the average aperture cover temperature decrease, this is due to the incident solar radiation intensity at the aperture reducing, due to the increasing cosine effect with increasing solar incidence angles.
5. CONCLUSION
A detailed analysis of the optical and thermal behaviour of untruncated and truncated ACPPVC systems have been undertaken. The angular acceptance was 100% within the range of incidence angles between the half acceptance
angles for the untruncated and truncated systems. Due to reflection of radiation at the reflectors, significant peak solar flux intensities were found on the PV cells at some incidence angles. With increasing solar incidence angle, the average temperature of both PV cells and aperture cover decrease.
TABLE 3: A VERAGE PREDICTED PV SURFACE AND APERTURE COVER TEMPERATURE AT DIFFERNET SOLAR INCIDENCE ANGLES
Solar incidence angle (°)
Average PV surface temperature(°C) Average aperture
cover temperature (°C) 15 87.7 38.3 30 81.1 36.2 45 70.4 32.9
6. ACKNOWLEDGMENTS
This work was supported by the School of Engineering, University of Warwick through a Departmental Scholarship to Yupeng Wu.
7. REFERENCES
(1) Boyle, G (2004) Renewable Energy: Power for a
Sustainable Future. Oxford U.K Oxford University Press.
(2) Rabl, A. (1976) Comparison of Solar Concentrators.
Solar Energy, V ol. 18, pp. 93-111.
(3) Winston, R., Mi?ano J. C. and Benítez, P. (2005)
Nonimaging Optics. London U.K Elsevier Academic Press.
(4) Eames, P. C., Smyth, M., Norton, B. (2001) The
Experimental Validation of a Comprehensive Unified Model for Optics and Heat Transfer in Line-Axis Solar Energy Systems. Solar Energy V ol 71, pp. 121-133.
(5) Duffie,J.A., and Beckman, W.A. (1991) Solar
Engineering of Thermal Process. John Wiley and Sons, New York. USA.
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