Hybrid Photovoltaic Thermal Collector (PVT) for the ...

Proceedings of the 7th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING (ICOSSSE '08)

Hybrid Photovoltaic Thermal Collector (PVT) for the Production of Hot Water and Electricity Adnan Ibrahim, K. Sopian, M.Y.Othman, M.H.Ruslan, M.A.Alghoul, M. Yahya, and Azami Zaharim Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor. MALAYSIA Abstract: - The main concept of developing the hybrid Photovoltaic/Thermal (PV/T) is to increase the efficiency of the solar and thermal collector. It is known that the efficiency of the Photovoltaic solar collector is decreases when the ambient temperature increased and vice verse. In this systsem, the heat, gain from the sun, is absorbed to the PV panel and convert it to electricity energy. Some of this energy is then transfered to the absorber collector underneath the PV module as a waste energy. Special design configurations of absorber collector has been designed, investigated and compared. Simulation has been performed to determine the absorption of the heat that influences the total efficiency of the system under certain range of conditions. The collector is represented as a flat plate thermal collector with single glazing sheet. The simulation results of the system shows that at certain particular time, it can generates the total efficiency of 63.55% with electrical efficiency of 11.39% at ambient temperature set between 21 to 36°C, fluid flow rate at 0.02kg/s and solar radiation between 700 to 800 W/m2 .(Typical clear day) Key-Words: - Hybrid Photovoltaic Thermal (PVT), Thermal and electrical efficiency, Absorber collectors design of the channel below the transparent PV, with PV-on-sheet and tubes design gives the best efficiency overall. Bergene and Lovvik [1] perform theoretical examination of a flat plate solar collector model that integrated with solar cells. They developed a series of algorithms for making quantitative predictions for both the electrical and thermal efficiency of a PVT system. They conclude that the system combination of both produced approximately about 60-80% efficiency. Huang, Lin et al. [8] have developed PV/T system using a polycrystalline solar PV module, adopted to be combined with a collector plate. The collector plate is directly contact with the commercial PV module using the thermal grease, for better contact. Underneath the collector, a PU thermal insulation layer is attached using a fixing frame. The collector was designed using the corrugated plate made of polycarbonate material. The water was flowed in the flow channel of the corrugated plate structure. He, Chow et al. [7] recently studied the hybrid PVT system which used natural convection to circulate the water, adopting a flat-box absorber design. They found that their system showed a combined efficiency of 50%, with the daily thermal efficiency contributing approximately 40%. Although they found that the thermal efficiency was less than a conventional

1 Introduction The inspiration of combining photovoltaic and solar thermal collectors (PVT collectors) to provide electrical and heat energy is not new, however it is an area that has received only limited attention. With concern growing over energy sources and their usage, PVTs have become an area which is receiving much more attention. Research in this field was carried out in the middle of 1970s to early 1980s. As mentioned by Zondag, De Vries et al. [15], the first inventor on the flat-plate PVT liquid system was carry out by the work of Martin Wolf [13] who analyzed the performance of combining the heating and photovoltaic power systems for residences. He concluded that the system was technically feasible and cost effective. In his experiment, the Hottel-Whillier model is used to analysis of combined photovoltaic/thermal flat plate collectors combined the traditional hot water system with the PV panel to minimize the usage of the installation area. It is proven that by combining the system, the installation area produce more energy per unit surface area than one PV panel and one thermal system. Florschuetz [5] Zondag [14] examined the various concepts of combined PV-thermal collector technologies. They introduced and evaluated nine different designs, ranging from the complicated to the simpler one, in order to investigate the maximum yield. They conclude that the

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β

thermosyphon solar water heater they note that the energy saving efficiency was much greater. Chow, Hand et al. [3] further investigate the hybrid PVT system of He, Chow et al. [7], developed a dynamic thermal model, which was based on a 260 m2 mono-crystalline silicon PV wall on a 30-storey hotel building. They suggested that this system could be improved by placing the PV cells on the lower portion of their collector. They noted that there was a larger temperature gradient between the water entering the thermosyphon tubes and the PV cells in this region and so the efficiency of the electrical and thermal could be improved by placing the cells there.

τα

η th ηr

In order to obtain the best results, proper design factors need to be considered. As mention by Ji, Lu et al. [9], the dual requirement of good thermal conduction and good electrical insulation between the solar cells and absorber collector need to be considered. The absorber collector conceptual design, as shown in Fig 1, is made of rectangular hollow tubes of stainless steel material. The tubes, with dimension of 12.7 x 12.7 x 1000 mm are assembled and connected together using a welding method.

m2 W/(m K)

m

Function of the collector area Conductance of the bond between the fin and square tube Specific heat of the collector cooling medium Hydraulic diameter Fin efficiency factor Corrected fin efficiency Heat removal efficiency factor Measured incoming solarirradiation on the collector surface. Solar radiation at NOCT (irradiance level 800 W/m2, wind velocity 1 m/s, ambient temperature 26°C) (06:00 to 20:00) Heat transfer coefficient of fluid Absorber thermal conductivity Photovoltaic thermal conductivity Absorber thickness PVT collector thickness System flow rate Mass flow rate

m m Kg/s Kg/s

Qu S Tc

Actual useful heat gain Absorbed solar energy Temperature of the solar cells

W/m2 W/m2 K

Ta To Ti UL

Ambient temperature K Fluid Outlet temperature Fluid inlet temperature K Overall collector heat loss W/m2 K coefficient Tube spacing Greek symbols

Cp dh F F’ FR G GT

hfi Kabs Kpv Labs Lpvt M .

W

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Reference efficiency of PV module (0.12)

2 Arrangement of flow pattern

Nomenclature Ac Cb

Temperature °C −1 coefficient( 0.0045 °C ) Average transmittanceabsorptance of the collector PV/T thermal efficiency

J/kg K m

W/m2

W/m2 K

Fig. 1 Spiral Flow Design

W/(m K) W/(m K)

The absorber collector is produced with near-flat in surface. This is to ensure that the collector is always parallel to the surface underneath the PV module. Once welded and assembled, the flat absorber collector is attached closely to the PV module with metallic bonded; this will ensure zero gaps or no gap at all between the tubes and the module, in which heat can be transferred. Rockendorf, Sillmann et al. [10] and later Chow [2] has discovered that the fin efficiency and bonding quality between the collector and the sheet underneath the cell (module), have been identified as the crucial factors, which often bring limitations to the overall efficiency achievable. Subsequently, the absorber collector is equipped with a pair of inlet and outlet on the opposite ends of the rectangular hollow tubes, these enable the trapped air in the absorber to be released during the cycle. One of the interesting features about the design is that the water fluid in the tubes is

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circulating in reverse after reaching the centre of the collector. This creating a better efficiency compared to the system with only one water fluid circulating on it. As the PV cells in PV module are exposed the sun, it generates electricity and at the same time absorbed the heat. As a result the cells temperature increases as they absorb radiation. Then the heat is transferred from the cells to the absorber, causing the absorber to increase it temperature. During this time, the water fluid passing inside the absorber tubes is heated by the contact underneath the PV module. The water is then flows along the absorber through a manifold and pipes and finally fed to a water tank. The system is considered to be a closed loop system, where water is fed continuously so that the fresh and cooler water that enters the hollow tubes is heated and so on. The result of the continuous operation of the flowing water consequently reduces the temperature of the PV cells and simultaneously increasing it efficiency (preferable for poly crystalline silicon cells usage). A schematic overview of the system is shown in Fig 2.

A

Converter/ Inverter

many system design parameters and operating conditions. In this simulation, the system is analyzed with various configurations of solar radiation, ambient temperature, and flow rate conditions. Also, it is assumed that the collector is represented as a flat plate thermal collector with single glazing sheet. Through this, the thermal performance η th of the PVT unit is evaluated for its thermal and photovoltaic performance, as such, the derivation of the efficiency parameters based on the Hottel-Whillier equations were used. Description Ambient temperature Inlet fluid temperature Tube length Collector width Tube diameter No of tube Tube spacing Collector parameter Collector area Number of glass cover Emittance of glass Emittance of plate Tilt (slope) Fluid flow rate Fluid thermal conductivity Specific heat Back insulation conductivit Back insulation thickness Insulation conductivity Edge insulation thickness Absorber conductivity Absorber thickness Fin conductivity Fin thickness Heat transfer coefficient fr to absorber Heat transfer inside tube Transmittance Absorptance

Grid connected

V

Thermal Storage Tank

Load

DATA ACQUISITION SYSTEM Thermal Storage

Auxiliary Heater

• • • • • • • • •

Anemometer Pyranometer Humidity sensor Thermocouples DC Current transducer DC Voltage transducer Flowmeter Thermostat I-V

Data Logger 24ch

Fig. 2 Schematic diagram of the system

3

PVT collector analysis

Ta Ti L b D n W P Ac N εg εp º mdot kfluid Cp kb Lb ke Le kabs Labs kf δ hca

Value 293 293 1 0.65 0.0127 1 0.0327 3.3 0.65 1 0.88 0.95 14 0.02 0.613 4180 0.045 0.05 0.045 0.025 51 0.002 84 0.0005 45

Units K K m m m m m m m²

kg/s

W/mºK m W/mºK m W/mºK m W/mºK m W/mºK

333 W/mºK hfi 0.88 τ 0.95 α Table 1 Photovoltaic-thermal collector characteristics

The performance of PVT collectors can be depicted by the combination of efficiency expression. He, Chow et al. [7]. It comprised of the thermal efficiency ηth and the electrical efficiencyη e and these efficiencies usually include the ratio of the useful thermal gain and electrical gain of the system to the incident solar irradiation on the collector’s gap within a specific time or period. The analytical characteristics of the PVT collector are presented in Table 1. The total of the efficiencies, which is known as total efficiency η o is used to evaluate the overall performance of the system. i.e.: η o = η thermal + η electrical (1) The thermal performance of the PVT is affected by

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Symbol

The thermal efficiency (η th ) of the conventional flat plate solar collector is calculated the formula below: ηth =

Qu

(2)

G

Under these conditions, the useful collected heat (Qu) is given by: .

(3)

Qu = m C p (To − Ti )

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The difference between the absorber solar radiation and thermal heat losses is identified by H.C Hottel [6]: Qu = Ac FR [S − U L (Ti − Ta )]

4.1 Simulation results As mention by Chow, He et al. [4], sites the work by Zondag, de Vries et al. [16] have grouped the design concepts of water type PVT collectors into four main types, which are, sheet and tube, channel, free flow and two absorber collectors. It is collectors have been identified as pump circulation types of collectors. From the viewpoint of the author, the single glazing sheet and tube hybrid PVT collector as the best design for overall performance and structural simplicity. The simulations were performed to investigate the system efficiencies for the thermal, cell and overall efficiency. Due to that statement, the simulation is performed using the single glazing sheet and tube hybrid PVT collector. In this simulation, the specific operation condition is set from 06.00 hours till 20.00 hours. Data for the ambient temperature were collected from local weather station shows that at 06.00 hours, the ambient temperature is at 21.49ºC and 26.49 ºC at 20.00 with the peak at 36.11 ºC at 14.00 hours. The average solar radiation level during the simulation period was 883.11 W/m2 respectively. In this simulation, the fluid flow rate is set to 0.02 kg/s is shown in Table 2. It can be seen that when Solar radiation (GT) is high (883.31 W/m2) and the Ti-Ta/G value is low, then the thermal efficiency is high and vice versa. The result of this simulation as seen in Fig 2 shows the cell efficiency against the Plate mean temperature (Tpm) and Fig 3 shows the efficiency plot of thermal and overall against reduced temperature. The regression lines were found as follows:

(4)

From the Esq. (3), the S is identified as: S = (τα ) PV GT

(5)

The heat removal efficiency factor (FR) can be calculated as: . ⎡ ⎡ ⎤⎤ mCp ⎢ AU F' FR = 1 − exp ⎢ − c. L ⎥ ⎥ ⎢ ⎥⎥ AcU L ⎢ ⎢⎣ ⎣⎢ m C p ⎦⎥ ⎥⎦

(6)

The corrected fin efficiency (F’) is calculated using: 1 ⎤ ⎡ ⎥ ⎢ 1 1 UL ⎥+ F =⎢ + ( ) ( + U d W d F Cb a b)h fi ( + − ) 2 ⎥ ⎢ L h h PV ⎥ ⎢ ⎦ ⎣ '

(7)

The fin efficiency factor F is then be calculated as: ⎛ ⎛ W − dh ⎞ ⎞ tanh ⎜⎜ m ⎜ ⎟ ⎟⎟ ⎝ ⎝ 2 ⎠⎠ F = ⎛ W − dh ⎞ m⎜ ⎟ ⎝ 2 ⎠

(8)

Where m: m=

UL K abs .Labs + K PV .LPV

(9)

Cell Efficiency

From these equations, it is then possible to calculate the useful heat gain by the solar collector. By rearranging Eq.(2), the thermal efficiency of the collector is expressed as: (Vokas, Christandonis et al. [12]) Ti − Ta (10) GT For temperature-dependent electrical efficiency of the PV module, (η e ) Tiwari and Sodha [11]the expression is given as below:

η th = FR (τα ) PV − FRU L

ηe = ηr [1 − β (Tc − Tr )]

10 24

26

28

30

32

34

36

38

40

42

Tpm

Fig 2 The Cell efficiency versus Plate Mean Temperature of the PVT

(11)

4 Results and discussion

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Time

Fluid flow rate

Temperature (°C)

t

Ta

Ti

To

GT

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21.49 21.42 22.03 23.66 28.73 31.80 33.44 34.56 36.11 35.14 34.44 33.42 31.98 30.21 26.49

25.00 25.00 25.00 27.00 30.00 33.00 35.00 36.00 38.00 39.00 38.00 35.00 33.00 32.00 29.00

25.00 25.00 25.24 27.91 32.71 36.94 40.07 41.52 43.42 43.47 41.45 36.86 33.95 32.11 28.93

5.27 5.63 56.89 165.90 437.13 631.40 812.72 883.31 871.43 734.70 570.34 304.93 157.70 28.81 5.98

mdot (kg/s) 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

Collector effiy. factor

Collector heat removal factor

Useful Heat Gain

F'

FR

Qu

(Ti-Ta) /G

ηt

ηc

ηo

0.9869 0.9869 0.9869 0.9868 0.9865 0.9862 0.9861 0.9860 0.9859 0.9860 0.9861 0.9861 0.9862 0.9864 0.9866

0.9665 0.9665 0.9664 0.9661 0.9653 0.9647 0.9644 0.9642 0.9639 0.9641 0.9642 0.9644 0.9647 0.9650 0.9656

0.0 0.0 19.8486 76.1889 226.6088 329.3261 423.6781 461.2735 453.1458 373.7999 288.2044 155.3181 79.6294 0.0 0.0

0.0 0.0 0.0523 0.0201 0.0029 0.0019 0.0019 0.0016 0.0022 0.0053 0.0062 0.0052 0.0065 0.0 0.0

0.0 0.0 34.887 45.924 51.840 52.158 52.131 52.221 52.000 50.878 50.532 50.936 50.495 0.0 0.0

0.0 0.0 11.981 11.844 11.603 11.387 11.229 11.155 11.052 11.039 11.138 11.370 11.518 0.0 0.0

0.0 0.0 46.868 57.767 63.443 63.545 63.360 63.377 63.052 61.917 61.670 62.306 62.013 0.0 0.0

Efficiencies

Table 2: Results of the simulation tests The efficiency:

4 Conclusion

η thermal

=

η cell

=

η overall

=

Ti − Ta GT Ti − Ta 11.24 – 11.39 GT Ti − Ta 57.77 – 63.55 GT

For the above case, the thermal efficiency of 52.16% under the zero reduced temperature condition, with a corresponding cell efficiency of 11.39%, is very much encouraging, based on the simplicity of this type of design. It is recommended that the PVT collector can be further improved by using other types of cell such as the amorphous silicon cell that provided higher thermal absorption due to it black mat surfaces. Besides that, by minimizing the gap between the absorber and cell, can improve the efficiency by direct contact between both, the absorber and the cell.

45.92 – 52.16

References: [1] Bergene, Trond, and Ole Martin Lovvik. Model calculations on a flat-plate solar heat collector with integrated solar cells. Solar Energy 55 (6):453462.1995 [2] Chow, T. T. Performance analysis of photovoltaicthermal collector by explicit dynamic model. Solar Energy 75 (2):143-152.2003 [3] Chow, T. T., J. W. Hand, and P. A. Strachan. Building-integrated photovoltaic and thermal applications in a subtropical hotel building. Applied Thermal Engineering 23 (16):20352049.2003 [4] Chow, T. T., W. He, and J. Ji. Hybrid photovoltaic-thermosyphon water heating system for residential application. Solar Energy 80 (3):298-306.2006 [5] Florschuetz, L. W. Extension of the hottel-whillier model to the analysis of combined

Fig 3 Variation of thermal and overall efficiency of the PVT collector with reduced temperature.

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photovoltaic/thermal flat plate collectors. Solar Energy 22 (4):361-366.1979 [6] H.C Hottel, A. Willier. Evaluation of flat-pate solar collector performance. Transactions of the Conference on the Use of Solar Energy vol 2 (University of Arizona Press, Tucson, Arizona).1958 [7] He, Wei, Tin-Tai Chow, Jie Ji, Jianping Lu, Gang Pei, and Lok-shun Chan. Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water. Applied Energy 83 (3):199210.2006 [8] Huang, B. J., T. H. Lin, W. C. Hung, and F. S. Sun. Performance evaluation of solar photovoltaic/thermal systems. Solar Energy 70 (5):443-448.2001 [9] Ji, Jie, Jian-Ping Lu, Tin-Tai Chow, Wei He, and Gang Pei. A sensitivity study of a hybrid photovoltaic/thermal water-heating system with natural circulation. Applied Energy 84 (2):222237.2007 [10] Rockendorf, Gunter, Roland Sillmann, Lars Podlowski, and Bernd Litzenburger. Pv-hybrid and thermoelectric collectors. Solar Energy 67 (46):227-237.1999 [11] Tiwari, Arvind, and M. S. Sodha. Performance evaluation of hybrid pv/thermal water/air heating system: A parametric study. Renewable Energy 31 (15):2460-2474.2006 [12] Vokas, G., N. Christandonis, and F. Skittides. Hybrid photovoltaic-thermal systems for domestic heating and cooling--a theoretical approach. Solar Energy 80 (5):607-615.2006 [13] Wolf, Martin. Performance analyses of combined heating and photovoltaic power systems for residences. Energy Conversion 16 (1-2):7990.1976 [14] Zondag, H. A. Flat-plate pv-thermal collectors and systems: A review. Renewable and Sustainable Energy Reviews 12 (4):891-959.2008 [15] Zondag, H. A., D. W. De Vries, W. G. J. van Helden, R. J. C. van Zolingen, and A. A. van Steenhoven. The thermal and electrical yield of a pv-thermal collector. Solar Energy 72 (2):113128.2002 [16] Zondag, H. A., D. W. de Vries, W. G. J. van Helden, R. J. C. van Zolingen, and A. A. van Steenhoven. The yield of different combined pvthermal collector designs. Solar Energy 74 (3):253269.2003

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