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Peer-review under responsibility of the scientific committee of the 8th International ... Keywords:CPC-PV/T, thermoelectric, constant temperature, constant flow. 1.
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ScienceDirect Energy Procedia 105 (2017) 869 – 874

The 8th International Conference on Applied Energy – ICAE2016

Experimental study of constant temperature operation and constant flow operation in concentrating PV / T system Zhang Henga, *, Li Mingjiea , Chen Haipinga, Ye Chentaoa a

School of Economics and Management, North China Electric Power University, Beijing 102206, China

Abstract Low-concentrating solar photovoltaic thermal (PV / T) system combines the solar cell module with a solar collector which is aimed at converting solar energy into both electricity and thermal energy. It can make good use of diffuse radiation and performs well under lower solar radiation. In this paper, a CPC-PV / T system which coupled PV / T collectors with the CPC was designed. And the PV / T collector was made by bonding and laminating polycrystalline silicon cells and flat cassette heat exchanger. Experimental study and analysis has been made on constant temperature operation and constant flow operation of this system according to first law of thermodynamics and second law of thermodynamics. Finally, the advantages and disadvantages of two operation modes have been analyzed to optimize the operation and further improve the efficiency of the system. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2017 2016The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.

Keywords:CPC-PV/T, thermoelectric, constant temperature, constant flow

1.Introduction Solar energy is the most abundant renewable energy source available on earth. There are several ways to harness solar energy, among which photovoltaics/thermal (PV/T) technology has received popular recognition. In the respect of numerical simulation, Garg and Agarwal established a steady state model of the water cooled PV/T panel collector to analyze the correlation between area of photovoltaic modules and photovoltaic efficiency [1]. Florschuetz has presented an extension of the Hottel-Whillier model to analyze the tube-sheet PV/T collectors by assuming a liner correlation between efficiency of cell and temperature [2]. Rosell et.al demonstrated a double axes tracking trough PV/T system with concentrators whose concentrating ratio was 11[3]. G Fraisse et.al studied PV/T system with glass covers and made comparative study of various domestic heating systems including a direct solar floor, PV and PV/T[4]. Kostic. Lj.T et.al analyzed the effects of concentrators on efficiencies of tube-sheet PV/T collectors and

*Zhang Heng. Tel.: +86 185 1014 6466; E-mail address: [email protected].

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.403

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calculated the highest energy-saving efficiencies. Their results showed that the efficiency was highest when angle of concentrator was 45° and that energy-saving efficiencies are respectively 60.1%and 46.7% with and without concentrators [5]. Ahmer Baloch et.al compared effects of different pass dip angles on the uniformity of surface temperature through CFD simulation. The results showed that in summer, the power output characteristics of panels with cooling system increase largest to an amount of 41% [6]. Ji Jie carried out theoretical calculation and experimental study on micro (need not tracking) CPC-PVT system. Compared with flat PV-T, micro CPC-PVT has lower heat loss factor with highest temperature to 70ć and electrical efficiency about 10% [7]. Tripanagnostopoulos et.al made experimental studies on 12 different forms of water/air cooled PV/T system and found out water cooled PV/T system’s performances are better than those of air cooled system. [8]. F. Hussain et.al. conducted an experimental study on a new type air cooled PV/T collector with honeycomb heat exchanger. The improved design is suitable to be further investigated as solar drying system and space heating [9]. Bhattarai et.al measured and analyzed the performance of a photovoltaic thermal system, conventional collector, and PV modules at different water storage capacities [10]. Sun Jian et.al built an experimental test platform for a low-concentrating PV/T system with a single air pass Its highest exergy efficiency was 16%, 5% higher than that of systems pure for power generation [11]. M.Y. Othman et.al conducted laboratory experiments on a PV/T system combined with water and air cooling system. The electrical efficiency achieved was 17% with average electrical power of 145W and thermal efficiency accomplished was 76% [12]. Ilhan Ceylan et.al chose paraffin as the phase-change material to reserve latent heat in CPV/T system. At irradiance of 828 W/m2, efficiency was 11% and highest temperature of backboard was 37ć [13]. In this paper, a novel CPC-PV/T system has been presented. And experimental study and analysis has been made on constant temperature operation and constant flow operation. 2.CPC-PV/T System Concentrating PV/T system is mainly constituted by three parts: CPC concentrating system, solar photovoltaic system and solar-thermal system. The electric power of experimental facilities in this experiment is 1kW, as illustrated in Fig.1. 











Water ,



MPPT

9

Tracking device

$

3

Battery

Load Anemometer

Dual-axis tracking

Data Acquisition Module



1. Entrance tank 2. Boosters pump 3. CPC-PV / T (2 × 6) units 4. Flowmeter 5. Solenoid valves 6. Storage tank 7. Industrial Personal Computer 8. Measure and control unit Fig.1 Concentrating PVT system



The working process of PV/T system is as follows: water runs through to cool down photovoltaic cells then warmer water flows into heat preservation water tank to provide domestic water. Dual-axis tracking system will keep following sunlight driven by electric motor to ensure receiving maximum sunlight. MPPT controller’s role is to maintain current and battery voltage at the maximum power point to

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ensure constant battery charging. Measurement and control system will record data including light intensity, inlet water temperature, outlet water temperature, panel’s temperature, environment temperature, wind speed and current and voltage output by panels and fill them in forms. Interface diagram of measurement and control system is displayed in figure 2.

Fig.2 Interface diagram of measurement and control system

3.Mathematical model According to data obtained from experiments, photovoltaic power, thermal power, photovoltaic efficiency and thermal efficiency are as follows: Photovoltaic power:

(1)

Pe U m ˜ I m Thermal power: Qth

mc(Tw,out  Tw,in )

Photovoltaic efficiency: Pe Um ˜ Im Ke CI sol APV CI sol APV Thermal efficiency: Kth

Qth CI sol Ap

mc(Tw,out  Tw,in )

(2) (3) (4)

CI sol Ap

In the equation: Pe is electric power, Um is the maximum voltage, Im is the maximum current, Qth is thermal power, m is water’s mass flow, c is water’s specific heat, Tw,in is inlet water temperature, Tw,out is outlet water temperature, ηe is photovoltaic efficiency, ηth is thermal efficiency, C is concentration ratio, Isol is solar irradiance, APV is area of panel and AP is area of collector. Considered grade difference between electric energy and thermal energy, this paper also introduces exergy efficiency which is closely related to second law of thermodynamics to further evaluate this system. Exergy efficiency of a system refers to the ratio between the usable part of gained energy and the input to do work. In the thermal system, the usable part of gained energy is:

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Zhang Heng et al. / Energy Procedia 105 (2017) 869 – 874

(4

PZ ˜ F g > 7Z RXW  7Z LQ  7 ˜ OQ

7Z RXW @ 7Z LQ

(5)

In the electric system, the usable part of gained energy is:

(HO

8 P ˜ ,P

(6)

Usable part of solar radiation energy gained in the system:

E sol

\s

C ˜\ s ˜ I sol ˜ APV 4 T 1 T0 4 ˜ 1- ˜ 0  ˄ ˅ 3 Tsol 3 Tsol

(7) (8)

Thermal exergy efficiency:

[t

E Q

C ˜\ s ˜ I sol ˜ Ap

(9)

Electric exergy efficiency:

[ el

E el C ˜\ s ˜ I sol ˜ APV

(10)

Where: EQ and Eel are thermal exergy power and electric exergy power respectively. ψS accounts for solar radiation available coefficient. T0 represents environment temperature. Tsol is solar temperature (5760K). ξt and ξel are thermal exergy efficiency and electric exergy efficiency respectively. 4.Comparison of Constant Flow Operation and Constant Temperature Operation Performances of two operation modes are compared to determine the best working plan. The flow rate 210L/h and outlet temperature 40ć have been set.

Fig.3 Comparison of electric and thermal power in two conditions

Figure 3 shows comparison of electric and thermal power under two kinds of operations. As it shows, electric power of constant temperature operation is higher than constant flow operation before 15:30. After 15:30, electric power of constant temperature operation declines faster. And the electric power of constant temperature operation is higher than constant flow operation. Figure (b) indicates that change trends of two operations are close to each other. When irradiation intensity is weaker, thermal power of constant temperature condition declines faster while that of another

Zhang Heng et al. / Energy Procedia 105 (2017) 869 – 874

operation is relatively smooth. And the thermal power of constant temperature operation is higher than constant flow operation.

Fig.4 Comparison of electric and thermal efficient in two conditions

Figure 4 shows change trends of electric and thermal efficiency under two kinds of operations. The electric efficiency of constant temperature condition is higher than constant flow condition. The electric efficiency of constant temperature condition is higher than constant flow condition.The thermal efficiencies are basically the same before 15:00. Then as irradiation intensity declines, thermal efficiency of constant flow operation keeps rising. Although lower irradiation intensity leads to cooler outlet water, the flow rate of constant flow operation maintains the same which has larger effects on thermal efficiency. Constant temperature operation produces a higher thermal efficiency while constant flow operation provides a higher electric efficiency. According to first law of thermodynamics, the latter one owns a higher overall efficiency. But thermal and electric grades are different, exergy efficiency can be calculated based on equation˄5˅to ˄10˅and results are shown in figure 5:

Fig.5 The comparison of exergy efficiency in two kinds of conditions

Average exergy efficiency of constant flow condition and constant temperature condition are calculated as 7.732% and 8.761%, respectively. The latter’s energy utilization efficiency is higher. Moreover, peak power of constant flow condition cannot reach 1000w and its outlet temperature can be lower than 40ć. Thus constant temperature system with set temperature at 40ć is the final working plan. 5.Conclusions Based on experimental data analysis, constant flow operation’s thermoelectric performance increases as flow setting value rises while the increase rate will gradually decrease. As for constant temperature operation, the lower setting outlet temperature lead to the better thermoelectric performance. After comparison, it is found that constant temperature operation produces a higher thermal efficiency while

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constant flow operation provides a higher electric efficiency. However, thermal and electric grades are different. Analysis of the exergy efficiency of the systems has been conducted in accordance with the second law of thermodynamics and constant temperature operation owns higher exergy efficiency. In the end, it is recommended to choose constant temperature system with set temperature at 40ć. Acknowledgements This work was financially supported by Beijing Municipal Science and Technology Project(Z151100003515002). References [1] Garg H P, Adhikari R S. Performance analysis of a hybrid photovoltaic/thermal (PV/T) collector with integrated CPC troughs [J]. International Journal of Energy Research, 1999, 23:1295-1304. [2] FLORSCHUETZ L W. Extension of the Hottel-Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors[J]. Solar Energy, 1979, 22(4); 361-366. [3] Rosell J I ˈ VallverdúX ˈ Lechón M A etal ˊ Design and simulation of a lowconcentrating photovoltaic/thermal system[J].Energy Conversion and Managementˈ2005ˈ46 (18-19)˖3034-3046. [4] Fraisse, G., C. Ménézo, and K. Johannes, Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type. Solar Energy, 2007. 81(11): p. 1426-1438. [5] Kostic. Lj. T, Pavlovic. T.M, Pavlovic .Z.T. Optimal design of orientation of PV/T collector with reflectors[J]. Applied Energyˈ2010ˈ(87): 3023-3029. [6] Baloch Ahmer A.B.; Bahaidarah Haitham M.S.; Gandhidasan Palanichamy. Experimental and Numerical Performance Analysis of a Converging Channel Heat Exchanger For PV Cooling [J]. ENERGY CONVERSION AND MANAGEMENT ,2015,103:14-27. [7] Li, G., et al., Numerical and experimental study on a PV/T system with static miniature solar concentrator. Solar Energy, 2015. 120: p. 565-574. [8] Tripanagnostopoulos,NousiaTH,SouliotisM,etal.Hybridphotovoltaic/thermal solar systems.Solar Energy,1972(3):217-234. [9] F. Hussain, M.Y.H. Othman, B. Yatim, H. Ruslan, K. Sopian, Z. Anuar,S. Khairuddin. An improved design of photovoltaic/thermal solar collector [J]. Sol. Energy ,2015,122:885-891. [10] Sujala Bhattarai,Gopi Krishna Kafle,Seung-Hee Euh,Jae-Heun Oh,Dae Hyun Kim. Comparative study of photovoltaic and thermal solar systems with different storage capacities: Performance evaluation and economic analysis[J]. Energy,2013,61:272-282. [11] Sun J, Wang YX, Shi MH. Experimental study on a CPC concentrating solar PV/T system[J]. Acta energiae solaris sinica,2012.33:86-91. [12] Othman, M.Y., et al., Performance analysis of PV/T Combi with water and air heating system: An experimental study. Renewable Energy, 2016. 86: p. 716-722. [13] Fraisse, G., C. Ménézo, and K. Johannes, Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type. Solar Energy, 2007. 81(11): p. 1426-1438.

Biography Zhang Heng is a Ph.D candidate in North China Electric Power University. His research interests focus on the development ,and analysis ,optimization of photovoltaic/thermal collector.