Performance of a double pass photovoltaic thermal solar collector suitable for solar drying systems

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Abstract

The photovoltaic thermal solar collector, sometimes known as the hybrid solar collector generates both thermal and electrical energies simultaneously. A double pass photovoltaic thermal solar collector suitable for solar drying applications has been developed and tested. A steady state closed form solution to determine the outlet and mean photovoltaic panel temperature has been obtained for the differential equations of the upper and lower channels of the collector. Hence, the photovoltaic, thermal and combined photovoltaic thermal efficiencies can be obtained. An experimental setup has been developed. For given sets of operating and design conditions the theoretical and experimental outlet and mean photovoltaic panel temperatures can be obtained. Comparisons are made between the experimental and theoretical results and close agreement between these two values are obtained.

Introduction

Changing to clean energy sources such as solar energy would enable the world to improve the quality of life throughout the planet Earth, not only for humans, but also for its flora and fauna as well. Therefore, there is a need to develop an ingenious method of solar energy conversion systems and then to substitute it where applications of fossil fuels are most vulnerable. One of the ingenious methods of solar energy conversion systems is the photovoltaic thermal solar collector or hybrid solar collector, which converts solar radiation directly to both thermal and electrical energies. It is very attractive for solar applications in which limited space and area related installation cost are of primary concern. The hybrid collector is also attractive when the space needed to install side-by-side solar thermal and photovoltaic collectors is not readily available. In a photovoltaic solar collector, the exposed surface (the glass cover or absorber plate) is partially or completely covered by photovoltaic cells, while a circulating air stream passes the rear and/or front sides of the collector, carrying away excess heat and thus maintaining the cell at an optimum operating temperature. Studies were limited only to simple single pass configurations [1], [2], [3], [4], [5], [6]. The design includes an air collector with the air flow passage between two metallic plates. The upper metallic plate was painted black, and the photovoltaic cells were pasted over it. The material used to paste the cells on the absorber plate should be a thermal conductor but also an electrical insulator.

A double pass system that can produce more heat while simultaneously having a productive cooling effect on the photovoltaic cells was proposed [7]. For the double pass collector, air first enters the flow channel formed by the glass cover and the photovoltaic panel. Next, it enters the flow channel formed by the photovoltaic panel and the back plate. This flow arrangement affects greater heat removal from the photovoltaic panel and also reduces the heat loss from the collector. This paper presents the theoretical and experimental studies on the double pass photovoltaic solar collector suitable for solar drying applications.

Section snippets

Theoretical background

Fig. 1 shows the double pass hybrid solar collector with the heat transfer coefficients. The energy balance equation at the glass cover isαgS=hrgsTg−Ts+hcgwTg−Tw+hrp1gTg−Tp1+hcf1gTg−Tf1The energy balance equation for the first air flow channel ismCpd1dTf1dx=hcf1gTg−Tf1+hcp1f1Tp1−Tf1The second air flow energy balance equation in the lower channel ismCpd2dTf2dx=hcp1f2Tp1−Tf2+hcp2f2Tp2−Tf2The energy balance equation on the photovoltaic panel isτgp11−Pgp1P1−ηpv−hrp1p2Tp1−Tf2−hrp2gTp1−Tg−h

Experimental setup

The experimental setup is shown in Fig. 2. The basic components are as follows: (i) the double pass hybrid solar collector, (ii) the support structure, (iii) the air flow measurement system, (iv) the temperature measurement system, (v) the wind speed measurement system, (vi) the solar radiation measurement system and (vii) the data acquisition system.

The photovoltaic panel has a width of 660 mm and a length of 1,476 mm. The total area of the panel covered by photovoltaic cells is 0.8505 m2.

Experimental procedures

Air is circulated for thirty minutes prior to the period in which data are taken. Data are sampled and averaged for a time period specified by the user using the application program of the data acquisition system. Data include the global solar radiation, temperatures (inlet, end of the first pass, outlet, ambient, cover (top and bottom), photovoltaic panel and back plate), mass flow rate and wind speed. The depths for the upper (d1) and lower (d2) channels are fixed at 5 cm. The mass flow rate,

Results and observations

The diurnal variation of the performance of the hybrid collector are shown in Fig. 7, Fig. 8. The diurnal performance can be presented by plotting the variations in the total power incident on the collector, the useful energy collected and the electricity generated by the photovoltaic panel for the day. Theoretical and experimental results are shown.

Fig. 9 shows the effect of temperature rise on the combined photovoltaic thermal efficiency at various flow rates and solar radiation levels.

Conclusions

Theoretical and experimental studies on the double pass photovoltaic thermal solar collector have been conducted. In this type of collector, both thermal and electrical energies are produced simultaneously. In addition, the packing factor or the area fraction covered by photovoltaic cells can be adjusted according to the requirement for electrical energy. Advances in the production methods of photovoltaic cells will reduce their initial cost and hence increase the demand. Under such favorable

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