E. Erdila, M. Ilkanb, F. Egeliogluc, and L. B. Y. Aldabbagh c

Electrical Engineering Dept., Eastern Mediterranean University, Magusa, Mersin 10, TURKEY

School of Comp. and Tecnology., Eastern Mediterranean University, Magusa, Mersin 10, TURKEY

Mechanical Engineering Dept., Eastern Mediterranean University, Magusa, Mersin 10, TURKEY

INTRODUCTION

Solar thermal collectors are commercially widespread in many countries. Cyprus, an Eastern Mediterranean island, has relatively high rates of solar insolation throughout a year. Summer days are rather warm and prolonged and the winter is fairly mild. The daily average sunny periods range from 5.5 -12 hours, [1] through the seasons, are shown in Fig. 1. The daily average global radiation is minimum during the months of December and January, at about 2.3 kWh/m2 (Fig. 2). The maximum is about

7.2

kWh/m2 in the months of June and July. Annual averages of daily sunny periods

Fig. 2 Daily sunny hours for each month (1981-1991)

and daily global radiation are about 9 hours and 5 kWh/m2 respectively.

Solar energy has been widely utilized in Cyprus for at least the past 40 years, [2]. People have enjoyed using solar energy for domestic water heating at no fuel cost. The average daily energy consumption of a typical household in N. Cyprus is about 7 kWh,

[3] as shown in Fig. 3. This amount of energy can be produced in electrical form, with a 1600 Watt PV system occupying an area of about 10 m2, [4].

The photovoltaic (PV) modules available in markets are now well developed. The trend is to generate even more power per m2 at a lower cost. The increase in demand and lowering of prices makes them even more attractive and relatively large systems are now installed at moderate prices. The growing worldwide awareness of the benefits of using clean energy has greatly increased the sales of PV systems by many folds in the last 10 years, [5].

C* Fax No.: +90392 3653715, loay. aldabagh@emu. edu. tr

Generally, depending on the type of solar cell used, some of the 70-95% of the collected energy is not converted to electricity. Hence, large area solar modules absorb considerable amounts of solar radiation that also generate excessive heat. This excessive heat leads to lowering of module efficiency, [6, 7]. Its found that the open — circuit voltages decreases by about 0.4% per centigrade (o C) for silicon cell [8], and hence the fill factor also decreases with increasing temperature. Many systems have been designed to remove this excessive heat and to utilize it as a source of energy for some other applications. This however requires some modification of the module structure to allow circulating a cooling medium with an aim to extract the excessive heat. Usuallly, the heat transfer fluid is either air or water. Use of water requires more extensive modifications to enable water-tight and corrosion free construction.

Such a hybrid module may combine the electricity generating function of a photovoltaic module with that of a hot-water collector, a system for space heating and any other low-temperature process. Performance of many different kinds of hybrid systems is extensively studied by many researchers [7, 9, 10, 11].

Generally, the cooling operation is applied at the rare of the solar modules. However, extraction of heat at the rare of the solar module may not be very efficient due to the often used reflective layer at the rare of the module. Such systems, require a complex design and fixures at the rare, where mounting frames are usually located.

The hybrid module we are studying is a variant of conventional systems with the cooling operation being applied at front cover of the solar module (Fig. 4). The photovoltaic modules are equipped with a transparent window at the front surface. The heat extracting fluid is circulated through a small gap between the front plate of the module and the attached window, by solar pumping. This arrangement may not provide an optimum method of cooling, but it is simple and practical to apply even to

I

Qin Qout

Fig. 4 Structure of the hybrid system.

an existing installation of modules. The attached window and the water-tight seal can easily be removed if and when not required, without replacing the module and its fixtures. The window at the front surface, off course, negatively effect the electrical conversion efficiency by absorbing/reflecting some of the incident radiation.