Yiping Wang*, Wei Tian, Yonghui Liu

(School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,China) Tel.:+86-22-27404674,Fax:+86-22-27404771,E-mail:xinxing@tju. edu. cn Abstract: Solar cells immersed in a dielectric liquid under certain conditions have an increased operating efficiency, and the liquid can provide an effective cooling to the solar cells. This kind of immersion operation of solar cells is also suitable for photovoltaic concentrator and photovoltaic/thermal systems. In the paper, a theoretical analysis has been presented to study thermal performance of solar cells immersed in silicon oil. The system has two different cooling types, which are free convection and forced convection respectively. Steady state heat transfer equations are derived and the average temperature of solar cells for these two different cooling types is calculated. The effects of various factors on the solar cell performance are analyzed. Further, the temperature distribution of solar cells was achieved by using aNsYS Finite Element Analysis (FEA) program. It is indicated that the experimental data fit well with the estimated values according to our models.

1. Introduction

Solar cells have been studied by various authors on efficiency improvement through different ways. An effective method that solar cells are immersed in liquids can increase conversion efficiencies and decrease the temperature of solar cells. The photocurrent and electrical characteristics are investigated for solar cells operated in liquids by Tadaki et al.

[1] . They found that photocurrent increases with the increase of the permanent electric moment of molecules in liquid and this effect is considered to be due to adsorption of polarizable molecules which reduce carrier recombination at the surface of solar cells. Russell [2] invented an optical concentrator and cooling system for photovoltaic cells which system is effective in concentrating the sunlight and in cooling the cells economically. The new system consists of an elongated tube with a curved transparent area for admitting sunlight. This elongated tube is filled with a clear nonconductive liquid having a refractive index suitable for concentrating the sunlight onto the solar cells mounted inside the tube and immersed in the liquid. Solar cells have been demonstrated to function satisfactorily when immersed in a clear nonconductive liquid. Abrahamyan et al. [3] also describe the effect of an increase in the efficiency of solar cells (in particular, common silicon solar cells) by their immersion in an isotropic liquid dielectric. The presence of a dielectric thin film results in an increase in the solar cells efficiency by 40-60% from the reference value. Authors think that the increase effect is caused by several reasons which include an increase in the barrier height of n/p junction, a decrease in the velocity of the surface recombination followed by an increase in the factor of the separation of charge carriers generated by light as well as a decrease in a part of the reflected radiation and the last two factors are main ones. Tanaka [4] thinks that solar cells operating in liquid have an increased operating efficiency resulting from two independent physical phenomena, an increase in output current from the solar cells from simply wetting the solar cells, and enhanced collection of light through refraction and inner reflection of light in the liquid. There are different explanations of increase effect in solar cell efficiency immersed in liquid.

The PV module cell temperature is a function of the physical variable of the solar cell material, the module configuration and the surrounding environment. Temperature is a key factor because the efficiency of solar cell decreases significantly with increasing temperature. The studies of thermal model of solar cells have been mostly concentrated on photovoltaic system and hybrid photovoltaic thermal system. Radziemska [5] summarizes the recent progress obtained in the field of the temperature performance of
crystalline and amorphous silicon solar cells and modules. Various authors have modelled the temperature of a PV module by evaluation of energy inputs and outputs thought radiation, convection, conduction and power generated. The energy balance of photovoltaic cells is modelled based on climate variables by Jones et al. [6]. Module temperature change is shown to be in a non-steady state with respect to time. A one­dimensional heat transfer model was derived by Davis et al [7] to improve upon the NOCT model. Garg et al [8, 9] have developed a computer simulation model for predicting the steady state and transient performance of a conventional photovoltaic/thermal (PV/T) air heating collector with single — and double-glass configurations. Lee et al. [10] concerns the development of a thermal model to predict the temperature profile of a typical building — integrated PV roof and comparison of the performance of this model to that of the simplified model for flat-plate PV arrays presented by Fuentes.

The aim of the thermal model presented here is to predict the average temperature and temperature distribution of solar cells immersed in silicone oil. Silicone oil is very suitable for immersion operation of solar cells mainly because of its low electrical conductivity and thermal stability. The liquid of the immersion operation system is usually chosen to be high electrical resistivity, optical filtering with suitable spectral response, low freezing point, high boiling point and high refraction index et al.