INVESTIGATIONS ON PHOTOVOLTAIC THERMAL SOLAR COLLECTORS USING DIFFERENT COOLING METHODS

Abstract

Solar energy has emerged as a vital source of both electrical and thermal energy over recent years. The rapid depletion of fossil fuels due to increased demand for energy attracted researchers toward solar energy as an alternative. The other main reasons for the adoption of this alternative are the eco-friendly nature and abundance of availability of solar energy. Solar PV panels and solar thermal collectors are two major technologies to extract electrical and thermal energy respectively from solar irradiation. The PV panel absorbs around 80% of incident irradiation but, fails to convert the majority of it into electricity. The conversion efficiency of a PV panel typically remains in the range of 15 – 20%. So, the rest of the absorbed irradiation generates heat, which is solely responsible for the high temperature of the PV panel front and back surface. This is the most undesirable operating condition for a PV panel as it reduces the conversion efficiency further as well as the life span of the panel. Hence, various cooling arrangements have been developed and investigated for the significant temperature reduction of the PV panel surface. The cooling techniques are mainly categorised into two types, one is passive cooling technique and the other one is active cooling technique. Passive cooling is also termed conductive cooling as conduction heat transfer by using phase change material (PCM) as heat transfer material is used. On the other hand, active cooling techniques use a heat transfer fluid that circulates continuously and absorbs the excess heat by means of convective heat transfer. The current study provides a more detailed performance evaluation of PV panel with respect to different cooling techniques performed under different selected system and performance parameters. A detailed literature survey was done to find the gaps in the previous research studies of PV cooling approach and to decide the objectives of present study. The major gaps that introduced for the current study were to investigate the effect of combined active and passive cooling, the effect of heat transfer thermal conductivity and the comparison of various cooling methods for same PV panel. The identified gaps were addressed in the current study through the objectives like selection of PCM material, selection of different heat transfer fluids and selection of different solar irradiation. Further the objectives were extended towards the designing and fabricating a PV/T-PCM system, performing different experiments on it and analysing the experimental results obtained in the form of temperature, electrical efficiency and thermal efficiency of PV panel.The water and nanofluids were used for the active cooling of the PV/T system. The Al2O3-water and TiO2-water nanofluids were engineered by the two-step method with two different nanoparticle concentrations of 0.05 and 0.1 %v/v to obtain different thermal conductivity. The thermal conductivity of the nanofluids was increased with concentration and the maximum enhancement of around 30 % is observed with Al2O3-water nanofluid for a concentration of 0.1 %v/v. On the other hand, Paraffin wax PCM was selected for passive and combined cooling investigations. The proposed system was studied at four values of solar irradiations in the range of 450 to 1050 W/m2. The maximum decrement in the temperature of panel was around 34 % at 450 W/m2 by using combined active and passive cooling. At 1050 W/m2 combined cooling was found most effective as it reduced the panel surface temperature by 36%. The electrical efficiency was also improved by applying various cooling methods to the PV panel. In the case of passive cooling at 450 W/m2 the efficiency at the peak surface temperature was found 42% higher. At 1050 W/m2 the combined cooling provided the highest improvement of 42% in electrical efficiency as compared to without cooling. The effect of thermal conductivity of heat transfer fluid was also studied during the current study. The use of nanofluids improved the system's thermal efficiency compared to the water due to their higher thermal conductivities. At 450 W/m2 of solar irradiation, the efficiency was improved by 11.4% with Al2O3-water nanofluid having 0.1 % v/v compared to pure water cooling. The thermal efficiency of the PV/T system was also found to be maximum in the case of simultaneous active and passive cooling with Al2O3-water nanofluid having 0.1 % v/v. The maximum enhancement of around 25% was found in this case compared to water cooling only. So, as a conclusion it can be said that the combined cooling along with higher thermal conductivity fluid was found the best solution to produce elevated electrical and thermal output from a PV panel. These results and conclusions are now more significant for the design of the PV/T system from the application viewpoint.