Improving the working efficiency of a photovoltaic module through the use of a photothermal (PV/T) hybrid system

Abstract

This work considers a method of improving the performance of solar photovoltaic modules, through the use of a photo thermal (PV/T) hybrid system. This is achieved by incorporating a cooling system in the PV module structure. The cooling system consists of a water tank (1.8 m diameter) supplying water to copper tubes. Two identical silicon solar PV modules were used in the work. The copper tubes placed in rectangular configuration were embedded at the bottom of one of the module using heat sink compound to ensure a good thermal contact whilst the second module was used without any cooling system. An electric potential divider circuit was built to facilitate the recording of the voltage delivered by each module by the Data logger.

Temperatures of both modules (bottom and top) as well as the ambient temperature variations, the solar radiation and the wind speed were recorded by a Data logger using copper constantine thermocouples while the cooling water flow was recorded by a propeller current flow meter. The Data collected were then used to calculate the power delivered by each module as well as the energy that could be extracted through the cooling water at the bottom of the cooled module. The total energy (electric and thermal) and the effective efficiency of each module are evaluated and the cost of the PV modules calculated based on their total energy delivered.

The results obtained confirmed that maximum solar insolation during the day occurs between 10 a.m. and 3 p.m. with intermittent fluctuations due to passing clouds and dusts in the atmosphere. Temperature of both photovoltaic modules (when they are not cooled) are greater than the ambient temperature. For ambient temperatures varying between 21°C to 25°C the PV module cell's temperature. For ambient temperatures varied between 41°C to 50.5°C. As the temperature increases, the module power falls by 0.4948% per degree centigrade while its efficiency decreases by 4%. The two modules output (voltages, currents and power) and their efficiencies have the same magnitude and their variations follow the same pattern. It is therefore possible to compare their outputs under the same climatic condition when one of them is cooled, and see the effect of the cooling process on the cooled module output as compared to the non-cooled module.

The cooling of the PV module bottom lead to a reduction of 30% in the temperature rise of the PV module. The electrical current increase of the cooled module varied between 0.2% and 3.63% while the voltage output varied between 0.4% and 3.30%. The electrical power output increased by 4.5% as compared to the non-cooled module power output. These results led to an increase in the cooled module efficiency varying between 3.21% and 14.93%.

The cooling of the module also allowed the extraction of thermal energy through the cooling liquid (water) as a result of the temperature gradient between the inlet water and the outlet water temperatures. The useful energy (Qu) extracted, in addition to the electrical energy has hence increased the overall efficiency of the cooled module.

The cost of the cooled PV module has been reduced as regard to the overall output (electric and thermal energies). A decrease of US$ 2.00 per unit Watt is obtained.

The output water from the cooling system could be used as input water for a solar water heater. Definitely, to achieve a maximum use of photovoltaic solar modules, there is need to cool the backside of the module.