Design of PCM-water storage for improving performance of Photovoltaic/Thermal module in residential building

Abstract

Photovoltaic thermal (PVT) module provides a conversion of solar energy into simultaneous electricity and heat. Unlike normal photovoltaic (PV) module, the nominal operating cell temperature (NOCT) used to evaluate PV module temperature and electrical power output is not given since the value for the PVT module also depends on mass flow rate and the inlet temperature of the module working fluid. In this study, a new method for calculating the NOCT of glazed and unglazed PVT modules having water as working fluid were presented. Four unglazed identical PVT modules in series connection were tested outdoor with various mass flow rates similar to solar collector testing. The water inlet temperature of the first PVT module of the system was varied in a range of 27 - 60 °C. A correlation for NOCT of unglazed PVT module with (Tfi - Ta)/IT and water mass flow rate, mi was developed. In case of glazed PV'I module, the evaluation of the NOCT is more complicated since the module temperature is rather difficult to be measured. The thermal characteristics such as F', FR, FR(𝜏𝛼), and 𝐹𝑅𝑈𝐿, and finally the NOCT at various water flow rates and inlet temperatures inlet temperatures could be evaluated. It could be noted that the calculated values of the module temperature, the outlet hot water temperature and the generated electrical power from the developed NOCT approach agreed very well with the experimental results. Use of phase change material (PCM) to generate temperature stratification in water storage tank coupled with unglazed photovoltaic-thermal (PVT) module for combined heat and power generation enhancement was investigated by experimental and numerical analyses. In this experiment. RT42 having a melting point of 38-43 °C, was filled in a packed-bed of 40 mm diameter spherical capsules in a water storage tank (220 L) connecting with four identical unglazed PVI modules (each of 200 Wp) in series. The experiments were performed outdoor with water mass flow rates of 2.4 and 5.8 LPM. The enthalpy method with the one-dimensional finite difference method was used to calculate the temperatures of water and PCM ball in the storage tank. It could be seen that the simulated results on the water and the PCM temperatures, including the module temperature of PVT modules, agreed well with those of experimental data. The model was also used to evaluate the net electrical and net overall (both thermal and electrical including pump power) efficiencies of the PVT modules by considering the position of the PCM ball, the PCM amount, the circulating water mass flow rate, and the PCM type. From this model, the PCM ball packed-bed position did not give an effect on the PVT module performances. But increase of the PCM amount at giving circulating water mass flow rate, higher water temperature stratification could be found, and higher performances of PVT modules, especially in the afternoon, could be obtained. Lower melting point PCM showed better performance of the PVI modules, but if the melting was too low, the water temperature might not be high enough for hot water utilization. RT42-water storage with 𝐻𝑏/𝐻 = 100% was recommended since the PVT performance could be improved compared with normal water storage, and the water temperature in the storage tank could reach 55 °C which was high enough for domestic hot water. The mass flow rate of 2.4 LPM was also recommended for actual production with the optimum overall efficiency when the pump power was included. Four PVT modules in series connection were selected since the storage tank temperature could be obtained at 55 °C and the average net exergy efficiency could reach 14.79%.