Thermal Analysis of a PV Module with Integrated Energy Storage System

An electric energy storage system fully integrated with a single photovoltaic PV module, realizing a modular generation-storage PV unit, is considered in this paper. Each PV module can be treated as a self-rechargeable battery unit, and the whole PV generation-storage system can be simply built by properly connecting the PV units. In particular, each PV unit is composed by a commercial PV module, a dc/dc chopper with both maximum power point tracking (MPPT) and battery charge regulator capabilities, a charge monitoring systems (BMS), and a set of lithium-iron-phosphate (LiFePO4) flat batteries for the energy storage (typical thickness 1 cm). The batteries are surface-mounted on the back side of the PV module by an aluminium-bar structure able to keep the batteries distant from the PV backsheet without to exceed the PV frame size (typical air-gap 2 cm). Battery disposition doesn’t exceed the original PV volume, being the thickness of air-gap and flat batteries equal to the PV frame height (typically 3 cm), and allowing natural air cooling of the backsheet. An additional low-emissivity sheet is introduced to shield the batteries from the backsheet thermal irradiance, being the maximum batteries temperature limited to 50-55 °C. For this purpose, an aluminium flat polished sheet (thickness 0.5 mm) has been used. The challenge is to evaluate the PV cell temperature increase, due to the reduced thermal exchanges on the back PV side, and to estimate the temperature of the batteries, in order to verify their thermal constraints. Two one-dimensional (1D) thermal models accounting for all the heat exchanges are proposed: one related to the original PV module, the other related to the PV module with the proposed energy-storage system mounted on the back side. Convective and radiative coefficients are calculated in relation to different configurations and ambient conditions. The considered models have been numerically implemented by using the thermal library of Simulink-Matlab. The model validation has been carried out by the PV module normal operating cell temperature (NOCT) given by the manufacturer, and by specific experimental measurements on commercial PV modules, including thermographic camera images, with and without the proposed energy-storage system.