On the Implementation of Two-Diode Model for Photovoltaic-Thermal Systems

Photovoltaic (PV) cells are known for poor efficiency within the range of only 6-15%, depending on the type of cells. As a result, those can convert only a small part of the absorbed solar energy into electricity, the rest is wasted as heat, which also contributes to rise in cell temperature. This heating up is undesirable for PV cells because it further decreases the electrical conversion efficiency. One viable solution to this problem is the combination of PV cells with integrated thermal collectors, known as photovoltaic-thermal (PVT) collectors. This combination usually improves the PV module efficiency compared to stand-alone PV modules, because the fluid circulating underneath the PV cells removes the heat from the cells and cools them. Among the studies concerning PVT, Delisle [1] provided a good mathematical model, where the electrical output was calculated simply by considering a linear dependence of PV efficiency with cell temperature. In the current study, following the Delisle's approach [1], a simple model configuration consisting of transpired collector absorber plate of corrugated type mounted underneath the PV cells is analyzed. The resulting mathematical system is solved numerically using multivariate Newton's method. To calculate the model output more accurately, a sophisticated model known as two-diode model is incorporated. This model provides current-voltage characteristics with maximum power point (MPP) tracker, and considers nonlinear temperature effect. The comparison of the model outputs with experimental data reveals that two-diode model behaves differently for different sets of data at different conditions.