High vacuum flat plate photovoltaic-thermal (HV PV-T) collectors: Efficiency analysis

Through a numerical model developed in MATLAB, we investigate the performance of a novel hybrid flat plate photovoltaic-thermal collector under high-vacuum (HV PV-T) to optimize the solar-to-thermal energy conversion and efficiently meet the thermal loads of industrial processes up to 150 degrees C along with additional production of electrical energy. In the proposed design, the photovoltaic (PV) cell is positioned directly above the Selective Solar Absorber (SSA) in a multi-layered PV-SSA structure. The performance analysis of the system has been first carried out by considering the theoretical Shockley-Queisser limit of the electrical efficiency, different values of energy bandgap (0.66 eV <= Eg <= 3.00 eV), emittance (0.1 <= e <= 1), and working temperature (25 degrees C <= TPV <= 175 degrees C) of the PV layer, and secondly by focusing on a wide variety of actual semiconductive materials. We analyzed the specific case of high bandgap materials, such as CdTe, CdS, and GaAs reported in previous publications. The analysis performed shows that full exploitation of the incident solar radiation with HV-PVT collectors can produce about 12.2% of electrical efficiency while 76.4% of the incident power remains available for thermal conversion at 100 degrees C. Moreover, obtaining the same annual energy using stand-alone solutions (i.e., a combination of the best PV and solar thermal collectors commercially available) would require up to 50% of the collector area more than the proposed HV PV-T collector.