Design of a microscale organic Rankine cycle for high-concentration photovoltaics waste thermal power generation

High-concentration photovoltaics (HCPV) is a highly promising technology to directly convert plentiful solar energy to electricity. However, even for the most advanced HCPVs, about 60% of the concentrated solar energy is rejected as waste heat; therefore, it is desirable to utilize the massive waste heat from HCPV modules. Considering the nature of low-grade waste thermal energy, a microscale organic Rankine cycle (MORC) offers a promising solution. In a subcritical MORC, subcooled refrigerant is usually pumped into a microchannel heat sink of each multi-junction photovoltaic cell. In this paper, a complete microchannel flow boiling model is developed based on distributed mass, energy and momentum conservation laws. Detailed MORC thermal-fluid analysis is conducted to evaluate the effects of working fluid, inlet subcooling, axial fluid/cell temperature distribution and critical heat flux on cogeneration efficiency. The performance analysis indicates that the HCPV/MORC system can achieve a net 8.8% increase of power generation efficiency in comparison to liquid-cooled HCPV at ambient temperature. The proposed HCPV/MORC configuration shows great promise in large-scale applications of HCPV solar power generation.