Development of an approach to compare the ‘value’ of electrical and thermal output from a domestic PV/thermal system

When considering the design of a PV/Thermal system, it is essential to determine the ratio of the values of the electrical and thermal output from the system. Otherwise there is no rational approach for optimising such a system, as there will be no single output to optimise. This paper focuses on methods that can be employed to develop a ratio between electrical and thermal output from a domestic style PV/Thermal system. Methods discussed include thermodynamic analysis using exergy; market analysis for both an open market and a renewable energy market; and environmental analysis using avoided greenhouse gas emissions. Ratios are developed for each method based on real data. An example is given comparing a PV/Thermal system that uses amorphous silicon cells with one that uses crystalline silicon cells. Levelised energy cost is plotted against the energy value ratio to show that there is a critical electrical-to-thermal energy value ratio below which a collector with a-Si cells is more cost effective than one with c-Si cells.