Thermo-economic multi-objective optimisation of a solar cooling system

A solar cooling system that combines the organic Rankine cycle and vapour compression refrigeration cycle, sharing the same working fluid and condenser, was optimised in terms of the total investment cost and payback period. A comprehensive thermo-economic model includes the optimal sizing of compact heat exchangers according to manufacturer's recommendations. Among the four considered working fluids, the system using R152a exhibited the highest solar energy conversion efficiency of 5.66 %, significantly lower that of a photovoltaic system. By optimising the system to minimise the number of solar thermal collectors required to meet the energy demand for achieving a cooling power of 7.3 kW, the total investment cost and payback period amounted to 13,740 EUR and 30 years, respectively. Such a system is unprofitable without considering the utilisation of waste heat from the condenser. The main drawback of using a single fluid in considered solar cooling system is the conflict requirements for working fluids in the two cycles. The best choice of working fluid for one cycle is not favourable for other one. The working fluid must be chosen from a group of working fluids more suitable for vapour compression refrigeration cycle, which is an unfavourable selection for the organic Rankine cycle resulting in the reduced solar energy conversion efficiency.