Investigation and optimization of a CO2-based polygeneration unit for supermarkets

Abstract

The objective of the present analysis is to investigate and optimize a solar-driven polygeneration system with CO2 as the working fluid. The studied idea is ideal for installation in supermarkets in order to produce refrigeration, electricity and heating. More specifically, the primary energy source of this system is solar irradiation and the unit produces, at the same time, cooling to cover the refrigeration demands of the store products, useful heat to cover the heating demands and electricity to cover the consumption of the refrigeration units and for sale to the national grid. Therefore, the proposed system includes a parabolic trough collector field, a storage tank, an upgraded CO2 cycle and an organic Rankine cycle. The CO2 cycle is based on an existing CO2 refrigeration cycle and includes additional Brayton cycle devices for power production. The unit is investigated and optimized in steady-state conditions and then the most suitable design is studied in dynamic conditions for all the year. According to the final results, the annual energy efficiency of the system is 67.8% and the annual exergy efficiency is 10.1%. Furthermore, the net present value is calculated to be equal to 543 k€ and a payback period of 7.55 years is found. So, apart from the food refrigeration, the other needs of the supermarket, such as electricity and heating, are met by a system, which is highly efficient and economically viable.