Selection of working medium for low-temperature ORC based on thermodynamic, economic and environmental criteria

Abstract

This paper concerns multicriteria analysis of the selection of working medium for the low-temperature ORC microsystem. The criteria for the selection of the working medium are: thermodynamic efficiency (efficiency and power of the cycle), environmental impact and safety of the substance as well as the cost of installation on the basis of the heat exchanger surface analysis. The analysis was done for five organic fluids: R1234ze, R1234yf, R152a, R143a and propylene, and the source of the powering heat for the power plant is oil heated to the temperature of 85°C while cooling the screw compressor. Based on the results of calculations and analysis of the working substance properties it has been found that the fluid that best meets the criteria mentioned above is R1234yf. The minimal environmental impact and the smallest heat exchange surface of the vapour generator, with comparable turbine power and the ORC efficiency compared to other fluids, make it the best choice. The disadvantage of this liquid is the relatively high price (∼ 140/kg). A cheaper alternative may be R1234ze (∼ 35/kg). Its environmental impact is also negligible and the calculated heat exchange surfaces of the vapour generator are only slightly larger.