Exergoeconomic analysis and optimization of organic Rankine cycles

Abstract

Heat sources such as biomass, industrial waste heat and solar thermal provide the potential to produce renewable environmentally low impact electricity. Using these resources efficiently within economic constraints is important for viability of these systems. This thesis explores a regenerative organic Rankine cycle for use in low temperature heat sources. A Bitzer model scroll expander is used for the prime mover for the system. This expander has a reliable model in which thermodynamic analysis can be done. Various working fluids are explored to investigate which one will provide the most power output and efficiency within system constraints. Using optimization, each fluid is tested within physical constraints for optimal operating conditions using system exergy efficiency as the objective function. An exergoeconomic analysis is performed to predict the cost rate of electricity of the system and is compared to current contract rates from the Ontario Power Authority. Dimethyl ether shows promising results with a system exergy efficiency of 11.76% and system energy efficiency of 2.84% at a source temperature of 120℃. The degree of superheat and pressure ratio are used as the independent variables in the optimization. Highest isentropic efficiency for the expander is 29.22%, showing large potential for improvement. Electricity cost rates for the system assuming 20 year life are 0.132 $/kWh to 0.197 $/kWh depending on the fuel input cost for dimethyl ether. At the current state the system shows merit with large potential for improvement in the expander.