Design strategy for component and working fluid selection in a domestic micro-CHP ORC boiler

Abstract

This paper presents an early design strategy for a micro Combined Heat and Power (micro-CHP with heat transfer rate less than 30 kWt and electrical power up to 2 kWe) based on organic Rankine cycle (ORC). The strategy consists of 3 steps: i) establish a set of pre-specifications; ii) conceive a thermodynamic steady-state model; and iii) formulate the optimization problem and develop a numerical algorithm to solve it. Through the described strategy, the selected working fluid for the domestic hot waters emulation test-rig was R245fa. The main components selected included a pre-specified scroll expander, a flexible rotary vane pump, an in-house direct evaporator coupled to the gas burner and a brazed plate heat exchanger as condenser. For last, a test-bench was built and the acquired measurement and control devices specified. The preliminary experimental results allowed to obtain a preliminary validation of the design procedure and revealed that the ORC components are not working far from the assumed nominal efficiencies. Also, the basic control variables are directly related to the model control variables and a reasonable agreement is obtained between the experimental and predicted overall thermodynamic efficiency.