Reversible high-temperature heat pump -organic Rankine Cycle System for industrial waste heat recovery

Abstract

The escalating demand for sustainable energy solutions in the face of global climate change has thrust industrial waste heat recovery into the forefront of energy conservation efforts. This doctoral thesis delves into the development and experimental investigation of a novel system, Reversible High-Temperature Heat Pump - Organic Rankine cycle for the efficient recovery of waste heat generated by industrial processes. Such a system can be then used for effective recovery of low temperature industrial waste heat. By operating in HP mode, the reversible system can be utilized to provide process heat for industrial applications or for meeting heating needs of an office/commercial building, and if the end user does not require process heat, the system can be operated in ORC mode to generate power.

This research work aims to develop a lab scale prototype of a reversible high-temperature heat pump - Organic Rankine cycle system which can effectively utilize waste heat in the lower temperature band (< 100 °C). As the concept of a reversible system is new, a comprehensive literature review of the system, equipment, and appropriate operating fluids/refrigerants is conducted.

Based on the literature review, a system design is undertaken that discusses the system's architecture, equipment selection, and working fluid. A compact, cost-effective system design is selected, and an automotive open drive scroll compressor is selected for both the compressor and expander roles respectively in HTHP and ORC modes. R1233zd(E), a HCFO refrigerant, is selected due to its low GWP and ODP values and its ability to effectively replace R245fa in heat pump and ORC systems.

The experimental investigation of the reversible system in HTHP was conducted at heat source temperatures ranging from 40-70 °C. Compressor overall isentropic efficiencies as high as 73.4% and COPmech of 4.75 (_Tlift,rside: 41 K, Tsource: 60 °C) is obtained. A validated HTHP model was used to simulate the performance of the reversible system at operating points outside of the experimental conditions. Model predictions showed that the reversible system has the potential to attain higher COP and better overall compressor efficiency values at higher source temperatures especially at 70, 80 and 90 °C.In ORC mode, overall compressor efficiencies as high as 54.6% (Tsource: 90 °C, rp: 2.23) were achieved. A maximum net power output of 512.4 W (Tsource: 90 °C, rp: 2.26) and an overall cycle efficiency of 3.01% was obtained. Technical constraints imposed by the valve arrangement (pressure drop at the expander discharge side) restricted the scroll expander from achieving higher operating pressure ratios to produce higher power outputs and the system from reaching better overall efficiency values. Simulation of the ORC mode, using a validated model showed that the system has the potential to produce a net power output of 903 W and achieve an overall cycle efficiency as high as 5.08% (Tsource: 90 °C, rp: 3.57). In addition, a comparative study of performance of different low GWP refrigerants showed that R1233zd(E),R1234ze(Z) and R1224yd(Z) can be effective substitutes for R245fa in HTHP and ORC systems.

Date of Award	May 2024
Original language	English
Supervisor	Neil Hewitt (Supervisor) & Ming Jun Huang (Supervisor)