Performance analysis of multistage high-temperature heat pump cycle

Abstract

High-temperature heat pumps (HTHPs) that can supply heat at temperatures at and above 200°C have the potential to increase energy efficiency and decrease carbon dioxide (CO2) emissions in industrial processes. In this study, three reversed Rankine cycles using water vapor (R-718) as the working medium, with different intercooling strategies, were proposed and their performance has been investigated. The thermodynamic performance was estimated under different operating conditions, and the optimal pressure ratio (PR) between compression stages was found to be where both compressors had the same PR. The thermodynamic efficiency, φ, and exergy efficiency, η_exergy, were also analyzed at the optimum PR. The cycles that employed an intercooler between the first and second compression stages (IC cycles) showed higher φ and η_exergy values compared with the spray-injection cycle. Among the IC cycles, the IC-in cycle, with an inward flow direction of heat sink to the IC, demonstrated higher efficiency and deliverable temperature, T_sink out, than the spray-injection and IC-out cycles. To assess the practical impact of the HTHP cycles on industrial CO2 reduction, the PR for each stage was limited to 2.5. Theoretically, the IC-in cycle could achieve a coefficient of performance of 5.86 with a T_sink out of 200°C or higher when T_evap and T_cond were at 90°C and 150°C, respectively. Additionally, the study demonstrated that the proposed HTHP system has the potential to reduce CO2 emissions by 8.1% in 2030 for industrial heat supply at temperature up to 200°C, by replacing existing industrial fossil boilers with high-efficiency HTHP.