Towards sustainable process heating at 250 °C: Modeling and optimization of an R1336mzz(Z) transcritical High-Temperature heat pump

Abstract

Fossil fuel boilers typically produce high-temperature process heating due to the impossibility of current renewable technologies. Vapour compression heat pump systems have been proposed up to 200 °C with subcritical cycles. To increase the heating production level, this paper proposes heating up to 250 °C through a transcritical high-temperature heat pump (THTHP) using R1336mzz(Z), which is not thermally degraded till this temperature, according to existing data. Computational models of a transcritical vapor compression cycle with and without an internal heat exchanger are developed for this work. The input cycle parameters, such as evaporation, production temperature, gas cooler output temperature, and the superheating degree, have been modified through iteration to consider possible scenarios in different industrial sectors. Moreover, the impact of using waste heat to increase evaporation temperature or superheating degree has been studied to optimize the cycle. The internal heat exchanger has also been considered for superheating the compressor suction. Other relevant parameters, such as compression ratio or volumetric mass flow rate, have also been assessed. The results show that a heat pump is feasible to reach 250 °C temperature using waste heat at 120 °C (evaporation at 100 °C and 20 °C of superheating degree), reaching a COP of 3.3. The variation of evaporator temperature from 80 °C to 140 °C increases COP from 2.5 to 5.9, while the increase of superheating degree from 20 °C to 100 °C increases COP from 3.2 to 7. Waste heat use optimization proves that it is more efficient to prioritize the increase of the evaporation temperature. If the industry requires lower production temperatures, the COP can reach a value of 7.0 at 180 °C. The decrease in gas cooler outlet temperature impacts less the COP than other input parameters. At baseline conditions, an optimum superheating degree with an internal heat exchanger of 45 K produces a maximum COP of 4.0. THTHP emissions are less than gas boilers for the same application, and in countries with greener electricity generation, these emissions can be 50 times lower.