High Temperature Heat Pumps
High temperature heat pumps (HTHPs) with heat sink temperatures in the range of 100 to 160°C are expected to become increasingly commercialized in the coming years. Major applications have been identified, particularly in the food, paper, metal and chemical industries, especially in drying, sterilization, evaporation, and steam generation processes. With the intensification of the F-gas regulations, only refrigerants with low GWP may be used in the near future. Replacement fluids for the currently applied hydrofluorocarbons (HFCs) R245fa and R365mfc are required. The actual research gap in the field of HTHPs is to extend the limits of efficiency and heat sink temperature to higher values, while using environmentally friendly refrigerants. This presentation reviews the current state of the art and the current R&D activities for HTHPs.
More than 20 industrial HTHP models from over 10 manufacturers have been identified on the market with heat supply temperatures of above 90°C. A few HTHPs exceed 120°C using mainly R245fa or R365mfc as refrigerants. The COPs range between 1.6 and 5.8 with a temperature lift of 130 to 25 K (40 to 60% Second Law efficiency).
Several R&D projects are running on an international level to demonstrate the technical feasibility of reaching even higher sink temperatures than 120°C. The research trend goes towards testing natural refrigerants (R718, R744), hydrocarbons (R600, R601) and synthetic HFOs (R1336mzz(Z), R1234ze(Z), R1233zd(E), andR1224yd(Z)) with low GWP (< 10). The experimentally obtained COPs of the various research studies reveal COPs in the range of 5.7 to 6.5 at 30 K temperature lift, and 2.2 to 2.8 at 70 K lift. Outstanding high temperatures of 160°C are reached with the HFO R1336mzz(Z).
Theoretical simulations of a basic heat pump cycle with various low GWP HFO and HFCO refrigerants revealed a tradeoff between performance (COP) and volumetric heating capacity (VHC). R1336mzz(Z) was found to be the next drop-in replacement for R365mfc, whereas R1224yd(Z), R1234ze(Z) and R1233zd(E) were closer to R245fa.
Based on those findings, a laboratory scale HTHP was designed, built and tested with the commercially available HFCO refrigerant R1233zd(E). Standard components were used to construct the experimental test setup. The developed HTHP is single-stage, operates with a variable-speed piston compressor and contains a continuously adjustable internal heat exchanger (IHX) for superheating control and efficiency increase. The use of the IHX increased the heating COP significantly (+14%) compared to a simple heat pump cycle. The HTHP provides 10 kW heating capacity and is relatively easy to control. The basic functionality of the developed HTHP system for potential industrial process applications (e.g. drying processes or steam generation) could be demonstrated by operating the heat pump at heat source temperatures of 40 and 80°C and heat sink temperatures between 70 and 150°C. At the reference point condition (W60/W110, 50 K temperature lift) a COP of 2.43 was reached. Future work is focusing on testing alternative HFO and HCFO refrigerants like R1336mzz(Z) and R1224yd(Z), as well as reducing of thermal losses at elevated temperatures by better insulation of the heat pump components and tubing. A further increase in the heating COP can be expected.
--- Datum: 15.10.2018 Uhrzeit: 13:45 - 14:14 Uhr Ort: Raum Kiew, NCC Ost