A CO2-ice water-to-water heat pump system has been found to operate more efficiently at lower compressor speeds and at higher evaporator water outlet temperatures, according to research conducted via the EU’s TRI-HP project.
The TRI-HP Project deals with the trigeneration (combined heating, cooling and power) systems based on heat pumps with natural refrigerants and multiple renewable sources. The development of these systems aims to couple heat pumps with photovoltaic cells to provide heating, cooling and electricity with an on-site renewable share of 80%, reducing the installation cost by 10-15%.
The TRI-HP researchers are from the Norwegian University of Science and Technology (NTNU), the SPF Institute for Solar Technology and Heim AG. The project has received funding from European Union’s Horizon 2020 research and innovation program under the grant agreement.
One of the researchers, Engin Söylemez, a post-doctoral fellow from NTNU, presented a research paper, “Performance analysis of a CO2-ice heat pump,” at the 10th International Institute of Refrigeration (IIR) held in Ohrid, North Macedonia, April 27-29.
TRI-HP project focuses on the development of three heat pump systems, two using propane (R290), and one using CO2. The CO2 heat pump system is equipped with a sub-cooler, a three-part gas cooler, and an ejector. The setup was arranged is such a way that it could supply domestic hot water (DHW), space heating (SH) or a combined supply of DHW and SH, also known as parallel mode (PA).
An ice storage facility, producing chilled water, is taken as a source in this model. The chilled water provides heat to evaporate liquid CO2, leaving the evaporator at a lower temperature.
Researchers did experimental testing for the CO2-ice heat pump protype at the SPF laboratory in Rapperswil, Switzerland. They measured power consumption, temperatures, pressures and mass flow rate across hydraulic loops, using simulation software known as Dymola. Validation against experimental test results was found to be accurate within a 15% deviation range, well within the acceptable limits for such study.
During the simulation, it was found that heating COP showed a positive correlation with the evaporator water outlet temperatures in both SH and PA modes. For example, in PA mode, the minimum COP value is nearly 4.4, and it goes up roughly 5.5 with rising evaporator water outlet temperature. Meanwhile, the hot water temperature reaches 40°C (104°F) and (70°C (158°F) for SH and DHW, respectively.
In addition, lower compressor speeds resulted in higher heating COP. For example, at a compressor speed of 30rps (revolutions per second), the heat pump was able to achieve a maximum COP of approximately 4.1. At higher speeds of 55rps, the COP remained impressive at around 3.8.
“Redefining high-speed efficiency, [the system] operates effectively even at a compressor speed of 55rps,” said Söylemez. “Yet, it’s true that this system gives a heating COP of 4.1 at compressor speed of 30rps.”
Working mechanism of CO2-ice heat pump model:
The flow direction is clockwise.
- 1-2: the compressor compresses the CO2 vapor to a high pressure
- 2-3: reheating of the domestic hot water (DHW) in GC (gas cooler) 1
- 3-4: heating the water for space heating (SH) in GC2
- 4-5: preheating DHW in GC3
- 5-6: sub-cooling,
- 6-7: further cooling of CO2 in the internal heat exchanger (IHX)
- 7-8: expansion in the ejector
- 8-9: the liquid CO2 is separated in the separator
- 9-10: expansion of the liquid CO2 to the evaporating pressure
- 10-11: evaporation in the evaporator
- 11-8: the CO2 vapor from the evaporator is pressurized to suction pressure
- 8-12: the CO2 vapor is separated in the separator
- 12-1: the CO2 vapor is heated in the IHX
“Redefining high-speed efficiency, [the system] operates effectively even at a compressor speed of 55rps. Yet, it’s true that this system gives a heating COP of 4.1 at compressor speed of 30rps.”
Engin Söylemez, a post-doctoral fellow from Norwegian University of Science and Technology
