Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE), based in Freiburg, Germany, have found ways to increase the COP and capacity of a low-charge propane (R290) heat pump, as part of a multi-stakeholder project.
Lena Schnabel, Head of the Department of Heating and Cooling Technology at Fraunhofer ISE, shared the latest results from the project – called “LC150 Development of a Refrigerant-reduced Heat Pump Module with Propane” – at the 14th Gustav Lorentzen Conference, held on December 9, 2020, in Kyoto, Japan, and accessible online.
The aim of the project is to develop an R290 heat pump with a charge of only 150g (0.33lbs), using market-available components and reaching a heating capacity between 5kW and 10kW (1.42TR and 2.85TR). This would help accelerate the adoption of heat pumps in residential buildings, thereby cutting use of fossil fuels for heating.
The project is funded by the German Federal Ministry for Economic Affairs and Energy. It is also supported by a number of European heat pump manufacturers, including Vaillant, Kermi GmbH, Bosch Thermotechnik GmbH, Viessmann Werke Allendorf GmbH, BDR Thermea and Ait-Deutschland GmbH.
“My hope is that this project, and this network between the manufacturers, us and the component suppliers, is a way to accelerate the production and the design of heat pump modules for low-charge refrigerant cycles for heat pumps for the European market,” Schnabel said.
The project, funded through March 31, 2023, began with simulations, with results that were “quite promising,” Schnabel said.
In 2019, as part of an ongoing feasibility study, Fraunhofer ISE developed a brine-water cooling circuit that employed 150g (0.33lbs) of R290 to achieve a heating capacity of 8kW (2.3TR), a refrigerant charge reduction of 75% compared to existing systems on the market.
An R290 brine-water heat pump with 150g (0.33lbs) of refrigerant, generating 5-10kW (1.42-2.85TR) of heating capacity, corresponds to approximately 20g/kW (0.01lbs/TR). By contrast, heat pumps on the market today employ 60-100g/kW (0.04-0.06lbs/TR).
The researchers achieved this charge reduction, she explained, by addressing the parts of the system with a high refrigerant content, such as by using asymmetric plate heat exchangers, reducing the length and diameter of pipes, and cutting the quantity of lubricant oil in cooperation with the compressor manufacturer.
“My hope is that this project, and this network between the manufacturers, us and the component suppliers, is a way to accelerate the production and the design of heat pump modules for low-charge refrigerant cycles for heat pumps for the European market”– Lena Schnabel.
“The combination of components and operation (compressor speed, superheat, different heat exchanger designs) gives a wide range of enhancing COP (coefficient of performance) and heating capacity within this limit of 150g,” she said.
Furthermore, the researchers found that COP can be further improved with more equalized flow distribution in heat exchangers, the insulation of components, and a well-designed configuration, Schnabel added.
A problem that occurred during the earlier experiments, noted Schnabel, was irregular distribution of the refrigerant in the heat exchangers, which led to higher temperature differences in the heat exchangers and also to reduced COPs.
Fraunhofer ISE followed the first experiments with another experimental evaluation. In these experiments, different configurations were used again, including two different compressors (scroll and rotary), and varied heat-exchanger design (such as the length of the condenser and the symmetry of the evaporator). There was an experimental variation of temperatures, superheat, compressor speed, refrigerant charge, and types and quantity of lubricant oil.
The researchers, said Schnabel, found that increasing the R290 charge between 150g and 250g (0.33lbs and 0.55lbs) increased the COP. The type of compressor also had an impact on the maximum COP and heating capacity: The rotary compressor shifted the COP and heating capacity to higher levels than the scroll compressor, but with a higher refrigerant charge.
The scroll compressor obtained a lower maximum COP and heating capacity than the rotary compressor, but at lower refrigerant charges.
In correcting the irregular refrigerant distribution in the earlier heat exchanger design, the researchers saw that a configuration with a short condenser could reach a higher COP and a higher heating capacity at a lower refrigerant charge, compared to one with a long condenser.
With regard to compressor speed, they discovered that a higher speed – 120Hz compared to 60Hz (7,200rpm compared to 3,600rpm) – increases the heating capacity; it decreases the COP, but not as much as it increases the heating capacity.
The project plans to continue generating design information and obtaining “stable knowledge about designing refrigerant cycles using just 150g (0.33lbs) of propane,” which can be used by the participating manufacturers, said Schnabel. This includes validating experimental results and discussing safety aspects.