Purdue conferences highlights: development of high-temperature heat pumps
All 474 papers from the three IIR-co-sponsored conferences at Purdue in July 2018 can now be downloaded from the Fridoc database1. Reminder: IIR members benefit from a quota of free downloads.
High-temperature heat pumps (HTHPs) was a topic of particular interest during the 17th International Refrigeration and Air Conditioning Conference. HTHPs are suitable systems for waste heat recovery in various applications such as the food, paper, metal, and chemical industries, especially in drying, pasteurizing, sterilizing, evaporation, and distillation processes.
In their review paper2, C. Arparagaus et al have identified about 20 industrial HTHP models on the market, which can provide heat sink temperatures of at least 90°C.
Today, only a few commercial available HTHP products manage to exceed a 120°C supply temperature, using mainly R245fa and R365mfc as refrigerants. The COP values range between about 1.6 and 5.8 with a temperature lift of 130 to 25 K, respectively.
Several R&D projects are currently running at an international level to push the limits of the achievable COPs and sink temperatures to higher levels compared to the state-of-the-art results of available commercial HTHPs. At a sink temperature of 120°C, the COPs are ranging approximately between 5.7 and 6.5 at a 30 K temperature lift and 2.2 and 2.8 at 70 K, respectively. Higher heat sink temperatures of up to 155°C have been achieved.
There is significant research towards testing new environmentally-friendly refrigerants with low GWP for use in HTHPs. Suitable substitutes like HFOs (R1336mzz(Z) and R1234ze(Z)), HCFOs (R1233zd(E) and R1224yd(Z)), and natural refrigerants, such as water (R718), CO2 (R744) and hydrocarbons, e.g. R601 (pentane) and R600 (butane) are intensively investigated. In addition, heat pump components are being developed to resist high temperature applications, e.g. compressors as the main driving device in a heat pump.
Due to the quick evolution in this research field, the authors consider that HTHPs with heat sink temperatures of up to 160°C will reach market maturity in the coming years. For a wider spread of HTHP technology, the transfer of the research results to applied demonstration projects in the industry is very important. Moreover, there is a need for better knowledge in characterization of the waste heat potentials classified by temperature ranges in the industrial sectors.
Usually, spa facilities such as saunas and steam baths are based on direct electric or fossil heating, which is extremely energy intensive and results in high operating costs. In order to establish a more energy efficient technology to provide heat to wellness areas, C. Seitz et al3 have developed, installed and tested a transcritical CO2 HTHP.
According to the authors, the system is convincing in terms of high temperatures on both the supplier (120-140°C) and consumer side (<119°C). Depending on the discharge pressure, the heating capacity on the high temperature level is 0.7-4.7 kW and on the medium temperature level 13.3-13.7 kW. A maximum COP of 3.6 has been reached. There is potential for improvement to reduce return flow temperature for a higher efficiency of the heat pump.
1 In order to download the papers of each of the three Purdue conferences, please login then click on the “See the articles” link at the bottom of the following web pages:
17th International Refrigeration and Air-Conditioning Conference at Purdue
5th International High Performance Buildings Conference at Purdue
24th International Compressor Engineering Conference at Purdue
2 Arparagaus C., High Temperature Heat Pumps: Market Overview, State of the Art, Research Status, Refrigerants, and Application Potentials. Available in Fridoc.
3 Seitz C., High-Temperature Heat Pump for Wellness Applications Using CO2 as a Refrigerant. Available in Fridoc.