Refrigerants for Heat Pump Water Heaters

For Heat Pump Water Heaters no alternative low GWP refrigerant at this moment fulfils all the requirements at once.

Alternative refrigerants for Heat Pump Water Heaters to the currently used refrigerants (R134a, R410A) are:

  • Carbon dioxide (R744) and Propane (R290) are natural refrigerants, which can be used to reach higher output temperatures up to 80°C.
  • R152a, R1234yf, R1234ze(E) and Ammonia are interesting alternatives for DHW HP, not yet broadly in use but already tested in R&D projects, while R32 is strongly promoted by a number of manufacturers.

An over of alternative refrigerants for HPWH’s is given by Nawaz in his presentation at the IIR Conference ICR2019 and the 13th IEA Heat Pump Conference:

Overview of alternative refrigerants (Oak Ridge National Laboratories)

In the choice of refrigerants the condenser configuration exchanging heat with the storage tank and the risks (like flammability) of the refrigerant play an important role. Oak Ridge National Laboratory (ORNL) has done a number of studies in this area. A great leap forward with Propane has been achieved with research by Kungliga Tekniska Högskolan (KTH) among others.

Needed R&D

The energy issue is now at the top of the agenda, moving to a low- GWP refrigerant, without a better energy efficiency, a new design … is not enough’.

Alternative refrigerants impose safety concerns, where these also can be an effective drop-in-replacement for conventional fluids (R134a). Refrigerants with no risks, like CO2 based systems can certainly compete well with conventional fluids. A focus for the R&D will be on appropriate system modifications with heat exchangers in relation to the hot water storage, which can provide further improvement in the system efficiency, with the energy issue on top of the agenda.

Although manufacturers are active and well aware of the need to develop new technologies with alternatives, further international collaboration on alternative refrigerants for Heat Pump Water Heaters is of importance.

R&D on refrigerants is done all over the world, however not so much especially for Heat Pump Water Heaters. The challenge is the transition towards natural working fluids for with opportunities and challenges:

Opportunities

  • Alternative refrigerants can be effective drop-in-replacement for conventional fluids (R134a).
  • Minor to no system modification is required to acquire an acceptable performance.
  • Appropriate system modification can provide further improvement in the system efficiency.
  • Selective fluids can lead to substantial reduction in system volume (refrigerant charge inventory).

Challenges

  • Alternative refrigerants impose safety concerns. Mild to high flammability can be an issue requiring system redesign to meet specific requirements.
  • Often times the alternative refrigerants are expensive and substantial reduction in volume is required.
  • CO2 based systems can certainly compete well with conventional fluids.
    • Requirement of split system (additional pump, heat exchanger)
    • Requirement to operate at elevated pressure.

GWP versus LCCP

The energy issue is now at the top of the agenda, moving to a low- GWP refrigerant, without a better energy efficiency, a new design … is not enough’.

For HPWH’s not much focused research is available in which results show which refrigerant is the ‘best solution’. The Global Warming Potential (GWP) can be a useful metric to compare different refrigerants. However, it may overestimate the benefits of low GWP refrigerant to environment, as it does not take into account many other affecting factors. Such factors, like the use of high efficiency components and system design, such as the optimal storage size, stratification and condenser design have a big influence on the overall energy efficiency of the system. Moreover, ensuring proper installation, optimised control and operation, under all common operating and climate conditions are factors not directly related to the technology itself and the choice of refrigerant. In the end, the overall energy use of the installation is an important factor in the calculation of the Life Cycle Climate Performance (LCCP) or the Total Equivalent Warming Impact (TEWI) factor. At the 2019 IIR Conference in Montréal a presentation on this topic was given by dr. Pavel Makhnatch (KTH).

The main policies worldwide focus on banning the use of refrigerants with a high GWP. However this does not necessarily result in lowering climate impact, expressed in term of LCCP value. Thus, LCCP evaluation can be necessary in order to account for the entire climate impact of a system when selecting an alternative refrigerant.

In 2018 the Swedish Kyl & Värmepumpföreningen (SKVP) requested that the Swedish state apply for exemptions from the European Union pursuant to Article 15.4 of the F-Gas Regulation for products listed in the Ecodesign Directive Lot 1 (boilers and heat pumps), as well as Lot2 (water heaters and accumulator tanks). This is to give the heat pump industry access to refrigerants outside the F-Gas Regulation quota system in order to ensure a qualitative and energy efficient transition to new low-GWP refrigerants.

Annex Report on Refrigerants

The Annex has in its collaboration analysed the work done on refrigerants for Heat Pump Water Heaters. The Final Report on this is available and can be downloaded.

State of developments

At the 2019 IIR Conference a number of papers were presented, one of these papers focusing on the application of R290 as refrigerant for Heat Pump Water Heaters.

At the 13th IEA Heat Pump Conference a number of papers was presented on refrigerants related to HPWH’s (only available to participants of the Conference):

  • 031 – Bo Zheng (Gree), Experimental Investigation on One-time Heating R32 Two-stage Air Source Heat Pump Water Heater, Xiangfei Liang, Rong Zhuang (Gree Electric Appliances, Inc. of Zhuhai)
  • 102 – Tyler Shelly, Dynamic Modeling and Charge Minimization Study of a Packaged Propane Heat Pump with External Flow Reversal for Cold Climates, Davide Ziviani, Riley Barta, Eckhard Groll (Purdue University)
  • 108 – Emilio Navarro-Peris, Analysis of the Refrigerant Maldistribution of Propane Evaporating in Brazed Plate Heat Exchangers as a Function of Operating Conditions, Lucas Alvarez, Paloma Albaladejo, Jose Miguel Corberan (Universitat Politecnica de Valencia), Lena Schnabel(Fraunhofer Institute for Solar Energy Systems ISE),
  • 117 – Yu-Jia He (Tongji University), Approaching Optimal High Pressure by Charge Management in Transcritical CO2 Heat Pump Water Heater, Xing-Yu Liang, Jia-Hao Cheng, Liang-Liang Shao, Chun-Lu Zhang, Tongji University,
  • 136 – Sergio Maria Capanelli – Benefits and reliability of air-to-water heat pumps in residential applications, using R-290 refrigerant and an alternative design solution to guarantee high safety with standard components, Carel Industries S.p.A., Italy,
  • 172 – Manuel Verdnik, René Rieberer, High-temperature vapor compression heat pump using butane (R600) – development of a prototype and first measurements, (Graz University of Technology),
  • 203 – Lena Schnabel, Evaluation of a Lab Scale Low Charge Heat Pump Circuit Using Propane, Clemens Dankwerth, Timo Methler, Simon Braungardt, Thore Oltersdorf, Christian Sonner, Marek Miara, Peter Schossig (Fraunhofer Heating and Cooling Technologies),
  • 204 – Kashif Nawaz, Alternate refrigerants for Heat Pump Water Heater Applications, Oak Ridge National Laboratory, Bo Shen, Van Baxter, Omar Abdelaziz (Oak Ridge National Laboratory),
  • 297 – Takaoki Suzuki, et al, Annual Performance Assessment of Heat Pump Water Heater Technology based on Thermodynamic Properties of Various Refrigerants, Zheng Ge, Muhamad Yulianto, Yoichi Miyaoka, Seiichi Yamaguchi, Kiyoshi Saito, Waseda University,
  • 354 – Gu Hwang Kang, Study on the Performance of Heat Pump by Mixing Ratio of Isobutane/Propane Mixed Refrigerants, Sun ik Na (Seoul National University), Min Soo Kim (Seoul National University),

This indicated the great interest by Universities in USA, Spain, China, Korea, Austria, Germany and Sweden, where also some manufacturers are involved.