Cold Climate Heat Pumps

Figure 2 – A single monobloc ASHP can provide domestic hot water as well as hydronic space heating. (Courtesy of Hisense.)

The HiSense product boasted low-GWP R32 but conventional R410A refrigerants were also common at the AHR Expo. The one major difference between the recent 2023 ISH Expo (Frankfurt, May 2023) and the 2023 AHR Expo was the lack of R290 ASHP products at the AHR Expo compared to ISH Expo. An exception was the PHNIX (Guangzhou, Guangdong, China) Greentherm Air-to-Water HP for space heating and domestic hot water. PHNIX claims stable running at ambient temperature down to –15 °F (– 26 °C) using R290 natural refrigerant [11].

Europe is on a faster track to phase out HFC refrigerants and also to permit the use of R290 in outdoor monobloc evaporators for ASHPs. This was evident in the presence of many units containing R32 as well R290 refrigerant at the ISH Expo.

The evaporators for the ASHPs on display at the AHR Expo typically were comprised of copper tubes with diameters of 9.52 mm (3/8 in.) or 7 mm. Yet there were several companies offering heat exchangers made with 5 mm diameter copper tubes. For example, Amber Enterprises (Haryana, India) [12], Bohn (Querétaro, Mexico) [13], Indus (Sharjah, United Arab Emirates) [14] and Lordan (Kfar Szold, Israel) [15] were promoting the manufacture of coils based on 5 mm or 7 mm diameter copper tubes [12-15].

Heat pump designs on display at the AHR Expo to some degree reflect the existing housing stock and the climate zones. As an economical alternative to ground source heat pumps (GSHP), different types and sizes of air source heat pumps (ASHPs) accounted for the wide array of products on display. Types of ASHPs include Air-to-Air (ATA) and Air-to-Water (ATW) heat pumps. Another distinction is made between split system heat pumps and (outdoor) monobloc heat pumps.

Also, midsize systems for multifamily homes and light commercial buildings were prevalent, for example, 20 refrigeration ton (RT) systems (equivalent to 240,000 BTU/h, or 70 kW). These systems can provide space heating and domestic hot water in apartment complexes. They lend themselves well to variable refrigerant flow (VRF) and the use of large outdoor evaporators. Indeed, they may be the low-hanging fruit in the push toward decarbonization.

In this case, the fluid passes inside the tubes of an air handler and forced air picks up the heat and distributes it through the home. This cascaded system combines an ATW heat pump with an air handler and the net effect is the same as an ATA heat pump.

An ATW heat pump is suitable for replacing the boiler in hydronic heating systems, which distribute hot water through pipes. Hydronic space heating is accomplished through conventional radiators, in-floor radiant heating or floorboard heaters. In the case of an ATW heat pump, the condenser may take the form of a brazed plate heat exchanger, which transfers heat from the hot compressed refrigerant to the water for use in space heating or domestic hot water.

For ASHPs with a secondary loop, the refrigerant path is restricted to the outdoors. These self-contained units are commonly referred to as monoblock heat pumps, or “monoblocs.” (See Figs. 2 and 3.)

Monobloc ASHPs can be used for space heating or domestic hot water (DHW) or both. These have different temperature requirements, and their efficiency depends on the outdoor temperature. Heat pumps that are specifically designed to be effective in cold climates are generally referred to as Cold Climate Heat Pumps (CCHPs).

Monobloc ASPHs with small-diameter copper tube heat exchangers have the potential to reduce refrigerant charge and enable the use of natural refrigerants such as R290, which have an ultralow GWP and do not pollute the environment.

Figure 3 – An outdoor ATW HP can be coupled with a multi-function tank to provide domestic hot water and space heating as well as space cooling and tap water. (Courtesy of PHNIX.)

Figure 4 – Smaller diameter copper tubes (5 mm or 7 mm) are in demand as OEMs discover their advantages in a variety of applications including evaporators for monobloc CCHPs. (Photograph Courtesy of Lordan.) Conclusion Heat exchanger engineers and policy makers from around the world met at the triannual IEA Heat Pump Conference in Chicago, and they will also meet at the quadrennial IIR International Congress of Refrigeration in August. It can be expected that the pace of product innovation, ramping up of production and market expansion will only increase in the coming years and the best designs will win over the market! Right now, it is still too early to pick the winning technologies and companies but it should be noted that ATW HPs have many benefits for space and water heating. The industry is bracing for a period of rapid transition, comparable to the transition to propane in the refrigeration industry and the perfection of R744 supermarket refrigeration systems. Much of this change will be driven by the existence of a robust supply chain, one that can meet the demand for smaller diameter copper tubes as well as the heat exchangers made from these tubes. On thing is certain. Cold climate heat pumps are coming to America … perhaps sooner than anticipated.