AIR SOURCE & GROUND SOURCE HEAT PUMPS
Revolution or evolution? heat pumps for hot water
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Bill Sinclair, technical director at hot water specialists Adveco considers the status of heat pumps for domestic hot water applications in commercial buildings
ith the consultation period now complete, expectations for the 2025 Future Building Standard will see the end of new buildings receiving gas
connections. In a move from the current five building types there will also be simplification to just two, sheds with direct electric heating and point of use, the remainder of buildings are set to be supplied by heat pump. In the drive to delivering net zero this not only helps usher in the beginnings of an all-electric future from 2026, it also cements the important role that heat pumps are to play in making it a reality. Despite this, there remains a challenge within the HVAC industry to provide better guidance on the use of heat pumps - especially for water heating - for achieving low carbon and ultimately net zero targets. There continues to be a discrepancy, often at the highest levels, when communicating requirements and obligations between residential (domestic) and commercial installations. Obligations that, amongst others, demand balances in legionella risks and other water quality issues. Air source heat pumps (ASHPs), offering significant carbon emission reduction over traditional gas boilers, have emerged as a compelling option for generating hot water in UK commercial buildings. But the technology also presents unique considerations. ASHPs can be a good fit for buildings with moderate or low hot water demand, but those with higher hot water requirements, typically seen in many commercial scenarios require either a larger ASHP unit or a hybrid approach that blends other technologies, such as electric boilers or solar thermal to meet peak demands. The efficiency of an ASHP is also impacted by ambient air temperature and the temperature of water an application must achieve. Efficiency can drop during colder periods as well as when a unit is pushed to deliver higher temperatures, this requires the unit to consume more electrical energy to achieve desired working flows. High temperatures in domestic hot water (DHW) systems for commercial buildings must maintain a minimum temperature (60°C) to prevent Legionella bacteria growth. For this reason, and to optimise the efficiency of the heat pump, the preferred application of ASHP has been to exclusively supply the pre-heat for water. This has notably driven a resurgence in undersized system storage, especially in like-for- like replacement of gas water heating, as larger thermal storage is now required to offset slower reheating after periods of peak demand. Greater system complexity with external units and larger thermal storage demands also means ASHP-based systems require more space and capital expenditure than traditional gas systems. They will also cost considerably more
to operate. The overarching advantage is the technology reduces carbon emissions, but many smaller businesses will continue to struggle to balance this against higher replacement and operational costs.
The challenge then is to efficiently deliver high-temperature water with a heat pump without necessitating complex system designs and major structural alterations. Standard heat pump components are generally unsuitable for very high temperatures, demanding larger specialised compressors and heat exchangers which add to the size and weight of a unit. High temperatures also require a change in refrigerant to achieve suitable efficiencies. The current default refrigerant in most ASHPs on the market is R-32 (Difluoromethane) which has been popular due to its efficiency and lower global warming potential (GWP) compared to prior R-410A refrigerant. Its ability to deliver DHW temperatures is realistically limited to 55°C before loss of efficiency is too great. For this reason, the next generation of ASHP is embracing R-290 (pure propane) which offers greater efficiency at moderate DHW temperatures and has the capability to reach DHW temperatures of 70°C with the bonus of an ultra-low GWP for an extra tick in the box for being better for the environment. Specifiers are seeking a singular heat pump application which can meet high- temperature demands creating a ready market for the next generation of ASHP. Our research with R-290 propane units indicates that whilst the refrigerant enables large 40-50 kW units to achieve a promising coefficient of performance, 2.4 at 65°C working flow/7°C ambient, there remains a propensity for output to fall steeply when the exterior temperature drops, lowering COP to as little as 1.8 at 70°C working flow and -5°C ambient. This brings the units back in line with existing smaller, and lighter R32 models. First-generation R-290 units are large and heavy, as much as ½ tonne for 40 and 50 kW models, which can introduce new limitations in terms of where and how the ASHP can be easily and safely installed. They also tend to be noisier which is a particular consideration should the building in question have a residential aspect such as a hotel, hospital, care home or school/university dormitory.
Looking further into the development cycle, variant blends of relatively new refrigerants such as 513 and 1234 (blended with R32 for 454) potentially offers a safer, more balanced performance at higher temperatures with a relatively low GWP making them well-suited for DHW. An R513 ASHP, for example, could be more compact and lightweight due to a smaller compressor and fan assembly, making them easier to install. Critically they are also likely to
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be cheaper to specify. This though assumes development is not influenced by new EU F-Gas regulations which, in an effort to transition away from higher GWP HFCs, are set to cease use of low GWP fluorinated refrigerants in favour of non-fluorinated alternatives. So where do we stand today? In terms of DHW design the building and its usage will always drive the application. R290 heats pumps alone are still not the complete answer. With carefully designed, balanced DHW applications heat pumps still lend themselves best to hybrid approaches, reducing emissions from existing gas-fired applications or helping offset direct electric demands from topping up heat with electric boilers in indirect systems. In truth, the current generation of R32 ASHPs offers an extremely viable way to introduce low-carbon, renewable technology into UK buildings and the technology meets all current BREEAMM requirements. They can also work in close conjunction with solar thermal acting as a mid- heater, this maximises the efficiency of the heat pump as it works with the cold in. The advantages in carbon reduction and
energy offsetting mean even complex systems can be cost-effective despite higher capital investment. In short, we need to accept that the hybrid approach continues to remain the optimal path to net zero water heating in commercial buildings.
BUILDING SERVICES & ENVIRONMENTAL ENGINEER JUNE 2024 27
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