| HEAT PUMPS environmental threat to the future of health in London, highlighting particulates and nitrogen dioxide (NO2) both produced from the combustion of carbon-based fuels. The policy explicitly supports the transition to low carbon and low-pollution heating alternatives such as heat pumps. So, there are good opportunities to investigate the use of alternative systems, and one of the most obvious is the new generation of heat pumps. The decarbonisation of the electricity supply grid in recent years has made a significant change when comparing natural gas fired water heaters with heat pumps as sources of building heat. The 2019 UK carbon factors8 for grid supplied electricity was 0.233kg. kWh-1 and for gas 0.184kg.kWh-1. If a directfired gas water heater is assumed as 98% efficient, then to obtain the same equivalent carbon emission per kWh of delivered heat in the commercial building would require a heat pump with a seasonal coefficient of performance (SCOP) of 0.233/(0.184/0.98) = 1.24. This is a relatively modest COP for a properly installed and operated heat pump. The expectation is that the carbon factor of grid electricity will continue to fall as the proportion of renewable sources are commissioned for example, with the proposed increase in the fields of offshore wind turbines. This will further improve the relative carbon performance of heat pumps. Typical commercial heat pumps are effective at providing low-temperature hot water suitable for space heating and, if installed and operated properly, are likely to provide season COPs of 3 and above. Current vapour compression heat pumps, though, have struggled to maintain good overall performance when producing significant amounts of domestic hot water at temperatures beyond 50C. However, transcritical carbon dioxide (CO2) refrigeration technology, as discussed in the 2012 CIBSE Journal CPD, Module 47, has now matured into commercial heat pump packages, so there are opportunities for efficient higher temperature operation. CO2 is a naturally occurring substance that is non-toxic, non-flammable and environmentally friendly, with a global warming potential of 1 and ozone depletion potential of 0. CO2 also has a low critical point of only 31C @ 7.3MPa, as shown in the pressure enthalpy diagram in Figure 2. At this critical point there is no separate liquid or vapour it is a homogenous supercritical fluid. When refrigeration cycles operate with heat rejection above this critical point they are said to be operating as a transcritical system. This is where heat is rejected to the All non-residential Retail Figure 1: Examples of UK non-residential building energy consumption 2015-167 Offices Hospitality Industrial Storage Health Education Emergency services Military Community, arts and leisure 0% 20% 40% 60% 80% 100% Space heating/cooling Heating water Cooled storage Catering Lighting Other cooling water by de-superheating the CO2 vapour at a supercritical pressure in a gas cooler. As can be seen in the pressure-enthalpy diagram in Figure 2, the operating pressures for a CO2 system are many times that of a traditional vapour compression refrigeration system, such as the R134a cycle shown. The heat pump illustrated in Figure 3 is a 40kW commercial air-sourced monobloc CO2 heat pump designed specifically to supply domestic hot water between 55C and 90C. (Monobloc indicates that the heat pump unit and its refrigeration cycle is contained within one unit. No refrigerant flows outside of the monobloc casing just water.) To provide high efficiencies, as well as to be able to work safely at the higher pressures required for CO2 transcritical operation, specifically designed components have been developed and optimised. Referring to the schematic of a commercial monobloc transcritical air source heat pump in Figure 4, there are several technological aspects that allow safe efficient operation: The gas cooler, where the heat is passed from the refrigerant to the water, is an important element where effective heat transfer is critical to efficient system operation. So, for example, the system in Figure 4 employs a proprietary twisted 20 CO2 Gas cooling 10 Expansion Compression 5 Pressure (MPa) CPD PROGRAMME Critical point 31oC, 7.3MPa Critical point 101oC, 4.0MPa Evaporation 2 Condensation R134a 1 0.5 Compression Expansion Evaporation Enthalpy (kJ.kg-1) Figure 2: Simplified typical refrigeration cycles on P-h diagram for R134a and R744 (CO2) 86 November 2020 www.cibsejournal.com CIBSE Nov 2020 pp85-88 CPD Mitsubishi 171.indd 86 23/10/2020 16:49