REFRIGERANTS | CO 2 Working fluid GWP CO2e for annual losses* R744 (CO2) 1 53kg R410A (HFC) 2,088 110 tonnes R448A (HFO) 1,387 74 tonnes R290 3 159kg *Calculation based on like-for-like volume of 354kg Table 1: Comparative fugitive emissions COP of R744 TC ASHP vs. R450A ASHP @ 5C ambient A polytropic process is a thermodynamic process that obeys the relation: n pV = C Where p is the pressure, V is specific volume, n is the polytropic index, and C is a constant. The polytropic process equation can describe multiple expansion and compression processes, which include heat transfer. The value of n is different in different thermodynamic processes. The polytropic index is a measure of the work done by the system. Given a value for n, then the heat of compression may be determined using the following equation. [(n-1)/n] T2/T1 = (p2/p1) Flow/return temperatures (C) Figure 3: Performance difference between R744 and R450A in a heat pump at 5C ambient temperature CO2 offers more efficient heat transfer A huge practical advantage of CO2 is the heat transfer in the gas cooler, which is typically a plate heat exchanger in a heat pump. In a condenser, a large fraction of the heat transfer surface is being used for phase change (condensation), during which the temperature remains constant, or only changes slightly, with some working fluid blends. With CO2, the temperature is changing during the gas-cooling process in transcritical mode. This helps to achieve very high water temperatures that are not normally achievable in HFC systems, or can only be approached by using a de-superheater, which further increases complexity. Where supply water temperatures are above 55C, CO2 heat pumps offer a better coefficient of performance (COP) compared with other technologies. CO2 is a preferable working fluid to supply heat at high temperature levels. The next energy transition is just starting and will be a period of rapid change in all respects technologically, socially and regulatory where T is the thermodynamic temperature. Where the numbers 1 and 2 denote the states at the beginning and end of the compression process, it can be seen that a higher value of n gives a higher differential in temperatures, thus CO2 has a greater temperature difference than HFCs. This means CO2 can deliver useful temperatures for heating applications while drawing heat from air at normal ambient temperature ranges, all year round. The performance difference between CO2 (R744) and a typical synthetic fluid (R450A) can be seen in Figure 3. CO2 systems producing high water temperatures have a higher efficiency than systems with other working fluids. However, many other factors in heat pump design also influence this. Different manufacturers will control the heat pump using proprietary algorithms and careful component selection, resulting in performance differences. CO2 is more dense than other working fluids, so the pipework, number of compressors, components and rack size are smaller in general (see Figure 4). For example, the required suction pipe cross-section diameter for CO2 is approximately half that required for R404A (for the same volumetric capacity). This is especially valid for largecapacity systems that are likely to require more compressors and much larger diameter pipework if designed for HFC/HFOs. Allowing for higher temperatures Potential hurdles for CO2 heat pump deployment The index of compression is very high for CO2, so the discharge temperature is higher than for HFCs. Also known as the polytropic exponent, the index of compression is 1.289 for CO2 and only 1.005 for R404A. CO2 is a great option for a heat pump working fluid. However, it faces two significant challenges from the buildings sector: building system return temperatures and oversizing. A CO2 heat pump in the factory destined for a school 70 November 2022 www.cibsejournal.com CIBSE Nov 22 pp67-68, 70, 72 CO2 paper.indd 70 21/10/2022 16:32