Alternative refrigerants and the environmental performance of heat pumps

Many of the refrigerants still in use have high global warming potential, so the environmental impact due to loss of refrigerant can be very high, as discussed earlier. Whilst the adoption of best practice in the design, build and operation of equipment can minimise leakage, it is difficult to eliminate leaks altogether, so there is increasing focus on alternative refrigerants that have much lower GWP, such as ammonia (R717), hydrocarbons such as propane (R290), carbon dioxide (R744) and the new class of refrigerant blends based on hydrofluoroolefins (HFOs). However, there are many potential issues in using such refrigerants: ammonia is highly toxic and attacks copper, so cannot be used in systems with copper pipe work; hydrocarbons are highly flammable; carbon dioxide refrigerant requires much higher operating pressures which increases the leakage potential end enhances the risk of mechanical failure, presenting a significant safety hazard. The issues around HFO refrigerants depend on the specific blend - some are mildly flammable and all tend to be very expensive.

The suitability of such refrigerants for use in specific applications also depends on their thermophysical properties, which determine the required operating parameters and settings for the cooling system. This can be a particular problem when seeking a replacement refrigerant for an existing cooling system, with limited scope for making major adjustments to the operating parameters. Table C-1 (Appendix C) summarises the thermophysical properties, environmental and safety issues of different refrigerants that are either used or have the potential to be used for heat pump applications. The boiling point (BP), critical temperature

(CT), freezing point (FP), critical pressure (CP), vapour pressures (VP) at different temperatures, vapour density (VD) and chemical stability state are given from the refrigerant material safety data sheets provided by refrigerant manufacturers and suppliers.

Refrigerants that are currently used in heat pumps include R134a and HFC blends R407C, R404A and R410A, for water heating and space heating. R290 has properties similar to those of R22 (HCFC and no longer permitted), apart from its flammability. Until 2004 almost half of the heat pumps sold in the EU used R290 but the use has declined due to the introduction of the Pressure Equipment Directive (PED) and low availability of R290 compressors. R744 heat pump water heaters were introduced to the market in Japan in 2001. R717 is used mainly for large capacity systems, since there are no compressors small enough for domestic heat pumps and copper cannot used with R717. Refrigerants with the potential for future use in air to water systems include R32 and R1234yf. Both R32 and R1234yf are mildly flammable whilst R1234yf has similar thermophysical properties to R134a. For water heating and space heating heat pumps currently using R22, R410A or R407C, significant design changes would be required to optimise them for operation with R1234yf. A recent new low GWP refrigerant with similar in thermophysical properties to R1234yf is R1234ze.

A comparison of the environmental impact of these alternative refrigerants in an air to water heat pump configuration was undertaken. The heat pump performance was calculated using Coolpack, a software tool developed by IPU and the Department of Mechanical Engineering at the Technical University of Denmark (IPU, 2012). COOLPACK is a collection of simulation models for refrigeration systems that includes cycle analysis, dimensioning of main components, energy analysis and optimization. The key assumptions were:

 Heat output from the condenser = 10 kW

 Condensing temperature (Tc) = 70°C and 45°C (2 separate calculations)

 Evaporating temperature (Te) = 2°C

 Compressor isentropic efficiency = 75%

 Suction superheat = 10 K

 Sub-cooling = 5 K

For the R744 system at 70°C condensing temperature (transcritical operation), a discharge pressure (Pc) of 85 bar was assumed, with a gas cooler output temperature of 40°C. Due to thermodynamic properties of R744, the performance at 45°C condensing temperature could not be calculated.

R32, R1234yf and R1234ze refrigerants were not available in COOLPACK, so their system performance was calculated using Pressure-Enthalpy charts, with the same assumptions. Using the Coolpack results the annual energy consumption, annualized refrigerant loss (assumed 6% for all refrigerant types) and their individual and combined environmental impact were then calculated in a spreadsheet to assess the TEWI over a 16 year life for each refrigerant type, at condensing temperatures of 45°C and 70°C. The spreadsheet also included a calculation of the energy and refrigerant replacement costs. A summary of the results is shown in Table 4-5, while the spreadsheet is included as Figure D1 (Appendix D). The results are also shown in graphic form in Figure 4-20.

Table 4-5. Heat Pump TEWI, COPs, energy consumption and annual leakage for different refrigerants

Refrigerant

name Calculated COP

Annual Energy consumption kWh System charge kg Annual leakage amount kg TEWI kgCO2(e) 70°C 45°C 70°C 45°C 70°C 45°C R22 3.02 4.94 13,328 8,148 3 0.18 110,313 69,548 R134a 3.00 4.99 13,417 8,066 4 0.24 111,302 69,196 R404A 2.45 4.60 16,429 8,750 3 0.18 141,050 80,624 R407C 2.77 4.80 14,531 8,385 3 0.18 119,671 71,311 R410A 2.60 4.69 15,481 8,582 3 0.18 128,089 73,800 R290 2.94 4.93 13,690 8,164 1.5 0.09 107,741 64,253 R600a 3.10 5.07 12,984 7,939 1.5 0.09 102,180 62,479 R717 3.22 5.01 12,500 8,034 1 0.06 98,368 63,223 R32 2.82 4.70 14,273 8,564 2 0.18 114,346 69,418

R744 2.41 N/A 16,701 N/A 1.5 0.09 131,431 N/A

R1234yf 2.70 5.32 14,907 7,566 4 0.24 117,329 59,555

R1234ze 2.20 5.05 18,295 7,970 4 0.24 143,999 62,746

In cooling (air conditioning) applications the more relevant TEWI results would be for the 45°C condensing temperature scenario (evaporating temperature 2°C). These suggest that R134a refrigerant would achieve a lower TEWI than R404A, R407C and R410A. Although refrigerants R290, R600a and R717 could achieve lower TEWI, their potential safety hazards could limit their usefulness. The HFO refrigerants could achieve a significantly lower TEWI with a 45°C condensing temperature, but are not believed to be readily available or commercially viable at this time.

Figure 4-20. Heat Pump TEWI calculations for different refrigerants over 15 year life

For a 70°C condensing temperature (heating/ hot water scenario) the systems using R290, R600a and R717 were the best, whereas, systems using R404A, R744 and R1234ze were the worst (the worst performing system producing 46% more carbon than the best). The impact of refrigerant leakage on total carbon emissions was relatively small in all cases particularly for those refrigerants with a GWP below 2500. However, the energy related emissions vary significantly and do not necessarily correlate with the refrigerant GWP, for example lower GWP refrigerants such as R32 and R1234yf have only average life cycle carbon performance, due to their lower efficiency.

For the best performing systems (R290, R600a and R717), there are reported concerns about the availability of components which consequently limit their immediate future application. For instance R717 can only be used with open type compressors (and cannot be used with copper, zinc or their alloys), whilst there are reported to be few compressors available for use with R290 and R600a. Of the existing HFC refrigerants R404A does not perform well. R134a systems perform the best, followed by R410A and R407C. Whilst hydrocarbon, R32 and R1234yf refrigerants may have potential for use in heat pumps in the future they do not currently appear to offer an attractive and commercially available alternative to current HFC

refrigerants. The 2014 assessment of the UNEP refrigeration, air conditioning and heat pumps technical options committee (RTOC) provides a more in depth review of the suitability of refrigerants for use in air to water heat pump systems (UNEP, 2015).