Propane as a refrigerant for the HVAC industry


19 October 2022

In August 2021, the Intergovernmental Panel on Climate Change (IPCC) released the first version of the AR6 report, Climate Change 2021 – The Physical Science Basis, which contains the new official GWP (Global Warming Potential) values for hydrocarbons, writes Edoardo de Pantz, managing director, Acquaria.

This report, using the 6th assessment, shows that propane (R290) has a 100-year GWP of 0.02, much lower than the traditional value of 3 using the 4th assessment (because of its three atoms of carbon (C3H8). Although 3 is a very low value if compared with most popular HFCs, it is still many times higher than the new value.

This news, together with the increased R32 GWP value from 675 to 771, could be a game-changer in the HVAC industry, especially if the 6th assessment is implemented by DEFRA as the reference point for F-GAS use enforcement,  as most likely it would be no longer possible to use R32 in new equipment from January 1st 2025 (for new single-split units, with a charge under 3kg at least).

The change to the 6th assessment has added weight to the fact that propane has become a valid alternative as refrigerant gas, on top of its effectiveness and energy-efficiency as a refrigerant. Why natural? Propane is mainly obtained as a by-product of natural gas extraction and oil cracking. Before being used in refrigeration systems, it is optimised through a specialist cleaning process.

Environmental impact

To demonstrate the environmental impact of a refrigerant it is sometimes clearer to use a CO₂ equivalent reference rather than using GWP figures, that is the refrigerant carbon footprint of the HVAC appliance that we put into the market.

A simple example can be worth a thousand words. A typical 50kW air source heat pump (ASHP) using a 10kg charge of R410A has the carbon impact equivalent of 21 tonnes of CO₂. The same system with R290 has 15kg of CO₂ equivalent, and now using the new AR6 classification the carbon impact can be as low as 90 grammes!

The majority of HVAC manufacturers are late in re-designing chillers and their heat pump ranges to use R290 as a refrigerant. However, a few of them started decades ago and now have significant case studies, practical experience and evidence of its successful use. It’s fair to say a large number of manufacturers are now playing catch-up and in the next few years we will see many new products entering the market. ASHP will be the focus, but water to water (W2W) appliances will also be of significant interest, especially when thinking of HP cascade systems and the system efficiency calculations now advised in CIBSE AM17:2022. 

In designing for the first time or re-designing a new heat pump, manufacturers will follow the detailed and specific Standard & Regulations (EN 378, ISO 5149), and will make sure to implement all the relevant safety measures required. But the real challenge will come from the thermodynamic performances of R290 compared with conventional refrigerants, and the advantages / opportunities that thermodynamics will give with R290 - as state of the art and as future developments – in perspective.

Benefits of R290

Interestingly, when we analyse R290 thermodynamic performances as a refrigerant we realise there are many advantages. Let’s see a few of them:

Compressor gas discharge temperature

R290 and especially R600a (isobutane) have a lower discharge temperature than other refrigerants. The benefit of a low discharge temperature has a positive effect on compressor materials, components and stability of the oil. If the discharge gas temperature is too high, decomposition of lubrification oil occurs.

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Higher heat transfer, low-pressure ratio

Thanks to the excellent thermodynamic properties of propane, such as the higher heat transfer and low-pressure ratio (condensation to evaporation pressure) it is possible to design appliances with smaller heat exchangers, this could lead to a reduction in physical space requirements, making planning and installation easier.

Lower density

The density of propane is much lower than of fluorocarbons (less than half than of R134a and R410a), and this, combined with higher heat transfer, gives a much lower refrigerant charge as a result. A number of analysis tests have been carried out in this regard using reciprocating compressors; this showed a potential half R290 charge compared with R32 and almost one third compared with R134a.

Efficiency comparison

Comparisons to R410A EERs for R290 and R600a are between 5% and 12% higher. This simplistic comparison of EERs is indicative of the strong potential for energy efficient operation of HC systems.

Compressor envelope comparison for different refrigerants

In this case we can see how propane is an efficient and flexible refrigerant in different operating conditions. It can go from very low evaporating temperature to high condensing temperatures. 

This makes possible to design a wide range of products from chillers (high, mid and low temperature) to heat pumps.
Euroklimat started using propane in 2006 on a wide range of R290 process and comfort cooling units up to 500kW cooling capacity, and later in 2018 extending to the R290 ASHP range up to 200kW heating capacity. However, time waits for no one and Euroklimat launched a 700kW heating capacity unit in early 2022. 

Overall, these comparisons can demonstrate that smaller sizes of compressors can be used with propane, as well as heat exchangers. Even considering medium GWP refrigerants such as R513a and low GWP options such as R1234ze and R1234yf, the comparison is in favour of R290, even considering the same reciprocating compressor model.

Early in July of this year I was in Milano MCE and I met with a number of scroll compressor manufacturers. They all declared: “Sure, we’re ready with R290”. Appreciating the massive effort required, this is not an easy process and in fact only the smallest compressors have been released. In many cases they are used for residential or small commercial HP projects. Effort and of course investment is required to develop a widely available commercial HP range, 20-30-50HP, tandem and trio configurations, high temperature versions for DHW production, and these are not yet available. But the demand to reach net zero and the obvious climate changes we have all experienced over the last few years mean we need it and we need it now. In the meantime, we enjoy the good old reciprocating compressor, so reliable, so efficient, full Inverter.

In conclusion, one of the ways for the HVAC industry to contribute to saving our planet is to promote an increasingly consistent use of natural refrigerant fluids such as propane R290 (GWP=0.02) and isobutane R600a (GWP=0.006).

The direction is well marked, the industry is definitely on the way and excellent results are expected soon from all major players in the market.