The search for low GWP refrigerants has revealed the intriguing possibility of the development of blends containing both 'natural' and man-made components.
With the air conditioning and refrigeration industry seemingly split into two distinct camps - those promoting and enlarging the application of so-called 'natural' refrigerants and those developing low GWP fluorocarbons - ACR News has learned of research being undertaken into blends of HFOs and carbon dioxide.

However implausible this combination might seem, Honeywell has patents for refrigerants containing these components as has fellow refrigerant manufacturer Mexichem.

Admitting that it was taking research into this potential blend very seriously, a Mexichem spokesman said that HFO/CO2 blends were not an unusual development.

'Chemical manufacturers have looked at HFC/CO2 and HCFC/CO2 mixtures in the past,' he said. 'The technology is pretty well known but it never previously had any commercial impact in part because of the high glide and limited benefits over other solutions,' he added.

With glide now better understood, refrigerant combinations with markedly different boiling points may be less of an issue.

While admitting that Honeywell had also been working on blends of various hydrofluoro-olefins with CO2, a Honeywell spokesman said that it was currently 'pursuing other blends which we believe offer a balance of properties more suited to the applications of interest to our customers.'

Honeywell's patents suggest past work with both HFO1234yf and ze in combinations with CO2 as a potential replacement for R134a and, in different percentages, for R22, R407C or R410A amongst others.

The Honeywell patents describe the blends as being non-flammable and suitable as a drop-in replacement or with some minor system modifications.

Additionally, Mexichem has been looking at three-component blends of 1234ze and CO2 with R32, R152a, R161, R134a and hydrocarbons. Mexichem also sees them as a potential replacement for a range of HFCs including the high GWP gas R404A. Mexichem is currently looking at using between 3% and 10% CO2 in the final blend.

'In principle, by varying the blend ratios you could match the performance of a range of refrigerants,' says Mexichem. 'CO2 boosts the pressure of the system, increasing refrigeration capacity.' So CO2 is not just being used to counter the mild flammability of the HFOs. 'CO2 will act to reduce flammability but it is not a good fire suppressant,' says Mexichem.

Blend developments are still only at the research stage so imminent introductions to the market are not likely.

'We are focussing on car air conditioning at the moment but will be looking at stationary systems in the future,' said Mexichem.