ASHRAE quotes R744 as efficient MAC refrigerant

The American Society of Heating, Refrigerating and Air-Conditioning Engineers has, for the first time, issued a public position paper about natural refrigerants, where it considers CO₂ to be a good alternative to R134a in mobile air conditioning. It also outlines the various uses R744 can efficiently answer to and cites it as a perfect substitute to CFCs.

A couple of weeks after organizing the first CO₂-only seminar “CO₂ Systems, Components and Applications”, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has published a position paper on the various uses, properties and potential of environmentally superior natural refrigerants in refrigeration and heating systems. With this paper, ASHRAE recognises the potential that natural refrigerants offer in improving the environmental performance of refrigeration systems and expresses its support for research, assessment, and strategic growth in their use.

Following a press release issued on 5 March, ASHRAE points out that the position document demonstrates the organisation’s commitment to the application of natural refrigerants, the development of strategic relationships to advance their use, and the consideration of natural fluids in existing and new guidelines, codes and standards. Moreover, ASHRAE’s paper will serve as the basis for providing guidance and education to policy makers and the public, for disseminating methods and tools for the environmental assessment of refrigeration systems, and for publicizing technical information on safety, reliability and efficiency issues. Further publications and seminars are therefore planned.

CO₂ as important refrigerant

Due to its low toxicity, non-flammability, zero ozone depletion potential and low global warming potential R744 has been recognized as an environmentally friendly substitute to chlorofluorocarbons (CFCs) over the last two decades. The document outlines how R744 has gained wide use in vapor-compression systems, from low temperature freezers to high temperature heat pumps. Its characteristics make it a broadly adopted secondary refrigerant, offering significant improvements in efficiency compared with traditional water, glycol or brine systems.

ASHREA also acknowledged that today there are many transcritical CO₂ systems in supermarkets. For about 90% of the year the Coefficient of Performance (COP) of systems with carbon dioxide is higher than in HFC systems. This is the reason that it is an attractive choice for beverage cabinets and vending machines.

High pressure, an asset for R744

The paper elaborates on the pressure/temperature characteristics of R744 requiring special equipment designs. ASHRAE acknowledges the benefits arising from high-pressure CO₂ systems, where the high pressure results in high gas density, which allows for a far greater refrigerating effect to be achieved from a compressor. It concludes that exceptionally good system performance is guaranteed in low temperatures plate freezers and multi-chamber blast freezers

When the pressure is raised above the critical point (7.3773 MPa) heat rejection is achieved by cooling the very dense gas which results in a temperature glide effect. This makes R744 a refrigerant of choice for water-heating heat pumps for a range of applications from domestic to industrial. The characteristics of R744 make it more suited for dense heat loads compared with smaller central heating units for example.

R744 versus R134a

The ASHRAE paper also mentions the hotly debated MAC issue where the currently used refrigerant R134a has a six times lower heat transfer capability than R744 at 14°C, the optimal temperature for transferring heat to R744. Thanks to its properties, the German Association of the Automotive Industry (VDA 2007) has confirmed that all mobile air conditioning (MAC) systems for cars will be fitted with R744 from 2011, when the European MAC Directive first deadline comes into force.

ASHRAE and AIRAH: on climate change

On 27 February, ASHRAE and the Australian Institute of Refrigeration Air Conditions and Heating (AIRAH) issued a joint resolution on climate change emphasizing the necessary use of renewable energy, education of the building industry and responsible refrigerant use to insure sustainable future of heating and refrigeration. Yet again, ASHRAE stresses the importance for the heating and refrigeration industry to evolve towards greater sustainability and greater environmental concern integration.

“The use of HVAC&R technologies is an essential element of contemporary life,” Bill Harrison, ASHRAE president, said. “Yet, HVAC&R systems contribute to greenhouse gas releases through energy-related effects and through the effects of refrigerant losses. ASHRAE and AIRAH are emphasising a variety of measures to decrease emissions associated with energy use and its effect on global climate.”