Modelling of an Integrated Supermarket Refrigeration and Heating System Using Natural Refrigerants

This paper describes the development of a computer simulation model of an integrated supermarket refrigeration and heating system using natural refrigerants (namely ammonia and hydrocarbons). Initial resultS of the model's performance are presented, showing the influence of some of the operating parameters on system thermal efficiency. This will be validated using data collected from monitoring of a similar plant. The model and monitoring of a novel refrigeration plant are part of the UK's contribution to IEA Annex 22 work program Compression Systems with Natural Working Fluids (NWFs). INTRODUCTION The growing accept..mce of TEWI (tot..'ll equivalent warming impact) as a me..'lsure of a systems contribution to global warming. puts greater emphasis on the energy efficiency associated with the system, rather than ',the direct contribution from the refrigerant. Natural refrigerants, such a<> ammonia, hydrocarbons and carbon dioxide. are widely seen as offering environmental benefit<; for retail refrigeration. The inherently high efficiency, and negligible direct global warming effect of natural refrigerants, in comparison to the widely used HFCs. will have a positive impact on the TEWI rating of a refrigeration system. Also. many see tighter controls on refrigerant emissions and potential restrictions in supply of HFCs as an impetus to move towards natural refrigerants. However, many have problems assocbted with their use ammonia for example is toxic, whilst hydrocarbons are highly flammable. Conventional supermarket refrigeration systems use direct cooling, with refrigerant circulated around the building to individual display case air coolers. The refrigerant leakage rate for this type of system is significantly greater than indirect systems, adversely effecting the TEWI rating for refrigerants with a global warming effect In comparison to direct systems, the charge of primary refrigerant for an indirect system can be reduced by as much as 80-90%. Furthermore, the risk of leakage is greatly reduced for an indirect system, since the primary refrigerant is contained in a factory assembled and leak tested unit. Ammonia and hydrocarbons are only likely to be acceptable a<; refrigerants for indirect systems, where a limited charge can be contained safely. Recent changes to the British standards has widened the scope for 'natural' refrigerant usage; these are favoured more positively in Europe than in the USA. The safe and effective use of ammonia, coupled with a secondary system, represents a very sound solution with the potential for excellent energy efficiency in supermarkets. Until recently ammonia has not been used widely in supermarket refrigeration plant. However, recent developments, and considerable investment, have seen a number of full scale field trials by the majority of the main retailers, and the demand is steadily increasing in the UK. SUPERMARKET REFRIGERATION MODEL A common approach to HV AC systems has been adapted to meet the chilling, freezing, store heating and cooling demand of a typical supermarket. A single phase secondary fluid is pumped around the shop floor ring mains acting as a heat sink and heat source for freezer, chiller and heat pump compressor packs. See figure L Any surplus heat from the secondary fluid is rejected to the