REFRIGERATION

Investigation of Vapour Absorption Refrigeration (VAR) systems for solar powered installations

Chapter 7: Research Methodologies

Introduction

Vapour compression-based systems are generally employed in refrigeration, air-conditioning and heat pump units. During the last century, the halogenated refrigerants have dominated the vapour compression-based systems due to its good thermodynamic and thermo-physical properties. But, the halogenated refrigerants are having poor environmental properties with respect to ozone depletion potential (ODP) and global warming potential (GWP). The international protocols (Montreal and Kyoto) restrict the use of the halogenated refrigerants in the vapour compression-based refrigeration systems.

As per Montreal protocol 1987, the use of chloroflurocarbon (CFCs) was completely stopped in most of the nations. However, hydrochloroflurocarbons (HCFC) refrigerants can be used until 2040 in developing nations and developed nations should phase out by 2030 1. Most of the developed nations reduced the consumption of HCFC refrigerants. The Kyoto Protocol of United Nations Framework Convention on Climate Change (UNFCCC) calls for reduction in emission of six categories of green house gas, which includes hydroflurocarbons (HFCs) as refrigerants 2. To meet the global demand in refrigeration and air-conditioning sector, it is necessary to look for long-term alternatives to satisfy the objectives of international protocols. HC and HFC refrigerant mixtures with low environment impacts are considered as potential alternatives to phase out the existing halogenated refrigerants.

Only a few pure fluids are having properties closer to the existing halogenated refrigerants. The refrigerant mixtures provide much flexibility in searching new environment-friendly alternatives to match the desirable properties with the existing halogenated refrigerants. The two alternative options are HC and HFC mixtures with lower GWP. HC-based mixtures are environment-friendly, which can be used as alternatives without modifications in the existing systems. But HC refrigerant mixtures are highly flammable, which limits the usage in large capacity systems 3. The HC refrigerant mixtures are preferred only for small capacity refrigeration units such as, domestic refrigerators, bottle coolers, visi coolers, deep freezers, etc., which require less refrigerant quantity compared with the halogenated refrigerants. HFC mixtures are ozone-friendly, but it has significant GWP.

HFC mixtures are not miscible with mineral oil, which require synthetic lubricants (such as polyolester). The synthetic lubricants used with HFC refrigerants are highly hygroscopic in nature, expensive, cause irritation when it comes in contact with skin, which leads to several service issues while retrofitting 4. To overcome the problems with HC and HFC refrigerant mixtures, hydrocarbons are mixed with HFC refrigerants, which improves the miscibility (with mineral oil) and also reduces the flammability (of HC mixtures). Earlier investigations reported that HFC/HC mixtures are miscible with mineral oil. The GWP of HFC/HC mixture was also reported to be less than one-third of HFC, when it is used alone. It is possible to mix HC refrigerants with HFC to replace the existing halogenated refrigerants.

Earlier reviews in this area include that of Wang and Li 7, who summarized the perspectives of natural working fluids in China for refrigeration and air-conditioning applications, which includes both compression and absorption-based refrigeration systems. Gryand 8 presented an overview of various pure HC refrigerants ...