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Air (R-729) is free and plentiful and there are no direct global warming, ozone depletion, and toxic potentials associated with it. In view of these attributes, air cycle air conditioning (referred to as the R-729 system) commends itself prima facie against the backdrop of today's environmental concerns. Presented in this paper are the historical developments and a critical assessment of the R-729 system for motor vehicles including its coefficient of performance (COP) under realistic operating conditions and the total equivalent warming impact (TEWI). Also included in the paper are the extensive performance comparisons of the R729 system with those of the highly-touted carbon dioxide (R-744) system and the baseline R-134a system. As elicited by the following tabular values of COP and TEWI, both the R-744 and R-729 systems are inferior to the baseline R-134a system.

As a prelude to the assessment of the global warming impact of automotive air conditioning, fundamental scientific facts about global warming are reviewed. A comprehensive mathematical model is next developed to quantify the total equivalent warming impact (TEWI) of the automotive air conditioning. Using this model, intensive calculations are performed to determine the TEWI of the present R-134a air conditioning system as well as the proposed alternate systems. The calculated results indicate that TEWI of the present R-134a system is higher than those of the flammable subcritical systems (R-152a, R-290 and R-717) but lower than those of the supercritical carbon dioxide system (R-744) and open air (R-729) cycle system. The calculated results show that TEWI of 303 million R-134a automotive air conditioning systems in the worldwide fleet is no more than 0.14% of TEWI of the greenhouse gases emitted annually into Earth's atmosphere due to human activity.

There is intense research activity to appraise the merits of the carbon dioxide (R-744) mobile air conditioning system due to its perceived amelioratory effect on the total global warming impact which comprises two components: direct global warming due to refrigerant leakage into the atmosphere and indirect global warming due to power consumption by the system. While the direct global warming impact of R-744 is negligible compared to that of R-134a, the indirect global warming impact of the R-744 system is intrinsically higher than that of the R-134a system. In order to quantify the indirect global warming impact of the R-744 system, an accurate assessment of its coefficient of performance (COP) vis-a-vis COP of the present baseline R-134a system is necessary.

The paper deals with potential augmentation of the present R134a automotive air conditioning system with the intent to lower its total equivalent warming impact (TEWI) which is a source of concern from the standpoint of environmental benignity of the system. It is identified that the most effective augmentation strategy includes (1) increase in compressor isentropic efficiency, (2) increase in condenser effectiveness, (3) decrease in lubricant circulation through the system, (4) decrease in air side pressure drop in evaporator through improved condensate management, (5) increase in condenser airflow, (6) decrease in air conditioning load via permissible increase in the amount of recirculated air through the passenger compartment and (7) reduction in direct emission of R-134a from the system through conservation and containment measures. The effect of each of these augmentations on the coefficient of performance (COP) of the system is quantified in a rigorous fashion.

The focus of the present paper is the energy efficient automotive air conditioning system, which ipso facto is also environmentally friendly from the standpoint of global warming impact. Two efficiency enhancement strategies are presented - one entailing the use of judicious amount of recirculated air and another relying on reduction in the amount of reheating of the chilled air employed in the conventional system. The first strategy, referred to as the air inlet mixture strategy, reduces the air conditioning load by mixing proper amount of recirculated air with the outside air. The second strategy, referred to as the series reheat reduction strategy, reduces reheating of chilled air under low to moderate load conditions. Analytical relations are presented for the determination of reduction in air conditioning load due to mixing of outside air with varying amounts of recirculated air as well as due to reduction in the series reheat.