Denso details CO(2) A/C system

Motor,  May 2002  by Nash, Tom

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Because of concerns regarding the effects of global warming, the refrigerant for automotive a/c systems was switched from R-12 to R-134a for ozone layer protection. However, the global warming potential of R-- 134a is still high. For the future, carbon dioxide (COQ) refrigerant is considered to be one of the most promising alternatives to help slow global warming.

At the recent Society of Automotive Engineers (SAE) World Congress in Detroit, Denso outlined its progress on developing an automotive air conditioning system based on COa as the refrigerant. However, there is much research and development to do before the system becomes technologically viable and financially feasible.

Why CO^sub 2^? The potential global warming effect of CO^sub 2^ is extremely low-- about 1300 that of R-134a. Even if CO^sub 2^ is accidentally leaked from an a/c system, the adverse effect on the environment is said to be negligible. Also, CO^sub 2^ is a natural element and readily available without extensive manufacturing processes.

Denso's a/c system (see the illustration above) differs from a conventional R-- 134a system mainly in the following ways:

*A gas cooler, which corresponds to a conventional condenser, cools CO^sub 2^ refrigerant discharged from the compressor. Because this refrigerant exceeds the critical point of CO^sub 2^ at the high-pressure side, it's not condensed by the gas cooler. Instead, the expansion valve condenses a part of the CO^sub 2^ refrigerant.

*An inner heat exchanger between the gas cooler and the heat exchanger further cools the CO^sub 2^ refrigerant, which has been discharged from the gas cooler, by exchanging heat with refrigerant flowing at the low-pressure side of the system. The inner heat exchanger increases the liquid refrigerant at the inlet of the evaporator to increase cooling performance.

*The accumulator is located at the low-pressure side, since the refrigerant pressure at the high-pressure side is much higher than that of a conventional system.

The Drawbacks. Carbon dioxide a/c systems are not without problems, however. CO^sub 2^ has a critical temperature lower than that of R-134a and a critical pressure that's higher. The operating pressure is seven to ten times greater than that of R-134a, so all components in the system, including evaporators, must be able to withstand and contain these high pressures. Because the potential for leakage is higher with increased pressure, components initially will need to be made of thicker, stronger materials, adding weight and manufacturing costs.

Also, the system must be designed to function and operate at substantially lower temperatures. New met allurgy will be required for such a system, including Orings, gaskets and seals. In addition, whole new sets of repair procedures and safety standards will need to be developed for CO^sub 2^-based systems.

It Works for Heating Too. Denso has developed a C02 a/c and heat pump system jointly with Toyota. (see the illustration on the bottom of page 46). It has excellent cooling and heating capacities and can be used on an electric or gas/electric hybrid vehicle. This system can be switched between the heating and cooling modes by opening and closing bypass valves 1 and 2. In the cooling mode, bypass valve 1 is opened and bypass valve 2 is closed. The air mix dampers of the interior gas cooler are fully closed. The CO^sub 2^ refrigerant circulates in the system basically in the same way as in the CO^sub 2^ a/c system.

During the heating mode, bypass valve 1 closes and bypass valve 2 opens. Then the air mix dampers of the interior gas cooler open. High-pressure and high-temperature CO^sub 2^ refrigerant discharged from the compressor exchanges heat with air to heat the air as it flows into the interior gas cooler. As needed, the system can perform dehumidification by closing bypass valve 2 and controlling the opening degree of expansion valve 2.

Keys to Success. The whole concept of using CO^sub 2^ as a refrigerant offers hope for the future, but before such an a/c system can enter the marketplace, the following issues must be resolved:

*System manufacturing costs must be lowered.

*Weight of the components must be reduced.

*Reliability must be improved.

*The service and maintenance procedures, as well as the procedures for handling CO^sub 2^ and related a/c systems, must be standardized.

*Diagnostic, service and maintenance equipment must be developed.

Only with the concerted, organized efforts of the automotive manufacturing and aftermarket industries can this concept become a reality. The reduced environmental impact would seem to make the effort worthwhile.

Copyright Hearst Business Publishing May 2002
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