Contact: Marlene Goldsmith
(248) 372-8222
marlene_goldsmith@denso-diam.com

FOR IMMEDIATE RELEASE

March 4, 2002

CO₂ Air Conditioning Systems

Concerns regarding the effect of global warming have been growing in recent years. Refrigerant of car air conditioning systems was switched from CFC-12 to HFC-134a in many countries for ozone layer protection. However, the global warming potential of HFC-134a refrigerant is still high, and car air conditioning systems are required to use a more environmentally benign refrigerant to prevent global warming. For the future, carbon dioxide (CO₂) refrigerant is considered to be one of the most promising alternatives.

Features of CO₂ Refrigerant

Extremely Low Global Warming Potential Value (GWP)

The global warming potential value of CO₂ is extremely low, which is about 1/1300 of HFC-134a. Therefore, even if CO₂ is leaked from an air conditioner system when it is used as refrigerant for the system, the effect caused by leakage to the environment is negligible.

High Operating Pressure

CO₂ has a critical temperature^*1 lower than that of HFC-134a and a critical pressure^*2 higher than that of HFC-134a. Therefore, in air conditioning systems adopting CO₂ refrigerant, a high-pressure side temperature exceeds the critical point. This results in a high operation pressure that is 7 to 10 times larger than that of HFC-134a.

*1) Critical temperature is the temperature above which a substance cannot exist in the

liquid state regardless of the pressure.

*2) Critical pressure is the equilibrium pressure of a fluid that is at its critical temperature.

DENSO CO₂ Air Conditioning System

DENSO, which is a global leader in air conditioning systems with the world’s top market share of 24 percent, has developed a CO₂ air conditioning system resistant to high operation pressure.

DENSO’s CO₂ air conditioning system shown in FIGS. 1 and 2 differs from a conventional air conditioning system using HFC-134a mainly in the following points:

A gas cooler, which corresponds to a conventional condenser, cools CO₂ refrigerant discharged from the compressor. Because CO₂ refrigerant exceeds the critical point of CO₂ at the high-pressure side, CO₂ refrigerant is not condensed by the gas cooler. Instead, the expansion valve condenses a part of the CO₂ refrigerant as a result of adiabatic expansion.

As shown in FIG. 1, an inner heat exchanger is provided between the gas cooler and the heat exchanger to further cool the CO₂refrigerant, 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 the cooling performance, resulting in increased Coefficient of Performance (COP) of the system.

The accumulator is disposed at the low-pressure side since the refrigerant pressure at the high-pressure side is about 10MPa or more, which is much higher than that of the conventional system. Because the accumulator is integrated with the internal heat exchanger and the expansion valve, the structure of the CO₂air conditioning system becomes simple and is easy to install in the vehicle.

Each component is designed to enable resistance to high operation pressure.

DENSO CO₂ Air Conditioning and Heat Pump System

Based on the CO₂ air conditioning system shown in FIGS. 1 and 2, DENSO also developed a CO₂ air conditioning and heat pump system jointly with Toyota Motor Corporation. The CO₂ air conditioning and heat pump system has excellent cooling and heating capacities and can be suitably used for an electric vehicle or an electric hybrid vehicle, a power source of which cannot act as a heat source for heating the cabin, unlike a conventional internal combustion engine.

This system can be switched between the heating mode and the cooling mode by opening and closing by-pass valves #1 and #2 (see FIG. 3).

Specifically, at the cooling mode, the by-pass valve #1 is opened and the by-pass valve #2 is closed. Further, air mix dampers of an interior gas cooler are fully closed. Accordingly, CO₂ refrigerant circulates in the system substantially in the same way as the CO₂ air conditioning system shown in FIGS. 1 and 2.

At the heating mode, the by-pass valve #1 closes and the by-pass valve #2 opens. Then, the air mix dampers of the interior gas cooler open. Accordingly, high-pressure and high-temperature CO₂ refrigerant discharged from the compressor exchanges heat with air to heat the air while flowing into the interior gas cooler. As needed, the system can perform dehumidification by closing the by-pass valve #2 and controlling the opening degree of the expansion valve #2.

Moreover, the compressor for this system is driven by an electric motor, and the compressor and the motor are hermetically integrated together. This results in good sealing performance, a simple structure and easy installation in a vehicle.

This CO₂ air conditioning and heat pump system has been installed in a Toyota fuel cell hybrid vehicle (Toyota FCHV-4) to undergo public road tests.

Outlook and Issues of CO₂ Air Conditioning Systems

Before the CO₂air conditioning system can enter the marketplace, the following issues must be resolved:

Reducing cost;

Reducing weight;

Ensuring reliability, and

Building infrastructures including:

-- Providing service and maintenance equipment, and

-- Clarifying and standardizing procedures of how to handle CO₂air conditioning

systems for service and maintenance while maintaining safety.

These issues are very difficult to be solved solely by one company. DENSO will strive to settle these issues in order to place the CO₂air conditioning system in the market in cooperation with automakers, other air conditioning system suppliers and governments worldwide.

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