| 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. |
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| Features of CO2
Refrigerant |
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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. |
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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. |
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| *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. |
|
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| 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. |
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As shown in FIG. 1, an inner heat exchanger
is provided between the gas cooler and the heat exchanger to
further cool the CO2 refrigerant, 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. |
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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 CO2 air conditioning system becomes simple
and is easy to install in the vehicle. |
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Each component is designed to enable resistance
to high operation pressure. |
|
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| FIG. 1 Basic Structure of CO2
Air Conditioning System |
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 |
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| FIG. 2 Basic Configuration of CO2
Air Conditioning System |
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 |
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| 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 for Toyota Fuel Cell Hybrid
Vehicle (Toyota FCHV-4). 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. |
| |
| The FCHV-4, equipped with this system, began public
road tests in August 2002. Limited sales are slated to begin at the
end of 2002. |
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| FIG. 3 Structure of CO2
Air Conditioning and Heat Pump System |
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 |
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| Outlook and Issues of CO2
Air Conditioning System |
| |
| Before the CO2 air conditioning system
can enter the marketplace, the following issues must be resolved: |
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Reducing cost; |
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Reducing weight; |
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Ensuring reliability, and |
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Building infrastructures including: |
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Providing service and maintenance equipment,
and |
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|
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Clarifying and standardizing procedures of
how to handle CO2 air conditioning systems for
service and maintenance while maintaining safety. |
|
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| These issues are very difficult to be solved solely
by one company. DENSO will strive to settle these issues in order
to place the CO2 air conditioning system in the market
in cooperation with automakers, other air conditioning system suppliers
and governments worldwide. |
| |
[Contact]
Shinya Omi, Yoko Suga, Miwa Kurokawa
Corporate Communications Department of DENSO Corporation,
+81-566-25-5594/5592
shinya_omi@denso.co.jp,
yoko_suga@denso.co.jp,
miwa_kurokawa@denso.co.jp,
jennifer_knoll@denso.co.jp
http://www.globaldenso.com |
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