Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)27

Development of Automotive Air-Conditioning Systems
by Heat Pump Technology

TOSHIHISA KONDO*1

AKIRA KATAYAMA*1

HIDEKI SUETAKE*1

MASATOSHI MORISHITA*2

In the future, the market for electric and hybrid vehicles is expected to expand against abackground of environmental protection and tightened CO2 emission controls. This report
describes the inspection of a heat pump air-conditioning system that uses thermal energy in theoutside air to heat the vehicle cabin. When applied to electric and hybrid vehicles, this system
reduces energy consumption by 20% to 60% compared to current systems.

|1. Introduction
The market for next-generation vehicles such as electric vehicles (EVs) and hybrid vehicles(HEVs and PHEVs) is expected to expand in the future. However, because of problems such as ashorter driving mileage, higher price, and undeveloped infrastructure, they have not yet becomecommonplace. In particular, improved energy efficiency is required for air-conditioning systems
used in next-generation vehicles to extend their driving mileage. Existing air-conditioning systems
significantly decrease the driving mileage due to their electric power-consumption during heating
operations. Therefore, air-conditioning systems that reduce electric power-consumption are
currently under development.

|2. Current Situation and Issues
2.1

Current situation

The heating operation of an air-conditioning system on a vehicle equipped with an internal
combustion engine uses exhaust heat transported by hot water drawn into the vehicle cabin fromthe engine cooling system. For an electric vehicle, however, exhaust heat is lower than that of aconventional internal combustion engine, and is insufficient for heating operation, since the electricmotor has a high energy-conversion efficiency, and the kinetic energy of the vehicle is recovered asregenerative electric power during braking. At present, systems equipped with an electric water
heater are commonly used to compensate for the heat shortfall (Figure 1).

Figure 1 Electric water heater*1

Automotive Thermal Systems Designing & Engineering Department, Air-Conditioning & Refrigeration Systems Headquarters*2

Nagoya Research & Development Center, Technical & Innovation Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
28

These systems offer advantages such as the following. First, development costs can be
reduced because parts and components can be shared with existing production vehicles (excluding
the electric water heater). Second, one need not route the high-voltage cables into the vehicle cabin
if the electric water heater is mounted outside, and hence a measure of passenger safety during acrash can be incorporated into the vehicle design.

2.2

IssuesAlthough an electric water heater system provides beneficial features such as those describedabove, it also gives rise to major issues when used in an electric vehicle. Its heating operation
requires a larger proportion of electric energy for the entire vehicle, and the driving mileageaccordingly decreases (Figure 2). Extending the driving mileage by increasing the battery capacity,
however, creates difficulties with regard to battery mounting space, weight, and cost.

Figure 2

Energy balance

In vehicles equipped with aninternal combustion engine,
exhaust heat from cooling loss isused for the heater. In electricvehicles, the electric power
required for the heater issupplied by the battery.

|3. Application of Heat Pump Air-Conditioning System
to Eco-Friendly Vehicles
Mitsubishi Heavy Industries, Ltd. (MHI) has chosen heat pump technology for automotive
air-conditioning systems as a solution to the problems described above, and carried out aninspection with particular emphasis on improvement of energy efficiency during heatingoperations.

3.1

Selection of heat source and refrigerant

The heating performance of a heat pump system is greatly affected by the heat source that isused for heating, and thus selection of the heat source is important. For a heat pump used in an
automotive air-conditioning system, possible heat sources for its heating operation are as follows:
・Thermal energy in the outside air;・Thermal energy from the ventilation air in the cabin.
In this research, existing refrigerant (R134a) was used because of its availability andaffordability. Table 1 compares heat pump systems using the two possible heat sources. As a resultof this comparison, MHI selected the system using outside air for further inspection, owing to its
simple structure and possible cost reduction, both of which are important factors in achieving wider
distribution. The details of this system are presented below.

3.2

Overview of the heat-pump system using outside air heat source

The heat-pump system using the outside air consists of a compressor, a condenser locatedinside of the cabin, a 3-way valve, a receiver, an expansion valve, a evaporator located outside ofthe cabin, a evaporator located inside of the cabin, a condenser located outside of the cabin, andplumbing that connects these components. It uses thermal energy in the outside air as the heatsource for heating the vehicle cabin (Figure 3). The heating performance depends on external
conditions such as the outside air-temperature, weather, and vehicle speed. Hence, adequately
investigating the influence of such conditions is important when a heat-pump using outside air heatsource is applied to an automotive air-conditioning system. For example, the performance of atypical heat pump system may be compromised by frost formation on the outside evaporator during
heating operation in cold weather. Thus, verifying that the system is unlikely to experience such
frost formation is necessary.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)29

Table 1 Comparison of systems
Heat source for the heater

Heat pump system A
Outside air

Heat pump system B
Ventilation air in the cabin

R134a

R134a

Heater capacity

System diagram

Heating medium
OutsideCapacity
temperature
0ºC

COP

OutsideCapacity
temperature
10ºC

COPCostThe heat pump system using outside air can improve energy
efficiency and reduce cost because of its simple structure, eventhough it is subject to an outside temperature limit.

Figure 3

good＞＞

badCircuit diagram of the heat pump system using outside air

This system uses thermal energy in the outside air as the heat source for the heater.

3.3

Component characteristics

The characteristics of the outside evaporator and inside condenser, both of which were newly
designed for the heating operation of this system, are described below.
(1) Evaporator located outside of the cabin (for heating operation)
The evaporator for cooling operation (located inside of the cabin) has a compact design
and uses a double-row refrigerant flow path to increase the heat exchange rate per unit area,
allowing it to fit into a constrained mounting space inside the cabin, while attaining thenecessary capacity. In addition, the heat exchanger surface has a special finish to prevent
condensed water from flying off. The evaporator for heating operation, however, must fit into
the constrained mounting space provided for the cooling components located outside the cabin,while attaining the necessary airflow. Accordingly, MHI adopted a thin, low-pressure-loss
design, whose heat exchange rate per unit area is smaller than that of the evaporator locatedMitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)30

inside of the cabin (Figure 4). Since one must consider frost formation during heating
operation, MHI designed the heating evaporator with headers at the top and bottom, and a
vertical refrigerant flow path, also taking account of dewatering (Figure 5).

Figure 4

Unit performance of the outside evaporator for the heaterComparison of characteristics of the inside evaporator and outside evaporator

Figure 5(2)Outside evaporator for the heater

Condenser located inside of the cabin (for heating operation)
The condenser for cooling operation (located outside of the cabin) has a thin,low-pressure-loss design to fit into the constrained mounting space provided for the coolingcomponents located outside the cabin, while attaining the necessary capacity and airflow. The
condenser for heating operation located inside of the cabin, however, must fit into a constrained
mounting space inside the cabin, while attaining the necessary capacity. Hence MHI adopted adouble-row structure, whose heat exchange rate per unit area is greater than that of thecondenser for cooling operation located outside of the cabin (Figure 6).

Figure 6

Unit performance of the inside condenser for the heaterComparison of characteristics of the inside condenser and outside condenser

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)31In addition, for the sake of compatibility with air-conditioning units on vehicles poweredby internal combustion engines, MHI designed the size and structure of the heating condenser
so that it can be replaced with a heater core (Figure 7).

Figure 7

3.4

Inside condenser for the heater

Vehicle level heating and defroster performance

Performance evaluation tests on an electric vehicle equipped with the heat-pump systemusing outside air heat source yielded the following results.
3.4.1 Heating performance
To evaluate the heating performance, the electric power consumption of the heat pumpsystem was compared to that of the electric water heater system as the outside temperaturechanged from 0ºC to 10ºC, while the cabin temperature was maintained at a comfortable 25ºC.
Table 2 lists the test conditions and Figure 8 shows the test results. The results verify that theheat pump system can reduce electric power consumption by 20% at an outside temperature of0ºC, and by 60% at 10ºC. The higher the outside temperature is, the greater the reduction rate
will be, presumably because the system recovers thermal energy from the outside air. Thus, lesselectric power is required for thermal energy recovery when the difference between the cabintemperature and the outside temperature is smaller. In addition, the experiment verifies thefeasibility of a continuous heating operation to maintain a comfortable cabin temperature,provided the outside temperature is within the experimental range.

Table 2 Test conditions used toevaluate heater performance
Test condition
VehicleElectric vehicle(A-segment)

Outside temperature

0ºC to 10ºC

Cabin temperature

25ºC

Vehicle speed

40 km/hFigure 8 Comparison of electric power consumption

The heat pump system can reduce electric power consumption by
20% to 60%, while maintaining a comfortable inside temperature.

3.4.2 Windshield defroster performance
A performance test of the windshield defroster in securing visibility at low outside
temperature was also carried out. Table 3 lists the test conditions and Table 4 presents the test
results. The results confirm that this heat pump system satisfies domestic regulations regarding
defrosters. In addition, the experiment verifies the feasibility of continuous defroster operation.

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Table 3Test conditions used to evaluate defroster performance
Test condition

VehicleElectric vehicle (A-segment)

Outside temperature–8ºC

Standard

Japanese regulations for defroster performance

Vehicle speed

0 km/hTable 4 Windshield defroster performance test results
Area

Required value

Test result

Judgment

Driver side

80% or more (after 20 minutes)

80%

Accepted

Passenger side

80% or more (after 25 minutes)

100%

AcceptedWind shield

95% or more (after 40 minutes)

100%

Accepted|4. Conclusions and Future Prospects
The performance evaluation of an actual electric vehicle equipped with the heat-pumpsystem using outside air heat source verified that the heating operation of the system can improveenergy efficiency, while continuously maintaining a comfortable cabin temperature at an outside
temperature between 0ºC and 10ºC. In addition, effective defroster operation was confirmed evenwhen the outside temperature dropped to –8ºC. In the future, MHI will improve the integrity of thesystem by verifying its practicality under various driving situations. MHI already possesses
development technologies for electric water-heater systems, and aims to combine the benefits of
the heat pump system and the electric water-heater system to devise enhanced heating systems andmeet user needs through cooperation with manufacturers of next-generation vehicles.