Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)92

Development of Two-Stage Compressor
for CO2 Heat-Pump Water Heaters

HAJIME SATO*1

YOSHIYUKI KIMATA*2YOHEI HOTTA*2TOSHIYUKI GOTO*1

HISAO MIZUNO*3

HIROYUKI KOBAYASHI*4

The use of CO2 heat-pump water heaters is becoming more widespread in response to energy
conservation requirements. This paper describes the development of a high-efficiency compressor
for commercial CO2 heat-pump water heaters. The new compressor is the world's first to employ atwo-stage compression structure, a combination of rotary and scroll mechanisms that can resolve
specific issues of CO2 refrigerant such as large pressure difference and high operating pressure,and it provides the most suitable compression mechanism for the operating conditions of aheat-pump water heater. Consequently, efficiency improvement and higher reliability can berealized under a wide range of operating conditions. MHI’s newly developed “Q-ton” CO2
heat-pump water heater is equipped with the developed compressor, and thus heating capacity andefficiency can be significantly improved in comparison with conventional water heaters. Theseimprovements have made it possible to introduce heat-pump water heaters in cold regions.

|1. Introduction
Nowadays, traditional fossil-fuel water heaters are being replaced by heat-pump systems in
response to energy conservation requirements and concerns about global warming. CO2 as arefrigerant has low global warming potential (GWP), is non-flammable and is appropriate forhigh-temperature hot-water supply due to its refrigerant properties. Accordingly, CO2 has been
widely utilized as a refrigerant for household heat-pump water heaters, and its use is also increasingin commercial and industrial fields. On the other hand, CO2 has a high operating pressure, leading
to larger leakage and mechanical losses at the compressor. Therefore, the reduction of these losses
is essential to improve the performance of CO2 heat-pump water heaters.
To resolve the issues described above, the authors have developed a novel compressor forcommercial CO2 heat-pump water heaters. The new compressor is based on two-stagecompression, using a combination of scroll and rotary mechanisms to enable the reduction ofleakage and mechanical losses. In addition, by employing intermediate gas injection, the newcompressor has attained high heating capacity and high efficiency under a wide range ofconditions.
This paper describes our efforts to improve the efficiency and reliability of the newcompressor and introduces a commercial CO2 heat-pump water heater equipped with the newcompressor.

|2. Structure of the New Two-stage Compressor
Table 1 compares the characteristics of CO2 (employed here as a refrigerant for heat-pumpwater heaters) and other common refrigerants. Alternatives to chlorofluorocarbon refrigerants(HFC refrigerants), which are typically used in current refrigeration and air conditioning systems,
have high global-warming potential (GWP). Therefore, the shift to refrigerants with lower GWP is*1Nagoya Research & Development Center, Technology & Innovation Headquarters*2

Air-Conditioner Designing & Engineering Department, Air-Conditioning & Refrigeration Systems*3

Group Manager, Air-Conditioner Designing & Engineering Department, Air-Conditioning & Refrigeration Systems*4

General Manager, Quality Assurance Department, Air-Conditioning & Refrigeration Systems

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)93

an important issue in the refrigeration and air conditioning fields. Although some equipment uses
natural refrigerants such as ammonia or hydrocarbons, these refrigerants have not been widely usedin refrigeration and air-conditioning systems because of their toxic or flammable properties. On theother hand, since CO2 has no flammability, low toxicity and is suitable for high-temperature
hot-water supply due to its refrigerant properties, it has been widely used as a refrigerant inhousehold heat-pump water heaters. However, CO2 has high operating pressure and a high pressure
difference compared with conventional refrigerants. This results in an increase of leakage andmechanical losses. Therefore, it is especially important for the development of CO2 compressors tomake the proper selection of its basic structure.
Table 1 Comparison of refrigerant characteristics
Alternative tochlorofluorocarbon
refrigerant (HFC refrigerants)
Natural refrigerants

Global-warming
Toxicity
potential
2 , 090
Low

R410A

oNone

Pressure (20 C)
(MPa)
1.34

None

0.47

High

Low

0.76

Low

High

0.74

Low

None

5.63

R134a

1 , 430Low

NH3 (ammonia)

1 or lessC3H8 (propane)

3CO2 (carbon dioxide)

1Flammability

In order to establish the basic structure of the new compressor, the number of compression
stages, the shell pressure and the type of compression mechanism are studied. Table 2 compares
compressor performances according to structure. Based on this study, the following structures are
employed for the developed compressor.
(1) Two-stage compression
The pressure difference between discharge- and suction-pressure of CO2 refrigerant isseveral times larger than that of conventional HFC refrigerants. This leads to an increase of gas
leakage and mechanical losses. However, by introducing two-stage compression, which
decreases the pressure difference in each stage, higher efficiency and higher reliability can beobtained.
In addition, the developed compressor is equipped with an intermediate gas-injection
mechanism to increase COP (coefficient of performance) and heating capacity. By employing
two-stage compression, gas injection can be introduced between the first and second stages, andthus mixture loss can be reduced in comparison with single-stage compression with direct
injection to the compression chamber.
Table 2Comparison of compressor performance according to structure

Number ofShellcompression pressure
stagesSingle stageDischargepressureSuctionpressure
DischargepressureIntermediatepressure

Two stages

Suctionpressure

Compression
mechanism
First
Secondstage
stage

Shell
Gas
thickness injection
--+--

+*1

Reliability

Efficiency

First
stage

Secondstage

First
stage

Secondstage

ScrollScroll++++*2+*2+++ScrollRotary+++++-++RotaryScroll+

++

++

++

++

++

RotaryRotary+++

++

-++

+

ScrollScroll++*1+*2+*2+++

ScrollRotary++*1

++

-++

*1

RotaryScroll++-++++++

RotaryRotary++*1--+++*1 Connection piping between first and second stages is required.
*2 An oil pump must be placed in the middle of the shaft.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
94(2)

Intermediate pressure shellSince the operating pressure of CO2 is much higher than that of conventional HFC, shellpressure has a great impact on the shell thickness and hence on the compressor weight. This
impact grows as the size of the compressor becomes larger, such as in commercial andindustrial applications. Because of this, it is effective to keep the design shell pressure low toensure a lightweight compressor. The developed compressor adopts an intermediate pressureshell so that the discharge pressure does not directly affect the shell, realizing downsizing and areduction in weight.
(3) Combination of scroll and rotary mechanisms
The compression mechanism is one of the most important factors in determining theefficiency and reliability of a compressor. Particularly in a two-stage compressor, it is necessary
to select appropriate compression mechanisms for the respective operating characteristics of thefirst and second stages. It is generally known that the mechanisms of a positive-displacement
compressor include reciprocating, screw, scroll, and rotary mechanisms. The developed
compressor employs rotary and scroll mechanisms for the first and second stages respectively
as the most suitable compression mechanisms for a heat-pump water heater. This innovative
combination of scroll and rotary mechanisms enables high reliability and high efficiency undera wide range of operating conditions.
Figure 1 shows the structure of the developed compressor.The compressor is driven by abrushless DC motor located in the middle of the compressor and has two compression chambers oneither side of the motor. The rotary-type first-stage compression chamber is located on the lower
side of the motor and is connected to a suction pipe via a suction accumulator. The scroll-type
second-stage chamber is located on the upper side of the motor and is connected to a discharge pipe
via a discharge chamber.
Refrigerant gas is introduced into the first-stage compression chamber through theaccumulator and is compressed to intermediate pressure. The compressed gas is then discharged
into the shell. Injection gas is introduced into the shell through the gas-injection port and is mixed
with compressed gas from the first stage. The mixed gas is then introduced into the second-stagecompression chamber and compressed to discharge pressure. The compressed gas is discharged tothe outside of the compressor through a discharge pipe on top of the shell.

Figure 1 Structure of the developed two-stage compressor|3. Efficiency of the Developed Two-stage Compressor
To verify improved efficiency, performance tests of the developed two-stage compressor anda conventional single-stage scroll compressor were carried out. Figure 2 shows a comparison of theefficiency with respect to operating pressure ratio between the new compressor and theconventional compressor. The vertical scale represents the values obtained by dividing theefficiency of a compressor by the efficiency of the conventional compressor at a compression ratioof 3. For the conventional compressor, the efficiency dropped significantly with an increasing
operating-pressure ratio due to increased leakage and mechanical losses. On the other hand, thedeveloped compressor showed little loss of efficiency, even with an increased operating-pressureMitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
95ratio, through the use of the two-stage compression mechanism. The developed compressor
resulted in a 15% improvement in efficiency at a compression ratio of 3 and more than 30%
improvement at a compression ratio of 4 or higher compared with the conventional compressor.
Hence, the developed two-stage compressor achieved significant efficiency improvement under awide range of operating conditions.

Differences in efficiency withFigure 2
respect to operating-pressure ratio

Ambient air Inlet/outlet watertemperaturetemperatureoo( C)( C)
Condition A
1627/65
Condition B
79/65
Condition C-76/65
Condition D
-20
5/65

Figure 3
Improvement of capability
according to operating condition

In order to evaluate the performance improvement of a heat pump system equipped with thedeveloped two-stage compressor, a performance comparison between a two-stage cycle withintermediate gas injection and single-stage cycle was conducted under four different ambient air
temperature and inlet water temperature conditions. Figure 3 shows the improvement in heating
capacity and COP (Coefficient of Performance) of a two-stage intermediate gas injection cycle
equipped with the developed two-stage compressor compared with a single-stage cycle equippedwith a conventional compressor. For each of the conditions, improvement over the conventional
cycle was observed. The improvement ratio increased as the ambient air temperature decreased
because the refrigerant flow rate of the gas cooler was increased by the gas injection, and theefficiency of the compressor was improved by employing the developed two-stage compressor. The
new compressor showed a 25 % improvement of heating capacity and a more than 50%
improvement of COP under deep-freeze conditions (cond. D).
These results indicate that the developed two-stage compressor offers special advantages forapplications operating under large temperature differences, such as refrigeration or chilling units aswell as heat pump water heaters in cold region.

|4. Reliability Assurance of the Developed Compressor
Since the pressure difference of CO2 refrigerant is several times higher than that ofconventional HFC refrigerants, the load on sliding parts such as bearings increases significantly incomparison with that of conventional refrigerants. For most existing CO2 compressors, special
efforts have been required to assure their reliability such as applying a hard coating to the sliding
surfaces and reducing the contact pressure on the bearings. However, by adopting a two-stagecompression mechanism, the developed compressor decreases the pressure difference in each stage
to a level equivalent to that of conventional HFC refrigerants. Thus, the load on sliding parts can bereduced to a similar level to that of conventional HFC compressors.
Providing a proper oil supply to all sliding mechanical parts in both the first and second
stages is one of the issues unique to the two-stage compression structure. Figure 4 shows thesupply pathways for lubricating oil and refrigerant in the developed two-stage compressor.
The developed compressor is equipped with a positive-displacement oil pump at the lower
end of the crankshaft. The lubricating oil is pumped from the oil storage at the bottom of theMitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
96

compressor and introduced into the oil channel in the crankshaft. The oil is then supplied to thejournal bearings of the first and second stages. Afterward, lubricating oil is supplied to the othersliding parts and the compression chambers. To assure the reliability of a compressor, it isnecessary to supply a proper amount of oil to all sliding parts. In the developed compressor, thecapacity of the oil pump and the dimensions of the oil channel are optimized in terms of the amountof oil required by each bearing and the pressure drop in the oil channel. As a result, proper
lubrication can be provided to all sliding parts.

Figure 4 Flow pathways for refrigerant and lubricating oil in thedeveloped compressor|5. Application of the Developed Two-Stage Compressor to a
Commercial Heat-pump Water Heater
The developed two-stage compressor is installed on our recently developed “Q-ton”
commercial CO2 heat-pump water heater.1 “Q-ton” now has the following features: Two-stage compression and an intermediate gas-injection cycle are adopted.
 The rated heating capacity is 30 kW and is sustainable at ambient air temperatures as
olow as -7 C.
 The COP on rated conditions reaches 4.3, which is the highest level in the industry.
o A 90 C hot water supply is available even at ambient air temperatures as extreme aso-25 C.
Figure 5 shows the variation in heating capacity and COP with respect to ambient airtemperature. Because conventional heat-pump water heaters are hampered by decreased heatingcapacity at low ambient temperatures, their applicability in cold regions has been limited. However,the developed system achieves both high heating capacity and high COP under the conditions at
low ambient temperatures by adopting two-stage compression and an intermediate gas-injection
cycle using the new two-stage compressor. Consequently, Q-ton makes it possible to introduce
heat-pump water heaters in cold regions, which has always been a difficult problem.

Figure 5 Variation in heating capacity and COP with respectto ambient air temperatureMitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
97|6. Conclusion
MHI developed a two-stage compressor for commercial CO2 heat-pump water heaters.
Employing the world's first combination of rotary and scroll mechanisms and intermediate gas
injection, the developed compressor realized high reliability and high efficiency operation under awide range of conditions. Q-ton, a commercial CO2 heat-pump water heater equipped with thedeveloped compressor, achieved significantly improved heating capacity and COP in comparison
with previous water heaters and makes it possible to introduce heat-pump water heaters in cold
regions, which has always posed difficulties. We would like to continue our pursuit of enhancing
compressor performance to offer energy savings and earth-friendly products to our customers.

Reference1.

Development of the “Q-Ton” CO2-Refrigerant Heat Pump for Industrial Water-Heater Systems for Use at
Outside Air Temperatures Down to -25oC, Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4(2011) pp. 86-88