Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
1

Development of Demonstration Unit of Residential
Air Conditioner Adopting Low-GWP Refrigerant toReplace R32

SEIJI HIRAMATSU*1

ATSUSHI ENYA*2

Wide-ranging regulations have been introduced to deal with global warming, and various
measures have been implemented accordingly. Under such circumstances, air conditioning andrefrigeration systems are required to not only have energy-saving features, but also start using
alternative refrigerants with less environmental impact.
Adopting R454C refrigerant, which has an extremely low global warming potential (GWP),
Mitsubishi Heavy Industries Thermal Systems Ltd. has developed a demonstration unit of aresidential air conditioner with enhanced environmental properties. The global warming impact of
this demonstration unit represents a reduction of more than 90% from that of our earlier models.

|1. Introduction
In recent years, the reduction of greenhouse gas emissions and enhanced energy-savingfeatures are in increasing demand from the standpoint of fighting back against global warming. TheKigali Amendment to the Montreal Protocol (2016) prescribes the international rules thatdeveloped countries will reduce, by 2036, the impact of refrigerants on global warming by 85%
from the average of the actual values between 2011 and 2013, while developing countries need toachieve the equivalent reduction approximately 10 years later (Table 1).
Table 1

HFC production/consumption phase-down schedule in the Kigali Amendment- The adopted Amendment adds the phase-down of HFCs (coming into effect on January 1, 2019).
- Developed countries are responsible for an 85% phase-down by 2036, while developing countries arerequired to fulfill a more than 80% phase-down by sometime around 2045.
October 10-14, 2016 at Kigali (Rwanda)

Baseline
years
Baseline
Baseline
calculation
HCFC
includedFreezePhase-downschedule

Developing countries
Group 1Developing countries
Group 2Developed countries

2020 to 2022

2024 to 2026

2011 to 2013

Average HFC
consumption

Average HFC
consumption

65% of HCFC baseline

65% of HCFC baseline

2024

2028

Step 1

10% reduction in 2029

10% reduction in 2032

Step 230% reduction in 2035

20% reduction in 2037

15% of HCFC baseline[*25%]―10% reduction in 2019[*5% reduction in 2020]
40% reduction in 2024[*35% reduction in 2025]
70% reduction in 2029
80% reduction in 2034

Step 350% reduction in 2040
30% reduction in 2042
Step 4――Finalreduction
80% reduction in 2045
85% reduction in 2047
85% reduction in 2036
goal
○ Developing countries Group 1: Developing countries that do not belong to Group 2○ Developing countries Group 2: India, Pakistan, Iran, Iraq and other Gulf countries○ (*) Belarus, Russia, Kazakhstan, Tajikistan and Uzbekistan
HCFCs: Designated fluorocarbons including ozone depleting substances (e.g., R22)
HFCs: Alternative fluorocarbons excluding ozone depleting substances, which have a high GWP under
existing circumstances and have become subject to the regulations as part of global warming
prevention measures (e.g., R32)

*1

Chief Staff Manager, Air-Conditioner Designing & Engineering Department, Mitsubishi Heavy Industries Thermal Systems,*2

Chief Staff Manager, Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)2In air conditioners, it is particularly necessary to switch to refrigerants with a lowGWP(Note 1). Nowadays, an increasing number of small-capacity air conditioners such as residential
air conditioners have changed refrigerants from R410A (GWP: 2090) to R32, which offers a low
GWP (675) and superior performance. However, R32 is slightly flammable. If it is applied tolarge-capacity air conditioners such as multi-split building air conditioners, some difficulties willarise in terms of facility design and construction. For example, safety measures such as shut-offvalves are required in many cases because the refrigerant is supplied in large quantities to a single
room. Since R32 is designated as a regulated substance (HFC), it has to be replaced by anotherrefrigerant with an even lower GWP to achieve the 85% reduction in 2029. We therefore took up
the challenge to develop a residential air conditioner demonstration unit, which can eventually lead
us to the development of air conditioners that use unregulated refrigerants.
Note 1: GWP stands for Global Warming Potential. It is expressed relative to CO2 (which has a GWP of 1)
Lower GWPs indicate a lower greenhouse effect and superior environmental properties.

|2. Selection of alternative refrigerants
As the requirement specifications in this product development, the low GWP refrigerant tobe adopted should satisfy the following:(1) Non-toxic(2) Able to maintain a pressure high enough to be clearly considered operable at anoutside air temperature as low as -15°C(3) In the case of zeotropic refrigerants, having a small temperature glide(Note 2) inconsideration of frost formation while heating using the outside air as a heat source
Table 2 summarizes the safety and environmental properties of several types of refrigerants.
The candidate refrigerants satisfying the aforementioned product requirement specifications include
R1234yf, R454C and R455A, all of which are comparable to R32 that is currently in use. However,considering use at an outside air temperature as low as -15°C, R1234yf was excluded because thepressure at the saturation temperature being close to atmospheric pressure would make its
application difficult. Figure 1 compares the properties of major refrigerants. Zeotropic refrigerants
with a large temperature glide during the phase change process have a risk of lowering the heating
capacity because of localized frost formation in the outdoor heat exchanger that works as anevaporator during heating. Therefore, in accordance with our selection criteria, R454C has been
selected as a low GWP refrigerant with a temperature glide as small as R407C, which has alreadybeen used in other products.
Note 2: Temperature glide is a phenomenon in which the phase change temperature(evaporation/condensation) is not constant. This phenomenon is particular to refrigerants with amixed composition.

Table 2Compositions, GWPs and safety classes of major refrigerantsRefrigerant nameComposition
GWP
Safety class
R410A
R32/R125=50/50Wt%
2090
A1
R32
Single675
A2L
R1234yf
Single
1A2L
R454C
R32/R1234yf=21.5/78.5Wt％
146
A2L
R455A
R32/R1234yf/CO2=21.5/78.5/3Wt%
146
A2L
Safety classification
A: lower toxicity, B: higher toxicity; 1: no flame propagation,
2L: slight flammability, 2: lower flammability, 3: higher flammability

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
3Figure 1

Property comparison of major refrigerants|3. Issues in securing performance and solution technology summary
If applied without any changes from the current model using R32, R454C is estimated tohave a high specific volume because of its properties as a refrigerant, consequently decreasing thecooling/heating capacity. The following improvements in the refrigeration cycle and frost
formation during heating have been made:(1) Increased displacement (approximately 64%)(2) Increased size of the outdoor heat exchanger (approximately 25%) and reviewed flow
path configuration (number of paths) in the heat exchanger(3) Increased size of the connected gas pipe diameter (φ 9.52 → φ 12.7) (reducedpressure loss)(4) Increased size of the EEV diameter (φ 1.5 → φ 1.8)
These improvements have succeeded in overcoming the performance decline caused by therefrigerant properties. The test results using the demonstration unit show a 0.5% reduction in thecooling capacity and a 0.2% increase in the heating capacity (both on a calculation basis), thus
indicating the likelihood of satisfying the performance regulations.

|4. Major challenges for practical application
The thermodynamic properties of zeotropic refrigerants with a low GWP have a potentialthat is comparable to R410A or R32 in terms of environmental performance. However, their use
requires larger compressors and heat exchangers than in the case of R32, increasing the size andcost. Therefore, it is necessary to achieve even higher efficiency and develop the technology fordownsizing. We will continue to develop highly-efficient compressors for low pressure refrigerants(other than positive-displacement compressors) and high-performance heat exchangers that cancontribute to a reduction of the refrigerant filling amount, and improve the cycle, for example, byintroducing internal heat exchangers to make frost formation less frequent.

|5. Conclusion
R454C has a GWP of 146. It is approximately 1/14th and 1/5th of the respective GWPs ofR410A and R32, which are currently and most widely used in small-to-medium-capacity airconditioners. As a result, the global warming impact(Note 3) from the refrigerant used in thedemonstration unit falls by 91.3% from the level of the current model(Note 4), which represents aMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)4highly-significant reduction. Since 2015, the European countries, where environmental awarenessis deeply entrenched, have gradually expanded regulations affecting the sale and use of global
warming substances including HFCs with a GWP of 150 or higher. The development of aresidential air conditioner adopting R454C is the fruit of our leading and positive response to such
movements.
Going forward, we will continue to dedicate our resources to developing and marketinghigh-performance products using low-GWP refrigerants, thereby contributing to the protection of
the global environment in markets across the globe.
Note 3: Impact of a refrigerant on global warming, which is calculated by multiplying the global warming
potential (GWP) of the refrigerant by its volume
Note 4: The top-of-the-line “SRK25ZSX-S” for the European market with a piping length of 5 m, which
adopts R410A refrigerant

References(1)(2)

NEDO Environment Department Activity Report Meeting: Activities to develop technologies related toglobal warming prevention measures in fluorocarbon field (27 July 2017)
JRAIA PAC Technical Committee: Task force of explanatory meeting increasing awareness on use of R32
for store-use air conditioners (16 March 2018)