Project Subjects
Highly-Efﬁciency Heat Pump
Background and Objective
Heat pumps, such as air-conditioners or water heaters, etc. are widely used as a measure to promoteenergy conservation and to reduce carbon emissions. The development of more efﬁcient heat pumps with low
GWP (Global Warming Potential) refrigerants and their expansion to new areas of application is needed.
The CRIEPI embarked on basic research on CO2 heat pump in 1995 and ﬁnally commercialized a CO2
heat pump water heater for residential use, with the pet name “Eco-cute”, in conjunction with Tokyo ElectricPower Company and DENSO Corporation.
In this project, we evaluate the performance of next generation Eco-cute and develop a more efﬁcient one.
Also we assess the potential for heat pumps used in commercial and industrial sector with low GWP refrigerants.

Main results
1. Establishment of the performance evaluation technology for Eco-cuteEco-cute produces hot water using inexpensive night electricity and stores produced hot water in a tank.
It is a heat storage system and its performance depends on daily hot water demands. The evaluation of consecutive three-day operation has been prescribed in Japan Refrigeration and Air Conditioning Industry Association
standards. But the more long-term performance evaluation is needed because of the daily change of hot water
demands. Setting up the heat pump performance test facilities (Fig. 1) and evaluating the performance of manykinds of Eco-cute, the CRIEPI established the performance evaluation technology, which consists of the technologies to keep air temperature and humidity settings in the chamber in which Eco-cute is installed and to set different daily hot water demands automatically and accurately over a consecutive 30-day operation period. Using
the technology, small sized new types of Eco-cute were evaluated and knowledge about the operation patterns toconsider the future direction of performance improvement was acquired.
2. Performance improvement of Eco-cute in winter conditions
To improve the efﬁciency of Eco-cute in winter conditions, which has a signiﬁcant impact on its annual
efﬁciency, evaporative heat transfer tubes for CO2 refrigerant were investigated experimentally. The result showedthat a tube with triangle ﬁn and a tube with slim high ﬁn were excellent (Fig. 2) [M09011]. Also, the study wasstarted to solve the frost problems of outdoor heat exchanger in winter. The wind tunnel for evaporator test was
installed to quantify heat pump performance degradation due to frosting and defrosting of outdoor heat exchanger
and to develop heat transfer tubes, heat exchangers and heat pump systems with frost resistant and easy defrosting(Fig. 3).
3. Development trends of heat pumps with low GWP refrigerants
To evaluate low GWP refrigerants potential, the development trends of heat pumps with natural refrigerants such as water and CO2 and with low GWP Freon were investigated and the challenges were summarized. The
CRIEPI will conduct research and development of heat pumps with water and CO2 as a refrigerant for residential
room heating, commercial room cooling and heating and industrial high temperature process heating.

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Environmental and Energy Utilization Technology
Control Unit for
Load of RoomHeating(Water Chiller,
Electric Heater,
Water Tank, etc.)
Heated Brine forRoom Heating

Eco-cute

Control Unit for
Tap Water Temp.
and Load of Hot
Water Supplying(Brine Chiller,
Electric Heater,
Water Tank, etc.)
Tap
Water

HotWater

Control Unit forOutdoor Air Temp.
and Humidity(Refrigerating
Machine, Brine
Chiller, Brine Tank,etc.)
Brine,
Refrigerant

WindowController
Air7m

Measuring
SystemDoor

Disribution Board

Heat Pump
Water Heater

Heat Pump
Water Heater

Environmental Test Room
（W4.5m×L7.5m×H3.5m）

HumidifierElectric
Heater,
Cooling
Coil,Fan, etc.

SteamAir

Dehumidifier
Door

Door

20m

ShutterFig. 1 Heat pump performance evaluation test facilities
This test facility evaluates the performance of heat pump water heaters under the various conditions of outdoor air temperature and humidity, tap water temperature and hot water demand.

No. and inner fin typeNo.0 smoothNo.1 triang

No.2 slim fin

No.10 slim high fin

Cross sectional shape(a) Heat source brine temp. Tbi 10℃(b) Heat source temp. Tbi 25℃

Fig. 2 Investigation of evaporative heat transfer tube best suited to CO2 refrigerant
In the ﬁgures, horizontal axis Qb and vertical axis Ts1 indicate respectively heat ﬂow and saturation temperature of CO2
at the tube exit. Higher value of Ts1 means more efﬁcient tube for CO2. The result shows that a tube with the triangle ﬁn(No.1) in low Qb and a tube with slim high ﬁn (No.10) in high Qb are excellent.

Fig. 3 Wind tunnel for evaporator test
The wind tunnel was installed in the environmental test room shown in Fig 1. Evaporators and heatpump systems with frost resistance and easy defrosting will be developed using this tunnel.

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