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Domestic EcoCute outdoor unit (front) and hot water storage tank (back)

The EcoCute (エコキュート, ekokyūto) is an energy-efficient electric Air source heat pump, only for water heating and supply system, not use for air conditioning or alike, that uses heat extracted from the air to heat water for domestic, industrial and commercial use. Instead of the more conventional ammonia or haloalkane gases, EcoCute uses supercritical carbon dioxide as a refrigerant. The technology offers a means of energy conservation and reduces the emission of greenhouse gas. It is a registered trademark of Kansai Electric Power Company.[1]


The name of the EcoCute comes from the Japanese phrase Shizen Reibai Hīto Ponpu Kyūtō-ki (自然冷媒ヒートポンプ給湯機), which literally means "natural refrigerant heat pump water heater". [2] Eco is a contraction of either ecology or economical and Cute is a near homonym to kyūtō (給湯); literally "supply hot water". [3]

Features and demand

In Japan in 1998, water heating (Kyuto (給湯, kyūtō)) accounted for 33.8% of typical domestic energy consumption, with air conditioner and kerosene heater heating accounting for another 26.9% and cooling by air conditioner another 2.3%. Most of the remaining 37% was spent on electrical home appliances, a field where 21st-century innovations in energy conservation began to make considerable energy savings.[4] This left hot water supply as the most difficult area for energy conservation, leaving a gap in the market for the EcoCute. By January 2005, 26 Japanese companies were producing more than 450 models of EcoCute machines, and sales of domestic units increased 130–150% each year between 2001 and 2005.[5]

Denso first introduced the EcoCute outside Japan at the COP9 Milan, Italy on December 9, 2003. From 2007, Denso began concentrating on marketing the EcoCute in the EU.[6][7] In Japan, the Japanese government incorporated the EcoCute into its CO2 reduction program under the Kyoto Protocol, mandating the installation of 5.2 million units in commercial and domestic properties by 2010.[3] [8]


EcoCute machine basics

An EcoCute machine or system consists of a heat pump and hot water storage unit. The sealed components are serially connected with refrigerant CO2 gas in circulation.[9][3]

  1. At the first stage, a heat exchanger collects heat from the air outside to use as energy for the refrigerant. Air flow is usually obtained using a centrifugal fan; in cold areas with ambient temperatures around −20 to −25 °C an auxiliary fan heater is attached.
  2. A gas compressor is used to increase the temperature of the gaseous CO2 refrigerant to around 100 °C under pressure of 10MPa via adiabatic compression. The carbon dioxide becomes a supercritical fluid.[3]Several types of compressors can be used, including dual layer cylindrical compressors, scroll compressors, and dual stage rotary compressors.[10]
  3. At the third stage a heat exchanger transfers energy from the hot refrigerant into water to produce hot water. Water temperatures around 50 °C and up are suitable at this stage.[11][12][13]
  4. Finally, ejector or expansion valvesreduce pressure on the refrigerant, letting it cool via adiabatic expansion and revert to CO2 gas from supercritical fluid, then circulation back to the first stage.[3]

Refrigerant carbon dioxide (CO2) is numbered or coded as R-744 by ASHRAE standard 34. The EcoCute can derive two to five unit ratio of COP (Coefficient Of Performance) to heated hot water energy from outdoor air with single unit input of electrical energy, resulting in reduced CO2 emissions compared to water heating via electricity or natural gas.[14] The COP value changes on temperature of outside air and water feed in.[15] To produce 90 °C hot water, an EcoCute consumes 75% less electricity than an electric water heater, and costs 80% less than heating water by natural gas in Japan.[16] Also, by reducing use of fossil fuels, the EcoCute results 50% reduction in CO2 emissions.[17]

Not considering upstream losses of input source energy, as one example reference value, the EcoCute's COP is 3.8 in industrial use, while electric power water heating is 1.0, and gas boiler is 0.88 including pilot light loss.[16]

See also


  1. ^ Report of Patents application trend on Heating and cooling by natural refrigerant Archived 2006-12-08 at the Wayback Machine Trademark No. 4575216 – Japan, third line 10. from bottom on page 3/24, May 15, 2003, Japan Patent Office (in Japanese)
  2. ^ "Fiscal year 2001, 12th Energy Conservation Award of Minister of Economy, Trade and Industry" (in Japanese). The Energy Conservation Center, Japan (ECCJ). Archived from the original on 2002-10-20. Retrieved 2008-07-10.(in Japanese) The Energy Conservation Center, Japan (ECCJ) (in Japanese) Fiscal year 2001 (Heisei 13), 12th Energy Conservation Award from Ministry of Economy, Trade and Industry. 5 companies with product named EcoCute and other 4 not named but conforming EcoCute requirement and performance, total 9 companies awarded (Denso, CORONA, Mitsubishi Electric, Sekisui Chemical, 四変テック Archived 2024-03-02 at the Wayback Machine, キューヘン Archived 2024-03-02 at the Wayback Machine, Former 日立空調システム Archived 2024-04-01 at the Wayback Machine, Tokyo Electric Power Company, Central Research Institute of Electric Power Industry).
  3. ^ a b c d e "Making hot water by CO2" (in Japanese). Central Research Institute of Electrical Power Industry. Archived from the original on 2009-08-10. Retrieved 2008-07-10. Archive indicates: Go to page 1-3, by bottom right selector ← Back [1] [2] [3] Next → Page 1: Naming. Page 2: illustrated EcoCute compositions and function. Page 3 shows, 2nd sentence from bottom in 2nd paragraph indicates 5.2 million (*3) unit diffusion by 2010. Back to page 2, top left illustration is detail of EcoCute closed refrigerant loop circuit, from left, the heat of outside air is taken into (pumping into) refrigerant gas CO2 via evaporation type heat exchanger (Electricity fan used for smooth air flow in/out for outdoor unit with negligible electricity cost in this stage). This is first stage in four stages. Step forward next stage in clockwise, compressor (electricity is mostly consumed here) compressing gas CO2 to higher heat temperature. Heated up CO2 circulates to another heat exchanger, hot refrigerant CO2 to hot water, then step to expansion valve (no electricity) and refrigerant back to gas CO2 as last stage, then step back to first stage. Noted that as EcoCute principle (<エコキュートの原理>), CO2 is state of matter of supercritical between compressor - heat exchanger (producing hot water) - expansion valve in sealed manner circulation.
  4. ^ 2.2 (2) Yearly home energy consumption by usage category Archived 2007-06-11 at the Wayback Machine Bar graph: Section (2) Energy consumption by purpose of usage per household、bottom, year '98 ((2) 世帯当たり用途別エネルギー消費量) The Energy Conservation Center, Japan (ECCJ) (in Japanese)
  5. ^ TOKUGIKON (特許庁技術懇話会) Archived 2005-05-12 at the Wayback Machine, Organized current and retired officers, with Japan Patent Office, technical discussion association (特許庁技術懇話会) since 1934 (in Japanese). Archive indicates: To select page 1-3, by bottom right selector ← Back [1] [2] [3] Next →. First page with page number 122: Context/content line 10-11. EcoCute put on market in 2001, approx. 450 types produced by 26 Mfg. as of now/today (2005.1.28. shown at bottom line, this pdf file/document issued date) and 130 - 150% annual increased in 2001 – 2005. (エコキュートは、2 0 0 1年に発売されて以来年成長率 1 3 0~1 5 0%と好調な売れ行きを見せており、今では2 6 社から約4 5 0種類の製品が販売されています)
  6. ^ EcoCute demand surges in Europe Archived 2008-10-19 at the Wayback Machine, European Heat pump network
  7. ^ Heat pumps offer huge potential Archived 2009-08-13 at the Wayback Machine, IEA Conference
  8. ^ 26 pages of the plan for energy saving and CO2 emission reduction: Archived 2024-03-02 at the Wayback Machine Page 16/26, the second block from left of bottom layer frame: Promote CO2 refrigerant heat pump for hot water supply 5.2 million unit by 2010. (CO2冷媒ヒートポンプ給湯器の普及台数<約520万台>) Official issue from Kantei (in Japanese)
  9. ^ ITOMIC Industrial use Eco-cute, Features, WorkflowArchived 2023-12-09 at the Wayback Machine
  10. ^ Cooling System Composed with Rolling Piston Type 2-Stage CO2 Compressor Archived 2006-02-19 at the Wayback Machine, Sanyo
  11. ^ Output of hot water supply temperature is preference of individual or family user, and EcoCute manufacture set temperature range by model for market, not industrial specific value.
  12. ^ EcoCuteArchived 2005-11-07 at the Wayback Machine, Japan Patent Office Society (in Japanese). Page 127, 2005.1.28. no.236 tokugikon, Fig 7 Heat pump configuration illustration: At top right, Output of hot water is possible to rise up to 90 °C. Under the condition of outside air temperature -10 °C, EcoCute can produce 90 °C hot water that could not obtain by chlorofluorocarbon refrigerant (5 °C water inlet at bottom right). Website is organized current and retired officers from Japan Patent Office technical discussion association (特許庁技術懇話会) since 1934. Accessed on 2024-02-06.
  13. ^ Referenced web site (in Japanese) of Sakaguchi Boiler ServiceArchived 2023-06-06 at the Wayback Machine, (in Japanese) Referenced web site of Sakaguchi Boiler Service in Kumamoto, Japan (坂口ボイラーサービス). Recommending temperature setting is boiling the hot water tank to be 65 to 80 °C, hot water supply is recommended to 50 to 60 °C then mixed with cool water suitable for human body. Avoid growth range of legionella in 20 to 45 °C, also under 40 °C tend to make the trouble on thermostatic mixing valve with hot and cool water. Temperature more than 90 °C is wasting heat energy and cost of electric cost. Accessed on 2024-02-06.
  14. ^ Japan Atomic Energy Relations Organization JAERO (ja:日本原子力文化財団) Archived 2023-10-05 at the Wayback Machine (in Japanese) Energy Encyclopedia (エネ百科), website supported by JAERO, describing: Mechanism of CO2 refrigerant heat pump, titled EcoCute (エコキュート), for hot water supply Electric energy (1) + Outdoor air energy (2 or more) = Obtained hot water energy (3 or more) (CO₂冷媒ヒートポンプ給湯器のしくみ): Three frames from left are Heat pump unit (ヒートポンプユニット), Hot water storage unit (貯湯ユニット) and House part. Within Heat pump unit, closed loop in circulation manner, operates to transfer Outdoor air energy (2 or more) (大気熱2以上) into the refrigerant CO2 through Air Heat Exchanger (空気熱交換器) via fan, then follow in clockwise, the refrigerant compressed by compressor (圧縮機/コンプレッサー) operates under applied Electric energy (1) (電気エネルギー1). Next step, heat energy transition into cool water and warm up to hot water (水加熱) through Heat Exchanger (熱交換器), then flow proceed down to Expansion Valve (膨張弁), the refrigerant CO2 back to Air Heat Exchanger (空気熱交換器), as total refrigerant CO2 cycle (CO2冷媒サイクル). The device, at bottom right in Heat pump unit frame, is cool/hot water circulate pump (ポンプ) feed in to Hot water storage unit. At this stage, 1 + (2 or more) = (3 or more) energy transition realized. Cool/hot water tank store obtained energy of more than 3 (得られる給湯エネルギー3以上). Top right is the temperature adjustment valve (温調弁) to maintain suitable temperature of storage hot water and to supply for household use for kitchen (キッチン), washroom (洗面所), bath (お風呂), underfloor heating (床暖房). Cool water supplied into Hot water storage unit at bottom right.
  15. ^ Website for example; EcoCute consideration of relation on electric cost and outside air temperature (エコキュート:電気代と気温の関係に関する考察) Archived 2021-04-12 at the Wayback Machine (in Japanese) Table shows COP value of four mfg.'s EcoCute and these average with outside air temperature (left most column), next to right, feed in water temp., COP value of Mitsubishi (三菱), CORONA (コロナ), Daikin (ダイキン), Panasonic (formerly 松下) and average, right most column is measured season with top line air 7 °C (気温) in warmer/high temp. winter (冬季高温), middle line 16 °C rated Japanese temp.;Average temperature in Japan (定格), bottom line 25 °C in summer (夏季). Top line shows; Warm winter air temp. 7 °C, water temp.9 °C, Average COP 3.32, for look at example. Fig.1 (COP vs. Temp. diagram (COP 0 – 6 vs. temp. 0-30 °C) indicates temp. between 0 and 16 °C; presents COP 2 to 5 in linear, and over 16 to 25 °; COP is saturated to 5. Line colour; Blue line=Mitsubishi, Pink=CORONA, Green=Daikin, Red=Panasonic
  16. ^ a b Feature of industrial use Ecocute Archived 2008-06-10 at the Wayback Machine, (in Japanese) Archive indicates: Information from Nihon Itomic Co., Ltd. Archived 2024-02-18 at the Wayback Machine for their EcoCute: EcoCute effectively heat up water to 90 °C. First bar chart shows comparison of COP of, from top, Itomic industrial EcoCute COP:4.0, electric heather COP:1.0, gas boiler COP:0.88. Itomic industrial EcoCute having marginal tolerance from COP 4.0 to 3.8 as specification value on delivery from factory. Chart also implicates four COP:1 machines brings the same thermal energy of hot water in volume of one COP:4 machine does. Ratio is (COP:4 – COP:1) / COP:4=0.75 so that EcoCute consume 75% less than COP:1 electric kW. Second bar chart shows CO2 emission comparison: EcoCute:50% is half compared to town gas or city gas burning boiler type 100%.
  17. ^ Research and development of EcoCute Archived 2005-09-24 at the Wayback Machine CRIEPI (in Japanese) Archive indicates: Titled - Development of home appliance hot water supply by refrigerant CO2. In 2/2 page, last two sentences in area inside the bold line: EcoCute reduce primary energy 30% and CO2 emission 50% compared to water heating by town gas or city gas burning boiler type. There points well evaluated and awarded with prizes, on Energy Conservation Award (省エネルギー大賞) by Minister of Economy, Trade and Industry, and from other organizations in 2001–2002.