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The Development of High TemperatureWater Heater using CO2 Heat Pump

1. Introduction2. Refrigerant choice 3. Advantages of CO2 water heater 4. Technological development of major components 5. Steps for the next generation CO2 water heater 6. Conclusion

Contents

April 5, 2008Shigeharu TAIRA, Product Development Group,Air Conditioning Manufacturing Division, DAIKIN Industries, Ltd.

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21.8%

3.4%

21.6%6.4%

46.8%

Lighting, appliances, etc:

Heating

Household direct CO2 emission in Japan2005 (Greenhouse Gas Inventory Office of Japan)

1. Introduction

●Water heating emits more than 20% of residential CO2 emission in Japan.⇒It’s a very important challenge to reduce CO2 emission from this usage. (CO2 emission due to residential energy is about 14% of Japanese total emission)

●Furthermore, more than 90% of water heaters are combustion-type ⇒There is a good chance for heat pump to reduce CO2 emission

●Mid-night power is surplus due to nuclear power plants. Electricity charge is determined according to the demand and the supply correlation. ⇒An efficient thermal storage system utilize mid-night power is ideal!

Background : Problem of Japanese GHG emission situation

Hot water supply:

Cooling

Kitchen Combustion type:90%

Electric type(heater, a heat pump):10%

3DAIKINDAIKIN 1. Introduction

●HFC is energy efficient and reasonable cost ⇒Effectiveness of heat pump water heater as a measure against global warming may be diminished by high GWP of HFC.

●Research to utilize CO2 for air conditioning have been under way for more than decade. (in Europe and Japan in particular)⇒For air conditioning, it has not been considered commercially

practical yet.However, CO2 for water heating has potential of equivalent or better performance than HFC

We decided to develop high temperature water heater for thermal storage system with CO2 heat pump. Compressor as well as water heat exchanger are developed to meetthe properties of CO2.

Background : Challenges in refrigerant choice

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Comparison of a refrigerant■ HFC R410A, R407C have considerably higher COP but far higher GWP than CO2.

Even if the heat pump water heater has lower CO2 emission, the effect to reduce global warming impact (against fossil fuel heating systems) would be deteriorated if HFC refrigerant is not fully recovered.

■ CO2 is not suitable for the reversible air source heat pump because of its low theoretical COP especially in cooling operation. But, it has good potential for high temperature heat pump due to trans-critical operation in high pressure side.

HFCsHFCs Natural refrigerantNatural refrigerantCO2R410A R407C A/C High Temp

90

ODP*1 0 0 0

Flammability No No No

Discharge Pressure [MPa] 2.7-3.0 1.8 -2.0 9.5 -11

GWP*2 1975 1652.5 1

Toxicity Low Low Low

COP [relative to 410A] 100 95 -100 60-80

Refrigerant

Note:*1: Ozone depletion potential，*2: Global warming potential（IPCC2001）

2. Refrigerant characteristics

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Refrigerant HFC CO2

Figure ofoutline

Fluorocarbons heat low-temperature water by high condensing temperature.⇒Irreversible loss is unavoidable, so the

theoretical efficiency is low.

Characteristics Fluid in a supercritical condition does not change the phase. The temperature of CO2drops gradually as the heat is transferred. ⇒Loss is smaller!

Comparison of a refrigerant■ CO2 is suitable for water heaters and other once-pass heating applications

with large temperature differences.

2. Refrigerant characteristics

Heat value

Temp.

1-Stage Cycle using typical Fluorocarbon

HFCs

Water

High

Low

HighLow Heat value

Temp.

Trans-Critical Cycle using CO2

CO2

Water

High

Low

HighLow

6DAIKINDAIKIN 3. Advantages of CO2 water heater

■ Cumulative sales targeted at 5.2 million units by 2010Result is lower, but expanded by 50% every year.

CO2 Water heater sales target and result in Japan

CO2 Water Heater sales target (as of 2005) and result (up to 2006)

Compact models

For cold areas

Current models

Cumulative sales: 5.2 mil. Units

Cumulative sales: 5.2 mil. units

For cold areas

Compact models

Current models

(1,000 units)

Year

Aim

sale

s num

ber

0.34 3.72 8.08 13.1 22.6 35.0 Result

1,400

1,200

1,000

800

600

400

200

0

01 02 03 04 05 06 07 08 09 10

5.0 35 80120

230

520

690

950

1,170

1,390

230120

520650 700 710 720

520

160 320

36

160

100

140990(19%)

440(8%)

3,770(73%)

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P

Heat exchanger and fan(common or similar parts with A/C)

Water heat exchanger Compressor

Heat pump unit Hot water storage tank

P

Tank

Bath

System of CO2 water heater & thermal storage■ The CO2 heat pump water heater heats up

water to high temperature at mid-night that is stored in storage tank.

■Appropriate temperature water is supplied by mixing hot stored water and cold water for day or evening time use.

■Bath water can be re-heated indirectly by stored hot water.

3. Advantages of CO2 water heater

Hot water storage tankHeat pump unit

Kitchen RC Bathroom RC

water

Hotwater

Hotwater

water

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■ Environment-friendly.CO2 emissions: about 2/3-1/2 of gas water heater

■SafeHeats water up to 90°C without flame

■Energy efficient against conventional resistive heater system ・Annual average COP = 3.0 or higher

■Incentive & Low running cost・ Heat water at mid-night when electric power

is cheaper under current electricity rate in Japan→Running cost is about 1/4 to 1/5 that of gas

・Cost comparison with European Average gas and electricity shows little cost merit.

(See right figure)

3. Advantages of CO2 water heater

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7

Resistive heater(EU)

Heat Pump(EU)

COP

Running Cost Comparison

Rel

ativ

e C

ost (

Gas

=100

)Gas

0

0.1

0.2

0.3

0.4

(kgC

O2/k

Wh)

1 2 3 4 5COP

Gas

Heat pump

Resistive heater

CO2 emission per heat in Japan

Heat Pump(Jp Mid-night)

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■ Electricity rate is determined according to the demand and the supply correlation. The water heater is operated to maximize the benefit of this kind of rate system.

Heating water at mid-night

0

5

10

15

20

25

30

35

43210 98765 1413121110 1918171615 23222120 24

Mid Night time¥7.22/kWh

DaytimeSummer:\29.76/kWhOther seasons:\27.06/kWh

Evening time\20.67/kWh

Morning time\20.67/kWh

Time [O’clock]

Pric

e [\/

kWh]

Mid Night time¥7.22/kWh

Electricity rates by time and season(KANSAI ELECTRIC POWER CO.)

3. Advantages of CO2 water heater

*web of KEPCO 2008

10DAIKINDAIKIN4. Technological development of major components

Development of swing compressor

Vane

IntakeCompression chamber

Roller

Crankshaft

At the tip of vane:・ Wear, abrasion・ Gas leak

Rotary compressor

Swing bush

CrankshaftPiston

Combined vane and roller:・ No wear, improved reliability・ Improved performance due to

less leakage

Swing compressor

■ Operating pressure of CO2 is higher than R410A, so it enhances the merit of swing compressor. Combined vane and roller of swing compressor achieves higher reliability and lower leakage than conventional design.

Refrigerant R410A CO2

Discharge pressure [MPa] 3.0 12.0

Intake pressure [MPa] 0.8 3.5

Pressure difference[MPa] 2.2 8.5Sound Velocity [m/sec at 50°C 1MPa]

192 273

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Traditional DevelopedSpec Double tube

１．００Weight ratio １．００ ０．７４ ０．６４Performance １．００ １．００ １．００

Smooth tube (initial)

０．８９

Dimpled (latest)

Outline of shape

Volume ratio ０．８９

CO2

Water

p

Dw

,iWater tubeCO2 tube

Dimple

Water

Leakage detection

ditchCO2

Comparison of water heater exchanger

Photo of appearance of water heat exchanger

■ The weight and volume of the dimpled water heat exchanger is about 10 to 30% less than those of a double-tube exchanger.

4. Technological development of major components

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Flow direction

Turbulent flow occurs by a dimple.

4. Technological development of major components

Visualization of flow in a dimple tube■ Performance of the water heat exchanger improved by adopting the

dimpled heat transfer pipe (SD tube) in the water side.・Heat transfer is doubled from smooth tube.・Enhanced heat transmission can be seen especially in low Reynolds flow.

Re

0

1

2

3

Nu/

Nu0

SD tubeInner grooved tube

0 2000 4000 6000 8000

Smooth tube

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Principle of expander cycle

5. Steps for the next generation CO2 water heater

■ Energy recovered by expander is transmitted to compressor to reduce power input.■ Cooling capacity increases because enthalpy of CO2 at expander exit decreases.

Enthalpy

CompressorInput

Heating CapacityExpanderCycle(IsentropicExpansion)

Pres

sure

Cooling Capacity

Conventional Cycle(Adiabatic Expansion)

EnergyRecovered byExpander

high

lowlow high

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■ The expander transmits the recovered energy directly to the compressorto reduce compressor power input.

Conventional type

Expander Compressor

Energyrecovery

M

Gas Cooler

Evaporator

Expansion valve

Adiabatic expansion (Energy is

not recovered)

Compressor

Gas Cooler

Evaporator

New Type

Function of expander compressor

M

5. Steps for the next generation CO2 water heater

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Expander-compressor

Cross Section Outlook

ExpanderInlet

ExpanderOutlet

CompressorDischarge

CompressorSuction

Expander

Compressor

DCMotor

■Connecting shaft transmit power from expander to compressor.⇒The technical development of water heater with this technology has

been carried out under NEDO (New Energy and Industrial Technology Development Organization of Japan) financial support project.

5. Steps for the next generation CO2 water heater

Connecting shaft

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- CO2 is suitable for high temperature water heaters because it has low GWP and equivalent COP. In addition, it can heat water to temperatures up to 90°C without a heater.

- For domestic use, both the user and the electric power company can benefit from CO2 heat pump operation at mid night.→ The user can reduce the operating cost while the power company can

level electric power load.

- In Japan, commercialization of residential CO2 heat pump water heater started in 2001. Accumulated sales exceeded 1 million units in 2007.

- By development of water heat exchangers, expander-compressor, other key components, and improvement of system technology, heat pump in the future may make a greater contribution to the challenge against global warming.

- This kind of thermal storage system may be useful to developing countries where nuclear or uncontrolable natural energy such as wind power is utilized.

6. Conclusion

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Thank you for your attention!

DAIKINWe want to contribute to a global environment through heat pump technology ⇒Slogan ； “Heat pump relieve the earth”

Acknowledgement ;・NEDO・Central Research Institute of Electric Power Industry・The Federation of Electric Power Companies of Japan・Tokyo , Kansai , Chubu , and Kyushu Electric Power Company