Experimental Testing of a Transcritical

Carbon Dioxide Heat Pump Water

Heater

Portia Murray and Stephen Harrison

Department of Mechanical and Materials Engineering

Kingston, ON, Canada

Solar Calorimetry Laboratory

Why use Carbon Dioxide as a Refrigerant?

• It’s a natural, non-toxic refrigerant with a negligible GWP and ODP

Type of Refrigerant

Refrigerant Name

Ozone Depletion Potential

Global Warming Potential

Flammability and Toxicity

HCFC R22 0.055 1700 -

HFC

R134a 0 1300 -

R407C 0 1600 -

R410A 0 1900 -

Natural

Ammonia (R717)

0 0 Toxic and flammable

CO2 (R744) 0 1 -

The Transcritical Cycle

• CO2 has a Low critical temperature of (31.1°C)

• The fluid transfers from a super-critical state to a sub-critical state and rejects heat sensibly rather than latently (gas-cooling vs. condensation)

• Working pressure typically 9-12 MPa on the discharge side and 4-6 MPa on the suction side

• Sensible heat rejection offers high temperature differentials and produces a higher efficiency and higher rejection temperatures

Conventional cycle

Transcritical cycle

Critical Point R134a: 4 MPa, 101°C

Critical Point R744: 7.3 MPa, 31.1°C

Japanese Eco-Cute Market

• Due to their advantages for heating, CO2 is particularly good for cold climates (i.e. Canada, Northern Europe, and Japan)

• Japan has spearheaded the commercialization and integration of CO2 heat pump water heaters (HPWH) by providing subsidies since 2002 and are currently selling over three million units per year

Eco-Cute Experimental Test Unit

• 4.5 kW Eco-cute heat pump water heater

• 2-stage hermetic rotary compressor

• Gas-cooler and evaporator retrofits

• Internal heat exchanger

• Electronic expansion valve

• Instrumented with pressure transducers, thermocouples, flow meters, and power meters

• LabVIEW data acquisition system

• 273 Litre hot water tank

• Controlled temperature water supply

Experimental Test Schematic

1st Stage

2nd Stage

Motor Shell Low Pressure High Pressure

Internal Intermediate Pressure

Compressor Operation

http://www.r744.com/assets/link/Sanyo_rotary_compressor.pdf

Advantages of Brazed-plate Gas-coolers

• Commercially available

• Compact design

• Promotes tank stratification

• Very good for natural convection flow

• Low pressure drop

Above: The factory capillary tube gas-cooler

Right: The new brazed plate

gas-cooler

Gas-cooler Retrofit

Results: COP vs. Flow Rate

• Although COP did increase with increasing flow rate, there was a diminishing return

• Beyond a flow rate of 2 L/min, there was no advantage in operating at a higher flow rate for these operating conditions

• Increasing the flow rate also decreases the temperature differential across the gas cooler and lowered the average temperature of the gas cooler and mixing the tank

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6

CO

P

Water Flow rate (L/min)

0

1

2

3

4

5

6

0 1 2 3 4 5 6

Heat

Tra

nsfe

r (k

W)

Water flow rate (L/min)

Qgc

Qevap

Wcomp

Results: COP vs. Average Gas-cooler Temperature

• A clear trend between the average temperature of the gas-cooler and the COP

• This average temperature increases with an increase in inlet temperature or a decrease in flow rate

• The effectiveness of the gas-cooler also increases with flow rate but shows a diminishing return after a flow rate of 2 L/min

2

3

4

5

6

7

0 10 20 30 40 50

CO

P

Average Temperature of Gas-Cooler (C)

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6

Eff

ecti

ven

ess

Water flow rate (L/min)

Results: Thermosyphon flow rate test

• A test was conducted using only natural convection (buoyancy driven) thermosyphon flow through the gas-cooler

• The thermosyphon flow rate slowly decreased throughout the test as the tank charged.

• the HP performance also dropped with the flow rate

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

00:00 01:12 02:24 03:36 04:48 06:00

Th

erm

osyp

ho

n f

low

rate

(L

/min

)

Elapsed Time (h:mm)

m load

Average

2

2.5

3

3.5

4

0.6 0.7 0.8 0.9

CO

P

Thermosyphon flow rate (L/min)

COP

Average

Results: Thermosyphon Test Tank Profile

• As the flow rate decreases, the average temperature increases

• The tank stays stratified for the full duration of the test

• Inlet temperature only increases when the tank is fully charged

• The heat delivery temperatures increase as the flow rate drops

2

2.5

3

3.5

4

34 36 38 40 42 44 46 48

CO

P

Average Temperature of Gas Cooler (C)

COP

Average

0

10

20

30

40

50

60

70

80

90

00:00 01:12 02:24 03:36 04:48 06:00 07:12

Tem

pera

ture

(C

)

Elapsed Time (h:mm)

TC10

TC9

TC8

TC7

TC6

TC5

TC4

TC3

TC2

TC1

Summary of Results

Evaporator Gas-cooler Comp-ressor

COP

Water CO2 Water CO2

Flow type

Tin (C) T out (C) Flow rate

(L/m)

T In (C)

T out (C)

Q ev (kW)

Low Press-ure (MPa)

T in (C)

T out (C) T

avg (C)

Flow rate

(L/m)

T In (C)

T out (C)

Q gc (kW)

High Press-ure (MPa)

Power (kW)

COP Cool

COP Heat

Forced 23 14.4 3.75 15 17 2.19 4.6 5.3 86.6 46 0.5 104 36.8 2.97 10 1.133 1.93 2.62

Forced 23 11.4 3.75 12 10 2.95 4.6 4.9 57.8 31.4 1 67 24.5 3.75 9 0.991 2.98 3.78

Forced 23 10.7 3.79 11 9.5 3.14 4.6 4.9 51.7 28.3 1.22 60 20.6 3.98 8.7 0.945 3.32 4.21

Forced 23 9.5 3.78 10 8.2 3.47 4.5 4.8 45 24.9 1.54 54 13.2 4.32 8.2 0.886 3.92 4.88

Forced 23 8.8 3.75 9 7.5 3.67 4.4 4.8 41.4 23.1 1.75 52 9.43 4.45 8 0.847 4.33 5.25

Forced 23 8.5 3.79 8.6 7.3 3.78 4.4 4.8 37.9 21.4 1.99 52 8.1 4.59 7.6 0.816 4.63 5.63

Forced 23 8.2 3.82 8.6 7 3.8 4.4 4.8 31.5 18.2 2.5 53 7.9 4.64 7.4 0.779 4.88 5.96

Forced 23 8.2 3.79 8.7 7 3.79 4.3 4.8 24.1 14.5 3.5 53 7.84 4.71 7.3 0.756 5.01 6.23

Forced 23 8.2 3.77 8.7 6.9 3.76 4.4 5.1 18.8 12 5 53 8.13 4.8 7.2 0.747 5.03 6.43

Thermo-syphon

22 12.8 3.79 14 12 2.43 5.16 6.2 70.1 38.2 0.75 83 30.8 3.28 10 1.042 2.76 3.47

Conclusion

• Carbon dioxide heat pumps have a greater potential for efficient operation in cold climates as they are particularly suited towards heating applications (hot water at 70-80°C)

• There is a non-linear relationship between the average temperature of the gas-cooler and the COP

• Beyond a flow rate of 2 L/min there was little benefit at operating at higher flow rates

• They are able to charge using thermosyphon flow due to the high temperatures in the gas-cooler at an average COP of 3.47 and a flow rate of 0.75 L/min

• In order to increase this COP further, the natural convection flow rate must be increased by lowering the pressure drop in the gas-cooler water circuit

Future Work and Publications

• Gas-coolers with a higher number of plates and a lower pressure drop are being tested to increase the natural convection performance

P. Murray, S. Harrison, B. Stinson and G. Johnson, "Experimental Evaluation of a Water Source CO2 Heat Pump Incorporating Novel Gas Cooler Configuration," in ASME 8th International Conference on Energy Sustainability, Boston. In Press, 2014.

P. Murray, S. Harrison and B. Stinson, "Passive Gas Cooler Anti-Fouling for Carbon Dioxide Heat Pump Water Heaters," in American Society of Mechanical Engineers International Mechanical Engineering Congress and Exposition, Montreal, In press 2014.

Acknowledgement

The authors wish to acknowledge the support of:

• NSERC and the SNEBRN

• SWEP International AB, Sweden