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CO2 vs. Fluorocarbons: Thermodynamic Comparison of Subcritical and TranscriticalHeat Pump Water Heater (HPWH) EfficiencyKyle R. GluesenkampKashif Nawaz

ACEEE Hot Water ForumNashville, March 13, 2019Session 4A

22

Acknowledgments

• Department of Energy (Contract DE-EE0006718.00)• Antonio Bouza, DOE Building Technology Office

DISCLAIMERThis material has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

33

Classification of HPWHs

Evaporator

HX

Wrap-around:Pumped (or external):

Evaporator

Immersed coil:

Evaporator

44

Classification of Stratification Types (Stratification During Heating)

Table 1. Tank Stratification Categories

Category

Heat exchanger (HX) used to transfer

refrigerant heat to water

Locations of tank water taps, OR

coil vertical span

High pressure refrigerant

glideWater or refrigerant flow type Stratifying

potential

1 Water pumped to HX Bottom and top Any Single pass (low water flow) Very strong2 Water pumped to HX Bottom; bottom Any Multi-pass (high water flow) None3 Wrapped around tank3 Spans tank height Low-glide1 Top-down refrigerant flow Moderate4 Wrapped around tank3 Spans tank height High-glide2 Top-down refrigerant flow Strong5 Wrapped around tank3 Spans tank height Any Bottom-up refrigerant flow None 6 Immersed coil – tall3 Spans tank height Low-glide1 Top-down refrigerant flow Moderate7 Immersed coil – tall3 Spans tank height High-glide2 Top-down refrigerant flow Strong8 Immersed coil – tall3 Spans tank height Any Bottom-up refrigerant flow None9 Immersed coil – short3 Bottom region Any Any refr. flow direction None

1for example, a subcritical HFC or HFO (condensing) cycle. 2for example, a transcritical CO2 cycle. 3it is assumed a wrapped tank refrigerant coil will extend for most of the tank height, whereas an immersed coil may have a shorter vertical span.

Table from: Gluesenkamp, Kyle R., John Bush (2016). “Impact on Water Heater Performance of Heating Methods that Promote Tank Temperature Stratification”, ASHRAE Annual Meeting 2016, Conference Paper Session 19, June 29, 2016, St. Louis, MO.

55

Condenser and Gas Cooler

Tem

pera

ture

[°F]

condensing

desuperheating

subcooling

refrigerant

water @ 2 GPM

Subcritical cyclecondenser

Tem

pera

ture

[°F]

refrigerant

Transcritical cycle gas cooler

heat exchange pinch point

0

58 58

0

65 65

heat exchange

pinch point

water @ 2 GPM

Cumulative heat transfer [kW, kBtu/hr]

Cumulative heat transfer [kW, kBtu/hr]

2, 6.8 2, 6.8

66

Condenser and Gas Cooler

Te

mp

era

ture

[°F

]

Subcritical cycle

Te

mp

era

ture

[°F

]

Transcritical cycle

heat exchange

pinch point

0

58 58

0

wate

r @ 0

.14 G

PM

155 155

water @ 2 GPM65 65

water @ 2 GPM

wate

r @ 0

.14 G

PM

Cumulative heat

transfer [kW, kBtu/hr]

Cumulative heat

transfer [kW, kBtu/hr]

2, 6.8 2, 6.8

77

Subcritical Cycle

88

Transcritical Cycle

Water in counterflow external HX

Performance relies on availability of cold water: stratification

99

Modeling assumptions

• Across all models:– 10 K (18°F) evaporator superheat– 3 K (5°F) closest approach at the

pinch– 0 fan and pump work– 95% motor efficiency– ηisen = 0.90 – 0.0467*PR– ηvol = 1.00 – 0.04*PR– 100 kPa condenser pressure drop– 50 kPa evaporator pressure drop

• CO2-specific:– High side pressure optimized for

COP

• HFC-specific:– 5 K condenser subcooling

1010

High side pressure optimization

• In transcritical cycles, the high side pressure is a free variable • Proper modeling requires optimization of this pressure

Tgcro = gas cooler refrigerant outlet temperature

Shown for Teri = 15°C

1111

Polynomial curve fit for optimum high side pressure

• For this work, capacity was ignored and pressure was chosen to maximize COP– In practice, this represents a small (~10%) sacrifice of capacity

• Optimum pressure as function of Teri and Tgcro:

• Poptimum [kPa] = a + bTeri + cTeri2 + dTgcro + eTgcro

2 + fTeriTgcro

– Where Ts are in °C

• Relative error <0.8% of optimum pressure

• Valid range:– 33 < Tgcro < 45°C

– -3 < Teri < 17°C

Coefficient Value

a -107.476661

b 30.9365

c -0.410714286

d 246.74575

e 0.165625

f -1.0285

1212

Computing closest approach at the pinchCounterflow heat exchanger

Approach temperature: leaving hot vs. entering cold

Approach temperature:

leaving cold vs. entering hot fluid

Closest approach at

the pinch

1313

Computing closest approach at the pinch

Closest approach at

the pinch

1414

Results: transcritical cycle performance

(animations – see slides 23-47)

1515

Results: subcritical cycle performance (R134a)Ambient air temperature = 15°C

Water entering heat pump = 18 °C

COP = 2.8059% of Carnot

COP = 6.4767% of Carnot

Tsupply = 48°C Tsupply = 96°C

1616

Results: subcritical cycle performance (R1234yf)Ambient air temperature = 15°C

Water entering heat pump = 18 °C

COP = 2.5358% of Carnot

COP = 6.2767% of Carnot

Tsupply = 48°C Tsupply = 96°C

1717

Results

Ambient air temperature = 15°C

1818

Results

Ambient air temperature = –5°C

1919

Results: crossover temperature

CO2 COP advantage over R134a

2020

Conclusions

• At 15°C Ambient, CO2 has higher COP:– Above 42°C for 18°C return– Above 56°C for 28°C return– Above 67°C for 38°C return

• At -5°C Ambient, CO2 has higher COP:– Above 47°C for 18°C return– Above 54°C for 28°C return– Above 62°C for 38°C return

The “crossover temperature”

2121

References

• Gluesenkamp, K., Abdelaziz, O., Patel, V., Mandel, B., deAlmeida, V. (May 2017). “High Efficiency Water Heating Technology Development – Final Report, Part II: CO2 and Absorption-Based Residential Heat Pump Water Heater Development” ORNL publication TM-2016/291. Available at http://info.ornl.gov/sites/publications/Files/Pub68329.pdf

• Kashif Nawaz, Bo Shen, Ahmed Elatar, Van Baxter. “Hydroflouroolefins (HFOs) as Low GWP Refrigerants for Residential Heat Pump Water Heaters”, ACEEE HWF 2017.

• Gluesenkamp, K., Bush, J. (2016). “Impact on Water Heater Performance of Heating Methods that Promote Tank Temperature Stratification”, ASHRAE Annual Meeting 2016, Conference Paper Session 19, June 29, 2016, St. Louis, MO.

• B. Shen, K. Nawaz, A. Elatar, V. Baxter, “Development and Validation of Quasi-Steady-State Heat Pump Water Heater Model Having Stratified Water Tank and Wrapped-Tank Condenser” International Journal of Refrigeration, 2018, 87,78-90.

• K. Nawaz, B. Shen, A. Elatar, V. Baxter, O. Abdelaziz, “Performance Optimization of CO2 HPWH System”, International Journal of Refrigeration, 2017, 85, 213-218.

2222

Thank You

2323

AnimationAmbient air temperature = 15°C

Supply water temperature = 46°C

2424

AnimationAmbient air temperature = 15°C

Supply water temperature = 50°C

2525

AnimationAmbient air temperature = 15°C

Supply water temperature = 54°C

2626

AnimationAmbient air temperature = 15°C

Supply water temperature = 58°C

2727

AnimationAmbient air temperature = 15°C

Supply water temperature = 64°C

2828

AnimationAmbient air temperature = 15°C

Supply water temperature = 68°C

2929

AnimationAmbient air temperature = 15°C

Supply water temperature = 72°C

3030

AnimationAmbient air temperature = 15°C

Supply water temperature = 76°C

3131

AnimationAmbient air temperature = 15°C

Supply water temperature = 80°C

3232

AnimationAmbient air temperature = 15°C

Supply water temperature = 84°C

3333

AnimationAmbient air temperature = 15°C

Supply water temperature = 88°C

3434

AnimationAmbient air temperature = 15°C

Supply water temperature = 92°C

3535

AnimationAmbient air temperature = 15°C

Supply water temperature = 96°C

3636

3737

AnimationEvaporation temperature = 15°C

3838

AnimationEvaporation temperature = 15°C

3939

AnimationEvaporation temperature = 15°C

4040

AnimationEvaporation temperature = 15°C

4141

AnimationEvaporation temperature = 15°C

4242

AnimationEvaporation temperature = 15°C

4343

AnimationEvaporation temperature = 15°C

4444

AnimationEvaporation temperature = 15°C

4545

AnimationEvaporation temperature = 15°C

4646

AnimationEvaporation temperature = 15°C

4747

AnimationEvaporation temperature = 15°C