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Thermodynamic Comparison of Subcritical and Transcritical Heat Pump Water Heater (HPWH) Efficiency

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Thermodynamic Comparison of Subcritical and Transcritical Heat Pump Water Heater (HPWH) Efficiency CO2 vs. Fluorocarbons: Thermodynamic Comparison of Subcritical and Transcritical Heat Pump Water Heater (HPWH) Efficiency Kyle R. Gluesenkamp Kashif Nawaz ACEEE Hot Water Forum Nashville, March 13, 2019 Session 4A ORNL is managed by UT-Battelle, LLC for the US Department of Energy Acknowledgments • Department of Energy (Contract DE-EE0006718.00) • Antonio Bouza, DOE Building Technology Office DISCLAIMER This 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). 2 Classification of HPWHs Pumped (or external): Wrap-around: Immersed coil: HX Evaporator Evaporator Evaporator 3 Classification of Stratification Types (Stratification During Heating) Table 1. Tank Stratification Categories Heat exchanger (HX) Locations of tank High pressure used to transfer water taps, OR Stratifying Category refrigerant Water or refrigerant flow type refrigerant heat to coil vertical span potential glide water 1 Water pumped to HX Bottom and top Any Single pass (low water flow) Very strong 2 Water pumped to HX Bottom; bottom Any Multi-pass (high water flow) None 3 Wrapped around tank3 Spans tank height Low-glide1 Top-down refrigerant flow Moderate 4 Wrapped around tank3 Spans tank height High-glide2 Top-down refrigerant flow Strong 5 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 Moderate 7 Immersed coil – tall3 Spans tank height High-glide2 Top-down refrigerant flow Strong 8 Immersed coil – tall3 Spans tank height Any Bottom-up refrigerant flow None 9 Immersed coil – short3 Bottom region Any Any refr. flow direction None 1for example, a subcritical HFC or HFO (condensing) cycle. 2 for 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. 4 Condenser and Gas Cooler Subcritical cycle Transcritical cycle condenser gas cooler F] F] ° ° desuperheating heat subcooling exchange pinch refrigerant condensing point refrigerant 65 65 Temperature [ Temperature58 [ 58 water @ 2 GPM water @ 2 GPM heat exchange pinch point 0 2, 6.8 0 2, 6.8 Cumulative heat Cumulative heat transfer [kW, kBtu/hr] transfer [kW, kBtu/hr] 5 Condenser and Gas Cooler Subcritical cycle Transcritical cycle heat exchange pinch point F] F] ° 155 ° 155 water @ 0.14 GPM water @ 0.14 GPM 65 65 Temperature [ Temperature58 [ 58 water @ 2 GPM water @ 2 GPM 0 2, 6.8 0 2, 6.8 Cumulative heat Cumulative heat transfer [kW, kBtu/hr] transfer [kW, kBtu/hr] 6 Subcritical Cycle 7 Transcritical Cycle Performance relies on availability of cold water: stratification external HX counterflow Water in 8 Modeling assumptions • Across all models: • CO2-specific: – 10 K (18°F) evaporator superheat – High side pressure optimized for – 3 K (5°F) closest approach at the COP pinch • HFC-specific: – 0 fan and pump work – 5 K condenser subcooling – 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 9 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 10 Shown for Teri = 15°C 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: 2 2 • Poptimum [kPa] = a + bTeri + cTeri + dTgcro + eTgcro + fTeriTgcro – Where Ts are in °C Coefficient Value • Relative error <0.8% of optimum pressure a -107.476661 b 30.9365 • Valid range: c -0.410714286 – 33 < Tgcro < 45°C d 246.74575 – -3 < Teri < 17°C e 0.165625 f -1.0285 11 Computing closest approach at the pinch Approach Counterflow heat exchanger temperature: leaving cold vs. entering hot fluid Approach temperature: leaving hot vs. entering cold Closest approach at the pinch 12 Computing closest approach at the pinch Closest approach at the pinch 13 Results: transcritical cycle performance (animations – see slides 23-47) 14 Results: subcritical cycle performance (R134a) Ambient air temperature = 15°C Water entering heat pump = 18 °C Tsupply = 48°C Tsupply = 96°C COP = 6.47 COP = 2.80 67% of Carnot 59% of Carnot 15 Results: subcritical cycle performance (R1234yf) Ambient air temperature = 15°C Water entering heat pump = 18 °C Tsupply = 48°C Tsupply = 96°C COP = 6.27 COP = 2.53 67% of Carnot 58% of Carnot 16 Results Ambient air temperature = 15°C 17 Results Ambient air temperature = –5°C 18 Results: crossover temperature CO2 COP advantage over R134a 19 Conclusions The “crossover temperature” • 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 20 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. 21 Thank You 22 Animation Ambient air temperature = 15°C Supply water temperature = 46°C 23 Animation Ambient air temperature = 15°C Supply water temperature = 50°C 24 Animation Ambient air temperature = 15°C Supply water temperature = 54°C 25 Animation Ambient air temperature = 15°C Supply water temperature = 58°C 26 Animation Ambient air temperature = 15°C Supply water temperature = 64°C 27 Animation Ambient air temperature = 15°C Supply water temperature = 68°C 28 Animation Ambient air temperature = 15°C Supply water temperature = 72°C 29 Animation Ambient air temperature = 15°C Supply water temperature = 76°C 30 Animation Ambient air temperature = 15°C Supply water temperature = 80°C 31 Animation Ambient air temperature = 15°C Supply water temperature = 84°C 32 Animation Ambient air temperature = 15°C Supply water temperature = 88°C 33 Animation Ambient air temperature = 15°C Supply water temperature = 92°C 34 Animation Ambient air temperature = 15°C Supply water temperature = 96°C 35 36 Animation Evaporation temperature = 15°C 37 Animation Evaporation temperature = 15°C 38 Animation Evaporation temperature = 15°C 39 Animation Evaporation temperature = 15°C 40 Animation Evaporation temperature = 15°C 41 Animation Evaporation temperature = 15°C 42 Animation Evaporation temperature = 15°C 43 Animation Evaporation temperature = 15°C 44 Animation Evaporation temperature = 15°C 45 Animation Evaporation temperature = 15°C 46 Animation Evaporation temperature = 15°C 47.
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