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Research Papers

Heat Pump Water Heater Control Strategy Optimization for Cold Climates

[+] Author and Article Information
Jayson Bursill, Cynthia A. Cruickshank

Department of Mechanical and Aerospace Engineering,
Carleton University,
Ottawa, ON K1S 5B6, Canada

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received July 27, 2015; final manuscript received November 24, 2015; published online December 22, 2015. Assoc. Editor: Jorge E. Gonzalez.

J. Sol. Energy Eng 138(1), 011011 (Dec 22, 2015) (8 pages) Paper No: SOL-15-1235; doi: 10.1115/1.4032144 History: Received July 27, 2015; Revised November 24, 2015

This paper presents a study which was conducted to evaluate the performance of a commercially available heat pump water heater (HPWH) with modified controls. The HPWH is first characterized experimentally under a series of different thermal conditions and draw parameters. The test tank contains a 1500 W electric auxiliary heater that provides on demand heat to the top 0.30 m (1 ft) of the tank, and a wrap-around heating coil. An air source heat pump (ASHP), using R-134A as the refrigerant, draws air from, and returns air to the surrounding space and provides heating to the whole tank through the coil. The tank has been tested using Canadian Standards Association draw profiles to characterize performance under different hot water demands. Electricity consumption and thermal flux is measured for each vertical tank section, and various performance metrics are calculated using energy balances. A trnsys model is then calibrated to the experimental data to allow for the flexibility of varying multiple parameters over various climates. Using this calibrated trnsys model, an optimal control strategy and tank setpoints can be determined for use in cold climates. As expected from previous work, there is a decrease in performance of the HP when heating the tank to higher temperatures to facilitate thermal storage, but the benefits from taking advantage of shifting electrical demand (of water heating) to space heating demand can outweigh the loss of performance.

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Figures

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Fig. 1

System diagram of hydronic components for the HPWH draw test apparatus

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Fig. 2

Rate of temperature rise relative to γ (setpoint–current node temperature)

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Fig. 3

Temperatures from node 1 (top) to node 14 (bottom) and instantaneous power of HPWH at 70 °C booster and 60 °C HP setpoints

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Fig. 4

Instantaneous power and COP over an experimental run at 70 °C booster and 60 °C HP setpoints

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Fig. 5

Comparison of selected node temperatures and HPWH instantaneous power for the experiment and model at 55 °C booster and 50 °C HP setpoints

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Fig. 6

Comparison of selected node temperatures and HPWH instantaneous power for the experiment and model at 70 °C Booster and 60 °C HP setpoints

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Fig. 7

HPWH energy consumption breakdown for various HP setpoints at a booster heater setpoint of 70 deg

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Fig. 8

HPWH energy consumption for various HP and booster heater setpoints

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