Heat Pump Water Heater Control Strategy Optimization for Cold Climates

Commercially available heat pump water heaters (HPWH) have been used successfully in warm humid climates (southern United States), and recently, have been proven effective in replacing electric water heater technology in cooler climates within Canada. Using an air source HPWH unit within a dwelling can yield electrical coefficients of performance that are indicative of significant energy savings, but can also add an additional load to the space heating system. Current control strategies do not attempt to mitigate the heating load added to the surrounding space, and only consider the water temperature in the tank. This is because, to date, the primary application has been in sub-tropical climates where cooling is frequently beneficial. Starting in 2015, the US Department of Energy is mandating that all electric water heaters have an energy factor (unit of heat applied to hot water per unit of energy applied to the system) greater than 2, which makes technologies that utilize electrical coefficients of performance, such as HPWH technology, mandatory. To ease the inevitable transition to heat pump water heaters in lieu of electric water heaters, modified control strategies that highlight using thermal storage to reduce space heating loads must be implemented. This paper presents a study which was conducted to evaluate the performance of a commercially available 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 wraparound heating coil. An air source heat pump, 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 set-points can be determined for use in cold climates. As expected from previous work, there is a decrease in performance of the heat pump 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.