Impact on Water Heater Performance of Heating Methods That Promote Tank Temperature Stratification (ST-16-C058)

During heating of a water heater tank, the vertical temperature stratification of the water can be increased or decreased, depending on the method of heating. Methods that increase stratification during heating include; bringing cold water from the tank bottom, heating it, and re-introducing it to the tank top at relatively low flow rate, using a heat exchanger wrapped around the tank, through which heating fluid (with finite specific heat) flows from top to bottom and using an immersed heat element that is relatively high in the tank. Using such methods allows for improved heat pump water heater (HPWH) cycle efficiencies when the heat pump can take advantage of the lower temperatures that exist lower in the tank, and accommodate the resulting glide. Transcritical cycles are especially well-suited to capitalize on this opportunity, and other HPWH configurations (that have been proposed elsewhere) may benefit as well.

In this work, the response of a tank that is stratified during heating is compared with the response of a tank that is mixed during heating, for first hour rating (FHR) and energy factor (EF) testing. Experimental results from FHR, EF, and UEF tests on a CO2-based HPWH with wrap-around coil and stratified tank are used to validate a simulation model. The implications on FHR, EF, and UEF of tank stratification are analyzed and discussed.