2016 ASHRAE Annual Conference

Sorption heat pump water heaters represent a breakthrough opportunity in residential and commercial water heating as the most economical use of primary energy, relative to any other method of water heating, including electrical heat pump water heaters or integrated heat pumps. To bring this technology to practice, it is necessary to understand and quantify the limiting parameters. Focusing on adsorption-based heat pumps, the paper discusses the use of inexpensive adsorbents, required boundary temperatures and pressures, and limiting factors such as the influence of thermal mass on efficiency. The working materials are stipulated to be non-toxic with zero GWP and zero ODP.

Gas heat pump water heaters (HPWHs) can improve water heating efficiency with zero GWP and zero ODP working fluids. The energy factor (EF) of a gas HPWH is sensitive to several factors. In this work, expressions are derived for EF of gas HPWHs, as a function of heat pump cycle COP, tank heat losses, burner efficiency, electrical draw, and effectiveness of supplemental heat exchangers. The expressions are used to investigate the sensitivity of EF to each parameter. EF is evaluated on two bases: site energy (as used by DOE for rating water heater efficiency in the US), and source energy.

For gas HPWHs, it is found that using typical component efficiencies, EF will be 75-90% of the heat pump cycle COP. The contribution of each parameter to the difference between EF and cycle thermal COP is as follows: burner efficiency accounts for 50-80% of difference, parasitic electrical draws for 10 – 30%. Independent of COP, the presence of a condensing heat exchanger can make a 5-10% difference in EF, and tank losses reduce EF by 6 – 10%, depending on the insulation level.

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.