Heat Pump Water Heaters: Technical Q&A
Earlier this month, Senior Research Engineer Ben Schoenbauer hosted a webinar to share findings from his recent heat pump water heater research and introduce a new calculator app. A number of viewers submitted technical questions about the project. Read on for Ben’s answers; and if you missed the live broadcast, you can download the slides and a video recording from our Resource Center.
Are there instant water heating heat pumps on the market?
Most currently available heat pump water heaters are heat pumps integrated with storage tanks and electric resistance backup elements. To my knowledge, the only other significant type of heat pump water heaters are add-on models designed to retrofit existing electric storage tanks.
If heat is drawn from the surrounding room, and not from the outdoors, how can the coefficient of performance be defended?
Total heating needs for water and surrounding air would then be the same with or without the heat pump. Water heaters are typically installed in semi-conditioned spaces in Minnesota. These locations include unfinished basements and unoccupied mechanical room and areas. In these cases, the temperature reductions in the surrounding air will not be directly replaced by the heating system (mechanical spaces are often not directly controlled and will operate at lower temperatures with a heat pump water heater installed than without). In cases where heat pump water heaters are installed in an environment where the air temperature will be maintained, the total heating needs will balance out with or without the heat pump. These conditions are taken into account in the savings calculator.
In the peak load energy consumption comparison chart shown during the webinar, is the heat pump water heater in 'heat pump' mode only or was it in 'hybrid mode' with electric resistance elements operating too?
The charts shown were for heat pump mode only. For large use homes or homes with grouped water uses, the electric resistance elements will be used in the heat pump water heater, these effects were included in the online app and in the white paper report.
What is the coefficient of performance and is there a correlation between it and electric resistance?
Coefficient of performance is the ratio of hot water energy output to energy consumed (in this case electricity). Coefficients of performance are typically measured values. Energy Factor is this same ratio under a specified rating methodology.
With the cost conditions you present, is it assumed that electric resistance is not relied on at all?
The energy use and savings numbers are determined from the estimated coefficient of performance. The applied coefficient of performance takes into account the ambient air temperature and the hot water usage profile. In large water use homes, the use of backup electric resistance increases and the savings are reduced. The electric resistance consumption is accounted for in our savings estimates.
New web-based calculators transform research results into a useful tool.
A homeowner inputs simple data about her home and water use, which the Heat Pump Water Heater Calculator uses to estimate her expected savings and payback if she installed a heat pump water heater. This gives her an idea of how much energy a heat pump water heater would save, and how it would affect her space conditioning load by reducing cooling energy by increasing heating energy. The calculator also plots how the heat pump would affect overall consumption and peak load. Do note that it only provides estimates for homeowners with electric water heating, not those with gas water heating.
The research team is also developing a similar tool to show electric utilities the potential effect on their peak usage if various percentages of their customers switch to heat pump water heaters. They calculated the ranges of expected savings based on energy models, but if utilities need fine details, the team will need to conduct additional research. This calculator uses Minnesota average values and water use patterns, and will be most useful to utilities with many customers who use electric water heaters.
Both calculators make the results accessible for non-technical audiences and could guide decisions involving heat pump water heaters. And our researchers found that the process of app development helped them in on the most important variables by forcing them to define simple inputs.
Image credit: Chris Smith and winterofdiscontent via Creative Commons.