Interest in heat pump water heater technology is rapidly growing, given its energy efficiency relative to gas-fueled equipment. For apartment building owners, especially those of large buildings, emerging heat pump technology now offers the possibility of using the stored hot water as a thermal battery, to better manage a building’s power use.
These systems, known as central heat pump water heaters, use largely the same technological approach as residential heat pump water heaters. If used to replace gas heaters in California multifamily buildings, a new study finds they could reduce greenhouse gas emissions by 1.7 million tons per year and produce $350 million in overall benefits annually from the increased efficiency of the systems, compared with gas water heaters.
The ability to use the stored hot water as a battery means that water can be heated in low-demand periods and later provided during high-consumption periods without adding demand to the grid during its highest use period – a strategy known as load shifting.
“Load shifting is going to be really important, not just to benefit the grid, but also to ensure that these technologies provide the most cost control for the building owners,” said M M Valmiki, an energy efficiency engineer with the energy engineering consulting firm ASK Energy. “When we are electrifying buildings, especially those with really big loads like hot water for a whole building, it is important to minimize the costs of electrification, especially in comparison with natural gas.”
To better understand the benefits and challenges of using central heat pump water heaters in large multifamily buildings, researchers installed customized systems in two buildings with 120 and 135 single-room units, respectively, that house low-income senior residents in San Francisco. The results of this study can be found in the report Commercial and Multifamily CO2 Heat Pump Water Heater Market Study and Field Demonstration. The study was co-led by experts at ASK Energy and Ecotope, an engineering firm focused on low-carbon building solutions, with funding from California’s emerging technology initiative, CalNEXT.
This study used central heat pump water heaters that utilize carbon dioxide refrigerants, which have the lowest global warming potential compared to other more common refrigerants on the market. Another benefit of carbon dioxide is that, unlike most refrigerants, it is not synthetic, thus avoiding environmental concerns about the impact of possible leakage.
The installations were retrofits in already occupied buildings, and a primary goal of the study was to assess the challenges involved in installing these heat pumps in existing housing stock.
Like their residential counterparts, central heat pump water heaters use refrigerants to move heat from the surrounding air to heat the water tank. Central heat pump systems need larger hot water storage tanks than natural gas-fueled systems, so they require more room than gas-fired boilers. “Heat pumps are not really that much more complicated than boilers,” said Scott Spielman, a research engineer at Ecotope and leader of the study. “It’s just that you need a bigger thermal storage system and thermal storage has costs: there’s a cost to have more water, both in the expense of the physical materials, like a storage tank, as well as in the floorspace that the tanks take up.”
For the study, the researchers considered different tank and related piping configurations to determine how to optimize the heat pump systems. For example, water heating can be done in single-pass or multi-pass stages — and tank sizing is important since oversizing can lead to lower efficiency and higher cost. Plumbing between storage tanks in the system created a challenge, because the length of time water circulates in the system impacts its temperature. These considerations and others were used to determine how to optimize thermal storage in the heat pump system as a whole.
“There haven’t been a lot of studies that show how thermal storage really operates in different, vibrant configurations,” Spielman said, noting that standardization of the design of these systems in the future will help make their installation easier and less expensive.
The research project demonstrated how the hot water storage volume could function as a thermal battery. The large volume creates the opportunity to store energy and serve as a battery. Heating water during periods of low grid demand helps to lower energy use during peak times and enhances grid reliability, especially as utilities increasingly integrate intermittent solar and wind energy into their power mix. The finding has implications for California’s grid, where concerns about additional load are sometimes used as an argument against electrification.
“Central heat pump water heaters are a much-needed way to get creative in terms of grid demand management,” said Madison Johnson, a data analyst with Ecotope, who assessed the results of the study. “By centrally managing that demand, we are not only using less electricity overall than an electric resistance system would use, but we are also able to control what time of day we would like to use that energy.”
Researchers found that access to hot water was no issue for residents, following the installation of the central heat pump water heaters, which consistently provided the 25 gallons-per-day residential demand without any issues, even during periods of load shifting.
They also found that shifting load to a lower demand time did not necessarily reduce the cost of the power to run the systems.
This result is based on the design of the test plan, which aimed to shift as much energy (that is, kilowatt-hours) out of the on-peak periods as possible. It did so successfully.
However, two issues kept the project from generating cost savings, according to Valmiki. Firstly, there were some slight increases in energy consumption outside of the peak periods. Secondly, the team did not test the controls that could reduce peak demand during those on-peak periods.
“Even though we were successful in shifting kilowatt-hours out of on-peak periods, the kilowatt demand spikes during on-peak periods were not reduced,” Valmiki said.
In a follow up study, the researchers plan to better manage how the system triggers demand charges.
“The demand charges in particular are something that we will try to limit in our follow-up study since they are a large portion of utility bills for buildings that have them in their tariff,” Valmiki said.
It is still early days for central heat pump water heaters for multifamily buildings, and this study aimed to document the technology’s readiness. Seeing how the technology performs in real-time is an important step, both for optimizing system design and for assessing how best to maintain the grid while increasing electrification.
To learn more about this project, read the report on the CalNEXT website.