The outdoor component of a residential heat pump
New research shows that residential heat pumps, essentially reversible air conditioners, may benefit homeowners while protecting the environment from harmful emissions. Credit: Dennis Schroeder/NREL, CC BY-NC-ND 2.0

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In 1855, engineer Peter von Rittinger was concerned with salt production. He was building a device that could evaporate water from brine more efficiently than available methods. Later iterations of this device, the heat pump, would become tools to slow climate change. Today heat pumps aim to replace a home’s in situ oil or gas consumption with cleaner electricity use.

Researchers recently found that wider installation of residential heat pumps for space heating could lower greenhouse gas emissions. The results, published in Environmental Research Letters, showed that heat pumps would reduce emissions for two thirds of households and financially benefit a third of U.S. homeowners.

But only around 10% of homes use heat pumps, which pump heat out of the house in summer and into the house during winter. “The majority of heating in buildings, as well as hot water and cooking, relies on fossil fuels burned on site,” said Michael Waite, associate research scientist at Columbia University who was not involved in the new study. To reduce emissions, homeowners need to replace such heating systems. “The only direct way of doing that is through electrification of those uses,” said Waite.

Pros and Cons

But wide-scale heat pump adoption may have unintended, undesirable consequences. Thomas Deetjen, a research associate at the University of Texas at Austin, and his coauthors wanted to see which circumstances make heat pumps a wise choice for homeowners and society.

Using tools from the National Renewable Energy Laboratory (NREL), they simulated outcomes of widespread heat pump adoption. They modeled 400 locally representative single-family homes in each of 55 cities. To model the electric grid, the researchers assumed moderate decarbonization of the grid (a 45% decline in emissions over the 15-year lifetime of a heat pump).

Researchers evaluated effects on homeowners, comparing costs of heat pump installation to energy cost savings. They also analyzed changes in carbon dioxide emissions and air pollutants, putting a dollar amount to climate and health damages. Climate damages included costs associated with climate change–driven natural hazards such as flooding and wildfire. Health damages include premature deaths due to air pollution.

“The key finding is that for around a third of the single-family homes in the U.S., if you installed the heat pump, you would reduce environmental and health damages.”

“The key finding is that for around a third of the single-family homes in the U.S., if you installed the heat pump, you would reduce environmental and health damages,” said Parth Vaishnav, an assistant professor at the School for Environment and Sustainability at the University of Michigan and a coauthor of the paper. Installing heat pumps would avoid $600 million in health damages and $1.7 billion in climate damages each year. It would also directly save homeowners money on energy costs. They also found that for all homes, assuming moderate electric grid decarbonization, heat pump use cut greenhouse gas emissions.

But heat pump installation did have other consequences. “Heat pumps are not necessarily a silver bullet for every house,” said Deetjen.

Although homeowners may trade a furnace for a heat pump, for example, the electricity for that pump could still come from a plant burning fossil fuels. The cost of generating electricity may be more than the cost of in situ fossil fuel use. “There are some houses that if they get a heat pump, it’s actually worse for the public,” said Deetjen. ”They end up creating more pollution.”

Heat pump benefits also depend on climate. Heat pumps operate less efficiently in the cold, running up electricity costs. In 24 of the studied cities, mostly in colder climates, peak residential electricity demand increased by over 100% if all houses adopted heat pumps, which would require grid upgrades.

“It could be challenging to meet that increase of winter peaking, because our system is not built that way,” said Ella Zhou, a senior modeling engineer at NREL not involved with this study. “We need to think about both the planning and operation of the grid system in a more integrated fashion with future use.”

Consequences of Widespread Electrification

The new research supported 32% of single-family homes converting to heat pumps. More widespread adoption came at much higher financial and health costs. If all U.S. houses adopted heat pumps, the study said, it would yield $6.4 billion in climate benefits. However, it would also cost homeowners $26.7 billion, and pollutants from increased electricity generation would result in $4.9 billion in health damages from financial burdens resulting from illnesses or premature deaths.

There is some uncertainty surrounding these findings. The study didn’t consider the cost of potential grid upgrades or what complete decarbonization would mean for heat pump adoption. Waite pointed out that as the grid evolves, future research should also determine whether renewable energy could even meet the demands of high electricity loads.

—Jackie Rocheleau (@JackieRocheleau), Science Writer

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Rocheleau, J. (2021), Heat pumps can lower home emissions, but not everywhere, Eos, 102, Published on 02 September 2021.

Text © 2021. The authors. CC BY-NC-ND 3.0
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