Could the humble heat pump be a decarbonization hero?
By Zach Mortice
September 6, 2017
Buildings are responsible for a bit fewer than half of all greenhouse gas emissions in the US. Break this figure down further and you’ll find that building heating, in particular, accounts for about a fifth of all US greenhouse gas emissions.
Building heating and hot water are a part of the climate change story people often miss, according to Jeffrey Schwane. An engineer in Arup’s New York office, Schwane has spent several years looking for hidden impediments to decarbonizing our building stock. “It’s the elephant in the room,” he said.
He believes that a relatively humble technology known as a heat pump could offer the solution. His prescription for climate-friendly buildings runs as follows. First, use passive energy-efficiency strategies (like tighter building envelopes and optimal solar orientation) to minimize the need for active heating, cooling, and lighting. Then make all active building systems as energy efficient as possible. Finally, supply them with electricity from the grid — a strategy known as electrification — and ensure that the power comes from renewable sources.
This includes heating systems, some of the most stubbornly carbon-intensive technology around. And that’s where the heat pump comes in.
An electrifying trend
Schwane isn’t the only one bullish on electrified heat pumps. “It’s a definite trend that we’re seeing: Electrification is a trend, of which heating is a subset,” said Martin Howell, an energy and sustainability consultant in Arup’s Los Angeles office. “It comes up a lot on projects.”
Several years ago, Seattle’s government asked Howell to investigate whether rolling out this solution on a large scale could help decarbonize the city’s building stock. The answer: yes. However, he’s careful to note that this solution isn’t necessarily the best one for every project.
Heat pump advocacy rests on two separate arguments: one, that the heat pump is the most efficient heating system on the market; and two, that electrification is the best way to produce carbon-free heat energy in most climate zones.
Building heating and hot water are a part of the climate change story people often miss.
But these concepts, along with the linkages between them, have many shades of gray. As a result, some experts believe that solutions such as district heating or low-carbon gas grids may be more appropriate than electrified heat pumps in very cold climates.
To understand more about the potential and limitations of this technology, let’s dive into the particulars.
Heat pump basics
A heat pump’s job is to extract heat from the environment and direct it wherever it’s needed — a living room, for example, or a hot-water tank. It can also create cool air and water by transferring the collected heat away from the target area. Window air conditioners and refrigerators are both variants of this technology.
Heat pumps that draw heat from the ground or water (geothermal boreholes in the first instance, lakes or rivers in the second) are more efficient than those that pull it from the air. But all can transmit more energy (in the form of heat) than they require to operate (in the form of electricity) — at least outside of regions that frequently experience extreme cold. This makes them generally more efficient than other heating systems. If powered by zero-carbon electricity, they can heat buildings without generating greenhouse gas emissions.
“The competitor technology for heating is a really efficient boiler. It works, but it’s never going to get you to zero carbon,” Arup principal Fiona Cousins wrote in an email. Heat pumps also have fewer negative side effects than some other alternatives, she noted. “In Massachusetts they argue that wood chips from a sustainable forest gets you to zero carbon, but it’s mostly awful from an air quality standpoint. Wood chips can be used in co-gen systems — [but] that doesn’t remove the inherent life-cycle carbon emissions in the wood chip, or the air quality issues.”
Another benefit of heat pump technology: it’s very modular and scalable. A large building can provide better individual control by using dozens of heat pump units than by relying on conventional solutions. Arup is currently working on a mixed-use Manhattan building whose heating, cooling, and hot water is designed to come entirely from heat pumps.
Now let’s explore electrification.
For Schwane, connecting heating systems to the grid makes more sense than the current default in many US buildings: relying on cheap, carbon-generating natural gas. He believes that gas boilers are at risk of becoming “stranded assets” — equipment that becomes obsolete before the end of its useful life if environmental regulations become stricter in the future. In this line of thinking, building designers and owners would be better off simply electrifying their heat in the first place.
“It’s relatively easy to swap in and out power plants on the grid,” Schwane said. “You can take a coal plant offline and replace it with a combination of solar, wind energy, and batteries. But it’s relatively hard to go back and retrofit buildings that weren’t designed to heat [themselves] in a certain way.”
It’s relatively hard to go back and retrofit buildings that weren’t designed to heat [themselves] in a certain way.
A grid-connected heat pump, on the other hand, can be as clean as the power source it comes from.
But not everyone is convinced that electrified heat pumps are the way of the future. Electrification remains controversial in some regions, partially because it would likely require costly upgrades to the electric grid.
Another caveat: electrified heat pump performance drops off in very cold climates. Schwane’s research focused largely on New York, a city that’s no stranger to frigid Decembers. But in places where temperatures fall far lower (Alaska, for instance), heat pumps may need to be supplemented by other temperature control sources during peak demand periods.
Heat pump technology has improved in recent years, allowing it to move into colder climes.
But extreme cold and cloudy days pose significant challenges for solar power, one of the most common forms of renewable energy. In the winter, when heating needs are greatest, solar PV panels typically produce far fewer electrons.
Stronger market penetration of other renewable energy sources and better energy-storage technology can help with this. However, current energy storage system projects focus on daily energy cycles, not the seasonal cycles required for winter peak heating.
Despite these challenges, Schwane believes that cost, not technology, is the main barrier to widespread heat pump implementation. While performance has been edging up, units haven’t seen the same dramatic price decreases as solar panels.
The total installation cost of a heat pump system can be less than a conventional heating and cooling system if it reduces the total amount of building systems needed — i.e., preventing the need to install separate cooling, heating, and hot water equipment and natural gas service.
On the operational side, however, heat pumps are generally more expensive than gas boilers. This is because natural gas, often supplied by fracking, is much cheaper than electricity per unit of energy in many parts of the US.
Heat pumps tend to be most economically feasible in places without access to natural gas service: rural New England, for instance. Outside of these regions, however, “There is no incentive for individual building owners [to install heat pumps] other than doing good, and mitigating the risk of having to rip out a boiler at some point in the future,” Schwane said.
Looking to the future
As a result, regulatory carrots and sticks represent the most likely path to widespread adoption of electrified heat pumps. With many state and municipal governments across the US committed to intensifying their climate change mitigation and adaptation efforts, Schwane believes that demand for electrified heat pumps will continue to grow.