In the United States, home heat pumps have been gaining traction (and government subsidies) as highly energy-efficient replacements for gas-fired boilers and furnaces. They vary in size, but most of the units being hyped by environmentalists and installed nationwide measure just a few square feet. In Stockholm’s Hammarbyverket plant, which is by some measures the world’s largest heat-pump plant, each of the seven electric-powered heat pumps is the size of a two-story house.
On the day I visited last fall, the motors and massive compressors hissed at a nearly deafening pitch. Pumps drew treated wastewater from an underground tunnel. As the water rushed through the pipes and cascaded down exposed panels, coolants in the machines sucked away degrees of warmth, until the water was so cold that it left small ice crystals behind as it poured into the Baltic Sea. Industrial-sized compressors, meanwhile, used that extracted heat to create a separate flow of 195-degree water that runs into a network of insulated pipes, supplying heat across the city.
The argument for heat pumps centers on their efficiency: Because they move warmth around, instead of generating heat directly, heat pumps can be many times more energy efficient than other heaters. In the U.S., heating alone accounts for more than half of the energy used in homes. Heat pumps sized for individual households can slash those emissions dramatically, and since President Biden signed the Inflation Reduction Act into law, more than 250,000 families have used one of the bill’s tax credits to invest in heat pumps, according to the Treasury Department. The larger heat pumps I saw at the Hammarbyverket plant are similar to the popular air-source household units, but a single heat pump there pushes out enough heat to warm thousands of apartments. And in recent years other European cities, too, have started switching large heating systems, that serve tens of thousands, over to heat pumps.
The efficiency advantage of small heat pumps holds for giant heat pumps. And giant heat pumps can also tap into heat sources—freshwater lakes, treated wastewater, heat exhaust from industrial plants—that wouldn’t be practical for smaller home units.
The first heat pumps in Stockholm’s system pulled waste heat from IBM mainframe computers, says Fabian Levihn, who heads research and development for Stockholm Exergi, the local utility that runs the citywide, interconnected heating system, a setup typically referred to as district heating. Modern data centers, which use huge amounts of energy to run and cool their servers, remain a major source of otherwise wasted heat. So are factories that produce excess heat in energy-intensive industrial processes.
My guide at Hammarbyverket was Bo Berndtsson, who oversees operations there. He began working in the control room just months after the giant heat pumps were installed in 1986. At the time, Sweden was more focused on weaning itself off volatile supplies of imported fossil fuels than reducing the carbon footprint of home heating. Similar district-heating systems elsewhere in Europe long relied on siphoning the extra heat from power plants that run on fossil fuels.
Stockholm decommissioned its last coal-fired plant in 2020, and its giant heat pumps are a major supplier of heat to the city, along with power plants that burn waste and scrap wood from Sweden’s forestry industry that would otherwise be left to rot. Levihn contends that generating heat and electricity from incinerated waste is more efficient than dumping it in a landfill, although these plants still emit carbon dioxide. Stockholm Exergi is working to install carbon-capture technology in the plants in hopes of making the system net carbon negative, he told me.
In Europe, interest in giant heat pumps like those at Hammarbyverket has been growing. The technology “has never really gotten traction because gas prices were always too cheap,” Thomas Nowak, a former secretary general of the European Heat Pump Association, told me last fall. In Europe, only a handful of massive heat pumps, such as those in Stockholm, are in operation, but more have been coming online as district-heating systems move to shut down coal-fired power plants and hit climate targets.
When we met, Berndtsson had been occasionally checking in on the progress of workers in Mannheim, Germany, who were building a massive new heat pump; it started heating thousands of homes right around the time we met. The Danish port city of Esbjerg is in the final stages of installing two giant heat pumps to use water from the North Sea to provide heat for about 25,000 households. Helsinki, meanwhile, is building a new plant that uses heat pulled from the Baltic Sea to cover the needs of as much as 40 percent of the Finnish capital (whose population exceeds 600,000).
And in Vienna, a series of heat pumps that can warm about 56,000 households opened in December. The city plans to double the capacity of the plant in the coming years. The Austrian capital’s district-heating system also uses heat pumps to recycle waste heat on a smaller scale from, for example, a local spa and a commercial bakery producing Manner wafer cookies, an beloved Viennese treat, Linda Kirchberger, the head of decarbonization and new technologies at the local utility Wien Energie, told me.
What Stockholm, Vienna, Helsinki, and other European cities installing giant heat pumps have in common is that they already have sizable district-heating systems. Many of those systems use hot water running in special insulated pipes to move heat from generation plants to buildings across the city. Shutting down the coal-fired plants that powered those systems, and instead installing giant heat pumps running on renewable electricity, can decarbonize tens of thousands of households with a single (albeit expensive) project.
In the United States, district-heating infrastructure is much less common and, where it does exist, is often far harder to convert to electric-powered heat pumps. Many of the systems in the U.S.—including Con Edison’s massive Manhattan system, among the largest in the world—run on high-pressure steam instead of hot water. For technical reasons, heat pumps are not designed to make steam. Switching over steam-based systems to water would be expensive and complicated, as would installing the network of pipes for a brand-new district-heating system. Density is a key factor in whether a district-heating system can be efficient and cost-competitive, so sprawl—or decisions by building owners to opt out of a newly launched system—poses major challenges to making one feasible.
Plus, the economic case for replacing furnaces and boilers with massive heat pumps is harder to make when natural gas remains relatively cheap and abundant in the United States. Higher gas prices in Europe, combined with carbon taxes, means the efficiency savings of large heat pumps will pay off far sooner.
Few places in the U.S. have tackled such projects, but several universities with campus-wide steam-heating systems have converted to hot water and installed giant heat pumps, in some cases to replace aging boilers. And for places with the right infrastructure—or for new campuses or other developments that provide their own heat—heat pumps can work.
And they have the bonus of solving two problems at once. Heat pumps can also work as cooling systems, an advantage in places that might not previously have needed air-conditioning. In Stockholm, only a few days before I visited the Hammarbyverket plant, workers were swapping out compressors and other parts from pumps that had been cooling buildings in the city center. The Swedish summer is still relatively mild compared with the rest of the world’s, but average temperatures have been slowly, steadily rising for decades.
theatlantic.com