By 2035, the city of Helsinki plans to be carbon neutral. But in a part of the world where the average winter temperature is a few degrees below freezing, one challenge of reaching that goal is heating hundreds of thousands of homes without using fossil fuels (or burning wood, which the city doesn’t see as a long-term solution). Because the city recognized a need for new innovations in heating technology, it sponsored a 1 million euro reward, the Helsinki Energy Challenge, for anyone who could come up with a viable alternative.
One of the competition’s winners, called Helsinki’s Hot Heart, envisions 10 floating reservoirs off the city’s coast that would store heat in seawater—and double as greenhouses with hot pools that residents can visit in February when the weather outside is bleak.
Water efficiently stores heat, and this type of thermal storage “has been used before, but never at this unprecedented scale,” says designer Carlo Ratti, founder of the international design and innovation office Carlo Ratti Associati, which developed the proposal. “Also, the concept of floating reservoirs is new and could be a game-changer. It could theoretically be used in any coastal city—facing the sea, lakes, or rivers.”
The system would use renewable electricity—at times of day when it’s most abundant and least expensive—to run heat pumps that would heat the seawater and store it in the reservoirs. Whenever the heat is needed, it would be sent to Helsinki’s district heating system, an existing network that sends heat to almost all of the city’s buildings. Four of the 10 reservoirs would have roofs and would be open to visitors, taking advantage of the heat to grow tropical plants and offer heated swimming pools.
“In the specific context of Helsinki, it is ideal to use seawater to store heat because it is readily available in the sea outside the city, does not require any costly transportation, which would result in CO2 emissions, and does not create interference with the natural ecosystem,” Ratti says. “It’s also easy to store seawater in basins in the sea because the water inside has a similar pressure to the water outside, simplifying the structural requirements of the construction.”
The idea was one of four winning proposals that the city is now evaluating further, and which could be deployed in combination. The Hot Heart designers say that the technology could fully meet the city’s heating needs by the end of the decade, with zero carbon emissions. “While contracts, design, permitting, and construction would take a while, we believe it’s feasible well in advance of Helsinki’s 2030 deadline to decarbonize its district heating system,” Ratti says. “One advantage that our proposal has is that it can plug directly into the city’s existing district heating system without needing to retrofit it.”