Carlo Ratti Associati proposes a group of floating cylindrical islands off the coast of Helsinki to store thermal energy and serve as a community recreation zone.

Carlo Ratti Designs a Floating Structure to Heat A City and Create Community

One of the winners of Metropolis‘ first-ever Responsible Disruptors program, Hot Heart, a recreation center, stores thermal energy for the city of Helsinki.

Disruption in technology is often associated with negative consequences like social disorder, environmental degradation, and economic marginalization. Facebook (now Meta) founder Mark Zuckerberg summed up this ethos with his early motto “Move fast and break things.” But disruption can also be beneficial. When done right, it can encourage health, wellness, efficiency, and equity. In that spirit, Metropolis is thrilled to share the winners of its first Responsible Disruptors program, honoring A&D technology projects that represent significant change for the better. 


In Finland, the concept Jokamiehen Oikeudet, or “right to roam,” refers to everyone’s freedom to enjoy nature respectfully. Hot Heart, the winning proposal for the Helsinki Energy Challenge—organized by the city to accelerate its transition to carbon neutrality in heating by 2030—takes its inspiration from this humble idea, though it is slated to become the largest infrastructure project of its kind in the world. Created by a cross-disciplinary team coordinated by Turin, Italy–based design firm Carlo Ratti Associati (CRA), Hot Heart consists of ten cylindrical basins that form a floating, archipelago-like structure situated off the coast of the metropolis. Upon completion around 2028, the unusual complex will serve the dual function of storing thermal energy and serving as a hub for recreational activities. 

Designed to act like a giant battery, the floating edifice harvests renewable energy from wind, solar, and other sources at low or negative cost when it is available abundantly. The energy is then converted into heat and stored in the ten tanks until it is needed during the winter. The tanks, which each measure about 740 feet in diameter, can collectively hold up to 2.64 billion gallons of water. “We know renewables are intermittent. Sometimes we produce too much, sometimes we produce too little,” explains founding partner Carlo Ratti. “If you use a thermal battery to store electricity, the cost is magnitudes less than other methods.” Hot Heart is projected to provide for all of Helsinki’s heating needs (estimated at 6,000 GWh) by 2028, with zero carbon emissions and at a cost that is lower than today’s expenditures.

Four of the tanks are enclosed in transparent domes that contain what the architects call “floating forests”—indoor ecosystems that will be naturally heated by the basins below and open to the public for relaxation and socialization. “In Finland, going to the forest and spending time in nature are very rooted in the local culture,” says Ratti. “But beyond that, we also see the forests as an opportunity to teach people about climate change, about sustainability, about energy flows. I think that engaging the public is going to be very important as we tackle the climate crisis together.”

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