Nearly 20 years ago, Andrew Bissell was sitting in the living room of his beachfront home in the Edinburgh area. He and his wife, Susan Lang-Bissell, had just sold their medical imaging startup, and he was pondering his future.

“Do we retire or do we go again?” Bissell told TechCrunch.

Around that time, he was leafing through a science magazine when he stumbled across an article about climate change, which said there could be between a 30 cm to 1 meter rise in sea level before the end of the century. 

“Hang on,” he said to himself. “If that happens, then the living room, the ground floor of this home that we live in, will be underwater twice a day. Then you start thinking about that being in hundreds of millions of other homes around the world. That’s not acceptable.”

Rather than join Greenpeace — “that’s not really what we do” — he and his wife founded Sunamp. They worked with a lab at the University of Edinburgh to devise a way to store solar power as heat that can then be used for household heating or hot water, what’s known as a thermal battery.

At the heart of Sunamp’s thermal battery are three compounds: sodium acetate trihydrate (SAT), a food flavoring used in salt-and-vinegar potato chips; water; and a sprinkling of so-called crystal habit modifiers. SAT has been used for years in hand warmers, which are prepared by heating the substance until it dissolves into a supersaturated solution. When triggered, SAT recrystallizes, giving off heat in the process. The reaction can be reversed, allowing SAT to store heat for later use, but not indefinitely. Eventually, the salt settles out of solution and refuses to recrystallize.

To improve the longevity of SAT, Sunamp uses a form of acrylic as a crystal habit modifier, which helps guide the SAT into the proper configuration time and again. “That’s both weird and exciting. Weird that it happens, but exciting because it means that now, instead of [SAT] getting worse and worse over time, it just stays the same,” Bissell said, adding that the substance, which Sunamp calls Plentigrade, will last up to 40,000 heating cycles, or more than 50 years of daily use.

The goal is to make use of excess renewable power, charging the thermal battery when solar is plentiful or rates are cheap because the wind is blowing. Overnight or when the wind stops blowing, the battery can discharge, releasing heat as the SAT crystalizes. 

Sunamp already has a foothold in the United Kingdom and Italy, and it’s working to expand into other Western European countries and the U.S. The company is in the process of raising a Series B, and Bissell said that it is generating revenue in the “tens of millions range.”

The startup’s first widely available product, a compact heat battery for domestic hot water, is targeted at customers for whom space is a premium. “Globally, maybe half of homes can afford the space to put a hot water tank in,” Bissell said. “We’re for the other ones, the ones that can’t afford that space.”

Nearly 20 years ago, Andrew Bissell was sitting in the living room of his beachfront home in the Edinburgh area. He and his wife, Susan Lang-Bissell, had just sold their medical imaging startup, and he was pondering his future.

“Do we retire or do we go again?” Bissell told TechCrunch.

Around that time, he was leafing through a science magazine when he stumbled across an article about climate change, which said there could be between a 30 cm to 1 meter rise in sea level before the end of the century. 

“Hang on,” he said to himself. “If that happens, then the living room, the ground floor of this home that we live in, will be underwater twice a day. Then you start thinking about that being in hundreds of millions of other homes around the world. That’s not acceptable.”

Rather than join Greenpeace — “that’s not really what we do” — he and his wife founded Sunamp. They worked with a lab at the University of Edinburgh to devise a way to store solar power as heat that can then be used for household heating or hot water, what’s known as a thermal battery.

At the heart of Sunamp’s thermal battery are three compounds: sodium acetate trihydrate (SAT), a food flavoring used in salt-and-vinegar potato chips; water; and a sprinkling of so-called crystal habit modifiers. SAT has been used for years in hand warmers, which are prepared by heating the substance until it dissolves into a supersaturated solution. When triggered, SAT recrystallizes, giving off heat in the process. The reaction can be reversed, allowing SAT to store heat for later use, but not indefinitely. Eventually, the salt settles out of solution and refuses to recrystallize.

To improve the longevity of SAT, Sunamp uses a form of acrylic as a crystal habit modifier, which helps guide the SAT into the proper configuration time and again. “That’s both weird and exciting. Weird that it happens, but exciting because it means that now, instead of [SAT] getting worse and worse over time, it just stays the same,” Bissell said, adding that the substance, which Sunamp calls Plentigrade, will last up to 40,000 heating cycles, or more than 50 years of daily use.

The goal is to make use of excess renewable power, charging the thermal battery when solar is plentiful or rates are cheap because the wind is blowing. Overnight or when the wind stops blowing, the battery can discharge, releasing heat as the SAT crystalizes. 

Sunamp already has a foothold in the United Kingdom and Italy, and it’s working to expand into other Western European countries and the U.S. The company is in the process of raising a Series B, and Bissell said that it is generating revenue in the “tens of millions range.”

The startup’s first widely available product, a compact heat battery for domestic hot water, is targeted at customers for whom space is a premium. “Globally, maybe half of homes can afford the space to put a hot water tank in,” Bissell said. “We’re for the other ones, the ones that can’t afford that space.”