How a sand battery stores summer heat for winter

HostIt's a strange feeling when you walk on a beach at night and the sand still feels warm under your feet. That heat stays there long after the sun goes down, and it turns out some people are using that simple trick to warm entire towns.

HostThere's this big steel tower in Finland that people are calling a sand battery, but how does a pile of dirt actually keep a whole neighborhood warm for months?

GuestIt's funny you call it a pile of dirt because that's pretty much what it is. Inside that tall gray tower is a hundred tons of low-grade sand. It's the kind of stuff builders don't even want to use for houses. But sand is amazing at one thing and that's holding onto heat. The big idea here is that we often have too much power during the summer when the sun is out and the wind is blowing. We usually have nowhere to put all that extra energy, so it goes to waste. These engineers in Finland decided to take that extra electricity and use it to heat up sand. They use big heating coils, kind of like the ones you would find inside a toaster. They run those coils through the sand until the whole pile is glowing at about five or six hundred degrees. Since the sand is tucked inside a thick, heavy shell, that heat has nowhere to go. It just sits there for months until the middle of winter hits.

HostI get the beach thing, but sand feels like such a low-tech choice. If we're trying to save the planet with new tech, why are we using dirt instead of the fancy stuff in our phones or cars?

GuestWell, the batteries in your phone are great for moving your car or running a screen, but they're very picky. They're expensive to build, they can catch fire if they get too hot, and they wear out after a few years. Sand is the opposite. You can heat it up and cool it down thousands of times and it never breaks. It's also dirt cheap. You can find sand almost anywhere, and you don't have to mine it in ways that hurt the earth as much as we do for lithium. Plus, sand can get way hotter than water. If you try to store heat in a big tank of water, it starts to boil and turn into steam once it hits a certain point. That creates a lot of pressure and makes things dangerous. Sand doesn't care. You can keep pumping heat into it until it's five times hotter than boiling water, and it just sits there. It's a very safe, very simple way to store a massive amount of energy without a huge price tag.

HostOkay, but there's a catch here. When I think of a battery, I think of something that gives me electricity back so I can turn on my lights. If this just stays hot, I can't really use it to charge my phone or watch TV, can I?

GuestYou're right, and that's a big point to clear up. This isn't the kind of battery that gives you sparks back. It gives you heat. When the town gets cold and the sun stays down all day in the Finnish winter, they pump cold air through pipes that run deep inside that hot sand. As the air moves through, it picks up that heat from the sand and gets incredibly hot itself. Then they take that hot air and use it to warm up water. In many parts of Northern Europe, they have these systems of pipes under the streets that carry hot water to every house and office. It's called district heating. So the sand battery is basically a giant heater for the whole town. It replaces the need to burn oil or coal to keep the radiators warm. You're basically using August sunshine to keep your toes warm in January. It's a shift in how we think about energy. We don't always need more electricity. A huge part of the energy we use is just for staying warm.

HostThat makes sense for Finland, but most places I know don't have a giant web of hot water pipes under the sidewalk. Does that mean this only works for a few specific towns?

GuestThat's the main hurdle right now. To make this work well, you need those pipes in the ground or a big factory nearby that needs constant heat. If you had to build all those pipes from scratch, it would cost a fortune. But we're starting to see people look at other ways to use it. Some factories need a lot of heat to make food or cloth. They could use a sand battery to stop burning gas. There's also work being done to see if we can turn that heat back into electricity using steam, but we lose a lot of energy when we try to do that. It's much smarter to just use the heat as heat. The real goal is to find a way to make this work in places that aren't as cold as Finland. Even in warmer spots, we use a lot of power to heat water for showers or to keep hospitals warm.

HostIt sounds like we're just relearning a lesson that people knew thousands of years ago about how stone stays warm.

GuestThe simplest versions of this were just rocks around a fire, and now we're just doing that on a scale that can save a city.

HostThe sand under our feet might be the most basic tool we have to finally bridge the gap between a sunny summer and a dark winter.

GuestOne of the first sand batteries ever built is still humming along in a small town, proving that a giant box of hot dust can do what the most advanced chemicals cannot.

HostThe beach stays warm long after the sun goes down, and now that heat is finally finding a way into our homes.