Making Solar Power Work at Night with Mirrors, Salt, and a Hillside

Solar-power systems are going to need to provide energy at night or on cloudy days. A team of MIT engineers has a new idea to do just that.

If solar power is going to replace conventional sources of energy like gas and coal, it's going to need to work at night or on cloudy days. Alexander Slocum, an engineer at MIT, and his team of researchers have come up with a new, more efficient system to do just that. Their idea involves hills, mirrors, salt, and a massive insulated storage tank. But to understand how it’ll work, you need to understand a little bit of the technology behind solar power.

There are two main ways that solar-power systems collect sunlight. The one most people are familiar with uses panels covered in photovoltaic cells to soak up the sun's energy. Most solar power in the United States comes from these systems. The other type of solar power system concentrates light at a smaller point of collection. Solar towers work this way: instead of fields of panels, solar tower projects use fields of mirrors that direct light to a single point at the top of a tower. The United States already has more than 400 megawatts of concentrated solar projects in operation, according to the Department of Energy.

Solar panels, on the other hand, currently provide more than two gigawatts of power across the country. Part of the reason that solar panels are so much more popular is that they’re cheaper. But there’s growing interest in making concentrated solar power commercially viable. The Department of Energy says it’s “ramping up” research in this area and aims to make it fully competitive by 2020. In April, Google put $168 million into BrightSource Energy’s Ivanpah project, one of the largest solar tower projects ever. It's going to generate 392 megawatts of energy, which could power 140,000 homes during the day.

The Ivanpah project, though, can’t store energy. Boilers sit at the top of the towers, and as the concentrated sunlight heats the water there, it turns to steam. The steam drives electricity-creating turbines. When the sun doesn’t shine, though, the water doesn’t boil, and the turbines stop.

But some solar towers—more expensive ones—can store energy. Instead of water, these towers heat molten salt, which still runs through a boiler system but can also absorb and hold on to large amounts of energy, enabling the boilers and the turbines to operate even when the sun’s not out.

The problem is it takes a fair amount of power to pump molten salt up and down tall towers. That’s where the MIT idea comes in: It turns the solar tower on its side. Instead of having mirrors pointing upward, the MIT team wants to situate mirrors on a hillside and have them pointing down. The light would concentrate at slim opening to a heavily insulated tank on the ground below. Generally, solar storage systems have a tank to hold cold salt and a tank to hold hot salt. This tank would have the hot salt on top and the cold salt on the bottom. As more salt absorbed the heat, the floor of the tank separating hot from cold would sink down, allowing more room for the hot salt. The water to drive the turbines would circulate around the tank, absorbing heat from the salt, which could reach up to 500 degrees Celsius.

According to Slocum and his team, a tank 25 meters across and five meters deep could generate 25 megawatts of energy, enough to power 20,000 homes. Given enough sunny days—10 in total—the system could also keep running through one entirely overcast day.

There are probably a limited number of hills available to create this system. But one maxim for the renewable energy future is that we’ll get our energy from a greater variety of systems. This could be one of them.

Photo courtesy of the National Renewable Energy Laboratory

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