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

via The Howard Stern Show / YouTube

Former Secretary of State, first lady, and winner of the popular vote in the 2016 presidential election, Hillary Clinton, sat own for an epic, two-and-a--half hour interview with Howard Stern on his SiriusXM show Wednesday.

She was there to promote "The Book of Gutsy Women," a book about heroic women co-written with her daughter, Chelsea Clinton.

In the far-reaching conversation, Clinton and the self-proclaimed "King of All Media" and, without a doubt, the best interviewer in America discussed everything from Donald Trump's inauguration to her sexuality.

Keep Reading Show less

Offering parental leave for new fathers could help close the gender gap, removing the unfair "motherhood penalty" women receive for taking time off after giving birth. However, a new study finds that parental leave also has a pay gap. Men are less likely to take time off, however, when they do, they're more likely to get paid for it.

A survey of 2,966 men and women conducted by New America found that men are more likely to receive paid parental leave. Over half (52%) of fathers had fully paid parental leave, and 14% of fathers had partially paid parental leave. In comparison, 33% of mothers had fully paid parental leave and 19% had partially paid parental leave.

Keep Reading Show less

Bans on plastic bags and straws can only go so far. Using disposable products, like grabbing a plastic fork when you're on the go, can be incredibly convenient. But these items also contribute to our growing plastic problem.

Fortunately, you can cut down on the amount of waste you produce by cutting down on disposable products. And even more fortunately, there are sustainable (and cute) replacements that won't damage the environment.

Coconut bowls


Who says sustainable can't also be stylish? These cute coconut bowls were handmade using reclaimed coconuts, making each piece one of a kind. Not only are they organic and biodegradable, but they're also durable, in case your dinner parties tend to get out of hand. The matching ebony wood spoons were polished with the same coconut oil as the bowls.

Cocostation Set of 2 Vietnamese Coconut Bowls and Spoons, $14.99; at Amazon

Solar powered phone charger


Why spend time looking around for an outlet when you can just harness the power of the sun? This solar powered phone charger will make sure your phone never dies as long as you can bask in the sun's rays. As an added bonus, this charger was made using eco-friendly silicone rubber. It's win-win all around.

Dizaul Solar Charger, 5000mAh Portable Solar Power Bank, $19.95; at Amazon, $19.95; at Amazon

Herb garden kit

Planter Pro

Put some green in your life with this herb planter. The kit comes with everything you need to get a garden growing, including a moisture meter that helps you determine if your herbs are getting the right amount of food to flourish. All the seeds included are certified to be non-GMO and non-hybrids, meaning you can have fresh, organic herbs right at your fingertips.

Planter Pro's Herb Garden Cedar Planter, $39.00; at Amazonedar Planter, $39.00; at Amazon

Reusable Keurig cups

K & J

Keurig cups are convenient, but they also create a ton of plastic waste. These Keurig-compatible plastic cups are an easy way to cut down on the amount of trash you create without cutting down on your caffeine. Additionally, you won't have to keep on buying K Cups, which means you'll be saving money and the environment.

K&J Reusable Filter Cups, $8.95 for a set of 4,; at Amazon

Low-flow shower head


Low-flow water fixtures can cut down your water consumption, which saves you money while also saving one of the Earth's resources. This shower head was designed with a lighter flow in mind, which means you'll be able to cut down on water usage without feeling like you're cutting down on your shower.

Speakman Low Flow Shower Head, $14.58; at Amazon

Bamboo safety razor


Instead of throwing away a disposable razor every time you shave, invest in an eco-friendly, reusable one. This unisex shaver isn't just sustainable, it's also sharp-looking, which means it would make a great gift for the holidays.

Zomchi Safety Razor, $16.99; at Amazon

The Planet