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Energy Efficiency MIT team develops solar power and energy storage system for 24/7 solar power delivery

Published on August 2nd, 2011 | by Tina Casey


Solar Power for Vampires: MIT Team Invents 24-Hour Solar System

August 2nd, 2011 by  

MIT team develops solar power and energy storage system for 24/7 solar power deliveryA team of researchers from the Massachusetts Institute of Technology has figured out a way to deliver low cost solar energy at night as well as during the day, by combining a concentrated solar power system with a molten salt heat storage system. Concentrated solar and salt storage are two familiar technologies and this would not be the first attempt to merge the two. However, the MIT team is focusing their system on bringing down installation, operating and maintenance costs. If the concept proves workable, it could help small towns and communities take themselves off the grid with a reliable, steady stream of clean energy.

Concentrated Solar Power and Molten Salt Storage

Concentrated solar power basically means using mirrors to focus sunlight on a central tower. When combined with molten salt in the conventional manner, the idea is to heat the salt with solar energy, then transfer the energy to water, which generates steam that turns a turbine to produce electricity. However, without a storage element this arrangement does not provide a steady stream of energy, and it requires pumps and piping that can be expensive to install and operate.

A Low-Cost Solar-Salt Combo

The MIT team resolved the cost problem with a ground-mounted tank instead of a tower. The low-rise system eliminates a significant amount of plumbing and related equipment, and it would be less expensive to install and maintain over the long run. Rather than positioning mirrors to focus sunlight upwards to a tower, the mirrors would be positioned on a hillside above the tank, and focus sunlight downwards to a narrow opening at the top of the tank, achieving temperatures of more than 500 degrees Celsius. The tank would double as storage by use of a floating barrier that adjusts itself during daylight hours to keep the heated salt separate from cold salt.

Solar Power as Cheap as Fossil Fuels

Earlier this year the Obama administration announced the SunShot initiative, designed to promote new technologies that bring the cost of solar energy down to a competitive level with fossil fuels. In a best case scenario, the MIT team estimates that the cost of their “Concentrated Solar Power on Demand” system would be about seven cents per kilowatt hour. Small-scale tests have already been conducted using sodium-potassium nitrate salt and the next step is to build larger demo facilities, potentially at sites in New Mexico or California.

Image: The moon by dingopup on flickr.com.


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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

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  • Wilmot McCutchen

    Getting power out of heat by steam in a Rankine cycle is conventionally 35% efficient. Does MIT know how to use the other 65% of the stored solar energy, or will it just be dumped into the atmosphere by the condenser? Another consideration is water consumption of thermal power plants. In a desert, where the CSP will be, wasting 25 gallons per kWh into the atmosphere with wet cooling does not sound like a sustainable practice.

  • Mac McDougal

    Very cool 🙂 The article linked to this post mentions a 10:1 operating surplus–10 sunny days allows power generation for one cloudy day. This is good, but is it economically realistic? Is there a more efficient storage mechanism (e.g., flywheels in a vacuum, or others mentioned in CT) that could store power longer than hot salt? Does anybody out there know these numbers?

    • Anonymous

      I think the first thing to look at is how likely is a two-day cloudy event in that location. This is the Southwest Desert – not Seattle. Periods greater than one cloudy day in a row might be so rare that extra days of storage would never pay for itself.

      If there are, say, ten times a year when the sun gets clouded out for 2+ days in a row the cost of that extra storage would have to be spread over only ten days – some very expensive electricity.

      There might be a lot more one cloudy day events. I expect someone has done the math and demonstrated that one day of storage makes sense. Might be that there’s a need to fill in for cloudy mornings along the coast.

      The big issue, IMO, is late afternoon/early evening storage. PV solar is going to get very affordable (and later on cheap). But it quits producing once the sun drops lower in the sky. And good wind often does not start up until later at night. I’m betting that down the road these thermal plants are going to be late-peak providers, not adding much to the grid when the sun is shining but storing up for the hours when it starts to set. That is going to be their best paying market.

      Other storage systems, flywheels aren’t likely a bulk storage solution, more grid smoothing. Utility scale batteries likely to be major players, but more likely for already generated power from wind. Pump-up is likely to be growing, we’ve got lots of existing dams that could be converted, thousands of them. And more solar/wind means that we can often reserve the hydro we have and use it as backup, rather than ‘base load’.

      And I’m really interested in semi-deep ocean CAES. Sticking air bladders at some depth and compressing air into them when we have extra power, pulling it out when we need it. One company now in the testing process is projecting $0.025/kWh for storage. Combine that with wind at less than a nickle and we’ve got some affordable very reliable power. Gas peakers could easily be twice, three, four, or more times as expensive.

      A little hard to guess how it will play out. We’re years away from needing significant storage. And when we add EVs to the grid we will need less, put the date off further. Parked, plugged-in EVs will mean a good market for what would otherwise be surplus power which means that it will be more profitable to install more solar and wind than otherwise.

      I’m liking the fact that we’re building a few of these thermal plants with storage now. This gives us time to tinker and get costs as low as possible. We need to be putting some money into all types of storage so that when the time comes we have mature technologies ready to go.

      The folks in Italy who are doing thermal solar with storage are talking some very inexpensive electricity. It’s going to be interesting to see how things work out.

    • Mac McDougal

      Thanks Bob, these are all excellent points. But I was assuming that this is a giant “bench test” for inexpensive solar storage that could be used in *any climate, not just the nearly ideal desert conditions of the initial tests. One number I want to make sure I understand: you mention CAES projected costs of ” $0.025/kWh.” 2.5 cents/kWh, right? Now that’s an exciting number.

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