Solar energy even without the sun

08. August 2011 | Applications & Installations, Markets & Trends, Storage & smart grids | By:  Nicholas Stone

A new technology emerging from the United States of America could provide solar energy 24 hours per day even without constant sunlight through improvements to heat storage capabilities.

Buildings from the MIT campus.

MIT researchers develop system to provide 24 hour energy. Image: flickr/InspiredinDesMoines.

Called CSPonD, or concentrated solar power on demand, it is a new and improved take on concentrated solar power (CSP) systems that are already being used to produce solar energy across the globe.

A group of researchers from the Massachusetts Institute of Technology (MIT) has been developing the technology by concentrating on bringing down installation, operating and maintenance costs.

Previously, the systems used a large array of mirrors to focus sunlight on a central tower, which then delivers high temperatures to heat molten salt that in turn heats water and turns a generating turbine.

Such tower-based CSP systems required expensive pumps and plumbing to transport molten salt and transfer heat, making it problematic when looking to develop it in large quantities. It also only worked well under direct sunlight.
Alexander Slocum, a Professor of Mechanical Engineering at MIT, and a team of researchers slightly altered the system so that it combines heating and storage in a single tank, which would be mounted on the ground instead of in a tower. It also concentrates on focusing the sunlight downwards, which helps to achieve temperatures of more than 500 degrees Celsius.

The heavily insulated tank admits concentrated sunlight through a narrow opening at its top, and would feature a movable horizontal plate to separate the heated salt on top from the colder salt below. Water in the tank then heats up and drives the turbine. 

Professor Slocum says the system would be able to provide a 24-hour activation for solar power and a huge improvement on traditional CSP systems.

"It is cheap, with a minimum number of parts," he explained. "It's the swings in temperature that cause [metal] fatigue and failure. Normally you have to way oversize the system's components. That adds cost and reduces efficiency."

Under the new system, these costs are removed and the temperature is maintained even at night or in cloudy weather.

The team analyzed two potential sites for their technology on hillsides near White Sands, New Mexico and China Lake, California in the USA. Using the MIT system with large tanks of sodium-potassium nitrate salt — each measuring 25 meters across and five meters deep — two installations could each provide 20 megawatts of electricity for an entire day or the equivalent to the amount of energy used in 20,000 homes.

At this stage, optimum system operation would see heat accumulated over ten sunny days allow power to continue to flow for a day without sun. Europe's largest CSP plants, Andasol 1-3 in Spain, have molten salt storage for only up to 7.5 hours.

While exact costs are difficult to estimate, an analysis using standard software developed by the U.S. Department of Energy and provided by MIT suggests costs between seven and 33 cents per kilowatt-hour. At the lower end, that rate could be competitive with conventional power sources. 

The group said via the MIT website that they hope to produce a 20- to 100-kilowatt demonstration system to test the performance of their tank at a larger scale.

Professor Slocum sees a problem, however, with being able to fully develop the technology, unless a major backer is found.

"It's going to take a company with long-term vision to say, 'Let's try something really different and fundamentally simple that really could make a difference,’" he said. 

"[We have so far done] assemblage and simplification of known elements,” Slocum said. “We did not have to invent any new physics, and we're not using anything that's not already proven."


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