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It may be wise to salt away our solar energy in the sand

By Dr. Nicolas Calvet 
December 15, 2013

Solar energy is everywhere. From dawn till dusk, the sun’s rays beat down on the earth, providing warmth and light. The solar energy hitting the earth each year exceeds the total energy consumed by humanity by a factor of over 20,000 times.

But once the sun sets, or clouds fill the sky, our supply of solar energy is cut off.

So how do we store solar energy to provide electricity at night?

The obvious solution of storing the energy in batteries is presently not economical at the massive solar power plant scale.
Other options, of compressing and uncompressing air, or pumping water up a slope to use the energy it generates when it flows downhill, are all energy intensive themselves and require large storage areas or riverbeds.

The best answer today lies in thermal energy storage (TES) and heat transfer – using the sun’s energy to heat a material up and then using that heat to create steam on demand that can power a turbine and so generate electricity.

In concentrated solar power (CSP) plants, molten salts have been the material of choice to store solar energy since the 1980’s.

The molten salt system uses concentrated solar energy to heat up nitrate salts and then, when solar energy is not available, using that heat to create steam to generate electricity from a turbine.

Our collaborating partner Masdar participated in the development of the Gemasolar plant in Spain, the world’s first CSP plant to produce electricity for 24 hours a day.

Scientists at Masdar Institute, however, think they may have an alternative to molten salts for the next generation of CSP plants.
It builds on a similar two-tank concept, but is cheaper, better for the environment and even more efficient because it operates at higher operating temperatures.

And the material they are looking at to store this thermal energy is one that is in plentiful supply in the UAE: sand.
Sand has many promising properties. It is cheap. And it can store thermal energy at higher temperature –1000°C, against molten salts’ 600°C. That means hotter steam for the turbine – and more efficient electricity production.

And the sand-based energy storage system the Masdar Institute is developing will do away with heat transfer fluids, pumps and pipes, resulting in a significant decrease in operation cost.
The technology Masdar Institute’s thermal energy storage research team is designing would use two tanks of sand, using gravity to transfer it from one tank to the other, as in an hourglass.

The upper tank will hold the “cold” (but still 250°C) sand, with the heated (800°C) sand in the lower tank.

This ‘cold tank’ will be in the shape of a hollow cylinder, with the beam of energy from the solar reflectors down the middle. When a valve is opened, the sand will flow into the path of this beam, and the concentrated solar energy will heat it up. Then the hot sand is recovered and stored in the lower tank until energy is needed.

To discharge the system, a heat exchanger is immersed in the heated moving sand, producing superheated steam that runs the turbine.

The cooled sand is then sent back to the top of the cold tank by a conveyer belt to close the loop of this continuous process.

This technology, once perfected, should provide the UAE’s solar ambitions with an efficient, cost-effective and environmentally friendly way to store energy for 24/7 CSP plants. It can also later be adapted to other industrial processes, such as steel making, that produce waste heat that could be used to heat the sand – and thus reduce the net energy use of these facilities.

With this research, we hope to help the UAE reach its targets for renewable energy integration and carbon footprint reduction while providing its economy with a lucrative and high-demand innovation.

 
Dr. Nicolas Calvet is an assistant professor of mechanical and materials engineering at the Masdar Institute and leads its thermal energy storage group.

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