A solar plant that can burn fuel when the sun is not enough

A startup called 247Solar has a deal with India to build and run a small solar plant that is its own backup. It stores the sun's heat for use after dark. When the sun is weak for long stretches, the same equipment can burn fuel instead. The plant will be tiny, at 1 megawatt, and it will also use ordinary solar panels.

How it works

The plant uses concentrated solar, also called solar thermal. Rows of motorized mirrors follow the sun and aim light at a tower. There, the light heats air to about 970° C. Most of that heat goes into storage: ceramic pellets about the size of a golf ball. The company says the stored heat can run power for 9 to 20 hours after sunset.

The backup trick

Hot air drives a turbine through a part called a heat exchanger. It sits where a fuel-burning unit would normally sit in a standard turbine. The system does not care how the air gets hot, so burning fuel can do the same job as sunlight. A 247Solar executive said adding this option costs less than 10 percent of the plant's total price.

• The first site pairs two 200 kW generators.
• Apart from the heat exchanger, the generator package is standard, off-the-shelf equipment.
• Leftover air at about 250° C could serve industrial processes or cooling.

Why it is a long shot

Solar panels have become the cheapest power in most places. Concentrated solar costs have not kept pace, and new plants have largely stopped being built. Grids now fill overnight and bad-weather gaps with batteries and natural gas plants, which means building extra facilities. 247Solar says its design avoids some of that. On a typical grid, though, there are few rewards for around-the-clock renewable power.

What is not known yet

The first plant is meant mainly to test the engineering. A company engineer said the biggest worry is the "unknown unknowns." There is no word here on how well it performs or what its power will cost. The company is already working on a 3 megawatt version that would share one tower. The small plant will show whether the money works out.