Building Storehouses for the Sun's Energy, for Use After Dark

The following was gleaned from a January 3 New York Times article with the above title.

Building Storehouses for the Sun’s Energy, for Use After Dark

The solar power industry must overcome a major stumbling block: finding a way to store it for use when the sun isn’t shining.

Solar thermal power makes electricity by using the sun’s heat to boil water. The water can be used to heat salt that stores the energy until later, when the sun is no longer shining.

The U.S. Energy Department recently gave a $737 million loan guarantee to a solar thermal company for a plant that will generate 110 megawatts at peak and store enough heat to run for eight to 10 hours when the sun is not shining.

One advantage of adding storage capacity has to do with the equipment that makes electricity being the most expensive part of a solar thermal system.  If it is connected to storage technology, it can run almost twice as many hours as a plant without storage. That means the unit cost of electricity drops.

Another has to do with the arcane economics of electricity. A utility must assure a supply of electricity in two forms: energy and capacity. The difference has never meant much to most consumers, who directly pay only for energy, as measured in kilowatt-hours.

But capacity, the dependable ability to produce power, is becoming more important as renewable energy forms a larger and larger part of the grid.

Wind and sun provide a lot of energy but not much capacity. Today, backup capacity for wind and solar power comes in the form of expensive gas-fired generators, which sit idle most of the year but operate when the wind stops blowing or the sun stops shining.

Storage could cut costs by 4 cents a kilowatt-hour, Mr. Denholm calculates — a considerable benefit for a commodity that retails for an average of 11 cents. A big part of the savings is not having to build the gas-fired generators for backup.