Ice or Molten Salt, Not Batteries, to Store Energy

Source: By MATTHEW L. WALD, New York Times • Posted: Wednesday, April 23, 2014

WASHINGTON — ENERGY storage is crucial to transforming the electric grid into a clean, sustainable, low-emissions system, the experts say. And it’s happening already, just not the way most consumers would expect.

The simplest idea for storage — charging up batteries at night when there is a lot of wind energy and not much demand for it, or at midday when the sun is bright — is years from being feasible, according to the experts.

The reason? It costs hundreds of dollars to store a kilowatt-hour of energy in a battery, while nationally the average retail price of a kilowatt-hour is about 11 cents. On the wholesale market, even buying low at off-peak periods and selling high could earn a battery owner perhaps 25 or 30 cents for each $400 or so invested. For that kind of transaction, “storage is not profitable,” said Jay Apt, executive director of the Carnegie Mellon Electricity Industry Center.

On the horizon is an exotic future of battery installations in places with plenty of conventional and renewable capacity that will need a bridge from daytime solar power. California anticipates that need in a few years, when solar power wanes at sunset, but natural gas plants are unable to awake fast enough from their afternoon nap, a problem the utility industry calls “ramping.”  Batteries will be needed to let the natural gas system start up early and supplement electricity supplies at twilight.

The storage field is so new that even executives in the energy business have trouble knowing how to think about it.

Take, for example, the strange tale of Dauren Kilish, an executive of AES Energy Storage, a company in Northern Virginia. Mr. Kilish’s firm set out to build a 40-megawatt battery in Texas, comprising several truck trailers. That amount of power would run about 4,000 window air-conditioners on a hot day or absorb about half the output of a small power plant running on natural gas.

Mr. Kilish called the grid operator to register the battery as an asset on the grid, since the battery was designed to be charged or discharged at the operator’s command. The grid keeps track of each generator as a resource, but also keeps track of businesses and can absorb extra energy when the system has a surplus.

So in which category  should it be registered? asked Mr. Kilish. Both, said the system operator, as a 40-megawatt source of supply and as a 40-megawatt source of load. AES said the system operator told the company to list the battery as an 80-megawatt resource.

AES also operates a big battery bank at the center of a wind farm in West Virginia. But it does not do what a consumer might expect; that is, storing energy from wind that blows mostly at night to meet heavier demand during the day when market prices are higher. Instead, on days when the wind is variable, it absorbs extra energy when a gust comes through, and pushes the energy back into the grid when the wind machines are suddenly calmed, to smooth out the variation in the farm’s output. It is far too small to store meaningful amounts from nighttime until daytime.

The value of storage, according to AES, is to add flexibility to the system.

But the way to store bulk amounts of energy for the grid lately is with cold or heat, as ice or molten salt. Hundreds of buildings now use their air-conditioning systems to freeze water into ice in the middle of the night,  taking advantage of low outdoor temperatures to help do the job. During the day, they melt the ice to cool the air.

In the Arizona desert, a major new solar plant uses the sun’s heat to warm sodium into a hot liquid. When the sun goes down, the sodium is piped into a steam generator, giving off its heat to boil water, which is used to spin a turbine and make electricity. The builders say using batteries would have made the costs many times higher.

A Houston company, TAS Energy, stores energy as cold. It chills water at night, and during the hot Texas summer, uses the cold water in a device like a radiator, to cool incoming air that is sent to a plant burning natural gas. When the incoming air is cooler, it is denser, so more of it fits into the combustion chamber, and power output is higher. The effect is to use nighttime energy to increase capacity in the hot afternoon.

Nearly all of these technologies lose energy on the way; that is, their round-trip efficiency, the ratio of energy put in to energy taken out, is well under 100 percent. But Darrell Hayslip, the chairman of the Energy Storage Association, said energy efficiency was not the point: dollar efficiency was. “You can’t really measure round-trip efficiency; it’s a function of economics,” he said. “If the charging electricity costs $2 vs. $20 for the electricity discharged, then it’s got greater round-trip efficiency.”

Mr. Hayslip’s trade group is 20 years old, but recently changed its name to “energy storage” from “electricity storage,” to better account for the variety of forms in which energy is being stored.