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Solar power is growing so fast in California - with installations by customers increasing tenfold since 2006 - the renewable energy source is turning the state's power system upside down.
In a twist that is being closely watched by power companies around the country, California utilities will install massive banks of batteries and other devices to store the power surplus created by solar panels in the afternoon, when the sun's rays are strong. The batteries are then to begin discharging power into California's electric grid in the early evening, around sunset, when the solar generation of energy dies down, but demand rises as millions of people arrive home and turn on air-conditioners, televisions and other electricity gobblers.
The new system is the opposite of an idea utilities have considered for years: Use batteries to store power at night from traditional sources, like natural gas and coal, and run them down in the peak heat of late afternoon.
"It is the reverse of the way we've always thought of storage," Gregory Reed, director of the Electric Power Initiative at the University of Pittsburgh.
The relatively new idea of using batteries - which could be bundled in packs, each about the size of an 18-wheel truck trailer - to store electricity during the day and discharge it in the evening is aimed at coping with rapid changes in supply and demand. The expense of the batteries, possibly in the billions of dollars for California, has limited their use.
But booming solar power in California has changed the equation and made the California Public Utilities Commission take a different path.
At the end of October, in the first mandate of its kind in the country, the commission ordered the utility companies it regulates to install some form of energy-storage equipment - exactly what was not specified. A critical purpose of the storage is to allow generators, which in California run largely on natural gas, to keep operating in the late afternoon, when the output from solar panels eliminates the need for their electricity.
With so many solar panels in California, "we may find ourselves in periods of time when we have oversupply, overgeneration," said Clyde Loutan, senior adviser for renewables integration at the California Independent System Operator, which runs the state's grid. That is just as destabilizing as shortage, he said.
The point of keeping the generators running is that they will be needed immediately after the sun sets, but the problem is that they cannot start instantly. By 2020, planners estimate, it will be difficult to balance supply and demand at sunset because the combination of the sun setting and evening demand picking up will create the instant need for a vast amount of power. The system would have to double its output in about three hours, faster than it can now manage.
The problem is acute in California because the demand for power is concentrated in a narrow, coastal band that runs north and south. "The sun sets on all of it at about the same time," said Philip Undercuffler, director of product management at Outback Power, a battery company.
This curve of the demand for electricity, formerly undulating like ocean waves, is projected to take an extremely steep pitch up every day around sunset, into a shape that industry insiders call the "duck chart" because a chart produced by the California Independent System Operator resembles the profile of a duck.
"There is no longer a peak during the day - that's now actually a valley," said Christopher R. Cook, president of Solar Grid Storage, a Pennsylvania company that installs batteries connected to solar arrays.
Batteries could bring far more value in adding flexibility to respond to changing positions than in other roles, specialists say, by providing what the utility industry calls "ramping" or helping the electricity supply increase or decrease quickly.
Installing batteries was once seen as a way to avoid building power plants because a utility could use the batteries to meet peak demand. So far, however, this has not proved profitable. In addition, batteries have never been inexpensive enough to pay for themselves with the profits from what utilities call "arbitrage" - buying power at off-peak prices and reselling it at higher prices.
Solar industry officials agree that the nation's electric grid must change to accommodate more solar power, but they contend that the problem is exaggerated. Ken Johnson, a spokesman for the Solar Energy Industries Association, said the duck chart assumes that solar panels are on platforms that change their angles to keep them pointed toward the sun, in which case energy generation would drop abruptly at sunset. In fact, most of the panels are in fixed positions, so generation falls more slowly, he said.
The Public Utilities Commission did not cite a cost estimate, but said it anticipated the price for storage would fall as storage equipment manufacturers developed new products to meet the mandate. It is not clear how costs would be shared among all rate payers and companies or individuals who have put solar power on the grid. Already, energy can be stored as heat or cold.
The utilities commission's order for storage, which will be phased in starting in 2014, represents shifting needs, said Laura Wisland, a renewables specialist at the Union of Concerned Scientists. "We're shifting from a need for capacity on the system to a need for flexibility on the system," Wisland said.
Some of the storage need could be met by the batteries in electric vehicles, said Ufuk Topcu, a specialist on storage and grid dynamics at the California Institute of Technology in Pasadena. But that would require more electric cars than are now on the road.
But none of this is bad news, Wisland said. "The duck chart is illustrating that solar is doing its job," she said.