People still need electricity when the wind isn't blowing and the sun isn't shining, which is why renewable energy developers are increasingly investing in energy storage systems. They need to sop up excess juice and release it when needed.
However, storing large amounts of energy, whether it's in big batteries for electric cars or water reservoirs for the electrical grid, is still a young field. It presents challenges, especially with safety.
The most recent challenge first appeared in May, three weeks after a safety crash test on the Chevrolet Volt, General Motors Co.'s plug-in hybrid. The wrecked vehicle caught fire on its own in a storage facility, raising questions about its lithium-ion battery.
Last week, after a series of additional side-impact crash tests on the Volt battery, the National Highway Traffic Safety Administration (NHTSA) launched what it called a "safety defect investigation" into the risk of fire in a Chevy Volt that has been involved in a serious accident.
Problems have also afflicted spinning flywheels, which allow power plants and other large energy users to store and release powerful surges of energy. In Stephentown, N.Y., Beacon Power's 20-megawatt flywheel energy storage facility suffered two flywheel explosions, one on July 27 -- just two weeks after it opened -- and one on Oct. 13. The company declared bankruptcy earlier this month.
In Japan, sodium-sulfur batteries at Mitsubishi Materials Corp.'s Tsukuba plant in Ibaraki prefecture caught on fire on Sept. 21. It took firefighters more than eight hours to control the blaze, and authorities declared it extinguished on Oct. 5.
NGK Insulators Ltd., the company that manufactured the energy storage system, said it is still investigating the incident's cause and has halted production of its sodium-sulfur cells, which are installed in 174 locations across six countries.
"Clearly, storing large amounts of energy is difficult from a physics standpoint; [the energy] would rather be somewhere else," said Paul Denholm, a senior energy analyst at the National Renewable Energy Laboratory.
He explained that energy naturally wants to spread out, so packing it into a small space like a battery or a fuel cell creates the risk of an uncontrolled energy release like a fire or explosion. Similar issues come up with mechanical storage, whether it's water behind a dam, compressed air underground or spinning flywheels.
Some storage risks are 'grandfathered'
However, these risks are not unique to storing electricity. Fossil fuels, which are technically forms of stored energy, pose plenty of problems in their extraction, refining, distribution and delivery.
"We basically have grandfathered these risk factors. Gasoline catches on fire all the time," said Denholm. Electrical energy storage systems aren't inherently riskier than petroleum or natural gas, according to Denholm, but their risks are different.
The NHTSA shares Denholm's assessment when it comes to cars. "Let us be clear: NHTSA does not believe electric vehicles are at a greater risk of fire than other vehicles," said the agency in a press release earlier this month responding to the Volt fire. "It is common sense that the different designs of electric vehicles will require different safety standards and precautions."
For batteries, the main issue is how they control the heat they generate. "What you really want to avoid is cascading failure," said Denholm. "A failure of any one of those batteries is not a huge event, but if you don't have proper thermal management, a failure in one battery can cause failure in another."
This condition, known as a thermal runaway, happens when a cell fails and releases its energy as heat. This heat can cause adjacent cells to fail and generate heat, as well, leading to melting materials and fires.