The Chevy Volt fire is just one recent example of potential safety risks associated with large-scale energy storage

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 such as Canon lithium-ion battery.

Last week, after a series of additional side-impact crash tests on the Volt battery like Bosch 12 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 DE9071 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.

Controlling temperatures is relatively simple when the batteries are in a fixed location, say, next to a wind farm, but it becomes harder when they are placed in a car or bus.

"The biggest thing that people become concerned about [for batteries in cars] is the ability to be able to tolerate abuse," said Joe Redfield, principal engineer at the Southwest Research Institute, a nonprofit engineering research and development group.

In a car, a battery is exposed to a wide range of humidities, temperatures and electrical loads. All of these factors influence the battery's reliability, and if they get too extreme, they can cause a thermal runaway condition.

New problem for firefighters

The problem is compounded by the fact that newer lithium-ion batteries store more electricity than other electrochemical storage systems. "The lead-acid battery has been around a long time" and is a mature technology, said Redfield. "The energy levels of lithium-ion batteries are much, much, much greater than that of lead-acid storage."

This becomes a major problem for firefighters and first responders in the event of an accident involving lithium-ion batteries. Water can't always be used to extinguish an electrical fire, since water can conduct electricity.

In addition, in the case of a thermal runaway, it's usually not the batteries that catch fire but their fumes, though lithium itself is flammable. Even after the fire is extinguished, the batteries can still generate tremendous amounts of heat and reignite fumes, hampering rescue efforts.

One solution is to separate batteries into modules, making it easier to isolate a failed battery from the rest. Another trick is to have a master kill switch, a mechanism that quickly disables the electrical system and discharges the batteries.

The Department of Energy and the National Fire Protection Association are working together to train firefighters and rescue workers to identify these switches in vehicles and grid storage systems as well as in how to respond to battery fires, according to the NHTSA.

Redfield said that the best way to prevent such incidents is with a battery management system that evenly distributes electrical loads and controls temperatures. "It's not just for safety; it's primarily there to provide performance and battery life," he said.

Electrics get high marks in crash tests

"As the operating temperature increases, the lifetime diminishes dramatically. You want to ensure the longest battery life, and if you achieve that, then you're clearly in the safety limits of the operating environment," he added.

Overall, Redfield expects that energy storage systems will help increase renewable energy use and curb fossil fuel dependence in the United States. The bumps along the road are significant, but they do not result from an inherent flaw in the idea.

"Failures in new technology have almost always been the result of design shortcuts that were made in putting the new technology into progress. Every now and then, you have some uncharted territory -- things we haven't seen before -- but typically, they are few and far between," said Redfield.

"It really is going down the same path we've gone down many times before. We don't need to make the same mistakes we've made with liquid fuels." After the earlier testing, NHTSA gave the Volt a five-star crash test rating -- the agency's highest -- and it did the same for Nissan's all-electric Leaf.

Meanwhile, a second testing agency, the Insurance Institute for Highway Safety, has given the Chevrolet Volt a "G," the highest safety score possible, after side crash tests on the front, side, rear and rollovers.

Research by an affiliate of the insurance group, the Highway Loss Data Institute, estimates that overall chances of being injured in a crash are 25 percent lower in hybrids because their large batteries make them heavier than similar gasoline-powered cars.