Storing energy for a rainy day

Battery fire caused by arcing

On a recent afternoon, Stephen W. Moore hurried through the USA's Dallas-Fort Worth International Airport on an important assignment. His destination: San Antonio, Texas, USA. His mission: perform forensic analysis on a battery that had burned during standardised abuse testing on the basis of IEC safety Standards.

Moore was called in to dismantle the battery pack and figure out what had happened. Using techniques similar to those seen on the popular television series CSI (Crime Scene Investigation), Moore picked through the blob of molten material and chipped away at the plastic hunting for fragments of melted metal.

'With that particular metal's melting temperature you can tell what temperature that part of battery reached,' he says. 'Like an arson investigation, you can tell where the fire started and how it moved, how hot it got and whether there was any structural damage.'

Moore's analysis was performed on a battery that had undergone routine abuse testing of the sort performed by any battery manufacturer or battery pack integrator. Manufacturers perform such standardised tests with expectations of learning from the results.

'From an IEC perspective, this illustrates a very good reason why IEC standards are important,' says Moore, who was the founding Chairman of SC(Subcommittee) 21A WG(Working Group) 5: Large format lithium batteries. Now owner of Lithium Power Solutions in Fishers, Indiana, USA, he's hired by companies to be their expert eyes and ears. His recent effort to autopsy a failed battery illustrates the importance and the pitfalls of finding safe, reliable energy storage options. As society becomes more complicated, the demand for portable energy is increasing astronomically.

'We will always need portable energy,' Moore says. 'Our bodies carry energy with us in the form of food that we eat. Appliances or consumer electronics carry energy in the form of batteries. For transportation, we are seeking ways to replace gasoline with a different form of portable energy that is not carbon-based. For the utility grid and for commercial power we want to supplement the power we already have. One way of doing that is putting distributed energy around the entire community. That reduces stress on transmission lines and power plants.'

Pump it up

Many storage technologies are either currently available or under development. These include pumped storage, compressed air, and hydrogen storage, and chemical storage, which includes Lithium-ion batteries and a host of other storage chemicals such as molten salt.

Batteries can reach the megawatt range for large-scale utility grid applications. For example, flow batteries contain liquid that is pumped through a manifold to metallic electrodes. Energy is released when the electrical charge is removed from the liquid. In another design, giant mechanical flywheels spin inside a vacuum storing energy in a mechanical form. These devices are hooked up to generators that take energy from the flywheels as needed.

Geography comes in handy when energy needs to be stored for commercial power plants. Some utilities can use underground mines to store compressed air. In this circumstance, electrically powered turbo compressors apply pressure to air and then store the compressed air underground in mines. Later, when electricity is needed, the compressed air can be run through a turbine to generate power.

More commonly, conventional hydroelectric plants generate electricity through the height difference between two bodies of water, either natural or engineered. Pumped storage plants take further advantage of this height difference to store energy. When energy demands are low, water is pumped from a reservoir at lower elevations to reservoirs at higher elevations. Then, during peak energy demands, water is released back to the lower reservoir through turbines, which in turn generate electricity. Such a plant is under construction in eastern Switzerland. The Limmern pumped-storage project in the Linthal Valley will utilise the proximity of two existing reservoirs – the Muttsee Lake (elevation: 2474 m; capacity: 25 million m3 water) and the Limmernsee Lake (elevation: 1857 m; capacity: 92 million m3 water) – by building an underground pumped-storage plant between the two lakes.

During the day, when power demand peaks, water will be released through the turbines. At night, as demand decreases, excess electric power is available on the grid. This power will be used by the new Limmern plant to pump backwater 'upstream', from Limmernsee into Muttsee. For these pump-and-turbine operations, the Limmern plant will be supplied with some 1000 MW of power through an underground access tunnel which is about 5 km long.

For parts of the world where solar farms support the energy grid, thermal storage solutions seem the best fit. Here, molten salt can be stored as an ultra-hot liquid. Unlike water, which boils at 100ºC and turns to steam as more heat is applied, melted salt remains a liquid. As more and more heat is pumped into the liquid salt, the temperature continues to increase and produces an ultra-hot liquid. The hot liquid in turn can be stored in the form of heat in solar concentrators until energy is needed. Then, the heat from the molten salt drives a steam turbine to create power.

Building a better battery

Chemical storage solutions reign in the transportation sector. For example, hydrogen fuel cells were used recently to power buses (see TC 105 prepares standards as fuel cells go commercial in March 2010 e-tech) at the 2010 Olympic village of Whistler, Canada, and will power a fleet of buses on a line expected to reach from Vancouver, British Columbia, Canada, to California, USA.

Often, scale determines the choice of chemical. A molten sodium battery can be used for hybrid locomotives. In this application, molten sodium works like a lithium battery. On a scale smaller than locomotives, present-day automobile batteries rely on nickel-metal hydride because this combination proves more robust to abuse, which may include short-circuit exposure to fire or overcharging, explains Moore.

On smaller scales, such as consumer electronics, lithium-ion dominates. A fire in an automobile would cause more potential damage than a fire in say, an MP3 player or other consumer electronics product. 'Because lithium-ion [batteries] have this vulnerability to abuse testing, they're not common yet in large-scale applications,' says Moore. The IEC committees test lithium-ion batteries exhaustively. The committees examine everything from overcharge, to short-circuiting, to penetration. They actually put a metal spike through the battery to determine the level at which each battery can be safely crushed.

No matter the form, whether chemical, thermal, or gravitational, each energy storage option has its drawbacks and its promise. One thing remains certain: energy storage technologies will remain as much a part of our lives as the food we eat.

'As our energy needs are always increasing, so will our needs for energy storage, both on personal and a community scales,' says Moore.

Reproduced from an article by Jeanne Erdmann in IEC e-tech, May 2010.

Published in energy.