Fuel input for hydrogen-fuel-cell-powered vehicle. Photo from Siemens PLM in Cypress California.
Storing energy is important for both long-term and short-term uses: to meet changes in energy supply and demand and to iron out irregularities in energy output, whether that’s in a car engine or on the power grid.
Unfortunately, we can only store a tiny fraction of the electricity we produce in a single day. Instead, power plants have to send their thousands of megawatts of electricity to the right place, at the right time. For more details about electricity transmission, see Power Grid Technology.
Our current electric grid has various quick storage solutions to help make energy delivey smooth, and we use energy storage in cars, phones, and anything else that needs to be moved around. However, batteries and other storing options leave much to be desired.
Devices like capacitors and flywheels can store energy for extremely short periods. Few technologies exist to store large amounts of energy over time periods ranging to several days: only pumped (water) storage is widely used to store energy on the scale of a power plant.
As energy sources are expanding to include more renewable and intermittent resources like wind and solar onto the electricity grid as we try to both meet growing energy demand and control greenhouse gas emissions. Likewise, there is increased interest in having reliable energy storage for vehicles, instead of gasoline and diesel fuels.
Depending on the type of battery, these devices can store energy on location, like at home or in the car, in a laptop or cell phone. Note that though batteries and fuel cells can help integrate renewable energy sources, most electricity is still generated from fossil fuels. Both charging batteries and making hydrogen for fuel cells thus produce greenhouse gas through reliance on the prevailing sources of electricity, and using these devices is less efficient than plugging into the wall because there’s always energy loss to byproducts like heat.
Batteries store chemical energy. Chemical reactions in the battery release that energy as needed. Eventually all the starting materials of the reaction are consumed and the battery is dead, or at least unusable until it’s recharged. There are many kinds of batteries, made of a wide array of chemicals. Polysulfide Bromide (PSB), Vanadium Redox (VRB), Zinc Bromine (ZnBr), Hydrogen Bromine (H-Br), and sodium sulfide batteries are some that the electricity industry has interest in.
Electric utilities use lead-acid batteries, which can be recharged, but there is research into other materials for utility and transportation use. Some of the best batteries used today for cars are nickel metal hydride and lithium ion.
HYDROGEN FUEL CELLS
Hydrogen fuel cells aren’t the same as batteries, but they can serve a similar purpose. Fuel cells are lumped with batteries because they both function through stored chemical energy. However, in practice, fuel cells are more like engines. They run off hydrogen “fuel” and produce energy and waste products, mostly water vapor. As long as hydrogen keeps being added, the cell can run, just like a gasoline engine can keep running as long as more gasoline is added. A battery has a finite amount of energy unless it’s recharged with electricity.
For more about how to make hydrogen for fuel cells, see The hydrogen economy, hydrogen sources, and the science behind these.
For a description of different hydrogen fuel cells in development right now, see here.
One way to smooth bumps in electricity delivery is through flywheels, which store energy in the form of rotational kinetic energy. A spinning potter’s wheel stores the energy of a good kick to be used moments later to mold a clay pot, and flywheels operate on a similar principle. In automobile engines, flywheels ease the transition between bumpy firing pistons and the drive shaft.
Flywheels can store energy for limited periods of time, from seconds to a few minutes.
Pumped storage (of water) is the only widely-used method for storing huge amounts of energy for long periods of time. The United States has a capacity of more than 20,000 megawatts of pumped storage, according to the National Hydropower Association.
During times of excess electricity production, that excess energy is used to pump water to a higher altitude, increasing its gravitational potential energy. When extra energy is needed, the water is allowed to flow back down by way of turbines, turning that potential energy back into electricity.
For a figure of pumped storage see the National Hydropower Association
OTHER WAYS TO STORE ENERGY
Other technologies are constantly being investigated for energy storage. Compressed air storage is when air is forced into spaces like mines or caves and held at high pressure, using up energy in the process. When the compressed air is let out again, it can turn turbines to generate electricity.
Thermal energy storage exploits the difference in temperature between a system and the environment. In the late 1800s, Americans used thermal energy storage by cutting blocks of lake ice during the winter and storing them underground packed in insulating wood shavings. When the summer rolled around, they retrieved that stored ice to make food cold, exploiting the difference in temperature to force thermal energy out of the food.
Thermal energy storage can also happen in the other direction. Electricity or other forms of energy can be used to heat various materials, which are stored in insulated containers. Later, when the energy is needed, the hot materials can heat water into steam, and that steam can push turbines, which in turn produce electricity.
Thermal energy storage can also be used through ocean energy.