Editor’s note: This article comes from MaxPower Weekly, a blog from Maxwell Technologies. It is authored by Mike Wilk, Sr. Systems Engineer.
Utilities and grid operators have a tremendous challenge every day—to produce enough energy to meet the ever-fluctuating demands on our electric grid. During the day there is peak demand—people, businesses and machines pull energy from the grid. At night, the world slows down, and the grid can prepare for the next spikes in energy use.
Over the decades, several different energy storage methods have been devised to capture and store energy so that it can be fed back to the grid when it’s most needed. There are a number of energy storage technologies available, and all come with their advantages and disadvantages. This is an overview of six energy storage methods available today.
1. Solid-state batteries Batteries are the most commonly understood form of energy storage. Solid-state batteries, which includes lead-acid and lithium-ion batteries, are energy dense. Lithium-ion batteries have superior energy density compared to lead-acid batteries. Batteries are ideal for backup, long-duration energy and use nighttime energy to recharge so that they can discharge energy during the day when peak rates are high. Batteries are an excellent fit for many applications that require long-term energy storage.
Power in a battery is what’s lacking. Lead-acid battery technology has reached its peak in terms of energy and power density. However, advances are being made in power density for lithium-ion batteries.
Another challenge with battery energy storage is cycle life. Batteries degrade with each charge/discharge cycle and with exposure to extreme hot or cold temperatures. Often they require complex heating and/or cooling systems, adding to the cost of the whole energy storage system.
2. Ultracapacitor or supercapacitor Ultracapacitors are also called supercapacitors or electric double layer capacitors (EDLCs). Ultracapacitors provide short-term, high burst power and typically perform hundreds of thousands of cycles with minimal degradation over time. Since ultracapacitors are electrostatic devices and contain no lead or acid, they are much less vulnerable to changes in ambient temperature.
Ultracapacitors perform optimally in industrial, transportation and grid applications that struggle to meet short-term, high power demands with current battery technology. Ultracapacitors do not provide long-term energy, so in many cases they complement batteries in applications that need to meet high peak power and long-term energy demands.
For example, ultracapacitors are an excellent technology for start-stop systems in cars, which require a high number of charge/discharge cycles to start and shut down the engine. The car’s battery performs best for supporting the car’s onboard energy loads, such as the radio and heating/cooling. For renewable energy challenges in the grid, ultracapacitors can be co-located with renewable resources or can be called upon from another location to help with stabilizing the renewables. The technology is flexible in that it can be stacked to match the requirements of the application.
3. Flywheels A flywheel is like a merry-go-round: when no one is on the merry-go-round, it spins easily and stores energy easily. When people get on the merry-go-round, they slow it down, pulling energy from it.
A flywheel is a high-speed wheel that rotates around an axis and stores energy mechanically in the form of kinetic energy. Very similar to the generators on the grid, the heavier the flywheel, the more energy it can store. The faster the flywheel mass spins, the more energy there is stored.
One challenge with flywheels that is well-known in the industry is that they can potentially become unstable. Typically, when a flywheel is installed, retaining walls must be built around it to contain the flywheel in the event it falls off its axis.
Flywheels can be installed in a car or can serve as a megawatt-scale energy storage system. They are similar to ultracapacitors in that they can be quickly charged and discharged. You can ramp up the speed of a flywheel by pushing energy into it and slow it down by taking energy out. Flywheels, like ultracapacitors, aren’t long-term energy storage devices; they are short-duration. They are used mostly for stationary grid applications.
The benefit of the flywheel is that it is a mechanical system, making it an eco-friendly solution, and it is a long-lasting device that offers many years of reliable performance. This NASA flywheel energy storage video does a great job of explaining how a flywheel works.
4. Pumped hydroelectric storage dams According to Department of Energy data, pumped hydro energy storage provides 95% of current U.S. energy storage capacity. The pumped hydro energy storage method uses two reservoirs, one at a higher elevation than the other. When the power demand is high, usually at peak hours during the day, water is released from the upper reservoir to the lower reservoir through a dam to generate electricity for the grid. When power demand is low and there is an excessive amount of power available on the grid, the water is pumped back up to the elevated reservoir. Pumped hydro is a long-term power generation system that supports multiple peak usage hours.
5. Rail energy storage The rail method is an iteration of pumped hydro. Rail cars full of rock are powered uphill during low grid demand, and when power is needed the rail cars roll downhill and recapture energy for later use via regenerative braking. Similar to pumped hydro, this method requires a lot of land usage. The Nevada Public Utilities Commission is building a rail energy storage system in the mountains of Nevada to stabilize the state’s electrical grid.
6. Compressed Air Storage This method compresses air into a cavern using motors powered by electricity or natural gas and when energy demand is high, the air is released through a turbine to generate electricity. This energy storage method has been in use for decades especially within the mining industry. The benefit is that it doesn’t use any toxic chemicals; the con is that it requires the cavernous space.
There are several other energy storage technologies in use, and all come with their pros and cons. The energy storage industry will continue to improve existing methods and innovate fresh concepts that can be implemented to deliver energy to consumers safely and efficiently.