My last blog explained how large-scale energy storage enables cleaner electricity grids by making wind and solar resources controllable and flexible. Now I will show how energy storage technology works in practice, and explore a few specific technologies that will play important roles in our clean energy future.
All energy storage technologies follow the same basic concept of reversible change, as shown in the diagram below. First, electricity is used to run a machine or process that puts energy into something, forcing it to change. The energy is then stored for as long as that change is held in place. When the change is allowed to undo itself, the stored energy is let out, driving another machine or process that produces electricity.
Due to the laws of physics, some energy is lost in each stage shown above. This means the electricity produced at the end will always be less than the electricity used at the beginning. The percentage of energy retained through the whole process gives the efficiency. An energy storage technology with high efficiency may retain 95% or more, while one with low efficiency may only retain 50% or less.
The ideal storage time also differs significantly for different energy storage technologies – from seconds to months. As I explained in my last blog, shorter storage times help make the electricity grid flexible, while longer storage times help make it controllable. Different energy storage technologies can therefore play different roles for the electricity grid.
Apart from efficiency and storage time, there are many other important considerations when choosing an energy storage technology, such as size, cost, operational life, and safety.
Now we will see how the energy storage concept applies to four different energy storage technologies. I’ve also provided links in the text so you can see real-world examples of each.
Pumped Energy Storage
Pumped storage makes up most of the existing energy storage on the world’s electricity grids. Electricity is used to run an electric pump, which pushes water up to a higher elevation (typically from a lower lake to an upper lake). Once the water is allowed to flow back down, it spins a generator to produce electricity. These systems have decent efficiency (80% or more), and are typically used to store energy for hours to days. You can see a pumped storage system in Alberta here.
Hydroelectric dams (such as the Hoover Dam pictured at the top of the page) also store energy, but they cannot take in extra electricity from the grid. Instead, hydroelectric dams rely on rain and rivers to fill up the water storage so electricity can be produced when needed. With pumped storage, we can store extra electricity from the grid and retrieve it later.
Battery Energy Storage
Batteries are now the most popular technology for new energy storage projects. When electricity is applied to a battery, it triggers electrochemical reactions that rearrange the chemicals inside. Once those reactions are allowed to reverse, electricity is produced as the chemicals change back. There are many different battery types, but lithium-ion batteries are used most often on the grid. These systems can have high efficiency (over 90%), and are typically used to store energy for seconds to hours. South Australia currently has the world’s largest lithium-ion battery, comparable to about 10 million cell phone batteries.
Flywheel Energy Storage
Flywheels are special wheels designed for storing energy. Electricity is used to run an electric motor, which accelerates the wheel to higher speeds. When electricity is needed, the spinning wheel slows back down as it transfers its energy to a spinning generator to produce electricity. These systems can have high efficiency (over 90%) and are typically used to store energy for seconds to minutes. You can see a flywheel system in the USA here.
Compressed Air Energy Storage
Compressed air technology has been developing for decades and may become popular in the future as the performance continues to improve. Electricity is used to run an electric compressor, which squeezes air into a container (such as a large tank or underground cavern). Once the air is allowed to flow back out, it spins a generator to produce electricity. Check out this unique project where compressed air is stored in giant balloons at the bottom of Lake Ontario.
These are just four technologies of many. There is a great deal of research being done around the world to develop new energy storage technologies for the clean energy transition. If you’d like to learn more, you can check out this page I helped create for Student Energy. You can also read about my own energy storage research project here.