Renewables are great until they aren’t. When the sun isn’t shining and the wind isn’t blowing, sources of renewable energy struggle to meet the baseline needs of our increasingly energy-hungry society. So we need to rely on non-renewable sources of energy like coal to make up the difference, or so the narrative goes.
In reality, there is a little-talked-about but very real alternative to using dirty sources of energy for our baseline needs: grid storage. The scale at which the grid operates allows engineers to use techniques that are inefficient or impossible for energy storage on a smaller scale, like in your watch or even your car. There are several highly efficient methods of grid storage in use now, some of which have been around in some form for over 100 years, and even more efficient methods are in development. As technology continues to improve, these methods of grid storage are likely to become more commercially viable.
Let’s take a look at some of the most exciting grid storage options in use today:
Pumped Hydroelectric Storage
One of the oldest methods of grid storage is pumped hydro. The idea is simple; it’s so simple, in fact, that energy officials in Britain came up with the idea in the 1950s. The idea then was to respond to peak demand times, but it would be no less effective at providing base load energy for a renewable grid.
Two lakes are either located or created, one at the top of a hill and another at its base. During the day, water is pumped up the hill to the lake on top. This is the storage phase of the design. Later, when the sun goes down or wind isn’t blowing, every gallon that has been pumped up the hill can be allowed to flow back down, turning turbines along the way.
The round-trip energy of pumped hydroelectric storage varies, but it is usually reported as between 70 and 80 percent. It represents, by far, the largest-capacity form of grid storage at present, with a worldwide nameplate capacity of 168 GW. The main disadvantage is the difficulty in siting a pumped hydro facility. Areas suitable for a facility require hilly or mountainous regions and access to water, meaning that locations are relatively rare and often very beautiful.
Rail Energy Storage
A much newer, but remarkably similar solution is being pioneered by a Santa Barbara startup Advanced Rail Energy Storage (ARES). Their solution still requires a hill, but instead of water, they are using heavily-laden rail cars.
The setup involves that ARES recently received approval from the Bureau of Land Management to set up in Pahrump, Nevada, involves several parallel train tracks set on a hill, each containing a train of cars loaded for maximum weight. During the day, the ARES system can use renewable energy to pull the cars to the top of the hill, storing electricity in the form of potential energy. At night, the cars are released, transferring potential energy into kinetic energy that is used to generate electricity.
The round-trip energy efficiency is anticipated to approach 80 percent, ARES claims, and the facility could be scaled to store 16 to 24 GWh of electricity.
Flywheel Energy Storage
Another form of energy storage pioneered long ago and being put to effective use on the grid is the flywheel. There are documented examples of rudimentary flywheels being used as a means of equalizing rotation speeds in mechanical devices almost a millennium ago.
For energy storage, the concept is relatively simple. A heavy wheel is made to spin at very high speed using electricity. This stores the electricity as kinetic energy, and because the wheel has only a very small point of contact, losses due to friction are low. Modern flywheels use vacuum chambers and magnetic bearings to further reduce losses, allowing them to achieve round-trip efficiencies of up to 90 percent. For example, a grid-level flywheel storage system pioneered by Amber Kinetics uses a 5,000 lb steel rotor to store 32 kWh of electricity. Losses over time are dramatically reduced from previous iterations, allowing these flywheels to store energy for up to four hours.
The advantages of flywheels come from their simplicity: They can charge and discharge extremely quickly relative to other grid storage options, and they can go through almost limitless cycles without needing significant maintenance. However, they will still need to be improved before they can store energy for more than a few hours.
Compressed Air Energy Storage
There have been energy systems designed to exploit the energy storage capacity of compressed air since 1870, but the first grid-level compressed air energy storage (CAES) project was completed in 1978. This method involves compressing and storing large amounts of air – often in an underground cavern – and allowing it to expand later in order to generate electricity.
The major challenge involved with CAES comes with storing the associated heat. Air pressure is a function of density and heat, so as air is compressed, it heats up. If that heat is allowed to escape, as it is under current methods, air pressure decreases, meaning that much of the energy is lost. For the air to be used, natural gas is burned to heat the air back up, seriously compromising efficiency. This is called diabatic CAES. Under adiabatic CAES methods under development now, heat is captured in the form of hot oil or molten salt and stored for later use or is retained in the compressed air through insulation. This can allow efficiencies of up to 70 percent to be achieved.
There are several grid-level adiabatic plants projected to come online this year. While the round-trip efficiency isn’t as good in CAES plants as other forms of grid storage, costs are potentially much lower compared to other grid storage options. Lower materials toxicity and a longer lifespan are also advantages.
Batteries designed for the grid are free from the constraints that other batteries face, like physical durability, volume, or mass. This frees them to explore more efficient chemical means of storing energy. The most important consideration for a grid battery is generally cost per watt or watt hour.
Because of the rush to improve battery technology for electric cars, lithium-ion batteries are being produced today that cost below $300 per kWh, and scientists are pushing to get this figure below $150 by 2020. One option in development is the sodium-ion battery, which could potentially be cheaper because of the relative abundance and low cost of sodium. Molten-state batteries and flow batteries, which use molten metals and dissolved transition metals, respectively, are also under exploration.
Another interesting idea is the use of electric vehicles as a distributed form of grid storage. Under this paradigm, the world’s fleet of electric vehicles could allow their 20 to 50 kWh batteries to help balance electric loads throughout the day. There are some disadvantages to this approach – notably the stress that frequent charge-discharge cycles put on a battery – but the sheer amount of energy storage capacity that will soon be on the road makes this an appealing concept.
The world is a long way off from building enough grid storage capacity to move off of non-renewables, let alone enough renewable generation capacity. As of June 2016, the U.S. had just 21.6 GW of energy storage capacity installed; that’s about 2 percent of the 1,068 GW of in-service generation capacity. Most experts agree that we will have to move to renewables sooner rather than later, though. When we do, grid storage will very likely form an important part of our power mix.