Solar panels and wind turbines were once very expensive, but technological innovation has reduced their cost to the point where they can compete with fossil fuels. However, these renewable sources still have a key obstacle to overcome: being capable of delivering electricity at any time, while using energy inputs that are variable – sunlight and wind.
Fossil fuels are still used extensively because they can produce electricity on demand. On the other hand, solar panels and wind turbines have a variable output that can be predicted to a certain degree, but not controlled. However, when energy storage is added, the installation can deliver electricity even when there is no generation.
Hydroelectricity stands out among renewable sources for its flexible operation, producing power any time with stored in the reservoir. However, hydroelectricity is demanding in terms of site conditions. Hydropower facilities also have a significant environmental impact, since they flood an existing ecosystem and interrupt a river, even if the water itself isn’t polluted.
In particular, lithium-ion batteries are characterized by their fast response when their power output is needed, and large-scale batteries can improve the stability of a power network. For example, if a large power plant is disconnected from the grid due to an electrical fault, batteries can supply electricity within fractions of a second.
Energy Storage Technologies for Large-Scale Applications
The concept of energy storage is often associated with batteries, but there are several other technologies with promising applications. Any of these technologies can be combined with a solar array or wind farm, to supply stored energy even when there is no sunlight or wind.
Pumped-storage hydroelectricity (PSH) is based on the same concept as conventional hydropower plants. The main difference is that water can be pumped into the reservoir to produce electricity at any time. Typically, the turbines used in a PSH facility can operate in reverse to act as pumps, so there is no need to use two different types of equipment.
Compressed air energy storage (CAES) uses a sealed space with a large volume, such as a cavern or a depleted mine, to store compressed air. When electricity is needed, the compressed air can be released to drive a turbine and generator. This technology is still experimental, but it has been deployed successfully in a few projects.
Thermal energy storage is also a promising concept, and the system can be based on either heat storage or ice storage. Surplus electricity from solar or wind farms can be used to heat water or to make ice, which is then accumulated with insulated containers. The system can then be used for space heating or air conditioning, depending on its design. For example, Chicago has a district cooling system with ice storage, which reduces the local grid demand by 30 MW.
Lithium-ion batteries have been deployed successfully in projects like the Hornsdale Power Reserve in South Australia, which has a capacity of 100 MW and 129 MWh, complementing the output of a 315 MW wind farm. The Laurel Mountain wind farm in West Virginia is a similar example, with 96 MW of wind turbine capacity and a 32 MW / 8 MWh battery system.
While lithium-ion batteries have received significant attention from the media, there are other emerging battery technologies that also show promise. Two examples are vanadium flow batteries and high-temperature sodium batteries.
The Scale Flexibility of Energy Storage Systems
Energy storage systems like PSH and CAES are viable in large-scale applications, but they cannot be scaled down at a competitive cost. However, thermal storage and batteries have the scale flexibility to be used in small- and medium-sized projects. In particular, lithium-ion batteries can achieve great synergy with solar panels. Both technologies have a modular design, and their capacities can be matched according to the needs of each project. Large numbers of batteries in homes and businesses can be interconnected and managed together with a smart platform, allowing them to mimic the behavior of a conventional power station – this concept is called a virtual power plant.