When compared with other commodities, energy stands out because the demand for energy is required to be met at the particular instant of that demand being created. In essence, this means that there must be a large resource which can be accessed quickly and delivered to the point of demand in required quantities. This is more or less true for fossil fuels and biomass, where large volumes of fuel can be accumulated and kept ready to meet the varying demand. With renewable energy, this has not been the case, as it occurs in different times and varies a lot with time.
The trouble with renewable energy is no matter how plentiful our energy resources are, there can be a gap between the varying demand and the varying supply. The imbalances between this demand and supply, as well as the efficiency of electrical systems can be improved through energy storage systems (ESS).
Renewable energy resources are variable and intermittent. Wind, solar for example, and even hydro for that matter, are sometimes available in plenty, sometimes they are not available at all. This intermittent nature of the resource requires an appropriate ESS to meet the demand. Many ESS have been developed in the recent past, which are for the support of electrical, mechanical and thermal energy systems. Generated energy can be stored as potential, kinetic, chemical and thermal energy, and can be released in various forms as necessary, most commonly, as electricity. They also play an important role in improving the stability and quality of power supply through electrical networks.
The energy storage market is set to explode globally, with the unfolding energy transition. The surge is such, the market for these devices are expected to grow over 40% annually in the coming decades.
There are various types of ESS. The most prevalent technologies are pumped hydro, batteries, thermal, compressed air energy storage (CAES) and flywheels. In the USA alone, almost 93% of energy storage is pumped storage.
In a CAES plant, air is compressed and stored under high pressure. This compressed air is stored in an underground cavern. When electricity is required, the pressurised air is expanded in an expansion turbine, driving a generator for power generation.
Large scale thermal energy storage like underground thermal energy storage and a system based on phase change materials named as latent heat storage, fall under the category of thermal energy storage systems (TESS). The common thermal storage systems like borehole TESS, aquifer TESS, tank TESS and pit TESS are examples.
The flywheel ESS is at present, an upcoming candidate among ESSs, since it can offer many advantages as an energy storage solution over others. It is stores the kinetic energy in wheels rotating at high speeds. Flywheels are known for their high lifecycles, long operational life, high power density, high round-trip efficiency, low environmental impacts and its capability to store mega joule levels of energy.
Electrochemical energy storage system is a general term applied for all types of secondary batteries. Batteries work by converting the chemical energy of its active material into electrical energy by an electrochemical oxidation-reduction reverse reaction. Today, there are many types of batteries available, ranging in size, prices and the spectrum of application. The recent advances in the lithium-ion battery is considered a cutting edge technology in ESSs. Almost all batteries used in mobile phones and laptop computers fall into this category. Recent studies on graphine based material for improving electrochemical performances in electrochemical energy storage devices, in terms of lifecycle and energy / power density, has warranted new opportunities for developing high performance electrodes. However, the use of chemicals in electrochemical ESSs needs to be looked at from the perspective of safety and environmental impacts as well.
The most promising ESSs in grid scale operations is seen as the flow batteries or more commonly known as redox batteries. These batteries can have quite long life and cost less than most other ESSs. Another advantage of these batteries is the ability to decouple power and energy rendered by the devices.
Pumped hydroelectric storage facilities store potential energy in the form of water in an upper reservoir, pumped from another reservoir at a lower elevation and released for power generation, when the need arises. It is mature and well established concept in ESSs and has been in operation since 1890s. It is also the largest available grid storage system in the world constituting 97% of the world’s total energy storage. Pumped storage is also being used to support standalone microgrid hybrid solar-wind systems, where this ESS effectively stores energy generated by wind and solar photovoltaic plants.