Electrical energy storage
Electricity grid operators have a number of options to match varying demand and supply including; turning flexible power plants up and down, demand management and pumped storage. It is likely that as the amount of variable renewables connected to the grid increases, increasing amounts of electricity storage will be required.
Pumped storage, where water is pumped to a high reservoir at times of low electricity demand and then used to generate electricity at times of high demand, is the main large scale electricity storage used in the UK.
Other potential ways to store large amounts of electricity are:
- Compressed air storage, where air is compressed to a high pressure and stored underground at times of low demand, and then used to generated electricity (by driving a turbine) at times of high demand.
- Flow cells or flow batteries which are electrochemical devices which can store large amounts of energy in tanks of electrolyte solutions. These solutions generate electricity when brought together.
Both of these methods have been demonstrated but are not yet in widespread commercial use.
Battery storage of electricity has been used for many years for a wide variety of applications, for example, lead acid batteries in cars, nickel metal hydride batteries in cameras, and lithium ion batteries in laptops. There is currently a great deal of development work on lithium ion batteries for use in electric vehicles. If a significant number of electric vehicles are adopted, they could be connected to the electricity grid to provide additional storage capacity when they are parked.
Supercapacitors store small amounts of electrical energy and can deliver this very rapidly. They are suitable for applications that require a fast response, for example, regenerative braking in vehicles.
Flywheels store energy in a rotating mass. They can be used to supply a short burst of energy, for instance to accelerate vehicles.
Thermal energy storage
Both hot water and cold water reservoirs can be used for storing thermal energy. Domestic hot water tanks are a familiar way to store energy. Making ice to be used for cooling at a future point in time is also a well established technique for thermal energy storage.
Thermal energy storage integrated into buildings can save energy by storing heat during the day for use at night, or allowing the effects of cooler air at night time to be used during the day. Phase change materials absorb or release heat at a relatively constant temperature as they change between the liquid and the solid state, and can be incorporated in building fabric.
Other potential methods of thermal energy storage include using large volumes of water underground, and reversible chemical reactions that absorb and then release heat.