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Solar and wind, the mainstay of future green power generation and also tidal and wave generation are all intermittent power producers. Therefore in addition to using diversity across the transmission systems and between countries substantial power storage will be required to match demand and generation. Diversity can be improved with 'smart metering.'
Energy storage systems make use of heat, pressure (air), potential energy (height/weight) , chemical (battery), fuel (H2). Developments of these will enable the best choice for each application.
Storage at High Temperature solar generating farms.
Solar heat collected during the day can be stored in liquid or solid media such as molten salts, ceramics or concrete. Phase-changing salt mixtures could be used in the future. After sunset, the solar heat can be extracted from the storage medium to maintain electrical generation.
Pumped Heat Electricity Storage System (PHES)Storage in rocks.
Isentropic (UK) have designed a system to store energy (as heat.) In times of available wind power or when grid power demand is low it takes electrical power to drive a specially designed heat pump using Argon to transfer energy as heat between a store consisting of rocks at -150 C and 500C. When excess power is demanded from the grid system, the heat engine reverses using heat to drive a turbine to generate electricity. Power out/in of 72 - 85%. (ref 217.)
It is claimed that the space required is 1/300 th of the space used for a large (hydro)pump storage scheme for equivalent power storage. Read more about Isentropic
Compressed air storage.
Compressors are driven at periods of off peak hours, or high wind capacity to compress air and store in underground caverns. At times of heavy load or when the wind power is low the compressed air drives a generator to feed the transmission system.
Locations of caverns need not be near wind farms. Typical pressure may be 68 times atmospheric and efficiency 75%; this includes the capture and use of heat generated in the process (otherwise efficiency would be under 65%).
Caverns are created where salts being extracted is dissolved and pumped out, by 'solution mining.'
Compressing air also produces a high temperature, eg up to 620 C. This heat can be stored by passing the compressed air over a solid material, eg ceramic bricks. When the compressed air is reused for generation, the heat store can be used to preheat it before the turbine stage.
Development of compressors to meet the conditions, including high temperature is needed.
Hydro / pump storage of water at high level.
Pump storage hydro systems have been in operation for many years. There are two reservoirs, high and low. In peak hours, water flows from high to low providing power. At times of low power demand water is pumped back up to the high level. The overall efficiency from electrical power in to electrical power out is around 65%(ref 6).
For a given size of reservoir the size of generators is considerably higher than those in a hydro generation station as, here, water is used periodically. It might be possible to convert some hydro plants to pump storage although this would mean constructing a second dam and creating a large lower lake.
An example is the Dinorwig North Wales UK 1320 MW (Max generation in 16 secs) or the Bath County Pump Storage scheme in Virginia USA with 30GWHr storage using 3.32 sq km area.
Dual Lagoon storage - hydro plan taken from ref 9.
A large area of shallow sea can provide more steady and flexible power on a large scale. As the tide rises, sluice gates allow the height of the high level lagoon to rise. Continuous or on demand generation is from the high to low level lagoon. The height of the high level lagoon can be boosted by further pumping at high tide (thus at low head -red in diag.) using part of the generation energy. As the tide falls, water is let out of the low level lagoon and can be further lowered by pumping at low tide. The power density at 4.5 watts/sqm is 50% higher than for a single lagoon (3 watts/sqm.)
Note: The width would be several km and height several tens of metres.
An alternative is to arrange to generate from the high level lagoon to sea at lower tide and from the sea to low level lagoon at higher tides:-
The Dual Lagoon plan can be used to augment the Pump Storage facility.
Battery storage
Use of batteries in cars to provide diversity. Smart meter management can provide for charging on a low tariff as the norm, overnight or when excess power is available.
Separately located battery back up such as Sodium- Sulphur with charge - recharge facilities will also provide high energy storage in a small space.
Hydrogen storage
Hydrogen could become a major source of energy storage (as secondary fuel) for transport in the future and also for generation. Hydrogen can be stored in a liquid state either at high pressure or low temperature.
The place for smart metering with fluctuating power generation.
Smart meters gives users in-home display of energy used and thus more control of it; and enables remote and thus accurate reading. (ref 205.) It also allows excess energy from users (generated by the user from intermittent sources wind,solar etc ) to be fed back to the grid and credited; thus excess intermittent energy is shared not wasted and overall the consumption of electricity (and generation of CO2) would be lower (ref 177).
A further development is a smart grid where load sensing devices could be arranged to reduce consumer load - eg turning down heating slightly - at times of excess load experienced by the national electricity supplier, with 'dynamic pricing' incentives. The timing of battery charging on battery cars of the future could also be adjusted to suit periods of low load demand eg when load falls at night.