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Energy storage technologies

by Dave Elliott

There are many energy storage options which can be used balance grid systems so as to compensate for the variable output from some renewables. They include existing and newly emerging electro-chemical and electro-mechanical systems (batteries, pumped storage and compressed air storage), as well as a range of thermal and hydrogen based systems 

Some are purely electrical, like super-capacitors and magnetic induction devices, which can provide short-term storage, but electro-chemical batteries are the most familiar option. However, conventional lead acid types are expensive and bulky and even the more advanced Lithium-Ion cells, although important for vehicles, are not that suited to bulk energy storage. The largest so far is the Zhangbei project in China, a giant battery array with 36 MWh of output capacity linked to 40 MW of wind and solar plants. Sodium Sulphur (NaS) batteries are also used at the multi MW scale. So are Nickel Cadmium (NiCad) batteries, despite their lower energy density. Metal-Air batteries are high energy density and very low cost, but are not directly rechargeable electrically, although some new liquid metal variants may be.

The various types of advanced flow batteries, with round trip efficiencies of 70% or so, only slightly lower than for lead acid batteries, show promise for larger scale applications. They mix separate chemical electrolytes to create a charge, in a reversible Zinc bromine and vanadium redox systems are some top contenders, but the US Sandia Lab is looking at electrochemically reversible metal-based ionic liquids, which are non-toxic.

In addition to these electro-chemical options, there is a range of electro-mechanical storage technologies. For example, advance flywheels can offer short-term storage and grid balancing possibilities. However, the most developed and widely used approach for storing excess electricity from grids is pumped hydro reservoir storage, with over 127 GW operating around the world, and more planned. One of the largest reservoir schemes is the 1.87GW Ludington plant on the shore of lake Michigan

There are also novel pumped storage ideas, like the Green Power Island proposed off the coast of Denmark, an artificial lagoon linked to a 150MW offshore wind farm.  A similar idea is being studied in Belgium. It would be a 3 km wide donut shaped island 3-4 km off the coast with a 30 meter deep reservoir at its center, and 300MW of pump/generator units. Excess electricity from nearby offshore wind farms would be used to pump water out of the reservoir into the sea. When energy demand was high and wind low, the water would be let back into the reservoir through the water turbines.

Similar ideas have been suggested for tidal lagoons, e.g. with pumped storage between segments.

Another electro-mechanical option is compressed air storage, for example in large underground reservoirs. In one version of CAES (Compressed Air Energy Storage), electricity from wind turbines is used to compress air, which is then stored in caverns underground, for use to supercharge the burning of gas in a conventional turbine.

In another, compressed air produced mechanically, using electricity from offshore wind turbines, is stored in large inflatable bags, mounted underwater around the turbine base, for subsequent use in a separate turbine to generate electricity.

MIT researchers have proposed a variant using huge hollow concrete spheres on the seafloor under the wind turbines: Researchers in Norway have come up with a similar idea, with a 80% round trip efficiency claimed from spheres at 400-800 metres depth

There also systems for converting electricity into heat. Heat is easier to store than electricity, and heat stores can have high energy storage densities. Hot water can hold about 3.5 times as much energy by volume as natural gas at atmospheric pressure and temperature. However, converting heat back to electricity (by raising steam to drive turbines) can be inefficient, so, depending on location and demand, it may be better to use the heat direct. Nevertheless, US IT company Apple are reported to be developing a system which uses a mechanical heat churn driven directly by a wind turbine, to create heat which is then stored, ready for use for electricity generation when needed.

Another approach is to use excess electricity from wind to heat water in a store via an immersion heater, for use when needed in a district heating network. There is a 200 MW system like this in operation in Denmark. In the SHEAP district heating project on the Shetland Isles, heat from a waste-to-energy incinerator is being augmented by the use of electricity from  6.9 MW of wind generation fed to immersion heaters in an expanded  135MWh capacity heat store,

UK company Isentropic have developed a gravel filled heat store system linked to a heat pump. They claim the round trip efficiency is 72- 80% Molten salt heat stores are also used with some Concentrating Solar Power plants and there are many solar heat stores linked to district heating networks in Denmark and elsewhere.

Any large temperature difference can be used to run a heat engine and there are some systems which use ice as an energy storage medium, although more usually they are part of an air-conditioning /cooling package.  and

Developing on that is the idea of cryogenic liquid air storage. UK company Highview Power Storage has demonstrated a 300 kW prototype which stores excess energy at times of low demand by using it to cool air to around minus 190 °C via refrigerators, with the resulting liquid air, or cryogen, then being stored in a tank at ambient pressure (1 bar). When electricity is needed, the cryogen is subjected to a pressure of 70 bars and warmed in a heat exchanger. This produces a high-pressure gas that drives a turbine to generate electricity. The cold air emerging from the turbine is captured and reused to make more cryogen. If waste heat from a nearby industrial or power plant is used to re-heat the cryogen, it is claimed the round-trip efficiency rises to around 70%

There are also many systems being developed for hydrogen storage, as a gas under pressure or cryogenically as liquid, or chemi-absorbed in metals. In Safe Energy’s system, hydrogen is absorbed as metal hydride in a molten mix, which can then be made to release its hydrogen, when required, in a reaction with water, producing hydrogen and heat.  That process converts the metal hydride to a metal hydroxide, which can be recycled back to a metal hydride. The magnesium hydride slurry can be stored safely in large quantities at ambient conditions.

Some hydrogen storage systems have been integrated with electrolytic hydrogen production from wind-derived electricity and hydrogen fuel cells.   This ‘wind to gas’ idea is now being developed rapidly in Germany,

Some small domestic-scale hydrogen storage systems are also on offer for use with PV solar and fuel cells.  Many other hydrogen storage options are also opening up, some nano-tech based

Although it will have to compete with other ways of balancing variable renewables, energy storage is a rapidly expanding field, offering options at a variety of scales and efficiencies, and


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