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Dictionary

Global Grid-Storage Market

Energy storage is an important element in the production, transport and effective use of electricity. Most electricity is generated in large power stations, and used on demand, and not stored. Today there is a great deal of interest in large-scale storage systems which can be a central part of the renewable energy revolution.

Energy storage is an important element in the production, transport and effective use of electricity. Most electricity is generated in large power stations, and used on demand, and not stored. Today there is a great deal of interest in large-scale storage systems which can be a central part of the renewable energy revolution.

The irregularity of renewable power generation from wind and solar has led to the need for an efficient way to store the excess energy for later use. Pumped hydropower and batteries are the traditional technologies, but researchers have put forward some intriguing and inventive alternatives.

Pumped-Storage Hydroelectricity PSH

Not a new technology, but undergoing a surge in interest. Excess energy is used to pump water up to a reservoir for later hydro-generation. It is the commonest system in use for load balancing, with an efficiency in the range of 70-80% (70-80% of the electricity used to pump the water to the reservoir can be recovered during later hydroelectrical generation). It is currently used to take advantage of price differentials between low and high-peak rates. Switzerland, for example, imports excess electricity from France and Germany, and when demand is high, exports it to Italy. This purely economic purpose increases the environmental load, and fails to encourage better organisation of generating capacities.

However, pumped storage provides a partial solution in areas which have the resources, especially alpine regions, for the intermittency of large-scale renewable energy. Unfortunately, most excess wind power is generated in regions far from most pumped storage facilities. Germany has very little pumped storage capacity, and is therefore looking at batteries and power-to-X solutions. Switzerland does not envisage largescale wind power generation, so its vast pumped storage capacity remains relatively poorly exploited. Norway has better possibilities in this field.

Worldwide there are 168 GW of PSH capacity, which is about 7% of total power generation capacity. As with all hydropower plants, there is controversy with regards impacts, such as flooding of the area behind the dam, earthquake provocation, biodiversity and visual/amenity disruption.

Chemical Batteries

Batteries took over the static charge storage Leyden jar when around 1800 Alessandro Volta discovered that two different metals separated by acid soaked parchment could release electricity. Today, batteries are a broad range of electrochemical storage systems, and include so-called 'advanced batteries' and capacitors (charged plates). High-capacity battery systems are being developed for grid load-balancing, and safety, environmental compatibility and cost are all improving dramatically.

Lithium-ion Batteries

Lithium batteries are the choice of Elon Musk and his $2-billion Gigafactory for Tesla electric vehicles. However, these may be too expensive and relatively inflexible for the needs of grid storage. The erratic feeds from renewables could cause too much variation in load for the batteries to be efficient. Price ex-factory: $250 per kWh. The total cost for a South Australian 100-MWh plant came to $500 per kWh, or 50 cents per watt-hour, to connect the batteries to the grid. AES, a global energy company, is also developing a large-scale lithium-ion battery project.

By the end of 2016, battery installations in America, led by utility-scale storage, grew to 336MWh. Much of this is in California.

Sodium as an alternative to lithium, and would be cheaper, since sodium is more abundant.

Source: The Economist, 18.03.17 p. 58: Elon Musk and batteries. See also GTM (consultancy) and Energy Storage Association (US).

Flow Batteries

Energy Storage Systems (ESS) in Portland, Oregon, proposes a solution using iron ions in water. Being liquid, this opens the possibility of cars 'retanking' their electrolytes, rather than waiting for a battery to recharge the usual way.

Thermal Storage

If energy is acquired as heat, such as solar-thermal radiation, the most efficient use of it would be as heat, rather then convert it to electricity, and then back to heat in an electric radiator. Heat can be stored for long or short periods in large bodies, or in tanks of molten salt. Salt is used in concentrated solarthermal plants to generate electricity after the sun has set or during cloudy spells.

Other Storage Systems

Flywheels

An ancient technology revived to provide small to large-scale power on demand from rotational energy.

Compressed air energy storage

Air takes energy to be compressed, and releases it when needed. The air can be stored underwater, where the water's weight provides the pressure containment needed. Alternatively, the air could be stored down abandoned mine shafts, or in salt basins. A project in Ticino, Switzerland, stores air in a horizontal shaft under a mountain, and releases it in a novel heat-recovery system to raise efficiency rates by as much as 30%.

Ice

The Portland General Electric (PGE) company in Oregon (USA) has a pilot programme (TAGES Thermal Approach to Grid Energy Storage), investigating the possible use of state change from water to ice (or slush) to store energy, and using a heat exchanger to extract the energy later when needed. This system could also take advantage of waste heat from a conventional power station. 80% efficiency of recovery is theoretically possible.

Molten Glass

Halotechnics in Emeryville, Calif., propose a system by which energy is stored by melting phosphate-based glass, which has a relatively low melting point. The very low-viscosity liquid (behaves like honey at 400°C) can be pumped as a liquid, and the thermal energy released as it cools used to evaporate steam for a traditional turbine. It plans to run a trial at an aluminium plant, using the waste heat from smelting.

Microgrids

A microgrid is a small, networked section of the grid that can generate its own electricity from distributed generation, and can be disconnected from the grid to operate autonomously. Energy sources are: solar panels, wind turbines, CHP, geothermal installations, biodigesters. Microgrids can feed excess power to the grid. They can also have large battery banks to store excess power for own use. Green Mountain's 7,700 solar panels generate 2.5MW to 3,000 households, and has enough lithium-ion and lead-acid batteries to store 3.5MWh.

ARES

ARES (Advanced Rail Energy System) is a pilot programme run by Valley Electric Association, Nevada, to store electrical power by a system where the gravitational potential of a heavily-laden train stores electrical power from renewable sources during low demand periods, and releases it during high demand periods by running the train down a 6-8% grade slope.

The test system is rated at 50MW, but claims to double the capacity of a 500MW system and increase capital costs (c. $1,100 per kW, $4,400/kWh storage) by only 20%. costs comparison with other storage systems:

  • ARES: $1,100 /kW
  • lithium ion batteries: $1-2,000 /kW
  • compressed air storage: $1,600-2,200 /kW
  • pumped hydro storage: $1,200-2,100 /kW

Legislation and policies

California passed a law in 2013 requiring energy companies to provide a total of at least 1.3 GW of storage capacity. At 1.75 kW per household (figure for the UK) this would be enough for 750,000 households.