Electrical Energy Storage and Transport
Electricity generation is one of the most important issues of our time, and key to a successful transition to a sustainable economy.
Energy storage systems
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. Batteries are the traditional technology, but researchers have put forward some intriguing and inventive alternatives.
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.
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.
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.
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.
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.
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 (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
Energy storage is an important element in the production, transport and effective use of electricity. Till recently, most electricity has been generated in large power stations, and used on demand, and not stored. Now there is a great deal of interest in large-scale storage systems which can be a central part of the renewable energy revolution.
The six main categories of energy storage system are:
- Pumped hydro-power
- Solid state batteries
- Flow batteries
- Compressed air energy storage
Switzerland is able to exploit its alpine location to pump water from low reservoirs to elevated reservoirs. This is done utilising electricity from wind power, or from imported electricity during periods when the price is low, allowing the water to be released to generate electricity during peak periods when the price is high.
Excess energy can be stored in large masses or salt bodies. Electricity can be generated or the thermal energy used directly later.
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 - the battery. Today, batteries are a broad range of electrochemical storage systems, and included so-called 'advanced batteries' and capacitors (charged plates).
Large-scale batteries can utilise tanks of fluid electrolytes. These can release energy quickly and efficiently.
An ancient technology revived to provide small to large-scale power on demand from rotational energy.
Energy can be stored by compressing air. By releasing it through a valve electricity can be generated.
Pumped-storage hydroelectricity (PSH) (also referred to as pumped hydroelectric energy storage (PHES)), stores energy in the form of potential energy of water. It is used to store excess electricity generated or imported during low-demand periods, and the water is discharged during high demand periods. At efficiencies of minimum 70% to more than 80% over a pump cycle, it is the most common large-scale electrical energy storage system in commercial use.
Pumped-storage hydroelectricity is a primary element in the planned exploitation of renewable energy in the GIR (Green Industrial Revolution). A major drawback in wind power is its intermittency. By allowing a wind turbine to generate power at any time the wind is strong, the electricity can always be used to pump water to the elevated reservoir. This water can then be used when demand is highest, irrespective of wind conditions. This pump cycle system is available in all weather conditions, any season, and 24/7.
The DOE Global Energy Storage Database estimates that total capacity of PSH worldwide exceeds 168 GW.
The effective efficiency is 65-70%: about 1.4 kWh generated by a wind turbine and stored in a pumped storage reservoir will generate 1.0 kWh when needed later. A hydraulic turbine generator has a high efficiency (more than 95%), and water pumps are less efficient.
Capital costs are likely to be in line with those for a conventional project, i.e. between $1000/kW and $3000/kW.
Batteries were invented by Alessandro Volta around 1800, as the first means of storing electrical energy. They remain a key technology for the transition to clean energy.
Lithium batteries are the choice of Elon Musk and his $2-billion Gigafactory for Tesla electric vehicles. However, these would be expensive and relatively inflexible for the needs of grid storage. The erratic feeds from renewables would cause too much variation in load for the batteries to be efficient. Price: $250 per kWh, total cost (including building the South Australian 100-MWh plant) is $500 per kWh to connect the batteries to the griid. The SA 100MWh plant costs $50m.
AES, a global energy company, is developing a large-scale lithium-ion battery project.
Sodium as an alternative to lithium would be cheaper, since sodium is more abundant.
By end of 2016, battery installations in America, led by utility-scale storage, doubled to 336MWh. Much of this is in California.
[The Economist, 18.03.17 p. 58: Elon Musk and batteries] See also GTM (consultancy) and Energy Storage Association (US).
Energy Storage Systems (ESS) in Portland, Oregon, proposes a solution of 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.