Grid energy storage

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Grid energy storage is the use of various energy storage techniques to complement electric power generation plants on the transmission grid. By using the grid connection, intermittent sources of power need not be installed with their own energy storage facilities. For example, a solar panel installed on a home may produce more energy than is required during the day, which can be exported to the grid; at night, the needs of the home can be met from the grid, as if the solar energy had been stored locally.

In the most narrow sense, grid energy storage does not involve actual storage. Instead, a grid-connected user/producer provides power to the grid that is surplus to its own requirements, and draws power from the grid when needed, netting out its usage. This is called grid energy storage as the user uses the grid instead of arranging storage, which may not be economically feasible. In this sense, grid energy storage is related to distributed generation. For it to function correctly, technical and economic arrangements (such as net metering) will be required, and often requires regulatory support. Use of the term in this limited sense is not consistent, however.

Demand for electricity from the world's various grids varies over the course of the day and from season to season. For the most part, variation in electric demand is met by varying the amount of electrical energy supplied from primary sources, usually hydroelectric power plants and natural gas-fired turbines. Increasingly, however, operators are storing lower-cost energy produced at night, then releasing it to the grid during the peak periods of the day when it is more valuable. In areas where hydroelectric dams exist, release can be delayed until demand is greater; this form of storage is common and can make use of existing reservoirs. This is not storing "surplus" energy produced elsewhere, but the net effect is the same - although without the efficiency losses.

Energy storage is economical when the marginal cost of electricity varies more than the energy losses of storing and retrieving it. For instance, 1.2 gigawatt-hours might be stored at night in a pumped-storage reservoir, at a cost of 1.5 cents/kilowatt-hour. The next day, 1 gigawatt-hour might be recovered, and 200 megawatt-hours lost, and sold at 4.0 cents/kilowatt-hour, for a profit of $22,000. If this profit can be realized on most days, and if the storage facility cost less than perhaps $100M, the operator makes a profit.

The marginal cost of electricity varies because of the varying economics of different kinds of generators. At one extreme, water from a dam can go through a turbine for little more than the cost of releasing it down a spillway, so the marginal cost of generation is nearly zero. Coal-fired and nuclear power plants are also low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, most peaking generators burn natural gas, which is expensive. Operators prefer cheaper electricity, so they run the low-marginal-cost generators most of the time, and run the more expensive ones only when necessary. This is called "economic dispatch".

Renewable supplies with prediction errors and variable production, like wind and solar power, tend to increase the net variation in electric load. Because they are not dispatchable and must run when available, power from these supplies is generally sold to grid operators for less than power available on demand. As renewable supplies become increasingly popular, this difference in price opens an increasingly large economic opportunity for grid energy storage.

1 Energy demand management
2 Pumped water storage
3 Compressed air storage
4 Thermal energy storage
5 Battery storage
6 Flywheel storage
7 Superconducting magnetic energy storage
8 Hydrogen fuel cells
9 See also
10 External links

[edit] Energy demand management

Main article: Energy demand management

The easiest way to deal with varying electrical loads is to decrease the difference between varying generation and demand. This is referred to as demand side management (DSM). For decades, utilities have sold off-peak power to large consumers at lower rates, to encourage these users to shift their loads to off-peak hours, in the same way that telephone companies do with individual customers. Usually, these time-dependent prices are negotiated ahead of time. In an attempt to save more money, some utilities are experimenting with selling electricity at minute-by-minute spot prices, which allow those users with monitoring equipment to detect demand peaks as they happen, and shift demand to save both the user and the utility money. Demand side management can be manual or automatic and is not limited to large industrial customers. In residential and small business applications, for example, appliance control modules can reduce energy usage of water heaters, air conditioning units, and other devices during these periods by turning them off for some portion of the peak demand time or by reducing the power that they draw. Energy demand management includes more than reducing overall energy use or shifting loads to off-peak hours. A particularly effective method of energy demand management is the installation of more energy efficient equipment. For example, many utilities give rebates for the purchase of insulation, weatherstripping, and appliances and light bulbs that are energy efficient. Companies with factories and large buildings can also install such products, but they can also buy energy efficient industrial equipment, like boilers, or use more efficient processes to produce products. Companies may get incentives like rebates or low interest loans from utilities or the government for the installation of energy efficient industrial equipment.

[edit] Pumped water storage

Main article: Pumped-storage hydroelectricity

In many places, pumped storage is used to even out the daily generating load, by pumping water to a high storage reservoir during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours, this water can be used for hydroelectric generation, often as a high value rapid-response reserve to cover transient peaks in demand. There is over 90 GW of pumped storage in operation, which is about 3% of global generation capacity. Pumped storage recovers about 75% of the energy consumed. The chief problem with pumped storage is that it usually requires two nearby reservoirs at considerably different heights, for which there are few suitable locations, and often requires considerable capital expenditure. Traditional hydroelectric dam configurations are far more common than pumped storage; in this configuration, release is delayed until needed. The net effect is the same as pumped storage, but without the round-trip efficiency loss.

[edit] Compressed air storage

Main article: Compressed air energy storage

Another grid energy storage method is to use off-peak electricity to compress air, which is usually stored in an old mine or some other kind of geological feature. When electricity demand is high, the compressed air is burned with natural gas to run a turbine and generate electricity.

[edit] Thermal energy storage

Main article: Thermal energy storage

Off-peak electricity can be used to make ice from water, and the ice can be stored until the next day, when it is used to cool either the air in a large building, thereby shifting that demand off-peak, or the intake air of a gas turbine generator, thereby increasing the on-peak generation capacity.

[edit] Battery storage

Main article: Battery (electricity)

Battery storage was used in the very early days of electric power networks, but is no longer common. Many "off-the-grid" domestic systems rely on battery storage, but means of storing large amounts of electricity as such in giant batteries or by other means have not yet been put to general use. Batteries are generally expensive, have maintenance problems, and have limited lifespans. One possible technology for large-scale storage are large-scale flow batteries. Sodium-sulfur batteries could also be inexpensive to implement on a large scale and have been used for grid storage in Japan and in the United States[1]. Vanadium redox batteries and other types of flow batteries are also beginning to be used for energy storage including the averaging of generation from wind turbines.

If battery electric vehicles were in wide use with modern high cycle batteries [2] [3], such mobile energy sinks could be utilized for their energy storage capabilities. Vehicle to Grid technology could be employed, turning each vehicle with its 20 to 50 kWh battery pack into a load-balancing device or emergency power source. This represents 2 to 5 days per vehicle of average household requirements of 10 kWh per day, assuming annual consumption of 3650 kWh. This quantity of energy is equivalent to between 40 and 300 miles of range in such vehicles consuming 0.5 to 0.16 kWh per mile. These figures can be achieved even in home-made electric vehicle conversions.

[edit] Flywheel storage

Main article: Flywheel energy storage

Mechanical inertia is the basis of this storage method. A heavy rotating disc is accelerated by an electric motor, which acts as a generator on reversal, slowing down the disc and producing electricity. Electricity is stored as the kinetic energy of the disc. Friction must be kept to a minimum to prolong the storage time. This is often achieved by placing the flywheel in a vacuum and using magnetic bearings, tending to make the method expensive. Larger flywheel speeds allow greater storage capacity but require strong materials such as steel or composite materials to resist the centrifugal forces (or rather, to provide centripetal forces). The use of carbon nanotubes as a flywheel material is being researched. The ranges of power and energy storage technically and economically achievable, however, tend to make flywheels unsuitable for general power system application; they are probably best suited to load-levelling applications on railway power systems.

[edit] Superconducting magnetic energy storage

Main article: Superconducting magnetic energy storage

Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature.

A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely.

The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2-3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%.

Due to the energy requirements of refrigeration, and the limits in the total energy able to be stored, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality. If SMES were to be used for utilities it would be a diurnal storage device, charged from baseload power at night and meeting peak loads during the day.

The high cost of superconductors is the primary limitation for commercial use of this energy storage method.

[edit] Hydrogen fuel cells

Main article: Hydrogen economy

Hydrogen is not a primary energy source, but a portable energy storage method, because it must first be manufactured by other energy sources in order to be used. However, as a storage medium, it may be a significant factor in using renewable energies, See Hydrogen storage. [4] It is widely seen as a possible fuel for hydrogen cars, if the problem of energy return on energy invested can be overcome. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, similar to the way the human body burns fuel. Making hydrogen requires either reforming natural gas with steam, or, for a possibly renewable and more ecologic source, the electrolysis of water into hydrogen and oxygen. The former process has carbon dioxide as a by-product, which exacerbates greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.

With intermittent renewables such as solar and wind, the output may be fed directly into an electricity grid, with a penalty due to the requirement to operate some extra conventional plant on part load to allow for the fluctuations in power output. At penetrations below 20% of the grid demand, this penalty is usually small and does not severely change the economics; but beyond about 20% of the total demand, the penalty may make the generated power uneconomic. If these sources are used for electricity to make hydrogen, then they can be utilized fully whenever they are available, opportunistically. Broadly speaking, it does not matter when they cut in or out, the hydrogen is simply stored and used as required.

Nuclear advocates note that using nuclear power to manufacture hydrogen would help solve plant inefficiencies. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods, and any not needed to meet demand being used to make hydrogen at other times. This would mean far better efficiency for the nuclear power plants. High temperature (950-1000°C) gas cooled nuclear reactors have the potential to split hydrogen from water by thermochemical means using nuclear heat (i.e. without using electrolysis).

About 50 kWh (180 MJ) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial. At $0.03/kWh, common off-peak high-voltage line rate in the U.S., this means hydrogen costs $1.50 a kilogram for the electricity, equivalent to $1.50 a US gallon for gasoline if used in a fuel cell vehicle. Other costs would include the electrolyzer plant, compressors, liquifaction, storage and transportation, which will be significant.