Grid energy storage

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, primarily hydroelectric dams and gas-fired turbines. Increasingly, however, operators are storing marginally cheap energy (usually at night), then releasing it to the grid during the day when it is more expensive.

Energy storage storage only makes sense 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 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 be let down the spillway about as cheaply as it can be run through a turbine, so the marginal cost of generation is nearly zero. Coal-fired 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 only run the more expensive ones when necessary.

Unpredictable and intermittent renewable supplies, like wind and solar power, tend to increase the net variation in electric load. Because they are not controllable or reliable, 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.

Contents

1 Reactive electrical demand 2 Pumped water storage 3 Compressed air storage 4 Battery storage 5 Flywheel storage 6 Superconducting Magnetic Energy Storage 7 Hydrogen fuel cells 8 External links

Reactive electrical demand

The easiest way to deal with varying electrical loads is to decrease the variation. 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. Usually, these time-dependent prices are negotiated ahead of time. In an attempt to save more money, some utilities are experimenting with selling electricity to large users at minute-by-minute spot prices, which allow those users to detect demand peaks as they happen, and shift demand to save both the user and the utility money.

Pumped water storage

Main article: Pumped-storage hydroelectricity

Already in many places pumped-storage hydroelectricity|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 80% of the energy consumed. The chief problem with pumped storage is that it usually requires a reservoir below another reservoir, which is not the usual configuration, and often requires considerable capital expenditure.

Compressed air storage

Main article: Compressed air energy storage

Another method is to use excess 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.

Battery storage

Main article: Battery (electricity)

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.

Flywheel storage

Main article: Flywheel energy storage

Simple physics is the basis of this storage method. A heavy rotating disc is accelerated by an electric engine 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 achieved by placing the flywheel in a vacuum and using magnetic bearings, making the method expensive. Larger flywheel speeds allow greater storage capacity but require ultra strong materials such as carbon nanotubes to resist the centrifugal forces (or rather, to provide centripetal forces).

Superconducting Magnetic Energy Storage

Main article: SMES

Superconducting Magnetic Energy Storage (SMES) uses the ability of certain materials to conduct electricity without resistance (superconductivity) to store electrical power. Currently speculative but has a large potential.

Hydrogen fuel cells

Main article: Hydrogen economy

Hydrogen as a fuel has been touted lately as a solution in our energy dilemmas. However, the idea that hydrogen is a renewable energy source is a misunderstanding. Hydrogen is not an energy source, but a portable energy storage method, because it must 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. 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, in the same way the human body burns fuel. Making hydrogen requires either reforming natural gas with steam, or, for a 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 (or at least does not improve) 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, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But 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 civil demand being used to make hydrogen at other times. This would mean far better efficiency for the nuclear power plants.

About 50 kWh (1.8 MJ) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial. ( At $0.10/kWh this means hydrogen costs $5 a kilogram for the electricity, equivalent to $5 a US gallon for gasoline if you use in a Fuel Cell vehicle, plus electrolyzer plant costs which will not be small.)

External links

• Electricity storage technologies
  • Graphical comparisons of different storage systems: [1][2][3][4][5]