Electricity must be generated as it’s being used, so that demand and supply are always kept in balance.
This is a critical factor with respect to unreliable, wind and solar. When the wind stops blowing or the sun stops shining, supply is cut off from these sources and another source must immediately be brought on line. Typically, the back-up source is a natural gas turbine that has been spinning, i.e., running, using natural gas, but not connected to the grid, which can be quickly brought on line. Using coal-fired power plants for back-up requires them to operate as load following units, cycling them up and down, which creates major thermal stresses on components, and increasing maintenance costs.
Standby power adds to the cost of wind and solar, but usually isn’t included when calculating their cost.
Storage is essential for widespread use of wind and solar, if large quantities of back-up power generation are to be eliminated.
It’s why the California Public Utilities Commission required utilities to add storage to their systems.
Unfortunately, storage of electricity isn’t cheap or easily accomplished.
It’s highly doubtful that storage of large amounts of electricity can be done cheaply.
Historically the most effective method for storing electricity has been pumped storage, which is actually an indirect method since it stores water that can be released from a reservoir to generate electricity.
Pumped storage, in so far as I can determine, was first used in the United States by Connecticut Light and Power in 1927 to pump water, using electricity generated by the hydro-power plant, back to the lake which was the source of water for their hydro-power. This application saved money by using a reservoir whose initial purpose was the storage of water.
Unless a reservoir is already in place, pumped storage requires building a dam, and there are only a limited number of locations for new reservoirs. Dams are expensive, and are usually objected to by radical environmentalists1.
But other methods of storage have been proposed, such as those listed here:
- Batteries, such as Lithium ion, sodium sulfur, lead-acid and flow batteries
- Hydrogen, produced from electricity when it isn’t needed on the grid, which can be used to power a gas turbine to generate electricity
- Compressed Air (CAES), where air is compressed using electricity when it isn’t needed by the grid
- Ice, where electricity is used to run a refrigerator to produce ice, which, when it melts, can return the energy to run a generator
- Other heat storage, such as salt beds, to store heat for use later, to run a turbine to generate electricity
As can be seen, most are indirect methods where the stored energy is in hydrogen or ice, etc., which can then be used to generate electricity.
Converting energy from one form to another, and then back again, always results in losses.
All of the above methods use a great deal of space per kWh of storage, and are very expensive.
One supplier of storage claims to have a cost of $1,000 / KW, but this uses some fancy logic. AES Energy Storage President, Chris Shelton, said that a storage facility performs two functions: First it absorbs energy and then it discharges energy. Voila, costs are cut in half because it can both absorb and discharge electricity, so a $2,000 / KW system becomes a $1,000 / KW system with the stroke of a pen. Of course, the storage facility still costs $2,000 /KW.
Battery life is usually overlooked when calculating the cost of battery storage, where the investment must be repeated every several years when the batteries die2.
Two CAES systems have been built, one in Germany, in 1978, the second in McIntosh, Alabama, in 1991. Both are large scale systems: Huntorf is rated 321 MW, McIntosh is rated 110 MW. Both were very expensive.
Underground caverns suitable for CAES are also not readily available.
The McIntosh CAES plant uses the compressed air in a natural gas turbine, so it is a hybrid system.
There are additional variations of CAES, and of other storage systems, but none, those listed here and others, singly or in combination, can economically provide the amount of storage needed to allow significant use of wind and solar on the grid.
It’s important to recognize that the cost of storage must be added to the cost of wind and solar. Storage makes electricity from wind and solar more expensive. Neither storage nor back-up generation are include in Levelized Cost of Electricity (LCOE) calculations, which is another reason why LCOEs can be misleading.
Equally important, storage is not essential for the grid to operate successfully without wind and solar. BUT, large scale storage is essential for wind and solar if the grid is to operate reliably without back-up power generation.
Currently, all storage methods are expensive, and none cost less than natural gas turbines for back-up power.
- The term radical environmentalist is used to distinguish between environmentalists, which includes most Americans who routinely support protecting the land and water, from those advocating extreme action. Many radical environmentalists are associated with extremist causes, such as stopping global warming; or stopping the use of nuclear power; or maintaining biological diversity.
- This may not be entirely the case with flow batteries, as the electrolyte can be replaced.
- Storage system size is usually published as Megawatts(MW), but this doesn’t accurately reflect the amount of electricity available for the grid. It would be more accurate to publish the rating in kWh, to reflect how much electricity is available. But, even this doesn’t indicate how long the electricity will be available. Is it consumed quickly, such as with Flywheel systems? Or spread out over hours, such as with the McIntosh, CAES hybrid system?
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