Storage can assist with renewables by smoothing the effect of variable energy output—typical for wind and solar—providing capacity firming such that a renewable resource can be seen as an almost constant source of energy, and with frequency regulation support for the transmission grid. Without storage, the grid needs to be able to deal with the effects of intermittent energy, and that can be done with other generation sources providing firming services. The issue historically with storage has been the cost.
Traditional generation can benefit from storage to the extent that storage technologies can provide cheaper and or faster-reacting support services. Flywheels and lithium-ion batteries, for example, can respond quickly and are typically used for frequency regulation.
The main technologies are: pumped-hydro storage; compressed-air storage below or above ground; batteries—sodium sulfur, vanadium redox, lead acid, nickel cadmium and lithium ion; molten salt; thermal peak shaving, aka ice storage; and flywheels. Of global installed storage capacity of about 125,000 MW, over 123,000 is pumped hydro. Other technologies lag by comparison: molten salt, 142 MW; compressed air, 440 MW; batteries, 451 MW; and flywheels, 95 MW. The different technologies have different applications. Pumped hydro has been used for centralized, utility-scale projects—being able to handle load with quick response.
Compressed-air projects are also being aimed at large utility applications, but there are also small above-ground, compressed-air assets, which can be teamed with a specific generation asset. Certain batteries—sodium sulfur, vanadium—have long duration and are better oriented to back up applications. Other batteries—e.g., lithium-ion—have faster response and are best used for renewable integration and frequency regulation, typically at the generation project level. Flywheels are used for frequency regulation and are being developed as stand-alone projects.
The more that the electric service model migrates from the central station generation-dumb meter consumer model—where it is today—to more of a distributed generation-smart meter consumer model, the greater the role for storage to play in smoothing of energy delivery, integration and regulation.
First Wind is using Xtreme Power batteries in wind farms in Hawaii. Energy storage makes a lot of sense in an island application where the load may not be large and the day and night demand may differ widely. In the case of the wind projects in Hawaii, First Wind would be facing the potential of curtailment at night when the wind still blows, given the reduced demand for energy. So a battery can be charged with off-hours electricity and discharged during peak demand during the day.
AES Energy Storage is developing storage systems using A123 Systems’ lithium-ion batteries to provide ancillary services. Primus Power is proposing to build a 25-MW battery storage project for the Modesto Irrigation District (MID). This is known as the Wind Firming EnergyFarm and is intended to replace a fossil fuel plant as the means of firming energy provided to the MID from wind power sources. Southern California Edison is building an 8-MW lithium-ion battery storage project to improve grid performance and to aid in integration of wind energy resources located in the Tehachapi area. Another example is a 20-MW flywheel project built by Beacon Power in Massachusetts. That project is a stand-alone project that will deliver frequency regulation services to the grid.
The Department of Energy launched a program to support energy storage technology in 2009. DOE is providing about $185 million to support over $775 million of energy storage projects; these aggregate about 537 MW of new storage.
Storage presents an interesting regulatory challenge. Depending on its use and the point of view of a regulatory agency, it may be considered transmission or generation, and as either a wholesale or a retail service. Those characterizations affect by which agency it is regulated—federal or state—and how an investment in storage can be recovered. Choice of a regulatory regime affects planning. Who approves? Ownership: Who can own, and by whom are they regulated, among other issues.
As a broad concept, electric vehicles can be used as energy sinks in the sense that they can charge at night while other electric demand is low. Of course, that doesn’t mean the vehicles are available as storage to be applied during the peak of the next day. I think electric vehicles ultimately will be another variable in the electric supply-demand mix that can’t be controlled other than in the broadest terms and so may present as many problems as they do solutions for grid operations.
A number of utilities are pursuing demonstration projects. The most interest seems to be in areas where there is significant penetration by renewables. Ultimately, the deployment of more storage technologies at the distribution level means that demand management can be more flexible because storage can be used to meet peak demands as opposed to relying on reduction of demand to trim peaks.
The principal benefits will be reliability of the grid, backup power when applied locally and lower costs because high on-peak prices can be mitigated with stored energy. (Electric Light & Power, Oct 2011)