The most common form of energy storage are conventional large-scale hydroelectric dams that create large reservoirs to capture maximum runoff in the spring and release it over the year to provide energy the following winter. This can be compared to run-of-the-river hydrokinetic systems, where energy is extracted from the river as it runs. In hydrokinetic systems power is reliant on the flow of the river. In Alaska, the flow is generally lowest in the winter when power demand is highest. However, conventional hydro systems are costly and can be built only where suitable geography and markets coincide.
In Alaska, some small scale hydro projects have been built that can provide power only during part of the year, but the hydro power can be extended by combining it with other large renewable power such as wind to allow optimal dispatching of renewable applications.
While conventional hydro extracts energy from flowing water, pumped hydro uses excess electrical power to move water from a lower elevation to a higher one. It then runs the water back through a turbine to generate power during times of increased demand. When power demand is low water is pumped into the reservoir using excess power. During the day, when power needs are high, water is released to generate power. When can be used to convert undispatchable renewable power into dispatchable power.
Pumped hydro systems require a suitable location where large storage capacity is required to be a cost-effective form of storage. A location should have adequate water, a relatively moderate climate, and a suitable variation between low-cost power and high-cost peak power. Most successful pumped hydro facilities have been large, and limited attempts have been made to integrate them into remote power systems. Capacity and rated power are generally determined by geographical considerations. Hydro power response is quite good, however, most conventional pumped hydro facilities cannot quickly switch from consuming energy pumping to providing energy.
Kodiak Electric Association (KEA) is considering pumped hydro as a way to permit greater penetration of wind on their grid as part of the Pillar Mountain Wind Farm project. KEA is a grid-isolated utility with generation including a combination of diesel generators and hydropower from their Terror Lake project. In 2009, KEA installed three General Electric 1.5 MW SLE wind turbines for the first phase of their Pillar Mountain project. A second proposed phase would incorporate three additional GE turbines and push penetration in excess of 60%, but this is expected to result in difficulty maintaining grid stability and frequency regulation. KEA is currently assessing options for incorporating pumped hydro, as well as a small, conventional battery storage system to absorb power variations and insure that power availability as a whole remains high.
Compressed Air Energy Storage (CAES)
Another viable option for reliable energy storage would use excess power to compressed air in a reservoir. Like pumped hydro air would compressed during times of excess energy and run through a turbine when power is needed. A major hurdle that has to be overcome, is finding a suitable storage container that could hold enough compressed air to create power. Currently, there are two operational plants that use this technology. One of these plants is in McIntosh, Alabama where large underground caverns in salt domes have been created by solution mining, which forms large scale, gas-tight volumes ideal for storing compressed air.
In Phoenix, Arizona, a compressed air storage system has been developed that uses solar energy to heat the air when it is being released, allowing more energy to be extracted from the stored air than the energy required to compress it, which can also be supplemented with natural gas. Some have suggested using mines in Alaska as reservoirs, but compressed air storage works best at high pressure in order to maximize turbine performance. For economical operation, leak rates cannot exceed 1%. In near-surface mines, porosity in rocks and fissures would likely allow too much air to leak; however, depleted natural gas wells could provide storage for compressed air. In most cases, current and planned CAES systems are installed in conjunction with natural gas-fired power plants, where the compressed air is not used to generate electricity directly, but is fed into the natural gas turbine to boost efficiency. As with pumped hydro, CAES capacity and rated power are generally determined by geographical considerations. Power response is also good and, depending on the design, can, in theory, quickly switch from generation to excess energy consumption. Because the storage capacity is determined by the volume of compressed air, CAES technology has been considered for long-term storage. At this point small-scale CAES systems are not commercially available, due in part to the cost of high pressure and the small storage capacity. For this reason, CAES is not likely to be useful for small communities until large-scale, cheap storage solutions are found.
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