Conventional Hydro
The most common form of energy storage is seen in conventional large-scale hydro where dams create large reservoirs that 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 can be extracted only from the river as it runs. Power is lowest when the river is lowest, which in Alaska is 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 with other large renewable power such as wind to allow optimal dispatching of renewable applications.
Bradley Lake Hydroelectric Dam provides energy
to residents from the Kenai Peninsula to Fairbanks.
Pumped Hydro
While conventional hydro extracts energy from flowing water as it is available, 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. Usually these cycles occur on the time scale of a day or week. When power demand is low, typically at night, water is pumped into the reservoir. During the day, when power needs are generally high, water is released to generate power. In this way pumped hydro can be used to convert undispatchable renewable power into dispatchable power.
Where a suitable location can be found and large storage capacity is required, a pumped storage system is the most cost-effective form of storage available. The right geographical location has adequate water, a moderate climate, and enough variation between times of low-cost power and high-cost power. Most 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, and power response is quite good; however, most conventional pumped hydro facilities cannot quickly go 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 planned 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. KEA is planning to install 4.5 MW of wind (from 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)
An idea similar to pumped hydro is compressed air storage. Air is compressed during times of excess energy and run through a turbine when power is needed. One major hurdle is finding a storage container of suitable size to hold enough compressed air to store a useful amount of energy. Two plants are currently operational worldwide. In the Mackintosh, Alabama plant, 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 if the sun is not shining. Some have suggested using mines in Alaska for compressed air storage, but compressed air storage works best at high pressures 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 might provide storage for compressed air in areas of the state where these wells occur, such as in the Cook Inlet basin. 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 at least 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.
