|Fig. 1: The electricity demand and supply, using data as reported by California Independent System Operator, for 28 Nov 10. |
As the hype surrounding 'clean' energy and sustainability grows both in the US and around the world we increasingly face the problem of finding ways to meet our energy needs consistently. Besides the fact that they are cheap and already have the necessary infrastructure, oil and coal are reliable - this consistency helps them gain favor in our energy industry. However, even traditional power plants, particularly nuclear plants, can have a hard time adjusting to the constantly changing demand of electricity. (See Fig. 1.) Coal powered plants, for example, run at an optimal production that does not always match the demand for electricity. Changing away from this optimum will decrease efficiency and increase the ratio of greenhouse gases emitted to energy produced.
Renewable energies such as wind and solar power have their own set of complications that demands a way to store the energy generated. Since wind and solar power plants are dependent on environmental variables outside of our own control there is no way to generate more or less electricity based on demand. When there is sun or wind there will be lots of available electricity, regardless of demand, and when there is no wind or no sun there won't be electricity. Thus, a method of storing electricity is necessary so that it can be generated at one time and used at another if necessary. Electricity itself cannot be stored, but it can be converted into other forms of energy and stored as chemical or gravitational potential energy and the be converted back to electricity when needed.
In order to meet these fluctuating demands pumped hydro storage facilities can be used to store electrical potential when demand is low and help supply electricity when demand is peaking. The efficiency of this system is typically between 70% and 85%, making it one of the more efficient methods for storing energy. 
|Fig. 2: A simple schematic of a pumped hydro storage facility.|
The principle of pumped storage are fairly simple - utilizing gravitational potential to store energy. You have two bodies of water, one more highly elevated than the other, and a system of tunnels and piping connecting them. When demand is low and electricity is cheap the plant uses energy to pump water from the lower reservoir to the upper reservoir. When demand is high and electricity is more expensive water from the upper reservoir is released back into the lower reservoir through the same system of pipes, this time the turbines acting as they would in a traditional hydroelectric plant and generating electricity. This type of plant actually has a net use of energy, the advantage comes from the fact that once the facility is operational it can quickly respond to energy demands. 
The pumped storage electricity generating capacity of the United States has stayed constant for the past several years at 21.5 gigawatts.  In the United States the majority of pump storage plants were built in the late 60s to the early 80s with a few built in the 90s. Table 1 lists a selection of major plants in the US, the date they began operations, and the maximum capacity they could generate. Many of these projects use natural bodies of water as one of their reservoirs. Northfield Mountain Station and Ludington Pumped Station use the Connecticut river and Lake Michigan as their lower reservoirs, respectively. [6,7] Additionally, most plants attempt to cater to the public by offering parts of their facilities as recreational areas, though often reservoirs cannot be used for recreational purposed because of the potential for large variations in water surface level.
|Table 1: A Selection of Major Hydroelectric Pump Storage Facilities in the US|
An additional complication in building these types of facilities is the initial environmental impact of creating the reservoirs. These plants can only be built where there is ample space and water for the reservoirs to be constructed. If rivers are damned in order to create sufficient water supply the land loss and environmental implications of decayed forest can be severe and cause community backlash. These have been the primary reasons stopping conventional hydroelectric power plants from being built and also apply to pumped storage plants. The advantages to pumped storage plants are that the water can be reused over and over again and thus smaller reservoirs are suitable. As mentioned above many facilities try to circumvent these issues by using one natural body of water as a reservoir, such as a river or lake. Other possibilities include pre-existing dams, as is suggested in the proposed Red Mountain Bar Pumped Storage Project, or the ocean, as in The Okinawa Yanbaru Seawater Pumped Storage Power Plant built in 1999. [13,14]
A final adverse factor to the construction of pump storage facilities is the upfront cost associated with building them and the time it takes to construct them. Building dams, miles of large quality tunnels, and the electrical infrastructure takes lots of time and money that many people consider prohibitive when other energy storage options are available.
Pump Storage Hydroelectricity improves the efficiency of coal and nuclear power plants by allowing them to run at maximum efficiency without wasting energy. They can also serve the electricity storage needs that renewable energy such as wind and solar require in order to provide a consistent and reliable grid which can match the demand. The current pump storage capacity of 21.5 gigawatts covers a considerable fraction of the energy difference between the peak energy and the base demand. It could be possible to build sufficient pump storage plants to generate the necessary capacity, however, other methods of storage, environmental concerns, costs, and other factors have prevented this from happening. But don't count it out yet, there are some technological advances that may bring pump storage back. One such example is a project that combines wind power, underwater transmission lines, and pumped storage which uses a river and an underground reservoir, this type of system would avoid the environmental costs of the typical above ground, pumping uphill storage.  Whether this project will see the light of day is yet to be seen, but ideas such as this one and other efficiency increasing, impact minimizing technologies may yet cause a resurgence in new pumped storage hydroelectric plants.
© Angela Boysen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Today's Outlook," California Independent Systems Operator.
 "Pumped Hydro," Electricity Storage Association.
 "Electric Power Annual," US Energy Information Administration.
 "Annual Energy Outlook 2009 with Projections to 2030," US Energy Information Administration, DOE/EIA-0383(2009), Table 9: Electricity Generating Capacity.
 "Muddy Run Pumped Facility Fact Sheet," Exelon Corporation.
 "Northfield Mountain Station,"+ FirstLight Power Resources.
 "Ludington Pumped Storage," Consumers Energy.
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 "Rocky Mountain Hydroelectric Plant," Oglethorpe Power Corporation.
 "Red Mountain Bar Pumped Storage Fact Sheet," TID Water & Power.
 H. Ota, T. Kageyama and T. Nishikawa, "Hydroelectric Power Generation. On-Line Operation of Adjustable-Speed System for Okinawa Yanbaru Seawater Pumped Storage Power Plant," Toshiba Review 54, No. 12, 54 (1999).
 S. Kraemer, "Here is the 21st Century Storage and Transmission System for Wind Power," Scientific American, 18 Mar 10.