Pumped Hydroelectric Storage

Executive Summary

Pumped hydroelectric storage facilities store energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... in the form of water in an upper reservoir, pumped from another reservoir at a lower elevation (Figure 1). During periods of high electricity demand1. The rate at which electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by..., powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. is generated by releasing the stored water through turbines in the same manner as a conventional hydropower station. During periods of low demand (usually nights or weekends when electricity is also lower cost), the upper reservoir is recharged by using lower-cost electricity from the grid to pump the water back to the upper reservoir. 

Reversible pump-turbine/motor-generator assemblies can act as both pumps and turbines. Pumped storage stations are unlike traditional hydroelectric stations in that they are a net consumer of electricity, due to hydraulic and electrical1. An adjective meaning “pertaining to electricity”. Electrical Engineer. 2. Related to, pertaining to or associated with electricity but not having its properties or characteristics. losses incurred in the cycleOne sequence of storage charging and discharging. Also known as charge-discharge cycle. of pumping from lower to upper reservoirs. However, these plants are typically highly efficient (round-trip efficiencies reaching greater than 80%) and can prove very beneficial in terms of balancing loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or... within the overall power system. Pumped-storage facilities can be very economical due to peak tand off-peakThose hours or other periods defined by NAESB business practices, contract, agreements, or guides as periods of lower electrical demand. price differentials and their potential to provide critical ancillary grid services.

 Discussion

 Pumped storage  hydroelectric projects  have been providing  energy storage  capacity and  transmission grid  ancillary benefits in  the United States  (U.S.) and Europe  since the 1920s.  Today, the 40  pumped- storage projects operating in the U.S. (shown in Figure 2) provide more than 20 GW, or nearly 2 percent, of the capacityThe rate at which equipment can either generate, convert or transfer energy. of the electrical supply system (Energy Information Admin, 2007).  Table 1 below presents a table of PSH capacities world-wide by country.  In 2009, the world’s pumped hydroelectric storage generating capacity was over 100 GW. 

Pumped storage hydropower can provide energy-balancing, stabilityThe ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances., storage capacity, and ancillary grid services such as network frequency control and reserves. This is due to the ability of pumped storage plants, like other hydroelectric plants, to respond to potentially large electrical load changes within seconds. Pumped storage historically has been used to balance load on a system, enabling large nuclear or thermal generating sources to operate at peak efficiencies. A pumped storage project would typically be designed to have 6 to 20 hours of hydraulic reservoir storage for operation at. By increasing plant capacity in terms of size and number of units, hydroelectric pumped storage generationThe manner in which electricity is generated. The electricity that flows through California can be concentrated and shaped to match periods of highest demand, when it has the greatest value. 

Pumped storage projects also provide ancillary benefits such as firming capacity and reserves (both incremental and decremental), reactive powerThe portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers. It also must supply the..., black startBlack start service is the ability of a generating unit to start without an outside electrical supply, or is the demonstrated ability of a generating unit with a high operating factor to automatically remain operating at reduced levels when... capability, and spinning reserve. In the generating mode, the turbine-generators can respond very quickly to frequency deviations just as conventional hydro generators can, thus adding to the overall balancing and stability of the grid. In both turbine and pump modes, generator-motor excitation can be varied to contribute to reactive power load and stabilize voltage. When neither generating nor pumping, the machines can be also be operated in synchronous condenserIn electrical engineering, a synchronous condenser (sometimes called a synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely.[1] Its purpose is not to... mode, or can be operated to provide spinning reserve, providing the ability to quickly pick up load or balance excess generation. Grid-scale pumped storage can provide this type of load-balancing benefit for time spans ranging from seconds to hours with the digitally controlled turbine governors and large water reservoirs for bulk energy storage.

Conclusions and Observations

In the U.S., the existing 38 pumped hydroelectric facilities can store just over 2 percent of the country’s electrical generating capacity.  That share is small compared with Europe’s (nearly 5%) and Japan’s (about 10%).  But the industry plans to build reservoirs close to existing power plants.  Enough projects are being considered to double capacity. (Scientific American 2012)