by Chad Redman
Damming natural flowing rivers is an ancient and effective method for generating renewable energy. However, sufficient rivers are a scarce resource and modern dams produce an array of undesirable environmental effects. In response to the drawbacks of traditional dams, the main commercial technique for storing potential energy in water is pumped hydroelectric storage (PHS). Traditionally, these facilities use a massive pump and two reservoirs, one elevated above the other. During off-peak hours, excess energy produced from sources such as wind farms and nuclear power plants is used to power a pump which moves water into the elevated reservoir. When energy demand rises, the water is released back into the lower reservoir, spinning the pump which effectively becomes a generator.
Two major forms of pumped hydroelectric storage exist. Open systems, which make up the vast majority of current PHS systems, use a body of free flowing water for one or both reservoirs. They do not require an initial filling of the system, the construction of artificial reservoirs, or perpetual compensation for evaporating water. They do, however, impact marine ecology. Closed loop systems, on the other hand, involve two reservoirs isolated from flowing water and allow for the use of waste water, such as grey water. These installations are inherently isolated from aquatic life.
Another advancement toward more efficient and effective PHS is the use of variable speed pumps. Historically, the transfer pump between reservoirs operates only at two speeds – on or off. This has meant poor matching of pump energy use with actual excess energy available, as well as lack of frequency control when the motor is used to generate electricity. Variable speed pumps solve these problems, allowing the pump to use precisely the excess energy available, as well as endowing the PHS facility with a tool for regulating frequency when in generation mode. This creates a safer and more efficient technology. However, variable speed pumps are new to the game and come with a couple of drawbacks. First, variable speed pumps require extra machinery expense, specifically in more advanced rotors and additional frequency converters. Second, variable speed pumps typically exhibit high parasitic loads compared to fixed speed pump systems. Given the overwhelming benefits of variable speed motors for PHS systems, it is likely that future installations will find solutions to the shortcomings of this technology.
“Pumped Hydroelectric Storage.” EnergyStorage.org. N.p., n.d. Web. 23 Feb. 2015.
“Variable Speed Pumped Hydroelectric Storage.” EnergyStorage.org. N.p., n.d. Web. 23 Feb. 2015.