Pumped Hydro

Pumped hydro storage (PHS) is currently the oldest, most deployed utility grade means of energy storage in spite of the substantial investment in equipment and land that is required (Figure 1) (Baxter, 2006). In simple terms, pumped hydro storage involves pumping water up to a high elevation reservoir so that it can flow back down through an electricity generating turbine. Consuming electricity, the pump elevates the water giving it potential energy. Often the charging electricity is consumed during off peak periods. To augment peak electricity production, this water is allowed to flow downhill through a turbine which turns an electric generator (Abele, Elkind, Intrator, & Washom, 2011). The energy rating of a PHS system has a direct relationship with the size of the reservoir (measured in units of volume, i.e. m³) while the power rating has a direct relationship to the water outflow rate (measured in units of volumetric flow rate; i.e. m³/s). Both power and energy also have a direct relationship with the hydraulic head. Hydraulic head is the liquid surface elevation (measured in units of length; i.e. m) between the upper and lower reservoirs (Eyer & Corey, 2010).

Figure 1 showing a diagram of the Ludington pumped hydro storage in Ludington, MI (Consumers Energy, 2012).

Employing a reversible pump-turbine (see Figure 1), the same equipment is often used to pump the water to the upper reservoir and to generate the electricity. Recently built plants have achieved a round trip efficiency higher than 75%, but most plants see efficiencies closer to 60% (Baxter, 2006). The head losses of the water traveling between the reservoirs and through the pump-turbine are the primary contributors to inefficiency in the PHS system. Another source of inefficiency centers around the fact that the pump-turbine must be designed to operate under a wide range of pressures as opposed to a single optimal pressure. This is because the pressure drops as the reservoir is drained (Mason, 2011). Though the power and energy rating of PHS can be enormous, with minimal environmental impact during operation and relatively low energy costs (~$10/kWh), PHS systems are very large requiring substantial capital investment and land use (Denholm P. , Ela, Kirby, & Milligan, 2010). System costs can exceed $2000/kW and often require prohibitively large tracts of land (Baxter, 2006). 

One example would be the 1 GW Northfield pumped hydro storage facility in Western Massachusetts. During times of low demand (where demand often dips below base load supply) water is pumped 800 ft. to a large hilltop reservoir on Northfield Mountain. During times of peak demand this water is released through four 270 MW pump-turbines to generate electricity. PHS systems with closed input and output reservoirs can be adversely affected by the water leaking or evaporating out of the system. The Northfield PHS facility avoids this leakage and evaporation by using the Connecticut River as the input and output for the system. Engineering studies began in 1964 and the plant went into commercial service in 1972 (FirstLight Power Resources , 2012). Supported by full-time maintenance staff, the Northfield pumped hydro storage facility is still in operation.

PHS systems have relatively fast response times compared to electricity generation equipment. The Northfield PHS facility switches from charge to full discharge in approximately 3 minutes (FirstLight Power Resources , 2012). The output can also be changed very quickly to match demand. This rapid response time coupled with the high power and energy ratings make PHS systems ideal for a wide range of applications including large scale power quality regulation, load following, arbitrage and renewable capacity firming (Baxter, 2006).

Works Cited

Abele, A., Elkind, E., Intrator, J., & Washom, B. (2011). 2020 Strategic Analysis of Energy Storage in California. Los Angeles: California Energy Commission. Publication Number: CEC-500-2011-047.

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

Consumers Energy. (2012). Ludington Pumped Storage. Retrieved from Consumers Energy: http://www.consumersenergy.com/content.aspx?id=1830

Denholm, P., Ela, E., Kirby, B., & Milligan, M. (2010). The Role of energy storage with renewable electricity generation. Las Vegas: NREL.

Eyer, J., & Corey, G. (2010). Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment Guide. Albuquerque, New Mexico: Sandia National Laboratories.

FirstLight Power Resources . (2012). Northfield Mountain Station. Retrieved from First Light Power: http://www.firstlightpower.com/generation/north.asp

Mason, T. (2011, October 11). Gravity Power: Utility-Scale Electricity Storage Systems. Retrieved January 12, 2013, from Definitive Solar Libary: http://www.youtube.com/watch?v=CujxJFXwOns