HIGHLIGHTS
• Pumped storage hydropower systems utilize gravity to generate electricity.
• Building prefabricated structural steel modules and then transporting them to the desired location may be a more cost effective approach for constructing pumped storage hydropower systems.
• To minimize environmental impact issues, the lower and upper reservoirs in a pumped storage hydropower system can be produced with a closed-loop pump storage system. 

The growth of renewable energy generation using solar and wind is leading to concern about having sufficient power available for consumer and industrial use when neither source is available. This has led to the development of energy storage systems to supplement renewable energy when the sun is not shining and the wind is not blowing. 

One option that has been under development is the use of redox flow batteries that store energy in large reservoirs. In a previous TLT article,¹ researchers increased the volumetric power density of a redox flow battery by developing a submillimeter bundled microtubular membrane. This membrane has a surface area that is an order of magnitude higher than currently used planar membranes. Including a zinc iodide redox system produced volumetric power densities that are 10 times higher. 

An alternative energy storage system utilizes gravity to generate electricity and is known as a pumped storage hydro- power system. Dr. Gordon Wittmeyer, senior scientist and hydrologist in Southwest Research Institute’s chemistry and chemical engineering division in San Antonio, Texas, says, “Pumped storage hydropower acts as an energy storage system by pumping water from a lower reservoir up to an upper reservoir (see Figure 1) at a time when the supply of electricity is higher than demand. The water in the upper reservoir is then released when demand for electricity is greater than supply. As the water moves down to the lower reservoir under the influence of gravity, potential energy is converted to kinetic energy, which then generates electricity.” 


Figure 1. A schematic of a closed-loop pumped storage hydropower system shows how upper and lower water reservoir sites can be positioned on relatively flat terrain, such as caprock mesas with level tops, steep side slopes and gently sloped valley floor. Figure courtesy of Southwest Research Institute.

The efficiency of a good pumped storage system is approximately 80% according to Wittmeyer. He says, “Efficiency is the ratio of the energy put into the electric grid to the energy taken from the grid. This figure can accurately be measured by metering.” 

Three issues make it challenging to build pumped storage systems in the U.S. Wittmeyer explains, “Cost is a factor as the price to build a new system is in the range of $5,000 per kilowatt installed capacity. The lack of a clear energy storage pricing signals increases the risk of building a system without knowing the return on the investment. A second factor is the time needed to build a pumped storage system, which can take more than a decade to design, construct and commission. Part of the issue is that these systems are usually present in locations away from large population centers. Environmental impact is the third issue, which can be a factor particularly if the pumped storage system is placed near a water source such as a river.”

These challenges have stymied the construction of pumped energy storage systems in the U.S. with the last one having been built 40 years ago. In contrast, subsidies from China, Japan and European countries have facilitated construction of these systems in the past and are encouraging their use into the future. 

Wittmeyer and his colleagues have now developed a new approach for building pumped storage systems that hopefully will reduce cost, reduce time needed for construction and reduce environmental impact.

Prefabricated structural steel modules
The researchers determined that the most cost effective approach for growing the use of pumped energy storage systems is to build prefabricated structural steel modules that range in size from 3 to 12 meters in length and 3 meters in height, transport them to the desired location using standard-sized flatbed trailers and then construct high buttress dams. An example of a dam under construction using prefabricated steel modules is shown in Figure 2.


Figure 2. Construction of a new modular steel buttress dam system using prefabricated structural steel modules that can be assembled onsite has the potential to make pumped storage hydropower systems cost effective. Figure courtesy of Southwest Research Institute.

Wittmeyer says, “These dams can be the basis for both upper and lower reservoirs that can be designed to a wide range of surface areas and water volumes to meet the needs of the specific application. The use of prefabricated modules will reduce costs and also reduce the time needed for construction. We estimate that this approach will reduce the construction time by less than 50% compared to traditional pumped storage hydropower units made using earthen embankment or concrete dams.”

The environmental impact issue is addressed by having the lower and upper reservoirs be in a closed-loop pump storage system. Wittmeyer says, “Water is pumped from the lower reservoir to the upper reservoir. When reversed, water will move due to gravity past the same pumps, which will act in a reversible manner as turbines generating electricity.”

Make-up water will be required for a closed-loop pump storage system. Wittmeyer does not consider water quality to be an issue. He says, “The Japanese operated a pump storage system on the island of Okinawa for 17 years using sea water without any difficulty. Pumps and other equipment need to be protected using specialized ceramic coatings that minimize corrosion.”

Closed-loop pump storage systems can readily be installed in topographies where the upper reservoir is at least 150 meters above the lower one. To maximize power output, Wittmeyer indicates that the upper reservoir should be as high as possible above the lower reservoir.

Future work will evaluate the effect of temperature on the operation of the closed-loop pump storage system. Wittmeyer says, “At higher elevations, the upper reservoir may have a significantly lower temperature than the lower reservoir. The temperature extreme where this system can operate is from -40 °C to 50 °C. We will be examining materials that can be used to handle these extreme weather conditions.”

Additional information can be found at the following link² or by contacting Deb Schmid, assistant director communications at Southwest Research Institute, at deb.schmid@swri.org.

REFERENCES
1. Canter, N. (2023), “Microtubular flow battery,” TLT, 79 (4), pp. 16-17. Available here.
2. Click here.