Hybrid solutions
Dr. Klaus Krüger, Head of Plant & Product Safety and Innovation at Voith Hydro, explains why hybrid solutions will become increasingly important for generating and storing clean, renewable energy.
In a forest overlooking the small town of Gaildorf in southwestern Germany, a unique renewable-energy project is underway. The “Naturstromspeicher” (natural electricity storage plant) combines a pumped storage hydroelectricity power station with a wind farm. The base of each of the farm’s four wind turbines acts as a reservoir. To generate electricity, this water is released downhill through pump turbines. If there is a surplus of power in the grid, the water turbines switch to pump mode and drive the water back up to the wind turbine reservoirs. An adjacent area of the Kocher River will be used for the lower reservoir, which can also be used as a retention pond in flood seasons.
Built-in flexibility
In September 2016, Voith was selected to supply three reversible Francis pump turbines for this groundbreaking project. Dr. Klaus Krüger, Head of Plant & Product Safety and Innovation at Voith Hydro, says the 16 MW Naturstromspeicher plant makes it easier to integrate renewable energy into the grid. “Pumped storage plants are a proven solution for flexible bulk energy storage, which is crucial when using more volatile sources of electricity generation like wind and solar.”
Combining pumped storage with a wind farm provides a new way to generate and store clean energy. The extra height provided by the reservoir bases results in taller wind towers that allow the wind turbines to generate more energy. Planning and infrastructure costs, and environmental impact, can be reduced as the two technologies use the same service access, grid connection and substation. “The storage, fast start-up times and load control options of the three speed-variable units will also allow the operator to profit across different energy trading markets, such as balancing energy, control power and spot markets. And pumped storage can avoid wind curtailment in case of negative energy spot market prices,” adds Krüger.
What’s more, by using the wind towers as upper reservoirs, geography becomes less of a limiting factor, opening the potential of decentralized pumped storage to more sites served by municipal utilities. “The hybrid concept could also be applied to small grids, on islands, for example,” Krüger explains. “Then you could even use salt water, with the ocean as the lower reservoir, instead of precious fresh water.”
Symbiotic saltwater applications
Using salt water for pumped storage plants presents other technical challenges, particularly protecting equipment against corrosion, which can be solved technically. Krüger and his colleagues are researching how to realize the huge potential of combining seawater and pumped storage economically, with the possibility to provide more than just electricity. This could be accomplished with symbiotic approaches.
One exciting symbiotic solution could help provide two vital resources: renewable power and clean fresh water. Around the world, desalination plants use reverse osmosis (RO) to turn seawater into drinking water by forcing it through semi-permeable membranes. However, it is an energy-intensive process that requires large amounts of electricity, often produced by fossil-fuel power plants. Instead, Krüger says, “Combining a seawater pumped storage system, powered by solar or wind energy, with a passive RO desalination plant could present an effective, environmentally friendly solution. The ocean would be used as the lower reservoir, with the upper reservoir in nearby coastal mountains. Rather than using high-pressure pumps and motors to push water against the membrane, as in classic RO plants, this would work passively: the pressure comes from the head of the upper reservoir. And the constant pressure allows the RO plant to run 24/7.”
Just like the hydro-wind hybrid, this concept reduces costs by co-locating and simplifying systems, and generates zero CO2 emissions during operation. Such large-scale pumped storage solutions could convert volatile wind or solar energy into an economical, controllable base load generation source, with quality and availability similar to coal- or gas-fired plants. Krüger summarizes: “They could bring greater grid stability, skilled jobs and hope for a decarbonized future. And solar photovoltaic (PV) and wind turbines feeding saltwater pumped hydro, co-located with passive RO plants near the ocean, could provide power and fresh water for coastal regions with water scarcity.”
