Using gravity for energy storage: viable idea or impractical?
Engineers know that there are three major parts of a large-scale energy system: generation, storage, and delivery. Each stage has unique characteristics and there’s often some overlap and interplay between them. However, energy storage is one area where there’s a serious need for viable options, especially as energy captured by intermittent renewable sources like wind and solar is wasted if it cannot be used immediately or stored for later use.
Among the storage options are electrochemical batteries, supercapacitors, flywheels, hydrogen from electrolysis, reversible salt states, compressed air, and pumped reservoir water. As you’d expect, there is no “best” way to store electrical energy, and each technology has pros and cons, depending on many factors. That includes size and scaling issues (as storing a few kilowatt-hours is very different from gigawatt-hours or even megawatt-hours, available space), safety concerns, and more. Long story short: storage at grid-level capacity is difficult.
Many of the active large-scale grid-level storage facilities are reservoir-based, pumping water to a higher elevation when power is available and allowing to run down through generators when power is needed. That seems simple enough in principle, but this scheme requires two large reservoirs at sufficient vertical separation.
Other alternatives being proposed are in various stages of prototype and trial run. Some are purely mechanical with no water involved while the others do use water in ways different than the classic two-reservoir technique.
The all-mechanical system from Swiss-based Energy Vault uses automated stacking and unstacking of blocks weighing up to 35 tons (one ton is 1,000 kilograms, about 2,200 pounds), all set in an open area with six crane arms (Figure 1). The sophisticated system uses advanced algorithms to decide what to stack where and also the optimum stacking order. Among the issues that must be accommodated are the effects of wind on the blocks and cranes while in motion, pendulum effects, cable stretch, and maintaining constant output by sequencing the block “dropping.” The company claims the installation is modular with ranges of 20 to 80 MWh storage capacity and a 4 to 8 MW of continuous power discharge for 8 to 16 hours. Energy Vault is building a large, 110-meter-high demonstration system in Ticino, Switzerland.
Figure 1 This storage system uses a coordinated array of six cranes and automated stacking and unstacking of blocks. Source: Energy Vault
In contrast, the Gravitricity system suspends individual weights of 500 to 5,000 tonnes, each in their own shaft, rather than stacking them out in the open (Figure 2). Each weight has a winch that either lifts the weight or releases it, so the dropping weight can power a generator. The company claims that each unit can produce between 1 and 20 MW peak power with output duration from 15 minutes to 8 hours. Construction and tests of a demonstration unit rated at 250 kW is underway in Scotland using 25-tonnes weights.
Figure 2 The Gravitricity storage system suspends individual weights of 500 to 5,000 tonnes with one weight per shaft. Source: Gravitricity
New Energy Let’s Go uses a combination of weights and water. Electrical pumps and hydraulics lift a large rock mass resting on a movable piston to store energy (Figure 3). To release power, the water, which is under high pressure from the rock mass, is routed to a turbine and generator. The claimed capacity of energy storage would be between 1 and 10 GWh.
Figure 3 The design of the storage system is based on a combination of weights and water, with a large mass resting on a movable piston. Source: New Energy Let’s Go
The Gravity Power approach also uses water, with a large piston suspended in a deep, water-filled shaft, along with sliding seals to prevent leakage around the piston and a return pipe connecting to a pump-turbine at ground level (Figure 4). The shaft is filled with water just once at the start of operation, is then sealed, and no additional water is required. To store energy, power drives the motor/generator pump to force water down the return pipe and into the shaft, lifting the piston. To produce electricity, the piston drops, forcing water down the storage shaft, up the return pipe and through the turbine, to turn the motor/generator. The company says that hundreds of megawatt-hours per shaft can be stored; a megawatt demonstration plant is under construction in Weilheim, Bavaria.
Figure 4 This storage approach is based on movement of a large piston in a closed-loop, water-filled shaft and return path. Source: Gravity Power
An important consideration in these designs is that they are capable of ramping up to full output in a second or so. That’s very useful for many grid applications where the source and load are both unpredictable.
Each of these options has interesting tradeoffs in size, siting issues, complexity, cost, potential reliability, and capacity, as well as perceived and real safety concerns. It will be interesting to see which one gains the most traction, or perhaps they all will do well, depending on the specifics of location, power level, local costs, energy needs, and alternatives.
Based solely on my “gut feeling” and without any data whatsoever, I am more skeptical of the multi-crane approach because it seems so complicated, so exposed to the outside environment, and requires the most complicated management and algorithms. But I have been wrong before: when USB ports were introduced, I said, “Who needs this? We have enough I/O options already.” We know how USB turned out.
What’s your sense of large-scale practicality of these options for grid-level energy storage? Do you see them as viable alternatives to battery farms, reservoirs, compressed air, or other in-use or proposed approaches?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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