Can You Store Renewable Energy In A Big Pile Of Gravel?
As the world grapples with transitioning away from fossil fuels, engineers are hard at work to integrate new types of generation into the power grid. There’s plenty of challenges, particularly around the intermittent nature of many renewable energy sources. Energy storage projects are key to keeping the lights on round the clock, even when the wind isn’t blowing and the sun isn’t shining.
Conventional grid-level energy storage has long made use of pumped hydro installations where water is pumped uphill to a storage reservoir where it can later be used to run a generator. More recently, batteries are being used to do the job. When you consider the cost of these installations and their storage capacities, there is a gap between batteries and pumped hydro. A recently published whitepaper proposes Mountain Gravity Energy Storage — gravity-based energy storage using sand or gravel in mountainous areas — is the technology that can bridge the gap.
They’re Storing Energy On Mountains Now
The concept of Mountain Gravity Energy Storage, or MGES, involves storing excess energy from the grid by raising sand or gravel to a higher elevation. This is achieved using a pair of cranes, which load the material into storage containers, before pulling them up to height on a cable. The material can then be held in storage at higher elevation until power is requested by the grid. At this stage, the material can be reloaded into storage containers, and lowered to the bottom storage site, with gravity doing the work to pull the weight back down, turning a generator in the process. Interestingly, the same electric motors that lifted the gravel in the first place can also be used as the generators.
If this sounds familiar, you’d be right. It’s not dissimilar from the basic theory of pumped hydroelectric installations, where water is pumped into a dam, and then allowed to flow out through a turbine when energy is required. However, these installations are typically only economically viable in larger installations of 50 MW and above. MGES systems are intended as an option for smaller installations, on the order of 1-20 MW output. For small islands or other isolated areas, an MGES could be a great way to support the local power grid in combination with renewable sources of energy.
Can You Spare a Mountain?
The viability of such an installation is dependent on the availability of suitable mountainous terrain. The higher the elevation difference between the top and bottom of the system, the more energy can be stored. An MGES would likely become viable for areas where the natural landscape enables an elevation gap of between 500-2000 meters.
Obviously, using sand or gravel as the energy storage medium brings its own set of challenges. It’s not possible to easily pump dry materials around like liquids, hence the need for cranes to move the material. Said cranes would need to be specially designed to carry heavy loads, and work reliably for high duty cycles during periods of rapid energy transfer. Loading and unloading of the sand or gravel would ideally be done with an automated system, with the paper’s authors suggesting this be handled by passing containers underneath the upper and lower storage areas. Thus, gravity could simply feed the material into the containers when required through a hopper. This adds a small loss of potential energy to the system, but minimizes the complication of the loading and unloading process.
On the other hand, pumped hydro has its own problems. One is the need for large amounts of water. Another is that storage reservoirs are prone to evaporation and freezing.
Is It Competitive?
While the basic physics of the system is sound, there are pitfalls to such a system. There’s a high degree of mechanical complexity involved, which not only complicates the design, but also adds to maintenance and running costs which could spoil the viability of a full-scale project. In large-scale electricity grids, pumped hydro is likely a more prudent choice, being an established technology that scales well to higher power levels. For smaller scale, isolated grids, MGES could have a place, but it is forced to compete with battery systems capable of delivering similar power levels. In these cases, batteries have the edge for short-term storage, while MGES could prove valuable where energy needs to be stored for weeks or months at a time. This has particular relevance to holiday spots, where there may be large seasonal changes in energy use.
If there’s a particularly bold company willing to invest in a pilot plant in a far-flung mountainous island, we may yet see such a system in action. It would likely make quite the spectacle, or eyesore, depending on your point of view. In the event that happens, we’ll be sure to cover it, but for now, MGES remains a novel and interesting concept that hasn’t quite reached fruition just yet.
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