Researchers Propose Using Mountains for Gravity Energy Storage
Mountains are advantageous in many ways as they provide habitation for life forms unique to the land form. Researchers from International Institute of Applied Systems Analysis (IIASA), Austria, Sustainable Energy Planning Research Group, Aalborg University Copenhagen, Denmark and FEEM — Fondazione Eni Enrico Mattei, Italy propose another use adding to the list of advantages provided by mountains.The idea is to use the height of a mountain for gravity energy storage.
Furthermore, the system is called Mountain Gravity Energy Storage (MGES). With this, mountains could potentially transform the lives of neighbouring residents in more ways than one can imagine.
Firstly, it is important to understand the use of gravity to store energy for power generation — gravitational potential energy.
Gravity Potential Energy and Gravity Energy Storage
Gravitational potential energy essentially refers to the energy accumulated within an object when raised to a height above the earth’s surface. For example, a bird flying 10 m above ground or a person climbing a flight of stairs.
In both cases, the bird and the person accumulated gravitational potential energy because they put in effort to overcome earth’s gravity and raise themselves above ground. This effort consumes energy and results in exhaustion or breathlessness. This is why you are not tired when climbing down the same flight of stairs.
Taking it a step further, in physics, we call this action of “putting in effort” as physical work or (simply) work.
Another example is throwing a ball up. You do work (put in effort) to throw the ball up against the gravity. However, when the ball comes down to you, you literally do nothing apart from avoiding the ball hitting your face.
Gravity energy storage systems use the height or the gravitational potential energy to store other forms of energy, particularly electrical energy. They are also called as gravity battery.
Mountain Gravity Energy Storage (MGES)
The MGES proposed by the research group consists of the following fundamental components:
- A mountain or a canyon to create a difference in height
- Sand and gravel
- The storage vessel to carry the sand and gravel
- Cranes
If you look closely, the MGES is analogous to an electrochemical battery. While the cranes act like the electrodes, the storage vessel carrying the sand and gravel is similar to the electrolyte. I’ve written more about the construction of a battery in Introduction to Batteries.
As shown in Fig 1, the crane located at the lower storage site (bottom of the mountain) transports the storage vessel containing sand and gravel to the upper storage site (top of the mountain). As the vessel moves up, it continues to accumulate gravitational potential energy until the motion stops.
When the storage vessel travels down to the lower storage, the stored gravitational potential energy drives a generator which in turn produces electrical energy. Subsequently, the generator converts the potential energy into electrical energy. Depending on the grid requirements, the electricity might as well support the local residents.
The amount of energy stored is directly proportional to three quantities:
- Mass of the sand and gravel carried between the two sites
- Height difference between the two sites
- And, the net system efficiency
One of the factors affecting net system efficiency is head loss. In simple words, when loading sand and gravel into the vessel at both the sites, how much of the height difference remains unused (wasted). Hence, if the height difference between the two sites is 100 m, the entire 100 m does not lead to energy storage.
Another factor affecting the net system efficiency is the speed of travel. Speeds less than 10 m/s enable maximum efficiency.
Engineering the System
In simple words, engineering directly implies building the system for practical use. Although the process may appear straightforward on paper, implementing the technology from scratch is a different ball game. Consequently, the researchers highlight the following pointers:
- Underground filling stations (shown in Fig 1) located at the upper and lower storage sites load the sand and gravel into the traveling storage vessel
- Positioning the generator on the upper site and placing it far away from the cliff reduces unwanted strain on the cables prevents damage to the system respectively
The research group simulated a model by varying multiple parameters to test the storage capability of MGES. The results are in favour of the storage system and show that MGES is extremely flexible. Though it can be used for short periods of time like a few days, it is best suited for longer duration viz. anywhere between 10–12 months.
Furthermore, the results also corroborate the fact that MGES works best for capacities between 1–20 MW. The storage capacity of different systems is shown in Fig 2.
Engineering any system is never complete without the cost analysis. The key assumptions in the cost analysis are:
- The Type of Cranes — Conventional cranes with a life expectancy of 15 years and a total power requirement of 31 kW approximately cost 31000 USD
- Cost of Sand — 50000 tons of sand costing 1 USD per ton
- Construction Cost — Civil construction costs amount to 30% of the total expense
Considering the above assumptions, the researchers estimate the cost of a 1 MW plant to be just about 1.2 million USD and a normalized cost of 52 USD/MWh for a storage cycle of 16 days. With the price of batteries falling, MGES is not economically viable for short time periods.
Conclusion
In conclusion, the researchers advocate that the most prominent advantage of mountain gravity energy storage plant is its ability to consistently generate relatively small yet constant amounts of energy for long-term time scales (several months or a year). In fact, this bolsters the cost analysis as well.
Moreover, this could be combined with other storage technologies, such as batteries, to balance the short-term variations of electricity demand, solar and wind generation. The link to the published paper is mentioned in the reference section for further details.
Although innovative energy storage technologies like MGES may not dominate the entire industry, their maturity supplement other primary technologies like pumped hydro-storage and batteries. This enables the construction of an ecosystem consisting of various inter-linked storage technologies. A future to look forward to!
Thank you for your time!
