By Julian David Hunt, Research Scholar in the International Institute for Applied Systems Analysis’ Energy, Climate, and Environment Program
As companies turn away from traditional thermal power capacity to embrace greener energy production options, mines across the country are being decommissioned and closed, leaving operators scrambling to find another use for them. A novel technique called Underground Gravity Energy Storage, developed by a team of researchers from the International Institute for Applied Systems Analysis (IIASA), turns decommissioned mines into long-term energy storage solutions, thereby supporting the sustainable energy transition.
Renewable energy sources are central to the energy transition toward a more sustainable future. However, as sources like sunshine and wind are inherently variable and inconsistent, finding ways to store energy in an accessible and efficient way is crucial. While there are many effective solutions for daily energy storage – the most common being batteries – a cost-effective, long-term solution is still lacking.
In a new IIASA-led study, an international team of researchers developed a novel way to store energy by transporting sand into abandoned underground mines. The new technique, called Underground Gravity Energy Storage (UGES), proposes an effective long-term energy storage solution, while also making use of now-defunct mining sites, which likely number in the millions globally.
The researchers released a paper detailing their findings and delving into the various elements of UGES.
How UGES generate electricity
UGES is a gravitational energy storage technology that consists of filling an underground mine with sand to generate electricity when the cost of electricity is high and then removing the sand from the mine to store energy when electricity is cheap.
UGES generates electricity when the price is high by lowering sand into an underground mine and converting the potential energy of the sand into electricity via regenerative braking, and then lifting the sand from the mine to an upper reservoir using electric motors to store energy when electricity is cheap.
The main components of UGES are the shaft, motor/generator, upper and lower storage sites, and mining equipment (Figure 1).
The crucial components of UGES
The UGES shaft has variable depths and diameters – the deeper and broader the mine shaft, the more power can be extracted from the plant. Additionally, the more space in the shaft, the higher the plant’s capacity.
To maximise power capacity, the sand containers in the shaft occupy around 50 per cent of the shaft’s volume. The other 50 per cent of space is required for filling and emptying the containers with sand. To reduce the costs and number of cables to support the sand containers and the forces exerted on the motor/generator, we propose several motors/generators throughout the shaft.
The containers in Figure 2 are independent, i.e. each module can be put into operation or removed independently (to complete loading and unloading), ensuring that the system does not stop due to the putting in or removal of carriers.
In addition to independence, the container must enable rapid loading and unloading of heavy objects (sand). So we consider setting up loading stations, which can be similar to ski lift stations.
The containers applied in the shaft are foldable to optimise the utilisation of the shaft.
During storage mode, the sand is removed from the container on the shaft’s top and then returned to the bottom of the shaft to be filled again. The container is folded to occupy the least space on the shaft, as shown in Figure 2.
Foldable containers that are not completely sealed could leak sand during operation. This would cause energy loss and damage equipment in the mine or block the shaft.
To mitigate this problem, we propose foldable containers with inner bags as carriers, where the bags act as liners for the foldable containers. This results in a space-saving foldable carrier while preventing the safety hazard of sand leakage.
Specifics of UGES motors and generators
The motor/generators are installed on both sides of the mine shaft, as shown in Figure 1. They should be installed on top of the filling and empty stations to minimise the risk of damage.
The total power capacity of the plant consists of the sum of the capacity of all motors/generators. Other advantages of having several motors/generators are that motors/generators with a small capacity are easy to find in the market and are cheap.
It should be noted that the motors in Figure 2 are electrically connected in parallel to ensure the independence of each motor input and removal – if one motor/generator brakes or requires maintenance, the others can continue operation.
Depending on the power requirements for energy storage, the system can alter the lift’s speed. The lift can raise its speed if the power requirements are high, but it might lower the system’s overall efficiency.
Lead author of the study and researcher in the IIASA Energy, Climate, and Environment Program, Julian Hunt, said, “When a mine closes, it lays off thousands of workers. This devastates communities that rely only on the mine for their economic output.
“UGES would create a few vacancies as the mine would provide energy storage services after it stops operations.
“Mines already have the basic infrastructure and are connected to the power grid, which significantly reduces the cost and facilitates the implementation of UGES plants.”
Other energy storage methods, like batteries, lose energy via self-discharge over long periods. The energy storage medium of UGES is sand, meaning that there is no energy lost to selfdischarge, enabling ultra-long time energy storage ranging from weeks to several years.
The investment costs of UGES are about US$1-US$10/kWh and power capacity costs of US$2.000/kW. The technology is estimated to have a global potential of seven to 70TWh, with most of this potential concentrated in China, India, Russia, and the US.
Behnam Zakeri, study co-author and researcher in the IIASA Energy, Climate, and Environment Program, said, “To decarbonise the economy, we need to rethink the energy system based on innovative solutions using existing resources.
“Turning abandoned mines into energy storage is one example of many solutions that exist around us, and we only need to change the way we deploy them.”
UGES versus other energy storage options
Electricity generation should operate in a continuous mode. The system should be designed to provide a constant power supply. However, due to possible power fluctuation as a result of the dropping and loading of the sand to the system, a battery or ultra-capacitor system should be implemented together with UGES to guarantee a constant power supply.
The proposed UGES design presented in the IIASA paper has multiple motors/generators, ensuring continuity in the generation profile. Slight deviations in the continuous supply are expected, of -10 to +10, which needs to be balanced with grid balancing measures.
The upper storage site of a UGES plant is designed to store as much sand as possible on the surface surrounding the mine shaft to minimise the energy required to store the sand on the surface.
We propose a circular sand pile surrounding the mine shaft, as shown in Figure 3a.
The sand pile’s outer diameter will depend on the availability and cost of land – if land cost is high, the sand pile can rise vertically as the trucks dump the sand on top of the sand pile. The sand pile can reach heights of 50m or more.
Figure 3b presents the upper storage site filled up and the UGES plant fully charged.
The lower storage site consists of filling the entire underground mine with sand. The mine is filled from its extremes until the channels reach its shaft. Figure 3c presents the lower storage site filled up, and the UGES plant discharged.
The mining equipment is essential to manage the sand in the upper and lower reservoirs. They consist of dump trucks, conveyor booms, bucket wheel excavators, and soil compactors.
Dump trucks or conveyor belts transport the sand from the mine shaft to the storage sites and back. The dump truck should be electric. This is because they can recharge their battery while driving down sand piles or tunnels in the underground mine, increasing the efficiency of the UGES plant.
Conveyor belts can also create a sand pile around the mine shaft. Conveyor belts should also generate electricity when lowering weights. The excavators and bucket wheel excavators extract sand from the upper and lower storage sites to load the dump trucks or conveyor belts. The soil compactor is applied to the sand piles to allow dump trucks to drive in the sand piles and increase their stability.
Cost-benefit analysis of UGES technology
The IIASA paper supposes that the mine shaft and the underground tunnels are already in place. However, there is still the need to buy the sand and the mining auxiliary equipment and install the cables, motor/generators, and foldable containers.
This IIASA paper does not consider the additional charge to rent the mine and its top to store the containers and sand.
The costs of UGES components are described in Table 1.
A mine with 40,000,000t of sand and an average height difference of 1,000m is being utilised to demonstrate the system’s cost. A US$1.6/kWh price tag is projected for energy storage. The cost of storing energy with UGES decreases with an increase in the height difference between the lower and upper storage locations.
Table 1 presents the UGES energy storage costs and power capacities at different depths. It should be noted that because natural underground mine caverns are irregular, there may be associated remediation costs, which are not considered in the paper due to the difficulty of assessment.
In light of the findings of this study, to produce a modest but constant amount of energy for a long time, UGES could be designed to store energy over weekly, monthly or seasonal scales, depending on the demand for energy storage.
To offset the short-term changes in electricity consumption of solar and wind generation, this modest but consistent electricity generation might be supplemented with other storage technologies, such as batteries and PHS.
The cost of installed energy storage for UGES is estimated in the IIASA paper to vary from US$1.0-US$10.0/kWh, assuming an average height difference between the upper and lower storage sites of 1,500 and 200 metres, respectively.
The project is less expensive the more significant the height difference. The power generation capacity varies with the mine’s depths, the sand storage capacity, and the sand moving speed.
The IIASA paper proposed constructing several motors/ generators along the shaft to reduce the cables’ costs and allow using smaller, more common/affordable motors/generators. The system’s technical lifespan can range from 20 to 30 years.
A precise description of the UGES system performance is outside this paper’s scope.
The IIASA paper proposes a more detailed analysis of the system’s performance and efficiency for future work as a precise description of the UGES system performance is outside the paper’s scope.
UGES is a particularly interesting technology for long-term energy storage to reduce seasonal fluctuations in electricity demand and wind and solar generation.
Reference
Hunt, J.D., Zakeri, B., Jurasz, J., Tong, W., Dabek, P.B., Brandão, R., Patro, E.R., Ðurin, B., Leal Filho, W., Wada, Y., van Ruijven, B., Riahi, K. (2023). Underground Gravity Energy Storage: A Solution for Long-Term Energy Storage. Energies. 16, 825. DOI: 10.3390/en16020825
