Metal Tech News - The Elements of Innovation Discovered

Economical Energy intends to mine cavities to house renewable energy storage systems.

An Australian startup has come up with an idea for energy storage mining that does not involve the production of lithium, cobalt, nickel, or any of the battery minerals and metals that have captured headline news over the past couple of years. Instead, Economical Energy proposes mining out a cavity that will house an innovative underground energy storage system that will keep the power on after the sun goes down or the wind stops blowing.

While wind and solar offer the world an abundance of low-carbon energy, the need for electricity does not diminish on calm days or after sunset. The intermittency of these zero-carbon energy sources demands a reliable and economical storage system that can bring balance to electrical supply and demand.

Economical Energy's underground gravity energy storage system, known as the Vertically Integrated Potential Energy Reservoir, or Viper, is an automated system that uses excess electricity generated from intermittent energy sources to lift buckets filled with rock pellets made from tailings, a waste product from mining.

To light up homes and businesses at night, gravity slowly pulls these pellet-laden buckets back down the shaft – transforming the stored energy into electricity.

The underground gravity energy storage concept is similar to pumped hydro, which uses excess energy to pump water into an upper reservoir and generates electricity as water flows back to a lower reservoir, but with a smaller footprint and increased efficiency.

This compact design is due to the pelletized mine tailings being roughly 2.5 times heavier than water and the ability to take advantage of a much larger vertical distance – up to 2,000 meters for Viper compared to 800 meters for hydro – which adds up to a lot more potential energy.

"We distinctly describe it as pumped hydro but with pellets," said Matthew Forrest, founder and managing director of Economical Energy. "You take the best aspects of pumped hydro and negate the adverse side effects."

Great minds

Earlier this year, a team of researchers led by the International Institute for Applied Systems Analysis (IIASA) published a scientific paper that makes a compelling case for underground gravity energy storage systems that transform existing underground mines into gravity storage systems.

"To decarbonize the economy, we need to rethink the energy system based on innovative solutions using existing resources," said Behnam Zakeri, a coauthor of the study and researcher in the energy, climate, and environment program at IIASA. "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."

More information on the underground energy storage system detailed in the IIASA paper can be read at Storing renewable energy in old mines in the January 16, 2023 edition of Metal Tech News.

Economical Energy was happy to see the paper published by IIASA garner media attention.

"Great minds think alike," Michael Short, a mining engineer who is part of the Economical Energy technical team, told Metal Tech News.

Intricately engineered battery

While the IIASA and Economical Energy underground gravity energy storage concepts are based on the same premise, Viper includes some intricate engineering details that set it apart.

The IIASA proposal envisions using existing mine shafts with equipment on the surface and underground to load and unload buckets with sand as needed during the charge and discharge cycles. The system, which is meant for seasonal-scale storage, could continue to store energy until the underground cavities reached capacity.

While Economical Energy sees the potential to utilize existing mines if they were available, the company believes that excavating underground shafts purpose-built for Viper may be the better solution for its energy storage system.

One of the reasons for this is Viper has a much more intricately engineered battery that automates the process for storing energy that matches the daily cycles of solar power generation.

The Viper battery involves a closed and automated system where buckets filled with roughly one-centimeter gravel pellets – a size selected to flow easily and prevent clogging when they get wet – are lowered down the mine shaft when generating electricity. As these buckets reach near the bottom of the shaft, they dump onto a conveyor that then transports the pellets underground. When storing excess energy, the buckets are filled and lifted back toward the top of the shaft – storing potential energy that can be turned back into electricity after sunset.

The exact amount of energy that could be stored would be determined by the depth of the shaft, the weight of the pellets, and the size of the stockpiles of the repurposed mine tailings.

"Economical Energy has developed a computational model of the Viper, which uses genetic algorithms (a type of machine learning) to optimise the system designs, and answer questions like 'how deep is optimal'?" Short told Metal Tech News in an email.

This computational model also provides insights into how much revenue a Viper system could generate at a specific mine and the potential energy cost savings that could be realized by using this underground storage system.

Commercializing Viper

With Viper proven with working lab models that have allowed engineers to simplify and lower the costs of the system, Economical Energy now has its sights set on scaling this underground energy storage solution up to commercial scale.

Toward its goal of commercializing Viper, the Australian energy storage startup has completed full technical engineering design and analysis for a 30-meter-tall demonstration unit that will have an output of five kilowatt-hours.

This demonstration plant, which is expected to be built near the South Australia city of Adelaide, will offer a real-world opportunity to endurance test system components.

The next step will be a roughly 1 megawatt-hour commercial demonstration unit that would be installed within an existing 300- to 500-meter-deep mine shaft.

Economical Energy has completed designs for the commercial demonstration unit and is in talks to secure an underground mine to install it in.

Whether Viper is installed in an abandoned underground mine or a purposely mined shaft, this closed-loop energy storage system can be installed anywhere along a grid that is being fed with renewable electricity, storing excess when available and delivering extra load when needed.

"Our solution, the Viper, is a cost-effective approach to storing this energy," said Forrest. "What this means is that we can keep the overall cost of the energy system down so that people and businesses can pay their electricity bills with the comfort of knowing they come from renewable sources."