From Apples to Electricity: The Underappreciated Potential of Gravity Storage

The need to completely transform our global energy system in response to the threat of climate change is an undertaking that demands technological, policy, and market innovation. It is easy during these times of unprecedented change to become fixated on equally unprecedented technologies. However, sometimes the best solution to a new problem is to simply rethink the tools already at hand.

Recently, I have been reminded of this fact by a remarkably unremarkable method of energy storage: gravity. Approximately 99% of grid-connected energy storage currently in use in the United States is pumped hydro, a system that uses gravity’s pull to draw water through a turbine. This method of storage is both cheaper and longer-lasting than batteries. That said, pumped hydro is constrained in its ability to meet the energy storage demands of a zero-carbon, renewables-dominated grid. It has relatively poor electrical efficiency, it requires many acres of land, and must be sited where there is a natural gradient in elevation. It requires an enormous amount of water, a precious and limited resource in many regions.

Fortunately, gravity is impartial, and our storage systems need not be limited to the movement of water. Two start-up companies—Gravitricity and  Energy Vault—believe that the gravitational potential energy of solid mass may hold the key to sustainable and economical storage of energy. These companies have subtly different designs: one proposes dropping a heavy weight down old mine shafts; the other proposes stacking hundreds of concrete bricks into a tower. But the fundamental concept is the same: pull something up with excess energy, and then generate electricity through a winch and generator as it falls.

Many of us are inclined to think that the best solution to the immense challenge of energy storage has to come from a groundbreaking development in batteries or hydrogen production. Perhaps this is a foolish mistake. Based on the technology available to us right now, deceptively simple gravity systems may be one of our most versatile options and could solve many of the limitations inherent in other methods of storage.

1. This technology is efficient. Energy Vault says that their system could reach 90% electrical efficiency, compared to approximately 70% for pumped hydro storage and ~35% for reversible fuel cell technology.
2. This technology should last for decades, and individual components could be easily replaced should they fail. This is also true for pumped hydro storage, but is a serious limitation of modern lithium-ion batteries and fuel cells which typically last for fewer than 10 years, depending on usage.
3. These gravity systems require very few resources. Unlike pumped hydro, they have no demand for water. Unlike fuel cells and batteries, they do not demand specific rare minerals and metals that will have to be mined and, eventually, recycled.
4. These systems are suited for long-term storage and can sit with full electricity potential for months. Hydropower is vulnerable to evaporation in dry weather, lithium-ion batteries degrade if left uncharged, and electrolysis requires the storage of compressed hydrogen—a potential safety hazard.
5. Lastly, these systems are small in terms of landuse and can be deployed just about anywhere, including within communities. This could make them a useful tool in the development of microgrids. This distinguishes them from both pumped hydro and compressed hydrogen storage.

The history of storage innovation is littered with failed start-ups that claimed to have the best solution to our load-balancing needs. However, as the demand for long-lasting and easily-deployable storage increases, gravity-based systems seem to have a distinct advantage over alternatives. In order for companies like Energy Vault and Gravitricity to succeed, they will need to be given the same opportunities as other seemingly more “groundbreaking” storage technologies. Gravity storage systems are technologically straightforward, but these companies are still vulnerable to the pitfalls of any new enterprise.

Oscar Serpell

Deputy Director

Oscar Serpell oversees all student programming, alumni engagement, faculty and student grants, and visiting scholars. He is also a researcher, writer, and policy analyst working on research initiatives with students and Center partners.