A New Energy Storage Option: Gravity Power


In my last two posts (#1 and #2) I explained that to truly make the transition to using renewable energy, we need the utilities on board, and to make that happen, the utilities need an affordable way to store energy.

Batteries are not environmentally or financially the best solution for grid-scale storage. Pumped Storage Hydro (PSH) — the only GW-scale storage technology deployed — and Compressed Air Energy Storage (CAES), with only two plants in operation globally, have given good results. But the construction of these energy storage options is costly, time consuming and wrought with environmental concerns.

So what’s the alternative? The answer may actually lie in digging deep rather than building up.

Pumped storage hydro uses gravity to store energy that is sourced from the grid by raising water to a higher altitude, creating potential energy. That potential is then converted to electricity when the water returns to its original level, passing through a turbine on the way. Storage capacity can be increased by adding mass and/or the storage height. An ideal site for pumped storage hydro would provide:

  • a large elevation difference between two reservoirs (hundreds of meters or more);
  • high power potential (1000 megawatts or more);
  • large energy storage capacity (4 hours or more at rated power);
  • negligible adverse environmental impact;
  • proximity to power transmission lines and a major electricity market, such as a city.

Unfortunately, such ideal sites for pumped storage hydro do not exist.

However, a new technology now being developed exploits widely available analogous sites, using the proven technological components of pumped storage hydro in a completely new way.

Gravity Power Module

The figure above illustrates the basic design of the “Gravity Power Module” or GPM, which is being developed by 21Ventures portfolio company Gravity Power. Full Disclosure: As is the case with any venture capitalist like myself, there is some self-serving message here.

The GPM uses a very large piston that is suspended in a deep, water-filled shaft, with sliding seals to prevent leakage around the piston and a return pipe connecting to a pump-turbine at ground level. The piston is comprised of pancakes made from concrete and iron ore for high density and low cost. The shaft is filled with water once, at the start of operations, but is then sealed and no additional water is required.

As the piston drops, it forces water down the storage shaft, up the return pipe and through the turbine, and spins a motor/generator to produce electricity. To store energy, grid power drives the motor/generator in reverse, spinning the pump to force water down the return pipe and into the shaft, lifting the piston. Hundreds of megawatt-hours per shaft can be stored with high efficiency, since pump-turbines have low losses and friction is negligible at modest piston speeds.

Tackling the cost issue, economic operation of the GPM system depends heavily on the construction cost of the shaft, which is surprisingly low. This is because the GPM system will require less excavation per storage capacity than many existing pumped storage hydro facilities and because that excavation can be automated. A small footprint and unobtrusive operation will allow multi-shaft installations to be constructed even in dense urban areas.

Advantages include: modularity; use of existing technology; environmental compatibility; flexible siting; fast permitting; rapid construction; low cost per megawatt-hour; long lifetime; high efficiency; and a short time from project start to revenue.

The pump-turbine is capable of ramping from zero to full power in less than twenty seconds and has a broad power range, making GPMs technically superior to gas turbine power plants for ancillary services such as frequency regulation. Larger GPMs built in arrays can replace gas turbine peaking plants, providing a substantially lower levelized cost of energy (LCOE), and can replace intermediate power plants at comparable LCOE. The general parameters of two GPM installation types are listed below.

Ancillary Service GPM GPM Peaking Plant
  • 6m shaft diameter, 500m depth
  • 8000-tonnes/shaft
  • 8 shafts @ 25 MW (200 MW total)
  • 68 MWh of energy
  • ~2 acres of land
  • 10m shaft diameter, 2000m depth
  • 210,000 tonnes/shaft
  • 8 shafts @ 150 MW (1200 MW total)
  • 4800 MWh of energy
  • ~2.5 acres of land

To really satisfy the world’s growing utility-scale energy storage needs, a technology must:

  1. Provide hundreds of megawatts for several hours, per installation, with the dynamic operating characteristics required by the grid. Many storage technologies could do this, in theory. So far, only PSH and Compressed Air Energy Storage (CAES) have.
  2. Achieve a competitive cost. Again, only PSH and CAES have met this goal, and few other technologies appear likely to.
  3. Be deployable on a truly gargantuan scale. The International Energy Agency, in its 2008 Baseline scenario, estimated a worldwide need for over 250 GW of new coal-fired and gas-fired power plant capacity, per year, from 2005-2050. Avoiding an environmental catastrophe will require replacement of much of this with renewable generation and storage. Many current storage technologies will have beneficial roles to play, but none of them can achieve this goal.

The GPM can achieve all three.

GPM construction is less complicated than conventional power plants and uses commodity materials with local labor, making them suitable for fast, wide deployment in both developed and developing countries. Market penetration rates will be constrained only by the availability of trained construction crews and project financing. No new equipment factories will be needed for at least the first decade of deployment.

The availability of massive electricity storage will free renewable generation from one of its most challenging constraints—variability.  And because that storage can be constructed in place of conventional power plants, the incremental cost will be very small, perhaps even nonexistent. Wind and solar can provide the energy to fuel the world economy, and advances in energy storage capabilities such as the gravity power module will help.

David Anthony is the Managing Partner of 21Ventures, LLC, a VC management firm that has provided seed, growth, and bridge capital to over 40 technology ventures across the globe mainly in the cleantech arena. David Anthony is also Adjunct Professor at the New York Academy of Sciences (NYAS) and the NYU Stern School of business where he began teaching technology entrepreneurship in 2009.

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I think the basic fundamentals of the GPM are sound and I like the concept. However, I do have one technical question: At 2000m below the surface as described in the example the rock temperatures can become very warm. That implies: the geometry of the piston; shaft walls; and sealing rings will change due to expansion and contraction as the piston moves from zones of high temperature and to lower temperature respectively and vice versa. I would recommend doing an extensive FMEA and prototype test at those depths. I would definately ask your friends in South Africa who have extensive deep shaft mining engineering expertese on how to manage this without getting the piston stuck or cracking the casing walls. Remember different materials have different expanding and contracting properties.

Jim Fiske

Yes, thermal management is an important consideration. The ground temperature gradient is quite different from location to location, and we will avoid those locations where the temperature rises too quickly with depth. Water flow through the shaft will tend to equalize temperatures, as well, and we will use adaptive seals, so we think the thermal issues are quite manageable. Nevertheless, we will do careful modeling and many tests.

Charles E. Campbell, Founder & CEO

Allen Hydro Energy Corporation (AHEC) http://wbsinccd.tripod.com/ahec is the renewable energy innovation of the decade. Why the White House and Department of Energy refuses recognize it is a mystery to many. AHEC is the reverse of this underground idea. Gravity Power Module” or GPM is a good idea, but AHEC’s Large-Scale 70-Story Hydro Generating Facilities is a great innovation. It’s the renewable energy solution the world has been waiting for. If you are a VC seeking to invest in a renewable energy startup, contact me.

Charles E. Campbell, Founder & CEO
Allen Hydro Energy Corporation

Shamil Ayntrazi

I propose the following as PG&E should not abandon its endouvors. Details at http://www.renewableenergypumps.com
1. Prototype
Information and a set of Drawings are available for manufacturing and installing a prototype for the REWGD systems as detailed under prototype.
2. Wave Gear Drive System-WGD
The WGD system provides wave power leveling to provide constant power through the whole cycle of the wave of 360 degrees. This is achieved by means of local energy storage above the “Construction Unit”, capacitors and/or high inertia fly wheels.
The WGD system consists of a buoyant float on the sea surface moving up and down with the wave motion developing an uplift force of 2,650.7 Kilograms. This uplift force is transferred through a set of pulleys and reduction gear to pumps or generators to generate electric power which is conditioned, synchronized and transmitted to the shore ready for connection to electric network. The equipment used is the following:
a. Wave Air Pump-WAP: Compresses a small quantity of air to a high pressure; collects and feeds it to the air inlet of a turbo generator. Fuel is injected as needed to maintain required turbo generator output at reduced fuel consumption irrespective of availability of waves. Adding a “fogging system” (injecting water vapor into the turbine inlet air) will further improve the efficiency of the turbo generator as detailed at:wbsite: http://www.meefog.com/downloads/GT_Comp_Guide.pdf
b. Wave Gear Drive Pump-WGDP: The float directly drives a pump through a set of pulleys and reduction gear to pump a small quantity of water to a high head; collect and feed it to a hydro-turbo generator to generate electric power. The wave water pump can be either:
I. A WWP, reciprocating wave water pump..
II. A WWP rotary wave water pump
c. Wave Gear Drive Generator-WGDG: The float directly drives a Generator through a set of pulleys and reduction gear, to directly generate electric power. The generator can be:
I. A synchronous AC Generator where Alternating voltage is conditioned; synchronized and feed to a step up transformer for transmission through a cable to the shore.
II. A DC Generator where Direct Current voltage is conditioned, converted to AC voltage, synchronized and feed to a step up transformer for transmission through a cable to the shore.


@David, the shaft would be lined and sealed the same way oil wells are lined. Sections of casing would be dropped into the hole and connected together.



You are correct with regard to an above ground structure. This is not at all practical, though we’ve been asked before to look at it.

With regard to lining the shaft, the mechanized approach over multiple shafts entails lining as we go. This ensures stability and accuracy in the process, fast advance rates (critical to make a GPM economically viable) and of course the safety of the workers – an ultimate priority.



@Bryan, To do the same thing above ground would require a tower thousands of feet tall to achieve the same head, obviously not practical.

Bryan Seigneur

This is basically a pressure vessel with a weight on top to give a more constant power output when being drained. Except for the large weight, then, this would make more sense above ground in a tower, since the hole must be lined with a strong material in order to be made water tight anyway. Very interesting.

I’m worried about the seal between the “piston” weight and the hole wall. I can’t think of any large seal like this except actually on very large ICE pistons, and I wonder what the cost would be to scale it up.

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