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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