One of the main barriers to the diffusion of renewable energy sources such as wind and solar power is their inherent variability. If excess energy produced could be stored cheaply and used during times of lower production, this issue could be largely mitigated. Several technologies are under development as possible options for storing energy from the grid, including batteries that store energy in chemicals, mechanical flywheels that store energy as rotational energy, and hydroelectric dams that convert mechanical energy into electrical energy by retaining and channeling rivers.
Heindl’s idea is to store potential energy by using water as a hydraulic fluid to transfer power underground. A project would involve carving out a gigantic cylinder of dense rock, such as granite, by drilling two underground circular tunnels with 500-meter radiuses, one tunnel several hundred meters deep and another at a 1-kilometer depth directly underneath the first. A saw mill would be lowered into the tunnels connected to a saw mill at the surface via a wire saw. The saw mills would work away at the rock to separate the cylinder from the deposit. A seal would then be placed within the first tunnel to close off the system to prevent the loss of potential energy.
Water would be pumped down an adjacent mine shaft to the base of the cylinder. Under enough pressure, the rock cylinder (representing a “piston” in hydraulic engineering terms) would then lift, storing potential energy in the system as pressurized water. Extra electricity produced on the grid could be stored within the system by converting electric energy to mechanical energy using a generator to power the pump. When the grid required energy, the process could be reversed to generate electricity for the grid.
Although drilling, tunneling, sawing, and sealing the cylinder would be expensive, the costs grow in linear proportion or proportionately to the surface area of the rock cylinder, while the possible amount of energy able to be stored in the system grows cubically with the volume of the rock cylinder. The economics of the project become increasingly more attractive with its size. The cost of the generator and pump would be directly proportional to the rate at which energy is transferred into the system and back out.
Assuming that power could evenly be bought at €20 (US$30) per megawatt-hour during peak production and sold at €70 (US$100) per MWh during peak consumption, and assuming that the system had 70 percent conversion efficiency, Heindl expects a return on investment in four years for a system with a 500 meter radius and 1 kilometer depth.
So far, utilities have focused on finding ways to make electricity generation from other energy sources play well with wind and solar. Accurate weather forecasting with technologies such as Doppler radar usually allows utilities enough time to balance the grid’s load when renewable energy generation drops. Utilities maintain the grid either by scheduling-in spare capacity from sources that are able to come on line quickly, such as natural gas plants and hydropower plants, or by signing contracts with industrial power users to pull their plugs at short notice in exchange for lower power rates.
Connecting grids over large distances with high-voltage transmission lines also helps account for wind variability and potentially even solar availability. The greater diversity of areas with high potential for wind and solar energy that are connected to the grid, the more likely the overall power output will stay close to its statistical average.
But as electricity generation from wind and solar continues to represent a larger share of the energy mix, energy storage will become increasingly attractive, and perhaps a necessary component of the energy system. One report suggests that the market for grid energy storage technologies will reach $2.5 billion by 2015.
Does it pass the laugh test?
It is difficult for a layperson to judge the feasibility of such a system, although it may be hard to stop anyone from laughing as they try. Hydraulic hydro storage remains utterly untested on the scale proposed by Dr. Heindl, and it is hard to take an idea seriously that involves lifting a cylinder of granite 300 meters or more across hundreds of meters into the air.
Does it have that WOW factor?
A wall of solid rock rising into the sky, too big across to see around? Anyone would stop and stare.
What does it bring to the table?
Artificial height, at low cost.
Hydroelectric energy storage is currently the cheapest way to store energy, but it requires a location with sufficient height differential. The hydraulic hydro storage system would create such a differential at any location with sufficiently dense rock, at theoretically far lower cost.
How scalable is it?
Scale certainly isn’t a problem.
Due to the costs of drilling, the system only comes in one size: extra large. According to Dr. Heindl, hydro hydraulic storage would likely become cost-competitive with hydroelectric storage only once it reaches storage capacies on the order of a country’s electricity consumption. Using Heindl’s price estimates, the system reaches cost-competitiveness around the 450 meter radius/900 meter height range, storing 1,000 gigawatt-hours (or more than half the daily electrical production of Germany).
How close is it to commercialization?
Close, yet so far.
Heindl’s system relies entirely on existing technologies, and at a 500 meter radius/1 kilometer height, Dr. Heindl predicts that the system would reach a return on investment in four years. But cost, of course, is not the issue here. Which brings us to…
What is the biggest obstacle to success?
How to pick just one?
Hydro hydraulic storage faces a number of technical challenges, including how such a system would manage any tectonic movement, how to maintain the seal between the rock cylinder and the hydraulic fluid, and how to reduce friction so as to prevent energy loss.
But clearly the biggest obstacle is public acceptance. Not only is this idea a complete departure from the energy technologies that currently exist commercially, but it seems downright ominous. No matter how many times I’m told that as long as half the massive floating rock cylinder is below the ground it can’t possibly tip over I’d probably continue to have nightmares of a giant rock rolling over my house. People raise Cain over living near wind turbines. Who would ever live near one of these?
The final word(s):
A neat idea.
As renewable energy becomes increasingly central to power generation, creative thinking will be needed to balance the load in the grid. At the very least, Dr. Heindl’s hydraulic hydro storage system embodies this spirit.