Storing Energy by Gravity

– A simple and efficient way for large scale and long duration energy storage.

Renewable energy technologies have come a long way since the first solar panels and wind turbines were built to harness the energy from sun and wind to meet the ever increasing energy demands of mankind. Advancements in renewable technologies and mushrooming of bigger and larger wind and solar farms are slowly reducing the unit cost of power generated using renewable resources. Also, the contribution by renewable technologies in many countries energy pie chart is growing in size, and is slowly reducing the world’s dependency on fossil fuels for electricity.

And yet, renewable energy technologies are still incapable of standing on their own and have to lean on the fossil fuel to meet irregular demands. Wind doesn’t blow with sufficient strength all the time and there are days when the sunlight doesn’t reach the earth’s surface at a given geographic location with sufficient intensity. Matching the irregular demand with an equally fluctuating power generation is the biggest challenge faced by renewable technologies today. Grid scale renewable farms often rely on gas-powered “peaker” stations to meet irregularities in supply and demand and are fired-up quickly when needed.

The solution to this problem lies in storing the surplus energy on the grid until it is required.
Traditionally, energy has been stored in a number of ways through devices like batteries, flywheels and compressed air. But other than pumped hydro, none of them really amounts to much from a utility stand point with a potential to match supply with demand.

For years, any surge in power demands has been predominantly met at the grid scale by pumped hydro stations. Around the world, 140GW worth of this is installed, with the last large scale pumped hydro project going online some 30 years ago. Mostly, all of these capacities are used as “peaking” power for base load sources like coal, oil, and nuclear power.

But building one of these stations is not easy. They demand a friendly geography with a vast land surface area for two reservoirs separated by a large difference in height. The system basically makes use of gravity to store energy. When available, excess power on the grid is used to pump water from the lower reservoir to the higher one, there by storing energy as potential energy. When needed, the water from the higher reservoir is made to turn the turbine blades to generate electricity.

Though these pumped hydro have good capacities to meet surges in power demands, setting-up of these is not possible everywhere, as not all places have complimentary hills and valleys. In addition, it takes a large amount of time and a lot of money to build one.

Gravity storage

What is ideally needed is a bulk electricity storage device which is scalable to gig watt, which is reliable, efficient and economically viable, but more importantly, it should be environmentally friendly. One such promising technology on the horizon with a capability to compete with pumped hydro and gas turbines for peaking and immediate power generation is storing energy by gravity.

A gravity storage system consist of a pair of two deep shafts one large, and the other smaller in diameter connected at the top and bottom, forming a closed formed circuit via a reversible pumpturbine, as seen in Figure 1. The shafts are filled with water, which acts as a medium for energy transfer, and the bigger shaft is fitted with a huge steel piston filled with reinforced rock and concrete. The whole device operates on the simple action of vertical motion of piston.

During the availability of surplus electricity, the reversible pumpturbine converts the grid power supplied by a dual purpose motorgenerator to potential energy, by pumping the water in the larger shaft to raise the heavy piston. At times of need, like during peak demand, this stored potential energy is converted back to electrical energy by allowing the piston to descend, which in the process energizes the water molecules to rotate the turbinepump blades, leading to power generation at the generatormotor end.

The speed at which the piston descends determines the rate of power generated, while the depth and diameter of the larger shaft and the mass of the piston determines the amount of energy stored. The low friction losses of the pumpturbine, along with negligible friction losses at low piston speeds, make the system to efficiently store energy worth hundreds of mega watt hours.

With a high efficiency of 75-80 percent, low cost, zero emissions, no continued water use, quick start and excellent dynamic response, experts in the field say the underground pumped storage system is on par with modern pumped hydro systems and much faster than fossil fuel based backup power sources. Unlike pumped hydro, the siting of these facilities is flexible with insignificant environmental impact and the modular nature facilitates to have clusters of shafts thereby enhancing the system’s potential to store 1000’s of megawatt of energy per acre of land. Experts foresee that such a facility can even be installed right in the middle of a dense urban area, to meet the fluctuating energy demands of a big city. Lastly, it takes just 2-3 years for installing an underground pumped storage system as against 9-15 years for a traditional pumped hydro.

This promising new technology is yet to take birth on a commercial scale. Gravity Power, a US based company is in the process of setting up the first commercial large scale gravity storage device in Penzberg, Germany. With a 30m diameter power shaft extending 500m meters deep, the facility will produce 160 Mwh or 40 MW for 4 hours of bulk energy storage in return for consuming 40 MW for about 5 hours.

GravityLight

It’s not only at the large scale grid level, that the potential of gravity to store energy is been realized and harnessed. Innovators and engineers are tapping into the gravities energy storing ability to come up with ingenious devices to light homes at a small scale level also.

One such innovative device which makes use of gravity to generate power is GravityLight. It is a small low cost device which was essentially designed to eliminate the dependence on kerosene for lighting houses by millions of people in developing countries, who lack the luxury of continuous supply of electricity to illuminate their houses during night time. After getting successfully crowd funded through Indiegogo, the project has now reached the stage of mass producing gravitylight in Kenya, Africa.

GravityLight’s working mechanism is very simple. The device is installed at a typical house ceiling height of around 1.8 m and a weight of about 12 kg is hoisted to the maximum height and released. As the weight starts to descend slowly at a rate of about 1 mm/second, it lights-up the room instantly. It is the muscle power of an individual which lights up the room.

https://youtu.be/iy43ys35a9w

What really happens is that, the potential energy of the falling weight is converted to kinetic energy as it powers a drive sprocket, which rotates very slowly with high torque. A gear train running through the device turns this input into a high speed, low torque output that drives a DC generator at thousands of rotations per minute. This generates about a deciwatt of energy, sufficient to power an onboard LED and ancillary devices.

Luminasense produced is five times brighter than a typical open-wick kerosene lamp and lasts for more than 20 minutes, which of course depends on how high the device is installed. On reaching the ground, the weight is simply lifted to repeat the process.

This device is very unique as it doesn’t need batteries or sunlight and costs nothing to run. It provides instantaneous light any time, as it doesn’t need a charge in advance and is ready for use when needed. Without a running cost, it pays for itself within weeks of switching over from a kerosene lamp. With no need for batteries, the energy can be practically stored indefinitely. Also, non reliance on sun makes it a dependable source of light on a cloudy day.

Concluding remarks:

Though the knowledge of gravities potential to store energy is known since many centuries, it is only in the last few years that engineers and innovators have started giving serious considerations to build devices to store energy using gravity. Unlike the pumped hydro or chemical batteries, gravity based devices can pave the way for a cleaner environmentally friendly way of storing energy. Without the need for any additional research, gravity based energy storing devices can be built with the available existing technology to meet the energy requirements of a large scale grid at one end and that of a small farm house at the other extreme. Ideally suited for renewable energy technologies like wind and solar power, they can easily iron out the hiccups in supply and demand and make renewable technologies stand on its own foot and also make them literally clean and green.

Additional Videos:

In the following two videos, Steven Dufresne shows how he builds a small light source based on falling weights.

1. Homemade DIY gravity light/gravity generator that powers LED’s. Part 1 (Duration: 8 min, 30 seconds)
2. Homemade DIY gravity light/gravity generator that powers LED’s. Part 2 (Duration: 2 min, 22 seconds)