The first system of its innovative gravity energy storage technology will be deployed in India in 2019

At the site, concrete blocks were turned into skyscrapers. Now, during the power-rich, energy-storage phase, a six-arm crane lifts the blocks, one by one, from the ground and stacks them high in the tower. The stacking was apparently orderly, and the crane was clearly well aware of it, operating autonomously and methodically. Potential energy is stored in the towers that form on the base as they rise and grow larger.

When the system is needed to release energy, a crane can pick up the right amount of concrete blocks and lower them back to the ground (where they are, of course), where the kinetic energy of the block's descent is converted into electricity for use. (The accompanying image shows the tower from left to right as it is fully loaded to progressively releasing energy.) Amazingly, the crane, which looks big and bulky, can power the system in 2.9 seconds. With a dedicated algorithm to calibrate and control the energy storage/discharge cycle, the combined round-trip energy efficiency of the system can be as high as about 90%.

Depending on the size of the system, the concrete tower can store up to 35 megawatt-hours of electricity, with a peak power of 4 megawatts. The first storage system will be deployed in India by Tata Power in 2019 and will be 35 MWH.

The system is modular, facilitating the deployment of multiple towers to increase capacity. Because it relies on blocks stacked, it does not degrade, or "leak" energy, as chemical storage systems do. Energy Depot says the concrete blocks do not degrade over time and should have a life span of 30 to 40 years. Of course, there are skeptics who believe that blocks exposed to the air over the years will wear down faster.

Another objection: Concrete is one of the most environmentally damaging materials, and building blocks can leave a large carbon footprint on their own. But the company says the blocks are made from concrete fragments originally used as landfills to cut costs and reduce material waste. It says it will also adapt for different regions, using more locally sourced waste in the composite.

There are already several options for storing electricity from intermittent renewable sources such as wind and solar, such as Tesla's giant lithium-ion battery in Australia. Germany's Brine4Power, the world's largest REDOX flow battery built in an underground salt cavern, stores electricity in liquid electrolytes and can power 75,000 homes. The energy storage system thinks its cost per kilowatt-hour should be lower than other storage systems.

Ling Qiyu (Photo: Swiss Energy Bank)