#Startup : Stacking blocks is an innovative way to store energy.

Thanks to the modern electric grid, you have access to electricity whenever you want. But the grid only works when electricity is generated in the same amounts as it is consumed. That said, it’s impossible to get the balance right all the time. So operators make grids more flexible by adding ways to store excess electricity for when production drops or consumption rises.

About 96% of the world’s energy-storage capacity comes in the form of one technology: pumped hydro. Whenever generation exceeds demand, the excess electricity is used to pump water up a dam. When demand exceeds generation, that water is allowed to fall—thanks to gravity—and the potential energy turns turbines to produce electricity.

But pumped-hydro storage requires particular geographies, with access to water and to reservoirs at different altitudes. It’s the reason that about three-quarters of all pumped hydro storage has been built in only 10 countries. The trouble is the world needs to add a lot more energy storage, if we are to continue to add the intermittent solar and wind power necessary to cut our dependence on fossil fuels.

A startup called Energy Vault thinks it has a viable alternative to pumped-hydro: Instead of using water and dams, the startup uses concrete blocks and cranes. It has been operating in stealth mode until today (Aug. 18), when its existence will be announced at Kent Presents, an ideas festival in Connecticut.

On a hot July morning, I traveled to Biasca, Switzerland, about two hours north of Milan, Italy, where Energy Vault has built a demonstration plant, about a tenth the size of a full-scale operation. The whole thing—from idea to a functional unit—took about nine months and less than $2 million to accomplish. If this sort of low-tech, low-cost innovation could help solve even just a few parts of the huge energy-storage problem, maybe the energy transition the world needs won’t be so hard after all.

Concrete plan

The science underlying Energy Vault’s technology is simple. When you lift something against gravity, you store energy in it. When you later let it fall, you can retrieve that energy. Because concrete is a lot denser than water, lifting a block of concrete requires—and can, therefore, store—a lot more energy than an equal-sized tank of water.

Bill Gross, a long-time US entrepreneur, and Andrea Pedretti, a serial Swiss inventor, developed the Energy Vault system that applies this science. Here’s how it works: A 120-meter (nearly 400-foot) tall, six-armed crane stands in the middle. In the discharged state, concrete cylinders weighing 35 metric tons each are neatly stacked around the crane far below the crane arms. When there is excess solar or wind power, a computer algorithm directs one or more crane arms to locate a concrete block, with the help of a camera attached to the crane arm’s trolley.

Once the crane arm locates and hooks onto a concrete block, a motor starts, powered by the excess electricity on the grid, and lifts the block off the ground. Wind could cause the block to move like a pendulum, but the crane’s trolley is programmed to counter the movement. As a result, it can smoothly lift the block, and then place it on top of another stack of blocks—higher up off the ground.

The system is “fully charged” when the crane has created a tower of concrete blocks around it. The total energy that can be stored in the tower is 20 megawatt-hours (MWh), enough to power 2,000 Swiss homes for a whole day.

When the grid is running low, the motors spring back into action—except now, instead of consuming electricity, the motor is driven in reverse by the gravitational energy, and thus generates electricity.

Big up

The innovation in Energy Vault’s plant is not the hardware. Cranes and motors have been around for decades, and companies like ABB and Siemens have optimized them for maximum efficiency. The round-trip efficiency of the system, which is the amount of energy recovered for every unit of energy used to lift the blocks, is about 85%—comparable to lithium-ion batteries which offer up to 90%.

Pedretti’s main work as the chief technology officer has been figuring out how to design software to automate contextually relevant operations, like hooking and unhooking concrete blocks, and to counteract pendulum-like movements during the lifting and lowering of those blocks.

Energy Vault keeps costs low because it uses off-the-shelf commercial hardware. Surprisingly, concrete blocks could prove to be the most expensive part of the energy tower. Concrete is much cheaper than, say, a lithium-ion battery, but Energy Vault would need a lot of concrete to build hundreds of 35-metric-ton blocks.

So Pedretti found another solution. He’s developed a machine that can mix substances that cities often pay to get rid off, such as gravel or building waste, along with cement to create low-cost concrete blocks. The cost saving comes from having to use only a sixth of the amount of cement that would otherwise have been needed if the concrete were used for building construction.

Via qz.com