State of the art technology and simple engineering principals are combining to generate a new method of flexible energy storage.

In Switzerland’s only Italian speaking canton, Ticino medieval castles and clear blue lakes sit against a backdrop of towering alpine mountains. But in 2019 a new addition was made to the landscape. There is now a 75 metre tall crane with six arms stretching out 66 metres wide. The crane is surrounded by dozens of giant 35 tonne blocks, and spends all day stacking them into a tower, before dropping them back down to ground. The reason behind this strange process is energy storage.

“Energy storage is what’s standing between us today in our clean energy transition between large scale decarbonisation and the migration from relying on fossil fuels,” says Robert Piconi is the CEO of Energy Vault, the company behind this innovative system.

A quarter of UK electricity now comes from wind energy and given that renewable generation costs are so much cheaper than electricity from fossil fuels finding long term, more flexible storage is vital. “The issue isn’t the cost of renewables being cheaper, it’s much cheaper. So what’s missing is the ability to store this energy when it’s not needed by the grid.”

Traditional storage

Energy storage is not new. Pumped hydro storage accounts for 95% of current global capacity. Other forms include compressed air, thermal storage, batteries and gravity storage but each has its challenges and limitations. According to the BloombergNEF H1 2022 Energy Storage Outlook this is rising fast. Globally 568 Gigawatts of new storage will be deployed every year by 2030. To put this into context the UK currently has 4GW of storage and 2.8 of that is from pumped hydro storage. And for the first time in over 30 years a new 1.5GW hydro pumped storage project is being built in the UK in Scotland.

However pumped hydro storage is  very expensive to build and requires specific geographic profiles, with two reservoirs at different heights passing water back and forth.

For this reason most of the recent UK investment into the industry has been put towards batteries and according to Renewables UK there is currently 27 GW of battery storage projects in the pipeline. But battery storage has a low average storage capacity. The average size was 45MW in 2021 and this degrades over time. The current range is short with 1-2 hours of discharge.

A new paradigm

Gravity storage could potentially provide both long duration discharge and have the high storage capacity that the energy storage market needs. This is exactly what the company behind the giant crane in Switzerland is hoping. “We did look at different ultra-low cost materials to try to suspend water at height and mirror what a pumped hydro dam does, but where you could build it anywhere,” says Robert explaining that an earlier iteration of the concept was using various types of structures, to essentially suspend water at height, drop it and the pump it back up to the top of a structure. “ We looked at steel cylinders, we looked at very interesting structures….” But they decided the structures would be too large and costly for a water based system. Instead of using falling water to turn a turbine they decided to use 35 tonne blocks. “We are utilising these blocks as the storage medium. It’s all potential energy, right? and that becomes kinetic energy, and then we convert that into electricity.”

As a wind or solar farm generates power, the excess energy is used to power a motor that lifts and stacks the 35 tonne block on top of one another making a circular wall around the crane.

 Then as the energy is needed, the crane one by one places the giant blocks back onto the ground while converting that kinetic energy to electrical energy. Since 2019 Energy Vault’s first gravity storage design has been delivering stored power to the Swiss grid.

Construction challenge

Building a 75-metre tall, 66 metre wide six-armed crane that lifts 35 tonne blocks required the design of a whole new kind of crane to overcome a series of challenges. The giant blocks had to be stacked and dropped with extreme precision or the wall of blocks stacked around the crane would be at risk of collapsing. It would also be very exposed to the elements.Even low levels of wind had to be taken into account when the crane was operating to allow the crane placement to be precise enough.The other thing we had to account for were the deflection of the crane, and even the elongation of the cable. So really amazing science, there was not a software out there that did it, nothing serving the crane industry. So we developed our own algorithms,” says Robert.

The other problem they faced were the blocks themselves which had to be structurally safe, cost effective and environmentally sensitive. Concrete is not only very expensive, but for every tonne produced 72.5kg of CO2 is emitted. So Energy Vault partnered with Cemex to make the blocks out of recycled materials. The mix they created was a combination of coal ash and wind turbine blades. One billion tonnes of coal ash is produced in the US every year and wind turbine blades have a lifespan of 25 years before they are thrown in a landfill.

The first performance factor that industry was interested in was the round trip efficiency, meaning for every unit stored, how much can be given back to the grid, given losses that take place in any thermodynamic process, or in this case, any mechanical system. So this was fundamental. “We proved that at just over 75% round trip efficiency, which in a system that was designed for speed and not to optimise for efficiency at that point. So that was important,” says Robert.

Design upgrades

If the design was scaled to up to full size with 7,000 individual blocks it would be capable of powering several thousands of homes over an 8 hours discharge time. But the team had some problems to solve. Exposure to the elements means it couldn’t operate in all conditions and the crane system itself was far too tall for most international building codes. This would massively limit where they could be built. In addition the design didn’t allow for easy scalability in a system’s storage capacity. It was time for a redesign. “That led to us redesigning the system to be essentially 40% lower and basically permissible on any International Building Code.”

The new design called the EVX can be built anywhere that a 20 storey building can be built and it eliminated crane arms lifting up the blocks. Instead the system works more like rows of freight lifts, that take the blocks to the top of the building. Then the blocks are slid along the top of the building on tracks before they can be dropped down the other side. The other element Energy Vault wanted to change in their second design was allowing for flexibility in the discharge duration. “We were really looking for something that could decouple energy and power,” says Robert so where a customer could say I need a four hour system, 100MW, so 400MWh, but over here, in this state, I want 100MW of power over two hours. So a 200MWh. “So that’s what led to us developing a modular system built out just like a building, and we clad it, as well, so that we could protect from the environment and, and have something not only shorter in height, but that would be more simplified in more of a vertical motion.”

Construction on the first EVX has been underway in China since March 2022 and is expected to be completed by the end of the year. The energy storage system is being built next to a wind farm and is then connected to the State Grid Corporation of China, the world’s largest utility company which provides power to 1.1 billion people in China.

Scaling up the number of these gravity storage systems can create what Energy Vault call resiliency centres tailored to the customer requirements. Plans are underway for a resiliency centre made up of the EVX systems in Queensland Australia and it will store renewable energy to power what will be the world’s first green zinc refinery.

Energy storage mix

Even with the new design able to adjust storage duration and capacity, Robert still believes that this is part of a mixture of storage solutions. “There is no silver bullet in storage. It’s a very difficult problem to solve,” he says noting that gravity storage will prove most useful for long duration storage at massive scales. “It is a very interesting technology for the large scale, large utility scale, longer duration, where you look at levelized cost over time, not just the initial capex, but because, for example, with our system, we don’t degrade over time. So you don’t need to replace and add augmentation. Also the operating costs are pretty low, you don’t have to air condition these buildings that we use, for example, so your operating costs are low, and therefore your levelized costs over time, a longer project, you know, 30 – 35 years, becomes very attractive.”

Meaning that gravity storage could work well when coupled with renewable energy generation. For places where gravity storage isn’t the solution Robert believes Energy Vault still has something to offer as they are now using their energy management software to help optimise other energy storage solutions. They announced a new software group last year.  “Software is an extremely important part of our solution. And I would say it’s the most important sort of tip of the spear now, for us as we talk to customers, whether we’re going to deploy gravity or another technology that might be a better fit.”

Along with building their first full scale gravity storage system in China they are also implementing their energy management software at three US battery storage sites to improve their efficiency. As the energy market grows there is space for new and creative forms of energy storage to compliment the hydropower and battery markets. Robert thinks continuing innovation and providing multiple different energy storage solutions will be crucial to allowing the transition to renewable energy resources. “We need a lot more innovation in this space. I maintain that we’re still far behind as an industry of where we need to be to really accelerate deployment of renewable energy to replace fossil fuel.”