Renewable energy sources like wind and solar have the potential to end damaging climate change. Deployed on a large scale, renewables will offset carbon dioxide-spewing coal and gas-fired power plants. But unlike the steady supply of energy generated by fossil fuels, electricity from renewables tends to be intermittent. Wind turbines cease production when winds die down, for example, and solar panels produce less on cloudy days and none at night.

The solution to this intermittency lies in storing electricity generated during peak renewable production and releasing it when renewable output falls. A number of existing and in-development energy storage technologies are aimed directly at achieving this outcome, from pumped hydroelectric to chemical batteries to a variety of gravity-based approaches.

Energy Vault

An Energy Vault tower in discharge mode, generating electricity to deliver back to the grid. Source: Energy VaultNew startup Energy Vault is advancing a concept that involves a tower of massive concrete blocks stacked hundreds of feet high that act like giant mechanical batteries, storing power in the form of gravitational potential energy. At first glance, the simple ingenuity and pure kinetic force of enormous concrete blocks hurtling up and down the tower offer a compelling argument for the concept’s success.

Energy Vault, however, may be fighting an uphill battle as it tries to bring its energy storage concept to market. A previous attempt at gravity-based energy storage by one of the company’s founders, Aaron Fyke, failed to gain traction. Mr. Fyke founded Energy Cache in 2009, based on a conceptual storage system that would carry gravel up and down a hill in buckets attached to a ski lift. Although the company built a 50 kW demonstrator, a full-scale project never made it off the ground. Mr. Fyke proved persistent, refusing to relinquish his vision for a gravity-based mechanical battery. He tweaked the concept, substituted concrete blocks for gravel and towering cranes for ski lifts, and established Energy Vault with the backing of his previous benefactor, Bill Gross’s Idealab.

The new company is pursuing a piece of the rapidly growing energy storage pie. From 2014 to 2050, grid-connected energy storage capacity must increase from 140 GW to 450 GW to keep temperature rise due to climate change below 2° C, according to the International Energy Agency. In a 2016 report, Bloomberg New Energy Finance projected more than $100 billion would be spent worldwide on energy storage through 2030.

Pumped hydro

The depth of the 555-acre lower reservoir of the Bath County Pumped Storage Station varies up to 60 ft during operation. Its upper reservoir covers 265 acres and rises and falls as much as 105 ft. Source: U.S. Geological Survey / Dominion Energy

But Energy Vault faces stiff competition from a number of other energy storage technologies. To start with, it must overcome the proven simplicity and vast capacity potential of pumped hydroelectric energy storage, which stores energy as gravitational potential energy by pumping water from a low reservoir to one at a higher elevation.

Pumped hydro accounts for 94% of U.S. electricity storage capacity as of March 2018, according to the Department of Energy. The largest pumped hydro plant in the world, the Bath County Pumped Storage Station in Virginia, has proven value and has operated since 1985. Its total storage capacity of 24,000 MWh and power output of 3,003 MW dwarf the specifications of a single Energy Vault tower. It would take more than 685 towers to equal the storage capacity of the Bath County plant at the standard tower rating of 35 MWh capacity and 4 MW peak power output).

In addition, the towers are competing against other alternative concepts, like energy stockpiled in the form of compressed air, large spinning flywheels and molten salts heated by concentrated sunlight.

Gravity-based alternatives

Another firm, Advanced Rail Energy Storage (ARES), is pursuing a different take on storing energy with the help of gravity. ARES plans to use excess grid energy to power electric trains which transport concrete containers filled with rocks and sand up an inclined track. On the way back down, regenerative braking turns the potential energy back into electricity. The company is in the process of securing permits to build a 50 MW system in California, which it hopes to have operational in 2020.

California-based Gravity Power’s energy storage approach involves raising a giant underground piston by pumping water beneath it (right). Electricity is generated by dropping the piston to push water back through the pump-turbine (left). Source: Gravity Power

A bevy of other companies are also advancing their own takes on gravity-based storage. Gravitricity’s concept is similar to Energy Vault’s tower, except that it relies on lifting and dropping a single heavy weight instead of stackable blocks, and it places the whole operation below ground in a deep shaft instead of a vertical building. Another pair of firms, Heindl Energy and Gravity Power, also locate their storage underground. Their ideas revolve around raising and lowering a gigantic rock piston with hydraulic pressure. Energy is stored by pumping water beneath the piston to lift it up a shaft, and generates electricity by letting the piston drop to force water through a turbine.

Chemical batteries

Energy Vault’s biggest upcoming rivals are, perhaps, a variety of chemical battery technologies – chief among them lithium-ion (Li-ion). Energy Vault asserts that its towers are superior in several ways. First, the endurance of the company’s 35 MWh tower outpaces Li-ion, delivering almost 9 hours of electricity at its full 4 MW of power output. Moreover, a tower’s lifespan exceeds 30 years, with no limit on the number of block-lifting cycles and no reduction in the amount of energy stored over its life. Chemical batteries typically have much shorter lifetimes and experience steadily degrading storage capacities as they age.

[Discover batteries on Engineering360.]

Energy Vault’s concrete blocks are comprised of a steel and high-strength concrete base topped by a mixture of rock and concrete recycled from old building materials. Source: Energy Vault

The storage medium at the heart of Energy Vault’s solution is also more environmentally friendly than that of chemical batteries. Made from waste debris salvaged from old construction material, the tower’s blocks recycle trash otherwise destined for a landfill. Blocks can be fabricated on site and combine concrete debris with special cement. Energy Vault is partnering with building materials supplier Cemex to develop cost-efficient, concrete-based composite material solutions for the blocks.

[Discover concrete products on Engineering360.]

Capex vs. LCOS

In comparing the costs of energy storage systems, Energy Vault highlights the importance of the initial capital expenditure for installation as well as the levelized cost of storage (LCOS), or the total cost of ownership. The upfront price to build a Li-ion-based storage system might be cheaper than Energy Vault’s system, but when the full costs of operating and maintaining the systems are considered, including longevity and replacement factors, the company says its tower approach pulls far ahead.

On a capital expenditure basis, Energy Vault prices its tower at $200-250 per kWh, claiming this is about 50% lower than other solutions currently on the market. When accounting for the levelized total cost of ownership, the company says its storage system costs 80% less than the competition.

Lithium-ion battery prices have fallen rapidly over the past decade and are expected to continue their downward trend. Source: Bloomberg New Energy FinanceStill, Energy Vault will need to contend with the falling price of Li-ion technology, as mass production of electric vehicle batteries drives down costs. In 2010, electric vehicle battery packs cost around $1,000 per kWh. That price has fallen to $190 per kWh for the battery packs Tesla is producing for its Model 3 car today.

Bloomberg New Energy Finance predicts that Li-ion battery prices may drop to as low as $73 per kWh by 2030. Although these prices do not represent the full cost of a Li-ion grid storage system, they underscore the fact that Energy Vault is competing against a challenger with a commercialized solution whose core component is rapidly falling in price.

In a positive sign for the company, Energy Vault announced it has already reached an agreement with its first customer, Indian electric utility Tata Power Company Limited, for a 35 MWh system to be deployed in 2019.

It remains to be seen whether Energy Vault’s storage concept proves to be more viable than competing technologies. But it is clear that as renewable energy gains an expanding share of the power market, the need for high-capacity energy storage will continue to grow to sustainably smooth the peaks and valleys of renewable energy’s intermittency.