Last week we dived into utility-scale battery energy storage systems (BESS) and presented the different storage technologies available today. It was mentioned that as of September 2020, the United States and Canada had over 37 GW of rated power in energy storage with 90% coming from pumped hydro.

Today, hydroelectric pumping technology is the most efficient system that allows to store energy in a large-scale for long periods of time. This technology compliments renewable energy resources providing a stable and reliable energy system with fast response times and without resulting in any type of emission into the atmosphere.

How Pumped-hydro works?

The system stores water in two reservoirs at different altitudes with a steep drop in between. During off-peak hours excess power from renewable resources is available to pump water from the lower-level reservoir to the upper one.

During peak hours, when the energy generated by renewable resources isn’t enough to meet the energy demand, the pumping station functions as a conventional hydroelectric plant: water accumulated in the upper reservoir runs to the lower one and spins the turbines which work as generators. In this way, water can be run downhill to generate electricity and pumped up hill to store its potential energy and run this cycle again and again.

Figue 1. Pumped-hydro storage plant scheme.

Other emerging technologies using gravity to store energy

Pumped-hydro is not the only mechanical-gravity energy storage system at rise in the market. There are tens of vendors offering their technologies to solve the problem of lack of long duration storage with high life expectancy (between 20 and 60 years). Among these we can find:

• Underground pumped-hydro

This solution is offered by companies like Gravity Power. When compared to conventional pumped hydro storage systems, Gravity Power removes siting constraints by moving the reservoir to underground, bringing more flexibility to where it can be installed. It will also utilize conventional and proven technology from the hydro industry and techniques from mining, bringing greater certainty to the company's performance claims.

Figue 2. Gravity Power's underground pumped-hydro.

Quidnet has also developed an underground solution but this one works with water stored underground and under pressure between rock layers. When electricity is abundant, it is used to pump water from a pond down a well and into a body of rock (1). The well is closed, keeping the energy stored under pressure between rock layers for as long as needed (2). When electricity is needed, the well is opened to let the pressurized water pass through a turbine to generate electricity, and return to the pond ready for the next cycle (3).

Figue 3. Quidnet's energy storage system with water under pressure between rock layers.

The entire Quidnet module is built on conventional drilling technology and off-the-shelf hydropower equipment. Facilities operate with closed-loop water systems, designed for conservation against evaporative loss. The need for specific geological rock layers can limit viable sites for Quidnet's projects, restricting applicability to address regionalized grid constraints and introducing interconnection costs.

• Hydraulic Lifting

Heindl Energy's Gravity Storage is based on the hydraulic lifting of a large rock mass using water pumps. The fundamental principle is based on the hydraulic lifting of a large rock mass. Water is pumped beneath a movable rock piston, thereby lifting the rock mass. Similar to Quidnet's solution, during times of insufficient generation of renewable power, the water which is under high pressure from the rock mass, is routed to a turbine, as in conventional hydroelectric plants, and generates electricity using a generator.

Figue 4. Henidll Energy's Gravity Storage scheme.

Gravity Storage allows for large quantities of power to be stored for long periods of time at a high efficiency rate and with no elevation required. Still, construction, maintenance and site-related aspects must be considered.

• Weight raising

Energy Vault's core product is a kinetic storage system that consists of multiple cranes and cement-like blocks. Energy is stored by lifting blocks and stacking them at a height, then utilizing their gravitational potential energy to fall back to the ground and drive a generator. Standard systems are built with 35 MWh of storage and a power rating of 4 or 8 MW, consisting of a 150 meter high tower and up to 7,000 blocks. The system can ramp up to its 4 MW power output in 2.9 seconds, and can be developed with storage capacities ranging from 20 MWh to 80 MWh.

Figue 4. Energy Vault System with pilling blocks.

• Gravity on rail lines

Advanced Rail Energy Storage (ARES) offers the Gravity Line, a system of weighted rail cars that are towed up a hill of at least 200 feet to act as energy storage and whose gravitational potential energy is used for power generation. Systems are composed of 5 MW tracks, with each car having a fixed motor to generate electricity. Installations can be scaled up to GW-level capacity by the addition of more tracks. Storage duration can be from 15 minutes to 10 hours with response time of 3 seconds.

Considerations

Long duration storage with long life expectancy, relatively low maintenance and low environmental impact certainly make these solutions worth considering. All of these technologies have managed to get financial back-up for their solutions which shows the high interest to solve the problem of lack of sustainable long-duration storage.

The benefits that each solution can bring are (with the exception of Gravity Power's module) deeply related to the site's characteristics. So, for particular cases these technologies are definitely promising.

Still, out of the few exceptions -for the time being- these technologies have complex mechanical systems which attempt to compete with electrochemical batteries, whose prices have been dramatically decreasing over the last decade and are expected to keep on going down. Besides, none of these energy storage technologies have yet reached the commercial stage being at pilot-phase and with no actual deployments.

This year and the following will be critical for these companies to take their first steps on actual deployments and prove their technologies and low LCOE models.