The world’s requirement for electric power is growing rapidly and according to the International Energy Agency estimates, an additional 250 GW of power will be required annually between now and 2050.

With the growing use of renewables, peaking power (able to provide the surge in power as needed), require the worldwide storage capacity to be increased by up to 10% per year in order to handle peak power usage periods.

In the US, despite the policy lag on LDES, both some federal and some state regulatory bodies are now pushing for energy storage. Some states have already moved forward on creating their own mandates for energy storage procurement, and some of these have also set 100% renewable targets for the future, making longer duration storage necessary. At the federal level, the Federal Energy Regulatory Commission (FERC) which regulates regional transmission organizations and independent system operators (RTOs and ISOs) is also moving to facilitate storage.

Among the storage options are pumped hydro, electrochemical batteries, supercapacitors, flywheels, hydrogen from electrolysis, thermal batteries, and many others that we have been covering throughout the year.

Figure 1.Grid-scale Energy Storage General Framework.

As you’d expect, there is no “best” way to store electrical energy, and each technology has pros and cons, depending on many factors. These include size and scaling issues (as storing a few kWh is very different from GWh or even MWh), available space, safety concerns, and more.

At the utility-scale level, most large energy storage projects appear to be little more than a shipping container filled with tens of thousands of small batteries. Bulk energy storage is probably the holy grail for the next generation of the electricity grid and, with remarkable ingenuity, different innovators have relied on a host of physical forces and states such as temperature, friction, gravity and inertia to keep energy locked up for later release.

In today’s article we will be focusing on mechanical storage. Which, with the exception of flywheels, is filled with technologies that focus on long-duration energy systems capable of storing bulk power for long periods of time.

Figure 2.Discharge times vs System Power Ratings for energy storage technologies

Mechanical Storage Solutions

The default mechanical storage solution we know of today is pumped-hydro storage. Pumped storage hydropower (PSH) is the world's largest storage technology, accounting for over 94% of installed energy storage capacity. The International Hydropower Association (IHA) estimates that PSH) projects now store up to 9,000 GWh of electricity globally.

PSH provides flexibility through system inertia, frequency control, voltage regulation, and black-start capability. Pumped storage excels at long discharge duration and its high-power capacity is crucial in avoiding VRE curtailment. In addition, PSH has a long asset life, low-lifetime cost, independence from raw material availability and no direct type of emissions into the atmosphere during its operation.

While the advantages of pumped hydro are numerous, the challenges involved include the extremely long time for development and the need of a mountain or a hill and a shed-load of concrete to build a dam to hold the water in. There are also environmental concerns about the concrete and the disturbance of natural habitats - plus it is not always easy to put your mountain-top reservoir next to the place where you are generating the power.

To avoid these environmental and ecological impacts, high CAPEX and siting constraints, other solutions started emerging in the mechanical storage space. We decided to show you the different mechanical storage technologies and innovators in the easiest way possible through the following Innovator Landscape

Figure 3. Mechanical Storage Framework. Find it here in Darcy Connect.

Mechanical Storage Innovations

A new breed of gravity storage solutions, using the gravitational potential energy of a suspended mass, is now coming to market and seeks to replicate the cost and reliability benefits of pumped hydro, without citing limitations, thus enabling a shift toward 100% renewable energy.

Marine-Pumped hydro All three companies (Fraunhofer, RCAM Technologies and Ocean Grazer) are developing scalable and modular solution for offshore wind farms. The technology is based on pumped hydro’s proven technology. It uses clean water as the energy carrier. When excess energy from renewables is available, it pumps water from a rigid underground reservoir to flexible bladders. When there is demand for power, water flows back from the flexible bladders to the low-pressure rigid reservoirs running turbines to generate electricity.

Figure 4. Marine-pumped hydro technology.

Underground Pumped-hydro

Gravity Power LLC - based in California - removes siting constraints by moving the reservoir underground. 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.

Power from the grid is used to pump water (the pump is shown in green in Figure 5) into the power shaft and raise the piston. When electricity is required, the piston drops, forcing water through the pump that now functions as a turbine, producing electricity from the motor that now functions as a generator. The penstock shaft is used to capture and return water to the system.

The difference between pumped hydro and this technology is that in the Gravity Power system, the water only acts as a carrier of hydraulic force, pushing up the singular, heavy mass, like a giant piston. The water itself does not provide energy storage; it merely allows the gravitational potential energy to be transferred from the weighted block through the water and pump, or turbine to the electric motor-generator and onto the grid.

Gravity Power claims it system can ramp from zero to full power in less than 15 seconds. The overall energy storage efficiency would exceed 80%. Also, siting of the facility is very flexible: 1,600 MW or more can be installed on less than three acres.

Figue 5.Gravity Power's solution.

A similar solution was developed by the German company Heindl Energy/Gravity Storage. The company filed for insolvency this year after running out of cash in late 2020. The technology has been sold to an interim investor New Energy Let’s Go which is now looking for a new strategic investor.

The technology uses a combination of weights and water. Electrical pumps and hydraulics lift a large rock mass resting on a movable piston to store energy. To release power, the water, which is under high pressure from the rock mass, is routed to a turbine and generator. The claimed capacity of energy storage would be between 1 and 10 GW.

Finally, Quidnet has developed a similar underground solution but works with water stored under pressure between rock layers. Quidnet Energy won a contract with the New York State Energy Research and Development Authority for a 2-megawatt/20-megawatt-hour demonstration project of its geomechanically pumped storage. Quidnet will use excess renewable energy to store pressurized water underground in dry oil and gas wells.

Figue 6.Quidnet's system: 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).

Gravity-based technologies Other novel gravity-based technologies are also actively rising in the market. The companies that have been showing significant growth over the past year are:

Energy Vault's core product is a 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 4 to 8 MW of continuous power discharge for 8 to 16 hours, 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.

Among the issues that must be accommodated are the effects of wind on the blocks and cranes while in motion, pendulum effects, cable stretch, and maintaining constant output by sequencing the block “dropping”.

In July 2021 the company has started operating a 110-meter testing facility in Ticino Switzerland where potential customers can try out the system at scale.

The company started the building process for its first gravity storage systems with customers in the second half of 2021. Each facility will take somewhere between nine to 15 months to build, so by the end of 2022 or the beginning of 2023 some energy customers may already have a gravity storage system in operation.

They will start with 40-50 megawatt-hour facilities, but they are modular so they can add to that.

To reduce costs they are doing a partnership with Mexican materials group Cemex to work with recycled construction material to use it for constructing their blocks.

Figure 7.Energy Vault's system.

The Gravitricity system suspends individual weights of 500 to 5,000 tons, each in their own shaft, rather than stacking them out in the open as with Energy Vault. Each weight has a winch that either lifts the weight or releases it, so the dropping weight can power a generator. The company claims that each unit can produce between 1 and 20 MW peak power with an output duration between 15 minutes to 8 hours.

The company has, in April 2021, completed a successful test in Scotland, paving the way for its commercial-scale roll out. Costing £1 million ($1,385 million) to build a 15-meter (49-foot) tower with a 25-ton weight, Gravitricity say the prototype can generate 250 kilowatt hours. The company is already in “advanced discussions” with mine owners in Britain, Scandinavia, Poland, and the Czech Republic over possible locations for initial European projects.

Figure 8.The Gravitricity system.

While Gravitricity is repurposing coal mines. Renewell is repurposing oil wells. Oil and gas companies are obliged to seal their wells at the end of their lifetime. The proposal from Renewell is that before doing that they extend the lifetime of these wells by 30 years, using them for energy storage purposes and letting Renewell operate them and finish the sealing process.

They have built a prototypebut haven’t tested it yet, they are on TRL level 6. They will test it in the field in 2022 with two different US utilities in Texas and California.

What is interesting about Renewell’s solution is not only the opportunity of extending the oil wells lifetime for 30 years more but also the flexibility that their system provides. Each well would be relatively small, and would only be able to store 100kWh per well with 1 hour discharge time. But by installing this system on different wells nearby, you can combine them and match whatever the market needs regarding power and discharge time.

CAES

Compressed air energy storage (CAES) involves taking electricity, using it to compress air to a smaller volumes, and then storing that air underground. Two larger-scale CAES sites have been built in Huntorf (Germany) and McIntosh (Alabama, US), so it’s among the few mechanical technologies that already has commercial deployments.

This is a space we have already covered in one of our Darcy Live Events this year. You can learn more about the different types of CAES types through this Framework and this presentation.

In our Darcy Live Event, we had Hydrostor as the main presenter of the technology which has a pilot operating in Ontario, and who this Wednesday filed an application with the California Energy Commission through a subsidiary to develop a 500 MW 4GWh storage facility in Southern California.

Other companies worth mentioning in this space are Corre Energy and Storelectric, which are both working on storing hydrogen instead of air to increase the storage capacity up to 30 times. In the case of Storelectric, they claim they could back up the entire UK demand in only 40 caverns if they used H2. Storelectric’s proposal is worth looking at as the key advantage they provide is the combination of diabatic and adiabatic CAES and a combination of these to increase the efficiency of their installation. Besides, as opposed to Hydrostor, they plan to work on dry caverns instead.

LAES

A variant of this technology is called liquid air energy storage (LAES), most notably being developed by Highview Power. Using existing technology of turbine compressors, expanders (all proven technologies), and various other cryogenic pieces of equipment, Highview has successfully built and demonstrated a 5 MW/15 MWh facility located near Manchester, UK, utilizing waste heat from GE Jenbacher gas engines.

They are currently building a full commercial plant expected to start operating by the end of 2022 or beginning of 2023. Time to build the plant: 18/24 months.

The main difference with CAES is that there is no underground work: that means lower environmental impact, no geographical constraint, no emissions, and it even cleans the ambient air. Clean air in and even clearer air out.

The storage startup has claimed that the levelized cost for its system is $140/MWh for a 200 MW/2 GWh (10-hour) system, with no use of waste heat or cold. The firm adds that its technology permits “weeks’ worth of storage,” with the use of additional tanks.

CO2 Energy Storage

Energy Dome has a very interesting approach by storing CO2 instead of air. In charging mode, the CO2 is compressed and stored under pressure at ambient temperature in a high density supercritical or liquid state. When energy is to be released, the CO2 is expanded into a turbine and stored back into an atmospheric gasholder, the Dome, ready for the next charging cycle.

By storing in the liquid phase at ambient temperature they significantly reduce the storage costs typical with CAES (Compressed Air Energy Storage) without having to deal with cryogenic temperatures, as in the case of LAES (Liquid Air Energy Storage).

Other mechanical solutions to keep an eye on

The Gravity Soil Batteries Concept

A new concept is proposed by University of Nottingham academics Professors Saffa Riffat (President of the World Society of Sustainable Technologies) and Yijun Yuan (EU Marie Curie Fellow Fellow), who filed a patent application in May 2019 claiming a novel gravity energy storage technology based on drums filled with soil.

The basic concept of Gravity Soil Batteries is shown in Figure 9. The technology uses storage cores (large drums filled with compacted soil) that could be shifted between lower and higher points. The soil for the storage device can be obtained locally by digging the ground to create deep channels for the system. The soil is also used as a filler for the central concrete support structure. Pulleys are mounted on the top of the central concrete structure. The drums are fitted with axial shafts and bearings and are mounted on a metal frame similar to tarmac rollers. The drums could be then pulled on the sloped central concrete structure using cables and a motor/generator. The motor/generator is mounted on the ground to provide a good stability and ease of maintenance. When heavy drums move down, they release potential energy (i.e. electricity generation) to the main grid system (Figure 9). During the discharge phase, the drums are moved upward to store energy supplied by photovoltaic solar power or wind turbines, using power when not needed by the grid, storing the energy for later use.

Figure 9.The Gravity Soil Batteries concept

Mountain Gravity Energy Storage (MGES)

Julian Hunt, an engineering scientist at the International Institute for Applied Systems Analysis in Austria and his collaborators have devised a novel system to complement lithium-ion battery use for energy storage over the long run: Mountain Gravity Energy Storage (MGES). Like hydroelectric power, MGES involves storing material at elevation to produce gravitational energy. The energy is recovered when the stored material falls and turns turbines to generate electricity. The group describes its system in a paper published November 6 in Energy.

MGES technology will be especially useful for grids that have small energy storage demands, says Hunt. These are typically microgrids utilizing less than 20 megawatts, or the amount of energy it takes to power 7,000 four-bedroom houses. The technology can potentially be applied to tiny or isolated islands such as Molokai in Hawaii, the Galapagos, and Cape Verde, where the cost of supplying energy is high, and demand is often seasonal due to tourism.

Figure 9.The Mountain Gravity Energy Storage (MGES) concept

Conclusions

The advantages of mechanical solutions, in general, are their low cost, long lifetime, long duration, and low technology risk. The challenges in some cases have been associated with round-trip efficiency, costs, and site-ability.

In summary, gravity storage technologies are demonstrating clear potential for broader impact. They are site-able anywhere, have fast response times, large storage capacities, high efficiency, low levelized cost of storage (LCOS), no waste materials, long lifetime, and generate jobs and economic movement in the region of deployment. Taking these factors into account, there is a pathway for gravity storage approaches to help provide the hundreds of GW of storage required to enable a planet primarily powered by renewable resources.

Each of these options has interesting tradeoffs in size, siting issues, complexity, cost, potential reliability, and capacity, as well as perceived and real safety concerns. It will be interesting to see which one gains the most traction, or perhaps they all will do well, depending on the specifics of location, power level, local costs, energy needs, and alternatives.

What’s your sense of large-scale practicality of these options for grid-level energy storage? Do you see them as viable alternatives to battery farms, reservoirs, compressed air, or other in-use or proposed approaches?