Gravity storage

Lately, a new definition has been given to energy storage systems based on using surplus energy to lift a certain mass, and to recover later it by returning it to its initial position. In fact, this is also the principle of hydropower and pumped-storage power plants, where the mass consists of a large quantity of water pumped to an upper reservoir and then, turbined to a lower reservoir.

Although this type of power plant is currently the most reliable type of large-scale energy storage; other types of systems are recently being proposed, where the mass activated consists of one or more solid bodies, what presents certain advantages:

• Materials with substantially higher density than water can be used, therefore it reduces storage dimensions for a certain energy capacity.
• This sort of mass can be easier to manage by maintaining their shape without the need of being put in storage in deposits or reservoirs.

It is usual these new systems use reversible motors, coupled to drums or pulleys, so that they consume electricity when the masses are lifted up to their upper position, while they are working as generators when the masses spin them as they descend in a controlled manner, at the same time as they produce energy (similar to regenerative braking in electric vehicles).

Now, we will mention some companies that are working on this type of storage and we will briefly review the concepts they propose.

Energy Vault

This company located in California and Switzerland, has patented a system based on the stacking and unstacking of large blocks. In 2020, it built a demonstrator prototype in Arbedo-Castione, Switzerland, the EV1 Tower, which consisted of a 120 m high crane with six booms, placed on a large esplanade where it was stacking and unstacking 35-tons concrete blocks.

They have recently dismantled this prototype and focused on the evolution of their concept, called EVx.

In this new design, instead of using a crane, they have built a huge metal structure (about 100 m high) where multiple lifting points have been installed, and horizontal rails set to allow each block to move towards its storage position while the next block is being lifted. In this case, they propose to compose the blocks of a composite made from local materials, to reduce their cost and avoid the environmental impact of concrete consumption

According to the technologist, the construction of the first plant of this type began a few months ago, with a capacity of 100 MWh, located in China next to a wind farm.

Gravitricity

It is a Scottish company that proposes to use vertical shafts to displace a large mass to a depth of between 150 and 1,500m. The initial approach is to take advantage of old, disused mine shafts to reduce the investment required, although the possibility of building new shafts is also being considered. The installation includes electric winches that work coordinated in pairs to raise or lower huge masses of between 500 and 5,000 tons.

The system is designed to raise and lower a certain number of mass successively, while a pair of winches move the mass in one direction, the other pair of winches bring back the hooks of the previous mass so that a new one can be picked up.

At the moment there is only a small 250 kW test bed in a Scottish port,  although the company says it is selecting a disused mine shaft for its first underground project, but still using a single weight, so the battery discharge time will be in the order of minutes. For the first multi-mass installation with they are considering building an ad hoc well in a 4 MWh project, intended for grid ancillary services.

ARES

The concept of this American company is based on moving the mass over an inclined surface, rather than vertically, as train carriages on their tracks.

The advantage of this system is that it can move successive masses between  two heights and, with the same means, bring them to the storage position, once the expected height has been reached. The disadvantage is that it requires more surface for the same accumulated energy, and its performance may be lower than a vertical moving system.

Two years ago, they started the construction of the first 50 MW plant in Nevada, 210 railcars totalling 75,000 tons were designed to travel on a 10-track system. However, since then there has been no news about its completion and operation, so it may not have been successfully put into operation.

Gravity Power

This company proposes what could be called a hydraulic-gravitational storage, as in this case, the movement of a large solid mass is carried out by injecting/extracting water into a closed chamber, as what happens inside a hydraulic cylinder when a piston is displaced, injecting or extracting oil under pressure. As in the case of Gravitricity, the installation would be carried out in an underground shaft, using the shaft of a disused mine, or drilling it expressly.

Probably the main advantage of this system is that, instead of moving a large mass by cables (which entails technical difficulties in terms of stability, safety and wear and tear, and requires the use of heavy and expensive machinery), it uses pressurised pipes and reversible turbines, a very reliable technology with fewer uncertainties. However, this concept also faces important challenges, such as the sealing of the borehole and the efficient sealing between the “piston” and the chamber where it is displaced inside.

In any case, it seems that this design is still at a conceptual stage and there is no evidence of any real installation of this type, even at a demonstration level.

Conclusions

Revising the essence of this sort of energy storage, seems obvious it has not reached yet a degree enough to demonstrate its commercial viability. 

However, compared to the technologies that currently dominate the industry, gravity storage has the attraction of combining some of the specific advantages of pumped storage altogether with those specific in electrochemical batteries.

• A gravity storage system can be installed in any location, with similar conditions as the ones found in any other industrial installation. It also happens for conventional batteries, but not for pumped storage plants, which are subject to topography and environmental constraints, what extremely limit viable locations.
• Degradation over time of their capacity is virtually zero and their lifetime can last many decades, with relatively low maintenance costs; being similar to pumped storage plants in contrast to electrochemical batteries, which gradually lose their charge capacity and have to be replaced by new ones after a certain number of discharge cycles. 

Moreover, the efficiency of the full charge-discharge cycle can be around 80%, a slightly lower amount than in lithium-ion batteries, but chiefly higher than reversible pumped storage plants and, above all, much higher than the amount achieved if hydrogen is produced from surplus electricity and then used in fuel cells to produce electricity again, which hardly gets to 50%.

The big problem with gravity batteries is their low energy density, i.e. they require a large amount of materials and strong infrastructure to store a relatively small amount of energy. This can be visualised if we consider storing the energy required by a standard (energy class A) refrigerator for a single day, it would be necessary to lift the weight of a large van (with the maximum authorised load) to the roof of one of the “Kio Towers” in Madrid.

Consequently, it is expected that the initial inversion required by this sort of installation will be significant, at least if we intend it has a capacity of several MWh (in a few words, it would be enough to keep thousands of refrigerators working during a day). Nevertheless, some companies such as Energy Vault claim that they are ready to reach a 40% less storage cost than the lithium batteries, generating expectation with their first pre-commercial projects.

In any case, as happens in Energy production, it would be likely that the optimal solution for the issue of Energy storage was combining different technologies as a combination where each of them will contribute with their strong points in certain situations. We will have to wait years to know if the gravity batteries are meant to be part of this solution so required by our planet.