Making Renewable Energy more Reliable with Giant Gravel Batteries

Published on Tuesday, 22 June 2010 10:19

By Vadim Sablo

An article published by Guardian.co.uk recently, suggested that Giant gravel batteries could make renewable energy more reliable. Arguments against other forms of renewable energy, such as wind or solar, state that these energy sources are not reliable enough to provide constant power.

This new technology may allow the storage of energy, accumulated by wind turbines or solar panels, using gravel. Ask The Experts spoke to Jonathan Howes, Technical Director at Isentropic, who was also formerly Head of Loads and Dynamics for the UK Civil Aviation Authority. He gives a deeper insight into gravel batteries.

Hi Jonathan, please explain how gravel batteries work? JH- The Isentropic PHES system utilises a highly reversible heat engine/heat pump-to-pump heat between two storage vessels containing particulate mineral (in its simplest form, gravel) In fundamental terms, charging is achieved by driving the machine as a heat pump via an electric motor and pumping heat from one vessel to the other, the first vessel then

becomes considerably cooler while the other vessel’s temperature is increased by the pumped heat. Discharge is achieved by allowing the pumped heat to flow back through the machine acting as an engine. In more detail, a gas, for example, argon, circulates through the machine and, to store energy, is first compressed, which raises its temperature (to typically 500deg c). It is then passed through one of the stores in which it heats the mineral by direct contact, which also cools the gas to somewhere close to the original temperature.

It is then expanded back to its original pressure, which cools it (to around -160 deg c) and it is then passed through the other store where it cools the mineral by direct contact and the gas is then warmed back to close to its original temperature. This process requires energy, which can be supplied electrically via a motor and is thus an energy storage process. Discharge is the opposite of the charge process and releases energy, during which the machine drives an electrical generator.

VS - What are the benefits of this technology, especially compared to other ways of storing electricity like pumped hydro (where excess electricity is used to pump water up a hill).

JH- The primary benefits are that the technology is independent of any specific geographical constraint, it is low cost, compact, safe, efficient and environmentally inert, that is, it does not use damaging refrigerants, chemicals or water. We can demonstrate a round trip electrical efficiency of 72% to 80% and a potential storage build cost of $10 per kWh for a utility-scale application, which is the lowest cost of any storage technology, including battery and pumped hydro.

Pumped Hydro is an excellent and well-proven technology where the local geography allows its use, however, there is a diminishing number of suitable sites and the sites are rarely co-located with the best grid points for storage. Other forms of storage such as flow batteries are significantly more expensive than PHES and also often involve the use of environmentally problematic chemicals, some of which are of very limited availability. Isentropic’s PHES uses conventional metals within the machine and the storage medium are very lightly processed (crushed and graded) mineral.

VS - What would be the cost to launch this electricity storage technology?

JH- Isentropic has developed the machine via three prototypes to date. The stores have been developed independently of the machine and the next project is to produce a full storage demonstrator. This is likely to be a scaled plant and the design will allow rapid scaling up to small utility sizes. The budget for development of a first utility scale plant of 2MW power with 16MWhr storage is estimated to be of the order of £10M sterling.

VS - Will it be possible to use it for household use (small “Gravel Batteries”) or it is aimed at large projects?

JH- Isentropic has considered domestic scale storage which has the appeal of being highly distributed with the ability to act as a system damper. This could have a significant grid load levelling effect. However, rather than a proliferation of micro-scale systems it is probably more effective to install storage on a district scale at sub-station level since the reduction in plant-count is likely to be both a cheaper and more readily maintained option. Large scale installations are also possible and require approximately 1/300 of the land area of an equivalent pumped hydro plant while providing equivalent capability and performance.

VS - When would we be able to see first prototype or commercial version?

JH- First prototype machines have already been built and tested. The third prototype is under construction at present. The timescale for the first fully integrated storage system is 18 months for a scaled device of 100 to 200 kW and two and a half years for a small utility scale machine.

VS - What is the reaction of investors/governments, are they ready to spend money on the technology?

JH - Isentropic has had considerable interest from both investors and government bodies. We are currently in final negotiations with backers for the next development steps.

VS - How feasible does it look, as there are fears that gravel batteries may not function?

JH- Any new technology has risks, however, the prototype machines have performed to specification or better and validated mathematical modelling. This de-risks further development. Some comments regarding mineral passing through the engine have been made elsewhere. This indicates a lack of understanding of the machine since the mineral is a static component and is subjected to very low gas flow velocities (typically less than 1/3 metre/second). The temperature range is easily achievable with a single phase; gaseous working fluid and the selected operational temperatures are within the capability of conventional metallic materials.

VS - What about other energy storage technologies? Is there anything similar in competition?

JH- Pumped Hydro is the obvious competitor on cost and efficiency grounds, however, Isentropic’s PHES appears to be equivalent or better in both respects. Current load peaking is normally addressed by open cycle gas turbines and these are still the primary competition, although they do not directly address the renewable energy intermittency challenge if the objective is to avoid the use of fossil fuels. Battery technologies are uneconomical at large scale although effective for short duration high power output applications, as are flywheel systems. The natural place for PHES is as a direct alternative to pumped hydro and in conjunction with renewable energy sources for load levelling.

VS - What do you think about the future of Gravel Batteries?

JH- Isentropic has the lowest cost storage option under current development. Being equivalent to pumped hydro but without geographical constraint gives PHES the ability to re-draw the energy storage landscape. By providing a cost-effective solution to the intermittency problems of renewable energy sources, it represents a key enabling technology for their widespread adoption. Without a storage solution, renewable energy sources cannot meet on-demand power unless excess generation is stored for later release.

Further reading:

Links

Guardian Article on Gravel Batteries

Green Energy News

Wordchanging Change Your Thinking article

Breakthrough in Energy Storage

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