After a quick holiday break over Christmas and the New Year, we get back to the blog series on energy storage technologies. I hope you all had an enjoyable time!

In this 5th episode, we cover storing electricity as electricity – not converting it into any other energy form. “At last!” you cry, “We are getting to batteries. That’s what I thought energy storage was all about!”. Well, not yet. As a chemist, I see lead-acid, lithium-ion and all the other battery types as storing the energy in chemical form, not electrical. We will get to battery technology in the next post.

In the meantime, let’s look at two forms of energy storage which do directly store electricity; supercapacitors and superconducting magnets. Both well-established technologies, and both used commercially for grid-scale energy storage and energy services.

How do they work?

Separate the charges

A basic capacitor consists of two conducting plates separated by a non-conductor. Perhaps two aluminium plates separated by a plastic film. Connect it to a source of electricity, and the plate on the negative side of the source accumulates excess electrons, and the plate on the positive side of the source loses electrons. Remove the source and you have electricity stored in the capacitor that you can use to drive an electrical circuit. The energy is stored by separating the charges, like static electricity. If you have ever rubbed a balloon on your hair and then used it to pick up small bits of paper, or witnessed a lightning strike, you have seen this process in action.

Structure of a supercapacitor

The size and material of the electrodes, the electrode spacing and material filling the gap all affect how much electricity you can store. Capacitors don’t hold as much electricity as chemical batteries for the same size and weight, but you can charge and discharge a capacitor faster. Capacitors get big fast if you want to store a lot of energy, so the recent focus has been on supercapacitors (also called ultracapacitors). These use a high-surface-area electrode material, such as carbon, a liquid electrolyte and a porous separator. Positively and negatively charged ions in the electrolyte solution are attracted to the oppositely charged electrodes, storing the energy in the electrical double-layer formed by charged ions and electrode. It is still a capacitor, not a battery, since no electrochemical reactions take place.

High surface area electrodes means you can pack more charge separation in and supercapacitors have about 100 times the energy density (in Wh/kg) of conventional capacitors. The price you pay for storing more energy is slower charge and discharge times and lower operating voltage. Charge and discharge times are in the range milliseconds to seconds, 1000 times slower than a conventional capacitor, but still fast compared to batteries. The problem of low voltage is solved the same way as a large lithium-ion battery; arrange multiple units in series and parallel to give higher voltages and more capacity.

In a direct comparison with lithium-ion batteries, supercapacitors have 20-100 times less energy density (Wh/kg), but 5 times the power density. They have about the same efficiency, but 10 times the operational life and can charge and discharge several hundred times faster. They are also safer and cheaper, as they cannot catch fire and don’t contain toxic materials.

Companies such as Maxwell Technologies, Skeleton Technology and Eaton all make supercapacitor modules for a wide range of applications where a modest amount of power is needed, but it must be delivered quickly. supercapacitors are very useful for dealing with sudden spikes in generation or demand on electricity grids. Helping to maintain clean and stable power supplies.

A growing area of application is in hybrid energy storage systems. Supercapacitors provide instant response and batteries or pumped hydro slower back-up. UPM Energy is using a supercapacitor at its Ontojoki hydropower plant in Finland to balance the load, giving the hydroelectric generators time to adjust.

Chemists, physicists and engineers are creative folk, so there are also various hybrid schemes where electrostatic energy storage combines with electrochemical storage in the same device. Creating something between a supercapacitor and a battery. As you would expect, these have higher energy densities and slower response times. There is now a continuum in performance between supercapacitors and batteries. You just chose the approach with the right characteristics for the job.

Cut your losses with superconductors

It is a feature of a superconducting coil that electricity can be stored in its magnetic field even after the power input is disconnected. The coil must be made of a superconducting metal alloy and cooled to below its transition temperature. Traditionally, this meant liquid helium temperatures (4K, -269°C), but materials are now available that are superconducting up to the boiling point of liquid nitrogen or even higher (77K, -196°C). With a superconducting coil, you can store and recover electricity with high round-trip efficiency, and quickly (milliseconds).

Diagram of a superconducting magnet energy storage system showing the key components - superconducting coil in a cryostat, refrigeration system, and connections to power conditioning and control system

Structure of a Superconducting Magnet Energy Storage System

This makes Superconducting Magnetic Energy Storage (SMES) an attractive possibility. Unlike batteries, SMES can be fully charged and discharged for thousands of cycles with no damage or performance degradation. They are environmentally friendly, and the high-value superconducting wire can be recovered at end-of-life. However, there are some obvious disadvantages. SMES systems are expensive to build, and also expensive to run because they must be refrigerated to very low temperatures. And whilst they can deliver very high output powers (10MW), they do not have a very high energy capacity.

SMES systems have been researched since the 1970s and in commercial use since the 1980s. At first there was interest in building large systems for long term storage of large amounts of electricity. Something to rival pumped hydro schemes. Unfortunately, the economics aren’t right yet. The current use is for smaller, distributed, quick response storage to provide a range of grid services. Things like load levelling, frequency stabilisation, preventing voltage sag, controlling fluctuations in high voltage transmission lines and interconnectors. They also find use in providing clean power to heavy industrial users, such as microchip fabs.

SMES is often considered as part of a hybrid energy storage system, just like supercapacitors. It provides ultrafast response backed up by slower, but higher capacity storage.

Work is continuing to reduce the costs, increase the energy density and simplify the manufacturing and operational complexity of SMES. There are still hopes for large scale storage, but for the moment SMES competes with supercapacitors for grid-support services requiring very fast response times. With less spinning reserve on the grid to provide the mechanical inertia to stabilise grid frequency, and the increase in renewable generation, these grid-support services are vital to developing a 100% renewable electricity grid.

Next up – Batteries and chemical storage

Previous articles:

Electricity Storage Options 5 – Pure Electricity
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