Highview Power cryogenic power plant for Vermont. (Courtesy image)
George Harvey
Our least expensive and least polluting forms of electricity generation happen to be intermittent or variable. We can bring them up to being highly reliable, highly responsive forms of energy by the simple addition of energy storage. But a question arises about what sort of energy storage we should use.
The answer to that question is not a problem because of a small number of difficult choices. It is quite the opposite. There is a huge number of choices, probably more than most people would care to count. In many cases, the question will arise only to find which of several choices is best. And there is a lot of research being done, so the situation is changing rapidly. Here is a look at what is new.
Batteries are only one type of choice. There are systems that involve gravity, including pumped hydropower and others. There are big flywheels to provide the grid with a quick power boost. Chemicals can be made and then used to generate power, hydrogen being one example. Compressed air is another choice with surprising potential. Energy can be stored as heat. And all of these are in use today for large-scale applications, and almost all of them are undergoing further development. Other technologies that are already used for smaller situations are also being developed for large ones, and capacitors are an example. Let’s look at some of these, and how they are being developed and improved. Please note that this list just touches on the subject, which is the object of intense scientific development worldwide.
Batteries
Rechargeable batteries have been around for a long time. Lead-acid batteries were invented in 1859 by Gaston Planté, a French scientist. Since that time, many more rechargeable battery types have shown up.
Today, a lot of the attention on batteries is focused on lithium-ion and lithium-iron phosphate batteries. The question of cobalt has been largely solved, from a technical point of view. But there are questions about the availability of lithium and environmental damage in extracting it.
One example of efforts to address a potential lithium shortage is to produce lithium by extracting it from sea water. Lithium compounds tend to be very soluble in water, so over the millennia most of our lithium has wound up in the ocean. Recent research shows that the concentration of lithium in the water can be increased from 0.2 parts per million (ppm) to 9,000 ppm at a cost of only $5 per kilogram of lithium, making it very worth extracting. The study on this was published in the journal Energy & Environmental Science (https://rsc.li/3qIOBHW).
Another approach is the sodium-ion battery, which does not use lithium at all. Part of the problem with sodium is that the atom is much larger and heavier than lithium, so the batteries are bigger also, possibly preventing their use in road vehicles. On the other hand, sodium-ion batteries promise to be much less expensive than lithium-ion batteries, because sodium is so very abundant. Also, sodium batteries will probably be very safe. The work on such systems is underway in many places. An article at Wikipedia covers the subject (). Other batteries, such as potassium-ion, also show potential.
Yet another battery technology worth mentioning is iron-air. These batteries produce power when iron in them rusts. They can be charged by applying electricity that converts the rust back to iron. The fact that the material in use is iron makes these batteries very inexpensive, non-toxic, and safe. A company in Sommerville, Massachusetts, Form Energy, has been developing such systems and is now installing its first system, a one-megawatt (MW), 150 megawatt-hour (MWh) battery in Minnesota. A second system, at 15-MW, 1,500 MWh is being designed for Georgia Power. These systems are expected to reduce to an “ultra-low cost” compared to that of lithium-ion systems, according to an article in Greentech Media ().
Flywheel for energy storage. (NASA image, public domain)
Flow batteries are very different. In them, solutions flow past each other, usually with a membrane between them. Electrons go through the membrane, allowing the battery to charge or discharge. Flow batteries tend to be too big for use in vehicles, but they have numerous advantages. There are many types, and a lot of work is being done to develop them. The article at Wikipedia may be helpful for those who want to know more ().
Gravity
There are several types of gravity storage systems. The one that has been getting attention in the news lately was developed by Energy Vault. This system has a tower with apparatus for lifting very large, heavy blocks to some height. When power is needed, the blocks are allowed to go back down, generating electricity as they do. A recent article in CNN explains this further (www.cnn.it/3Nxnd9A).
Chemicals
A lot of work is going into what is called “green hydrogen.” Most hydrogen is currently made using natural gas or coal, but green hydrogen is made by hydrolysis of water using renewable energy. Hydrogen can be used in a number of ways to create electricity or be used for fuel, but it is also important as a chemical. Some green hydrogen projects are intended to replace natural gas. A post at Rethink Technology Research says Fortescue Industries, an Australian industry giant, pledged to put up $50 billion Australian ($32.5 billion US) to replace one third of the UK’s natural gas with hydrogen ().
Heat
We should consider heat for storage. Energy has been stored as heat for conversion to electricity later for some time. A lot of research is being done to use heat as much as months after it is stored. We might feel that this technology is still young. An article on the subject is in Wikipedia ().
Compressed air
There are different ways to store energy by compressing air. One is to compress it to the point that it liquefies, then boil it to turn turbines, using just ambient heat. Highview has a project underway in Vermont to do just that. An article in Energy Storage News says that Highview has raised $70 million to build such projects, including one in northern Vermont ().
The cost of storing energy is declining rapidly. As it does, more options are becoming available.
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