By George Harvey
Flow batteries have long offered a hope of low-cost, safe energy storage. With many potential designs for flow batteries, we have been waiting for one to come along that may have an effect on the ability of the grid to integrate renewable power easily. ViZn Energy Systems (pronounced “vision”) is now offering a flow battery that we believe is worthy of serious consideration.
In a flow battery, two different liquids flow past each other, usually separated by a membrane. Electrons can pass between the liquids. When the electrons move in one direction, power is produced. If a current is applied to the battery, the electrons move the other way, storing energy. There are a wide variety of combinations of chemicals that can be used as the liquid electrolyte in such batteries.
The power of a flow battery with a given chemistry depends on the total area of the membranes it has. The amount of energy the battery stores depends on the volumes of the liquids. If you are using the battery to run a motor, a bigger membrane can drive a more powerful motor. However, bigger tanks for the liquids means more energy storage, which can make the motor run longer.
Flow batteries are not perfect for all applications. For example, they tend to be rather large. We do not see much discussion of using them to power automobiles or even single households. The ViZn batteries are available in sizes that fit in shipping containers. The smallest fits into a single 20-foot container, provides 160 kilowatt hours (kWh) at 48 kilowatts (kW). The largest fits into two shipping containers of 40-feet each, which are set up as a two-story unit in the field. Of course, multiple batteries can be set up together.
Nevertheless, they have some really wonderful attributes. They last longer than nearly all other battery types, and they are simpler to repair. Generally, they can be fully discharged without damage. In fact, they can see some really hard use for a long time without any reduction in reliability or power. The ViZn battery comes with a twenty year warranty and optionally can be guaranteed to provide at least 95% of its rated power for that time.
Historically, one problem with flow batteries has been that the chemicals used in the liquids can be expensive. For example, vanadium flow batteries have been proven reliable, but vanadium is quite expensive. ViZn’s flow battery is based on zinc and iron, two very abundant metals, making for a comparatively inexpensive battery, with a cost about 75% to 80% of a vanadium battery’s.
Some other flow battery chemistries use bromine or iodine, which are not especially expensive but are powerfully corrosive. The ViZn battery liquids are alkaline, which makes them less corrosive to hardware than the acids used in some other batteries. Other benefits there are no compounds in the battery that are environmental hazards; there is nothing that is explosive or flammable.
Another consideration for any battery is a question of how well it delivers power or energy – how big can the motor be and how long can it run. Most batteries are designed for one or the other. For example, the drive batteries in a Prius are designed to provide a lot of power, but not for more than a couple minutes. Some batteries can provide a lot of energy, provided that you do not call for too much at one time. ViZn’s flow battery is designed to provide both power and energy, so it can be discharged fully, multiple times each day, for its full lifespan.
Under certain circumstances, large batteries pay for themselves rather quickly. One example is a remote site, such as a mine or military base, where the cost to transport diesel fuel can be up to $10 per gallon. In a combat area, transportation of diesel oil also exposes a convoy to attack.
Another example is support for a large solar array. A recent solar auction in Dubai saw a power purchase agreement bid at under 3¢ per kilowatt hour for 800 megawatts. A transmission line providing that amount of power would cost $1 million per mile. If half the energy is stored in batteries, not only does the delivery time for the solar power go well into the night, but the amount that must be delivered at any given time is cut in half, cutting the cost of the transmission line in half with it.
Another potential use of the ViZn battery is arbitrage, a financial practice of making money on the difference of prices between two markets. The cost of the battery is low enough that it can make money for its owner by storing energy purchased when the price is low, usually at night, and selling it during high demand times, usually during the daytime, when the price is high.
After seven years of research and development, ViZn began installing their batteries two years ago. They are now installing a 2-megawatt, 8-megawatt-hour system in Ontario, which will be the largest flow battery in North America – at least until something bigger comes along. It will be interesting to watch this company develop.
ViZn’s website is viznenergy.com.