The world of energy storage is full of non-intuitive metrics that can be hard to compare, visualize or place a value to. Engineers love the precision of metrics like amp hours, farads and megajoules, but do you know many people who can tell you how far they can drive on 20 megajoules of energy? Worse yet, what would you see if I asked you to close your eyes and visualize a 50-kilowatt battery pack? Did you imagine something the size of a toaster or was it more like a trash can? What would you use such a device for, and better yet, what would you be willing to pay for it? My point is that in the highly technical world of energy storage, it can difficult to develop an intuitive sense of how the engineering metrics translate to final user value.
One way to build an intuitive understanding is to equate batteries and capacitors to their energy equivalent all in the size of a gallon jug. We all know the size of a 5-gallon gas can or a gallon of milk, and most of us are intimately aware (more than we would like to be) of the price of a gallon of gas. While gasoline may be a very energy-dense material, it cannot be directly converted to electrical energy; however, you’ll soon see why the reference will be useful, taking a few liberties with calculations. Using a rough calculation of 20 percent conversion efficiency, we can uncover some interesting insights. For example, 1 gallon of lithium-ion batteries only stores the electrical energy equivalent of about 2.5 cups of gasoline. Thought about this way, my truck’s 20-gallon gas tank would only hold 3 gallons of gas if it held the equivalent amount of energy as 20 gallons of batteries. This clearly illustrates why electric vehicle designers and consumers have range anxiety. I too worry about range when my truck gets down to 3 gallons in the gas tank.
This problem isn’t any better when you compare higher-power devices, such as lithium titanate batteries or ultracapacitors. For lithium titanate batteries, a gallon of cells stores the energy equivalent of about 1.5 cups of gas. One gallon of ultracapacitors, which is about eight of Maxwell’s largest cells, would give just 2 teaspoons of gas equivalent or less than a tenth of a gallon in my 20-gallon tank. Visualizing that nearly empty gas tank, you can see why no one is proposing to build electric vehicles powered solely by power batteries or ultracapacitors.
But storing energy is only one way that batteries and capacitors provide value to users. In many applications, such as hybrid vehicles or drilling equipment, the key metric is not how much energy a device can store, but rather how much it can save over its lifetime, by recuperating energy that would nominally be lost when the vehicle brakes or the drill lowers. In these cases the gallons of gasoline saved over the lifetime of the battery or capacitor is the value of that device to the end user in a very real sense. Of course, to save a gallon of gas, its energy equivalent must be first stored and then released back into the electrical system so both energy density and cycle life heavily influence the energy storage system’s value to the customer.
For example, that gallon of lithium-ion batteries that stored about 2.5 cups of gas? It will wear out after a few thousand cycles, saving the equivalent of about 200 gallons, or four barrels, of gasoline over its lifetime. Not bad, but still far short of high-power technologies like lithium titanate, for which a gallon of cells would save nearly 20 barrels of gas over their lifetimes. Here, then, is where ultracapacitors shine; with cycle life measured in millions, a gallon of ultracapacitors can save the energy equivalent of about 100 barrels of gasoline over their lifetimes. Saving 100 barrels of gasoline is about the same as annual greenhouse gas emissions from 13.4 tons of waste sent to the landfill or the carbon emissions from five homes’ electricity use for a year, according to the Environmental Protection Agency. That is a value that is easy to visualize.