The Energy Information Administration has done us an enormous favor by producing a simple chart to make sense of where the development of energy storage technology is going. Energy storage, as the EIA defines it, includes heat storage, and a quick look at the chart reveals that those forms that involve sheer physical mechanisms – pumped storage, compressed air and heat reservoirs – are much further along than chemical means of storage, particularly batteries.
The EIA divides the development of technologies into three phases – “research and development,” “demonstration and deployment” and “commercialization.” It also ranks them according to a factor that might be called “chances for success,” which is calculated by a multiple of capital requirements times “technological risk.”
As it turns out, only two technologies that could contribute to transportation are in the deployment stage while three more are in early development. The two frontrunners are sodium-sulfur and lithium-based batteries while the three in early stages are flow batteries, supercapacitors and hydrogen. The EIA refers to hydrogen as one of the ways of storing other forms of energy generation, particularly wind and solar. But hydrogen is also being deployed in hydrogen in hydrogen-fuel-cell vehicles that have already been commercialized.
Other than building huge pumped-storage reservoirs or storing compressed air in underground caverns, the chemistry of batteries is the most attractive means of storing electricity, which is the most useful form of energy. Batteries have always had three basic components, the anode, which stores the positive charge, the cathode, which stores the negative charge, and the electrolyte, which carries the charge between them. Alexander Volta designed the first “Voltaic pile” in 1800 by submerging zinc and silver in brine. Since then, battery improvements have involved finding better materials for all three components.
Lead-acid batteries have become the elements of choice in conventional batteries because the elements are cheap and plentiful. But lead is one of the heaviest common elements and becomes impractical when it comes to loading them aboard a vehicle.
The great advantage of lithium-ion batteries has been their light weight. The lithium substitutes for metal in both anode and cathode, mixing with carbon and iron phosphate to create the two charges. Li-ion, of course, is the basis of nearly all consumer electronics and has proved light and powerful enough to power golf carts. The question being posed by Elon Musk is whether they can be ramped up to power a Tesla Model S that can do zero-to-60 with a range of 300 miles.
Tesla is not planning any technological breakthrough, but will use brute force to try to scale up. Enlarging li-ion batteries tends to shorten their life so the Tesla will pack together thousands of small ones no bigger than a AA that will be linked by a management system that coordinates their charge and discharge. Musk is betting that economies of scale at his “Gigafactory” will lower costs so that the Model X can sell for $35,000. According to current plants, the Gigafactory will be producing more lithium-ion batteries than are now produced in the entire world.
In the sodium-sulfur battery, molten sodium serves as the anode while liquid sodium serves as the cathode. An aluminum membrane serves as the electrolyte. This creates a very high energy density and high discharge rate of about 90 percent. The problem is that the battery must be kept at a very high temperature, around 300 degrees Celsius, in order to liquefy its contents. A sodium-sulfur battery was tried in the Ford “Ecostar” demonstration vehicle as far back as 1991, but it proved too difficult to maintain the temperature.
Flow batteries represent a new approach where both the anode and cathode are liquids instead of solids. Recharging takes place by replacing the electrolyte. In this way, flow batteries are often compared to fuel cells, where a steady flow of hydrogen or methane is used to generate a current. The great advantage of flow batteries is that they can be recharged quickly by replacing the electrolyte, rather than taking up to 10 hours to recharge, as with, say, the Chevy Volt. So far flow batteries have relatively low energy density, however, and their use may be limited to stationary sources. A German-made vanadium-flow battery called CellCube was just installed by Con Edison as a grid-enhancement feature in New York City this month.
Supercapacitors use various materials to expand on the storage capacity devices in ordinary electric circuits. They have much shorter charge-and-discharge cycles but only achieve one-tenth of the energy density of conventional batteries. As a result, they cannot yet power vehicles on a stand-alone basis. However, supercapacitors are being used to capture braking energy in electric trams in Europe, in forklifts and hybrid automobiles. The Mazda6 has a supercapacitor that uses braking energy to reduce fuel consumption by 10 percent.
The concept of “storage” can be also be expanded to include hydrogen, since free hydrogen is not a naturally occurring element but can store energy from other sources such as wind and solar. That has always been the dream of renewable energy enthusiasts. The Japanese and Europeans are actually betting that hydrogen will prove to be a better alternative than the electric car. Despite the success of the Prius hybrid, Toyota, Honda and Hyundai (which is Korean) are putting more emphasis on their fuel cell models.
Finally, methanol can be regarded as an “energy storage” mechanism, since it too is not a naturally occurring resource but is a way to transmit the potential of our vast reserves of natural gas. Methanol proved itself as a gasoline substitute in an extensive experiment in California in the 1990s and currently powers a million cars in China. But it has not yet achieved the recognition of EVs and hydrogen – or even compressed natural gas – and still faces regulatory hurdles.
All these technologies offer the potential of severely reducing our dependence on foreign oil. All are making technical advances and all have promise. Let the competition begin.