2/2/2015 The limits of energy storage technology | Bulletin of the Atomic Scientists BE A DONOR COLUMNISTS (/FEATURETYPE/COLUMNISTS) 1 01/20/2009 13:27 The limits of energy storage technology Tweet 43 Kurt Zenz House Like Editor's note: The following column was coauthored by Alex Johnson, a postdoctoral fellow at Harvard University. 1 For the past several yearsuntil the credit crisis investors have flocked toward renewable energy. Their hope is that solar radiation can be harnessed directly and through intermediaries such as the wind and biosphere to power the global economy into perpetuity. This hope is understandable since renewable energy has benefits that range from the environment to geopolitics. Nevertheless, care and scientific rigor should be used to quantify the challenge of converting society to renewable energy. 2 The maximum theoretical potential of advanced lithiumion batteries that haven't yet been demonstrated to work is still only about 6 percent of crude oil." (/bio/kurtzenzhouse) KURT ZENZ HOUSE S (/BIO/KURTZENZ HOUSE) House is the Chief Executive and a cofounder of C12 Energy. For nearly a decade, House has studied the physics, chemistry, and economics of capturing and storing anthropogenic carbon dioxide in... More (/bio/kurtzenz house) SUBSCRIBE (/BIO/196/FEED) The most significant challenge to renewable energy is competition from fossil carbon the world's predominant source of primary energy for the past 150 years. Fossil carbon has dominated the energy market for many reasonsnot the least of which is its intrinsic mass and volume energy densities. Indeed, 1 kilogram of crude oil contains nearly 50 megajoules of chemical potential energy, which is enough to lift 1 metric ton to a height of around 5,000 meters. Furthermore, crude oil happens to be liquid at Earth's surface conditions, making it easy to store, transport, and convert. The energy densities of natural gas and coal, around 55 megajoules per kilogram and 2035 megajoules per kilogram respectively, are similar to those of crude oil. Fossil carbon is packed with chemical energy because carbon and the hydrogen it stabilizes in a condensed form react strongly with oxygen to form carbon dioxide and water. In addition, geologic processes have concentrated large quantities of fossil carbon into relatively small geographic areas such as coal mines and oil fields. Biofuels such as ethanol and biosynthetic diesel can have volume and mass energy densities equal to that of fossil carbon, but since they're regularly harvested, their areal energy densities are substantially lower. Renewable energyunlike fossil carbonis harnessed dynamically from the http://thebulletin.org/limitsenergystoragetechnology 1/3 2/2/2015 The limits of energy storage technology | Bulletin of the Atomic Scientists environment. Therefore, it won't be as useful as fossil carbon until it can be stored and transported with similar ease. Many companies and scientists are diligently trying to improve energy storage technologies, and we're confident that substantial progress will be made. We can, however, use thermodynamics to calculate the upper limits of what's possible for a variety of technologies. And when we do this, we find that many technologies will never compete with fossil carbon on energy density. Let's start with batteries. Today's lead acid batteries can store about 0.1 megajoules per kilogram, or about 500 times less than crude oil. Those batteries, of course, could be improved, but any battery based on the standard leadoxide/sulfuric acid chemistry is limited by foundational thermodynamics to less than 0.7 megajoules per kilogram. Due to the theoretical limits of leadacid batteries, there has been serious work on other approaches such as lithiumion batteries, which usually involve the oxidation and reduction of carbon and a transition metal such as cobalt. These batteries have already improved upon the energy density of leadacid batteries by a factor of about 6 to around 0.5 megajoules per kilograma great improvement. But as currently designed, they have a theoretical energy density limit of about 2 megajoules per kilogram. And if research regarding the substitution of silicon for carbon in the anodes is realized in a practical way, then the theoretical limit on lithiumion batteries might break 3 megajoules per kilogram. Therefore, the maximum theoretical potential of advanced lithiumion batteries that haven't been demonstrated to work yet is still only about 6 percent of crude oil! But what about some ultraadvanced lithium battery that uses lighter elements than cobalt and carbon? Without considering the practicality of building such a battery, we can look at the periodic table and pick out the lightest elements with multiple oxidations states that do form compounds. This thought experiment turns up compounds of hydrogenscandium. Assuming that we could actually make such a battery, its theoretical limit would be around 5 megajoules per kilogram. So the best batteries are currently getting 10 percent of a physical upper bound and 25 percent of a demonstrated bound. And given other required materials such as electrolytes, separators, current collectors, and packaging, we're unlikely to improve the energy density by more than about a factor of 2 within about 20 years. This means hydrocarbonsincluding both fossil carbon and biofuelsare still a factor of 10 better than the physical upper bound, and they're likely to be 25 times better than lithium batteries will ever be. What about storing energy in electric fields (i.e., capacitors) or magnetic fields (i.e., superconductors)? While the best capacitors today store 20 times less energy than an equal mass of lithiumion batteries, one company, EEstor, claims a new capacitor capable of 1 megajoule per kilogram. Whether or not this claim proves valid, it's within about a factor of 2 of the physical limit based on the bandgap of the dielectric material. Electromagnets of hightemperature superconductors could in theory reach about 4 megajoules per liter similar to our theoretical batteries given a reasonable density; existing magnetic energy storage systems top out around 0.01 megajoules per kilogram, about equal to existing capacitors. Here again, both the realized technology and its ultimate physical potential are far behind the energy density of common hydrocarbon fuels. That brings us to the option of storing chemical potential energy as fuel that can be oxidized by atmospheric oxygen. We do it today, but with two differences: We generate this fuel renewably and convert it to work more efficiently than in combustion engines, either by fuel cells or air batteries. Zinc air batteries, which involve the oxidation of zinc metal to zinc hydroxide, could reach about 1.3 megajoules per kilogram. But if we take elemental zinc all the way to zinc oxide, then we can theoretically beat the best imagined batteries at about 5.3 megajoules per kilogram. Zinc has proved interesting enough that several writers (not us) have imagined a "zinc http://thebulletin.org/limitsenergystoragetechnology 2/3 2/2/2015 The limits of energy storage technology | Bulletin of the Atomic Scientists economy." To get really ambitious, we imagine storing energy as elemental aluminum or elemental lithium. Those two highly electropositive elements yield a theoretical energy densitywhen oxidized in airof 32 and 43 megajoules per kilogram. At least now the theoretical limit is between 60 percent and 80 percent to that of hydrocarbons; we just have to figure out how to extract a large fraction of the energy from that oxidation. A more promising approach is to use fuel cells with liquid and gaseous fuels. The two obvious choices for such fuels are hydrogen and hydrocarbons; in terms of energy per unit mass, hydrogen beats crude oil and natural gas by a factor of almost 3. Alas, hydrogen is a gas at surface conditions, so its volume density is horrible unless it's compressed to several hundred atmospheres of pressure. At 700 bars, for example, hydrogen has an energyvolume density of around 6 megajoules per liter, while gasoline at 1 bar has about 34 megajoules per liter. Both hydrogen and hydrocarbons can be produced from renewable energy sources, though doing so economically and at a global scale remains a challenge. There is one more energystorage approach that theoretically beats hydrocarbons. Energy density comparable to lithiumion batteries has been demonstrated with flywheels, and a theoretical device composed solely of toroidal carbon nanotubes could reach 100 megajoules per kilogram. But the fabrication and safety challenges inherent in such a device render it unlikely that even a small fraction of this potential will ever be realized. The bottom line is that nature has given us hydrocarbons in the form of fossil carbon and biomass, and their energymass and energyvolume densities are superior to the thermodynamic limits of nearly all conceivable alternatives. Thus, it's quite likely that hydrocarbons of one form or another will be humanity's primary energy storage medium for quite a long time. Tweet 1 Like 43 1 2 S (/ fe 0 Comments Bulletin of the Atomic Scientists e d Login s)★ Share ⤤ Favorite Sort by Newest Start the discussion… Be the first to comment. ✉ Subscribe d Add Disqus to your site http://thebulletin.org/limitsenergystoragetechnology Privacy 3/3
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