Interesting write up on the limitations of energy storage technology when harnessing energy from renewable sources like wind, solar, tidal.

Let’s start with batteries. Today’s lead acid batteries can store about 0.1 mega-joules per kilogram, or about 500 times less than crude oil. Those batteries, of course, could be improved, but any battery based on the standard lead-oxide/sulfuric acid chemistry is limited by foundational thermodynamics to less than 0.7 mega-joules per kilogram.

Due to the theoretical limits of lead-acid batteries, there has been serious work on other approaches such as lithium-ion 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 lead-acid batteries by a factor of about 6 to around 0.5 mega-joules per kilogram–a great improvement. But as currently designed, they have a theoretical energy density limit of about 2 mega-joules 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 lithium-ion batteries might break 3 mega-joules per kilogram. Therefore, the maximum theoretical potential of advanced lithium-ion batteries that haven’t been demonstrated to work yet is still only about 6 percent of crude oil!

But what about some ultra-advanced 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 hydrogen-scandium. Assuming that we could actually make such a battery, its theoretical limit would be around 5 mega-joules 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 hydrocarbons–including both fossil carbon and biofuels–are 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.

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 energy-volume density of around 6 mega-joules per liter, while gasoline at 1 bar has about 34 mega-joules per liter. Both hydrogen and hydro-carbons can be produced from renewable energy sources, though doing so economically and at a global scale remains a challenge.

Tags: e=mc2, Fundamentals // 1 Comment »