On Board Energy Storage

Thursday, July 24th, 2008

Rod Adams explains how on-board energy storage is the reason why automobile engineers chose fossil fuel so many years ago — and why we still use fossil fuel today:

When you look at the products of the reaction in a balanced equation — where all of the input elements are accounted for in the outputs — you will discover that the products weigh about 4.5–5 times as much as the hydrogen and carbon input.

The rest of the weight comes from oxygen. Here is the chemical equation often used to describe gasoline combustion (gasoline is actually a complex combination of various hydrocarbons each with different numbers of carbon and hydrogen atoms, but C8H18 is representative of them all.)

C8H18 + 12.5 O2 ? 8 CO2 + 9 H2O

By mass, only 114 units out of 514 units are in the gasoline, while the rest is in the oxygen. This is important for vehicles because oxygen does not need to be carried — it can be sucked in as needed. There is also no technical requirement — in the absence of new regulations — to capture and store the waste products and carry them around.

The people who developed the internal combustion engines were seeking a way to eliminate the weight of the water, piping and pressure vessels that limited the portability of steam engines. They figured out that they could use the hot products from combustion to directly move pistons and turbines as long as the input fuel did not have too many contaminants that could damage the engine parts. Coal and wood contain a lot of contaminants and both of those solid materials cannot be moved with pumps.

Batteries have to contain all of the chemicals on both sides of their energy releasing equation. The very best batteries available today can store about 0.4 MJ/kg (0.05 kw-hr/lb) including the cases and safety systems. In contrast, gasoline carries about 46 MJ/kg (5.7 kw-hrs/lb).

Even with a 20% efficient IC engine, a gasoline tank stores 20 times as much energy as a battery of equal weight. As the vehicle is moving it gets rid of some of that weight. Battery powered vehicles must carry the full weight of their energy source.

The energy density difference also plays a key role in the time that it takes to put more energy back on the vehicle once a fuel load is consumed. A two minute fill-up of a 12 gallon tank puts the equivalent of 87 kilowatt-hours into the vehicle, again, taking into account the 20% thermal efficiency.

87 kilowatt-hours in 2 minutes works out to 2.6 MegaWatts. Even with a 220 volt connection, that would require about 11,800 amperes of current. Just imagine the size of the electric cables for that current.

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