Solar Fuel - Storing Energy in Metals

October 17, 2004 – Vol.9 No.30

SOLAR FUEL - STORING ENERGY IN METALS.

When they come off the assembly line, primary batteries, like disposable, non-rechargable flashlight batteries, have an initial charge. It is the is only charge they will ever have.

The charge they get is not from passing electric current flowing through them, but is from the electrochemical reaction between the materials used make the batteries. One of those materials is typically a sacrificial metal anode made of zinc.

When a primary battery is put into a circuit (a flashlight for instance) and the circuit completed (flashlight on) the reaction begins and will continue until the zinc is fully oxidized or all the oxygen in the electrolyte is used up. So much for that battery. Throw it away.

Metal-air fuel cells are a spin-off of primary batteries. The difference is that the sacrificial metal anode can be replaced when fully oxidized, corroded, or spent if you will. And the electrolyte is continually re-oxygenated so it isn’t thrown away either. Most of the device is reused - cathode and the container the components are packaged in.

Wind, photovoltaic solar energy, wave and tidal current energy are all intermittent energy suppliers that need some kind of energy storage device or energy back-up so that power can be supplied on calm days, when the sun doesn’t shine, or between tides.

Aside from rechargeable batteries, hydrogen is being considered to be a possible energy storage device, or a carrier of potential energy. The difficult problems, however, of hydrogen storage and transportation are well known, and expensive.

So, if hydrogen is so difficult to deal with, why not consider pure metals as an energy storage medium? Metals, as long as they are protected from corrosion - oxidation - could store energy almost forever at room temperature and under atmospheric pressure. Transportation would be by common means: train or truck.

Conceptually, the simplest from of this idea is a primary or disposable battery manufacturing plant run on renewable power: a wind-powered flashlight battery plant for instance.

But the world can’t be run on stored energy in disposable flashlight batteries.

A more complex version would be for a renewable energy facility to be built specifically to deoxidize, or recycle metals into sacrificial anodes to be used in metal-air fuel cells.

The idea is not so far-fetched. The SolZinc project is a four- year feasibility study to investigate the possibility of building a solar thermal chemical reactor to produce Zinc (Zn) from Zinc Oxide (ZnO). In simple terms concentrated solar thermal energy is used to melt zinc so it will release its destructive oxygen.

The end product would be a solar fuel: The Sun’s power stored in zinc that could be used in a zinc-air fuel cell.

In the process being studied, carbon would be used to lower the melting point of zinc to make it easier for the Sun to do its job. There’s a side effect too, which would make hydrogen buffs happy. Hydrogen can also be generated from zinc when it’s used as fuel.

The SolZinc project is one of the collaborative efforts of the International Energy Agency. Visit SolZinc at http://www.solarpaces.org/solzinc.htm