Solar Fuel

Solar engineers know how to make petrol, diesel, jet fuel, and other hydrocarbon products out of sunlight, water, and carbon dioxide. They use solar energy to strip oxygen out of water and carbon dioxide, reducing them to hydrogen and carbon monoxide, the raw materials for Fischer-Tropsch synthetic crude oil.

Hydrogen

Solar thermochemical hydrogen production involves a cycle of chemical reactions that converts water (or steam) into hydrogen and oxygen.

There are many chemical cycles that can do the job. One of the simplest is the two-stage Iron Oxide Redox Cycle, which is a good illustrationof the general idea. The first reaction converts Wüstite (chemical symbol FeO) and water into Magnetite (Fe₃O₄) and hydrogen. The hydrogen goes to a tank or another process, such as Fischer-Tropsch synthesis. The magnetite goes to a second process, in which intense solar heat drives out oxygen, converting the material back into Wüstite ready for another cycle.

The chemical reactions do not occur spontaneously. They require heat. Just as a schoolboy might set fire to grass or paper by focusing sunlight with a magnifying glass, solar engineers focus sunlight onto their solar furnaces. But solar engineers do far better than schoolboys, regularly achieving temperatures up to 1500ºC inside their reactors.

It is vital to separate hydrogen and oxygen. They are hot. If they mix, they quickly and noisily convert themselves into steam. A multi-stage chemical cycle makes it possible to solve this practical problem using a multi-chamber reactor. For example, Dr Rich Diver Jr and his colleagues at Sandia National Laboratories in the USA mount their iron oxides onto a rotating ring. As it rotates, the ring moves iron oxide from chamber to chamber.

Carbon Monoxide

Hydrogen is only one half of the Fischer-Tropsch equation. Practical solar fuel factories will also require carbon monoxide.

Researchers have identified two possible strategies for solar carbon monoxide production. The first is to exploit a thermochemical cycle that splits carbon dioxide. For example, just as Wüstite can strip oxygen from water, it can extract oxygen from carbon dioxide, converting it into carbon monoxide* . Other chemical cycles can also be adapted for carbon monoxide production.

The other approach is based on the Reverse Water-Shift Reaction, which converts a mixture of hydrogen and carbon dioxide into water and carbon monoxide**. This reaction runs at about 800ºC, with heat supplied by concentrated solar energy.

Energy Conversion

Solar hydrocarbon fuel production involves the conversion of solar energy into chemical energy in the form of hydrocarbon fuels. The first step is to capture solar energy in the form of hydrogen and carbon monoxide. There are hundreds of chemical processes suitable for making hydrogen, including metal oxide redox processes. The serious candidates are about 40-50 percent efficient, which means that 40-50 percent of the solar energy absorbed by the chemical process can be recovered by burning the hydrogen. Carbon dioxide splitting cycles will be similarly efficient. The Fischer-Tropsch process converts hydrogen and carbon monoxide into synthetic crude oil containing most of the energy from the raw gases, though as with every practical process, some energy is lost as heat. Still, solar fuel production will be immeasurably more energy-efficient than the process by which nature captured and stored solar energy in fossil crude oil.

Commercialisation

Solar engineers will need at least a couple of decades to commercialise this technology, provided their research is adequately funded.

They require an economic, energy-efficient process for extracting carbon dioxide from the atmosphere. Such a process will be hot property, not only for solar fuel production, but also for removing carbon dioxide from the smokestacks of coal- and gas-fired power stations: and for extracting carbon dioxide from the atmosphere and burying it.

Solar engineers must also solve some technical problems associated with the operating conditions inside their reactors. For example, the iron oxide cycle involves temperatures up to 1500ºC, hot enough to crack some of the ceramic tiles in Sandia’s prototype reactor. Engineers might crack this problem by using better materials. Or, they might choose a reactant that works at a lower temperature. Besides iron, numerous other metals and metal oxides might work, including magnesium, zinc and cerium. Engineers can also trim operating temperatures by adding carefully chosen impurities to the reactant, a procedure known as Doping.

Other thermochemical cycles present different problems: for example, the Hybrid Sulphur Cycle requires a reactor capable of withstanding both high temperature (about 800ºC) and very strong acid.

In late 2008, a team at the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt) successfully made hydrogen in a 100-kilowatt reactor. Like the Sandia reactor, the German device exploits the iron oxide redox cycle. Its high-temperature components are made of silicon carbide, the same material used in cutting tools, and for brake discs on high-performance sportscars.

Prospects

Of all the methods by which the world might produce renewable liquid fuel, the solar thermochemical process will require the smallest land area by an enormous margin.
The world produced four billion tonnes of crude oil in 2007. Solar thermochemical facilities capable of producing this amount of synthetic crude oil per year would occupy a maximum of 775,333 square kilometres: Just 0.52 percent of the Earth’s total land area.

Suitable sites tend to be concentrated in the northern hemisphere. The process depends on mirrors that focus sunlight onto a solar furnace. This requires direct sunlight. It doesn't work in cloudy conditions. Southern Europe, some parts of North Africa, and deserts in the Americas and Australia are perfect. Solar thermochemical developers will avoid areas plagued by sandstorms. Most parts of New Zealand are far too cloudy for solar fuel production, but some local technologists think New Zealand may have one or two suitable locations.

Solar fuel is expected to be considerably more expensive than synthetic fuel made from wood chips. In 2007, solar thermochemical engineers were hoping they might eventually make fuel for less than $US 10 per gallon ($NZ 3.80 per litre). That's about twice the expected price of forestry biofuel, and it was based on a desert location. It would be more expensive in a less sunny spot.

I have not seen a detailed analysis of the process’s EROEI. However, EROEI doesn't seem to be a major problem for solar concentrating electricity systems. There's no reason to believe EROEI will be a problem for solar thermocehmical fuel facilities, provided they are built in the right locations.

(from Chapter 12, From Smoke to Mirrors.)

* The iron oxide carbon-dioxide splitting reactions are:
3FeO + CO₂ → Fe₃O₄ + CO
Fe₃O₄ → 3FeO + ½ O₂

** The reverse water-shift reaction is:
CO₂ + H₂ → CO + H₂O

Buy the Book

Spread the Word