Scientists say: go solar for answers on climate change

Ben Selinger Tuesday, 6 February 2007 (Published Canberra Times)

The debate over climate change and our energy future must take place within a framework set by the natural laws of the physical world.

There are only three main sources of energy on earth. There is solar energy captured by plants in earlier eons and now stored as coal, oil and gas.

There are radioactive elements, like uranium and thorium, from which energy can be extracted.

Nuclear also includes "hot rocks" that derive heat from underground nuclear energy, and nuclear fusion mimicking the sun, both of which might some day provide high-density, relatively clean energy.

And there is an unlimited supply of low-density solar energy which continually downloads on to earth.

The most relevant natural law by which we are bound is the second law of thermodynamics. In its simplest form it says, "There are no free lunches."

When we burn fossil fuel, carbon and hydrogen in the fuel react with the oxygen in the air to form carbon dioxide, water and heat. This process is not easily or quickly reversed. Carbon dioxide and water are very stable. The bonds between the atoms are very strong, so to split CO2 back into carbon and oxygen, or water back into hydrogen and oxygen, requires more energy than was released by the original burning. Using fossil fuels to power such a step would be self-defeating, so we need to think about other sources of energy.

As this process takes lots of energy, and solar is really diffuse, perhaps what we need is a factory that's really spread out. Hey, let's design a forest.

Up till now we have relied on plants to break down carbon dioxide and water, but we have been overloading the system ever since we invented the steam engine.

So what are the options for continued use of fossil fuel?

1)Invent solar reversers that are better and faster than plants. Plants have had the odd billion years start but we might improve marginally through genetic engineering. We would need to do this on a huge scale, which would require a lot of agricultural land.

2)Collect CO2 from burning fossil fuels, compress it and store it forever. I doubt if anyone sees this as a realistic solution that will make a difference globally. Mega-quantities of stored CO2 represent a lethal hazard if ever released by accident or intent. And compression of a gas as anyone who has pumped up a bike tyre knows requires a lot of energy that is wasted as heat. Piping away excess carbon dioxide is a pipe dream aimed at sustaining an unsustainable industry.

3)We can use solar energy directly via solar water heating, solar thermal, photoelectric, wind, tide energy, and bio-fuels. Although fossil energy is needed to produce and maintain the mechanics for these energy options, solar provides by far the best answer, for relatively low-intensity domestic use. Because solar energy is low intensity and widely dispersed it is secure from natural and unnatural disasters, and contiguity of supply and use means low transmission losses. How about hydrogen? What holds for carbon, holds equally for hydrogen. There are no free lunches, just bludging off the sun. Some exotic plant species might allow us to tame them into using solar reversal to convert water to hydrogen. Chemists have devised synthetic water-splitting solar cells that show promise. But it is all very small bickies.

Say someone invents a brilliant hydrogen fuel cell. This still needs a supply of hydrogen. Another finds a good solvent for CO2 but then still needs to do something with this greenhouse gas. Emperor Coal's new cleaner clothes can't disguise the fact that the Emperor is mainly carbon and still ends up as carbon dioxide. He will need to be taxed on the damage he does.

To sustainably counter global warming we need a solar process for either energy generation or fuel regeneration. It can be natural plants or synthetic plants which split CO2 into carbon and oxygen or water into hydrogen and oxygen. There is no sustainability without the sun.

The apparent exception to the thermodynamic rule is the one energy source that most are prejudiced against nuclear. It's not a free lunch but a significant packed lunch provided when the earth was formed and as it matured. We can draw down on it without affecting the overall equations that keep us in compliance with the physical laws. The lunch menu nuclear fuel is "eat once and throw away". Nuclear reactions are not recyclable. Nuclear energy is high intensity and generates about 16 per cent of the world's electricity. Its efficiency is increasing and fuels like thorium make it safer and weapon-free. There is no doubt that it is a necessary technology, at least in intermediate timeframes. It requires fossil fuel energy to make its hardware, dismantle it safely, and deal with the waste.

Solar also requires hardware built using fossil-fuel energy. While the energy payback for wind energy is very good, for silicon photo-voltaics it is very poor, and this may not change much. A significant investment in solar photo-voltaic energy would require huge amounts of silicon. Remember the idea of a charcoal factory on the South Coast to be used to produce silicon from sand near Lithgow? Charcoal becomes CO2 as sand silica becomes silicon.

Further necessary purification of silicon is also energy intensive and, ironically, the service of the Lucas Heights nuclear reactor is required to neutron-dope silicon for photo-voltaics, iPod memory chips and the like.

Large ingots, each a single crystal of silicon, are exposed to a neutron beam which causes one in every billion silicon atoms to gain a proton and an electron and thus turn into a phosphorus atom . This irradiated silicon is sliced into chips and used for a wide variety of applications. The sun runs a series of nuclear energy reactions and will maintain its enormous energy output for a few more billion years. Except for our minute solar lunch, it all appears to be "wasted". To enjoy a bigger lunch not to mention breakfast, dinner and heavy snacking between meals we need only pay for the energy cost of the hardware needed to collect and use it. And that energy might eventually be solar as well.

So the long-term outlook for sustainable energy points to solar. Significant investment should flow that way, but the pressing question is "what is the best intermediate strategy?" Nuclear investment does make sense. And on-going scare campaigns about nuclear power-plant placement provide a market mechanism for bringing down seaside property prices to more realistic levels.

Visit secondlaw.com for more info

Ben Selinger is Emeritus Professor of Chemistry at the ANU.
This story originally appeared in The Canberra Times