Dec 28 2010

Artificial Photosynthesis

A new report in Science details a possible new way to harness sunlight for energy. Researchers Chueh et al exploit the properties of the metal ceria to create a chemical reaction from sunlight that splits CO2 and H2O into CO (carbon monoxide), H2 (hydrogen gas), and O2 (oxygen gas). The hydrogen could be collected and used as fuel. The CO is a nasty byproduct, but it could be used in further chemical reactions to create other types of fuel, such as methane.

The ability to derive useful energy from sunlight is very appealing. More energy from the sun falls upon the surface of the earth than we use, by far. In fact, this energy in one year is greater than all our fossil fuel reserves.

The trick is how to efficiently and cost-effectively access this energy. (The term “exergy” is used to refer to the portion of energy that can be used to do work – and that is what we are talking about when we discuss harnessing energy.) While sunlight is abundant, free, renewable, and clean – is also has some drawbacks. Right now our methods of utilizing this resource are inefficient and not cost-effective. Further, sunlight is an intermittent source of energy – available only during the day and limited by cloud cover. It also varies by latitude, and so may become cost-effective in Florida and Mexico long before it is in Canada.

The intermittent nature of sunlight is very limiting – it means no energy at night. This, therefore, requires an alternate energy source, or a method for storing the solar energy. Energy storage adds another layer of cost and inefficiency limiting the utility of solar energy. Also, for massive centralized energy production, solar power plants would need to be located in remote sunny (probably desert) regions, and then not only would have to be stored but also transported great distances.

Researchers are tackling these issues from many different angles. Each time an incremental advance is made the media tends to report it as a breakthrough finally bringing the promise of solar energy – until the next “breakthrough” really does. There is somewhat of a “boy who cried wolf” syndrome with the public – yeah, yeah – solar energy. We heard it all before. I am a solar energy enthusiast, and so I tend to read any news report about solar energy – such as the current one. I find them interesting, as long as you look past the hype and realize that someone makes a scalable, practical, and cost effective device, the basic science advances are just that – interesting.

There are a few ways to approach the harnessing of solar exergy. The one most people think of when they think of solar energy is photovoltaic cells – the direct conversion of photons into electricity. Current photovoltaic cells are approaching (barely) cost effectiveness, if you live in Arizona. I read conflicting reports, and it depends on how you calculate it. If you consider the costs of manufacturing the system, installing it, and maintaining it compared to the real-world efficiency of the system, the cost-effectiveness is marginal. You also have to consider the energy effectiveness – the energy costs of manufacture, transport, installation, and maintenance compared to the total lifetime energy production of the system. Again, your mileage may vary, depending on many variables, mostly climate. Even the best systems require years of use before cost and energy efficiency are achieved.

Advances in photovoltaic cells include making them cheaper to manufacture and install, and increasing the efficiency of energy conversion (not necessarily both at the same time). But even the best photovoltaic system is only a supplemental energy source, unless there is also an energy storage system, which usually means batteries. Then, of course, you have to consider the energy and cost effectiveness of the batteries.

Another way to harness solar energy is to directly heat water (or another medium) with sunlight. You can have a water-heating system installed on the roof of your house, and this is appealing if you want to be as green as possible, but is not very cost-effective. There are solar power plants that are basically an array of mirrors shining sunlight on a tower, heating water, and running a turbine.

There are also solar chemical systems – and this new research falls into this category. The analogy to photosynthesis is valid, as far as it goes, but it is very superficial in many cases – essentially the only similarity is the use of solar energy to drive a chemical reaction. Plants use photosynthesis to make CO2 and water into sugar and O2. But that is only one possible chemical reaction.

In the current study the researchers developed a prototype that uses solar energy to make hydrogen and carbon monoxide from CO2 and water. The efficiency of the device is only 0.7-0.8% – which is tiny, and probably not economically viable. However, in their paper they argue that this inefficiency is due primarily to the scale of the prototype,and not a limitation of the chemical reaction itself. They predict efficiencies of up to 19%, once the tweak and scale up the prototype. But again – I have heard that promise many times before. Until they actually do it, it’s just a bold claim.

This is precisely where the crying of wolf comes into play. What I often see are prototype devices, reactions, batteries, or whatever exploiting some new and promising material or technique, and perhaps even having some property that is highly desirable. What the news reports rarely mention (or if they do it is downplayed) is that the new system also has some deal-breaker weaknesses. Low efficiency is just one example. There is always the promise that these weaknesses will be worked out, and we will be on our way to cheap-energy nirvana. But in follow up they rarely do, and when they do it seems to take 20 years more than anyone predicted.

An energy generation or storage system needs to have a reasonable combination of the following properties: high efficiency, high capacity, adequate energy output rate, physically practical, uses widely available and affordable materials in its construction, adequate lifetime of use, and affordable maintenance. It would also be nice if it did not contain loads of toxic materials that will eventually end up in landfills. These features add up to three overall calculations – energy effectiveness, cost effectiveness, and environmental friendliness.

What reports often do is focus on the one or two features that are very promising, but ignore the others, even those that are crippling. I have seen many news items for batteries, for example, that promise high capacity, but neglect to mention that the process is dependent on platinum, or that the batteries break down after a few uses, or the rate at which they can store and release energy is cripplingly slow. The researchers always promise that these issues can be worked out with a few tweaks, but usually they are flaws inherent to the design and require massive breakthroughs to fix.

I am not being nihilistic. To the contrary, I think all of these small advances will add up to practical gains. I also think there will be real breakthroughs in the future and we are on our way to a significant contribution from solar energy to the running of our civilization. But it is helpful to put the news reports of these incremental advances into a proper context. The risk is that people will become distracted and disillusioned by the hype and miss the real progress that is being made. This is true in many areas of science. For example, the public keeps getting promised a “possible cure for cancer” or some other dreadful or annoying disease, when the real story is that we are making slow but steady progress.

The same is true of energy production – slow but steady progress. This latest study is one more step. Who knows if it will pan out. Maybe it will just give the next research team an idea that will lead to some other solar chemical process. Eventually we will creep over the line of energy and cost effectiveness.

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