I’ve been wanting to celebrate my 100th blog post with something significant. I know this one is rather long, and in some parts might be confusing to those without a physics background. However, on this day when Australia prices pollution, and takes the largest, although far from last, steps towards a future built on clean energy, I thought I would discuss two pieces of renewable energy research.
Not only do these two create the prospect of cheaper (and in one case more reliable) clean energy, I think the research, and the context, is pretty cool as well and could build support for the cause. You can read about them from a slightly different angle in the July Australasian Science.
Spotting the wind
The Cooperative Research Center for Contamination Assessment and Remediation of the Environment (CRCCARE) is a proactive creature. They are much more interested in publicizing their research than most of the CRCs, and now it seems they’re expanding well beyond their original ambit. And a good thing too.
They’ve taken a technology originally developed for helping planes take off and land safely and utilised it to reduce pollution. But then they realised it has potential in other areas as well, and they’re not afraid to apply it.
LIDAR involves bouncing lasers off objects to find out how far away they are and how fast they are moving (amongst other things). It’s in fairly widespread use, but one application, which requires a particularly developed sort of LIDAR, is to observe the movement of dust particles in order to track the behavior of the wind. The first use for this particular form of LIDAR was to provide air traffic controllers of warning of sudden gusts of wind that might be a problem for planes taking off or landing.
CRCCARE saw another opportunity using it to track dust produced in mining operations and mineral loading. Apparently there are cases where no one really knows the scale of the problem, let alone where most of the dust is going, so dust tracking is the first step to tackling the problem.
But having bought up this particular form of LIDAR for such purposes, CRCCARE didn’t see why they shouldn’t put it to other uses as well. One of these is working out where to site wind turbines. At the moment much of the siting is done based on computer models of wind flow over the landscape, but it seems these models haven’t been validated that well, so checking how the wind really behaves is quite important. Better turbine placement can be the difference between a windfarm being economic or not. Moreover, the scientist I spoke to said a lot of windfarms have trouble getting finance because the banks don’t trust the models, and are not convinced the location will work. This could help a lot.\
The other thing about this particular LIDAR is that it can track what the wind is doing out to a distance of 20km. As such it can give windfarm operators knowledge of what is coming up, so they can stow windmills before they get damaged by extreme gusts, or bring on back-up power if the wind drops suddenly. The Canadian company Catch the Wind are using much less powerful LIDAR systems to adjust turbines in gusty conditions, turning them so they, well…catch the wind. They’re claiming an 8% increase in energy on trial sites, for a tiny increase in costs. CRCCARE’s system works on a much larger scale, and they see the two as highly compatible.
The first application of this LIDAR is particularly exciting Northern Kenya hosts probably the most remarkable wind resource in the world. A combination of the high altitude, a dip in the jet stream a this point and the local geography means that average wind speeds at Lake Turkana are 11m/s. If that doesn’t mean much to you, consider this – in Australia, 6m/s is considered a good spot for a turbine.
Still not impressed? Well the energy in the wind is proportional to the cube of the speed. Double the wind speed means 8 times the energy. The wind is so fast at parts of the site they’re not planning to put turbines, at least initially, at the fastest locations because they’ll wear out too quickly. The wind is so constant that the site will be providing something close to baseload power (more reliable than the coal-fired power station at Yallourn, which has been running at 25% capacity for weeks as a result of flooding).
Construction will begin this year, but the site is so remote they’re going to have to build roads just to get the turbines up there. And of course plenty of high energy transmission capacity to get the power to Nairobi. This will make the initial 300MW project very expensive, so finding the best spots to place the turbines is quite important.
However, if the first stage works it will provide close to 30% of Kenya’s power. What is more, the road will be built, and there will still be plenty of unutilized wind up there (particularly if they work out how to build turbines that can stand the toughest conditions). Future stages could produce something like a quarter of East Africa’s energy demand. CRCCARE’s Lidar system is a crucial component if this dream is to come to fruition.
Red and Red Makes Yellow
The next example is much further from being put to use. Indeed it may never be more than a laboratory curiosity. But the physics of it are truly fascinating, and if it can be put into practice the implications are a whole lot bigger than LIDAR.
First up, some background. One of the problems for solar power is that light comes in different energy levels (indicated by colour), and photovoltaic panels can only use photons with more than a certain amount of energy. Moreover, they can only use the minimum energy of the photons they can use. It’s a bit like putting on a concert. Your costs are fixed irrespective of how many people come. You can set the price low and get a lot of people, but not much money from each of them. Or you can charge a higher price, but exclude all those without the money to pay.
Each red photons has about 1.1 electron volts of energy. Violet photons carry almost twice that. So if you build a solar panel to capture all the visible photons you’re simultaneously throwing away those too low energy to see (ie Infrared), and half the energy of the violet photons.
This is one of the major reasons for the Shockley-Quisser limit, which says that solar panels can only capture a theoretical 34% of the energy in sunlight. Since practical models always fall short of the theory, we’re always likely to be a fair way below this.
Or so it seemed until NASA came up with a way around this, producing triple junction solar cells. The first junction absorbs blue and violet photons, the second yellow and green and the third captures what is left. It’s like selling front row tickets at the concert to the seriously well off, while the seats at the back go for half price.
This is very impressive, and it’s great for satellites, where the cost of the cells is almost irrelevant compared to the cost of getting them up there. As long as the triple junction cells don’t weigh much more there is no reason for NASA not to use them.
For most purposes however, such cells are pretty useless. Efficiency takes second place to cost, and triple junction cells just can’t compete. There have been some projects that focus sunlight from large areas onto small triple junction cells. Besides the awkward architecture involved these rely on highly directional light, making them useless under cloudy conditions.
The University of Sydney however, have another alternative. They figured what was needed was a two for the price of one offer tempting enough to capture even the most impecunious red photons. Imagine if there was some way you could combine two low energy photons into one higher energy one; remarkable as it seems, there is.
Under certain conditions two photons will combine to produce one of higher energy (under very rare conditions indeed three can be made to combine, but that’s never likely to happen on a scale that will challenge the energy crisis). In most cases however, this requires two photons of identical energy. That’s fine when you’re dealing with laser light – where all the photons are identical – but useless for sunlight.
A/Prof Tim Schmidt however, has been working on a way to combine photons of different energy. The physics of the next bit might seem a little difficult, but Yvette Hancock, one of the scientists in my book, did her PhD in the area and turned it into a reader and play for five year-olds, so don’t give up too easily.
When a photon strikes an atom its energy is absorbed, and in the process one of the atom’s electrons is knocked into a higher energy state, or shell. Usually what happens is that shortly afterwards, the electron drops back down to its previous state, releasing a photon of the same energy it absorbed. So no gain there. But what if you could keep the electron in the higher shell until another photon came along, putting it into a higher shell still? In this case, when the electron eventually dropped back down, it would go all the way, releasing both photon’s energy in one go – one high energy photon instead of two low energy ones.
Schmidt has found a way to do this. So you could take a conventional solar cell and cover it’s back with the photon-combining material, and back that with a mirror. The high energy photons would be captured by the solar panel. Low-energy photons would pass through the panel and hit the material where they would be absorbed and combined into one higher energy photon. In theory two red photons could become one blue or violet one, but there is some loss of energy in the combination process, so you’re more likely to get yellow, but that is still enough to be captured by most solar panels. Some of the photons would leave the material in the wrong direction, but that is what the mirror is for, sending them back through the solar panel to be collected.
Now there are a lot of potential obstacles to this being used in practice. For one thing, some sorts of solar cells are not transparent to red photons, so the whole idea is a non-starter for them. However, there are plenty of cell types on the market or under investigation where that is not an issue. A bigger problem is that currently the efficiency is way, way too low to be useful. However, Schmidt is optimistic. Not only does he see plenty of ways to increase efficiency, he went to the unusual step of laying them out in the article announcing his research in Energy and Environmental Science (the legitimate journal, not the schlock publication of a similar name).
Doing so may allow other scientists to steal Schmidt’s glory by beating him to at least some of these achievements, but his view is that the most important thing is that we get better solar panels. If someone else gets there faster than him, well he hopes they will at least quote his initial article so he gets a little credit. And if that doesn’t make you feel warm inside on this cold, but hopeful, day I’m not sure what will.