Update on Solar Energy

Stephen DeAngelis

August 13, 2009

Anyone who follows U.S. politics knows that the Obama administration has tied much of its recovery program to breakthroughs in renewable energy. Part of its efforts to spur energy development involves the creation of an Advanced Research Projects Agency-Energy (ARPA-E) modeled on the Pentagon’s Defense Advanced Research Projects Agency (DARPA). For a brief history of DARPA, read my blog entitled Happy Birthday DARPA. ARPA-Energy began operating in March with a budget of $400 million [“A Sparkplug for Energy Innovation,” by Elise Craig, BusinessWeek, 3 August 2009 print issue]. According to Craig, the agency has “been flooded with thousands of applications, with ideas ranging from super-high-efficiency solar cells to new battery materials.” Craig continues:

“Proponents say ARPA-E’s dollars will help technologies bridge the chasm between basic research and commercial development. ‘It fills a major gap in R&D,’ says Rafe Pomerance, president of the nonprofit Clean Air-Cool Planet. The private sector is already pouring billions of dollars into clean-energy technologies, but companies tend to bet on things with a high probability of success, notes venture capitalist John Denniston, partner at Kleiner Perkins Caufield & Byers. The government can take more risks. ‘If the government doesn’t do it, it won’t be done in the U.S.,’ Denniston says.”

Although ARPA-E may stimulate research into solar energy, a lot of research has been and continues to be done in that area. Nevertheless, a lot more research needs to be done since “solar is still several times more expensive than wind or natural gas and many times pricier than coal” [“Catching Rays Gets Cheaper,” by John Carey and Mark Scott, BusinessWeek, 22 June 2009 print issue]. The good news, Carey and Scott report, is that “solar is where costs are improving the fastest” because “supplies of crystalline silicon, the base material used in most panels, are plentiful, thanks to climbing production capacity.” As a result, predictions are that “it will be possible to make crystalline solar panels for $1 per watt in five years, down from about $1.90 today.” Of course, solar panels are not the only way to convert solar energy into electricity [“The other kind of solar power,” The Economist, 6 June 2009 print issue]. The introduction to The Economist‘s article states: “Think of solar power, and you probably think of photovoltaic panels. But there is another way to make electricity from sunlight, which arguably has even brighter prospects.” The article continues:

“In the past few months BrightSource Energy, based in California, has signed the world’s two largest deals to build new solar-power capacity. The company will soon begin constructing the first in a series of 14 solar-power plants that will collectively supply more than 2.6 gigawatts (GW) of electricity—enough to serve about 1.8m homes. But to accomplish this feat BrightSource will not use photovoltaic cells, which generate electricity directly from sunlight and currently constitute the most common form of solar power. Instead, the company specialises in ‘concentrating solar-thermal technology’ in which mirrors concentrate sunlight to produce heat. That heat is then used to create steam, which in turn drives a turbine to generate electricity.”

It might seem counter-intuitive that generating electricity by creating steam (the motive power that launched the industrial revolution) has a brighter future than generating electricity directly from sunlight. The article explains.

“Solar-thermal power stations have several advantages over solar-photovoltaic projects. They are typically built on a much larger scale, and historically their costs have been much lower. Compared with other renewable sources of energy, they are probably best able to match a utility’s electrical load, says Nathaniel Bullard of New Energy Finance, a research firm. They work best when it is hottest and demand is greatest. And the heat they generate can be stored, so the output of a solar-thermal plant does not fluctuate as wildly as that of a photovoltaic system. Moreover, since they use a turbine to generate electricity from heat, most solar-thermal plants can be easily and inexpensively supplemented with natural-gas boilers, enabling them to perform as reliably as a fossil-fuel power plant.”

Scale, of course, is an important advantage where electrical distribution grids exist. For many places in the developing world where there are no distribution grids, photovoltaic systems may prove more useful. Both systems require mostly uninterrupted access to sunny skies. Any young child with a magnifying glass has probably experienced the heating effect of concentrated sunlight.

“The modern history of solar-thermal power began after the oil crises of the 1970s, which prompted many nations to start to investigate clean and renewable energy sources as alternatives to fossil fuels. Over the following decades America, Spain and a handful of other countries built solar-thermal pilot plants for research purposes. The first company to implement the technology on a commercial scale was Luz International, an Israeli company founded in 1980. Drawing on prior research, Luz began building a series of solar-thermal power stations in California’s Mojave desert in the mid-1980s. … By 1990 Luz had constructed nine plants with a total capacity of 354MW. At the time, solar-thermal power was producing about 90% of all solar electricity in the world, says Arnold Goldman, the former chief executive of Luz, who is now chairman of BrightSource. … For nearly two decades no new commercial solar-thermal plants began operating. In the meantime, solar-photovoltaic technology slowly took over the market, and by 2007 worldwide installed capacity reached 9.2GW. Although it is more expensive per kilowatt-hour, solar panels can be deployed in small, modular systems, and thus require much less capital investment. Moreover, they can generate power off the grid, which turned out to be an important market for solar power in its early days. Now, as the solar-thermal industry is experiencing a revival, parabolic-trough projects are garnering much of today’s investment money because of their proven track record. To improve the economics still further, SkyFuel, a firm based in New Mexico, is replacing curved glass mirrors, which are expensive to make, with a thin, reflective low-cost film. And other competing solar-thermal technologies that were developed in parallel with trough-based systems, but never commercialized, are also ready to be deployed.”

The article observes that past systems have primarily relied on parabolic arrangements of reflecting surfaces that concentrate solar heat on tubes containing water that is heated into steam. BrightSource uses a different arrangement the focal point of which is a “power tower.”

“[The ‘power tower’ system uses] a field of small, flat mirrors called ‘heliostats’ [that] redirect and concentrate sunlight onto a central receiver at the top of a tower. The tower contains a fluid, typically water, which boils and the resulting steam is then transferred to a nearby ‘power block’, where it spins a conventional turbine. The advantage of this ‘power tower’ approach is that it can produce steam at a temperature of 550°C and can thus achieve a higher thermal-to-electric efficiency than trough-based systems, says John Woolard, the chief executive of BrightSource. In addition, he says, power-tower systems suffer from fewer pumping losses than trough-based designs.”

The thing that makes solar/thermal plants offspring of the information age is that thousands of cheap mirrors can be directed by computer software to ensure maximum effect. Cost, however, remains a problem even with these hybrid systems.

“Both power-tower and parabolic-trough systems can store thermal energy in the form of hot, molten salt. It is then possible to generate steam, and thus electricity, even when the sun is not shining. Solar-thermal plants without storage can operate about 30% of the year; but with storage that number could climb to 70% or higher. Unfortunately storage is expensive, and is only economical when regulators provide incentives.”

The article goes on to report on a third solar-thermal approach that has not received as much attention.

“In addition to parabolic troughs and power-towers there is also a third solar-thermal technology, which combines curved, dish-shaped mirrors with heat engines. In a dish-engine design, the mirrors concentrate sunlight to generate heat, which then typically powers a Stirling engine—a machine that converts heat into mechanical energy by compressing and expanding a fixed quantity of gas. The change in pressure drives the engine’s pistons, which drive a shaft that turns a generator to produce electricity. Although they are highly efficient, Stirling engines have seen little practical use since their invention nearly two centuries ago, and so far there are no commercial solar-thermal systems that use this approach. Critics of the technology say it involves too many moving parts, making it more complex and expensive to operate and maintain than competing technologies. Stirling Energy Systems, based in Phoenix, Arizona, hopes to prove the doubters wrong.” (Click on the image to see an animated version of how a Sterling engine works)

There is one other big problem with solar-thermal systems. They require water and lots of it. Since most of these systems are likely to be built in desert environments, finding the water is no small challenge. That is why research into photovoltaic systems continues apace. As noted above, supplies of crystalline silicon are becoming more widely available and the price is coming down. But it remains relatively expensive [“Seeing red,” The Economist, 10 January 2009 print issue].

“The temptation, therefore, is to use less of it. As a result, the makers have developed a generation of cells whose silicon layers are only a micron or two deep, as opposed to the usual thickness of 200-300 microns. The thinner the cell, however, the less efficient it is. In particular, thin cells fail to capture much light at the red end of the spectrum. That means they produce up to 20% less electricity than standard cells of equivalent area. And that negates some of the advantage of their initial cheapness. To remedy this problem, Kylie Catchpole of the Australian National University in Canberra and Albert Polman of the Institute for Atomic and Molecular Physics in Amsterdam have been trying to redirect the light that falls onto the surface of a cell in such a way that all colours are efficiently absorbed. Their chosen tools for this task are tiny particles of silver.”

The silver particles redirects the light in such a way that more of the light entering the cell is captured and turned into electricity.

“Indeed, Dr Catchpole and Dr Polman report in Optics Express that their system increases the absorption of red light tenfold—bringing the efficiency of thin cells much closer to that of the traditional sort. Of course, silver is expensive. But so little is used that the new technique would add only a few cents to the price of a solar panel. And it would bring the day closer when solar electricity is as cheap as that generated from coal.”

Companies in Israel are also experimenting with ways to alter silicon layers to improve the efficiency of solar cells [“It’s a knockout,” The Economist, 25 July 2009 print edition]. The article observes that Israel is perfect location for R&D on solar energy. It has lots of sun, lots of sand, and lots of entrepreneurs.

“The physicists and chemists at GreenSun Energy, led by Renata Reisfeld, think the way [to make solar cells cheaper] is to use less silicon. Traditional solar cells are made of thin sheets of the element covered by glass plates. In GreenSun’s cells, though, only the outer edges of the glass plates are covered by silicon, in the form of thin strips. The trick is to get the light falling on the glass to diffuse sideways to the edges, so that the silicon can turn it into electricity. Dr Reisfeld’s team do this by coating the glass with a combination of dyes and sprinkling it with nanoparticles of a metal whose nature they are not yet willing to disclose.”

The technique sounds similar to the one being explored in Australia that uses silver. The dyes help capture sunlight and the metal uses plasmons (like those created by silver) to keep light from escaping out of the silicon layer.

“These, as their name suggests, propagate over the surface of the glass until they are intercepted by the silicon at its edges. Not only does all this make GreenSun’s cells cheaper than conventional ones, because they use so much less silicon; it also makes them better. In a conventional solar cell much of the energy is lost. The energy of light varies across the spectrum (blue light is more energetic than red) but only a certain amount of energy is needed to knock an electron free. If the incident light is more energetic than necessary, the surplus disappears as heat. Unlike the sun, which scatters its energy across the board, the dye/nanoparticle mix delivers plasmons of the right energy to knock electrons free without waste.”

The new approach promises to reduce the cost of photovoltaic generated electricity from about five times the cost of traditionally-generated electricity to around twice the cost. That’s a vast improvement. The other process being explored in Israel doesn’t use silicon at all.

“Jonathan Goldstein of 3GSolar hopes to get rid of silicon altogether. 3G’s ‘dye-sensitized’ solar cells use titanium dioxide (more familiar as a pigment used in white paints) and complicated dye molecules that contain a metal called ruthenium. When one of the dye molecules is hit by light of sufficient energy, an electron is knocked out of it and absorbed by the titanium dioxide, before being passed out of the cell to do useful work. This is a well-known process (it was invented 20 years ago by Michael Grätzel, a physicist at the Federal Polytechnic School in Lausanne, Switzerland) and several firms are trying to commercialize it. Dr Goldstein, however, thinks 3G has an edge over its rivals because of the way it draws off the power—though he is reluctant to go into details. One thing that is clear, though, is that dye-sensitized cells will be cheap to make. Both silicon cells and a third technology, so-called thin-film cells (which use novel materials such as cadmium telluride deposited onto sheets of glass or steel), have to be made in a vacuum. That is expensive. Dye-sensitized cells can be made by a process similar to screen printing, which is cheap. Dye-sensitised cells are not as efficient as silicon ones, but their cheapness may outweigh that in many applications. As Barry Breen, 3G’s boss, points out, more than a billion and a half people have no access to grid electricity. With people like Dr Reisfeld and Dr Goldstein around, soon that may not matter.”

Both grid dependent and grid independent solar systems are needed to meet the world’s needs for electricity. There are a couple of other ways to use solar power that are being explored and that deserve a brief mention. The first method uses solar power to generate electricity using “an age-old chemical process called electrolysis, which cleaves hydrogen atoms from water [“Hydrogen power on tap,” by Adam Aston, BusinessWeek, 10 August 2009 print issue]. “The idea is to store [hydrogen] gas in tanks, then burn it—like natural gas—in turbines, making power on demand. Jetstream Chief Executive Henry Herman, who is building a $219 million pilot plant in Truth or Consequences, N.M., admits that selling solar power straight to utilities is simpler than going through the storage steps. But adding storage will yield a higher return on investment, he argues.”

 

The final method of generating electricity from solar power that I’ll discuss involves the use of balloons [“Party time!The Economist, 7 March 2009 print issue]. The process is being advanced by Cool Earth Solar of Livermore, California.

“Cool Earth’s insight was that if you coat only one half of a balloon, leaving the other transparent, the inner surface of the coated half will act as a concave mirror. Put a solar cell at the focus of that mirror and you have an inexpensive solar-energy collector. Cool Earth’s balloons are rather larger than traditional party balloons, having a diameter of about 2½ meters (eight feet), but otherwise they look quite similar. The solar cell aside, they are ridiculously cheap: the kilogram of plastic from which each balloon is made costs about $2. The cell, the cost of which is a more closely guarded secret, is 15-20cm across and is water-cooled. That is necessary because the balloon concentrates sunlight up to 400 times, and without this cooling the cell would quickly burn out. Like a conventional mirror, a solar balloon of this sort must be turned to face the sun as it moves through the sky, and Cool Earth is testing various ways of doing this. The focus of the light on the solar cell can also be fine-tuned by changing the air pressure within the balloon, and thus the curvature of the mirror. The result, according to Rob Lamkin, Cool Earth’s boss, is a device that costs $1 per watt of generating capacity to install. That is about the same as a large coal-fired power station. Of course, balloons do not last as long as conventional power stations (each is estimated to have a working life of about a year). But the fuel (sunlight) is free. When all the sums are done, Mr Lamkin reckons his company will be able to sell electricity to California’s grid for 11 cents a kilowatt-hour, the state’s target price for renewable energy, while still turning a tidy profit.”

All of these methods are interesting and are likely to have a place in energy’s future. As I’ve noted before, development requirements electricity and the more of it that can be generated cheaply in environmentally friendly ways the better off we all will be.