Solar Power: Taking Advantage of Desertification?

Stephen DeAngelis

August 31, 2010

Many emerging market countries have notable deserts that historically have been more of a curse than a blessing. In past posts about solar power, I’ve noted that as technologies improve opportunities may emerge for these countries to put their desert lands to use producing much-needed electrical power. A German physicist is now seriously touting such a plan for the Sahara [“Dune Boon? Solar Power From the Sahara Is Debated,” by Patrick McGroarty, Wall Street Journal, 11 August 2010]. McGroarty reports:

“German physicist Gerhard Knies thinks he’s found the answer to Europe’s long-term energy needs—in the Sahara. Dr. Knies’s idea seems quixotic in every other sense: Build dozens of solar plants across the North African desert and put thousands of miles of power cables under the Mediterranean Sea to carry the electricity produced to Europe. He says the plan could supply 15% of the Continent’s energy by 2050 at an estimated cost of €400 billion ($529 billion)—more than the gross domestic products of Algeria, Egypt, Libya, Morocco and Tunisia combined. ‘I see a solution, so the problem is solved,’ the white-haired scientist says with an earnest shrug of his shoulders. ‘Of course, to implement it is something else.’ What distinguishes Dr. Knies’s brainchild from a mere idealist pipe dream is the nominal backing of some of Europe’s biggest corporate names. Last summer, Munich Re, Siemens AG, Deutsche Bank and nine other mostly German companies formed a consortium called the Desertec Industrial Initiative to promote the alternative energy plan. For now, each company is contributing €150,000 to Desertec annually, just enough to open an office and pay a chief executive to chase additional backers.”

There are plenty of skeptics who believe such a plan will never get off the ground. Among their objections are the fact “solar thermal plants use mirrors to concentrate sunlight and create steam to drive turbines that generate electricity. Most require water for condensation and cooling—a scarce resource in the Sahara.” Proponents counter that “the plants could be built near the sea and incorporate desalination facilities to generate fresh water, but analysts say that could prove cumbersome.” Solar concentrators are generally more efficient than arrays of solar panels, but solar panels are getting cheaper and more efficient very quickly. The Economist reports, “The price of solar panels is falling fast enough to hurt Western manufacturers, but it is not yet low enough to make the sun a competitive source of electricity” [“Growing pains,” 15 April 2010]. The article reports:

“Solar energy is popular because it is clean and abundant. The problem is that it remains expensive. According to recent calculations by the International Energy Agency, power from photovoltaic systems (solar cells) costs $200-600 a megawatt-hour, depending on the efficiency of the installation and the discount rate applied to future output. That compares with $50-70 per MWh for onshore wind power in America, by the IEA’s reckoning, and even lower prices for power from fossil fuels, unless taxes on greenhouse-gas emissions are included. The costs of solar are dropping; in some sunny places it may, in a few years, be possible to get solar electricity as cheaply from a set of panels as from the grid, and later on for solar to compete with conventional ways of putting electricity into the grid. But for the moment there would be no significant market for solar cells were it not for government subsidies.”

There are plenty of reasons why people continue to pursue solar technologies. The most obvious reason, however, is that there is plenty of sunlight to be had for free [“Solar energy offers a vast supply of power, but harnessing it is a challenge,” by Brian Palmer, Washington Post, 22 June 2010]. Palmer writes:

“We have a solar-based economy, whether or not we realize it. Ninety-four percent of the world’s energy comes from the sun, even energy that doesn’t at first glance seem solar. Coal, oil and natural gas are mostly the products of ancient plants that grew with the sun’s help. The sun drives hydroelectric power by evaporating low-lying water, then dumping it at higher altitudes. Windmills turn because the sun warms the planet’s air unevenly. Fortunately, there’s plenty of sun to go around. Our local star is continuously transmitting 180 quadrillion watts of energy to the Earth, 14,000 times our requirements for generating power. So the question isn’t where to get our energy, but how to capture it.”

Rather than debate the plausibility of producing electricity from sunlight in the Sahara or whether solar power can currently compete with other forms of energy, I want to use this post to explore a number of purported breakthroughs in solar technology that could decrease the cost of solar power and increase its efficiency. Let’s begin with the efficiency of solar cells.

 

The average solar cell converts 15 percent or less of the sunlight it captures and into energy. Conversion rates of over 40 percent have been achieved in the lab using concentrated sunlight and multi-junction solar panels, but that efficiency is gained at a high cost [“New solar cell efficiency record set,” by Noel McKeegan, Gizmag, 26 January 2009]. McKeegan reported:

“Researchers at the Fraunhofer Institute for Solar Energy Systems ISE have set a new record for solar cell efficiency. Using concentrated sunlight on a specially constructed multi-junction solar cell, the research group lead by Frank Dimroth has achieved 41.1% efficiency for the conversion of sunlight into electricity. The breakthrough, which surpasses the 40.7 percent efficiency previously demonstrated by Spectrolab, involved the use of sunlight concentrated by a factor of 454 and focused onto a small 5 mm multi-junction solar cell made out of GaInP/GaInAs/ Ge (gallium indium phosphide, gallium indium arsenide on a germanium substrate). Even at a higher sunlight concentration of 880, an efficiency of 40.4% was measured.”

In May of this year, a company called Innovalight claimed a record of 19 percent efficiency from its solar cell (obviously a record using non-concentrated sunlight) [“Record 19 percent efficiency achieved with low-cost solar cells,” by Jeff Salton, 5 May 2010]. A little over a month later, a company called SunPower claimed to have surpassed that record by achieving a mark of 24.2 percent [“SunPower claims new solar cell efficiency record of 24.2 percent,” by Darren Quick, Gizmag, 24 June 2010]. In order to increase efficiency, researchers are looking at what kinds of material can be used in manufacturing the cells as well as how the cells are constructed. Let’s look first at some of the materials with which researchers are experimenting.

 

A Canadian researcher is working with cobalt sulphide [“Breakthrough in low-cost efficient solar cells,” by Ben Coxworth, Gizmag, 8 April 2010]. Coxworth reports:

“A researcher at [Université du Québec à Montréal] … has come up with … an effective, low-cost solar cell. [Professor Benoît] Marsan’s invention builds upon work done in the early ’90s by Professor Michael Graetzel of the Ecole Polytechnique Federale de Lausanne in Switzerland. Graetzel designed the dye-sensitized solar cell, which is still considered to be one of the most promising types of solar collection technology. Graetzel’s cell is composed of a porous layer of titanium dioxide nanoparticles, covered with a molecular dye that absorbs sunlight, like the chlorophyll in green leaves. The titanium dioxide is immersed under an electrolyte solution, above which is a platinum-based catalyst. As in a conventional alkaline battery, an anode (the titanium dioxide) and a cathode (the platinum) are placed on either side of a liquid conductor (the electrolyte). Sunlight passes through the cathode and the conductor, and then withdraws electrons from the anode, at the bottom of the cell. These electrons travel through a wire from the anode to the cathode, creating an electrical current.”

Is it just me or does anything involving platinum sound a bit on the pricey side? Coxworth admits that a problem and points out some of the other drawbacks with this approach:

“Although Graetzel’s cell is easy to manufacture and can be used in a variety of applications, Marsan says it has two major problems that have held it back from large-scale commercialization. For one thing, the electrolyte is extremely corrosive, resulting in a lack of durability, and it’s densely colored, preventing the efficient passage of light. The other problem is the platinum cathode. Platinum is expensive, non-transparent, and rare – hardly a low-budget substance.”

Marsan indicates that he has overcome these drawbacks. Coxworth explains:

“Marsan’s patented electrochemical solar cell has neither of these problems. For the electrolyte, UQAM created a new liquid/gel that is transparent and non-corrosive, increasing the cell’s output and stability. For the cathode, the platinum has been replaced with much less expensive cobalt sulphide. This substance is also more efficient, more stable, and easier to produce in a lab. Using this approach, the researchers have demonstrated an efficiency of 6.4% under standard illumination test conditions. While this is lower than the efficiency of conventional PV cells, the manufacturing and durability advantages of the Marsan electrochemical solar cell still make it an attractive proposition.”

So our first candidate solar cell is low cost, but not efficient. The next approach uses metamaterials [“New metamaterial could lead to more efficient solar cells,” by Ben Coxworth, Gizmag, 10 May 2010]. Coxworth reports:

“Metamaterials are manmade substances designed to do some very weird things that natural materials don’t. The path of a beam of light through a natural material like glass is predictable, but scientists from the California Institute of Technology (Caltech) have engineered an optical material that bends light in the wrong direction. This new negative-index metamaterial (NIM) could have several valuable uses including invisibility cloaking, superlensing (imaging nano-scale objects using visible light) and improved light collection in solar cells. … Previous NIM’s have used used multiple layers to wrongly refract light. The Caltech material, by contrast, utilizes just one layer of silver permeated with ‘coupled plasmonic waveguide elements.’ Basically … plasmons are light waves, which get coupled to waves of electrons at the point where the silver meets the air. The waveguide elements then guide these plasmon/electron waves through the material. This relatively simple material would be much easier to produce than previous efforts. By changing its composition or waveguide geometry, the NIM could be tuned to respond to different wavelengths, coming from almost any angle with any polarization. This quality makes it particularly well-suited to use in solar cells. ‘The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells,’ explained team leader Harry Atwater. ‘The fact that the metamaterial has a wide-angle response is important because it means that it can “accept” light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or “wasted’ light”.'”

Silver is cheaper than platinum, but it is still a precious metal; which means that this second approach is higher cost but also higher efficiency than the last one discussed. A third approach being experimented with uses solar cells with thin sheets of silicon on the outside, and a mixture of copper, iron and nickel between them [“Silicon that melts while cooling could improve solar cells,” by Ben Coxworth, Gizmag, 2 August 2010]. Coxworth reports:

“You might think it was a simple law of physics that most solids melt as they get hotter, and harden as they get colder. A few materials, however, do just the opposite – they melt as they cool. Researchers at the Massachusetts Institute of Technology (MIT) have recently discovered that by dissolving certain metals into silicon, they can add that silicon compound to the relatively short list of exotic substances that exhibit retrograde melting. Their accomplishment could ultimately result in less expensive solar cells and electronic devices. The team started by creating a ‘sandwich’, made from two thin sheets of silicon on the outside, and a mixture of copper, iron and nickel between them. This was heated to a point that was below the normal melting point of silicon (which is 1414C/2577F), but high enough to cause the filling to dissolve, thus causing the silicon to become supersaturated with the metals. Not unlike clouds that become saturated with dust particles, when the supersaturated silicon cools below 900C(1652F), the metals precipitate out in liquid form, turning the material as a whole to a slush-like consistency. Impurities originally present in the silicon tend to migrate into the liquid, leaving purer silicon behind. The MIT team believe[s] that this means manufacturers could use less pure, less expensive grades of silicon for items such as solar cells or electronics, and purify them in the production process.”

Although this approach may marginally increase the efficiency of solar cells, it’s not clear whether it would make the manufacture of solar cells any cheaper (even though silicon is abundant). Two other approaches that Coxworth has written about involve nickel/lithium fluoride and selenium/zinc oxide [“Nickel and selenium could be used for cheaper, more efficient solar cells,” by Ben Coxworth, Gizmag, 5 August 2010]. Coxworth writes:

“In two just-released studies, scientists have announced new ways of making solar cells less expensive and more efficient. In one of the projects, researchers from the University of Toronto demonstrated that nickel can work just as well as gold for electrical contacts in colloidal quantum dot solar cells. In the other, a team from California’s Lawrence Berkeley National Laboratory added selenium to zinc oxide, dramatically increasing the oxide’s efficiency in absorbing solar light. Both developments could result in more practical, affordable solar technology.”

Coxworth first describes the approach using quantum dot solar cells and nickel.

“Quantum dot solar cells are already on the less-expensive side, as the dots themselves (also known as nanocrystals) are nanoscale bits of a semiconductor material, created using low-cost chemical reactions. The current produced by those dots has traditionally been collected via gold electrical contacts, but researchers from the U of T’s Photovoltaics Research Program have now successfully used nickel contacts to get the job done just as well. When the team first tried nickel, it intermixed with the quantum dots, forming a compound that blocked the flow of the electrical current. By adding just one nanometer of lithium fluoride between the nickel and the dots, however, a barrier was created that kept the two from mixing, while still allowing the current to flow from the dots to the nickel. The scientists claim that by using nickel, the material costs of quantum dot solar cells will be reduced by 40 to 80 percent. They plan to commercialize the technology once they can boost the cells’ overall efficiency.”

Coxworth next turns to the approach using selenium/zinc oxide technology.

“Like nanocrystals, zinc oxide is also a relatively inexpensive material. While it has shown promise as a solar power conversion medium, the challenge has been to boost its efficiency at collecting solar energy. The team from Berkeley Lab has succeeded in doing that, by embedding selenium in it. Even just a nine percent concentration of selenium in a mostly zinc oxide base was found to have a pronounced effect on its ability to absorb light. ‘Researchers are exploring ways to make solar cells both less expensive and more efficient; this result potentially addresses both of those needs,’ said Marie Mayer, of Berkeley Lab’s Solar Materials Energy Research Group.”

Another approach uses plastic (that’s right, the miracle substance touted in the movie The Graduate) [“Lower cost solar panels using plastic electronics,” by Jeff Salton, Gizmag, 30 March 2010]. Salton reports:

“Engineers at Princeton University have developed a new technique for producing electricity-conducting plastics that could dramatically lower the cost of manufacturing solar panels, making alternative power within reach of more consumers and industry. The Princeton researchers have successfully produced plastics that are translucent, malleable and able to conduct electricity, and are now looking at applications for the materials, most likely in a wide range of electrical devices. They believe plastics could represent a low-cost alternative to indium tin oxide (ITO), an expensive conducting material currently used in solar panels.”

Finally, researchers in Japan are using gallium nitride/manganese to help them capture the energy of light wavelengths beyond those provided by visible sunlight [“New PV cell generates electricity from UV and IR light,” by Rick Martin, Gizmag, 14 April 2010]. Martin reports:

“Last month at the meeting of the Japan Society of Applied Physics, a research group from the Kyoto Institute of Technology introduced a new photovoltaic cell that is capable of generating electricity not only from visible light, but from ultraviolet and infrared light as well. The research group, led by associate professor Saki Sonoda, hopes that this will lead to a more efficient PV cell that can be single-junction rather than the more conventional multi-junction. These new PV cells were made by ‘doping’ a wide bandgap transparent composite semiconductor — in this case, gallium nitride (GaN) — with a 3d transition metal such as manganese. For the atomically impaired among us, the other metals of that family would be scandium, titanium, vanadium, chrome, iron, cobalt, nickel, copper, and zinc. … With such advancements in the field of photovoltaics it’s exciting to see that the much hyped potential of thin film solar is not only coming to fruition, but it can also get even better still. Add to this the recent news that carbon sheets might possibly be used in solar panels, and the future looks bright for the industry indeed.”

Now let’s turn from the materials used in solar cells to how solar cells are constructed. There are a number of interesting new approaches being researched. For example, solar cells printed on paper [“MIT unveils first solar cell printed on paper,” by Darren Quick, Gizmag, 6 May 2010]. Quick reports:

“MIT researchers used carbon-based dyes to ‘print’ the cells, which are about 1.5 to 2 percent efficient at converting sunlight to electricity. … But Vladimir Bulovic, director of the Eni-MIT Solar Frontiers Research Center, [claims] any material could be used to print onto the paper solar cells if it was deposited at room temperature. It will still be some time before solar cells can be installed with a staple gun, however, as the paper variety are still in the research phase and are years from being commercialized.”

Because solar cells are now able to be constructed in a number of shapes, such as flexible panels, designers are finding innovative ways of incorporating solar cells into structures even as windows [“Sphelar cells are the new ‘power windows’,” by Rick Martin, Gizmag, 28 March 2010]. Martin reports:

“Developed by Kyosemi Corporation, Sphelar solar cells are one of the most intriguing solar solutions that we have seen in a while. On display at the recent PV Expo 2010 in Tokyo, these tiny spherical cells gave us a glimpse of how windows in buildings might be used to collect solar power in the not-so-distant future. Sphelar cells are solidified silicon drops measuring 1.8 mm in diameter and are highly transparent, which is advantageous for a number of reasons. They can be embedded in glass to create a transparent solar cell window, capable of absorbing light from any direction or angle. Because both sides of the glass can collect light, this should translate into highly efficient energy harvesting. The cells can also be embedded in flexible surfaces, allowing for them to take on unusual shapes or be bent if necessary. The Sphelar Dome is one such example, designed to absorb more energy in the early morning and late evening unlike a flatter design.”

Like the Sphelar that differentiates itself from flat panel designs, researchers at Boston College are experimenting with a design that is closer to a coaxial cable than a panel [“Highly efficient ‘nanocoax’ solar cell inspired by coaxial cable,” by Mike Webb, Gizmag, 9 June 2010]. Webb reports:

“Traditionally, the goal of high power conversion efficiency in thin film solar cells has been compromised by opposing optical and electrical constraints – while a cell needs to be thick enough to absorb adequate amounts of light, it must also be thin enough for the extraction of current. Rising to this ‘thick and thin’ challenge, researchers at Boston College have designed a nanoscale solar cell based on the age-old technology that created the coaxial cable, promising a higher conversion efficiency than any thin film solar cell yet seen. Dubbed ‘nanocoax’, the cell features solar architecture that makes it thick enough to capture light while being sufficiently thin to promote a more effective elicitation of current. Using the coaxial concept initially conceived in the 1800’s, the Boston College research team devised a method of cell creation that does not require crystalline materials. … ‘Therefore, [According to professor of physics Michael Naughton, it] ‘offers promise for lower-cost solar power with ultra thin absorbers. With continued optimization, efficiencies beyond anything achieved in conventional planar architectures may be possible, while using smaller quantities of less costly material.’ Representing a new possibility for low cost, high efficiency solar power, the amorphous silicon nanocoax cells provide in excess of 8% power conversion efficiency, the highest of any nanostructured thin film solar cell to date.”

A group of researchers from the University of Minnesota are trying to capture a significant amount of the heat that is lost by current solar arrays by using quantum dots and thereby boosting efficiency [“Harnessing ‘hot’ electrons could double efficiency of solar cells,” by Tannith Cattermole, Gizmag, 11 July 2010]. Cattermole reports:

“Researchers from University of Minnesota have removed a barrier to improving solar cell efficiency by showing how heat energy currently lost from semiconductors can be captured and transferred to electric circuits. They hope manufacturers will use the results to produce solar cells with twice the output of current solar cells and at a lower cost. In current solar cells, the topmost layer is made from a crystalline semiconductor, usually silicon, which absorbs excess solar energy in the form of ‘hot’ electrons, and radiates the energy away before it can be harnessed. Up till now, capturing this heat into an electric circuit before it cools has not been possible, however the team constructed semiconductors of lead selenide only a few nanometers wide called ‘quantum dots’, and transferred the heat into wires of titanium dioxide successfully before the hot electrons cooled. The next step will be to build and improve solar cells with quantum dots, but a further obstacle will need to be addressed; there is still heat loss from titanium dioxide.”

One final approach also uses both light and heat to improve solar power efficiency [“New process that harnesses heat energy could double efficiency of solar cells,” by Darren Quick, Gizmag, 2 August 2010]. Quick reports:

“Photovoltaic solar cells convert light energy from the sun into electricity. Although significant strides have been made in increasing the efficiency of photovoltaic technology, they usually only result in incremental increases. Researchers at Stanford University have come up with a way that could more than double the efficiency of existing solar cell technology and potentially reduce the costs of solar energy production enough for it to compete with oil as an energy source. Instead of relying solely on photons, the new process, called ‘photon enhanced thermionic emission,’ or PETE, simultaneously combines the light and heat of solar radiation to generate electricity. Unlike photovoltaic technology currently used in solar panels – which becomes less efficient as the temperature rises – the new process excels at higher temperatures. The Stanford engineers who discovered it say the process promises to surpass the efficiency of existing photovoltaic and thermal conversion technologies. And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable. ‘This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak,’ said Nick Melosh, an assistant professor of materials science and engineering, who led the research group. ‘It is actually something fundamentally different about how you can harvest energy.”

I’ll end on that bright note. Someone once said, “We will have solar energy as soon as the utility companies solve one technical problem – how to run a sunbeam through a meter.” All I can say is stand by. Barriers once considered too high to make solar energy competitive with other means of generating electricity are being lowered every day. Who knows, all that desert wasteland may yet be put to good use if the cost of solar power continues to decrease.