Developments and innovations in solar power technology

Perhaps it has something to do with the high cost of fuel these days, or the increasing concern over greenhouse gas emissions from the burning of fossil fuels, but, for whatever reason, there appears to have been an abundance of articles about solar power reported in recent weeks. The subject is approached from various angles, focusing on different aspects of the technology, but whether the stories are regarding research or development, a common thread among them is that they're all striving to make solar power as cheap as any conventional power source that currently feeds the electrical grid.

The solar power technology that most people are familiar with is the one that uses photo-voltaic (PV) cells to convert light into electricity. Often found on pocket calculators or seen gracing the roof of the occasional building, they're now showing up in all sorts of places, including parking meters, emergency roadside telephones and lights to illuminate your garden path at night. The current state of the technology, however, is such that it cannot compete with power generation which uses conventional sources, such as coal and oil, therefore there is a lot of interest in finding ways to improve its cost effectiveness.

Photo-voltaic solar cells generate electricity by absorbing photons of light with sufficient energy to raise an electron from the valence band (VB) to the conduction band (CB) of the semiconductor material. Conventional PV cells are tuned in such a way that only the visible wavelengths of the electromagnetic spectrum have sufficient energy to do this, which leaves a lot of energy in the incident light from the sun untapped. Researchers in Madrid, Spain have been working with a new material which has an increased absorption spectrum, allowing it to exploit the longer wavelength infra-red light¹. The material has an intermediate band (IB) partially filled with electrons, which the researchers calculate would give a solar cell made from it an ideal efficiency of up to 63%, which is a significant gain over conventional, silicon solar cells which have a theoretical limit of only 40%. By increasing the power conversion efficiency of PV cells, the researchers claim that their technology could become the next generation of photo-voltaics.

There are also advances in the development of commercial solar power technology. First Solar are one of many companies striving to produce PV cells that can generate power more economically. Richard Stevenson, reporting in this month's edition of IEEE Spectrum Online², has been investigating this rather media-shy company. It appears they have succeeded in making cadmium telluride (CdTe) into a commercially viable PV material. Their cells are fabricated on glass substrates; they use significantly less material for the active element than conventional cells; and they can be produced in a fraction of the time. CdTe is less efficient at converting solar power into electricity than its silicon counterpart and, up until now, it has been difficult to manufacture with dimensions any larger than a postage stamp, but First Solar are making them on glass sheets measuring 1.2 by 0.6 metres and are doing this cheaply in large-scale manufacturing facilities, giving it a worthwhile economic advantage over silicon cells. The trade secrets behind their technology are why the company keeps so quiet, for they clearly hope to dominate the market with their product by generating power at highly competitive prices.

Another approach to making solar power more economically viable, is to capture more sunlight and produce more electricity from it per unit area, or, putting it another way, doing the same job but with less material. SUNRGI are doing this with their Extreme Concentrated Photo-voltaics (XCPV) technology, which uses a combination of "solar concentration, heat removal, component reduction and highly accurate solar tracking". By making the most of the active area of a PV cell, and by effectively removing the heat that is generated so that it doesn't melt the cell, the entire cost of a system, which includes the land it sits on, can be substantially reduced. SUNRGI claim their technology is scalable from large commercial plants to residential systems that power your home. In the latter case, one can envisage a day when the costly infrastructure we know as the power grid, may no longer be necessary.

Nanotechnology also has a few tricks up its sleeve in the effort to increase the power conversion efficiency of solar cells. Once dismissed as a candidate material to supersede bulk semiconductors in the PV market, semiconductor nanocrystals are once again receiving attention. Photons with energies greater than the band gap of the semiconductor produce charge carriers with excess energy which is normally lost to heat production, however these novel materials can exploit that energy to produce more charge carriers, in a process called carrier multiplication. Scientists from the Netherlands and the US recently demonstrated this effect using lead selenide (PbSe) nanocrystals and they suggest its efficiency could perhaps reach as much as 44%³.

Photo-voltaic solar technology is not the only method of turning sunlight into usable energy; it can also be used to generate electricity indirectly by heating water and using the steam to drive turbines. Solar thermal power plants typically use large, expensive parabolic mirrors to direct concentrated sunlight to heat oil or water, but researchers from the Fraunhofer Institute for Solar Energy Systems have developed a method for vaporising water in tubes using linear Fresnel reflectors, which are far cheaper and more reliable⁴. Such a financial gain could be enough to make the technology more widely acceptable as an alternative source of power generation.

Even a fraction of a percentage increase in the efficiency of the technology to produce electrical power from sunlight is of importance in this industry. XeroCoat have recently entered the market, targeting both the solar thermal and the solar PV technologies. They claim to be able to increase their efficiencies with a cost-effective anti-reflection (AR) coating. By reducing the amount of light reflected at the surface of a solar cell, whether it be PV or a fluid for thermal transfer, a greater proportion of the incident light is absorbed and can be converted into useable power. Most AR coating processes are prohibitively expensive when compared with the small gains in efficiency that they provide, however XeroCoat believe they can do so affordably. They use a low temperature coating step followed by curing at atmospheric pressure, to create a mesoporous silica structure with AR properties. With a theoretical increase of 0.3 - 0.6% power conversion efficiency, they claim this benefit is worth the additional cost of the coating.

Solar power is all well and good when we're facing the sun and it's not hidden behind some clouds, of course, but what do we do when its not shining. Storing sunlight would be great, but even the multi-million dollar tanning industry haven't come up with a way to solve that one. MIT researchers, however, have come up with a possible way of efficiently storing the electrical energy that it produces⁵. Their concept involves using any unused electrical energy to split water into its constituent parts of hydrogen and oxygen, which are then stored until needed, at which time they are recombined inside a fuel cell to generate electricity. The researchers demonstrated that a new catalyst, made from cobalt and phosphate, could be used to extract the oxygen from the water; this can be combined with the better-known process of using a platinum catalyst to extract the hydrogen. The technology could be applied in large-scale solar power plants or even in household power generators.

The amount of sunlight that falls upon the Earth in one hour is enough to satisfy the energy needs of the world for a whole year. This is an often repeated claim and one worthy of repeating again, because it is just the sort of statement that will inspire humanity to free itself from the finite and dirty fossil fuels that pollute our world today, in favour of this almost limitless supply of clean energy. So put away your placards declaring "the end is nigh" and have a little optimism, because future power generation is going to rely a lot more on this one resource that has sustained life on Earth since day one. All we need to do is harness it.

1) Transition-Metal-Substituted Indium Thiospinels as Novel Intermediate-Band Materials: Prediction and Understanding of Their Electronic Properties.
P. Palacios, I. Aguilera, K. Sanchez, J. C. Conesa, and P. Wahnon, Phys. Rev. Lett. 101, 046403 (2008), DOI: 10.1103/PhysRevLett.101.046403

2) First Solar: Quest for the $1 Watt

3) In Spite of Recent Doubts Carrier Multiplication Does Occur in PbSe Nanocrystals.
Trinh, M. Tuan, Houtepen, Arjan J., Schins, Juleon M., Hanrath, Tobias, Piris, Jorge, Knulst, Walter, Goossens, Albert P. L. M., and Siebbeles, Laurens D. A.
Nano Lett., 8, 6, 1713 - 1718, 2008 DOI: 10.1021/nl0807225

4) Fraunhofer-Gesellschaft

5) 'Major discovery' from MIT primed to unleash solar revolution