Mainstream Solar PV research remains focused on the development of more efficient multi-junction PV cells – and with 50% efficiencies in sight, it is easy to see why. But efficiency at this level comes at high cost.
Concentration is potentially something that could be added to products to change the solar cost metrics. Such devices would need to be mass producible, affordable and requiring neither tracking nor cooling. Present solar concentrators need both these ‘add-ons’ to maintain all-day harvesting and to avoid the fall-off in conversion efficiency that accompanies rising cell temperature.
A technology promising these benefits is luminescent solar concentration (LSC). An LSC cell has two transparent plates (in a single-layer device) sandwiching a fluorescent (luminescent) material. This acts as a waveguide that traps light rays by total internal reflection and guides them to the plate edges, where they are therefore concentrated and can be absorbed by underlying solar PV cells.
Because PV cells only have to be located at a panel's edges and not uniformly across the whole panel, there is a great saving in PV material. Natural light entering the panel is in a waveband that does not get trapped, so it would pass right through were it not for the luminescent material. This fluoresces in response to the incident light, emitting at longer wavelengths that are trapped within the guide and subsequently concentrated.
In a quest for the most effective materials, researchers have achieved some success by embedding organic dyes into plastic sheets, with multicrystalline silicon PV cells being attached to the concentrator edges. When, for instance, workers at the University of California achieved a concentration factor of 4.3, it was recognised that, while modest, this could be useful in a low-concentration system – energy generating windows for instance – and could, in any case, be doubled using optical means.
Their system works for both direct and diffuse light, but dye longevity has been a problem – most organic dyes breaking down in weeks or months rather than years. Efforts to address this issue continue.
Researchers at the Massachusetts Institute of Technology (MIT) have improved efficiency by substituting a type of glass for plastic sheets, and by tackling the problem of light re-absorption in the waveguide caused by the fluorescent dyes absorbing a proportion of the light they generate.
Modifying the dye molecules with a form of aluminium has provided an answer, causing the light emitted to be at a slightly different wavelength that does not get re-absorbed. Reports suggest that panels using the MIT technology can deliver ten times more power than a conventional panel. The technology, it is claimed, can be retrofitted over existing solar panels, or used for windows in building integrated PV (BIPV) schemes.
At MIT and the University of Michigan, the use of micro-ring lasers has been tried as a means of narrowing the waveband of the light produced so that less escapes from the panel. Work elsewhere is investigating rare-earth emitters and wavelength-selective filters as means to enhance concentration factors.
Another development attracting interest is the use of semiconductor nanocrystals as the luminescent species. These, it seems, absorb sunlight and re-emit it at red-shifted wavelengths with high quantum efficiency. A large fraction of the emitted light is trapped in the sheet where it can be collected by one or more mono- or bi-facial solar cells, with minimal loss due to prior absorption.
LSC is not the only novel technology able to reduce the use of costly semiconductor material. One intriguing solution is the application of a holographic film overlay, as championed by US company Prism Solar Technologies Inc. This approach is claimed to cut the amount of PV material needed to produce a given amount of electricity by 50-75%, thereby halving module price compared with conventional technology. A film of holographic material applied over a PV array favours the passage of light that is convertible to electricity while rejecting heat-producing radiation.
According to the company, a module using Prism's film will have the efficiency of a good ‘standard’ solar PV panel, approaching 20%, but the cost of a thin-film module (around US$1/W).
Installed as a flat-plate module, it has no costly lenses or trackers. It performs even in cloudy conditions and, with bi-facial cells, can capture light on both front and rear sides of the module – useful in such items as canopies and electric vehicle charging stations. Recently, the firm obtained a US$4.4 million financing facility, enabling it to ramp up its manufacturing plant to an expected 3 MW of capacity by later this year.
A further line of concentrator research is electro-wetting, a technique whereby a surface layer of liquid changes shape with applied electric charge so that incident light is concentrated. This electro-opto-fluidic alternative to mechanical tracking has been investigated by the University of Maryland and Teledyne Inc.
Meanwhile, IBM has used its computing and processor expertise to develop a way of overcoming the undesired side-effect of solar concentration whereby the PV material becomes heated, with accompanying loss of conversion efficiency and possible damage.
IBM's research has shown that interposing a liquid metal, such as vanadium or a compound of gallium and indium, between the PV cell (or processor) and a heat sink, dramatically improves the conduction of thermal energy away from the semiconductor material. In the solar application, the thermal interface layer would allow very high levels of solar concentration to be used without damaging the PV material.
Team members believe that about five times the power density generated by present concentrated PV (CPV) systems is realisable by these means. An experimental system that used a really powerful lens to concentrate the sun's energy in excess of 1500 times was able to hold the temperature of the sample semiconductor material at 85°C.
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