Blending Conventional And Concentrated Solar Captures Light Under Cloudy Skies

Concentrator photovoltaics such as these are only viable in locations where most of the light is direct sunlight. However, a new development could make them viable in more cloudy locations. Jim Parkin/Shutterstock

A new design for concentrator photovoltaic (CPV) systems could take them out of the deserts and into locations where clouds are a common occurrence. Don't expect them in London or Seattle any time soon, but modestly sunny locations may benefit.

CPV solar modules produce electricity from sunlight with an efficiency normal solar panels cannot match. Their fatal flaw is to require direct sunlight. However, a team led by Dr Kyu-Tae Lee of the University of Illinois at Urbana-Champaign have demonstrated a new design that enables power production even in cloudy conditions. Their work, published in PNAS, is another in a string of developments announced in the last three months that should make solar power much cheaper than fossil fuels.

Most solar electricity is produced using flat-plate photovoltaics (FPPVs), where light falling on semiconductors excite electrons to produce a current. A less common but growing method concentrates the Sun's rays to boil a liquid, whose steam drives a turbine. Concentrator photovoltaics take a little of each, using lenses or curved mirrors to focus light onto small PV cells. This has the advantage of meaning that small, but highly efficient, multi-junction cells can be exposed to the light that would otherwise be spread over large areas.

Unfortunately, however, lenses and mirrors can only effectively focus light coming from one direction – that is directly from the Sun. Even thin clouds scatter light so much that it falls on the concentrators from many different directions. Flat-plate PVs produce less electricity in such conditions, but often still make enough to be useful. The fact that concentrator photovoltaics produce almost no power under such conditions has kept them at less than 1 percent of installed solar power and restricted to deserts where clouds are rare.

The authors tackled this by producing a further hybrid, combining concentrator PVs with features of their flat-plate cousins. They cut conventional silicon solar cells into strips with lasers and mounted them on the backplates of modules. An alternative design put multi-junction cells and lenses on top of conventional FPPVs.

Either way, in sunny conditions the system operates like a normal concentrator system, but when clouds appear, some of the otherwise wasted light is picked up by the FPPVs.

Testing at three sites in North America, all between 35.9º and 40.2º north, produced efficiency improvements of between 1.0 and 8.4 percent compared to traditional CPV modules. With an estimated cost increase for an installed system of 7.5 percent, relative to unmodified CPV, the authors believe there may be a large number of sites where the economics stack up. This is particularly the case in locations where land is in short supply, making it important to squeeze the most power out of a small area.


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