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solar energy Simulation shines light on


Simulation and optimisation are vital if solar energy is to be efficient, as Tom Wilkie found out


E


ven here in Britain, the sun sometimes shines. But my wife and I were surprised to discover that the photovoltaic solar panels newly


installed on the roof of our house don’t reach as high a peak power output on hot summer days as in spring and autumn. Sunshine is free and there is a lot of it: more


than 86,000 terawatts of solar power reaches the Earth’s surface each year (although a lot more hits the upper atmosphere). In principle, this would be enough to satisfy current global demand a thousand times over. But making use of the sun’s bounty is more complicated than it seems, as our little domestic example shows. So it is no surprise that simulation and optimisation plays a critical role in ensuring that green energy is also efficient energy. Even when it shines, the sun is a problem.


It radiates light, which photovoltaic (PV) cells convert to electricity, but it also radiates heat. Te efficiency of PV cells decreases as their temperature rises. Ten the sun moves (or, as Galileo pointed out, it appears to move), and so the angle of incidence and the total flux of radiation changes as the day wears on. In Britain and elsewhere, the sun seldom


shines from a cloudless sky, so part of an array may be shaded while another part is in full sun – can the load across all the panels be balanced for maximum efficiency? Te winter may bring snow – again partially shading some, but not other parts, of the array. But snow has a mechanical effect as well as electrical. Will the panels stand the extra weight and does it matter how that is distributed? It rains a lot in Britain, and the wind blows, producing stress in the panels and their support structures. In addition to the temperature effect, PV cells have a non-linear response to variations in


44 SCIENTIFIC COMPUTING WORLD


BrightSource’s thermal solar-to-steam plant is supporting enhanced oil recovery at Chevron’s oil field in Coalinga, California


solar radiation flux. Tey also produce direct current, whereas most of the world is set up to use alternating current. Te output from the panels needs to be stored if not immediately needed, and converted to AC before being sent out for consumption. Roof-top PV cells are not the only way of harvesting energy from the sun. But the


PV CELLS HAVE A NON-LINEAR RESPONSE TO VARIATIONS IN SOLAR RADIATION FLUX


problems of thermal stress and distortion, and seasonal and diurnal variability, apply just as much to industrial-scale concentrated power plant systems where moving mirrors track the sun across the sky and concentrate the reflected radiation to a central point into a heat-transfer fluid, feeding either a steam turbine or a Stirling engine. Many of the issues apply even to the ‘intelligent’ design of buildings that tries to take


advantage of solar thermal radiation to heat the building when needed, and bring down shutters or blinds to keep it cool in the summer.


Thermal analysis Soſtware for thermal analysis is what animates Ron Behee, director of development for MSC Soſtware, headquartered in California. Te company’s thermal analyser package, Sinda, has a distinguished pedigree – it can trace its roots back to NASA’s Apollo programme in the early 1960s, Behee said – and in its modern guise it has found successful application in space programmes such as Astra, ERS 1-2, Gomos, Mars Express, Silex, and Soho, as well as more down to earth applications in the aircraſt, automotive, and electronics industries. According to Behee, one important characteristic of solar power is that the sun radiates both in the visible and the infrared areas of the spectrum; and engineers have to model these two fluxes separately, something which traditional finite-element analysis codes do not easily do. ‘You need a ray-tracing code to model sunlight bouncing off shiny surfaces,


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©2011 Jamey Stillings, All Rights Reserved


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