Parabolic Dish Collectors

The high efficiency of parabolic dish collectors is partly offset by the fact that they require two-axis rotation to track the Sun.

The parabolic dish collector (or concentrator) is a solar system that concentrates direct solar radiation falling on its reflective surface onto a single focal point. The high concentration of solar radiation allows temperatures of around 700 °C to be reached at the receiver. Dish/engine systems have achieved some of the highest solar-to-electric conversion efficiencies among concentrating solar power technologies, with record efficiencies exceeding 30%.

Manufacturing large parabolic mirrors several metres in diameter is technically demanding. Larger and more powerful units are therefore usually assembled from smaller mirror facets which together form a parabolic reflecting surface. The disadvantage of this system is the need for two-axis tracking. The supporting structure must carry not only the weight of the dish but also the receiver located at its focal point.

The receiver absorbs the concentrated solar radiation and transfers the resulting heat to an engine, most commonly a Stirling engine. A fixed amount of working gas, usually hydrogen or helium, is enclosed in the engine and is alternately heated and cooled. The resulting pressure changes drive the pistons, producing mechanical work that is converted into electricity by a generator.

The size and mechanical complexity of parabolic dish collectors limit the practical output of individual units. Dish/engine systems typically produce several to about 25 kW of electricity. Larger installations can consist of dozens or hundreds of individual units. Because each dish requires a tracking mechanism and other moving components, such installations have relatively high maintenance requirements.

The reflecting surface is usually made of a thin layer of highly reflective metal, such as silver or aluminium, protected by a transparent layer of glass or plastic. The solar reflectance of silver-glass mirrors can exceed 90%.

A solar cooker concentrates sunlight onto a focal point, where a cooking vessel can be placed.

Dish/Stirling systems have achieved solar-to-electric conversion efficiencies of around 30%.

Temperatures exceeding 1,000 °C can be reached at the focal point of parabolic dish collectors.

At a direct normal irradiance of 1,000 W/m², a parabolic dish collector 10 metres in diameter can power a 25 kW engine.

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6 pictures