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Solar thermal concentrators can generate temperatures above 2,000℃.Gas turbines can operate above 1,000℃, giving potentially high efficiencies.However, compression and expansion are adiabatic, rather than isothermal, giving efficiencies much less than the theoretical limit.This can be improved by reheat, etc, resulting in a nearly isothermal cycle, increasing efficiency.However,the turbine is more complex, with heat transfers between heating stages.The resulting losses can be as large as the gains from improved turbine efficiency.For solar turbines, heat transfer over long distances would result in large heat losses.Primary optics concentrates radiation to the focal point, as in other systems.Secondary optics directs radiation through windows in the turbine casing.Most of this radiation is directed into the primary heating chamber.The remainder is focused onto the turbine discs, heating the gas passing through them.The resulting expansion is close to isothermal, producing higher efficiency.For smaller systems, parabolic concentrators can be used.For large systems, a heliostat array provides the area required.The heliostats focus radiation onto lenses mounted around the central tower, which focus the radiation into light pipes, and then into the turbine.A gas turbine rotates at 10,000 rpm or higher, and for a solar-powered turbine, this will vary with insolation.A generator rotates at 3,000 or 3,600 rpm.A gearbox can reduce the speed (this varies with insolation), but incurs transmission losses.Modem inverters can operate in the power range required, and with lower losses than a gearbox.Therefore the generator is directly coupled to the turbine, and rotates at high speed.Its output is rectified, and inverted to 50 or 60 Hz, which is connected to the grid.A multi-level inverter can produce an output voltage with relatively little harmonic distortion.