Actively shaped focusing heliostat

We describe a technology that uses a computer-driven, active servo control to change the shape of a heliostat reflector, in order to keep an image of the solar disc focused on a fixed distant target. The heliostat reflector is made with glass mirrors bent and rigidly attached to a support frame to form an initial specific concave toroidal shape. The different toroidal shapes needed throughout the day to maintain focus, despite the changing angle of the sun, are obtained by bending the frame by means of a truss of stiffening struts behind it. The struts are connected in pairs to a central back structural node, which incorporates linear actuators to change the truss geometry. For a hexagonal reflector, a total of three linear actuators suffice to adjust the amplitudes of the three lowest order orthogonal bending modes of the frame, and thereby to obtain all the different toroidal shapes needed for accurate imaging through the day. A recently constructed 1.6 m2 hexagonal prototype with three actuators has demonstrated this concept by producing sharp solar images throughout the day at a 40?m distant target. The measured flux concentration ranges from 90% to 98% into a square target measuring only 1.44 times the ideal disc diameter. Active heliostats of this type open new possibilities for nighttime solar electricity generation and solar industrial process heat, allowing fields of even relatively small numbers of heliostats to generate higher than current concentrations, as needed for high temperatures and more efficient energy conversion. A design for a scaled up hexagonal heliostat with 48 m2 reflector surface with eight petal mirror segments has been optimized using ANSYS. A 440?m diameter field of 776 of these heliostats would yield 25 MWth at 1,500x concentration by area, averaged over a cylindrical central receiver 2.75?m in height and diameter.