Issues with Beam-down Concepts

Most beam-down central receiver systems replace the usual central tower, receiver, and heat transfer vertical piping and pump with a hyperbolic reflector located below the aim point of the field. This reflects the impinging light toward the ground. It is shown that this also expands the image which would have been produced at the initial aim point by several fold, to the extent that an array of CPC's is required to restore some of the concentration. It is suggested that the costs of the towers to support the secondary reflector assembly, the reflector and its strong-back, and the CPC's may well equal or exceed that of the elements eliminated. The requirement that secondary size and cost be constrained also limits the boundary of the heliostat field to the extent that, for a given aim point height, typically half or less of the optimum power to the tower top receiver can be achieved in the beam-down configuration.