Philipp Schramek

Heliostats for maximum ground coverage

Most conventional heliostats consist of a rectangular reflector which is moved around a fixed vertical axis tracking the azimuth of the sun and a second moving horizontal axis which rotates around the vertical axis to allow tracking the elevation of the sun. The maximum ground coverage possible of fields of such heliostats without colliding neighbouring reflectors is 58%. Applications of a Multi Tower Solar Array (MTSA) may need heliostat fields with ground coverage over 90%. We propose a new type of heliostat which can be set up in fields with a ground coverage of up to 100%. The maximum possible ground coverage depends on the unimpeded space volume needed by the reflector of each heliostat. The shape and the orientation of this space volume depends on the orientation of the tracking axes and on the shape of the reflector. A horizontal orientation of the fixed axis allows higher ground coverage and heliostats with rectangular reflectors can be lined up in rows with the highest density. The row density increases the longer the reflector becomes relative to its width and approximates 100% maximum ground coverage for infinite long and slim heliostats. With specially shaped hexagonal reflectors, ground coverage up to 100% is possible.

Design of the Heliostat Field of the CSIRO Solar Tower

A close-packed heliostat field of more than 800 m 2 reflector area has been installed by Solar Heat and Power for the CSIRO solar tower at the Energy Centre in Newcastle, Australia. The heliostat field has been designed with significantly greater field packing density than normally associated with heliostat fields. It can be shown that even though a heliostat field with a high ground coverage exhibits more blocking and shading, a higher annual performance can be achieved up to a certain point. The optimum ground coverage calculated for the CSIRO solar tower configuration is in the range of 53%. Other heliostat field designs usually have ground coverage below 30%. The annual optical performance of the CSIRO field per square meters of deflector is about 9% higher than a radial stagger field of 30% ground coverage for a research tower, which was optimized to have the highest perfor- mance for the time frame from 10 a.m. until 2 p.m.