Heliostat Desalination

Concept
The standard power tower heliostat layout consists of a field of mirrors forming a parabolic reflector with a central tower carrying a collector positioned at the focus of the reflector.

The mirror field is physically flat, usually built as many self contained structures carrying several plane mirrors arranged to reflect on the collector, thus forming a parabolic reflector.

Usually the focus is placed some distance above the ground in order to reduce the

effect of mirrors shading each other - this allows the mirrors to be placed closer together, reducing waste space between them and thus the overall area of the mirror field.

In practise the sunlight arrives at an angle related to the latitude of the location of the heliostat, rather than coming straight down as shown in this diagram. The angle also varies with the time of year, and these facts do introduce some problems.

There are about a dozen operational heliostats in the world, mostly constructed in the eighties - mostly designed to produce electricity and most with outputs of less than 5 Mw.

Solar II
The best known of these is the Solar II heliostat located in the Mojave Desert near Daggett, California. The project was begun in 1982 and achieved an output of 10 Megawatts.

The concentric circles are individual heliostats making up a 32 hectare field of 1,818 mirrors giving a total mirror area of approx 72,500 square metres.

This is a photograph of part of a Solar II mirror unit which has been rotated to vertical just for the photo. Other mirrors are visible in the lower left of the image, and are in working position.

Each mirror unit is approx 7 metres square, and is carried on a pedestal mount with electrically driven rotation in both horizontal and vertical axes.

The reflection of the group of engineers gives an idea of scale, and the image of the collector tower can be seen in the background.

The actual collector is the cylinder glowing white near the top of the tower.