Pictures of Mini-Heliostats


Introduction

I have made the following pictures to give people an idea of what a field of Mini-Heliostats might look like.

Since some of the pictures are rather large, thumbnails will be presented first. If you want more detail, click on the thumbnail and the bigger picture will load.


Views of a small rectangular field and a tower

POVRAY performance

The pictures on this page were generated using the ray-tracing program POVRAY running under Linux. Versions of POVRAY exist for most major operating systems. If you are interested in ray-tracing with POVRAY, take a look at the official Persistence of Vision Ray-Tracer Home Page. POVRAY takes a long time to calculate a scene if there are many objects in it, so my first efforts have been to render small fields of heliostats rather than something more practical in size. Even so, these pictures took about 45 minutes each to generate on a AMD486DX4 running at 120MHz!

Heliostat field specs

What is a small field? The field in these pictures consists of 36 arrays of 8 mini-heliostats each (288 mirrors total). Since the mirrors are 30 cm by 30 cm, this gives a maximum total flux of about 25 kW of solar energy (assuming a direct insolation of 1 kW/m2 and ignoring cosine factors.) The cost of the arrays alone in this small field would be between 300 and 650 Swiss francs. I expect to deploy fields with about 16 times more collection area per tower than what is shown here.

In the center of the field is a 4 meter receiver tower made of metal with a 1 meter high, 60 cm diameter steam boiler on top. The boiler could be a recycled high pressure gas bottle or a big coil of steel tubing. The boiler is white due to the concentrated solar radiation falling on it. The sun is at an elevation of 45° and an azimuth of 45° and the mirrors are all angled to reflect the light to the center of the boiler. In addition to making pretty pictures, POVRAY has allowed me to simulate and debug the heliostat targeting code.

rectangular field, side view rectangular field,  eye-level view rectangular field, top view
LEFT: A side view, as seen from the sun. The mirrors are white since they are reflecting the image of the white-hot boiler from this angle.

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CENTER: A side view, as seen from eye-level about 11 metres away from the center of the field.

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RIGHT: A top view, seen from above the tower. The mirrors are mostly blue because they are reflecting the sky from this angle.

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Environmental impact

Admittedly, the pictures lack a person or a house to give some idea of the scale, but one can imagine that the tower is as high as two or three people. The mirrors would come up to your knees. The receiver would be a brilliant white and it would hurt to look directly at it if you were closer than a hundred metres away. Now the most important question (from the Swiss point of view): would it disturb you to see this construction in some farmer's field?


Views of a small circular field and a tower

Blocking and shadowing

Heliostat mirrors can interfere with each other if they are set too close together. If the sun is low in the sky, some mirrors can cast their shadows onto others and some mirrors can block the reflected rays of others. Since heliostats were very expensive engineers in the past have studied the ideal spacing of heliostats in order to reduce interference. The general rule that they came up with is that the distance between heliostats should be between 1.5 and 1.9 times the width of their mirrors with greater spacing at the edges of the field. A hexagonal layout was found to reduce the amount of time that a mirror would be blocked by its nearest neighbors.

Spiral layout

Finding the ideal layout of mini-heliostats is not so critical due to their lower cost. Nevertheless, one can easily do better than the rectangular arrangement above by placing the mini-heliostat arrays in a sort of spiral pattern that approximates a hexagonal layout of mirrors over most of the field.

spiral field, side view spiral
field, eye-level view spiral
field, top view
LEFT: A side view, as seen from the sun. The mirrors are white since they are reflecting the image of the white-hot boiler from this angle.

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CENTER: A side view, as seen from eye-level about 11 metres away from the center of the field.

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RIGHT: A top view, seen from above the tower. The mirrors are mostly blue because they are reflecting the sky from this angle.

JPEG 29k

--

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Erik Rossen <rossen@rossen.ch>
OpenPGP key: 2935D0B9
Tel: +41 22 362 45 08
Home URL: http://www.rossen.ch

Copyright © 2000 until the heat-death of the Universe (thanks, Mickey!), by Erik Rossen
Last modified: 2008-10-06T09:02:27+0200

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