An Easy to Build Earth-Sun Simulator for Testing Reflector Designs

This page covers building a simple mechanical earth/sun simulator that will allow you to build and test small scale models of reflector designs and see how they do at various times of the day and year for any point on the earth.  It can be built in a couple hours.

This earth/sun simulator can also be used for modeling shadow patterns, modeling light on collectors (especially ones of odd shape), modeling light on home models, shadows of mountains on valleys, ..

earth/sun simulator for reflector design
The simulator showing reflector light pattern on a vertical collector at 3 pm on the
summer solstice.

Back to the main page on reflector design and use here...

How it Works

The picture shows the basic parts and operation. 

earth-sun simulator for testing reflector designs

The earth's equatorial plane is represented by the "equator board" (see picture).  The equator board is hinged so that it can tilt through plus/minus 23.4 degrees to model the tilt of the earth's axis toward or away from the sun as the earth goes through the seasons (23 degrees toward the sun in summer, 23 away from sun in winter, and flat at the spring and fall equinox).

The "turntable board" is pinned to the center of the equatorial board and can be rotated to model the hour of the day.  The picture shows the turntable board at about 2 hours before solar noon.

The "ground board" is mounted to the equatorial board at a tilt of (90 - latitude) degrees.  This board represents the surface of the earth at the latitude the collector will be located.  In the picture, the ground board is set for a latitude of 45 degrees.  
If you are located at latitude 30, then the ground board would be mounted at a tilt of (90 - 30) = 60 degrees to the equator board.

The sun is represented by a spotlight that remains in a fixed position as the equator and turntable boards are rotated to model different seasons of the year and times of the day.

showing sun
Showing the "sun" spotlight and the simulator. 
Normally sun is mounted further away to reduce light angle errors.
I use a 75 watt halogen "spot" light.

The height of the sun should be the same as the height of the middle of the collector/reflector model.  The sun should be aligned with the south or solar noon axis of the turntable board with the turntable set in the 12 noon position.

To use the simulator, you mount a scale model of your collector and reflector on ground board.  You then darken the room, and turn on the spot light that acts as the sun.  You can easily see how well your reflector design actually reflects light onto the collector by varying the tilt of the equator board to represent the different seasons, and by rotating the turntable board to model the time of day.

Some more pictures of the simulator in use:

winter solstice position
In the winter solstice position.
In the equinox postion
In the equinox position.
From the back
From the back toward the sun.
Measuring light levels
Measuring light levels on reflector,
collector, and ground using
a photographic light meter.

The simulator is very fast and easy to use -- you can go from 2 pm on the summer solstice to 2pm on the equinox or winter solstice with just one quick hand movement -- you can easy flip back and forth between different times of day and times of year in seconds and see how the pattern of light reflected onto the collector changes.  By building the reflector out of aluminized Mylar cemented onto cardboard, you can try different reflector shapes, orientations and curves very easily.

Note that this earth/sun simulator could also be used for modeling shadow patterns, modeling light on collectors (especially ones of odd shape), modeling light on home models, shadows of mountains on valleys, ...

Light meter readings can be taken on the various surfaces of the model collector and reflector, and I see no reason why the relative values of these light light levels would not be in the same ratios of the actual sun light levels on the corresponding real objects.  If these light meter readings are taken on different test runs, then the exact distance of sun to model must be duplicated, and things like aging or the sun spotlight and line voltage variations should be considered.

Building the Simulator

There are no doubt many good ways to build this kind of simulator, but here is what I did.  If you have any improvement suggestions, please send them in.

Make the base and equatorial board

See picture just below.

 The equatorial board support should be high enough to allow the equatorial board to tilt 23 degrees down toward the front and also toward the back.  When the equatorial board is parallel to the base board it is in the equinox position, when it is tilted 23 degrees down toward the spotlight it is in the summer solstice position, and when tilted 23 degrees down toward the back, it is in the winter solstice position.

simulator base

Make the turntable

See the picture just below.

Turntable board is the circular board to right.
It pivots about a bolt that is just threaded into the equator board.
Turntable board mounted in position.
The edge of the turntable board that faces the spotlight is
trimmed off to keep it from shadowing the model when
in the winter solstice position.
Note the 15 degree hour lines on the turntable board -- turning the turntable board one of these 15 degree increments is one hour.
wax equator board
If you sand and wax the equator board before
installing the turntable board on it, the turntable board
will rotate through the hours more easily.

Make the ground board

See the picture just below.

Ground board
Ground board mounted to the turntable.
Ground board
The ground board is held at the correct angle by the two
triangular pieces that are cut to an angle of 90 - latitude.
The 1 by 1 along the back just provides a handy way to
screw the ground board to the turntable.

Make the ground board so that it tilts up at an angle of (90 - latitude) degrees to the turntable.  This board will then accurately represent the ground surface at your latitude.  For example, if you live at 40 degrees latitude, the ground board should be tilted up at 90-40 = 50 degrees from the ground board.  Think about the angle your front yard makes with the equatorial plane of the earth -- that's the angle you want (assuming your front yard is not a hill).

Add the angle stops so that the equatorial board is at the summer solstice position when against the front angle stop and at the winter solstice when against the back angle stop.  For the equinoxes, I just us wedge piece of plywood of the right height between the equatorial board and the base.  Assuming you live on earth, the stops will be cut at an angle of 23.4 degrees.

Finished simulator Finished
Board wedged in between equator board and base
holds precisely in equinox position.

Setup the spotlight so that it is a good ways away from the model.

Sizes of stuff

You can set up the size of the simulator and the model to about anything you want or have a large enough shop for.  On mine, the diameter of the turntable is 24 inches.  This allows me to have models of collectors and reflectors that are in the 10 inches tall or wide area.  This model size allows me to work in a scale of  about 1 inch per foot  compared to the actual collector and reflector.   This seems like a good size to work with to me in that the models of this size are quick and easy to make out of plywood or cardboard, but are large enough that you don't have to be overly fussy to get good accuracy.

Since the spotlight is a lot closer that the 90 million miles our sun sits away from us, there will be some error in the angle that the light comes from on some parts of the model.  The part of the model that the spot is centered on will have no error.  But, if the spot light is 20 ft away, then a model with a radius of (say) 5 inches will have a maximum error in the angle of light of arctan(5/240) = 1 degree -- for most things this is not really a problem.


Some Examples

I will add more examples of using the simulator here as time goes on -- if you build one, please send pictures of it and how you use it.


Best reflector for vertical collectors

Just to get a feel for what a good reflector geometry is for a vertical collector for different times of year, I did this exercise using a 9 by 9 inch reflector below a 9 by 9 inch collector (model size) to see what the best angles were for the different times of year, to see the pattern of reflected light on the collector throughout the day, and to try to get a rough idea what the benefit might be....

square reflector on vertical collector

Reflector for the $2K system

This is an effort to solve a problem with my $2K solar space and water heating system of not as much output in the spring as I would like to see by using a reflector to boost output...

The collector modeled in the picture just below is designed to reflect more light towards the center of the collector.  In this way, the full reflected pattern stays on the collector for a long period of time -- in the case of the configuration shown below, the full reflected light pattern is on the collector from 10 am to 2pm solar time.  A reflector without the side to side tilt up would have half of the reflected pattern off the collector by 2 pm.

$2K system reflector

Got any other ideas -- Comments??

blog comments powered by Disqus

Gary June 1, 2011