Build a Heliostat for solar heating and lighting

Introduction

My solar tracking mirror array or “death ray” as it is affectionately referred to by my friends is actually a heliostat. A heliostat is technically any device that tracks the movement of the sun, but most often the term refers to a device that orients a mirror to reflect sunlight continuously onto a specific target.

My heliostat consists of an array of 7 mirrors, each 1?x4? in size mounted to a 4?x8? plywood backing reinforced by a 2×3 frame to prevent warping. The frame is supported by a welded steel gimbal mount allowing rotation about both horizontal and vertical axes. Each axis is equipped with a stepper motor and leadscrew to adjust the position. In spite of the size of the heliostat, relatively small motors can be used because the system is well balanced. Using two stepper motor controllers wired to a laptop inside my home, I can control the orientation of the mirror array to reflect sunlight continuously onto a specific target on the north side of my home.

Heliostats are not a new concept. They have been used for very large projects such as two 10MW solar thermal electric power plants, Solar One and Solar Two. See The Solar Project for more details. I’m also aware of their use in some residential water heating applications; however, I have not seen any reference to their use for heating a home’s interior by reflecting sunlight directly into the home as I am doing.

Heliostats provide both light and heat (much like the sun itself… go figure). On warm days, I target a relatively small office window, providing plenty of natural lighting for the occupants (normally electric lighting would be used all day). On cold days I target the glass sliding door of my bedroom. The door is large enough to accept all the sunlight reflected from the heliostat. On a clear day at solar noon, this amounts to just under 2kW of power, and lights up the room like nobody’s business. It’s quite a surreal experience waking up to sunlight streaming in through a north facing window. One of the benefits I hadn’t anticipated is the device’s use as a solar alarm clock.

Gallery

Here’s an image of the front of the heliostat. This is about all you ever see from inside the house. One thing that surprised me about the project was how pretty it turned out. People who see it for the first time often don’t realize they’re looking at mirrors.

This is the back of the array showing the gimbal mount and the motors. Note the gimbal mount is fixed to a 2×4 with a point cut on the end that I simply pounded into the ground with a sledge hammer. This is just a prototype. Were I to install it permanently, I’d use something a little more rigid (the 2×4 tends to wobble a bit in the wind).

Here is a closeup of the drive system showing the stepper motors and lead screws. The stepper motor controllers are built into the same black boxes that contain the stepper motors. I build my own stepper motor controllers (my business is motion control, so this was the easiest solution for me) but they could have been purchased off-the-shelf too. Because the array itself acts as a roof, I haven’t found it necessary to provide much in the way of water proofing, though this may prove to be an issue over the long term. I do keep the lead screws well greased so they don’t rust

Here is an image of the initial software I wrote quickly in VB for Applications in MS Excel. For initial testing, I just calibrated manually by adjusting the mirror on target at a few different times during the day. I then matched a 3rd order polynomial to the calibration points to calculate proper motor position for any time of day. Every 10 seconds, the computer sends an instruction to each motor controller to move to the calculated position. This type of calibration doesn’t account for seasonal variation so I had to re-calibrate every few days as the heliostat drifted off target. Eventually I added all the proper math to calculate desired motor position based on the linkage geometry and solar position so the system doesn’t require any adjustment. See the software section below for more details.

Here are two images showing what it looks like near mid day when I’m targeting the upstairs office window.

This is a picture of a smaller solar project, my solar hot dog cooker, with the heliostat visible in the background, targeting the sliding glass doors of my bedroom at the right of the image.

And last but not least, here’s yours truly, standing next to my solar house cooker to give a better idea of scale.

Parts

This article isn’t meant to be a detailed set of instructions for building a heliostat. I built mine with parts that were available or easily sourced by me, which may be very different from the parts available to others. However, since a few people have asked where they can get the components, I will provide some information.

Support Frame

The support frame for mounting the mirror in such a way to allow rotation about horizontal and vertical axes was welded from 1? x 1? square tubing with 1/16? wall thickness as well as some round and flat bar. Below is a rough sketch showing the way I cut and assembled the pieces.

Motors

The motor I used for turning the heliostat left/right is an ordinary size 17 rotary stepper motor with a piece of 1/4-20 threaded rod attached to it’s shaft. This is the lower of the two motors in the images. A suitable motor is Jameco part number 155460. This is a little weak for the application, though it is adequate. For a little more thrust, try Jameco part number 238538. The price for either of these is under $25/ea US. These are not the exact motors I used but they are the least expensive I’ve found that are available to the average consumer.

You could use the same part number for the motor that tilts the heliostat up/down, but I found it more convenient to use a motor with non-rotating leadscrew. This is essentially a motor with a threaded hole through it’s center and a matching leadscrew. Rotating the motor pulls the leadscrew in or out (assuming the leadscrew is prevented from rotating by some means). A suitable motor is EAD Motors part number L1MGE-H12XX-0. This part number is for just the motor with no leadscrew. The leadscrew required is an ACME 1/4-20 leadscrew, which I already had (you cannot use 1/4-20 threaded rod). Since the average joe probably doesn’t have ACME 1/4-20 leadscrew lying around, you’d likely need to order a different part number that includes a length of leadscrew. Price for this motor is around $70 US.

Note the “tilt” motor is a lot more expensive than the “turn” motor, so if you’re on a tight budget, you might consider using a rotating leadscrew for both axes. If you do you’ll have to change design a bit. In the current design, I mount both motors to the T-frame. If you use a motor with rotating leadscrew for the “tilt” motor, you should mount that motor to the back of the mirror array and mount the leadnut to the T-frame. Then as the leadscrew extends through the nut, it won’t interfere with anything else.

Controllers

I used my own controller designed for my work (my company specializes in precision motion control products). This controller isn’t available as a stand alone product. It’s built-in to various products we sell. However many stepper motor controllers can be purchased off the shelf. To work with the above motors, you will want something capable of driving a 12V bipolar stepper motor with up to about 750 mA per phase. You also want something that can be interfaced to a computer via serial port, USB, or some other means.

A suitable controller that I’m aware of is the Allmotion EZ10EN. It sells for $175 US each. You can also try a Google Product Search on Stepper Motor Controller.

Mirrors

The guiding principle behind my mirror sourcing efforts was “cheap cheap cheap”. For this type of application, precision and/or flatness is not much of a concern. I found some 1?x4? mirrors on sale for around $8 each and snapped them up. I’ve since seen similar mirrors go for as little as $5 each. Mine are glass, which is not the most durable material (an array of mirrors may make a tempting target for any kid with a slingshot). If you can find mirrors made of acrylic or polycarbonate or another more durable material at a reasonable price, go for it.

Software

I automated my heliostat by adapting this program which is an Excel workbook with custom functions for calculating dawn, sunrise, solar noon, sunset, dusk, solar azimuth, and solar elevation. It’s based on formulas from http://www.srrb.noaa.gov/highlights/sunrise/azel.html. This program only calculates solar position so I had to adapt it to calculate the mirror position required (essentially aiming the mirror exactly half way between the solar vector and the target vector).

Update 2009-03-02: In the comments below you can also find reader contributed code in BASIC and C for calculating solar position and mirror position based on time and target position. See the comments for more information.

Performance

Power

On a clear day, the solar power available at the earth’s surface is about 1 kW/m^2 assuming a collector oriented towards the sun. Where I live (around the 49th parallel) the sun reaches a maximum of about 60° above horizontal at mid day in the summer. The sliding doors of my bedroom are about 20° above horizontal, relative to the heliostat. Therefore, when the heliostat is targeting the bedroom, the mirror surface is about 20° ((60°-20°)/2) from perpendicular to incoming rays. Therefore, the captured area of sunlight is about cos(20) times the area of the mirror array. The area of the mirror array is 1.22m*2.13m=2.6m^2 .

Therefore, the power available to the array is:

Power Available = cos(20) * 2.6m^2*1 kW/m^2 = 2.44 kW

The array is constructed of common glass mirrors which typically reflect around 90% of the light that hits them. Ordinary window glass typically reflects about 4% per surface (ie it transmits 96% per surface). A double-paned glass door has 4 glass surfaces (two for each pane). Therefore, the transmitted power can be calculated as:

Power Transmitted = 2.44 kW * 0.90 * 0.96^4 = 1.87 kW

Efficiency

The efficiency is simply the power transmitted divided by the power available.

Efficiency = (1.87 kW)/(2.35 kW) = 80%

Note that flat panel solar water heaters (which are the most common method of active solar heating) have an efficiency of only 30-40%; less than half that of a very simple heliostat. In addition, most flat panel collectors or passive solar heating methods don’t track the sun, and therefore the “power available” per m^2 of collector area diminishes more rapidly on either side of mid-day. A heliostat, on the other hand, can capture early morning and late evening light, and multiple heliostats can be used to reflect light through a single, relatively small, north facing window. For these reasons, heating a home by this method could be an even more efficient process than passive solar heating through large south facing windows. A home designed to take advantage of such a solar heating system would have multiple heliostats reflecting sunlight through a single relatively small north facing window onto a thermal storage and distribution medium (ex a brick wall or a large water tank) inside the home.

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