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The Learner's Heliostat

A heliostat to provide experience in making greater use of sunlight for heating, lighting, and drying, thus reducing my carbon footprint.

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The main problems with making greater domestic use of sunlight are clouds and the Earth's rotation. To begin tackling the second problem, I built a heliostat prototype, pictured. A heliostat is a big, pivoting reflector that keeps the sun's image falling on the same spot all day (e.g., a window or photovoltaic panel) despite the Earth's rotation. The close-to-finished project is the main gallery image. The materials of construction maximize the use of wood, to minimize the carbon footprint of the project itself. Wind and water resistance were major design considerations. Another was adaptability to diverse installation situations. I aim for automatic, computerized control in a future embodiment, to be fitted to the present project. The computer may be programmed to protect the heliostat from high winds by placing the reflector horizontal; ice and snow, vertical. The project may thus become robot-like.

Relevance:

“Residential energy use is responsible for about 20% of total greenhouse gas (GHG) emissions in the US.”

From

https://wcec.ucdavis.edu/analysis-of-greenhouse-gas-emissions-from-residential-heating-technologies-in-usa-2/

Last accessed 6 Nov 2022.

Figure not found in a peer reviewed source but probably partly based on

Dichter, N., & Aboud, A. (2020). Analysis of Greenhouse Gas Emissions from Residential Heating Technologies in the USA. UC Davis Western Cooling Efficiency Center. https://wcec.ucdavis.edu/wp-content/uploads/GHG-Emissions-from-Residential-

Comment:

The carbon emissions on these shores appear to be roughly evenly distributed across sectors. (See: “Overview of greenhouse gasses”, publisher: EPA, govt, USA) Therefore, meaningful reductions will require a combination of collective action and household-level action: neither will suffice by itself.

From Zig Ziglar, we have this: “Whatever you need to do, do a little every day.”

Safety: Do not use warped plywood for the reflector or it may focus sunlight and cause a fire. Replace or reverse it if it becomes warped. Waterproof it carefully before putting it into service. Inspect for foci periodically and after each rain and renew or reverse the reflector if any are found.

To put my safety note in perspective, products already on the market have the same issue, like this concave, magnifying shaving mirror. The combination of concave, reflector, and sunlight is to be avoided.

Linseed oil safety:

X

https://www.simplemost.com/linseed-oil-can-fire-hazard-need-know-stay-safe/

X

https://www.flinnsci.com/sds_450-linseed-oil/sds_450/


I plan to initially use push-rods made of wooden dowels to set the angles. These can be marked with a pencil to show the daily series of settings that must be reproduced by a future computerized controller. The dowel can be rotated to get a fresh surface for marking the results for the following month. In future, the dowels will be replaced by lead screws powered by stepper motors, probably with a worm gear included in the drive train. I may instead try a length of aircraft cable that takes a few turns around a motor-driven drum, and the ends connect to opposite edges of the reflector. I once had a go-kart that was steered in this manner.
The design in the sketch has a clash between the azimuth-adjusting push rod and the right inclined brace.

Clash is fixed.

Dowel hardware
Shown are the manual clutch assemblies for the fixed ends. The adjustable ends are very similar. In addition, each dowel passes through a guide assembly with a slip fit. The dowels will have one-hole sections of mending plates on the ends to keep them from getting pulled out of the clutch by gusts of wind. They will probably also need springs on the motor paddles to support them when maximally protruding.
Tools and raw materials. The long cuts were made in-store for free (a Home Depot service). Will try to do all other cuts with the miter box. Two of the dowels are for extension feet if necessary.

  • 2 × plywood sheet 4' x 2' x 0.5"
  • 2 × pushrod dowels 0.75" dia. x 48" long
  • 1 × wooden block, for joining pieces at 45 degrees 6" x 1.5" x 1.5"; cut into 4 wedges, 3" long, with a 45 dgree angle.
  • 2 × long carriage bolts, with matching nuts and washers 1/4" dia. x 2" long
  • 19 × short carriage bolts, with nuts and washers 1/4" dia. x 1.5" long

View all 28 components

  • Math aspects

    David Matthew Mooney10/08/2022 at 04:55 0 comments

    protr = the length of the push rod for azimuth (compass direction) that protrudes past the control point. 

    open angle = the angle between the reflector and the boom on the universal joint (it is entered divided by 2 for mathematical convenience).

    Take-homes:

    1) Proportionality can probably be assumed between the angle/2 and the pushrod position, with the constant = 0.0315.

    2) The end of the pushrod goes 3" sideways as it is being pushed in, then straightens out again.

    3) Reminder: the law of reflection means that to follow the sun through 180 degrees, the reflector only has to change orientation by 90 degrees.

    the Excel formulae (I couldn't import a working spreadsheet):
    B2=A2*3.14159/180
    C2=1.4142-2*SIN(B2) [comment: from the right-triangle relationship sin(angle)=opposite side/hypotenuse]
    D2=C2*COS(B2)
    E2=C2*SIN(B2)
    F2=E2*12
    G3=(C3-C2)/(A3-A2)
    H2=G2*12

    How to use Excel for math:
    1) Write your headings in row 1 and format as text ("format" is in the cursor menu).
    2) Fill in column A with your x-values (use "flash fill" if manual entry would be tedious), select these values, and format as numbers with reasonable numbers of decimal places. 
    3) Place the cursor in the cell where you want the first result (e.g., B2).
    4) Place the cursor in the formula box and write = followed by the formula.
    5) Hit return to evaluate.
    6) Place the cursor in the cell with the first result.
    7) Drag the lower-right box corner down to the bottom of the x-values and release. 
    Voila!

    To graph: select the columns with x and y, go to the insert menu, and select scatterplot. 

View project log

  • 1
    Cuts required

    One of the plywood sheets is for the reflector and the other is to be cut lengthwise into eight 4-foot strips 3” wide to make the base and universal joint assembly (UJ). Each sheet weighs 5 kg (11 lb).

    From the strips, cut the following 23 lengths, all in inches. The UJ hypotenuse is chamfered on one long edge on one end.

    Dowel lengths: azimuth, 30"; elevation, 34".

    Length, in

    Part no.

    Function

    Strip number (shows suggested tiling)

    3

    7

    Spacer for diagonal base brace

    1

    6

    18

    Spacer for elevation motor paddle

    1

    6

    17

    Elevation motor paddle

    1

    6

    17

    Azimuth motor paddle

    3

    11.5

    14

    UJ side

    3

    12

    3

    Horizontal crosspiece

    2

    12

    15

    UJ side

    3

    16.3

    16

    UJ hypotenuse

    3

    11.3

    10

    Left vertical brace

    4

    19

    9

    Diagonal base brace

    4

    24

    6

    Cross base element

    5

    24

    4

    Cross base element vertical

    5

    24

    8

    Side base element

    6

    24

    5

    Side base element vertical

    6

    14.9

    11

    Right vertical brace

    4

    30

    1

    Left vertical strut

    2

    30

    2

    Right vertical strut

    1

    48

    19

    Hinge strip for attaching reflector

    7

    2.625

    20

    Azimuth spacer

    scrap

    48

    21

    Reflector

    Sheet 1

    24

    22

    L strut reinforcer

    8

    24

    23

    R strut reinforcer

    8

    6

    24

    UJ "gusset plate"

    scrap

    Other wooden parts:

    18

    25

    Base brace reinforcer

    1.5 x 1.5” x 24” hdwd

    12

    26

    Body diag. upper

    1.5 x .75” pine scrap

    10.25

    27

    Body diag. lower

  • 2
    Detailing for fastener placement

    Excuse the amateurish Powerpoint CAD.

    The big disks are carriage bolts and a foot; small rectangles are furniture brackets. Blue arrows indicate members that continue out of the overlap area. A red dot indicates an end-on view. The UJ triangle will use a wooden block in each corner for fastening. The vertical braces will both use furniture brackets with their angles widened as necessary in a drill press vise. The right brace will use a connection block at the lower end instead of furniture braces, because I wanted to use up the fourth block, which are made in pairs.

    Figure 1. The heads of the carriage bolts face the reflector. The crosspiece is in front of the verticals when you stand behind the reflector. The UJ hinges go on the front of the crosspiece and outside the triangle of the UJ, at the bottom of a side of the triangle.

    Figure 2. The base brace and spacer are not shown in the napkin sketch.

    Figure 3.

    Figure 4. 
    Figure 5. 
    Offset the hypotenuse holes 1/4”, not 1/8” as shown.
  • 3
    Miter-box cuts (mostly 90 degrees but precise)

    Frame the UJ as shown except that the chamfer is on the wrong end.

    Complete the cuts and keep an eye on the humidity. I gather that for best results, you condition all lumber in the room where it will be cut (done) and cut everything the same day. (Or at least at the same humidity?) Right after cutting, I sanded all edges and corners, (outside and while wearing gloves) because I was getting tired of infected splinters. I put on a P100 respirator halfway through sanding. I finished by brushing off the sandings with a new shoeshine brush. Now everything I cut feels like a wooden kid's toy.

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