Building a parabolic reflector with mirrors part 1. The goal of this project is to build a parabolic using mirror to focus the reflected solar energy to power a stirling engine kit. The second goal is produce a temperature above 500F.
I picked up 6 12" X 12" mirror tiles from a local glass and mirror shop. Next was to pick up a glasscutter to cut the mirrors down to size. I think that an oil filled stained glass cutter works best. In cutting the first mirror tile down to 2" X 2" squares, I found the need to build a cutting jig so all the mirrors would be 2" X 2". With out the jig most mirrors where off by just a little.
I picked a 2" mirror size because I wanted a relative short focal point. The Sun as seen from Earth is about .5 deg wide. The mirrors are reflecting and image of the sun on the focal point target. If the focal point is too far away the reflected image of the Sun blurs, and when the image of the Sun blurs the cooler the reflection will be.
So using 2" X 2" mirror at 19 feet the Sun's image fully blurred.
(2 / tan(.5)) / 12 = 19 feet. I found that with a 2" mirror that I got the most reflection before getting blurry edges at 18". 1 in. 9 ft.
2 in. 19 ft.
3 in. 28 ft.
4 in. 38 ft. The first row started out with 8 2" mirrors. I marked out the center point and calculated the circumference needed to put 8 mirrors in a circle. The stirling engine that I had mounted at the focal point would spin very slowly. Both sides of the engine used flat black disks, so the top disk was pointed directly at the Sun and did not provide enough cooling. It did run better when I blocked the Sun as seen in the pictures. But once again cooling became a issue, where the cardboard cutout I used to block the Sun was helping the top plate to retain the heat. I mounted the mirrors to the board using a bead of clear silicone caulk. Before the mirrors where mounted noted their positions and pre drilled holes for screws to adjust mirrors. On this parabolic I use single screw per mirror. At the time of the single mirror setup I did not have a thermometer so I do not have any temperature data. If you want to calculate a theoretical temperature of an object at the focal point you can use Stefan-Boltzmann law of blackbody. This is my estimation on the power out of the parabolic. This is not the Stefan-Boltzmann law.
Using 2in^2 mirrors
solar constant 1000W/m^2 on the earth surface
sc = .1W/cm^2 = .6452W/in^2
J = (mW/cm^2 X area X (time in seconds))/1000
working in in^2
J = ((mW/in^2) (area) (time in seconds)) / 1000
J = ((.6452) (area) (900))/1000
mirror reflectiveness: 75%
The Heliospheric Labs Gallery: Building a parabolic part 1 10 Images.
(2 / tan(.5)) / 12 = 19 feet. I found that with a 2" mirror that I got the most reflection before getting blurry edges at 18". 1 in. 9 ft.
2 in. 19 ft.
3 in. 28 ft.
4 in. 38 ft. The first row started out with 8 2" mirrors. I marked out the center point and calculated the circumference needed to put 8 mirrors in a circle. The stirling engine that I had mounted at the focal point would spin very slowly. Both sides of the engine used flat black disks, so the top disk was pointed directly at the Sun and did not provide enough cooling. It did run better when I blocked the Sun as seen in the pictures. But once again cooling became a issue, where the cardboard cutout I used to block the Sun was helping the top plate to retain the heat. I mounted the mirrors to the board using a bead of clear silicone caulk. Before the mirrors where mounted noted their positions and pre drilled holes for screws to adjust mirrors. On this parabolic I use single screw per mirror. At the time of the single mirror setup I did not have a thermometer so I do not have any temperature data. If you want to calculate a theoretical temperature of an object at the focal point you can use Stefan-Boltzmann law of blackbody. This is my estimation on the power out of the parabolic. This is not the Stefan-Boltzmann law.
Using 2in^2 mirrors
solar constant 1000W/m^2 on the earth surface
sc = .1W/cm^2 = .6452W/in^2
J = (mW/cm^2 X area X (time in seconds))/1000
working in in^2
J = ((mW/in^2) (area) (time in seconds)) / 1000
J = ((.6452) (area) (900))/1000
mirror reflectiveness: 75%
| row | # mirrors | total mirrors | area | watts | Joules/15min |
| 1 | 8 | 8 | 32 | 15.4848 | 13.93632 |
| 2 | 16 | 24 | 96 | 46.4544 | 41.80896 |
| 3 | 24 | 48 | 192 | 92.9088 | 83.61792 |
| 4 | 33 | 81 | 324 | 156.7836 | 141.10524 |
| 5 | 42 | 123 | 492 | 238.0788 | 214.27092 |
| 6 | 50 | 173 | 692 | 334.8588 | 301.37292 |
| 7 | 58 | 231 | 924 | 447.1236 | 402.41124 |
| 8 | 66 | 297 | 1188 | 574.8732 | 517.38588 |
Latitude: 26° 10' North
Longitude: 127° 40' East
The Heliospheric Labs Gallery: Building a parabolic part 1 10 Images.