How to collect more sunbeams ?
Capture of sunbeams through the bottom of the oven
At first, having studied the subject on several Roger Bernard's books (a professional of the solar cooking), we wanted to create a system allowing to get more sunbeams. The first objective was to admit some light by the bottom of the oven. To do it, we glazed the base of the oven, by removing the insulating wall. The heat rising naturally, we emitted the hypothesis that the losses by the base were less important, what infrared images had proved. We added a double glazing and put the black plate over by means of small brackets.
Afterward, the problem was to reflect sunbeams towards this window. We wished to use a TV parabola covered with paper mirror, because it would allow to get back a more important power than reflectors plans. Before pursuing our works, we looked for the parabola focus F (that is 70 cms of the summit S) which is the point where converge parallel incident beams. Indeed, it was necessary, because it needs to avoid concentrating light on the window.
Light reflexion by a parabola – Breaking of the glass result in a concentration of energy
Unfortunately during our first tries, due to the lack of supervision of beams reflected towards the window, this one broke further to a too important concentration of the light in a point as we can see on the photography above. On second thought, we arrived at the conclusion, not finding parabola of different focal length, which it was necessary:
- Let be to heighten the oven, to take away the parabola and avoid that the focus is too close to window. To move closer to it is not possible because then we would have been bothered by the shadow of the oven and would have collected less sunbeams.
- Let be to move slightly the parabola forwards to increase the distance between the window and the parabola. However this solution will decrease clearly the efficiency of the device, pulling a slope of beams on the window which will not be convenient but also of parabola, thus a loss of collected solar power because the effective solar section will decrease.
We wished to model these situations by means of GéoGébra software to be able to envisage them and plan the spot obtained at the bottom’s oven. We created three cursors to modify at the same time the beams’ angle a, the parabola angle Of compared with the ground and a cursor to modify the oven’s height.
With an oven heightened by a meter, a parabola with 70 cms focal length, tilted by 10 ° or by 20 ° with compared with the ground and the sunbeams arriving with an 45 ° angle compared with the vertical line, we thus obtain modellings below. Sunbeams (in yellow) are reflected (in red) by the parabola). We can expect to have a spot which covers all the oven’s bottom because the green segment represents it. We note that with a parabola tilted with a 20 ° angle, this one has to be situated under the oven. This situation seems more favorable because allows to collect a more important solar power, and to reflect sunbeams with a more favorable angle to cross both windows without too much reflection. Final device
Modeling of solar reflexion by the parabola depending to its inclination
If beams arrive with one more short lens, it will be necessary to tilt undoubtedly more the parabola. For example for beams which arrive with a 60 ° angle, a 20 ° angle for the parabola is the most appropriate. Thus the parabola must be removable..
Experience and hypothesis 16: the parabola is going to allow to increase considerably the internal temperature of the open oven downward.
Protocol: we let go up the oven’s temperature until stabilization with a normal incidence on the window. Then we add the parabola in the same conditions while verifying that the illumination does not vary.
Results and interpretations:
- Device without contribution downward with a normal incidence on the window: 145°C;
- With contribution downward: 175°C with a 20°C outside temperature, are an increase of 30 ° on a gradient of temperature of 125°C, thus a 24 % gains.
The parabola was tilted of a=7 ° with compared with the ground, then beams arrived an angle b=37 ° with compared with the vertical line, that is 53 ° with the horizontal. We thus deduct according to the plan above that from it sunbeams arrived with an angle of d=b-a=30° compared with the optical axis of the parabola. So, at best, the parabola gets back a solar power Plum =E.S’=E.S.cosd =E.p.R².cosd = 850xpx(0,47)²xcos 30 = 510 W = 0.51 kW..
This power did not enter altogether the oven because the incidence.