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Solar Cooker (english version)
When energy savings but also the ecological problems are important, the use of the solar cooker seems a good compromise in the sunny and developing countries. We decided not to build our own solar cooker because the researches around this device are plenty. Actually, we decided to deal with the particular characteristics of the model BISS developed by Bolivia Inti Sud Soleil, an association with which our high school is linked since the beginning of the French-Moroccan exchange in 2012. So, we decided to try to optimize the temperature inside the oven.
In a first part we shall see how the oven was built to optimize the solar energy captured to warm its inside. Then we shall see the means which were implemented to limit the losses towards the outside. We shall finally deal with some possible improvements to optimize its use in countries not as sunny , like France while not losing sight of the fact that this oven was built for developing countries, thus its cost and its maintenance have to remain reasonable.
In Morocco, or in other developing countries, numerous farmers cultivate plants for their essential oil, however for lack of ways, they cannot buy the material necessary for the distillation to produce oil and have to pass by companies which distil in their place, which reduces their thin margins. We thus looked for a way of distilling which requires few resources at the request of the association DARNGH. Yet to distil, it is necessary to boil some water, and to use some wood to warm it would be a very bad idea for the environment. We thus turned to the solar oven. However the sun being not with us this winter, all the measures requiring solar furnaces were taken in Marrakesh.We left with another group which studies only the functioning of the solar oven with whom we had to share three ovens.
Our product does not have anything to see with that of the laboratory, and it was necessary to carry out a research work of materials and understanding of the plant itself. Our results proved that to obtain some essential oil of lavender from the solar furnace is completely possible. We also notice that in France, our assembly is valid only in summer, because in winter the period of sunshine is too low. A solution to distil throughout the year would be to use a solar satellite dish, known to reach powers superior to those of the solar furnace.
This request was made by the president of the association DARNGH, Ali OUAUZIZ. This association aims at developing economically and socially the rural and poor areas of the region from Marrakesh to Morocco. The solar oven had to keep its first function, worth knowing to cook, but was to be able to be transformed into solar airdryer thanks to detachable parts.
What is the principle of the solar dryer?
The solar dryer allows ,as its name indicates, to dry food and keep it in a " natural " and " healthy " way. The drying is made by a heat not exceeding 60°C otherwise food cooks. However, every food possesses different time of drying according to the quantity of water which it contains. For example, a fig will be dry in less time than a tomato which contains a lot of water. The solar hairdryer possesses numerous advantages due to its construction and its use: It is easy to build ; it requires simple and little expensive materials (wood, laminated) ; it is economic and autonomous because it requires only the presence of the sun ; it is simple of uflisafon ;it is adaptable to the oven ;it is neither cumbersome, nor heavy
The stages of the construction:
We thought first of all about plans which allowed the adaptation of the hairdryer to a solar oven. Helped then by a volunteer close to the foundation Dar Bellarj in Morocco, further to the discussions with the volunteers of the association Bolivia Inti and our companions, we completed these plans which allowed us to arrive at the final idea rather quickly. For that, we had to make modifications on the solar oven. Indeed, it was necessary that of the air can circulate, that's why we created openings on the top and the bottomof the oven with an adaptable system established by a wire netting.
It was then necessary to begin the construction and to find materials. We chose some wood and the plywood for the manufacturing of the structure of the dryer. We built the box with several railings inside where we can put food as well as a door to open if needed and to have access to food.
It was also necessary to plan an air outlet at the top of the structure. For it we left an opening protected by a fine and adjustable wire netting to regulate the air outlet according to the needs.
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.
How to limit the energy losses towards the outside ?
How to limit the heat losses by convection?
Thanks to our infrared pictures, we were able to notice that there were important losses due to the lack of air impermeability of the double glazing.
Experience and hypothesis n°18: So, we wondered if the use of joints would limit these losses due to convection ?
Protocol: to verify it we have to renew the experience of 3) d) intended to estimate the new thermal resistance of the oven.
Measures / interpretation: we can notice that the guiding coefficient, thus the thermal resistance, passed from 0.45 to 0.47 K.W-1. So, the resistance is bigger with the joints which led more low losses thus our hypotheses is verified.
How to collect more sunbeams ?
Thus reflectors could be much more impressive than they are. However the oven would become fast unstable and cumbersome.
However, all the beams which come to strike the usual reflectors, are not reflected towards the window as we were able to notice with a laser. All the cornersplaces surfaces, refelected the beams towards another reflector, reaching the window with a not convenient incident angle. Some of them, are reflected towards the atmosphere.
Experience and hypothesis n°15: Reflectors with "broken corners" should allow to reach a more important temperature because they would allow to reflect more beams towards the oven.
Protocol : To test this hypothesis, we decided to put simply corners in the existing reflectors, before buiding more sophisticated reflectors. We installed two ovens with the same reflectors and in the same conditions at first to be sure that they reached the same average temperature, then we added corners.
Results: When that we installed settled both ovens in the same conditions, we were able to notice that the reached 145 °C maximal temperature ( 21°C outside temperature). With corners in one of the ovens, we observed that its maximal temperature reached 151.5 °C. So, the temperature difference increased from 124 °C to 130,5 °C, that is to say a 5 % increase.
Conclusion: corners arranged in reflectors allowed to reflect more sunbeams in the oven and so to get back more energy.
How to limit the losses of light by reflection towards the outside?
As we have just seen it before, it is possible to let enter a big quantity of visible and infrared beams to the oven. However some of them won't reach the bowl or the black plate at first time and will be reflect by the wall. Because of these reflections, they loose power before reaching the bowl or redirecting towards the outside. We thus thought of using a lens of Fresnel to concentrate the entrance light towards the bowl of cooking.
The Fresnel’s lens principle is quite simply because it bases on a convergent lens. Indeed, when it is crossed by beams of parallel light, it makes them converge in a point called focus of the lens F '. The distance between the focus and the lens’ center is called lens focal length f '. The advantage compared with a single lens, is that its thickness is clearly reduced by cutting it a set of concentric rings of prismatic section known by the name of “zones of Fresnel”. For each of these zones, the thickness is reduced, so the global surface of the lens is not smooth any more but consists of several surfaces of the same curvature, separated by discontinuities.
Experience and hypothesis : So, we thought that a Fresnel’s lens, would allow to concentrate beams towards the cooking bowl without absorbing too much energy. We would just have to place it on the first window of the oven. Of course, it make us ask many questions on the properties of the lens : certainly we wish to converge sunbeams entering towards the bowl, but they must not be concentrated in a point, at the risk of melting the bowl, or only burning the food. That is why, a focal length of 30 cms, seemed to us a good compromise whether it is for a pressure-cooker which can be located to only 10 cms of the window or a cake plate which will be it at the bottom of the oven or in 50 cms of the window.
Protocol: We exposed an oven until stabilization of its temperature. Then we added a Fresnel‘s lens concentrating beams on the black plate.
Results: The oven did not reached a higher temperature. On the contrary, it would have even lost two or three Celcius degrees.
Conclusion: the lens of Fresnel does not allow to increase the internal temperature of the oven, probably because it absorbs a not insignificant proportion of light.
Hypothesis and experience 17 bis: it is possible that if we concentrate the light on a black bowl containing some water, the watter’s temperature will grow faster.
Protocol: we let go up in temperature two ovens in the same conditions (normal incidence on the window + reflectors), then we introduced 1,00 L of water into everyone. On the window of the first one, we added a lens concentrating the light on the black bowl.
Case n°1: in 20 min, the water’s temperature increased from 21,3°C in 41,7 °C with a 106°C average temperature and an 21°C outside temperature. So we can found the power received by the water:
Case n°2: in 20 min, the temperature of the water increased from 21,3°C to 36,9 °C with an 112°C average temperature in the oven and a 21°C outside temperature.
Conclusion: By concentrating more beams on the bowl, the water absorbs more heat, what decreases the internal temperature of the oven at the same time, thus the losses towards the outside.