Info

We now have the angle of incidence of the light as it hits the glass to oven-air interface. To figure out how much of the wave is reflected or transmitted, we simply repeat the process we used to find out how much of the light wave was transmitted into the glass from the outside-air, except we use graph 2 which shows the solutions to Maxwell's equations for electromagnetic radiation passing from glass into air. We find that, at an angle of incidence of 35 degrees, 10% of the energy is reflected, and 90% is refracted (and transmitted). So of our original light, 91% was transmitted into the glass, and of that 91%, 90% was transmitted through the glass into the oven-air.

At an original angle of incidence, i, of 60 degrees, about 82% of the radiation reaches the oven-air.

You have probably noticed, however, that the reflection and refraction curves are not linear. As the angle of incidence increases past a certain point, the percentage of the energy which is reflected begins to grow quite quickly. Using 60 degrees as a center point, if you decrease the angle of incidence, i, to 45 degrees, 90% of the energy is transmitted, but if you increase the angle of incidence, i, to 75 degrees, only 63% of the energy is transmitted.

Conclusions

This is a gross approximation for a number of reasons. Visible light is a spectrum of wavelengths, not simply 589 nm, and each wavelength has its own index of refraction for a given medium. Some UV and infrared reaches the earth, as well, thought they are largely absorbed by the atmosphere (assuming we don't destroy it). In addition the light reflected from the glass

Figure 2:

Light wave A is perpendicular to the surface of the earth and is equivalent to the sun being directly overhead. Light wave B travels through the atmosphere almost twice the distance that A does.

Figure 2:

Light wave A is perpendicular to the surface of the earth and is equivalent to the sun being directly overhead. Light wave B travels through the atmosphere almost twice the distance that A does.

Atmosphere

to oven-air interface can be re-reflected back towards that interface from the glass to outside-air interface (a diminishing but repetitious cycle). This would increase the percentage of transmission very slightly.

What is more important is to get a general sense of how the angle at which the light strikes the glass affects how much of that light is reflected. Other factors also effect how much radiation passes through the glass. The angle of the glass also determines how much light strikes the surface in the first place. The aperture, or exposed area, decreases as the angle of incidence increases (cosi). Also, if the glass surface is dirty, light will reflect off the dirt particles, so keep your glass clean.

Also, the atmosphere absorbs radiation, and the amount of atmosphere the sunlight travels through to reach the oven changes as the sun passes overhead. When the sun is directly overhead, the sunlight has the shortest path through the atmosphere. The further the sun is from directly overhead, the more atmosphere the radiation must travel through. The further the radiation travels through the atmosphere, the less intense it will be when it reaches the earth. This becomes increasingly significant as the sun approaches the horizon.

While this information applies to both PV panels and to solar ovens, the conclusions one comes to are very different. PV panels are comparatively expensive and any additional efficiency you can wring out of them helps to give you bang for your buck. Because of this, you always want your PV panels working, even when the sun is low on the horizon and the intensity of the solar radiation is low. To combat the reflection problem, however, there isn't too much you can do. A dual-axis tracker keeps the angle of incidence as close to zero as possible, but trackers are expensive. Routine maintenance and cleaning keeps the panel surface free from dirt and deposits, but you should be doing that anyway. Most PV panels already have an anti-reflective coating impregnated into the cells, so you cannot improve the reflective properties that way.

With a solar oven, however, there are lots of low-cost ways to make sure you're getting the most out of your oven. It is also very important to get as much as possible out of the sun when solar cooking because, while you may be able to generate power at 50% insolation, you cannot cook in any reasonable amount of time at 150 degrees F. Cooking at such a low temperature is also unsafe because it may not kill bacteria or parasites. The first, easiest, and most important thing you can do is to keep your oven pointed directly at the sun. This involves moving the oven from time to time. Unless you're making a lot of money (or having a whole lot of fun with your oven), a tracker is hardly cost effective; this is also true of anti-reflective coatings. To cut down on reflection you might want to invest in low iron glass, however, which is relatively cheap and is less reflective than regular glass.

You can also build reflectors for your solar oven to focus sunlight into the oven. This is not a simple option with PV panels because you can easily overheat the panels and reduce, rather than increase, their efficiency. Unless you plan to be cooking all day, try to keep your cooking hours as close to noon as possible when the suns radiation will be the most intense. Of course, like PV panels, you'll want to keep the glass clean.

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Written by: Lowrey Brown, Intern at Solar Energy International, PO Box 715 Carbondale, CO 81623 Phone: 970-963-8855 • Fax: 970-963-8866 E-mail: [email protected]

Web: www.solarenergy.org ||)y

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