Glazing
From Solar Cooking
Glazing is the term used to describe the glass or plastic used in a solar cooker. All glazing blocks some sunlight. Thicker glazings generally block more sunlight than thin ones. Glazings may be regular window glass or certain synthetic transparent plastics.
In many solar cookers, it is possible to cook using a dark pot with a lid without any glazing. However, glazing creates the green house effect, which leads to much greater cooking efficiency.
While two layers of glazing work better than one, in most solar cooking applications a single layer of glazing works very well and is much simpler.
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[edit] Glass
Clear window glass, standard single thickness (2.5 mm), has proved very satisfactory. Thinner glass has less tendency to crack from heat than thicker panes, probably because it heats more quickly and evenly, reducing thermal strains. Plate glass is very heavy and has been satisfactory only when tempered. Some SBC designers choose single glazing for portable models with temporary second glazing as an option for poor weather. Often permanent double glazing is chosen for all-weather cookers and this is probably ideal where materials are available.
Window glass, although fragile, is usually available in urban areas, while tempered glass and synthetic glazings are specialty materials requiring unique supply lines. Window glass is available around the world and therefore in that respect is preferable. Glass does not degrade in sunlight and if protected from thermal strains and impacts, is more durable than most plastic glazings, even those that are treated against degradation by ultra-violet rays.
Low iron glass allows more radiation to penetrate into the box and makes a hotter oven but is not essential. One way glazing has been found to reduce the function of a box cooker. Recycled automobile glass, particularly flat pieces from the side windows of vans, has been used successfully but even pieces that appear clear may be tinted to some degree to cut down on heat in the vehicle or have added materials to make safety glass. Both tints and all added materials block some solar radiation.
Glass may be tempered if desired although the cost of tempering is so high that occasionally replacing glass may be cheaper in the long run for homecrafted SBCs. Glass that has simple heat cracks may be held in place by silicon sealant or nontoxic glue or a narrow strip of tape. Mass produced SBCs customarily use tempered glass.
In the original experiments that led to the invention of the CooKit, inventor Roger Bernard used a large glass salad bowl placed upside down over the pot to let in light and trap heat. Solar Cookers International has heard of people using large, clear, low-cost plastic or glass fish bowls instead of bags. (Try to check to be sure the plastic does not include flouride, chloride or iodide additives). A Japanese solar cook uses a 25 cm diameter clear plastic dome that is sold as protecting young plants from frost.
With glass domes, etc., there is a danger that a lot of condensation with form inside the dome, run down the sides, and soak the floor of the CooKit around the bottom of the dome. This could damage the CooKit. Therefore, people who use such bowls place the pot on a tray and fit the bowl onto the tray, so any pools of condensed water form on the tray, not on the foil/cardboard CooKit. To not interfere with reflection and absorption, the tray should be no bigger than necessary. Something thin and black would absorb sunlight and conduct heat into the pot. Something shiny would bounce a little more light to the pot. Something thick and metallic would work as a heat sink, absorbing and holding heat that would be more useful if conducted into the pot and from there into the food. In fact, this whole discussion about protecting the CooKit from dripping condensation is actually one of the best arguments for sticking with plastic bags.
On the other hand, if you are cooking in a panel cooker in windy conditions or when ambient temperatures are cold, a glass or rigid plastic cover will give you a definite edge. Sharon Cousins, who cooks in northern Idaho at 47 degrees north, high on a southwest facing ridge that is open to a fifty mile sweep of the prevailing winds, has put together several rigid cover options that do not risk damage to her cookers. One of her favorites is a one-gallon clear pyrex bowl/casserole for the bottom and a large clear acrylic bowl for the cover. This will take her one-gallon black-painted enamel pot, her round graniteware roaster, and various other pots, and performs much better in cold or windy conditions than an oven bag. It also gives easy access to the food which is particularly helpful if the food will be improved by giving it an occasional stir (rice pudding, for example) or if foods will be added to the pot later in the cooking process (adding tender vegetables after tougher ones have begun to soften). A raised grate underneath allows light to strike the bottom.
Another favorite is to use a glass plate for the bottom (choices include heavy glass plates sold as holders for giant pillar candles or a larger glass round from a microwave oven turntable, purchased at a thrift store) and a clear bowl or vase or jar for the cover. A tall, heavy, straight-sided vase from a craft store does a terrific job of covering quart or half-gallon cooking jars and has performed well in cold and windy conditions with low sun. Another woman in northern Idaho has found a one-gallon glass jar that works well with a glass plate base as a cover for cooking jars. Yet another option is a clear glass apothecary jar with lid that will hold a quart cooking jar securely. A rigid cover also makes it much easier to clear condensationn periodically, which can often make it difficult to keep foods at a good boil when using an oven bag. If you are cooking in the north, in cold or windy condtions, rigid covers will give you a big edge and are well worth the trouble to rig, and if you are on a budget, thrift shops and yard sales may provide creative and inexpensive options.
[edit] Plastics
[edit] Using plastic bags as glazing for solar cooking
Polypropylene and high density polyethylene (HDPE) are both used in autoclaves which sterilize medical instruments at much higher temperatures than solar cookers normally get. Thus, these plastics are usually available in most countries. HDPE is also widely used for grocery bags worldwide--not the soft, clear plastic, but the crisp, "noisy," slightly milky transparent bags. These HDPE bags are generally quite inexpensive. If nearly transparent, these work almost as well as the much more expensive "oven roasting bags" made of polyester or nylon. The thickness of the plastic film does not affect the heating of the oven, but may affect its strength and lifetime.
In the USA, and possibly other parts of the world, oven cooking bags are available in most supermarkets. These bags are made to withstand temperatures up to 600F.
The main issues with plastics for box cooker windows are availability and durability. John Roche, an SCI board member, is a retired engineer who worked for the 3M company. His specialty was designing plastic films that could be used as windows for solar box cookers. It is ultraviolet light that plays the most havoc with the chemistry of clear, plastic films, so he tried to develop plastics that could endure the ultraviolet light. The best he got was a clear plastic that would last for a year or two before it got too yellowed and brittle. However, 3M never brought that product to market, and even if it had, there is little reason to think it would be available in stores in Rwanda, for example. The point of this little story really is that plastics vary greatly in how well they hold up as box cooker windows. You can investigate sources of plastics in the capital, visit the major suppliers, talk to them about the durability and yellowing issues, and then obtain and test the most promising options. You may find that there are relatively affordable plastics that will hold up long enough to be economically and environmentally cost effective.
The ideal bag for a solar CooKit or any other types of Panel Cookers lets the maximum amount of sunlight through, but does not melt or break down when exposed to the hot pots and the ultraviolet light in sunshine. In practice, bags do break down. Alternatively, some less-than-fully-transparent bags will work with your CooKit.
The bags packed by Solar Cookers International with the CooKits they sell are called "autoclavable polypropylene bags" which we purchase in boxes of 1000. They are 2 mils in thickness and 19 inches by 24 inches in size. Thicker bags would tend to last longer but be stiffer and harder to work with,, but performance of the cooker will not be substantially different. The size of the bag need only be large enough to completely surround your pot with a very small amount of room to spare.
In the United States, generally the easiest way to find replacement bags is by visiting your grocery store and looking for "oven roasting bags" or "turkey roasting bags." If you live in an area where stores only carry the turkey sized Reynolds bags at Holiday time, or if you use so many bags for your projects that you would like a better quantity deal, or if you want a size larger than the Reynolds turkey bags, you can buy oven bags in boxes of 100 (there is an additional discount if you buy five boxes at a time, which might be of interest to solar cooking clubs) from: The Webstaurant Store (click on "disposables" and then on "specialty bags" to get to the oven bags, which are labled "plastic cooking bags"). The smaller size they offer is 18"x24", roughly comparable to the Reynolds turkey bags (19"x23 1/2"), and the larger size is 24"X30", which is significantly larger, which could be especially useful for using as glazing in a larger box oven or enclosing a whole small cooker in a bag.
HDPE (high density polyethylene plastic) bags can be used with a solar cookers. They handle high temperatures well. In the United States, they are in common use as grocery bags, often milky white or tan in color, and rustle noisily when handled. These do not appear to work quite as well as completely transparent high-temperature bags, but except on marginal solar-cooking days, the difference may not be noticeable.
In many parts of the world, heat resistant plastic bags are difficult to find in smaller villages and towns. However, this is not always the case. For example, our team found the bags for sale in the remote village of Kakuma, Kenya. If the bags cannot be found in the smaller towns, one should check in the larger cities. Organizations planning a solar cooking project may want to check with manufacturers of plastic bags in their countries to see whether the bags can be made to order, as one of our correspondents from Burkina Faso has done.
Where plastic bags are most difficult to replace, one answer being tried is to make small frames that hold the plastic bag around the pot to insulate it, but keep the bag from actually touching the hot pot, so that the bag does not heat up as much and thus lasts longer. A frame designed by Mr. Gnibouwa Diassana of Mali, by twisting stiff electrical wire into shape keeps the plastic bag away from the hot pot when cooking in a CooKit solar cooker.
[edit] Transparent heat traps for panel cookers
- Polypropylene bags
- Nylon (polyamide) bags, a common type of “oven bag”
- Polyester bags, a common type of “oven bag”
- High-density polyethylene (HDPE) bags — the crinkly-sounding bags with handles used to carry goods from stores in many countries, often bearing the #2 recycle symbol — are acceptable if mostly clear
- Inverted glass or Pyrex® bowls over the cooking pot, if big enough to rest directly on the cooker to seal the air space around the pot. (Note: over time, moisture released during cooking can damage cardboard solar cookers. If you use an inverted bowl, consider placing the pot and the bowl on a clear glass tray or dish with a slightly raised edge to prevent accumulated moisture from running onto and damaging the cooker.)
[edit] Plastic sheets may offer alternative to bags as “greenhouse” for pots
The polypropylene (PP) bags that are distributed with CooKits in the United States are typically reusable 10-20 times before they become brittle. More durable alternatives have been tested over the years, including polyester sheets with ultraviolet (UV) inhibitors, formed into bags using tape.
Recent studies by Dr. Dale Andreatta, a mechanical engineer, and Stephen Yen, a graduate student in electrical engineering, indicate that perfluoroalkoxy fluorocarbon (PFA) may also be a good alternative. Though expensive, PFA can withstand temperatures over 250°C and is UV stabilized.
Andreatta obtained a 60 centimeter by 60 centimeter sheet of the transparent material and tested it for transmissivity and longevity. On a clear day, the PFA sheet was nearly as transmissive as the PP bag (0.95 vs. 0.97) when perpendicular to the sun, and equally as transmissive (0.87) at a 45 degree angle. Over a testing period of several months, PP bags degraded with exposure to sunlight alone, and faster when exposed to sunlight and heat. PFA held up well across both types of exposures and appears to be much more durable than PP in typical solar cooking environments.
Andreatta performed side-by-side solar cooker tests of the plastics, in which two identical black pots were filled with equal amounts of water. One pot was placed in the standard PP bag and cinched at the open end. The other pot was wrapped with the PFA sheet as follows: the sheet was centered on the top of the pot and wrapped beneath the pot in such a way that the weight of the pot held the bottom of the sheet closed, preventing most air leaks. (Any air leaks would be under or near the bottom of the pot where little warm air escapes.) Both pots were then placed in CooKits and exposed to sunlight. Temperatures were measured throughout the day using thermocouples located approximately in the center of each pot. The cookers were re-oriented twice during the measurement period. As the graph shows, the water in the PFA-wrapped pot rose to a higher temperature faster, and held that temperature longer.
Since transmissivity of the two plastics is essentially the same, Andreatta believes that the performance difference is mostly due to volume air that surrounds the pot when placed in a bag versus wrapped with a sheet. A thin layer of air between the plastic and the pot is ideal because it insulates the pot from heat loss to ambient air. However, if the layer is too thick, air begins to circulate around the pot and convective heat loss increases. Also, heat losses to ambient air increase as the surface area of the plastic increases. Therefore, Andreatta theorizes that the PFA sheet performed better because the air layer created when the plastic sheet is wrapped around a pot (1.5 to 2.5 centimeters) is smaller than when a bag is used, as is the overall surface area of exposed plastic.
A theoretical study of heat loss confirms this. Using reasonable assumptions regarding pot temperature, and ignoring possible effects of evaporative heat transfer, Andreatta found that heat loss ranged from about 84 watts for a 1.0 centimeter air space to 115 Watts for a 10.0 centimeter air space. The bulk of the heat loss was by radiation.
Andreatta concludes that plastic sheets, wrapped carefully around pots, can be used as an alternative to plastic bags and may marginally increase performance. He also suggests that less expensive, UV-stabilized plastics may outlast PP bags even if they are not quite as heat resistant as PFA, though further tests should be conducted under actual cooking conditions.
See also: The complete report on this innovation
[edit] Are plastic bags harmful to the environment?
Production of plastic bags consumes almost no energy, because the chemical changes from oil to these plastics are minor molecular changes.
The amount of fossil fuel (oil) needed to produce a plastic bag is a tiny fraction of that consumed when someone instead cooks a meal with paraffin (kerosene).
Not all plastics give off harmful fumes when heated or burned, only those containing chlorides, fluorides or iodide additives, such as PVC pipes and styrenes (styrofoam) where there is insufficient oxygen. Others, including all plastic bags used in solar cooking (polyethylenes, polypropylenes and polyesters), are all simple hydrocarbons which, when heated or burned, give off only minute quantities of carbon dioxide and water (steam), but no toxic fumes. After bags are worn out, they can be safely burned as fuel--just like paraffin or wood. They can also first be re-used. For example, in the solar cooking field projects sponsored by Solar Cookers International in East African refugee camps, refugees have used traditional weaving skills to make baskets, pot hangers, mats, ropes and other useful items from worn out solar cooking bags.
[edit] Using plastic for glazing in box-style cookers
A large size cooking bag (heat-resistant nylon) can be cut open and glued on a frame to lay on top of the oven under the lid, or may be glued directly onto the lid like glass.
To work a plastic glazing, lay glue around the window opening, place the plastic, pull it tight and hold with masking tape until the glue sets. If desired, make a second glazing on the other side. Masking tape alone will not hold in the heat.
Pull it very tightly to reduce flutter on windy days as that will fan heat out of the oven. Such heat resistant glazings make very light ovens and are moderately durable but in time kitchen-grade plastic glazings become brittle. Thicker, oven-safe polyester glazings, some of which are treated to resist degradation by ultra-violet light, may be available as special order items.
Single strength window pane glass overlapping the window opening by ¾" attached with silicon sealant or glue may be used if preferred.
An oven having a permanent, double glazed window, and a side door, resists wind chill best. Where the top is used as the door, the lid may be built with either a plug fitting down into the oven, or with a cap folding over the edges to help prevent wind from blowing heat out.
Most solar box cooker builders simply glue their glass or plastic glazing onto the cardboard of the lid. The cardboard is slightly flexible, reducing the incidence of thermal cracks in glass, yet providing good protection during handling. Some ovens have been made with a narrow strip of cardboard sandwiched on the outer edge between two pieces of cardboard frame thus providing space for glass. Such frames are either glued or stitched on the outer edge a fingerwidth from the edge of the glass. Glass is allowed to move slightly within this casing and sealant is not essential.
Flexible glazing may be wrapped around a cardboard frame to give the two layers. Cardboard frames are simply laid on top of the oven under the lid.
Alternatively, frames for the glazing may be made of wood. For single strength glass in a wooden frame, a small (1/8 inch or .3 cm) clearance around the edge of the glass as it lays in the frame reduces breakage from thermal expansion. Glass is secured with a flexible sealant. The inner glazing must be well sealed to prevent heat leaking from the oven. Second glazings need a maximum of half an inch (2.5 cm) separation from the inner pane. Many home builders do not permanently seal two glazings together, but simply lay the second glazing, either glass or plastic, above a well sealed inner pane. This arrangement allows easy access for cleaning. Double glass panes if permanently sealed, require at least one small opening from the space between panes to the outside to relieve pressure which otherwise might break the glass.
Frames will be true if the pieces from opposite ends are squarely cut and equal. With equal lengths for opposite pieces, a frame can be squared by using the corner of any book, magazine or commercially cut piece of paper as a guide.
An insulating strip between the glazing frame and top of oven is not essential if the top surface is smooth. For irregular tops, solid commercial insulation strips work well if heat resistant. To make a crafted gasket, lay a bead of silicon sealant, paper mash or similar material on the top edge of the oven, cover it with waxed paper and close the lid. Remove the waxed paper when the sealant has hardened. The Dhauladar design by Didi Contractor uses a double fold of woolen fabric glued to the surface beneath the edge of the glass with a final fold brought up to rest on the top edge of the glass thus thoroughly wrapping the edge of the glass and binding it to the frame. It is a permanent seal as access in this design is through a door in the side.
