The choice of glazing material for solar dryers is a balance of several factors:
Transparency (aka transmissivity) to the sun's light. The energy from the sun has to get into the dryer.
Trapping of infrared radiation. An ideal glazing for any solar application that wants to capture heat will allow the visible wavelengths of the sun's radiation through (see transparency above. Those wavelengths carry most of the energy. In turn, the warm insides of the collector release energy in the form of infrared radiation. Ideally, the glazing is more opaque to these infrared wavelengths and keeps more of the heat in. This is known as the greenhouse effect.
Durability. A good glazing cover, especially if it is expensive, will hold up against the stresses of moving the dryer or unintentional impacts from hard and/or falling objects (like mangos from trees, balls from the nearby football pitch, or the hooves of goats who like to climb on things).
Ease of installation. A material that is hard cut or to install with good air sealing wiil make building the dryer difficult.
Safety. The material should be safe to work with and not be a hazard either in use, or when it breaks or reaches the end of its life.
Cost. A high performing material that is very costly works against the goal of making the dryers affordable and cost effective.
Availability. Can this material be readily found, including in more rural areas far from commerical centers. A dryer with broken or torn glazing will not work well, and won't get fixed if the material isn't readily available.
The current round of solar dryer development has looked at the following materials. With each item, there is a summery of pros and cons.
Glass. Sheet glass has good transparency. If the glass has low iron content, it has good infrared trapping properties. The green color seen on the edge of the glass indicates higher iron content and much of the glass I have seen in shops in Tanga has greener color indicating high iron content. Glass is quite expensive and if used will be the first or second most expensive part of the collector. It is heavy and can be difficult to handle without breaking, and cutting it to the right size requires some skill. Broken glass can cause severe cuts and injuries, and broken glass can be a hazardous waste. It is probably also difficult to get in rural areas.
Fiberglass roofing sheets. While the transparency is lower, it is still very transparent and from references it appears to have good IR trapping properties. While it can be cracked and torn, its light weight and flexibility reduce the stresses on the material. While it is probably only available in larger towns, the light weight makes it easier to transport to rural areas. The cost is less than that of glass, especially if the dryer is sized to use the material efficiently. It is easily cut, but the fibers released by cutting can be a health hazard if workers don't take precautions when doing a lot of cutting. Safe disposal at the end of life requires sending it to a landfill. The biggest drawback is that it hard install with a good seal because of the wavey profile. There are forms of this material made as a flat sheet, but so far they do not appear to be available in Tanzania.
Polyethylene sheets. This material tends to be very light, easily cut, fit, and very low in cost. Several replacement covers could be included with a dryer and the glazing cost would still be less than glass or fiberglass. It is not durable, a goat walking on it or a large object would dropped on it would tear the plastic. It can stretch and sag under weight like pooled rainwater on the surface. We have been putting multiple wood strips under it to support it to prevent pooling of rainwater. The long term durability is a question.--how often does it have to be replaced. Some forms are susceptible to damage from ultraviolet (UV) radiation from the sun. At the end of life it is plastic waste, although its light weight reduces the amount of waste. Preliminary attempts to use polyethylene show that it can work for food drying, but it is not durable enough. In one case, it appears that a large bird (perhaps a crow) landed on the film and created holes with its claws.
Polycarbonate sheets. This material is often used in a corrugated form and used to cover greenhouses. It is lightweight, has good light transmission and IR trapping, is easy to cut and install. It is somewhat flexible and can handle some impacts. I have used in solar air heating collectors and found it to work well. It is available in Tanzania, but so far we have only found it in very large sheets. We would like to get a small sample for testing before we commit to a larger purchase.