Ordinary glass is not very good as a cover for an enclosure designed to keep wind chill to a minimum because it exhibits several unwanted characteristics. It is unstable, brittle and thermally conductive. Furthermore, because it is green it attenuates light, sometimes considerably.

Borosilicate glass AKA Pyrex is completely opaque to both visible and UV light. Other properties are that it withstands rapid heat changes and high temperature differentials over an extremely small area, hot spots; it is tough, extremely stable and does not allow the passage of long wave infrared radiation.

The majority of the photons reaching the Earth are in the visible and ultraviolet light spectrum. Although some infrared light reaches us most is reflected back into space by the earth’s CO2 layer so there is not enough to be of great use. Heat, however, is produced when high frequency light, UV, is converted into low frequency infrared radiation.

UV and visible light photons easily pass through borosilicate glass, and when they strike a darkened surface they are converted into long wave infrared radiation, which heats the material and, by conduction, the water within. This means that the glass does not heat up because infrared radiation cannot pass through it. Therefore a high temperature differential between the outside wind chill world and the inside sweltering temperature of the enclosure is maintained, under sunny or at least lighted conditions.

Yet another design problem

Light radiation arriving at the panel at low angles such as in the early morning and late afternoon is reflected off the surface, which reduces the potential efficiency of the enclosure, and therefore the solar heater, by restricting the daily number working of hours that the appliance can operate. Happily, there is a solution.