Is there any reason why twin walled polycarbonate won't work?
It will work, but one needs to know the spectral response for the material. Some materials work better than others because they allow passage of near IR, in sunlight, through into the receptacle, but are more opaque to the longer IR re-radiated by the cooler bodies within the structure (think here difference of black body radiation between 'Earth items', at around 300-400K and the spectral output from the Sun at around 6000K). Glass is very good in this respect, which is why a double glazed unit works well.
In addition, one needs to vector in the coefficient of conduction for the material. One millimetre of polycarbonate will conduct more thermal energy than eight millimetres of glass, I would guess.
So, the term 'work' is not a straightforward comparison of the construction (ie both are effectively double skinned).
The problem of the inner leaf cracking is a function of thermal expansion, I would suggest. If both sheets were able to move relatively freely, instead of being constrained, the inner glass leaf would withstand the elevated temperature. After all, they do not often crack when fitted to our houses (which do not reach that range of thermal gradient).
On the point about flat plate solar thermal collectors, we again have cherry picked data. Yes, flat plate collectors are both cheaper and as good as (or better than) evacuated tube technology in bright direct sunlight at higher ambient temperatures. It is not the case for all conditions which actually translates to evavuated tubes are far better collectors,for the UK climate, when evaluated over the whole year. Ie flat plate collectors are pretty poor in comparison to evacuated tubes, when in less bright light and at low ambient temperatures - like we get in the winter months. Now think when that improved thermal gain is more valuable - in the winter months, I would suggest!
So don't believe all you are told on a non-technical forum like this one! If you don't believe my points above, go and look it up on some technical/scientific websites, where you would be able to find the real truth in peer-appraised papers and not just the cherry picked points put forward by persons with rose tinted spectacles or wearing blinkers!
RAB
It will work, but one needs to know the spectral response for the material. Some materials work better than others because they allow passage of near IR, in sunlight, through into the receptacle, but are more opaque to the longer IR re-radiated by the cooler bodies within the structure (think here difference of black body radiation between 'Earth items', at around 300-400K and the spectral output from the Sun at around 6000K). Glass is very good in this respect, which is why a double glazed unit works well.
In addition, one needs to vector in the coefficient of conduction for the material. One millimetre of polycarbonate will conduct more thermal energy than eight millimetres of glass, I would guess.
So, the term 'work' is not a straightforward comparison of the construction (ie both are effectively double skinned).
The problem of the inner leaf cracking is a function of thermal expansion, I would suggest. If both sheets were able to move relatively freely, instead of being constrained, the inner glass leaf would withstand the elevated temperature. After all, they do not often crack when fitted to our houses (which do not reach that range of thermal gradient).
On the point about flat plate solar thermal collectors, we again have cherry picked data. Yes, flat plate collectors are both cheaper and as good as (or better than) evacuated tube technology in bright direct sunlight at higher ambient temperatures. It is not the case for all conditions which actually translates to evavuated tubes are far better collectors,for the UK climate, when evaluated over the whole year. Ie flat plate collectors are pretty poor in comparison to evacuated tubes, when in less bright light and at low ambient temperatures - like we get in the winter months. Now think when that improved thermal gain is more valuable - in the winter months, I would suggest!
So don't believe all you are told on a non-technical forum like this one! If you don't believe my points above, go and look it up on some technical/scientific websites, where you would be able to find the real truth in peer-appraised papers and not just the cherry picked points put forward by persons with rose tinted spectacles or wearing blinkers!
RAB
