Dear Nassr Hussein , what you are interested in is the temperature of the heated air, so the thermocouple should be inside the chamber where the air is heated. If you place it outside, it will bring you back information about the air temperature in the exterior of the heater or at best the surface temperature of the glass cover. I´m not an expert on these kind of devices, but a quick search on the web shows that often air heaters have a double chamber, one for the fresh ("cold") air and another for the hot air and these designs show a double temperature control, with one thermocouple by chamber (or a series of them), probably because what really matters is the temperature difference the device can provide between the input gas and the output one, like in many heat exchange units.

In some way you can imagine your solar air heater like a car exposed to the Sun with the windows closed, after a while the inside temperature will be hotter than outside, to measure how much hot it is, you will place a thermometer (or thermocouple) inside the car, not outside.

Here I included some publications which could be useful to you, both because the information provided and because the clear schemes or pictures showed there, please have a look at them :

Article Thermodynamic performance of forced convection solar air hea...

Article Performance analysis of a double-pass solar air heater syste...

Article A thermodynamic review of solar air heaters

Article A review on thermal energy storage systems in solar air heaters

On internet you also can find videos where the construction of such solar air heaters are build. Also you can use the search box at RG for publications on this field.

Hope this helps.

Hello Nassr,

the temperature is determined by the equilibrium of heat generation and heat transport. It is a complex behaviour, described by the heat equation. Some general statements can be done:

The radiation is absorbed in the depth d* below the surface. This depth is determined by the coefficient of absorption alfa. Here, especially the infrared radiation is important, because the glass is a window for visible radiation. If the thickness d of the glasslayer

d > d* = alfa-1, then the temperature maximum lies near the surface. In the opposite case, the temperature maximum lies below the glass cover.

With Regard

R. Mitdank