Tri Try Tro Trombe

We've been having a heat wave the last couple weeks in Annecy, and Marika and I were discussing the lack of air conditioning here. In spite of that, some buildings remain reasonably cool. Marika, being both an observant nurse and coming from a family of engineers, noticed that most buildings here are made of concrete, rather than the wood/steel buildings we have in the US. While talking to Papou a few nights ago, we mentioned this, and he identified them as Trombe walls.

Trombe walls consist of a concrete bulk, with a layer of glass or reflective coating on the outside.

Simplified from Wikipedia

The key is that UV rays from the Sun are able to pass through the glass and get absorbed by the wall. The wall heats up, which releases infrared light, but these are reflected by the glass, and remain inside. We can see this looking at the spectral reflectance:

Aleksandra Gardecka

Ultraviolet light has wavelength around 0.4 microns, the left edge of this graph, and infrared light is anything from 0.7 microns to 1 mm, beyond the right side of the graph. You can see that in between, the glass goes from mostly transmitting, to mostly reflecting. This means that the UV light will get in, but the re-emitted IR light will be reflected back.

It isn't all about the glass though: The concrete provides a large mass to store the heat when it comes in, and acts as a buffer between the inside and outside. I had hoped to make a simulation of this, but my concept of what happens is that as the weather outside heats up in the day, and cools down at night, those changes are transmitted inside with a delay, as the concrete absorbs and re-emits the heat. That results in the inside being cooler during the day, and warmer at night.

Air conditioning will still cool things better than this method, but this has the advantage of being more eco-friendly. I didn't expect international living to inspire blog posts, but thanks to Marika's observational skills, I've learned something new!