Solar Absorbtion reduces performance!?

September 9, 2009

What's up guys, :lol2 Hope you all are making out ok in this economy...

Been thinking about this one for awhile but have been too busy to pursue it fully...

Thought maybe I could get some help. As I understand it Solar Energy can either be 1) transmitted 2) absorbed 3)reflected. And reflection is what we are aiming to do with film. But some is always going to be transmitted and absorbed. Now when heat is absorbed it then moves to the cooler area, which in most cases in summer is inside the home. If this is true, then say a film that has a 50% TSER but has a 45% solar absorption has reduced performance.? By how much? I would think this has been discussed somewhere and I read VC and EWF's (sorry if that's not right) thread going back and forth but it was a bit different and honestly lost me at points. I have even had some difficulty finding solar absorption rates on some mainstay films...

Has anyone else thought about this and how it impacts the consumer? :unsure: I don't know if any of this makes sense but it's been rolling around in my brain for awhile and I need to get it out. :lol

September 9, 2009

Yes, absorption affects film performance. It is the "dirty little secret" of some "high performing" products as well. It is true that as a thermodynamics principles, thermal energy between two sources of connected mater, energy transfers from the warm object to the cool object. If solar radiation is striking it, a window will be warmer than the internal or external air. It will transfer that heat in both directions- some will go back outside, but some will also go inside.

The confounding factor is convection- more of that absorbed energy will be released to the outside if there is convective cooling (air circulation) at the window. If it is a still day, less will go outside.

This is why SHGC and TSER are the only reliable performance characteristics in relating film performance to cooler homes. They take into account not only transmitted radiation, but also absorbed radiation that heats the glass and enters the home anyway.

Solar Gard has a more detailed technical bulletin on this exact topic, called "Reflection and Absorption in Film" or something like that, on their dealer corner.

September 10, 2009

Solar Gard has a more detailed technical bulletin on this exact topic, called "Reflection and Absorption in Film" or something like that, on their dealer corner.

I read that one, it was a sleeper, they could have done better. :beer

September 10, 2009

Solar Gard has a more detailed technical bulletin on this exact topic, called "Reflection and Absorption in Film" or something like that, on their dealer corner.

I read that one, it was a sleeper, they could have done better.

How so?

I successfully used it to demonstrate the concept to the Denver fire department. They chose Hilite 70 over the absorbing competitor product as a result.

EWF · September 10, 2009

TSE-a

TSE-ref

TSE-rej

Combined will equal 100.

Because visible and infrared light are right next to each other in the spectrum, films with a high TSE-ref tend to be visibly reflective. Some customers find high VLR films objectionable. Some measure of performance will be sacrificed, but an acceptable film will be found for that customer. There are plenty of good choices.

Without question, some of the absorbed heat will be re-radiated into the room - but it is still preferable to contain it within the close proximity of the glass. The other factor involved with absorption is the risk of thermal shock and/or seal failure. Always follow the guidelines. But you know that already.

SHGC and TSER numbers take these factors into account. Give those parameters extra weight when making your film vs. film decision. Lean toward the darkest film the customer can tolerate.

What was the customer's primary objective? Heat control?? I need a scotch. Make it a double. Agrrrrrrrr.

-Howard

September 10, 2009

EWF, I certainly agree. It is all about what the customer wants.

Some don't like a high visible light reflected. Others want the highest energy rejected without concern for appearance. Others still want to let a lot of light and not change the appearance of their windows, and also have a good solar energy rejected. Most of the film companies have a good mix of products that meet various sets of requirements. I think the main point is if a custmer wants low heat gain, the TSER/SHGC is the number to look at since it takes into account all effects.

One thing though- thermal energy transmitted into a room does not stay in the vicinity of the window, it just feels warmer there. A window re-radiating 500 watts of absorbed energy into a room is no different than a square 500 watt heater attached to the wall. It just feels warmer near the window/heater, but it warms the whole room.