AERC of WCMA has been created to test the effectiveness of window attachments and to design an Energy Star type rating system to cover these devices.  The work will occur over the next 4 years or so and I'm told Solar Screens will be included in the testing.  The selection of testing candidates depends on factors such as technical merit and market presence. 

A product similar in savings to Solar Screens is Solar Grates.  Each has it's advantages and both are mounted external to the window.  Screens are a woven product mounted on a custom frame.  The weave is typically 80+% closed and blocks all light frequencies, thus darkening the room some.  Screens are sometimes recommended for permanent installation to avoid handling labor and reduce the risk of wrinkling.  Grates are 80% open and operate via SELECTIVE REFLECTIVITY wherein impinging direct sunlight reflects off the grate surface.  The visible light fraction largely reflects and enters the room through the window whereas the infrared portion is largely blocked, leaving the room cool and bright.  The view through screens is better at an angle but the night view can be problematic.  Grates have an unimpeded view head-on but the view becomes blocked as the viewing angle increases.  Grates are recommended for seasonal installation to optimize energy performance.  Maintenance over 10 years has been nil and the Grates have proven to be quite robust.

Images of Solar Grates can be viewed at the "Solar Grates" website.  (I installed the LINK address but I got an error message stating files need to be under 5mb.)  It's http//    -followed by-    hjcochran5.wix.com/solargrates101   .

An insufficient market presence may prevent Solar Grates from being on AERC's testing schedule.  There are many other candidates for the testing schedule set at LBNL.

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could please someone explain for a layman, how is it possible for a solar grate made from a common material like metal, plastic or wood (that's the material used according to the author's site) to reflect solar radiation SELECTIVELY ?

i have read about films having strong reflectivity in NIR compared to the visible region but that is achieved by special pigments they contain.

thank you

Romeo,

I'm not sure what would constitute a good explanation for you.  Here are a couple of thoughts from a fellow layman:

1)  The prototype grates were made of polystyrene containing a UV stabilizing additive.  The egg crate diffusers forming the grate are the common ones found it lighting fixtures of acoustical ceilings. 

2)  Because of some yellowing over a 10 year period, occasional touchup with flat white spray paint has been used to maintain the grate appearance.  Other colors work as well.  That spray paint could be considered a "special pigment" in this application.  From experience, painted surfaces like this also reflect selectively like the unpainted polystyrene.

3)  All surfaces reflect selectively in the sense that certain frequencies in the visible range reflect better than others.  This is why surfaces have different colors as we view them.  A surface absorbing reds and blues while reflecting greens will appear green to our eyes.  How it does this requires an atomic level view of things.  I don't have this capability and I'm not sure I could explain it even if I did. 

4)  The sky is blue.  Blue light reflects better than red light.  Red light (lower frequency) is selectively absorbed by small particles in the atmosphere and the light reaching our eyes contains more of the higher frequency (blue) light.  There is no special pigment involved. 

5)  Actual experimentation with grates shows that when a grate is placed external to a window, the room remains bright because much visible light is reflecting inward but the room is also cooler because it is no longer being heated by the radiant infrared so much.  We don't see the drop in IR because our eyes are not tuned to IR.  The glass of the window is also cooler with the grate in place.  Try it.  Go to a window in the sunlight and it will feel warm.  If it has a film or IR absorbing material (ex: some office building windows) it may even feel hot. 

6)  Not any material will do.  If the grate is made of metal and the metal is polished to a mirror-like finish, this won't  work because a polished surface tends to reflect most all frequencies.  A metal grate that is painted (or dull) will have the selective reflectivity property because it is the paint doing the reflecting.  If the paint is black, it will tend to absorb even more IR but it will also absorb a lot of visible light.

7)  It should be possible to improve the performance of a grate using a special coating.  The only coating I've tried is black paint but I don't prefer it because I like the room to be cool and bright rather than cool and dark.

You probably deserve a better answer but as a layman myself, this is about the best I have.

thank you w d !

...you seem to be the inventor... who experimented with solar grates over a long time... sorry for not undertanding that from your first post.

i would like to make a try myself, i am afraid i wiil not be able to find  the necessary egg crate stuff where i live.

is it possible please to provide some photos and additional info on your experiment ?

if what i am asking for is not appropriate to be posted here (i do not know, am a completely new member) i could give you an email address.

Romeo,

The egg crate diffusers are sold at home building products stores like Loews, Home Depot, and Menards (as examples).  They may be sold at lighting or hardware stores.  These products are priced in line with lighting products.  As I understand it, there are only two remaining actual manufacturers of these products in the U.S., one in Illinois and one in Missouri.  There are numerous distributors, however.

The test equipment we are talking about includes a test rig shown here: 

 http:// .... followed by ...       hjcochran5.wix.com/solargrates101#!ir-blocking--savings-2/cnbl

The test rig has 3 chambers, each with an acrylic face.  External or internal "window attachments" can be mounted for testing.  Typically this is done and then the assembly is placed in sunlight at varying angles.  The temperature rise due to radiant heating is monitored using a temperature probe or non-contact "laser" meter.  The purpose of the test rig was to help guide a learning path, not so much to scientifically document the results for peer review. 

When it comes to detailed testing of window attachment performance, I believe LBNL will set the standard in their work with AERC.  LBNL has a new testing facility called Flexlab and it was designed just for scientific studies such as this.  They also have a talented staff to fashion the testing protocol and analysis. 

The real issue for energy savings is not the test rig data but the live experience of going external to the window to absorb some of the IR before it enters the room.  That requires a mounting technique and some care must be taken not to damage the window sash or frame or cause possible warranty issues with the window.  Some ideas on mounting are shown here:

http://     ...  followed by ....   hjcochran5.wix.com/solargrates101#!mounting-1/c1ur0

Both the L screw and Pop-in methods have their advantages.  Because of the 80% open area of the Grates, the wind has difficulty moving the Grate.

A manufactured Grate product would resemble the Grate Prototype in appearance and performance but there would be important differences in order to reduce the material and labor costs.

If you can't acquire materials for a DIY Solar Grate, you might consider instead a Solar Screen.  The screens have been in the market for several years and are popular in the Sunbelt.  There are a number of websites where you can learn about them and order the materials needed for a DIY screen.  The screens save energy comparable to Grates.  They work on a shading principle and block all frequencies in sunlight, reducing IR gain and making the room some darker.

 

w d,

thank you for your new info.

i will look again at it, more carefully, tomorrow.

i do not live in US but in Romania. it's around 45 degrees northern latitude and continental cilmate over here, for almost all of the country. so sun problems of considerably less magnitude compared to Sunbelt.

still, some efficient exterior see-through shading device, cheap and easy to deploy,  safe and removable, would be great stuff.

w d,

i visited your site... interesting...

could you please say something more... re the following:

- what was the latitude of your experiment location ? ... to have an idea about the sunpath overthere.

- what was your conclusion re the effciency of the grates for various window orientations ? South compared to East and West. and vertical orientation compared to inclined (there is a photo with a skylight).

were the same grates used  for all orientations ? were they mounted the same, parallel to the window ?

- did you test grates made from metal or plastic or only polystyren  ? (the grate covering the right chamber of your test rig in the shown photo seems to not be polystyren... but maybe i am wrong). were they covered with some coating ?

- did you test egg crate grates having different cell edge sizes ? not just the same cell edge size and different depths of the cell.

- did you experiment with other shapes of the cell cross section ? e.g triangle instead of square.

- do grates not require cleaning the dirt in the cells in order to do their job ?

and any other info you think might be of help to someone wishing to make a grate, DIY style.

thank you very much for your shared work and knowledge

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