mirrors for Passive solar...

I'm not talking about the mirrored parabolic dish-trenches that are used for solar water heating, but was wondering... how effective would mirrors be to reflect heat onto say a black Trombe wall intended to radiate especially winter heat into a home over the night?  I suppose you'd have to pay special attention to angles and such for what time of the day the winter sun is even available, keep snow and the path to the black wall obviously clear, and maybe it's more hassle than it's worth...  or maybe it would work.

I don't know how efficient normal/inexpensive mirrors are at reflecting heat (as opposed to light) and some part of me wonders whether such a design might serve more than one good...  like the mirrors that help warm the Trombe wall in the middle of the winter months are aimed at solar panels the rest of the year or something, the tradeoff being do you need the heat or the power more and how much of a difference it makes.


Would this idea have any potential or is it cracked?  : P

Regular mirrors, in the house, reflecting sunlight, do make a hot spot on the floor where it hits. So I think what you suggested would work, to one extent or another, depending on other factors of the design. Something to take into account if you put them outdoors to reflect is cheap mirrors are very fragile, break easily (and badly, sharp shards all over) and would need to be protected, or good heavy mirrors used. Perhaps polished metal.

Yes, mirrors can improve performance by reflecting more sunlight than what would normally shine through window.  They are called heliostats and can also be put on sun trackers.  They also don't  necessarily need to be put adjacent to the window...such as 50' away. You could also build "shutters" having a mirror on one side, insulated, and a tracker to shutter the window over night....

I was wondering how well just something like polished aluminum might work, with or without glass covering/protecting the reflective surface (something more readily cleaned so i'm not digging snow out of a parabolic trough).

My thinking is that a given piece of land only gets so many square meters of solar light and thermal.  It's just a question of how is one best using that - raising food is one use, but if its shining on the land i'm wondering how useful directing around some of the peak day sunlight would be - while trying to not either blind myself from multiple sun intensity light I walk into unthinkingly or start a forest fire.  :^)

I know for solar electric they talk of concentrators and how many suns of concentration one can use, i'm referring to something meant to primarily be employed in winter...  like maybe there's 1/3 the sun wattage in the middle of winter, so designing "3 suns" of concentration brings you back to summer intensity level into a Trombe wall - which in reality is 1 sun because it's reduced to 1/3 to start I mean.  It wouldn't be for as many hours per day, so maybe you boost it a bit more to make up for that...  say "6 suns" (aka twice the summer peak sun intensity) aimed at a black wall?


This is me thinking out of the box again of how to equalize seasonal variations in solar thermal gathering ability...

  If you have snow on the ground you don't need mirrors on tromb wall.  On the other hand if you have snow on the ground you are probably to far north to use a tromb wall.    (they leak as much heat as they collect)

 A reflecting pond or flat snow collector south of a solar heat collector is effective at increasing the heat gain.  If you use an actual mirror be very careful of hot spots as they can set plastics and wood frames on fire.  When I needed more heat for hot water collectors I made batwing reflectors painted white.  It added 50% to the area of the collector (cheap) and added 10f to my water temp.  Tom

If you put a mirror 10' away from where you want the sun to shine, you'll have to change the angle of the mirror all the time to keep the reflection going where you want.  So the closer it is to your target, perhaps the less you'll have to adjust the angle.

I'm pretty sure that they reflect radiant heat as well as light (98%+).  Polished metal may not reflect as fully since I imagine the metal would get hot in the sun itself, thus showing that it's absorbing some of that heat instead of passing it along.

Thomas Michael wrote:   If you have snow on the ground you don't need mirrors on tromb wall.  On the other hand if you have snow on the ground you are probably to far north to use a tromb wall.    (they leak as much heat as they collect)

The leaking as much as they collect isn't correct.  That is about good design.  Here is mine.

Vertical Warm air collector

Now here is a link to another good set of thinking although it needs the newer information added to its design.  Using internet way back because the original website has been down for several years.

passive tromb wall system with passive double thermal trap.

Adding the window screen collector should increase absorbtion and the raising the window slot relative to the glass so it goes back to a reflector to you don't have it as a visible thermal loss path.