This goes along with my other post but its a bit different. I'm looking at staining the concrete floor so its basically a med/light grey. I'm wondering what the best option is for a concrete floor for Passive Solar design? Is it ok to stain it that color, or something darker better? Also, should I get it polished? I'm hearing that polishing it would diminish the floor storing the energy? I was looking at polishing, nice and shiny but if it's going to hurt the system than I might not want do it. If I don't do the polish what are my options?

Thanks,
Ryan

Flat black would be best - but you might find that when you have a good solar day, you have so much heat that you don't care.

Hmm I can't do black. I prob could do Med/Dark grey or something. What about polishing, is that ok? My concern is after polishing the surface seems very reflective.

Almost any color except bright-white would be fine. The performance difference in performance between flat-black and a light tan might be measurable in a lab, but won't be much noticed in the heating bill. The IR emissivity will be about the same no matter what you do to it, but the absorption factors only somewhat altered with color & finish. The brighter the luster of the polish, the more light & heat gets reflected, but most of that reflected light/heat gets absorbed by other materials in the room (walls, furniture, people.) Again- in a lab it'll be measurable, but in the heating bill, not so much. Stain it any color you like except titanium-white (or aluminum-bright), and polish it to whatever finish works for you from an aesthetic & ease-of-cleaning point of view.

Flat black or very dark surfaces suck the light out of the room- it's depressing, and it takes more lighting power to get the ambient lux levels to comfortable/usable/aesthetically pleasing levels. Humans aren't designed for living inside an IDEAL solar collector, but the compromises for human-factor don't add up to a big performance hit. There's a wide range of acid stains for concrete these days, with lots of room for pattering/marbling the finish to make it more interesting to look at than "neutral-density field color-X".

Bouncing the radiation around the room vs absorbing it negates much of the effect of your passive solar storage. You want to heat the slab as much as possible and the room as little as possible. The difference between the solar reflectance of different colors and polishes is large - several 100%.

If you want numbers, you could make up some concrete test plates and apply a heat lamp at the angles you expect.

You want a matte, rough finish in a dark color. Ceramic tile or staining are typical solutions. But you also need a room design and furniture that won't shade the floor excessively. If you model the room with Google Sketchup and plug in Google Earth, you'll be able to see how the sun strikes the floor through the heating season.

As Dana suggests, livability is as important as heat absorption, and reason enough to avoid reflection and glare: http://www.ncsc.ncsu.edu/include/_upload/media/pubs/00decor8.pdf