I have been looking at house plans incorporating passive solar in the design. Several pictures of finished houses show white carpeting on the floor inside of the patio doors that face south (master bedrooms). Without the sun striking thermal mass how is solar gain acheived ? Wouldn't dark tile on the floor be better ?

Absolutely. The three pillars: southfacing glass, thermal mass, preferably with dark matte finish, and control via overhangs, window treatments, awnings and the like. You may need a distribution system, fans or convection, as well.

Your taste will vary, but Frank Lloyd Wright has many passive solar designs dating from in the 1930s. It's best to crib design features from timeless architecture IMHO. You can find design tools and advice at builditsolar.com.

Frank Lloyd Wrong was not a building scientist, and built many pretty houses that were uncomfortable or had ongoing moisture issues. Don't crib TOO closely without checking out the results.

The PassiveHouse USA has decent and easy modeling tools for projecting performance, and BeOpt (a freebie download from the US D.O.E.) is pretty good too. With better windows and better modeling/simulation the passive solar trend has been moving away from excessive thermal mass and excessive glazing, and more toward super-insulation with more moderate thermal mass. More glass needs more mass, and getting the heat into that mass at a high enough rate to avoid comfort problems isn't always possible even if you trade in the white rugs for black slate. Many certified PassiveHouse designs work great in winter most of the time (you'll sometimes have to open some windows for some passive cooling in January), but are often uncomfortable during the summer, even with properly designed roof overhangs.

We have a suite of DIY passive solar design software on our website that you may find useful:

Borst Passive Solar Altitude Angle Software

Borst Passive Solar Roof Overhang Design Software

Borst Passive Solar Fenestration Exposure Software

Borst Passive Solar Heat Gain Software

Borst Passive Solar Thermal Mass Performance Software

Here’s a sample of what needs to be determined as a minimum when designing a passive solar building:

Sample Passive Solar Analysis

I'd like to see online data from a well instrumented passive solar house. Something like this (unfortunately an active system). For example, I've seen "our passive solar works great" and after digging deeper, "we see 30F+ temp swings inside".

My passive solar house has had a range of 67 to 73 this winter with a hydronic concrete slab held at 75 degrees, hiked to 80 with outside temps in the teens or lower. if. As a practical matter, that means comfortable at 67 with warm tootsies, and tee shirts and bare feet at 73. The bigger complaint is being kept in the dark on cold, overcast days by thermal shades. Windows exist for reasons other than energy. (People like to look out through them.)

With properly sized thermal mass, there's no reason for a 30-degree swing. Yes, it's a difficult calculation, but it's quite easy to overdo mass as I did. A plus is freeze protection. I left this house unheated in early 2015 for a week with two, rare subzero nights. Obviously you'll want a means of reheating the mass directly and quickly.