I’m designing a new home and want to incorporate passive solar. My preliminary heat loss calculations based on an outside design temperature of -10F is 18,000 BTU/hr. Solar heat gain from the south facing windows is 71,350 BTU per day during January. What I am trying to understand is the amount of heat that can be stored in the concrete slab that measures 30’ x38’ with a depth of 5”. I understand that concrete can store 28 BTU per cubic ft. Thus the 475 cu. ft. slab can only absorb 13,300 BTU? The sun will only be able to penetrate 16 to 18’ in to the structure. Would Pex running through the slab and connected to a storage tank be beneficial? I was thinking that a Mitsubishi heat pump would suffice for heating and cooling the space with perhaps radiant cove heaters for backup. I want to keep it simple and cost effective. Geothermal is too expensive. Is there a good book or reference for designing passive solar?

Any time you look at heat storage, you have to determine by how many degrees you are going to heat it up. In this case of passive solar, this pretty much means "how hot and cold are you willing to tolerate in the room?". If you want a steady 72F, there is no heat storage.

In most of the passive solar homes that I design (usually 1200-2000 ft²)  I incorporate a small on demand boiler to run domestic hot water and the slab with pex in the slabs.... It helps to keep the slab temp even on those cloudy cold days.  Now I dont usually do all the heating calcs.... I let the guy that is installing the boiler do that work.  A digital thermometer with in the slab sensors is rather manditory when you do hydronic in conjunction to passive solar.... and their is some fiddling around to get the settings correct. 

 I find as long as you have the correct overhangs to minimize overheating in the summer, the right percentage of glass to floor area, good venting, and that the slab is insulated correctly.... you're good to go.  Its not rocket science like a lot of people would like you to believe.  Most old schoolers that dont have radiant hydronic integrated in just fire up their wood stoves to supplemental heating.  

If I was designing your home for you it would not be as square as you are aiming for a number of reasons.  1. difficult to get good convection and ventilation 2. you will get uneven heating....which will make that pex necessary .  I prefer to have a more length along east west axis.  Have operable windows on north and south sides.... low windows on north side.... high windows on southside.  Dont do flat 8' high ceilings.... have vaulting... have windows up high to dump heat, I use motorized awing or crankpole.

Where it gets tricky is when you are trying to do no heating other than the passive solar direct gain slab (carbon neutral).... there you have lots of math to do.  Hope this helps some.  I know that I am not giving you as scientific of an answer as some might.  The beauty of passive solar with some supplemental heating is that it really is pretty simple.   Keeping the scale of the home down helps too.  You can see  a write up of some of the other aspects to think about in the designing of passive solar homes on my blog here.     The trick to doing a nice passive solar home is how to make it beautiful, functional, comfortable, and a good roll model in the neighborhood.  There have been too many incorrectly designed ugly passive solar homes built which has ruined potential home builders perception of it worth.  I wrote an article about this here. 

Where are you building this?

Great feedback! I would be building it near Waterloo, IA.

Thus the 475 cu. ft. slab can only absorb 13,300 BTU?

That is per degree Fahrenheit. So, if the sun charged your slab up to 76F during the day and overnight it was to cool to 68F, the slab would have released 8X (76F - 68F) the 13,300 BTU you calculated.

Is there a good book or reference for designing passive solar?

I would recommend two books; "The Passive Solar House" by James Kachadorian and "The Solar House" by Daniel Chiras. Both are worth purchasing.

Yes, those books are good reading material. So is “The Passive Solar Energy Book, Expanded Professional Edition” by Edward Mazria.

Properly designing a passive building involves being able to accurately determine the maximum instantaneous solar heat that can occur each hour of the year, the forecast climatic solar heat gain that will likely occur each hour of the year, and having a control methodology for addressing the heat variance between the actual climatic solar heat gain (which can be anywhere between zero and the maximum instantaneous solar heat) and the actual building heat loss. Don’t use obsolete rules of thumb for designing passive solar buildings. Always sort out the BTUs and have a good control methodology for dealing with the variances. In addition to fenestration area and SHGC, you should consider and include the effect of your roof overhang design and any local solar terrain obstructions too.  These days nearly all our designs are integrated passive solar and hydronic radiant floor heating systems.  In addition to enabling maximum solar heat gain capture and achieving minimum heating system operational cost, this also allows any excess heat to be efficiently conveyed to the northern rooms (or completely removed if necessary) which ensures that the desired comfort level in all the rooms can be easiliy and perfectly maintained.

You might find our suite of free DIY passive solar building design software to be useful:

http://www.borstengineeringconstruction.com/Passive_Solar_Altitude_Angle_Calculator.html

http://www.borstengineeringconstruction.com/Passive_Solar_Roof_Overhang_Design_Calculator.html

http://www.borstengineeringconstruction.com/Passive_Solar_Fenestration_Exposure_Calculator.html

http://www.borstengineeringconstruction.com/Passive_Solar_Heat_Gain_Calculator.html

http://www.borstengineeringconstruction.com/Passive_Solar_Thermal_Mass_Performance_Calculator.html

sailawayrb thanks for posting those calculators... I will give them a try .... I agree hydronic integration helps spread the heat around.  

Sure thing Greenovision and hope you find them useful. Sustainable by Design also has some great passive solar visualization tools:

http://www.susdesign.com/tools.php

Agagent, you may find our thermal mass performance software useful for your specific question/problem. Please keep in mind that concrete slabs don’t actually store heat…they heat up when they are exposed to irradiance, they convect and radiate heat to the living space when their temp becomes greater than the room temp, they cool down when they are no longer exposed to irradiance, and they result in living space heat loss when their temp becomes less than the room temp.  The rate at which passive solar concrete slabs heat up and cool down can be significantly influenced by the material used in their construction and the slab temp can be accurately controlled by the hydronic radiant floor control system.