We are building a backyard patio outside of Dallas that will have about 400 square feet of concrete.  The summers can get very hot here and we are looking at some options to make it more bearable.  We will be putting in a retractable shade with a mister system, but we are also thinking it might help if we were to try to cool the concrete down some as well.

Our thoughts are to run PEX through the concrete when it is poured and put in a geothermal horizontal loop.  I have tried to research as much as possible online, and am trying to find a contractor in the Dallas area, but I think the biggest question I have at this point is how to determine how much cooling would be needed.  If we could drop the temperature 10 or 15 degrees would be a big help.  We aren't trying to cool the air, but just cool the concrete down so it is not so hot to walk on.

Does anyone have any thoughts on how much cooling would required?  I know a rough rule of thumb for a house is about 1 ton per 400 square feet.  I have no idea how that translates to the outdoor concrete slab.

I am new to this forum and this is obviously not a critical application.  Any thoughts on how much would be needed would be greatly appreciated.

Thanks

Before answering the question what are your thoughts about the geothermal loop.We have done several jobs like this but normally the heat extracted is transfered to a pool, spa or preheating the domestic hot water.

I'm trying to figure out what is green about trying to cool off a 400 square foot concrete slab that you have intentionally placed out in the intense sun. Your numerical answer is somewhere between zero and 800,000 BTU per day, which is the peak insolation in Dallas. 800,000 BTU is otherwise known as 240 kW, or the amount of cooling that a 5 ton system can put out in 12 hours. Out here in the Pacific Northwest, we have invented a solution to that problem. It is called cloud cover. You can simulate that with the retractable shade you talked about. You can also flood the slab with water periodically to take advantage of natural evaporative cooling.

Thanks for the responses.  I realize that there is no good answer to this.  I guess a better way to phrase the question would be something like - will say 1 ton of cooling have any noticeable effect on the temperature of the slab during the summer? 

Since we are going to have an excavator out redoing a lot of the backyard anyway I thought it would make sense to go with a geothermal loop for this rather than use some sort of chiller.  I was thinking of a slinky coil 4 to 6 foot deep with the trench being around 70 feet in length.  From what I have seen that would be around 800' of pipe.

If it might help on the very hot days I'm willing to risk some time and money to see what could happen.  On the other hand, I have no experience with radiant heating in concrete, so if a loop that size or say a 1 or 2 ton chiller would have no effect, then there is no point in doing it.  I'm just trying to get a feeling on whether it might be worth a try.

You're willing to put in a one or two ton geothermal system and use energy to cool off a patio slab that you could shade?

Using sunny slabs as pool heaters works in some climates, but during recent years' torrid weather in TX people are looking for pool COOLERs.

ICFHybid: In the original post on this thread find:

"We will be putting in a retractable shade with a mister system, but we are also thinking it might help if we were to try to cool the concrete down some as well."

With shade and evaporation cooling the slab wont need a lot of cooling, but I'll agree than using a geothermal heat pump solution to achieve that end isn't usually in the green part of the spectrum. Pumping water into slinky heat exchanger WITHOUT benefit of compressor isn't going to cut it either, but you might get something out of a sufficiently deep well or a large pond.

The greenest approach might be to shade it fully from direct sun with a photovoltaic array, which may even cost less than a geothermal heat pump system.