Underfloor solar heating
Ecnerwal gave you a VERY good answer, and I'd like to underscore one point in a separate answer ("accept" Ecnerwal's answer, mine's just supplemental).

I have a neighbor who did something similar, laid pipes into a well-sunlit concrete floor. He learned a strong lesson from doing that: while heat RISES through masonry very very well, heat does NOT travel downward through masonry very well at all; you can't count on the blazing-hot upper surface to heat any pipes below it if the pipes are more than one or two inches below the surface. Experiment by (reversibly) peeling up a little of the terrace surface on a blazing hot sunny day, and feel its underside. I think you'll find that if that stone is more than a couple of inches thick, its underside will be easily 50*F cooler than the top surface, and possibly as much as 100*F cooler.

My neighbor never DID get any benefit to his under-floor pipe, so he abandoned it. You, too, may get no measurable benefit. Too, you should know that the long run of pipe may slow down the hot-water flow by virtue of friction.