Perhaps bury the water tank/s in with your rocks under the concrete. I would think the water will still heat as well as all those rocks you were ging to put there for the same purpose. Then you can still move the water, unlike rocks. Or just let it sit there like a rock. Your option. Bring pipes out to a convenient location for future use even if you don't plan to use them now. Later you could tie into them at anytime if you want.
Although maybe not quite as good as other things, I have seen a system used to heat water, the house, then store the left over heat in the under ground tank. Hot water rises, cooler water settles, and it worked. He tapped the tanks at the lowest point (alittle above actually to aviod any sedimant), takes the coolest water and runs it through pipes in the sun to heat (at the windows), it rises and flows over head to the house, through the wall and a convection heater, as it cools it runs back out to the top of the under ground tank.
As long as the sun is hot and heating the outer pipes the water rises hotter, then as it cools on the other side away from the sun it runs back into the tank. This system then works in reverse at night. Hotter water in the top of the tank rises up through the pipes at the house, cools and runs down the pipes on the outer side and back into the bottom of the tank. The room is always warm if the sun has been shining durring the day, and that keeps the outer house wall warm at night
Because the tank is tapped at top and bottom, it will flow heat in either direction. One thing you need to be careful of is that the house is not warmer than the water and you end up sucking the heat out of the house. I think he had some type of simple thermastat setup to shut off a valve in the system when the house was warmer than the incoming water.
If you are just heating that room (like you would be with rocks only) you could just let it freeflow and not worry about it. You will always have a warm room with only the same losses as with rocks and concrete. And you still have the option of piping it elswhere if wanted. Pretty sure you would capture more heat durring the sunny days for storage anyway.
Run the lines from the bottom of the tanks up the sunny area near the window, across the ceiling, down near the inner wall back to the top of the tank. The water should heat well in the sun, cool in the rest of the room while heating it also.
To keep the wife happy, use a nice solid strong pipes and put some plant hangers on them, just don't shade the front of them with leaves and flowers.
