pipe
Plain pipe is notorious for not absorbing many therms, except when the water is stationary. Once the water begins to flow, you can calculate how much water was stored in the pipe, and that is the amount of hot water you will inject into the tank. Now if the pipe had "radiator" fins on it and was painted black like in a solar collector it would absorb more heat, but probably would not save as much as the pipe would cost. A true solar collector on the roof next to the heater would probably cost very little more plus the controls and would do a much better job.