Many types of solar hot water systems are possible. I personally like the serpentine, closed loop system, with a multi-tank, non pressurized heat storage vault and double barrier heat exchange or even better the new improved MTD design. A multi media show the drain down system and more shows are available from the Multi Media Page
Since the drain back system is popular and easy to install I made a partial screen web based show available for FREE that illustrates the nature of this hydronic system. View & here the show by clicking this Drain Back graphic .
Every Solar hot water system has it's own plumbing advantages and disadvantages.
Here are the basic solar hot water heating systems available today.