I got interested in solar water heating a year or so ago, and decided that the campground was the ideal location to install a system. There are a total of 6 showers, 10 sinks, and three washers that all use hot water. An 80 gallon propane water heater provided (or at least attempted to) all of the needed hot water. A solar system to produce enough hot water for all of the needs was considered to be prohibitively large and expensive; however, building a solar water pre-heater to raise the temperature of the incoming water to the propane unit was more feasible.
A system modeled after the one constructed by Gary Reysa at http://www.builditsolar.com was chosen. BuilditSolar has many great projects, and Gary is an excellent resource for do-it-yourself renewable energy projects. He has detailed construction information for many projects, including a solar water heater like the one we constructed.
(above: completed solar collector, constructed from parts available at many local hardware stores)
An active drain-back system was constructed from materials mostly available at any hardware store. The system consists of a solar collector, solar storage tank w/ heat exchanger, differential temperature controller, pump, and all of the plumbing, wiring, and other parts to get the thing working. Basically the temperature controller turns a pump on to circulate water from the storage tank through the collector whenever there is solar heat available. Incoming water to the propane heater passes through a heat exchanger in the solar storage tank to transfer heat from the solar system to the existing hot water heater. The system is considered to be closed-loop in that water passing through the solar collector is never physically used as consumable hot water, only the heat is transferred for use.
(above: poly storage tank and PEX heat exchanger, suction and return lines to solar collector)
Systems in cold climates need to offer some kind of freeze protection. Some use antifreeze, ours uses a drain-back design in which all of the water returns inside to the storage tank whenever the pump shuts off. I installed the system this past spring and have been out of the country since cold weather has set in. The system has not turned into a giant sprinkler due to freezing pipes as I feared, and seems to be functioning and draining as designed.
(above: differential temperature controller that turns the circulation pump to the collector on and off based on collector and storage tank temperatures)
(above: tank insulated with 1.5″ of polyiso rigid insulation, another 1.5″ was added later for a total of 3″ of insulation or about R=20-24)