The National Renewable Energy Laboratory's Guide to Solar Water Heating

B. Types of Solar Water Heating Systems

Solar water heating system types are classified as follows:

Active requires electric power to activate pumps and/or controls.
Passive relies on buoyancy (natural convection) rather than electric power to circulate the water. Thermosyphon systems locate a storage tank above the solar collector, while integrated-collector-storage collectors place the storage inside the collector.
Direct heats potable water directly in the collector.
Indirect heats propylene glycol or other heat transfer fluid in the collector and transfers heat to potable water via a heat exchanger.

C. Design of a Solar Water Heating System

Solar water heating systems should be designed to minimize life-cycle cost. It is never cost-effective to design a system to provide 100% of the load with solar because of the excessive investment in collector area and storage volume. Minimize life-cycle cost by designing a system that meets 100% of the load on the sunniest day of the year. Such a system will usually produce about 70% of the annual load. Other design considerations include maintenance, freeze protection, overheating protection, aesthetics of the collector mount, and orientation. Also, utility rebate programs may impose additional design requirements. For example, a solar water heating system must meet 90% of the load in order to qualify for Hawaiian Electric Company rebates.

1. Properly locate the solar collectors The best annual energy delivery is achieved by facing toward the equator with a tilt up from the horizontal equal to the local latitude. Recent studies show that adequate performance may be obtained with tilt angles and orientations that vary from this considerably. In the continental United States, for maximum performance, collectors should be rotated within 30° of true (not magnetic) south. Also, optimize the tilt of the collecting array. Surfaces tilted up from the horizontal at an angle of latitude minus 15° maximize summer solar gains, but reduce winter gains. Surfaces tilted up at latitude plus 15° maximize winter solar gains and result in a solar delivery that is uniform throughout the year; such a tilt angle may be the best choice for solar water heating systems, as it can reduce the risk of summertime overheating while maximizing use of limited wintertime sunlight. It is usually acceptable to mount the collectors flush on a pitched roof close to the optimal orientation as possible in order to reduce installed cost and improve aesthetics. Resource maps and tables of solar resource information throughout the U.S. are posted at the Solar Radiation Resource Information Center.
2. Protect against freezing Damage can be caused if water freezes in the collector flow passages or connecting piping. There are several strategies for prevention of freeze damage. The most common is to circulate a solution of propylene glycol (never use toxic ethylene glycol) and water in the collector loop of an indirect system. Another strategy is to drain the water from the collector back into a small drain-back tank. This drain-back configuration has the added advantage of protecting the system from excessive temperatures if hot water consumption is reduced due to seasonal use patterns, remodeling, or vacations. Where freezing is uncommon, a controller function that simply circulates water in the collector loop when temperatures approach freezing in conjunction with freeze protection values may be adequate.
3. Provide a tempering valve and bypass capability The tempering valve is very important to assure consistent temperature water is delivered at the taps. Bypass piping and valves allow the conventional system to provide hot water if the solar heating system is down for any reason.
4. Provide periodic maintenance for all systems Check for obvious damage such as broken collector glazing or wet pipe insulation. Check pH and freeze point of heat transfer fluid. Check control temperature sensors against thermometers to be sure sensors are functioning. Check proper pump operation, etc. For a simple comprehensive test, check the preheat tank temperature, it should be hot after a sunny day.