Solar water heating systems have certain features in common:
Solar collectors that absorb heat from the sun. These collectors are generally mounted outside on a roof or sun-facing side of a building.
A storage tank for holding the heated water before it is used in the home. This tank can be placed outside or inside the residence as appropriate for local zoning restrictions and aesthetic considerations.
Connecting pipes to feed water into the system and convey fluid between the collectors and the balance of the system.
The specific layout of solar water heating systems can be either an active or passive configuration.
Characteristics of active solar water heating:
- Includes electrical components to operate a pump that circulates water throughout the system.
- Allows full thermostatic control.
- Forces water through the heating apparatus as needed.
- Can be more efficient than passive systems, but also more expensive.
- Use of a pump allows the solar collectors to be situated at a higher elevation than the pipes and storage tanks.
- Use of a pump also means that the water, or heat transfer fluid, can be drained from the collector when the system is turned off.
Characteristics of passive solar water heating:
- A passive configuration does not include electricity, electronics, or pumps
- Does not forcefully move water through the heating apparatus or storage unit.
- Is designed to make use of natural laws (i.e., thermosyphoning, which incorporates gravity and pressure of hot versus cold) to heat, move, and store the water.
- May be less efficient than active systems.
- Can be less expensive to build than active systems.
- Requires the storage tank to be near and above the collectors.
- Collectors have water, or heat transfer fluid, in them at all times.
- Can require intervention to prevent excess hot water from building up in the system. For example, opening and closing a valve daily to fill and drain an external tank of water heated on a rooftop during the day.
Potential issues with solar water heating systems:
- Pressure within the system will rise if the water gets too hot; this is most likely in directly heated solar tanks.
- Certain passive systems can heat water to a dangerously high temperature, requiring discharge and safety mechanisms.
- Weight of a full storage tank may be too heavy for mounting on the roof.
- Need to prevent freezing of the system. For example, un-insulated storage tanks kept outside can freeze when temperatures drop too low.
- Need to prevent heat loss from the storage tank into cool ambient temperatures.
- Need to release excess heat and pressure in over-heated systems.
- If water sits too long at temperatures between 68 and 140 degrees Fahrenheit, dangerous bacteria can proliferate.
- Systems that directly heat fresh cold water can become clogged with mineral deposits, limescale, and debris from the water.
- Any cold water heated through these systems is no longer drinkable quality, and backflow needs to be prevented for safety from contamination.
- Collectors and other components need to be able to withstand long-term exposure to the elements.
For more information, see:
Laughton, C. Solar Domestic Water Heating: The Earthscan Expert Handbook for Planning, Design and Installation. Earthscan, 2010.