Active Solar Water Heater Systems
Solar water heater systems are designed to be active, with pumps or fans to promote the gathering of heat, or passive, with no mechanical parts added to the system. Solar water heaters have been around since the late 1800's. Standard natural gas water heaters took their place in the early 1900's. For the last four decades, newer technologies have fixed many problems with solar water heaters. Problems like excessive oxidation and just plain not having enough hot water are not problems now.
The difference between the different kinds of active solar water heaters is the way the heat collector protects itself against cold conditions. These systems are called: Recirculate, drain-down, drain-back, and heat exchange.
The recirculate system
This system uses an electrical pump to move water through the collector to prevent it from freezing. Moving water will not freeze, but some heat is sacrificed to do this.
The drain-down system
This system simply takes the water out of the collector by opening a valve when a thermostat indicates that the water is near freezing. If electricity is down, then the collector empties all water.
The drain-back system
This system uses a heat exchanger to store drained water from the collector when it nears freezing. The heat exchanger coordinated with a storage tank, pushes the water through to the water heater and then the rest of the house. The drain-back system has an electrical pump that works most of the time. If the pump is not working, the heat exchanger stores warm water from the collector and uses this water itself as the heating fluid to heat the water. The heat exchanger can lose up to 10% of its heating efficiency.
The heat exchange system
This system is the only system which does not drain out any water from the collector when it nears freezing. The heat exchanger uses an oily fluid that prevents freezing to travel to the collector and keep the water warm. Some of these systems use compressed air to heat the water instead of an oily fluid.
All active solar water heater systems have pumps and fans working to keep them heating and moving water. These systems need maintenance on a regular basis. All solar water heater systems should be installed so that they can be bypassed when they aren't needed by your regular water heater.
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