There are different types of solar domestic hot water (SDHW) systems available. In most of Canada, for year-round water heating, it is important to pick a system that can be protected from freezing.
SDHW systems come in a variety of designs with different types of collectors and circulation systems.
- Evacuated tube collectors are the most efficient collectors available. They can work well in temperatures as low as -40°C (-40°F) and in overcast conditions. Each tube works like a thermos. Individual tubes can be replaced as needed. These collectors can cost twice as much per square foot as flat plate collectors.
- Flat-plate collectors typically consist of copper tubes fitted to flat absorber plates. The flat plate assembly is contained within an insulated box, and covered with tempered glass. Two collectors provide roughly half of the hot water needed to serve a family of four.
- Batch collectors, also called Integrated Collector-Storage (ICS) systems, heat water in dark tanks or tubes within an insulated box, storing water until drawn. These collectors are incompatible with closed-loop circulation systems. Thus, they are generally not recommended for cold climates.
- Closed-loop, or indirect, systems use a non-freezing liquid (antifreeze) to transfer heat from the sun to water in a storage tank. The sun heats the fluid in the solar collectors. The fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.
- Direct systems circulate water through solar collectors. The heated water is then stored in a tank, sent to a tankless water heater, or used directly. Freeze protection is necessary in cold climates.
- Active systems use electric pumps, valves and controllers to move heated water from the collectors to the storage tank. These are common in Canada.
- Passive systems require no pumps. Natural convection moves water from the collectors to the storage tank as it heats up.
- A solar preheat unit typically has one tank that stores solar heat and another tank connected to a conventional water heater—or, in some cases, a tankless water heater.
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