page_title: Storage water heater
section_title: Solar
hierarchical_section_title: Storage water heater / Solar
caption_attribution_description: English: Photothermic modules (or solar heat flat plate collectors) are increasingly used in the world to struggle against green house gas emissions.
context_page_description: A storage water heater, or a hot water system, is a domestic water heating appliance that uses a hot water storage tank to maximize heating capacity and provide instantaneous delivery of hot water. Conventional storage water heaters use a variety of fuels, including natural gas, propane, fuel oil, and electricity. Less conventional water heating technologies, such as heat pump water heaters and solar water heaters, can also be categorized as storage water heaters.
context_section_description: Solar heat is clean and renewable. This is the most modern system. Increasingly, solar powered water heaters are being used.  Their solar collectors are installed outside dwellings, typically on the roof or walls or nearby, and the potable hot water storage tank is typically a pre-existing or new conventional water heater, or a water heater specifically designed for solar thermal.
  The most basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector.  Many such systems are said to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector.  Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any), can easily heat water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days.
  By contrast, indirect or closed-loop systems do not allow potable water through the panels, but rather pump a heat transfer fluid (either water or a water/antifreeze mix) through the panels.  After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger, transferring its heat to the potable hot water.  When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the controller in closed-loop systems will stop the circulation pumps.  In a drainback system, the water drains into a storage tank contained in conditioned or semi-conditioned space, protected from freezing temperatures.  With antifreeze systems, however, the pump must be run if the panel temperature gets too hot (to prevent degradation of the antifreeze) or too cold (to prevent the water/antifreeze mixture from freezing.)
  Flat panel collectors are typically used in closed-loop systems.  Flat panels, which often resemble skylights, are the most durable type of collector, and they also have the best performance for systems designed for temperatures within 100 °F (38 °C) of ambient temperature.  Flat panels are regularly used in both pure water and antifreeze systems.
  Another type of solar collector is the evacuated tube collector, which are intended for cold climates that do not experience severe hail and/or applications where high temperatures are needed (i.e., over 200 °F (93 °C)).  Placed in a rack, evacuated tube collectors form a row of glass tubes, each containing absorption fins attached to a central heat-conducting rod (copper or condensation-driven).  The evacuated description refers to the vacuum created in the glass tubes during the manufacturing process, which results in very low heat loss and lets evacuated tube systems achieve extreme temperatures, far in excess of water's boiling point.
caption: Photothermic modules (or solar heat flat panel collectors) are increasingly used in the world.
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