Eric´s active solar water heating

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Also known as Thermosiphon System, Passive type solar hot water systems are those which do not utilize an outside power source. Passive Solar is a term that is applied to much more than just solar hot water. For example, painting a swimming pool black so as to absorb more energy, or putting windows on the south side of a house (in the northern hemisphere) in order to let in more of the sun's energy are both considered Passive Solar.

A Passive Solar water heater must take into account the physics surrounding hydrology and thermodynamics. This type of system utilizes a thermosiphon. A thermosiphon uses the energy of the sun to move water or other collector fluid in a vertical direction. This occurs when a liquid has varying temperature, and thus density differences. Because the hottest water will always move upward in, thus displacing the colder water downward, the hot water storage tank or Heat Exchanger must be above the solar collector.

Advantages[edit | edit source]

No external energy is needed in order to move liquid through the system. Less moving parts equals less things to break.

Disadvantages[edit | edit source]

If a hot water tank or heat exchanger is used, it must be positioned above the solar collector in order for the thermosiphon to flow. This can cause problems as most solar systems are on roofs of houses. Many design options are now available that facilitate the storing of hot water above the system.

Anecdotal Stories[edit | edit source]

I've got a couple of 4'x8' flat plate collectors up on my roof. It circulates ethylene glycol with a 1/4 HP pump, closed loop, to pre-heat an 80 gallon tank, which my primary (electric) water tank pulls from.

It is a little more complicated plumbing, but the system works great - I probably don't have to use any electricity to heat water at least 7 or 8 months out of the year (in central United States latitudes). I don't know that I have saved any money though - In five years, I have had two service calls, one to braze over a pinhole in one of the copper pipes, another to replace the pump). I could have heated a lot of water for the $250 I've put into maintenance on an existing system. If I had to retrofit an existing system, the payback would be even worse.

It is also not a thermosyphon system - if I lose electric power, I lose hot water. --Eric Blazek 14 April 2006 (CST)


External links[edit | edit source]

DIY Solar Hot Water Articles can be found here