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Residential Solar Thermal Options |
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| If the temperature drops below freezing in
your area, then the two systems below are your best options
for solar thermal. However, some people successfully use
cheaper, less freeze-protected solar thermal systems in areas
that experience only sporadic periods of freezing temperatures,
such as Middle and South Georgia. System options marked 3,
4 and 5 below are typically cheaper, but they are at risk
of freezing and causing considerable plumbing damage. |
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Only a qualified installer can determine what system will
prove reliable where you live. |
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| Freeze-Protected Systems |
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| 1 |
Indirect, Freeze Protection System
(Indirect)
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| This system: |
| • |
will not freeze unless you’re in Alaska |
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uses a pressurized loop that circulates a food-grade water-glycol
solution |
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indirectly transfers heat from the water-glycol solution
to the potable water entering your hot water tank |
| • |
requires that the glycol solution be changed about every
few years |
| Features: |
| • |
flat plate collector(s) and mounts |
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heat transfer unit |
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connectors and valves |
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small expansion tank |
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differential control and thermostats |
| • |
DC or AC pump(s) |
| • |
water-propylene glycol solution |
Conclusion:
This system is the most reliable unit available, but it’s expensive
compared to non-freeze protected units. If you want to never worry about
freezing damage, and are not concerned about a quick payback, this system
is for you. Changing fluid modestly increases maintenance time and costs. |
|
| 2 |
Drainback System (Indirect)
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| This system: |
| • |
will not freeze unless you’re in Alaska |
| • |
uses a non-pressurized, closed loop that circulates a heat-transfer
solution—either distilled water, glycol-water or alcohol |
| • |
indirectly transfers heat from the heat-transfer solution
to the potable water going into the hot water tank. This
heat exchange takes place in a special heat-transfer unit |
| • |
dumps the heat-transfer solution from the solar collector
loop into a reservoir tank (thus, drainback system) when
outside temperature reaches freezing |
| • |
does not require that the heat-transfer solution be changed |
| • |
requires that the solar collector and all related piping
be located above the drainback tank |
| Features: |
| • |
flat plate collector(s) and mounts |
| • |
heat-transfer unit |
| • |
connectors and valves |
| • |
differential control and thermostats |
| • |
DC or AC pump(s) |
| • |
heat-transfer solution (water, water-glycol
or alcohol) |
| • |
10 to 15 gallon reservoir tank |
Conclusion:
This system is pretty much comparable to the Indirect, Freeze Protection
system, though it is usually 10 to 15 percent more expensive. It requires
less maintenance because the heat-transfer fluid is not in a pressurized
loop. But an extra reservoir tank (the drainback tank) does mean possible
plumbing problems. |
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Non-Freeze Protected
Systems
If you live in an area that never experiences freezing, there are three
more options available to you. |
| |
| 3 |
Integral Collector Storage System
(Direct)
These systems can either preheat all water that goes into the hot water
tank, or can stand alone as collector-storage units. |
| This system… |
| • |
is very simple in design and operation |
| • |
can combine collector and storage into
one heavily glazed unit |
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requires a roof that can support hundreds
of pounds or unshaded space on the ground |
| • |
will not provide a consistent hot water
supply unless backed up with a conventional heating system |
| • |
is also known as a “batch” or “breadbox” system |
| • |
will probably freeze and cause considerable
plumbing damage in areas that experience freezing temperatures |
| Features: |
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a batch collector, typically 30 to 60 gallon volume |
| • |
mounting hardware |
| • |
connectors, valves and controls |
Conclusion:
This system is extremely simple to use, but does not provide a dependable
source of hot water unless sunlight is extremely consistent. When used
to preheat all water going into an existing hot water tank, the system
is more dependable because gas or electricity will keep tank water at
a constant temperature. |
|
| 4 |
Thermosiphon System (Direct or Indirect)
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| This system: |
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relies on the fact that cold water is more dense than warm
water |
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pulls cold water from the hot water tank down into solar
collector, then sends sun-warmed water back up into the hot
water tank |
| • |
requires that the solar collector be located on the ground
or on the roof, below the hot water tank |
| • |
costs less to purchase and maintain than an active system |
| Features: |
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flat plate solar collector(s) and mounts |
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piping to connect the collector with the water tank |
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oftentimes, a specialized thermosiphon water tank |
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controls and valves
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Conclusion:
Thermosiphon systems are typically less expensive than active, pump-driven
systems, and they’re more reliable than stand-alone integral collector
systems. They should be connected to and backed up with electric or gas
water heaters. The most cold-resistant models can only be used in areas
that never experience ambient air temperatures below 20 degrees Fahrenheit
for more than 18 hours. |
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| 5 |
Active Direct System (Direct)
|
| This system: |
| • |
circulates potable water through solar collectors with
a low voltage pump |
| • |
operates faster than a thermosiphon system, minimizing
heat loss |
| • |
can function whether the hot water tank is above or below
the collectors |
| • |
uses a differential thermostat to decide when to run the
pump and use the solar collectors |
| • |
should only be used in areas that never experience freezing
temperatures for more than 18 hours |
| Features: |
| • |
solar collectors and mounting hardware |
| • |
small pump (This pump can be powered with a photovoltaic
panel—therefore only when sun is shining. This eliminates
the need for a differential thermostat) |
| • |
connectors, controls and thermostats |
Conclusion:
Functions better than a thermosiphon system, but relies on an electrical
pump. A direct system is not advisable in areas with very hard water,
as buildup can clog some collector tubes. This system needs gas or electrical
backup for the water tank and cannot withstand freezing temperatures. |
