| Commercial Solar Thermal
Options |
Because office and retail buildings don’t
use significantly more or less hot water than homes, the
possibilities for solar hot water systems overlap. But
if you’re building uses an antiquated boiler system,
then be sure to consult with an installer to determine
if it can be retrofitted for a solar thermal (hot water)
addition.
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. Non-freeze
protected systems are typically cheaper, but more likely
to freeze and cause 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: |
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will not freeze |
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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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expansion tank |
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differential control and thermostats |
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DC or AC pump(s) |
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water-propylene glycol solution |
| Conclusion: This
system is the most reliable system on the market, but it’s
considerably more expensive than non-freeze protected units.
If you want to never worry about freezing damage, and are
not terribly concerned about a quick payback, this system
is for you. Changing fluid modestly increases maintenance
time and costs. |
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|
| 2 |
Drainback System (Indirect)
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| This system: |
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will not freeze unless you’re in Alaska |
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uses a non-pressurized, closed loop that circulates a heat-transfer
solution—either distilled water, glycol-water or alcohol |
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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 |
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dumps the heat-transfer solution from the solar collector
loop into a reservoir tank (thus, drainback system) when
outside temperature reaches freezing |
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does not require that the heat-transfer solution be changed |
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requires that the solar collector and all related piping
be located above the drainback tank |
| 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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differential control and thermostats |
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DC or AC pump(s) |
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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 above, but the drainback system is 10 to
15 percent more expensive. It requires less maintenance because
the heat-transfer fluid doesn’t need to be changed.
But an extra reservoir tank (the drainback tank) does mean
another possible problem spot. |
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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. |
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|
| 3 |
Integral Collector Storage System
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 |
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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 ground space |
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will not provide a consistent hot water supply unless backed
up with a conventional heating system |
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is also known as a “batch” or “breadbox” system |
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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 |
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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 and building demand quantity and temperature are
low. 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. |
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| 4 |
Thermosiphon System (Direct
or Indirect)
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| This system: |
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relies on the fact that cold water 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 |
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requires that the solar collector be located on the ground
or on the roof below the hot water tank |
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costs less to purchase and maintain than an active system |
 |
Thermosiphon solar water heaters on employee housing
at Yosemite National Park.
Photo: Jim Schwerm
Installation: Solahart
America
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| 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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controllers and valves |
| 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)
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| This system: |
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circulates potable water through solar collectors with
a low voltage pump |
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operates faster than a thermosiphon system, minimizing
heat loss |
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can function whether the hot water tank is above or below
the collectors |
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uses a differential thermostat to decide when to run the
pump and use the solar collectors |
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should only be used in areas that never experience freezing
temperatures for more than 18 hours |
| Features: |
| • |
solar collectors and mounting hardware |
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small pump (can run on 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. An indirect system is not advisable in areas with very
hard water, as buildup can clog some collector tubes. This
system needs electrical backup and cannot withstand freezing
temperatures. |
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