Green Collar Careers Solar Heat
Component Design Review
- System Flow - Open Loop, Drainback & Closed Loop
- Solar Collectors
- Loop Pump
- Storage Tank
- Supplemental Electric Water Heat
- Installer Related Materials
- System Diagram
- Open Loop, Drainback, Closed Loop
feeds the potable water directly to the to the solar collector for
heating. The advantage is its simple and slightly more efficient
then a Closed Loop system. Of course if the ambient temperature
drops below about 28° the collector will almost certainly be
damaged. When the sun is not heating the collector the water flow
is bypassed from the collector to avoid loosing stored heat.
is an Open Loop system with a series of valves that allow the
collectors to be drained rather then just bypassed. Over 95% of
America has some risk of freeze temperatures below 28° at least
once every 3 years and if the system poses enough risk in the
event of failure that they are not preferred.
Loop uses an antifreeze solution to circulate between the
collector and storage tank which has to be pumped. Simple
circulation pumps are relatively low cost, use very little power
and have a very long life. For that reason we strongly recommend
a Closed Loop system.
Flat Plate Solar Collectors are a simple low cost design that
has good efficiency in moderate climates.
Evacuated Tube Solar Collectors are about 50% higher cost
then Flat Plate and in moderate climates are less efficient.
Other designs including Parabolic Dish and Trough Solar
Collectors are currently only gaining popularity in commercial
Flat Plate Solar Collector
Evacuated Tube Solar Collector
Both types of collectors work well however Flat Solar Panels tend
to provide higher efficiency in warm-moderate climates while
Evacuated Solar Tube is more efficient in moderate to cold climates.
Hail damage is another consideration. Evacuated Solar Tubes are more
susceptible to damage from direct hits of hail (usually 1" or
larger) if the hail strikes it just about dead center - otherwise
the hail is deflected. When they do break, its next to impossible to
replace a single element. Flat Panels have more flat area, and are
generally resistant up to about 1-1/4" hail.
One advantage the Evacuated Solar Tubes do offer is that they do tend
to be more efficient when generating higher temperature loop fluid
which can increase a Closed Loop efficiency. Closed Loop
Antifreeze will be heated 30° - 40° higher then the water in the
storage tank; if the hot water is set to 135° that means the solar
collectors will be heating up to 175°.
The Collector Efficiency chart below shows the efficiency of both
solar collectors based on the difference (or delta) between the air
and fluid temperatures. When the delta is greater then about
107° the efficiency of the Flat Plate drops below the efficiency of
the Evacuated Tube.
|Delta Temp °F
If the antifreeze has to reach 175°, then in ambient temperatures
above (175°-107° =) 68° the Flat Plate is more efficient, and below
68° the Evacuated Tube is more efficient.
To circulate antifreeze through the solar collectors and back
to the storage tank heat exchanger requires a pump. Pumps are
available to run on 12/24 volts powered by a Solar Cell, and there are
conventional 120/240 VAC pumps. The pump doesn't take much power
to run, generally about 10-20 watts.
To switch the pump on/off the Controller monitors temperatures
of the high side and low side of the antifreeze in the loop.
Usually there are two thermocouple inputs to the Controller and a
relay output that switches the pump circuit. In some systems
(usually an Evacuated Tube in hotter climates) if no hot water is used
for a period of time (often referred to as "vacation mode") the
temperature in the storage tank can reach close to boiling. To
overcome that risk, some Controllers have a bypass switch that
controls added valves to bypass the storage tank. This mode may
not be needed with a Flat Plate as the temperatures of the antifreeze
rarely gets close to boiling. Consult the supplier for your
application as they will have specific information on this.
The general rule of thumb is each member of the household will
use 20 gallons of hot water a day. That may be a bit excessive
depending on the family but its a safe number to use. Storage
tanks are similar to a hot water heater except they don't have a
heater element and the have an internal heat exchanger the antifreeze
While a non-toxic antifreeze is used it there were to be an
internal leak the possibility of the antifreeze mixing with potable
water is eliminated by using a double wall of protection. Early
systems mounted the heat exchanger coils inside the storage tank.
Current storage tanks have the heat exchange tubing coiled around the
outside of the tank.
Sizing the system is generally based on a daily 24 hour cycle.
Trying to store some extra hot water for cloudy days isn't going to
hurt except that to return to temperature means it will either take
extra time or more solar collectors. Plan on a 24 hour cycle for
an every day dwelling, although for weekend retreats etc, it may be
more cost effective to plan on a ratio 7 days of heating for every 2
days of storage.
Tanks are generally sized to 80 gallons and 120 gallons. A
family of 4 would need an 80 gallon tank.
Size and location of the storage tank may require squeezing an
extra tank in the mechanical room where the water heater was. It
would make sense that someone should make an electric water heater
with a heat exchanger right? Well someone has. Rheem
Supplemental Electric Water Heat
More about the Rheem Solaraide HE - its the first dual-purpose
system built to accommodate both a closed loop solar water heater that
includes a heating element. Available in 80 and 120 gallon sizes
this system is lower in cost then buying both a storage tank and a
good quality water heater. If you choose the Rheem
Solarade HE option we would recommend the 120 gallon system for 4
people as this will keep a more uniform temperature and provide a
longer cushion for depleted hot water before the heater switches on.
Unless your customer can go without hot water during extended
periods of inclement weather they still will need a conventional hot
Installer Related Materials (IRM)
Mounting provisions will need to be included no matter
where you plan on placing the solar collectors. Mounting
provisions will include stilts that angle the solar collectors for
optimal alignment with solar altitude.
Not all solar collectors are roof mounted as not all homes have the
right geometry or conditions for that. Some solar collectors are
ground mounted which actually has a lot of advantages if the space is
available. Some solar collectors have been built into cleverly
designed awnings to provide shade over patios or south facing windows.
Different roof styles take different mounting hardware. Tile roof
construction can be compatible with a roof mounted solar collector.
Plumbing will need to be tallied up in lengths and sizes of
piping, along with any elbows, couplings, reducers, caps, and hand
valves required. During the site survey you'll get an idea of
how long the runs are of what sizes and how may turns it takes.
All of the loop plumbing will need insulation. Also hanger iron
fasteners and more little parts.
We've put together an "IRM Kit" of suggested materials that should
have more then enough materials to do the job, and when the job is
done, merely replenish the kit.
Section 402 will cover System Configuration.