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Application Statement:
An apartment complex in Sacramento California wants solar. The
complex is a single building with 55 units. 10 of the units are
triple bedroom, 10 units are double bedroom, and the remaining 35 are
single bedroom. Design a suitable system.
| 1
Sizing the Array |
Using the table from section 1, we see
that triple bedroom units have a daily draw of 45 GPD, doubles 35
GPD, and singles 20 GPD. The total draw for the complex is
therefore 1,400 GPD [10*45+10*35+35*20]. Well water
temperatures in this region are approximately 62 F so the sizing
ratio would be 0.71 ft2/GPD [1.15*8.34*(135-62)/984]
assuming we used painted collectors in this temperate region.
The total array area is therefore 996 ft2
[0.71*1,400]. Assuming we used EP-40 collectors with an area
of 40.8 ft2, the total number of collectors is 24
collectors.
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| 2
Choosing the System Type |
Because Sacramento does experience
frequent winter freezes, we will want to go with a closed loop
system. While we have the option of drainback or glycol, we
will assume there is a need for simplified piping runs at this site
and will choose glycol.
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| 3
Laying out the array |
Because it is a 24 collector closed loop
system, it would look exactly like the glycol diagram shown in
section 3
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| 4
Sizing the Heat Exchanger |
24 Collectors at 40.8 ft2 each
yields an installed array area of 979 ft2. For the SPL31
series, the number of plates would be 108 (979/9), which we would
round up to 110 plates. For the SPL110 series, the number of
plates would be 20. Since the minimum number of plates
available on the SPL110 Series is 30, we would opt to go with a 110
plate SPL31 unit (Model # SPL31-110)
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| 5
Sizing the pump and piping |
With a design flowrate of 1.04 GPM/collector,
the total array will require 25 GPM of flow. Using the chart
in section 5 we see that the smallest pipe size that can handle this
flow rate is 1.5" piping. This will be the size of the
main supply and return runs. The pressure loss per 100' of
1.5" type M tubing at 25 GPM is approximately 2.6 psi.
Because we have 3 rows of 8 collectors each, we can use 1"
headers. From section 5 we determine the pressure loss in the
collector row to be 0.32 psi (4 GPM average going through 64 feet of
1" header). From section 4 we see that average pressure
losses through the SunPlate heat exchanger are 1.5 psi. If the equivalent pipe run length is 100' round trip accounting for all
fittings and valves, then the overall loss is 4.42 psi
[2.6+1.5+0.32], which is equal to 10.2 feet of head. Since the
fluid is glycol, we will add 20% to the head to account for
viscosity. The design pump pressure is therefore 12.3 feet
at 25 GPM. Looking at the pump curves in section 3, we see
that these pumping requirements can be met by a Grundfos 43-75F
circulator or a Taco 0013.
For optimal heat exchanger performance we want to pump the potable
side at the same flow rate as the glycol side (25 GPM).
Because the flow rate is the same, we will use 1.5" pipe with a
pressure drop of 2.6 psi per 100'. If the potable recirc loop
has an equivalent length of 30' then the total potable loop pressure
drop including the heat exchanger would be 2.3 psig
[30/100*2.6+1.5], or 5.3 feet. For this low head application,
suitable pumps would be a Grunfos 26-96BF or 43-75BF, or a Taco 0012
or 0014.
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| 6
Sizing the Storage |
According to section 6, the ratio of
storage volume to collector area would be 1.2 gal/ft2
[120/(160-62)]. The total storage for the array would be 1175
gallons (979*1.2). We would likely choose the closest size
ASME tank greater than 1175 gallons that would fit at the
site. If that size storage could not fit, we would have to
reduce the size of the array for a smaller tank.
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| 7
Sizing the Drainback or Expansion Tank |
To determine the size of the expansion
tank we need to determine the closed loop volume. At 1.2
gallons capacity per collector, the array holds 29 gallons
(1.2*24). For 1.5" type M piping, the fluid capacity is
9.5 gallons per 100'. The total loop volume is therefore 38.5
gallons (29+9.5). Looking at the chart of expansion tanks in
section 7, either an Extrol #90 or an Extrol SX-30V would be
sufficient for the application.
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| A
Review of System Performance |
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| The 2 graphs above show the
simulated operation of the system we just designed.
Solar tank temperatures illustrate the difference in
Sacramento's cool cloudy winters and hot central valley summers
when it reaches the design value of 160 F. Likewise, the
system energy output shows over a 90% solar contribution
through the summer falling off to 20-40% in the cloudy winter
months. The annual solar fraction for this system would
be 64%, which is in line with the most that could reasonably
be expected for that climate. |
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