Solar Pool Heating

Posted by CT on January 6, 2006, 2:35 am

All,

I am looking to add solar heating to my pool and my local installer
will not give me a straight answer.  I think it is because he is only
trained on book answers...  So here it goes.

I have a NW facing roof (46 deg N of West), I live in Central FL, the
roof pitch is 6" drop for every 1'.  I would like to put about 225 sq
ft of panels on that roof.  How can I figure how effective they are
going to be?  I have a potential to add another 225 sq ft on the NW
to a SW facing roof.

I am not looking to add much temp in the winter, most of the swimming
is from Apr to Oct.  Gas heater takes the spa up during the winter. I
would perfer to leave the panels flat on the roof and not on a rack.

So my questions are,

How can I figure out how effective the panels are going to be?  I was
told that for every 20% of pool surface area on a south roof you will
get 4 degrees of increase.

Will having a split system NW and SW be an issue?  One side cooling and
one heating = 0 btu.

Posted by nicksanspam on January 6, 2006, 12:21 pm

NREL says 1020 Btu/ft^2 of sun falls on the ground and 1280 falls on
a south wall on an average 59.9 F January day with a 69.8 F daily max
and a 0.0083 humidity ratio in Tampa. Deep ground is 72.3 F. A square
foot of flat R1 glazing with 90% transmission over 80 F water might
collect 918 Btu and lose 6h(80-65)1ft^2/R1 = 90, for a net gain of 828.

With ambient vapor pressure 29.921/(1+0.62198/0.0083) = 0.394 "Hg and
80 F water vapor pressure e^(17.863-9621/(460+80)) = 1.047 "Hg, 1 ft^2
of uncovered pool might lose 24hx100(1.047-0.394) = 1568 Btu/day.

You might trickle water over the roofs with 1 or 2 layers of greenhouse
polyethylene film suspended above. Or put a plastic film greenhouse over
the pool for the winter, which would reduce the heat loss by evaporation.

Nick

Posted by CT on January 6, 2006, 3:04 pm
Thanks.

How do I convert 1280 btu from a south wall to a NW pitched roof?

Posted by John on January 6, 2006, 4:29 pm

I live in south Jersey.  I had to split my panels, 120sq ft facing east, and
120 facing south.  Pool surface area is 512 sq ft and 21,000 gallons.  The
full 240 sq ft of panels only get about four hours of full sun due to
neighbors tree's on the south side and my oaks on the west side. The full
surface of the pool get's about one hour of direct sun. Prior to the
addition of the panels the pool fluctuated between 74 and 78 degrees.  After
adding the panels last spring it fluctuates between 80 and 85.  I lay them
flat on the roof (30 degree angle - I don't know roofing pitch terms)

I would suggest maxing out coverage on the SW roof and 120 sq ft or so on
the NW see what happens. They can always be moved.

You said you had a NW and SW facing roof? Is that correct or is it NW and
SE.  IS there no pitch on the east side?

John

: All,
:
: I am looking to add solar heating to my pool and my local installer
: will not give me a straight answer.  I think it is because he is only
: trained on book answers...  So here it goes.
:
: I have a NW facing roof (46 deg N of West), I live in Central FL, the
: roof pitch is 6" drop for every 1'.  I would like to put about 225 sq
: ft of panels on that roof.  How can I figure how effective they are
: going to be?  I have a potential to add another 225 sq ft on the NW
: to a SW facing roof.
:
: I am not looking to add much temp in the winter, most of the swimming
: is from Apr to Oct.  Gas heater takes the spa up during the winter. I
: would perfer to leave the panels flat on the roof and not on a rack.
:
: So my questions are,
:
: How can I figure out how effective the panels are going to be?  I was
: told that for every 20% of pool surface area on a south roof you will
: get 4 degrees of increase.
:
: Will having a split system NW and SW be an issue?  One side cooling and
: one heating = 0 btu.
:

Posted by Gary on January 6, 2006, 4:45 pm
CT wrote:

A NW facing tilted surface is not ideal for collecting.  If you can't do
south facing, then flat would be better that NW facing and tilted.

I pasted in some Radiation On Collector runs below -- this should give you some
idea
how it will work out.
All the runs are for 1 sqft of collector at a latitude of 30 deg.
You will want to set your display for a fixed width font, and wide enough not to
wrap the
lines.

Run 1: The first run is for a surface facing south (ideal) and tilted at 27 deg
(6/12).

Runs 2: The 2nd run is for a surface facing NW (yours) and tilted at 27 deg.
By comparing the two, you can see how much of hit your taking for the NW
orientation.

Run 3: The third run is for a flat (0 tilt) collector.

The "Total" in the last column of each run shows the radiation on 1 sqft of
collector area for a full day for the month listed (BTU/day).

So, comparing the NW facing 27 deg tilt collector with a south facing
27 deg tilt:
In mid summer the NW facing one does pretty well -- 2300 BTU/sqft facing NW
vs 2400 BTU/sqft facing south.

In mid winter the NW facing collector is way down -- 600 BTU/sqft facing NW
vs 1900 BTU/sqft facing south.

By Oct 21 the NW facing collector is down to half what a south facing
collector would give.

--
The very last run gives the hourly output for sept 21.  It shows the pattern of
sun on the collector throughout the day.

combinations yourself:

**Bear in mind that these runs give solar radiation on a surface with FULL SUN.
To get the heat that will go to your pool, you need to account for less than
full sun,
and the collector efficiency.  The runs are only good for comparing one
collector orientation to another, not for estimating collector output.

If all this "does not compute" feel free to drop me an email.

South facing collector titled at 27 degrees  <---------------------
Month by Month Summary of Sun on Collector
(100% sunny weather)

Collector Area:        1.0 (sqft)
Collector Azimuth:     0.0 (deg) measured from South
Collector Tilt:       27.0 (deg) measured from horiz
Latitude:             30.0 (deg)
Altitude above SL:     0.0 (ft) Above Sea Level

Date ----  Sun ----------------  Collector -----------------------
Month Day  Direct   Di-   Total    Direct      Difuse     Total
Normal   fuse
1   21   2537     147   2684       1873        139       2012
2   21   2896     173   3070       2119        164       2283
3   21   3031     215   3246       2214        203       2418
4   21   3071     299   3370       2112        283       2395
5   21   3096     376   3472       1987        355       2342
6   21   3066     410   3476       1914        388       2302
7   21   2995     407   3402       1924        385       2309
8   21   2894     352   3245       2005        332       2337
9   21   2829     263   3092       2079        249       2328
10   21   2731     198   2929       2012        187       2200
11   21   2468     155   2623       1825        147       1972
12   21   2409     138   2547       1750        131       1881
Sum        34022    3134  37156      23815       2963      26778

NW facing collector tilted at 27 degrees <------------------------
Month by Month Summary of Sun on Collector
(100% sunny weather)

Collector Area:        1.0 (sqft)
Collector Azimuth:  -135.0 (deg) measured from South
Collector Tilt:       27.0 (deg) measured from horiz
Latitude:             30.0 (deg)
Altitude above SL:     0.0 (ft) Above Sea Level

Date ----  Sun ----------------  Collector -----------------------
Month Day  Direct   Di-   Total    Direct      Difuse     Total
Normal   fuse
1   21   2537     147   2684        619        139        758
2   21   2896     173   3070        986        164       1150
3   21   3031     215   3246       1383        203       1587
4   21   3071     299   3370       1736        283       2019
5   21   3096     376   3472       1960        355       2315
6   21   3066     410   3476       2016        388       2404
7   21   2995     407   3402       1910        385       2295
8   21   2894     352   3245       1656        332       1988
9   21   2829     263   3092       1311        249       1559
10   21   2731     198   2929        924        187       1112
11   21   2468     155   2623        598        147        745
12   21   2409     138   2547        485        131        616
Sum        34022    3134  37156      15583       2963      18547

Flat collector (horizontal)
Month by Month Summary of Sun on Collector
(100% sunny weather)

Collector Area:        1.0 (sqft)
Collector Azimuth:     0.0 (deg) measured from South
Collector Tilt:        0.0 (deg) measured from horiz
Latitude:             30.0 (deg)
Altitude above SL:     0.0 (ft) Above Sea Level

Date ----  Sun ----------------  Collector -----------------------
Month Day  Direct   Di-   Total    Direct      Difuse     Total
Normal   fuse
1   21   2537     147   2684       1234        147       1382
2   21   2896     173   3070       1592        173       1766
3   21   3031     215   3246       1920        215       2136
4   21   3071     299   3370       2117        299       2416
5   21   3096     376   3472       2208        376       2584
6   21   3066     410   3476       2215        410       2625
7   21   2995     407   3402       2147        407       2554
8   21   2894     352   3245       2015        352       2367
9   21   2829     263   3092       1812        263       2075
10   21   2731     198   2929       1507        198       1704
11   21   2468     155   2623       1200        155       1355
12   21   2409     138   2547       1082        138       1221
Sum        34022    3134  37156      21050       3134      24185

------------------------------------------------------------------------
Sample HOURLY output for sept 21.
NW facing at 27 degree tilt on Sept 21 (the equinox).
Shows the pattern of sun on the collector for one day -- little sun in
the morning, but pretty good in the afternoon.
Solar Radiation on Collector for day of:  9/21

Collector Area:           1.0 (sqft)
Collector Azimuth:     -135.0 (deg) measured from South, + is to East
Collector Tilt:          27.0 (deg) measured from horizontal
Latitude:                30.0 (deg)
Altitude:                 0.0 (ft) Above Sea Level
Sun Rise:                 6.0 (hr) Sunrise in solar time

Time    Sun --------------------------------    Collector
------------------------------
Hour    Az   Elev  DirNormal  Diffuse  Total    HorzTot IncidAng Direct Diffuse
ITotal

4  106.5  -25.4      0        0        0        0      90        0        0
0
5   98.0  -12.7      0        0        0        0      90        0        0
0
6   90.3    0.2      0        0        0        0      90        0        0
0
7   82.7   13.2    167       16      182       54      90        0       15
15
8   74.3   25.9    243       23      265      128      88        8       21
29
9   63.8   38.0    273       25      299      194      78       57       24
81
10   49.5   48.9    288       27      315      244      68      108       25
133
11   28.5   57.1    295       27      322      275      59      151       26
177
12    0.0   60.4    297       28      325      286      52      183       26
209
13  -28.5   57.1    295       27      322      275      47      200       26
226
14  -49.5   48.9    288       27      315      244      46      200       25
225
15  -63.8   38.0    273       25      299      194      48      181       24
205
16  -74.3   25.9    243       23      265      128      54      143       21
164
17  -82.7   13.2    167       16      182       54      62       79       15
94
18  -90.3    0.2      0        0        0        0      71        0        0
0
19  -98.0  -12.7      0        0        0        0      81        0        0
0
20 -106.5  -25.4      0        0        0        0      90        0        0
0
-----   ------   ------    -----            -----    -----
-----
Day Total            2829      263     3092     2075             1311      249
1559

Angles in degrees, Radiations in BTU/hr, DayTotal in BTU/day
1 BTU/hr-ft^2 = 3.152 W/m^2

--

Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects

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