Hybrid Car – More Fun with Less Gas

A large pool heater

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Posted by nicksanspam on July 13, 2008, 1:03 pm
The local YMCA gas bill varies from $K/month in the summer to $1K in
the winter. A lot of that heats an indoor pool. They also have a 75'x100'
flattish EPDM roof near the pool. We might cover the roof with 2 layers of
4-year UV polyethylene greenhouse film to heat the pool to 84 F and also
preheat water for showers with a large pressurized plastic pipe coil in
the surge tank. And heat the building, with an HVAC mod for better
dehumidification with heat recovery.

The roof pitch is about 1' in the 75' direction, so the lower film would
always be covered with water if we glue 2x4s to the roof on 8' centers
in the 100' direction with silicone caulk. The materials cost would include
about $500 for 2 75x100' rolls of film (replacing the upper one every
4 years) plus $50 for 350' of perimeter aluminum extrusions with stainless
steel spring wires to fasten the film plus $50 for 1000' of 2x4s, plus
about $0 for an 50 watt static inflation blower, and a few more dollars
for some webbing straps above the upper film to avoid wind fatigue.


10 PI=4*ATN(1)
20 PHI=PI*39.88/180'Phila north latitude (radians)
30 DATA 30.4,37.9,620,1000,33.0,41.0,870,1080
40 DATA 42.4,51.6,1200,1070,52.4,62.6,1520,950
50 DATA 62.9,73.1,1760,830,71.8,81.7,1940,790
60 DATA 76.7,86.1,1890,820,75.5,84.6,1710,940
70 DATA 68.2,77.6,1380,1070,56.4,66.3,1020,1150
80 DATA 46.4,66.3,680,990,35.8,43.4,530,900
90 GASBILL00'summer monthly gas bill ($)
100 PPTHERM=1.5'price per therm ($)
110 HTREFF'pool heater efficiency (%)
120 POOLHEAT=GASBILL/PPTHERM*100000!/30*HTREFF/100'pool heat (Btu/day)
130 GPM0'heater water flow rate (gpm)
140 DT=POOLHEAT/GPM/60/8.33/6'heater dT with 6 hour solday
160 DCOST!00'heater cost ($)
170 AREAu*100'heater area (ft^2)
180 FOR MONTH=1 TO 12
190 READ TA,TP,HS,SS'avg temp,peak temp, horiz sun, south wall sun
200 TD=(TA+TP)/2'average daytime temp (F)
210 N+30.42*(MONTH-1)'day of year
220 DELTA=PI*(23.45*SIN(2*PI*(284+N)/365))/180'declination (radians)
240 ACOS=-ATN(ARG/SQR(1-ARG^2))+PI/2'arc cosine (radians)
250 DAYLEN=2/15*180*ACOS/PI'day length (hours)
260 SOLDAYYLEN-3'solar collection day length (hours)
270 GAIN=AREA*(.9*HS-(84+DT/2-TD))'pool gain (Btu/day)
280 SOLARPC0*GAIN/POOLHEAT'solar heating %
290 TGAIN=TGAIN+30.42*GAIN'yearly gain (Btu)
320 PRINT"600'heater area (ft^2):";AREA;"       heater cost ($):";DCOST
330 SAVINGS=PPTHERM*TGAIN/100000!'annual solar savings
340 PAYBACKOST/SAVINGS*365'simple payback time (days)
350 PRINT"601'savings ($/year):";SAVINGS;"     payback (days):";PAYBACK

month   pool heat             solar heat           % solar

1       1.422222E+07 Btu/day  3633276 Btu/day      25.54647
2       1.422222E+07          5342151              37.562
3       1.422222E+07          7644651              53.75146
4       1.422222E+07          9883401              69.49266
5       1.422222E+07          1.158215E+07         81.437
6       1.422222E+07          1.286278E+07         90.4414
7       1.422222E+07          1.256015E+07         88.31356
8       1.422222E+07          1.133503E+07         79.6994
9       1.422222E+07          9053901              63.66024
10      1.422222E+07          6537276              45.96523
11      1.422222E+07          4204776              29.56483
12      1.422222E+07          3066651              21.56239

heater area (ft^2): 7500          heater cost ($): 2100
savings ($/year):   44583.33      payback (days):  17.19252


Posted by PCPaul on July 13, 2008, 1:14 pm
On Sun, 13 Jul 2008 09:03:58 -0400, nicksanspam wrote:


That's a nice payback time you've got there.. are you in for a cut of the
first years savings as well?

I'm curious though (being a solar n00b..) is this a thin film of water
being heated across the entire roof? If not, is the roof up to the weight
of it?

Do they know it will need to be redone every few years?


Posted by nicksanspam on July 13, 2008, 6:07 pm

No. It's nice enough to be listened to :-) They just installed in power
factor correction capacitor with a 2 month payback, after I sent email
to a YMCA VP for 2 years.

The 2x4s would make terraced pools with an average 3/4" depth. Greenhouse
shadecloth might also make a water film.

Yes. Businesses accept that more easily than homeowners.


Posted by Ed on July 13, 2008, 4:00 pm

Hmmm...   Why reinvent the wheel and have the potential for algae etc.
Apricus has a very efficient tube based water heating system that can heat
the pool and the water for ALL the summer showers.   (along with many other

Also, if they were to just cover the pool during the hours that the pool is
closed, it would probably cut the current energy bill by 30-60%.


Posted by nicksanspam on July 13, 2008, 6:02 pm

I just priced an Apricus... $800 for 30 tubes with about 25.8 ft^2 of net
solar collection area, ie $0/ft^2, ie $23K for 7500 ft^2.


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