[Greenbuilding] [BULK] Re: Solar Water Heater Tilt
Lawrence Lile
LLile at projsolco.com
Mon Feb 9 14:07:47 CST 2009
Great link, Gary! I'll try this program when I get time.
I did finally have time to sort through using RETscreen. Here are the results for several collector tilt angles using RETscreen:
Base case: High efficiency Electric tank hot water heater, Squanderous, hot water wasting family: $401 per year for hot water
Entering the parameters for the system I am planning to put in, I find:
Collector Tilt: $/Yr for hot water
0° (horizontal) $195
10 $178
18 (= 4:12) $169
20 $168
30 $162
38.9 (My Latitude) $161
40 $162
50 $168
54 $172
64 $186
90 $249
North Facing 4:12 roof $227
It doesn't really even matter if the collectors are facing South - they still work halfway decent on a North Facing roof!
So from this analysis, it makes essentially no difference whether you tilt the collector anywhere between 10 degrees and 54 degrees, over a year. The difference is smaller than some of my assumptions. For example if I over-estimated the efficiency of the water heater by 4%, this would make as much difference as changing the tilt angle through this wide swing. If I could get my family to conserve 2.5% of their hot water use, this would save as much as tilting the collectors. This is from the perspective of cash flow over an entire year.
This also busts the "Latitude plus 15" myth. I get the best yearly efficiency at the tilt angle equal to my latitude, which I have also seen used as a rule of thumb.
Looking at your RadOnCollector run, I see that the 55 degree angle is 38% better at making hot water in the January. Although there are few sunny days in the winter here, you have a much better chance of getting any solar water in the wintertime if the collector is tilted steeply. Also, my family is more liable to take baths in the winter, using up scads of hot water compared to showers. This argues for tilting the collectors.
The other consideration is winter/summer electric rates. Winter rates are cheap here - as low as 3c in certain blocks, in summer it can go up to near 10c. It is most economical to save energy like mad in the summer, because I am on a summer peaking utility. From this perspective it isn't worth the extra effort of propping up the collectors to a radical tilt. I am not sure, but this looks like I am getting 100% of my hot water from Solar in the summer 6 months, and maybe 20% in the winter. Not too far off target.
Tilted collectors are a much bigger wind sail. Bolted flat to the roof, they present less area to catch Dorothy's next flight to Oz. So there is a practical reason to bolt them down flat.
Hmm - right now I am leaning toward just clamping them onto the roof at the same 4:12 angle as the roof, seeing as it is simpler and doesn't make much difference in the long run.
--Lawrence Lile,
>
> Hi,
> You can get a pretty good idea just by running Radiation On Collector
> for the two tilt angles and do the "calculate for months". Free
> download here:
> http://www.builditsolar.com/Tools/RadOnCol/radoncol.htm
>
>
> Radiation On Collector Runs:
> 18 degrees tilt:
> 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: 18.0 (deg) measured from horiz
> Latitude: 38.9 (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 2238 130 2368 1353 127 1480
> 2 21 2639 158 2797 1713 154 1867
> 3 21 2932 208 3140 2015 203 2218
> 4 21 3095 302 3396 2148 294 2442
> 5 21 3155 383 3538 2187 373 2561
> 6 21 3162 423 3585 2173 413 2586
> 7 21 3054 415 3469 2124 405 2529
> 8 21 2914 354 3268 2041 345 2386
> 9 21 2729 254 2982 1893 247 2140
> 10 21 2467 179 2646 1613 174 1788
> 11 21 2168 136 2304 1310 133 1443
> 12 21 2038 117 2155 1179 114 1294
> Sum 32591 3059 35649 21749 2984 24733
>
> Radiation in BTU/day
>
> 55 degree tilt:
> 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: 55.0 (deg) measured from horiz
> Latitude: 38.9 (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 2238 130 2368 1879 102 1981
> 2 21 2639 158 2797 2078 124 2203
> 3 21 2932 208 3140 2072 164 2236
> 4 21 3095 302 3396 1811 237 2049
> 5 21 3155 383 3538 1575 301 1877
> 6 21 3162 423 3585 1465 333 1798
> 7 21 3054 415 3469 1520 327 1847
> 8 21 2914 354 3268 1713 279 1992
> 9 21 2729 254 2982 1934 199 2134
> 10 21 2467 179 2646 1965 141 2105
> 11 21 2168 136 2304 1824 107 1931
> 12 21 2038 117 2155 1735 92 1827
> Sum 32591 3059 35649 21573 2406 23979
>
> Radiation in BTU/day
>
> Gary
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