# Capacity calculation questions...?

Posted by John on November 27, 2005, 8:28 pm

Hi Guys:

I live about twenty miles north of Dallas, TX, USA around 33.1 N lat,
96.9 W. long, and about 550 ft elevation.  If I read the chart right, we
receive approximately 4.5 kWh/m2-day of sunlight.

Present plans are for a closed loop active water heating system.
Initially the target is domestic hot water, but if all works well, the
intent is for future expansion with additional panels for winter
heating.  I'm starting with four 4x10 panels arranged in Parallel
(thanks to the forum members' advice.)

Assuming I added correctly (and didn't forget to carry...) I have about
1500 BTU's per square foot per day to play with in this geographic
region.  My collector area is approximately 160 square feet.  So:

(1500 BTU's)(160 ft2) = 240,000 BTU's per day

1 BTU = 1 lb of water 1 degree F

and at a 100 degree temperature difference

[(240,000BTU's/100deg F)]/(8.4 lbs per gallon of water)

which should be about 285 gallons of water at 165 deg F (assuming a
starting temperature of 65 deg F.)

I don't know is how much temperature differential to design the system
for or even the design temperature limit.   What should one reasonably
expect.

Also what seasonal variations can be expected from ambient outside
temperature changes.  If the system is designed for optimal gain in the
Winter, is there a serious danger of overheating in the Summer?

BTW, I have been searching the web all weekend and have found some good
sources of information, but if you guys know any favorites sites for
info (sunlight, seasonal heating stats, sources for heat exchangers,
tanks, solar collector panels - PV or thermal, DC pumps, etc...) please
let me know.  Every little bit helps at this point.

Thanks
John

Posted by David L. Jones on November 27, 2005, 8:49 pm

John wrote:

That figure will vary a *lot*
If you want any degree of accurate calculation you have to have access
to the real data on a daily basis so you can plot real trends,
averages, and best/worst case calculations.
For example, in Sydney Australia where I live, the solar radiation can
vary anywhere from 0 to 10KWh/m^2 year round.
data like we do in Australia?

Dave :)

Posted by caraug on November 27, 2005, 9:31 pm
John,

You might want to visit the Canadian website
http://www.retscreen.net/ang/t.php .  They have a free Excel spreadsheet to
calculate domestic hot water heating system and they have solar data for many
US and Canadian city, perhaps European too - I'm not sure.  This is pretty
much all you need.  Check it out.  Let me know.

Good luck!

Gilles

"David L. Jones" wrote:

Posted by SJC on November 27, 2005, 10:00 pm
Here is a link for Dallas.
http://www.thermomax.com/DallasTX.htm

"1500 BTU's per square foot per day"

That is assuming 100% efficiency. Depending on the difference between
water and air temperature, the efficiency will go down dramatically.

If your air temperature is 65F and you want to get 165F, you would have to
look at your manufacturers specifications. At that difference, I would guess
less
than 10% efficient, if you could get the water that hot in the winter with flat
plates.

Hi Guys:

I live about twenty miles north of Dallas, TX, USA around 33.1 N lat,
96.9 W. long, and about 550 ft elevation.  If I read the chart right, we
receive approximately 4.5 kWh/m2-day of sunlight.

Present plans are for a closed loop active water heating system.
Initially the target is domestic hot water, but if all works well, the
intent is for future expansion with additional panels for winter
heating.  I'm starting with four 4x10 panels arranged in Parallel
(thanks to the forum members' advice.)

Assuming I added correctly (and didn't forget to carry...) I have about
1500 BTU's per square foot per day to play with in this geographic
region.  My collector area is approximately 160 square feet.  So:

(1500 BTU's)(160 ft2) = 240,000 BTU's per day

1 BTU = 1 lb of water 1 degree F

and at a 100 degree temperature difference

[(240,000BTU's/100deg F)]/(8.4 lbs per gallon of water)

which should be about 285 gallons of water at 165 deg F (assuming a
starting temperature of 65 deg F.)

I don't know is how much temperature differential to design the system
for or even the design temperature limit.   What should one reasonably
expect.

Also what seasonal variations can be expected from ambient outside
temperature changes.  If the system is designed for optimal gain in the
Winter, is there a serious danger of overheating in the Summer?

BTW, I have been searching the web all weekend and have found some good
sources of information, but if you guys know any favorites sites for
info (sunlight, seasonal heating stats, sources for heat exchangers,
tanks, solar collector panels - PV or thermal, DC pumps, etc...) please
let me know.  Every little bit helps at this point.

Thanks
John

Posted by John on November 28, 2005, 4:02 am
Hey Guys:

Thanks for the information and links, that should keep me busy for a few
days... especially since tomorrow its "back to the day job..."

BTW, the panels are "hand me downs."  I have no idea who the
manufacturer is or was and doubt I could ever find out.

The 65 deg F mentioned in the example was the estimated entering water
temperature at start up.  I didn't know where to start so I just used
the temperature of the tap water I would be filling the system with.

better questions next time...

Thanks
John

SJC wrote: