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Temperature expectations from solar hot water panel

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Posted by Doug on May 9, 2005, 6:16 pm
 
Hi,

I'm thinking of augmenting my radiant floor heating system (water) with
a solar panel on the roof of my house.  I live near Ottawa and my house
has some aspects of passive solar heating - direct gain with lots of
thermal mass.  On typical January days, the floors retain their heat
until late at night when our propane boiler kicks in to heat the
floors.  The temperature of the water entering the floor is ~110F I
believe.  Perhaps a bit lower.  I'd like to store solar heated hot
water for use at night so that I reduce our use of propane.  I would
temper the hot water with a mixing valve to achieve the 110 F
temperature.

The higher the temperature of the solar hot water, the less water I
would need to heat and store.  I would like to know what I could expect
to achieve in terms of temperatures with a roof mounted solar water
panel during average Dec-Jan days in the Ottawa area.

I'd really like to know how various solar panel designs compare with
each other and how they perform on cloudy days versus sunny dayes, etc.

thanks,
Doug


Posted by DJ on May 11, 2005, 12:04 pm
 

Doug wrote:

Near, or *in*? *In* would be bad. I've been doing alot of interviews
and the like with CBC radio and TV over this whole prohibition the city
has with regards to SDHW systems.


Usually the way we do this for clients is to have the passive solar
panels heat the boiler reservoir, but a cistern system is occasionally
used.


Depends on who you ask ;-). Dec-Jan is a bad time of the year to be
trying to get decent output out of SDHW systems.


Again, caveat emptor.

Doug, have you seen the Natural Resources Canada literature on this
stuff?

http://www.canren.gc.ca/prod_serv/index.asp?CaId 6&PgIdu5

DJ


Posted by Doug on May 11, 2005, 3:31 pm
 Thanks for the response.  I live outside of Ottawa in a rural setting.

My main concern is not solar "domestic" hot water but solar heated
water for my radiant floor.  Although it wouldn't be a difficult
addition to add a pre-heat exchanger for domestic hot water.  (My
boiler is a dual use Baxi system.)

Realistically, I was hoping that two 60 gal water tanks would be a
practical size to substantially reduce my usage of propane.  I am very
worried about the rise in costs of propane in the short term future.

I've looked but cannot find measured data that compares various systems
at various ambiant temperatures at various flow rates at various solar
radiation levels (grey days vs sunshine).

How many BTUs can I collect in ~120 gallons of water with a minimum
temperature of 90 degrees F on a typical day in Dec or Jan in Ottawa
using a flat panel collector.  How would this compare to a sunny day?
Or, what maximum temperature could I expect to achieve if the input
temp of the water is 90 degrees and there is 120 gallons of water.

-Doug


DJ wrote:

occasionally


Posted by DJ on May 13, 2005, 12:16 pm
 
Doug wrote:

setting.

If you want to save money, though, Doug, you might get better bang for
your buck if you looked into swapping out the AC drive pumps on your
system with DC ones.


systems

Um, ok, here's the thing. I took the NRCan's course on this stuff,
using the... hmm, some course about doing output estimates based on
insolation values. It was good, it was well engineered.
But, after all the numbers settle, grey days are bad. And November to
January has alot of grey days.


Ok, here's the thing. Realistically, from about November to January,
you've got paperweights on the roof. To be honest. They will
occasionally work. True. But if you are basing your purchase on that,
don't. The rest of the year, they'll be magnificient. Up to the end of
October, and from February on, they'll save you mucho bucks. So you
need to burn a bit more propane in the dead of winter. True. But you'll
be burning ALOT less for the rest of the year. And if you install them
(or get them installed) so you can manifold them into making your
domestic hot water in the summer, you'll love them even more.

DJ


Posted by nicksanspam on May 21, 2005, 1:40 pm
 Doug wrote:


My 'ol NRC Solarium Workbook says 1603 Wh/m^2 of sun falls on the ground
and 2943 falls on a south wall on an average -10 C January day in Ottawa,
vs 1214 and 2257 on an average -8 C December day. A solar collector with
optimal tilt would get sqrt(1603^2+2943^2) = 3351 in January and 2563 in
December. The best tilt would be atn(2943/1603) = 61.4 degrees in January
and 61.7 in December.


Two 60 gal tanks would store about 1000 Btu/F, vs 4"/12"x25x1000 = 8333
(1 gallon per square foot :-) for a 4" x 1000 ft^2 concrete floorslab.
Those tiny tanks might not add much, unless the water is very hot on an
average day. How about a large unpressurized tank? Could a 24'x4' deep
113K Btu/F pool store heat for 5 cloudy days in a row? It wouldn't need
a big collector, if a sunspace warmed your house on an average day.


January has 3351/(32-(-10)) = 80 Wh/DD, vs 2563/(32-(-8)) = 64 in December,
so December is the worst-case month. A square meter of collector with a US
R1 (metric U5.68 W/m^2C) cover with 90% solar transmission and a large 90 F
water flow and a large 90 F water tank perfect back insulation would gather
0.9x2563 = 2307 Wh and lose about 6h(32-(-8))1m^2xU5.68 = 1363 Wh, for a net
gain of about 944 Wh/day, ie 3220 Btu, or 299 Btu/ft^2. You might raise that
with a reflector or some extra glazing (a parasitic air heater) below the
collector. A 2-cover collector might gain 1869 and lose 6h(32-(-8))1x5.68/2
= 682, for a net gain of 1187 Wh/day.

Energie Solaire (a Swiss company) sells unglazed stainless steel collectors
with a selective surface for about $2/ft^2. They might work well in this
low-temp application. The efficiency equation on their web site indicates
100(0.959-8.91(0.375)-0.047(0.375)^2) = 62% with an average 30 C collector
and 0 C outdoor temp in full sun (800 W/m^2) in December in Toronto.

Nick


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