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re: solar options
29 jan 2003
bill kreamer  wrote:

>"nick pine"  wrote

>> ...a smaller receiver can have less loss at a higher temp: a square foot
>> of flat plate with one layer of r1 glazing with 90% solar transmission
>> might collect... 175 btu of net gain. with 3 suns and 130 f water and
>> a 90% reflector... 608 btu enters and 100 leaves, for a 508 btu gain.
 
>...but this is collector performance.  isn't it downstream where
>higher temps give the higher losses?
 
your question was "why concentrate, for space heating?" imo, collection is
the main problem. higher temps and water vs air seem to make other things
easier, but that's hard to say without more application details.

>> 2. higher temps can make heat storage easier.
>
>best to both collect at low temps (lower equipment costs)

the equipment can be cheaper if the reflector is fixed, eg a downward-
reflecting linear parabola putting 3 suns into a shallow water trough
near the north wall of an attic.

>and eliminate the heat exchanger stuff (needed for separate hi-temp
>receiver fluid), and the storage (lower costs) and its losses (more savings).

water seems like a good "hi-temp receiver fluid," circulating though
a hydronic slab or baseboard radiators with no heat exchanger or inside 
a galvanized tank inside an unpressurized tank to preheat water for showers. 

>> 3. water can make heat transportation easier.
>
>sure, smaller diameter tubing to north side rooms, right?

i was thinking more about how to transport heat from an attic to a basement
or a garage or outbuilding to a house. moving heat in water is easier, over
significant distances. pipes are smaller and cheaper "ducts" with more
effective insulation and less pump vs blower power. zoning is easier.

>but then, 98% of installations will have only enough collector area to heat
>a few south side rooms. go directly from vertical wall collectors into the
>room - eliminate the piping _and_ the ductwork.

as in the example below. but 99.9% of us houses have no solar heating systems
at all :-) this may be a matter of taste or ambition or available systems. 
some people want higher solar heating fractions, as well as hot water. they
ask themselves: should i try to do 20% of the job, heating a couple of rooms
for 8 hours on sunny days, or should i set my sights higher? 
 
>> 4. water can make heat distribution easier, with less noise and drafts
>> and smaller "ducts" with more insulation and less pump vs blower power.
>
>than a small fan?

low-temp hydronic floors and baseboard radiators (which might be installed
as efficient convectors in basements or closets, with hot air above them...)

>> 5. higher temps can heat water for showers.
>
>i guess i'm commenting only on space heating.  it's a nice big load just
>crying to be served by simple non storage fan-driven air collectors.

a good first step, but why stop there?

>> 6. mirrors are cheaper than collectors.
>
>not if you overwhelm those savings paying for critical receiver
>construction, dual fluid circuits, hi-temp piping, pumps, heat exchangers,
>storage construction, and electronic controls.

agreed. let's not do that. the example below could use a 16'x32' reflective
patio in front of the wall, made with 2" of modified stone over landscaping
cloth, sprayed with permanent whitewash. this $200 mirror would cut down on
lawn mowing and add 20% more sun in january. 

>> >what about storage?  storage is rarely a paying proposition.
>>
>> how about 12" diameter greenhouse poly duct (38 cents/ft) over 2"x4"
>> welded-wire fencing (10 cents/ft^2 in 48" rolls) screwed under basement
>> rafters on 2' centers, in a return air path to a large air heater...

israeli greenhouses store heat in water-filled poly ducts on the ground. 

>that may be worthwhile, when you make more heat than you can use;

which is easy to do with a plastic film sunspace, if we want to do that.

a foot of 12" diameter duct with c = 2.8"x16"x1'x64/144 = 20 btu/f and
g = 1.5pi = 4.7 btu/h-f has rc = c/g = 20/4.7 = 4.2 hours. if we cover
the basement ceiling of a 32'x32'x8' house with 10k btu/f of water and
expose it to 70 f air for 6 hours per day, and the house cools to 60 f
by dawn, how much insulation does the house need to keep itself warm
on an average january day in phila? how does this change if the water
sees 100 f air for 6 hours per day? should we add more insulation to
make cloudy day heat storage easier?

>most people will be constructing 20-100 sq. ft. systems, i.e., enough to
>supplement some south side rooms during the day.

that may be true for your target market. i can't speak for most people.
most of my neighbors burn oil. a few burn wood, and would welcome less work
and less mess. new houses tend to have natural gas or air-air heat pumps.
what people do is not an infallible guide to what they can or want to do.
 
>> >this is non-trivial controls territory.
>>
>> simple dampers and thermostats come to mind.
>
>with some decrease in systemn reliability.

compared to what?

>> >the materials expense, design time, controls, space, installation, and
>> >maintenance can be avoided in an air system by venting excess heated air.

most people avoid solar heating systems entirely :-)

>> a nearby house has about 3k/year of heating and water heating and electric
>> bills. it's 2 years old, well-insulated and airtight... 

>> part of the south wall is about 27'x9' tall, with few windows. it might
>> provide heat and help heat water year-round, with a 24' wide x 8' tall
>> x 6" thick passive air heater with a $100 2-watt motorized damper at one
>> corner and $200 worth of fin-tube pipe near the top for water heating.
>>
>> the air heater might have $300 worth of redwood 1x6 sides and $100 of
>> black aluminum window screen inside to act as a transpired mesh absorber.
>> room air would slide past the basement ceiling and enter the heater via
>> a hole at one corner and return to the house via the upper damper when
>> the sun is shining and the house needs heat (using a $20 thermostat.)
 
>this size could be reduced, and a fan added, for the same output.
 
the owner dislikes fans, except for summertime ac. he's a tough cookie.

>> the heater could stagnate from june through september, heating the
>> fin-tube with the damper closed...
 
>the fin tubes could be eliminated, and the outlet could be diverted on
>rising room temp, for some welcome cross flow ventilation...

we could move hot air through the house this way in summertime, forgoing
year-round water heating... we could piss in the soup... :-)

nick




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