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re: solar pool heating
13 oct 2004
n. thornton  wrote:

>> >but the pipe surface is not at 80f during use, its more than 60c. over
>> >140f. i know that from using a hose/polythene panel, the water is hot
>> >enough to burn if it stagnates. we drew the water off it every 40
>> >minutes, and it was most hot.
>> that's shooting yourself in the foot to start with, efficiency-wise.
>> efficient collectors run cool, with moving water. 
>that isnt the point, a collector producing a lake of tepid water would
>have been a failure.

can you say "swimming pool"?

pool collector water might enter at 70 and leave at 90.

>the fact that it would have been more thermally efficient is not relevant.

how annoying. i seem to recall that a) you suggested that someone use
this pipe for heating a swimming pool (see subject: above) and b) you
claimed it would be more efficient with a polythene cover. i said use
two pieces of polyethylene film and forget the pipe. 

>the purpose was to impress people with what you can do easily...

well then, forget the cover. perhaps you can easily wiggle your ears.

>that means impressing people who dont already have much knowledge of
>the subject. impressing was achieved by tapping off a collectorful of
>steaming scalding hot water every 40 minutes, and using it to wash the cups.

so they will say "wow" to themselves with one voice and go buy
a practical pool heating system?

>something that dogs the solar world today is not putting enough
>emphasis on the real world considerations. the question for the end
>user is not thermal efficiency, but rather output per input, with the
>input being measured in dollars, not sunlight. thermal efficiency is
>obviously important, but it is not the goal itself.

agreed. joules per day per pound of investment might be nice.

>lets illustrate. if for example one could mass produce 10% efficient
>solar water heaters at $1 per square metre, solar heating use would
>increase, they would be useful. if otoh you perfect 99% efficient
>collectors for $1000 per sqaure metre, no-one is buying. iow thermal
>efficiency is not the number one issue.

you seem to be playing a different game, with impressive ignorance. 

>> >plus winds go well above 4mph some days... that translates to an
>> >amount of heat loss i wouldnt know how to calculate.
>> that doesn't interfere with your process of coming to conclusions :-)
>it doesnt interfere with my objecting to your analysis, no, since i
>could spot the numbers looked questionable.

weather data will always be questionable.

>it does unfortunately stop me from calculating the relative merits
>of differing covers.

you might learn how to do that before offering technical opinions.
>> >> a         vt       q         t     
>> >> 
>> >> 0.0833    59.8     -0.256    79.3   (1" poly over 1" pipe)
>> >> 0.1666    88.1     +6.617    96.8    2"        "
>> >> 0.3333   102.2    +12.394   111.5    4"        "
>> >> 0.5000   106.9    +14.951   118.0    6"        "
>> >> 1.0000   111.8    +17.964   125.6    1 foot    "
>> >> ...
>> a is the number of square feet of glazing that covers a 12"x1" tube.
>well, i didnt make much sense of that. not enough information there:
>what dimension is getting varied in order to achieve this variable
>area per amount of tube? tube spacing? if thats the right guess, when
>a=1/12 the tubes are touching, when a=1 theyre spaced a foot apart.


>> q is the btu/h of sun the tube collects.
>> a = 1 (the most efficient of your designs) has a mere 100x17.9/250
>> = 7.2% solar collection efficiency in full sun, and it loses lots of
>> heat at night.
>it doesnt lose anything at night cos water isnt pumped thru it then.

if water stays in the pipe, it stores and loses heat. 

>>solar heating is an exacting science. if you want to experiment with
>>less education, why don't you take up surgery?
>are you seriously suggesting that we should all have no involvement in
>or discssion of the things that interest us if they dont happen to be
>our area of expertise?

i'm suggesting you learn more solar physics.

>> you might think of this as basic electronics, if you'd ever heard of a
>> thevenin equivalent circuit :-) i suggest you look it up, 
>thats not where the problem lies. one has to be able to describe the
>model before being able to analyse it.

full sun (250 btu/h-ft^2) shines on 1 foot of 1" pipe. the pipe intercepts
21 btu/h of sun and converts it to heat. it has 80 f moving water inside,
so the pipe is close to 80 f outside. it loses 2+v/2 btu/h-f-ft^2 of the
heat from its 0.262 ft^2 of surface to v mph air at ta (f). the rest of
the heat goes into the water. clear enough?
    ---        rp                rp = 1/(0.262(2+v/2))
|--|-->|-------www--- ta
    ---    |  i -->
 21 btu/h  | 80 f    you say you know something about electronics?
           -         what's the net current into the battery
           |                if v = 0 and ta = 70? 
                     hint: what's i?


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