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re: increase solar flux and pv lifetime
1 jun 1997
conpute  wrote:
 
>hello,

 hello 

>      i am installing a solar pv system in my cabin this summer.  however,
>i am also interested in installing a hot tub heated also by solar energy.

nice. 

>first a word about the hot tub.  i have found a supplier of a vinyl/foam
>tub which also comes with a thick insulating cover. 

who dat? how much?

>the normal way that the water is heated is just by being continously pumped
>by a 150 watt pump.  no real heater element is needed.

good insulation. about $150/year at 10 cents/kwh... i suppose it doesn't
pump air into the water continuously as well. 

>second the solar panel.  i am looking at a 50 watt panel that has a metal
>(sheet aluminium) backing.  i am hoping to make two extra reflecting
>panels that should raise the solar flux to 2.0 to 2.5 suns in mid summer.

sounds nice, and not too hard to do with fixed reflectors.

>you can guess the rest.  i want to cool the solar panel using water from
>the hot tub, gaining two benifits.  1) more power for the batteries, 2)
>hot water for soaking in.

lovely.

>in normal cases i am sure that doubling the solar flux will cut the life
>of a panel in half,

the output may be reduced while hot, but i'm not sure about the lifetime,
despite gene townsend's experiments.

>will active cooling of the panel then counter the effects of extra sunlight?

i guess so. otherwise dr. richard komp (sunwatt@juno.com) wouldn't be selling
his 2:1 hybrid panels that make 150 w electric and 1600 watts of water heating.

since the hot tub doesn't require much heat, and its comfortable temperature
range is small, say 102-104 f, and pvs work better at lower temps, you may
want some sort of "shunt regulator" for the tub heating, ie some way to dump
excess heat from the water heating part of the solar panel, eg by turning on
a small fan to blow some outside air under the cover. you may want to run the
pump or an immersed resistor from the batteries on cloudy days to keep the tub
warm. a 100 ah 12 v battery could keep the tub warm for 8 hours. or, you might
use a higher temp water store for cloudy days, say 4 55 gallon drums full of
134 f water for 4 cloudy days in a row.

nick




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