[Greenbuilding] Water heaters/Solar

[Keith Winston]

Oops, I brain-hiccuped and totally misexplained delta-T. My central 
points still stand!


> They have 30 Btu/sf/day on a clear day with a delta T of 36F. That 
> means, let's say it's 50F out, you don't require water any warmer than 
> 86F. As soon as it gets any colder, or your require hotter water, or it 
> gets even mildly cloudy, their performance drops to zero. Literally, 
> according to the tests.
>
>   
What this really should say is, when the INPUT water to the collector is 
86 on a 50F day, you'll be gathering 30 Btu/sf/d. Let's say 40 sf, 
that's 1200 Btu/d. Let's say you have a 60 gallon tank (I'm being 
generous, bigger tanks would look worse). That's a daily temp increase 
of 2.5F. Woo hoo! That's for the entire 40 sf, for the entire day. Of 
course, these are often done on an entire roof, so you might have 
400-800 sf, or even more (I'm thinking 1/2 of a gable roof). Still, if 
you can only use it for water heating, could it possibly make sense? 
We're STILL only talking 12,000 Btu/day, the output of a single 
flat-plate collector on a mediocre day. Is it THAT important to hide 
your collectors? Oh, and performance STILL drops off much faster than 
with the flat-plate...

BTW, don't let anything shade that roof! Even though the energy saved 
(in cooling climates) by shading would almost certainly far outweigh 
what this system is going to gather!

Keith

>