[Greenbuilding] Water heaters/Solar

[Keith Winston]

Do you have install costs for an average Dawn Solar install? Do you 
think it would do well in colder/windier areas than Florida? Like they 
said, it's only 1/3 or 2/3 as effective as an unglazed pool collector, 
and we can't use those to heat water cost effectively where I am (DC), 
even though they're dirt cheap. We can use them to heat pools, since 
we're only trying to heat them when it's basically warm out...

Keith


barbara deane-gillett wrote:
> yeah, i caught that hiccup. now when you are trying to heat from 40 to 
> 120 half of the water temp gain is from inlet temp of 40 to 84 where 
> the dawn collector excells (outperforming evac tube and glazed 
> collectors)  in terms of savings, a btu is a btu .
>  
> second the typical dawn solar dhw collector is 400-500 sq.ft.
>  
> using your number 400 sq ft would raise it 25 degrees (86 to 110 and  
> 800 sq.ft to 60 to 110 ( the collector perf is better at a lower than 
> 86 input and less at higher) ) and this on the average day, not the 
> good day.   the method assumes every day is an average day rather than 
> a series of good days and terrible days which is the way the real 
> world works.
>  
> for the year, your 12000 is about 40 gallons of fuel oil displaced or 
> about $120 .  actuall performance is about double that or $240 for the 
> reasons above and in the other email.  or about 75% of the $ spent on 
> dhw heating.   
>  
> my point is that the actual output is reasonable (not perfect or even 
> good per sq.ft) and should be considered by any customer..
>  
> and yes it is that important for many people to hide the collectors.
>
> > Date: Tue, 6 Nov 2007 12:31:08 -0500
> > From: keith at earthsunenergy.com
> > To: greenbuilding at listserv.repp.org
> > Subject: Re: [Greenbuilding] Water heaters/Solar
> >
> > 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
> >
> > >
> >
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