[Greenbuilding] Water heaters/Solar

[barbara deane-gillett]

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> > > > > _______________________________________________> Greenbuilding email list> Environmentally-preferable design, construction, building elements> List info: http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> List email: Greenbuilding at listserv.repp.org> Managed by BuildingGreen, Inc. http://www.buildinggreen.com> publisher of Environmental Building News and GreenSpec> Hosted and archived by REPP / CREST http://www.crest.org> To get on the list:> http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> or mailto:greenbuilding-request at listserv.repp.org?subject=unsubscribe> To get off the list:> http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org> or mailto:greenbuilding-request at listserv.repp.org?subject=unsubscribe