[Greenbuilding] Water heaters/Solar

[barbara deane-gillett]

when you recognize that it is cold (20 versus 70) dark (much shorter days)  and cloudy (70% versus 40%) in winter and that there is basically no or very little  solar input energy to not use, you realize that a system operating ok during the spring summer and fall gets 90% of the energy you can expect to get and so don't worry about winter performance. check it out with f chart and the absurdly low collector performance  curve ( .8 minus 3 ) numbers and you will see the effect.> Date: Tue, 6 Nov 2007 23:22:52 -0500> From: keith at earthsunenergy.com> To: greenbuilding at listserv.repp.org> Subject: Re: [Greenbuilding] Water heaters/Solar> > 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> > >> > > >> > >> > > _______________________________________________> > > 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> >> > _______________________________________________> 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