Article 30359 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Re: Overnight air heater storage Date: 27 Apr 2008 18:08:27 -0400 Organization: Villanova University Lines: 119 Message-ID: References: <8846b87c-3b43-4ef7-90b6-d644b82d5f66@t12g2000prg.googlegroups.com> NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1209330495 28093 153.104.44.130 (27 Apr 2008 21:08:15 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Sun, 27 Apr 2008 21:08:15 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:30359 wrote: >> > | | >> >-------------------------------------------- >> > sfd d | >> > ==> sa d ceiling mass d | >> > sn d d ----- | >> > s | | | | >> > | | | | >> > | r | h | a | >> > | e | o | i | >> > | t | u | r | >> > | u | s | | >> > | r | e | h | >> > | n | | e | >> > | | w | a | >> > | d | a | t | >> > | u | l | e | >> > | c | l | r | >> > | t | | | >> > | | | | >> > | | ----- | >> > | v | >> > | | >> >----------------------------------------- >> >> Hey Gary, how about trying this with a 12/06 MEN air heater? :-) >> >I have observed that there is definitely a tendency for the hot air >from the exit vents to keep the first couple joist cavities very >warm. Under sunny conditions the entry air temp tends to run about >60F and the exit about 120F -- a paint can full of water and hung in >the first joist cavity reaches about 110F by the end of the day if I >am recalling correctly. Sounds promising. A shallow water tray would have more surface/volume. >I guess I would be inclined not to include the extra wall that returns >air to the inlet vent. It seems like the some kind of tray in the >area of the first two joist cavities with the skirt and a reflective >coating on the tray might be a good thing to try? Ceiling mass with a slow ceiling fan still seems good to me, but after a calculation, letting hot air slide out from some shiny mass to allow a non-shiny ceiling to radiate downwards seems less good than a slow ceiling fan or a vertical mass wall with thermosyphoning airflow into the room. An 8' R15 cube with a 4'x8' mass and a 4'x8' non-shiny ceiling needs a 92 F (min) mass to stay 70 F on a 30 F day, vs 78 for an 8'x8' 2-sided wall with separate overnight and cloudy day heat stores. >I did try a sort of closet arrangement on one bay of the collector. >It had an inside insulated wall to return exit air to the inlet, and >gallon bottles hung in this cavity -- maybe 30 gallons in all for the >one bay. The inside "wall" was hinged so that you could close it >during the day to heat the water bottles, and open it later when heat >was wanted. It did not work very well -- flow around the loop was >weak, and only the top couple rows of bottles heated up. I remember that. >I think that mass near the ceiling will work better. Sure, but how do we keep it hot without overheating the room? >I'm a bit skeptical that you can get good flow around the loop -- Maybe not so good is still OK. ("Le mieux est l'ennemi du bien.") More mass surface is good. A 100 F ceiling mass would not accept any more heat from a 100 F air heater, but 7' of 100 F wall above 1' of 90 F wall would still accept some heat, with an average cool column air temp that's less than the average hot column air temp. 20 AC=6*8*8'cube surface (ft^2) 30 RV=15'US R-value of cube walls 40 GC=AC/RV'cube conductance (Btu/h-F) 50 TA=30'outdoor temp (F) 60 SUN=1000'average sun on south wall (Btu/ft^2-day) 70 CH=8'solar collection hours 80 TD=70'day room temp (F) 90 TN=60'night room temp (F) 100 PD=(TD-TA)*GC'daytime heatflow (Btu/h) 110 QH=8*(TD-TA)*GC'daytime heat (Btu) 120 PN=(TN-TA)*GC'night heatflow (Btu/h) 130 QN=16*PN'overnight heat (Btu) 140 QD=QH+QN'daily heatflow (Btu) 150 AS=8*8'south glazing area (ft^2) 160 RS=1'south glazing resistance (per layer) 170 TS=.92'south glazing transmittance (per layer) 180 SSS=TS^2*AS*SUN'sun entering south glazing (Btu/day) 190 TCA=TA+RS*(SSS-QD)/CH/AS'average cool cavity temp (F) 200 AV=8*8/12'vent area (ft^2) 210 H=8'vent height difference (ft) 220 DT=(PN/11.7/AV/SQR(H))^(2/3)'night thermosyphoning dT 230 AN=8*8'1-sided overnight heat store area (ft^2) 240 UA=1.5'slow airfilm conductance (Btu/h-F-ft^2) 250 TMINN=TN+DT+PN/(2*UA*AN)'predawn heat store temp (F) 260 TMAX=2*TCA-TMINN'max cool cavity temp (F) 270 CN=QN/(TMAX-TMINN)'overnight heat capacity 280 DN=12*CN/AN/62.33'overnight store depth (inches) 290 PRINT TMINN,TMAX,DN 300 DT=(PD/11.7/AV/SQR(H))^(2/3)'dawn thermosyphoning dT 310 TMIN=TD+DT+PD/(2*UA*AN)'dawn heat store temp (F) 320 THA=TMAX+SSS*RS/AS/CH'hot store temp (F) 330 CC=5*QD/(THA-TMIN)'hot store cap (Btu/F) 340 DH=12*CC/AN/62.33'hot store depth (inches) 350 PRINT TMIN,THA,DH 66.66535 124.9347 .6343741 78.56218 230.7347 2.024268 A 60 F night setback seems to help a lot, with the same average outer glazing cavity temp but a higher overnight mass swing than with a constant 70 F day/ night temp. This would also allow using the inherent mass of the cube to store some overnight heat, vs having to store all overnight heat in the cool store, if there's no night setback. Nick