Article 31697 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Re: Passive solar air heater(s) Date: 20 Jan 2009 09:24:57 -0500 Organization: Villanova University Lines: 68 Message-ID: References: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1232461498 22275 153.104.44.130 (20 Jan 2009 14:24:58 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Tue, 20 Jan 2009 14:24:58 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:31697 2Hirondelles wrote: >Am in planning stages for installing two of these units on the SE end >of our cottage. (Just so we know what we're talking about, it's the >'Simple Solar Heater' design by Gary Reysa that appeared in Mother >Earth News and Home Power.) The location is a climate essentially >the same as N. Maine... Gary's heater is 8'x20', with 5 ft^2 of vents at the top and bottom. >The SE wall has a tiny (5x7) sewing room in the centre, with a window. >The remaining wall space either side is blank and unobstructed by shading, >and seems a good option for installing one of these units on each of two >11' sections of wall. You might put 8'x8' heaters on those sections. >The intake/outlet vents would end up being in each of two bedrooms, >one 11 x 16, the other 11 x 11. The doors of these bedrooms give onto >the central hallway. > >1) I'm wondering how much the heat produced by these units will move >on it's own from each of the bedrooms to the central hallway, and how >to decide to size the units. Essentially, I am concerned about possibly >overheating these bedrooms if the heat doesn't circulate on it's own. Gary measured his heater's output, but I like to calculate things. If 0.9x250x160 = 36K Btu/h of (full) sun enters 160 ft^2 of R1 glazing with 90% solar transmission on a 30 F day with a 70 FF room and average heater temp T and (T-30)160/R1 + 16.6x5(2(T-70)^1.5sqrt(8) leaves (using an empirical chimney formula), T = 70+((255-T)/4.15)^(2/3) = 82 F, after some iteration, ie 1150 cfm of 70 F room air enters the heater and leaves at 94 F, with a 27.7K Btu/h output and a 69% solar collection efficiency. An 8'x8' version might produce 64/160x27.7 = 11.1K Btu/h. Some heat will leave through the walls of the room, but if they were perfectly insulated and you left a 3'x7' door from the T (F) room to a 70 F hall open and 11.1K = 16.6x10.5sqrt(3.5)(2(T-70))^1.5, T = 70+(11.1K/922)^(2/3) = 75.2, after some iteration. Not too hot. On an average 14.8 F day in Caribou, Maine, 780 Btu/h-ft^2 falls on a south wall. If 0.9x160x780/6h = 18.7K Btu/h enters over 6 hours and (T-14.8)160/R1 + 16.6x5(2(T-70)^1.5sqrt(8) leaves, T = 75.7 F, with an 8979 Btu/h output. (R2 ThermaGlas Plus twinwall polycarbonate glazing with 80% solar transmission would probably work better, with simpler edge sealing.) With an open door and perfect wall insulation, the room temp would be 74.6. >There is the possibility of installing a ceiling fan in the central >hallway, if that would help. Sounds like it wouldn't, in a one-story house. A slow ceiling fan in the room could be interesting, under a massy ceiling. With some loss of solar collection efficiency, you might store 30K Btu/day at 120 F in 600 pounds of water in an 8'x8'x2" plastic film pillow on an overhead tray with foil underneath. A room temp thermostat with an occupancy sensor could run the fan as needed to keep the room exactly 70 F when it's occupied. >What we are looking for is to reduce our use of baseboard heating when >the house is occupied and to increase the temperature however we can >when it is not. If the heater output is small compared to the house heating requirement, you don't need to store solar heat, just mix it around the house. Nick