Article 30782 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.energy.homepower,alt.solar.thermal Subject: Massy floors for solar heat storage Date: 3 Jul 2008 08:21:08 -0400 Organization: Villanova University Lines: 129 Message-ID: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1215083968 10190 153.104.44.130 (3 Jul 2008 11:19:28 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Thu, 3 Jul 2008 11:19:28 +0000 (UTC) Xref: news.misty.com alt.energy.homepower:138799 alt.solar.thermal:30782 don wrote: > I am looking at a new build and considering building a heat sink into > the material under the basement floor slab. I'd call that a heat store. To me, heat sinks just dissipate heat from transistors and other things. >This would involve excavating down an additional depth in a 2000 sf ft >slab... The walls up to the eventual basement floor grade would >possibly be an ICF... to provide an insulated below grade wall that >will be completely buried when completed. Within this structure I would >first put 1 ft of silty till... and then lay loops of heat exchange >coil(at a spacing of about 3 ft?). Next cover with 3 ft of silty till >(and then lay another course and then perhaps another 3 ft of silty >till?) and a final set of heat exchange loops finally covered by the >final 1 ft of silty till and then gravel... and then poured concrete >floor (no insulation under the concrete pad). The basement walls would >then be poured concrete with R-40 on the exterior below grade to >provide a good insulated thermal mass surrounding the full basement. >(This heat sink would thus have heat exchange loops cubing about each >cu m of till - is this overkill given the thermal conductivity of a >silty till - whatever that may be?) That depends on how fast you need to withdraw the heat, which increases with the house thermal conductance and indoor-outdoor temperature difference. > The question is how many BTUs' can I store in dry in this till... Dry soil has about 30 Btu/F-ft^3 of thermal capacitance and about R1 per foot of resistance, > The climate is southern Ontario and normal deep ground temperature > approx 8 deg C. I am assuming that... I can get the temp up to near 25 > deg C by the end of fall when I need to start to withdraw heat and drop > it down to near 5 deg C (in late spring)for a 20 deg C seasonal swing. That's 36Fx2000ft^2x8'd = 17.3 million Btu. Is that enough? Too much? > I am premising this on the fact that this type of system would be > easier (and less expensive) to install and control than the typical > vertical or horizontal heat exchange loop system for a conventional > ground source heat pump (and in the event of any prolonged power > failure keep the building well above freezing for an extended period > of time). A 48'x48'x8' house with 96 ft^2 of R4 windows with 96/4 = 24 Btu/h-F of thermal conductance and 3744 ft^2 of R40 walls and ceiling with 3744/40 = 94 and 30 cfm of air leaks with about 30 Btu/h-F totaling 148 would only need 1.8(0-(-3))148 = 800 Btu/h to stay 0 C indoors on an average -3C January day in Toronto, so the basement might keep it from freezing for 17.3M/(24hx800) = 901 days :-) >... I may need to do a heat loss calculation first and then work >backward to calculate heating requirements That's a good idea. If cloudy days are like coin flips, storing heat for 5 days can make a house 97% solar-heated. If it's 70 F for 12 hours and 60 for the other 12, it needs 24h(65F-27F)148 = 135K Btu/day, or 675K Btu for 5 days. With lots of surface, a 48'x48'x8" deep floor with 12 $25 42"x48'x6" deep water-filled polyethylene film greenhouse air ducts laid flat on the ground among 144 hollow concrete blocks under 4'x4' plywood floor slabs with 1008 ft^3 of water and C = 1008x62.33 = 68.2K Btu/F can provide that with no heat pump if the floor is 70+675K/68.2K = 81 F on an average day. My 1981 NRCC Solarium Workbook (one copy that was not burned by the Canadian government :-) says 2785 Wh/m^2 (883 Btu/ft^2) of sun falls on a south wall and 1321 (419) falls on east and west walls on an average January day in Toronto. If equal windows on 3 sides transmit 50%, the house gains 0.5x32ft^2(883+2x419) = 27.5K Btu/day. We can get the remaining 135K-27.5K = 107K from A ft^2 of R1 $1/ft^2 corrugated polycarbonate Dynaglas south wall "solar siding" with 90% solar transmission over a 100 F air gap if 0.9x883A -6h(100-27)A/R1 = 107.5K, ie A = 301 ft^2. With $2 R2 Therma-Glas Plus twinwall polycarbonate with 80% transmission, A = 220 ft^2. A solar attic could also work. A $35 1000 cfm car radiator with its 2 fans in series (20-watts total) could move solar warmed air down under the floor through a duct near the slab center with a separate siding cavity air return duct. We could heat the house by allowing floor air to flow up naturally through the same duct and back into the floor near the perimeter, using a 2-watt motorized damper. The car radiator could also heat water for showers with a $60 1"x300' pressurized black plastic pipe coil in a 4'x8'x3'-tall 140 F box with a folded 10'x14' EPDM rubber roofing liner. Ted writes: >... are you confident in the "1000 CFM @ 20w" figure you quote at the end? >From what I've seen of blowers, that seems off by an order of magnitude. Nathan Hurst measured 1000 Btu/h-F in water heat gain in Melbourne (see http://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf.) That requires about 1000 cfm of matching air movement. He ran his fans at 16 W total with PWM controllers. Nathan and I ran my car radiator and 2 fans in series from a 20 W PV panel at the PA Renewable Energy Fest last September. They use 36 W from a 12 V battery. >For example: http://www.waveplumbing.com/store/index.php?main_page=product_info&cPath=55_459&products_id=3455 >rate about 8 CFM per watt. Longer blades tend to be more efficient. I like Lasko's $50 2155A 20" window fan, which can move 2470 cfm with 90 watts, ie 27 cfm/W.. >Though the study on the gossamer ceiling fans indicate flow rates of >150-200 cfm/watt for "typical ceiling fans". I suppose that's with >essentially zero static pressure. http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1059-99/ With large ducts and low airspeeds (<400 lfm) and few turns, we can get close to zero pressure. Grainger's $149 3C690 48" industrial ceiling fan moves 21K cfm with 86 watts, ie 244 cfm/W. Their $221 4C761 60" fan moves 46K cfm with 105 watts, ie 438 cfm/W. >With lots of surface, a 48'x48'x8" deep floor with 12 $25 42"x48'x6" >deep water-filled polyethylene film greenhouse air ducts laid flat on >the ground among 144 hollow concrete blocks under 4'x4' plywood floor >slabs with 1008 ft^3 of water and C = 1008x62.33 = 68.2K Btu/F can >provide that with no heat pump if the floor is 70+675K/68.2K = 81 F on >an average day. PhD Rich Komp (author of "Practical Photovoltaics") solar-heats his Maine house with a similar "hypocaust" hollow mass floor (a concrete slab over lots of hollow blocks) and a small PV-powered fan. Nick