Article 30445 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal,alt.energy.renewable Subject: Re: Geothermal Cooling Date: 11 May 2008 07:42:11 -0400 Organization: Villanova University Lines: 50 Message-ID: References: <1210470774.911.1210467714@bayman.org> NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1210502500 30556 153.104.44.130 (11 May 2008 10:41:40 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Sun, 11 May 2008 10:41:40 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:30445 alt.energy.renewable:137538 David Williams wrote: >-> ... Experience has shown earth tubes to be unfeasible in North Carolina >-> for several reasons. The chief problem is the fact that the air introduced >-> through the earth tubes is typically humid Raleigh has an average wo = 0.0035 #H20/#air humidity ratio on an average 38.9 F January day with a 28.8 min and wo = 0.0149 on a 78.1 F average July day with a 68.1 daily min. The deep ground temp is 59.3. A tight house won't need much cooling, but it needs dehumidification. With 80 F and wi = 0.0120 indoors and 30 cfm, it would need 24hx60mx30cfmx0.075lb/ft^3(0.0149-0.0120) = 9.4 pints per day. >If the tubes are well sealed, and just conduct heat though their walls, >then no moisture, radon, insects, etc., should be able to enter. >However, the *relative* humidity of the air will rise, simply because >it is cooled... Concrete stores about 1% of its weight in water as the RH rises from 40 to 60% (its "sorption isotherm") which makes for a low RH in January, if we don't let the tubes get too cold. Psat = 0.522 "Hg at 100% RH at 60 F, and w = 0.0035 makes Pa = 29.921/(1+0.62198/w) = 0.167 with 100Pa/Psat = 32% RH. If the concrete reaches 70 F at 0.0120 (77% RH) in July, it can store about (77-32)/(60-40) = 2.25% of its weight in water. If we only need dehumidification in July, we might store 31x9.4 = 291 pounds of water in 13K pounds of concrete, eg a 4'ID x 13' earth tube with 6" walls, or smaller tubes with the same weight and more surface. >This kind of system works best if you use it in both directions, for >heating in winter and cooling in summer. That way, the heat you take >out of the soil in winter is replaced in summer. We might solar heat a house and just use the tubes for dehumidification and cooling, with a passive thermal chimney and a 1-way passive flapper valve at the tube inlet that lets outdoor air flow up through the chimney until the start of the cooling season, then circulate air through the tubes during the cooling season, then start cooling them with winter air again. To remove 291 pounds of water with an average w = 0.060 inside the tubes, we need to move about 291/(0.075(0.060-0.035)) = 155K ft^3 of January air, eg 5K ft^3/day (3.5 cfm :-) for 31 days, with a 3.5/(16.6sqrt(8(65-38.9)) = 0.015 ft^2 x 8' tall chimney :-) Or a larger chimney, for more cooling. Simulating lots of 4' tubes and 2 1' rings of soil surrounding them using hourly local TMY2 weather data, I see a 3-month phase shift with a 2-day time constant for the soil rings. It looks like a 2'-diameter x 24'-tall chimney will work. Nick