Article 30500 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Greenhouse design Date: 21 May 2008 05:24:26 -0400 Organization: Villanova University Lines: 104 Message-ID: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1211358221 28013 153.104.44.130 (21 May 2008 08:23:41 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Wed, 21 May 2008 08:23:41 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:30500 michael writes re: >... someone who grows fruits and veggies year round in Nebraska... >... The greenhouse itself is built somewhat below grade, 4 or 5 feet... NREL says 540 Btu/ft^2 of sun falls on the ground and 990 falls on a south wall on an average 24.4 F in Omaha in December. The deep ground is 50.1 F. Then again, plants need headroom. A 10'x10'x13'-tall greenhouse with a 12/12 roof and 2 layers of R1 south glazing with 80% solar transmission would collect 0.8x10'(13'x990+5'x540) = 124.6K Btu/day. With insulated non-south surfaces and a thermal conductance G = 10'(8+5/cos(45))/R2 = 75.4 Btu/h-F, 124.6K = 24h(T-24.4)G would allow an average indoor temp T = 93.3 F on an average December day. >Black drainage pipe is laid in patterns in the soil below the root zone. >Air from the upper portion is moved by a fan at slow speed through the long >sections of corrugated pipe and releases moisture and heat back to the soil. So the pipe has holes? And mold isn't a problem? If most of the water condenses in the pipe vs on the inner glazing, it must be cooler than the glazing. If greenhouse air is (say) 80 F and it's 24.4 outdoors and (80-24.4)/1ft^2/R2 = 27.8 Btu//h flows through 2 layers of R1 glazing, an R2/3 indoor air film makes the inner glazing 80-27.8x2/3 = 61.5 F. If the pipe is (say) 60 F at dusk and it keeps the house 50 F for 16 hours at night and 16h(50-24.4)G = 31K Btu = C(60-50), C = 3100, eg 3100/30 = 103 ft^3 of damp soil, eg 2 4'x10'xi15" deep beds on each side of a 3'x10' walkway, with lots of pipe. But tomato plants want to be warmer at night. Benton Jones says (on page 18 of "Tomato Plant Culture," CRC Press, 1999) they like to be 80 F max during the day and 65 at night. If the pipe is more than 61.5 F at dusk, there will be condensation on the inner glazing film with more heat loss to the outdoors (how much?), but we can keep the house warmer at night... 65 F for 16 hours takes 49K Btu. >The movement speed of the air is critical to make the dump of moisture >AND heat. It's probably "critical" to move enough air to store enough heat to keep the house warm at night. If Q cfm of 80 F air moves through a large mass with a large conductance for 8 hours and 8h(80-65)Q = 49K, Q = 408 cfm. C = 5000 Btu/F (2 4'x10'x2.1' beds) and 5000(T-65) = 49K makes T 74.8 F, which makes RC = -8/ln(74.8-80)/(65-80)) = 7.6 hours and R = 7.6/5000 = 0.00151 = 1/cfm + 1/(1.5A), approximately. With a 1000 cfm fan, we need A = 1307 ft^2 of pipe surface, eg 131 10'x4" pipes :-) A 1000 Btu/h-F (R = 0.001) $35 20-watt car radiator seems more practical: http://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf If a C Btu/F tank reaches T (F) after 8 hours and T = 80+(65-80)e^(-8/RC) and C(T-65) = 49K Btu, C = 3267/(1-e^(-8000/C)). Plugging in C = 3267 on the right makes C = 3576 on the left. Repeating, C = 3657, 3679, 3686, and 3687, eg 59 ft^3 of water in a 2'x4'x7' tank in the trench. The radiator could also go in the trench, for freeze protection and minimal pump power, with a downflow duct from the top of the greenhouse. During a long string of cloudy days, we might keep the greenhouse about 40 F with the radiator and (40-24.4)G/(50.1-40))/8.33/60 = 0.23 gpm of wellwater. >The walk paths are also built so the walking height is waist high to >help the ease of working the soil and plants. We could wear stilts to pick indeterminate tomatoes :-) >The walkway itself is expanded steel or wooden slats with an empty air >space below of a couple of feet deep. When cold air overcomes the greenhouse >through the night, it sinks to the sinkholes below the walkways and slows >considerably the cooling of the soil. The trench could warm the greenhouse. With 2x3x3+3x10+2x3x10 = 108 ft^2 of 50.1 F soil surface (at best) and (T-24.4)G = (50.1-T)108x1.5, T = 42 F. Lots of wet rocks in the trench could help. Warm air rises. Humid air rises. >Heaters would be optional midwinter if desired but not necessary according >to the designer. Freeze-tolerant plants could help, as in Eliot Coleman's unheated houses. Dan writes: > I've seen drums that doubled as supports for beds with condensate on > the surface while in full shade. The condensate indicates that the drums > are absorbing water at a high rate (due to phase or state change). The drums are absorbing water? > A rather large water tank underground connected to radiator fins in > the greenhouse will supply thermal mass for extended times of little sun. Freeze-tolerant fins could help. Warm water stops rising at 39 F. > A properly designed and constructed solar greenhouse in any part of > the continental US that gets good winter sun does not require back-up heat. Bubblewalls could help... >Make the greenhouse about 12 feet wide and only one story high, with >vents equivalent to 20 percent of the glazed area. My 100' tomato greenhouses are about 12' tall, with 4' rollup sides. Nick