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a 16' non-cube 8 mar 2001 looks like we can save some serious bucks by removing the glazing and putting the heat storage tank in the basement. less "roof" required, 10% more sun, the unpressurized target can be a lot simpler, the tank can have less insulation, and the heat loss from the tank warms the house, but... we need a pump. nrel says a 1-axis ew concentrator can collect 729 btu/ft^2 over 3 hours in phila on an average 30.4 f january day. with a 90% reflector and a layer of r1 clear polycarbonate glazing with 90% solar transmission, we might collect 0.9x0.9x729x16'x16' = 151k btu/day with a 16'x16'x16' tall open-sided box with a parabolic reflective north wall and roof. the reflector would bounce dawn sun down onto a 4'x16' horizontal target at the base of the north wall. the focus moves closer to the north wall during the day until the reflector begins to shade the base of the wall at a 45 degree sun elevation (at noon on april 4 and september 28, at 40 n latitude.) we would still have lots of hot water in summertime. with a 4' strip near the north wall dedicated to solar collection and 12'x16' = 192 ft^2 of clear floorspace, the structure could also be something else, eg a run-in shed for horses or cows or a place to park a car temporarily when it's snowing. the target might be a $10 16'x30" round uv polyethylene film greenhouse air duct laid flat on the ground over black plastic film. it might heat water for a 4'x12'x4' tall 1,024 gallon epdm-lined tank in a basement corner, with 9 inboard 4' studs on 2' centers and 1" of r7.2 foamboard underneath inside 4' and 12' 2x4s attached flat on the floor. we might put 3.5" fiberglass r11 insulation between the studs with a 1" layer of foamboard over that and a 1" layer of foamboard between the tank and the basement walls and another 2 layers over the tank. its thermal conductance would be 48ft^2/r17.2 = 2.79 for the floor plus 48ft^2/r14.4 = 3.33 for the cover plus 64ft^2/r7.2 = 8.89 for the outside walls plus 64ft^2/r18.2 = 3.52 for the inside walls. g = 18.53 and rc = 8192btu/f/18.53btu/h-f = 442 hours, or 18.4 days. so far, we have a $10 duct (griffin greenhouse and nursery supplies, 978-851-4346) plus 64 ft^2 of tank osb, $12 at $6/4x8 sheet. we've also used a $72 20'x12' piece of epdm for the tank liner, 64 ft^2 of fiberglass worth $16, 272 ft^2 of foamboard at $85 and 52 ft of 2x4s at $13. these materials subtotal $196. the reflector needs 384 ft^2 of osb and another 384 ft^2 of epdm for its roof ($187), and 9x24' = 216 ft of 2x4s on 2' centers with a dozen (11 :-) kerfs in each bow. the lower and upper edges could use 32 ft of 2x4s. we might support the open south wall with 3 16' posts with 2 2x4s attached at right angles with deck screws, with some upper diagonal bracing. that's 344 ft of 2x4s worth $86. the bottom part of each post could be pressure- treated wood bolted between the sidewalls of 2 vertical underground tires filled with stone. gluing 384 ft^2 of mylar ($0.09/ft^2 from http://www.snomo.com/mylar.html) under the parabola adds $35. grainger's 4pc89 120v 0.7a pump adds $135 and $7/year at 10 cents/kwh, making the total materials cost $639. the target has about 1.5x4'x16' = 96 btu/h-f of thermal conductance to outdoor air. we might collect 151k btu/day of beam sun over 3 hours at 200 f and lose 3h(200-30)96 = 49k btu, for an average daily hot water production of 102k btu at $639/(102k/3.41) = 2.1 cents per peak watt. at t degrees f, the tank loses heat at a rate of (t-70)18.53btu/h, and hot water use removes heat at a constant 102k/24h = 4250 btu/h, so dt/dt = -((t-70)18.53+4250)/8192 = -0.002262t -0.3605 = -ct +d, so d/c = -159 and t = -159+(200+159)exp(-0.002262t) = 120 f when t = 111 hours, after 4.6 cloudy 30 f days in january. now how can we make it frugaler? nick |