Article 31240 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Still more comments on "Solar Water Heating" by Bob Ramlow Date: 3 Sep 2008 16:58:25 -0400 Organization: Villanova University Lines: 116 Message-ID: References: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1220475507 1477 153.104.44.130 (3 Sep 2008 20:58:27 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Wed, 3 Sep 2008 20:58:27 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:31240 page curiosity 133 Why not get closer to 100% solar house heating? If cloudy days are like coin flips, a house that can store enough heat for 1 day can be at most 50% solar-heated; 2 days make 75% possible, 3 make 88%, 4 make 93%, and 5 make 97%. More than 5 becomes uneconomical. 135 Used utility kWh meters are cheap at about $20. 136 Storing 5 gallons vs 1 of hot water for every gallon of daily use makes more sense to me. As an alternative, a large unpressurized space heat storage tank that cools from 140 to 90 F over 5 cloudy December days might preheat water in a pressurized plastic pipe coil 100% to 110 F for the first 3 days, 91% to 100 F on the 4th day, and 82% to 90 F on the 5th, supplying a weighted average 98% water heating. A simple greywater heat exchanger with storage could return most of the water heat to house air. Then Bob talks more about "the risk of overheating," as if we can't just turn off the pump in a simple draindown system :-) 137 A large well-insulated tank doesn't require a lot more collector area for the same use. Why not store 10 vs 1-2 gallons/ft^2 of collector? 138 Overheating fears again. Turn off the pump! :-) 139 "In most climates it is nearly impossible to heat a dwelling 100% with solar energy..." Given the random nature of weather, that's true in every climate. PE Norman Saunders says he's never quite reached 100% in New England, but having to "purchase heat" every 35 years isn't bad :-) 140 Sizing a solar energy system IS a precise science, given good local weather data, including NREL's TMY2 and TMY3 hourly files. 141 "It is impractical to size a solar heating system to heat your building on the coldest day of the year, because then for the rest of the winter your system would be oversized." But that's how conventional HVAC systems are sized, with a local "winter design temperature." Economics may favor "purchasing heat" on very cold days, eg once every 35 years. 142 The gas bill analysis needs to include a furnace efficiency. 145 The big issue is NOT overheating! And the number of sunny days in a month is less important than the average temperature and the average amount of sun, eg 21.7 F and 810 Btu/ft^2 of sun on a south wall on an average December day in Madison, WI. The average daily max is 29.8, so the average daytime temp is about 26. Twinwall polycarbonate on a low-mass sunspace would collect 0.8x810 = 648 Btu/ft^2 and lose about 6h(100-26)1ft^2/R2 = 222, for a net 426 Btu/ft^2 on an average day. 148 No mention of slow-draindown systems with less powerful pumps, nor 2-speed pumps nor sequenced pumps in series. "Be aware that centrifugal high-head pumps can only pump fluid to a height of 30' above the pump..." shows serious confusion. They can only suck water up from a 30' height above the water. 149 Why not "oversize" an air collector? :-) Bob suggests 10% of the floor area as collectors. My 1536 ft^2 1820 stone farmhouse has 16'x32' of sunspace glazing and 20'x32' of Dynaglas attic glazing, ie 75% of the floor area. 160 Another way to mount a "collector" (or studs on masonry walls) is to glue it to a house with spray foam. I just bought 94 23 oz. Great Stuff Pro canisters near the end of their shelf life for $1.25 each from Home Depot :-) 161 Collectors with clogged weep holes can explode? :-) 165 The book's S = 2Lsin((T-P)/2) equation produces a standoff length of 12.53 inches with L = 48" and collector tilt T = 60 degrees and roof pitch P = 45 degrees. Simple geometry makes S = Lsin(T-P) = 12.43". Fig 8.11 on page 166 says S = 13"(?) 167 "All collectors should be pitched slightly towards the supply end to facilitate system draining"? :-) 171 Ice can be a good solar mirror, leaving collectors frozen in full sun. We might melt it off by manually turning on the circulation pump instead of scraping it off. 173 In most parts of the US, collector frames have to be strong enough to withstand a 100 mph wind. Building codes specify max local windspeeds and basic velocity pressure P = 0.00256V^2, eg P = 16 psf at V = 80 mph. Gary Reysa and I are working on a simple very low cost ground-mounted ICS "solar pond" heater with a pump. 178 Gary and I found that even small pumps destroy stratification. 190 Why commercial solar pool heaters, vs very simple systems, eg trickling water between a roof and a large piece of 5-year 5 cent/ft^2 greenhouse polyethylene film? I'm helping a local YMCA with a 50'x75' version of this. 204 Pump horsepower and head capability are not necessarily related. A low-power gear pump might move a trickle of water up 300'. 209 ICS systems CAN have pumps. 215 A smart differential thermostat might circulate glycol to keep it from overheating and continue circulation at night to cool the tank to some upper limit. IIRC, some swimming pools work this way. 218 We need to OPEN- vs short circuit a negative tempco thermister storage sensor to make the controller think it's cold. And ICS systems can work fine in freezing climates. SRCC's OG300 spec includes a test procedure for this. 222 Buy Tom Lane's book instead of this one :-) 228 Water vapor does not scatter light, and degree-days are not temperature differences. 230 Orientation includes tilt, and selective surfaces are poor heat vs solar energy radiators. Nick