From nick@vu-vlsi.ee.vill.eduSat Dec 23 00:06:07 1995 Date: 22 Dec 1995 15:34:45 -0500 From: Nick Pine To: london@sunsite.unc.edu Subject: Solar closets in a nutshell Fred A wrote in sci.engr.heat-vent-ac: >amuller@dca.net (Alan Muller) writes: >>I'm concerned about the electrical consumption of my oil >>burner "gun," though I have yet to measure the consumption. >Nothing is free. If you had a gas or electric hydronic system you would >still have the circulator pump, and the zone valves. With gas you may >still have the vent damper, burner motor; and, or other accessories. >Electrical consumption for heating systems is a fact of life, and not >a big deal... It is for us energy misers :-) The sun is free, and plastic film backdraft dampers use no electricity. If the only electrical power used in your mostly passive solar heating system is to move two Honeywell MLS642LS dampers and D6161A1001 actuator motors, and they only consume two watts when they are moving, that's pretty close to free :-) Important points: o Glass is a very poor insulator, compared to an insulated wall. o Trombe walls are very inefficient, because they store solar heat behind glass, and that heat leaks back out to the cold outdoors all night. o Low-thermal-mass sunspaces, eg those made with thin polycarbonate glazing, make better solar heaters than Trombe walls. We want an insulated wall between the sunspace and the house, and dampers in about 5% of the insulated wall, or perhaps fans (1-2 cfm/ft^2 of glazing) that move warm air from the sunspace into the house on sunny days. The sunspace fan or motorized damper should be in series with a cooling thermostat in the sunspace and a heating thermostat in the house. At night, the sunspace should get icy cold, leaving the heat stored inside the house, in the thermal mass of the house. o Water stores 3 times more heat than masonry by volume, with lower thermal resistance, so a box full of water containers can have 1/3 the volume of a rock bed, and lower airflow resistance as well, so it can transfer heat better by natural convection, or with minimal fan power, vs a rock bed. o It is good to have a separate heat battery, so we can charge it up to a high temperature on sunny days, and discharge it in a controllable way to heat the house to a constant temperature during a week of cloudy days. If we _live inside_ the heat battery, we cannot do that... o The heat battery should have some sealed containers of water, and the total surface area of the water containers should be 5-10 times that of the glazing. o Ohm's law for heatflow tells us how much heat we need for a house on an average day. U = hours x (Tin - Tout) (sum of A's over Rs) Beware of thermal bridges! o This tells us how much low-thermal mass sunspace glazing we need, since each square foot of sunspace glazing gathers about 1000 Btu or 300 watt-hours on an average day. o The daily heat loss also tells us how much water we need in the heat battery--enough for 5 days without sun, say (this needs further study.) A pound of water raised 1 degree F stores 1 Btu. o Solar closets can also heat water for houses, and serve as saunas or clothes-drying areas. Other points: o What's wrong with Trombe walls? (They are 25 times less efficient than low-thermal-mass sunspaces.) o What's wrong with direct gain houses? (Large temperature swings from day to night, steadily decreasing temperatures over a few days without sun, large backup heat requirement in cloudy climates, lack of privacy, excessive glare, can't put rugs on the floor, expensive masonry construction, no solar water heating, "optimal ratio of glass to floorspace" dilemma, etc.) o Temperature swings in a solar house (U delta t = C delta T, RC time constant) o What's wrong with "solar panels"? (Expensive, need pumps, antifreeze, heat exchangers, roof climbing--> broken bones, roof mounting-->inefficient, surrounded by cold air and wind, with back losses lost to ambient, vs back losses that heat the house, roof mounting-->leaks in roof penetrations and reroofing difficulties, roof mounting-->expensive rigid framework and installation labor, etc, etc.) o Efficiency vs. cost-effectiveness (Do you want a Mercedes or do you want transportation?) o What's wrong with PV? :-) (100 times less cost-effective than passive solar) o Ohm's law for heatflow (U = (hours)(Tin-Tout) x area/R-value) o Heat capacity and storage (Rocks, 22 Btu/ft^3/degree F Water, 62 Btu/ft^3/degree F Air, 0.02 Btu/ft^3/degree F A cubical solar closet L feet on a side has a time constant of L^2 days.) o Why water vs. rocks? (Cheaper, easier to move, lower thermal resistance, 3 times more heat capacity, lower airflow resistance.) o Why fans vs. blowers? (Blowers use hundreds of watts. Fans use 10s of watts.) o Is it wrong to waste solar energy? (No, if using a bit more sunspace glazing and a hotter sunspace lets one use a damper instead of a fan, and no, if it costs less to underinsulate the sunspace wall and open a house cooling damper during the day.) o Why dampers vs. fans? (Some motorized dampers only use 2 watts when moving, and zero watts in a fixed position. Using these with thermostats allows very accurate room temp control, with very little electrical power consumption.) o Convective loop heat flow (The amount of air in cfm that flows through a damper in an unrestricted chimney with height h feet and openings of Ad ft^2 at the top and bottom and a temperature difference of delta T from top to bottom is approximately Q=16.6 Ad sqrt(h*delta T). The amount of heat that moves through the dampers in Btu/hour is approximately U = Q delta t.) An example: 10 'ball park solar closet house design 20 TA=32'average December ambient temp 30 TCOLD=-10'coldest December ambient temp 40 SUN=1000'average amount of sun falling on sunspace glazing (Btu/ft^2/day) 50 DL=6'average number of hours of sun per day 60 TR=68'room temp 70 L=12'house length 80 W=8'house width 90 H=8'house height 100 AW=2*(L*W+L*H+W*H)'outside surface area 110 RW=23'R-value of outside surface of house 120 DHL=24*(TR-TA)*AW/RW'daily heat loss 130 DHLC=24*(TR-TCOLD)*AW/RW'heat loss on coldest day 140 AV=8*8/144'damper area (ft^2) 150 DF=AV*161.6*SQR(H)'damper factor 160 TSS=TR+(DHL/6/DF)^(2/3)'minimum ss temp to supply average daily heat 170 PRINT TSS;"F, minimum sunspace temp to supply average daily house heat 180 C=4000'pounds of water in solar closet 190 ATM=200'thermal mass surface area 200 RM=2/3'thermal mass surface R-value 210 LC=4'closet length 220 WC=4'closet width 230 HC=8'closet height 240 RC=20'R-value of solar closet surface 250 'find min solar closet temp ness to heat house on an average day w/o sun 260 TCM=TR+(DHL/24/DF)^(2/3) 270 PRINT TCM;"F, closet temp ness to heat house on an average day w/o sun" 280 'find min ss solar closet temp to provide heat for 5 days w/o sun 290 TCS=TCM+5*DHL/C 300 PRINT TCS;"F, min steady state solar closet temp for 5 day heat storage 310 'find avg closet cfm to keep house warm on coldest day w/o sun" 320 CFM=DHLC/24/(TCS-TR) 330 PRINT CFM;"cfm, avg closet airflow to heat house on coldest day w/o sun 340 'find average daily solar closet loss at that steady state temp 350 DCL=24*((TCS-TR)*(HC*WC+HC*LC)/RC+(TCS-TA)*(HC*WC+HC*LC)/RW) 360 AC=2*2'solar closet damper area 370 DFC=16.6*AC*SQR(H)'solar closet damper factor 380 TCP=TCS+(DCL/6/DFC)^(2/3) 390 PRINT TCP;"F, sunspace temp ness to make 5 day closet air temp" 400 'find min glazed area to make tcp, while providing heat and closet heat 410 AG=(DHL+DCL)/(SUN-DL*(TCP-TA)) 420 PRINT AG;"ft^2, min glazed area to make 5 day closet air temp" RUN 74.29121 F, minimum sunspace temp to supply average daily house heat 70.49667 F, closet temp ness to heat house on an average day w/o sun 94.53841 F, min steady state solar closet temp for 5 day heat storage 65.42774 cfm, avg closet airflow to heat house on coldest day w/o sun 97.65994 F, sunspace temp ness to make 5 day closet air temp 41.99063 ft^2, min glazed area to make 5 day closet air temp