Ohmwork

Notes for ASES Workshop on Solar House Heating and Natural Cooling Techniques

Portland, OR July 9, 2004

Written by Nick Pine, with Steve Baer, Drew Gillett, and Rich Komp.

Debatable Conclusions

  1. Heat flows like electricity.
  2. Solar heat can be 100 times cheaper than solar electricity.
  3. Superinsulated houses have to be very small or very large.
  4. Direct gain houses can be improved.
  5. Indirect gain can be more efficient.
  6. We might store heat in the ceiling.
  7. We might have a separate cloudy-day heat store.
  8. Low temp heat storage and distribution are difficult.
  9. Shurcliff's lung might be a good air-air heat exchanger.
  10. Greywater heat exchangers can help.
  11. We might also gather heat from PVs.
  12. Smart ventilation can be helpful.
  13. Swamp cooler controls can be improved.

0.0 It's a snap to save energy in the US. As soon as more people become involved in the basic math of heat transfer and get a gut-level, as well as intellectual, grasp on how a house works, solution after solution will appear.

This workshop aims at improving that grasp, which we can control better than our US cheap energy policy... If we paid related costs of healthcare and air pollution and Gulf wars at the pump, gasoline would be a lot more expensive. Drew says this writeup needs exercises for the reader. OK:

Exercise 0.1: The US consumed 20 million 42 gallon barrels of oil per day in 2003. What was the real cost per gallon? (Debatable answers appear at the end of these notes. :-)

Most people think "electricity" when they hear "energy," even though most houses need more heating energy than electrical energy (the ratio is 1:1 in Hawaii and 5:1 in Vermont.) It's easy to shrink the small electrical slice of the home energy pie with compact fluorescent (CF) lights and more efficient appliances. Solar heat can be very inexpensive compared to solar electricity. PV panels at $3 per peak watt cost 150X more than polycarbonate glazing at $1/50W = 0.02/Pw. And sunspaces add floorspace to a house.

A square foot of "solar collector" only collects about $1/year at today's oil prices, so anything (except PVs??? :-) that costs more than $10/ft^2 (half of that being labor) and only collects energy with no other useful purpose seems economically-doomed...

Exercise 0.2: Should we a) replace a 60 W bulb with a 14 watt CF or b) buy 60-14 = 46 additional watts of PV power or c) caulk the house?

Most of us "know" how to design passive solar houses with well-established rules of thumb, but let's relax and take a fresh look from a standpoint of basic physics...

Berlin is a nice town and there were many opportunities for a student to spend his time in an agreeable manner, for instance with the nice girls. But instead of that we had to perform big and awful calculations.

Konrad Zuse, inventor of the 1936 Z1 computer

Overview

This is a workshop on "Ohm's law for heatflow" with applications to solar water and house heating and natural cooling. We'll also discuss a simple greywater heat exchanger, a $60 300' piece of 1" plastic pipe coiled inside a 55-gallon drum. With hot water bursts of 13 gallons or less, it could be

97% efficient. If it is, why bother with solar hot water?

We'll provide arithmetic tools and data and strategies needed to site-build effective house heating and cooling systems using inexpensive materials and skills. We expect workshop participants to have some familiarity with high school algebra.

We'll discuss power, energy, heatflow, and overnight and cloudy day heat storage at the high-school math and physics level, with insulation values and heat capacities of materials, simple equations involving time constants, evaporative and night ventilation cooling, passive and low-energy solar heating, climate data, and schemes for houses that are 100% solar-heated and naturally cooled, by design. We'll provide a calculator (Steve Baer says "Throw away your calculator." :-) and a CD-ROM. Promising techniques include solar closets, trickle collectors, "pancake houses," soap bubble foam insulation, and solar attics, including systems to collect heat and electricity from water-cooled standard PV panels.

I'm an EE by training, Steve Baer is a well-known solar inventor, Rich Komp is president of the Maine Solar Energy Association and a PV author with a PChem PhD, and Drew Gillett is a Professional Engineer with civil engineering and architectural degrees.

Disclaimer

Some of the techniques we describe are experimental. Some have never been tried. We do not accept responsibility for their safety or functionality.

  1. Power and energy

Energy is the stuff we pay for, measured in Joules or watt-hours or kilowatt-hours (kWh) or Calories or "British thermal units" (Btu), no longer used in Britain :-) The British now use joules or kWh. A Btu is a quantity of heat, about the same as the energy in a kitchen match or a mouse-hour. One Btu can heat one pound (16 ounces) of water one degree F.

Exercise 1.1: How many Btu are needed to heat 8 ounces of water from 50 to 200 F to make a cup of tea?

Power is the rate of energy flow over time. A mere number, vs the stuff we pay for. Energy is power times time. One watt-hour of energy is equivalent to 3.41 Btu. If energy were miles traveled, power would be miles per hour. If energy were a paycheck, power would be an hourly rate of pay.

Exercise 1.2: How long would it take to heat the tea water with a 300 W immersion heater?

We might check this with an immersion heater and a watch and a $100 Raytek IR thermometer. Or a HOBO from Onset Computer Corp (1-800-LOGGERS.) Their $119 battery-powered U12-013 HOBO is about the size of a matchbox. It can record 43,000 12-bit samples at 1 second to 18 hour intervals of its own temperature and relative humidity (RH), with jacks for 2 more temperature probes or other devices on cables, and upload them to a PC spreadsheet via a USB port.

People often confuse power and energy, as in "My house uses lots of power" (vs energy) or "My furnace capacity is 50,000 Btu," vs Btu/h. Power is measured in watts or kW. Unlike energy, it can't be used or consumed.

People confuse heat and temperature, too. A bathtub full of hot water contains a lot of useful house heat, compared to a candle, but the candle is much hotter. A lower minimum usable temperature increases useful heat. Temperature is a measure of heat intensity. A 12-volt 100 amp-hour 50 pound automobile battery stores 267 times more energy (12Vx100Ah = 1200 Wh) than a 9-volt 500 milliamp-hour (9Vx0.5Ah = 4.5 Wh) 2 ounce transistor radio battery, at a lower voltage (ie "electrical temperature.") The $40 battery can store about 200 kWh over its lifetime, at 20 cents/kWh. A $1 cubic foot of water cooling from 130 to 80 F stores (130F-80F)64Btu/F = 3200 Btu, ie about 1 kWh, with a much longer lifetime and simpler I/O.

  1. Rich Komp, Ohm, and Newton

Rich Komp (who is still alive) says heat moves by conduction (a hot frying pan handle), convection (including air movement), radiation (the sun brings about 1000 watts per square meter or 300 Btu per hour per square foot on a clear day at noon in the Sahara), and phase change (144 Btu melts a pound of ice and 1000 Btu evaporates a pound of water.)

About 300 years ago, Isaac Newton said the amount of heat that flows through a wall is proportional to its area and the temperature difference from one side to the other and its thermal conductance. About 100 years later, Georg Ohm said the same about electricity: V = IR, ie a current I in amps times a resistance R in ohms produces a voltage difference V. Electrical conductance is measured on "mhos," which is ohms spelled backwards (or "siemens" which is snemeis spelled backwards :-) If 6 amps flows through a 2 ohm resistor, we'll see V = IR = 6Ax2ohms = 12 volts across it. Another example: 120 volts across 48 ohms makes I = E/R = 120V/48ohms = 2.5 amps flow, with electrical power P = IE = 120Vx2.5A = 300 watts.

Exercise 2.1: If 24 A flows through a 12 V headlamp, what's the resistance?

  1. Thermal ohms?

Ohm's law for heatflow (aka Newton's law of cooling) uses temperature instead of voltage difference. Heatflow is measured in units of power, in watts or Btu per hour. There's no such thing as a thermal "ohm." The closest thing is the US "R-value" stamped on foamboards and batts in hardware stores.

Beadboard (expanded white polystyrene coffee cup material) has an R-value of

4 (ft^2-F-h/Btu) per inch. Blue or pink or green Styrofoam board has R5 per inch. So does air, for downward heatflow. The slow-moving indoor and windier outdoor air films near a single layer of glass have a combined R-value of about 1. A smooth square foot in slow-moving air loses about 1.5 Btu/h-F, with a U1.5 or an R2/3 airfilm resistance. A rough square foot in V mph air loses about 2+V/2 Btu/h-F. Larger objects have lower film conductances.

Tiny cold soap bubbles (1/16" at 50 F) have an R-value of about R3 per inch. Bill Sturm's Calgary greenhouse filled the space between two polyethylene film covers with air during the day and soap bubble foam insulation at night and measured an 82% propane energy savings with and without the foam on -20 F nights, without much solar energy storage. He thinks about heating poly film refugee shelters this way in a cold climate.

Fiberglass is R3.5/inch, or half that, if it contains 2% moisture (which it might, in a cold climate, with some holes in a vapor barrier), or even less, if air flows around or through it, in very cold places. But R-values are not quite thermal ohms. To find the thermal resistance of a wall, we need to divide the R-value by the wall area. An 8'x10' R20 wall has a resistance of R20/(8'x10') = 0.25 F-h/Btu. We might call this "0.25 fhubs" or "4 buhfs." We can add buhfs for different kinds of house walls and windows, ie resistors in parallel, and add fhubs for series layers of wall insulation.

Exercise 3.1: If 6 Btu/h of heat power flows through a wall with a 12 F temp diff, what's the wall's thermal resistance in fhubs?

Exercise 3.2: What's the thermal conductance of an 8'x16' R10 wall?

If it's 70 F indoors and 30 F outdoors, (70F-30F)/0.2F-h/Btu = 200 Btu/h of heat power will flow through 1 ft^2 of R5 wall. But stud walls have lower stud resistances (R1/inch "thermal bridging") in parallel with the insulation resistance and heat collecting stud fins on each side of the wall in the form of sheathing and drywall, which lowers the overall R-value.

Ignoring the fins, we can look at a 10.66 ft^2 16"x8' wall section with R13 insulation framed with R1 per inch 2x4s on all edges as a "stud conductor" with 2(16/12+8)1.5/12 = 2.33 ft^2 of surface in parallel with a 10.66-2.33 = 8.33 ft^2 "insulation conductor." The studs have a 1/(R1x3.5") = U0.2857 U-value and the insulation has 1/R13 = U0.0769. The combined conductance is 2.33x0.2857+8.33x0.0769 = 1.306 Btu/h-F, for an effective R-value of

10.66ft^2/1.306buhf = R8.2, vs R13, without the studs.

Structural Insulated Panels (SIPs, glued plywood-foam-plywood sandwiches) have less thermal bridging and fewer air leaks. So do walls with plywood I-joists used as studs and loose-fill insulation (as used by ME Al Eggen, snipped-for-privacy@mindspring.org) and walls with a layer of foamboard insulation over the studs. An 8'x24' R24 6" SIP wall with a 40 F temperature difference would allow (70F-30F)8'x24'/R24 = 320 Btu/h of heatflow.

Metric (European, Canadian, Australian...) U-values are 5.68 times bigger than US U-values. A metric U1 window has 1 W/m^2C of thermal conductance. It's like a low-loss US R5.68 window. A 1 m^2 metric U1 window with 20 C air on one side and 0 C air on the other would pass (20C-0C)1m^2x1W/m^2C = 10 watts of heat power.

Exercise 3.3: How much heat flows through a 2mx6m metric U0.5 wall with a 20 C temperature difference?

Exercise 3.4: How much heat would flow through a 3'x4' US U0.25 window with 70 F air inside and 30 F outside?

Exercise 3.5: How much heat flows through a 24'x32' R64 ceiling with a 40 F temperature difference?

  1. A three-dog house?

Now we can talk about superinsulated houses. They need to be small, like sleeping bags. Can a person heat a house with the help of a few dogs? An ASHRAE- (American Society of Heating, Refrigeration and AC Engineers) standard 154 pound 19.6 ft^2 human at rest makes about 350 Btu/h, like a 100 W light bulb...

Exercise 4.1 How big can an L' R10 cube be, if it's 70 F inside and

30 F outside, with six outdoor faces, and it's heated by one person?

With R20 walls, L = sqrt(350/12) = 5.4'. Better, but cramped...

Exercise 4.2: How big can it be with the help of a 100 Btu/h dog? With two dogs? With three dogs? With three dogs and R50 strawbale walls?

If internal heat gain grows with volume, superinsulated apartment houses have minimum sizes. If a family of 8 1.65 Btu/h ASHRAE-standard 0.046 lb mice in a 6" cubical room make 8x1.65 = 13.2 Btu/h-ft^3, what's the smallest cubical "mouse motel" with R20 walls that can stay 70 F on a 30 F day? If

13.2L^3 = (70-30)6L^2/R20, L = 12/13.2 = 0.91', with 48 mice in 6 rooms.

Exercise 4.3: How big is L, with R10 walls?

Germans build "Passive Houses" like this (for people), with a "Passive House Institute" that certifies them. Jackson Lab in Bar Harbor, Maine is heated by 972,000 mice, even on -11 F cloudy days :-) A 6.61 pound ASHRAE-standard cat makes 68.02 Btu/h. A 50.0 pound normally-active ASHRAE dog makes 354.9 Btu/h. A 1,000 pound Jersey calf makes 4,000 Btu/h. Larger creatures with lower surface-to-volume ratios produce less heat per pound. In "The Return of the Solar Cat" (Patty Paw Press, 2003) PE Jim Augustyn mentions DOE's secret $1.6 trillion 100 megaton solar cat demonstration project "designed to mimic a solar cat's basic method for capturing energy on a grand scale."

A frugal 100 kWh per month of indoor electrical use (vs the US average of

833 kWh/mo) would add 100kWhx3412/(30dx24h) = 474 Btu/h to a person's 300 Btu/h, making an internal heat gain of 474+300 Btu/h = (70F-30F)6L^2/R20, which makes L = sqrt(774/12) = 8.03 feet, for a cube with R20 walls. With R50 strawbales, 474+300 Btu/h = (70-30)6L^2/R50 makes L = 12.7 feet max.

Exercise 4.4: If N families each use 774 Btu/h in 1024 ft^2 of floorspace inside an L' R20 cube, what's L, min?

But some people define a "solar house" as "one with no other form of heat," not even electrical usage or creatures-in-residence...

  1. The sun, and weather

Suppose the R20 8' cube has an A ft^2 R2 window that admits 200 Btu/h-ft^2 of sun. If 200A = (70F-30F)(A/2+(6x8'x8'-A)/R20) = 18A+768, we can make it

70 F indoors on a 30 F day with an A = 4 ft^2 window, but the temperature would instantly drop to 30 F when night falls, unless the cube contained some thermal mass. Of course the sun doesn't put a constant 200 Btu/h-ft^2 into the window all day. This is a crude but useful approximation.

Exercise 5.1: What's the steady-state 24-hour temperature of an 8' R20 cube with a *huge* thermal mass and a 4 ft^2 window with 80% solar transmission, if 1000 Btu/ft^2 falls on the window over a long string of average 30 F days?

Keeping it warm at night requires a larger south window. The NREL (National Renewable Energy Lab) Solar Radiation Data Manual for Buildings (the "blue book" at

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implies that January is the worst-case month for solar house heating in Boulder, CO, when 1370 Btu/ft^2 of solar heat falls on a south wall on an average 29.7 F day, and 1370/(65-29.7) = 38.8. December is the worst-case month for solar house heating in Portland, OR, where solar heating is harder, with 470/(65-40.2) = 19.0. January is the worst-case month in Albuquerque, where solar heating is much easier, with

1640/(65-34.2) = 53.2.

PE Norman Saunders says the "worst-case month" is the one with the least solar heat per degree day, on average. This is usually December or January. Aerospace engineers often use "worst-case" specs instead of saying "I know you are going to be very happy with this airplane" or "It will go VERY far and VERY fast and carry a LOT of weight." :-)

People often wishfully overestimate passive solar performance, saying things like "we only burn a few cords of wood per year." How vague. What's "a few"? What kind of wood? A 4'x4'x8' cord of oak with 20% moisture content burned at 60% efficiency delivers 158 therms of heat (158x10^5 Btu), about the same as 158 gallons of oil. White cedar can only deliver 72 therms. And wood is ongoing work, compared to solar heat. Was it a cold year or a warm year or an average year?

What's the solar heating fraction of this house?

Every day is sunny for George and Charlotte Britton of Lafayette Hill. Their 2,900-square-foot house is blessed with energy bills 20 percent lower than one of comparable size... The design incorporates "passive" solar principles. There are large double pane windows and sliding glass doors on the south side. Inside, tile floors and a Trombe wall absorb the sun's heat during the day and radiate it at night... A fireplace on the south wall of the living area provides additional heat during colder months... Britton says "We have a fire every day of the winter."

Solar houses with no other form of heat are 100% solar-heated, with no doubt. Then it's just a matter of comfort, or temperature swing. You might buy an inexpensive 100% solar-heated tent with a -20 to 120 F temperature swing :-) A house that can keep itself warm in the worst-case month should do fine in other months. Should America's Cup boats have outboard motors? Should solar architects pay clients backup heating fuel bills, if any?

NREL's "blue book" contains long-term monthly average solar weather data for 239 US locations. NREL's Typical Meteorological Year (TMY2) hourly data files can be useful for simple solar heating spreadsheet simulations with equations from high-school physics. PE Howard Reichmuth says a house that can pass a 1 year TMY2 simulation with no backup fuel requirement can also survive a 30-year hourly simulation for the same location. But a calculator seems like enough, with simple worst-case monthly average weather data. Can we connect these basic insights and understandings with LEED criteria which seem to have more to do with feng-shui than fossil fuels? People cared more about heating bills in 1975.

I wish we could circulate hot air between a house and sunspace and stop the airflow at night with an Energy-10 simulation, but it looks like that's not part of the present package (available from

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or future Energy-10 improvements. I still think of SBIC (aka PSIC) as crooked masonry salesmen out to cripple performance of solar houses by loading up sunspaces with thermal mass, but people can change.

An 8' R20 cube might be 70 F (eg 75 at dusk and 65 at dawn) over an average Boulder December day with an A ft^2 R2 window with 80% transmission that admits 0.8x1370 = 1096 Btu/ft^2. If 1096A/24h = (70-30)(A/2+(6x64-A)/R20) = 18A+768, A = 28 ft^2. Some thermal mass and a 4'x8' window would do, with a cube conductance of 32ft^2/R2+(6x64-32)/R20 = 34 Btu/h-F, about half window and half walls, like an early Los Alamos Labs passive solar test cell.

Exercise 5.2: What's A in Portland, OR (where 470 Btu/ft^2 falls on a south wall on an average 40.2 F December day)?

  1. Thermal mass and direct gain, aka "direct loss"

Water is cheap and heavy, but easily moved. When moving, it has a low thermal resistance. A square foot of 1" drywall (a "C1" board foot) stores 1 Btu/F, like a pound of water. A 95 lb cubic foot of dry sand with a 0.191 Btu/F-lb specific heat can store 0.191x95 = 18 Btu/F. Adding 40% water would increase this to about 24 Btu/F-ft^3 and let it support something above. Moving water would greatly lower its R0.4 per inch resistance. A cubic foot of concrete stores about 25 Btu/F, like wet sand, vs 64 for a cubic foot of water or steel, but concrete's R0.2 per inch resistance makes it hard to move heat in and out. A 32 pound 8"x8"x16" hollow concrete block with lots of surface has a specific heat of 0.16 Btu/F-lb and stores about 0.16x32 = 5 Btu/F.

An R fhub cube outside C Btu/F of thermal capacitance has a "time constant" RC = C/G in hours (dimensionally, (F-h/Btu)/(Btu/F) = h.) In RC hours, the indoor-outdoor temp difference decreases to about 1/3 (e^-1) of its initial value. If RC = 24 hours and T(0) - 75 F, T(h) = 30+(75-30)e^(-h/24) after h

30 F hours and T(d) = 30+(75-30)e^(-d) after d days, where e^x is the inverse of the natural log ln(x) function on a Casio fx-260 calculator. T starts at 75 F when e^(-0) = 1 and ends up at 30 F (the outdoor temp) much later, when e^(-oo) = 0. Between those times, the exponential factor gradually squashes the initial temperature difference of (75-30) = 45 degrees.

Exercise 6.1: What's T(d) after 1 day?

A liter of water weighs 2.2 pounds, so it stores 2.2 Btu/F. N 2-liter bottles inside the cube would make RC = 4.4N/34/24 = 0.00539N days. Starting at 75 F, T(d) = 30+(75-30)e^(-d/0.00539N) = 30+(75-30)e^(-185d/N) d days later. When

65 = 30+(75-30)e^(-185d/N), ln((65-30)/(75-30)) = -185d/N, and N = 738d.

Storing heat for 1 day requires 738 2-liter bottles, 2 days takes 1496, and so on. If clear and cloudy days were like coin flips, storing heat for 1 day would make the cube's solar heating fraction 50%, with 75% for 2 days, 88% for 3, 94% for 4, and 97% for 5. The chance of 5 cloudy days in a row would be like the chance of 5 tails in a row, ie 2^-5 = 0.03. Real weather has more persistence, like a 3-state (clear, average, cloudy) Markov chain.

But 738x5 = 3,690 is 4 pickup trucks full of bottles, and 16,236 pounds of water is a good ballast foundation :-) And PET bottle walls leak water vapor. They might need topping up once a year. We can fit about 9 4" diameter by 12" long bottles into a cubic foot, so they would occupy 3690/9 = 410 ft^3 of the cube's 8^3 = 512 ft^3, ie 80% of the living space, rather intrusively. This fraction shrinks with larger cubes with larger potential heat-storage volume to heat-losing surface ratios.

Exercise 6.2: What's the time constant of an "empty" 8' cube with a 1/2" layer of drywall inside R20 foamboard insulation? If it's 70 F indoors and 30 F outdoors when we turn off the furnace at 10 PM, what will the temperature be at 7 AM? What would it be if we add 1000 concrete blocks?

Exercise 6.3: What would it be if we filled it with water?

  1. Indirect gain and night insulation

Why look out a black window at night? With an R20 wall between the cube and a "low-thermal-mass sunspace" containing a south window, we can circulate warm air between the sunspace and living space during the day and stop the air circulation at night, so we can have the best of both worlds, the daytime gain and views and of the window without nighttime and cloudy-day heat loss, and the window and thermal mass can be smaller, compared to direct gain, but it's hard to store solar heat from warm air, compared to mass in direct sun, because of the high airfilm resistance at the mass surface.

Exercise 7.1: Can we use less than the 3690 bottles in section 6, with indirect gain? How many are needed to store heat for 5 cloudy days?

We can stop air circulation at night with a one-way passive plastic film damper hung over a vent hole in the R20 wall, with a mesh that only allows the film to swing open in one direction. I think Doug Kelbaugh invented this "7-cent solution" in Princeton in 1973. Drew thinks he described it in the proceedings of the First Passive Solar Conference in Albuquerque. PE Norman Saunders says they need inspecting every two weeks for folded or torn or hung-up films, but automatic systems are nice, in general. Over the years, people usually stop moving movable insulation that requires twice-a-day people power, even the outdoor shutter over the wall of Steve Baer's house.

Gary Reysa 466 Btu/h 466 Btu/h

where (72-65)96Q/(96+Q) = 466, so Q = 217 cfm. Grainger's $120 48" 315 rpm

86 W 21K cfm 4C853 ceiling fan might move 217 cfm at 217/21Kx315 = 3.3 rpm with 86(217/21K)^3 = 100 microwatts, according to the fan laws :-) Large slow fans can be very efficient and quiet. The Gossamer ceiling fan developed by Danny Parker at the Florida Solar Energy Center can move 1907 cfm with 9.1 watts, so it could move 217 cfm with 1 watt, running 11% of the time.

  1. Even less mass?

On an average day, we might only store 13.8K of overnight heat in C Btu/F in the ceiling, with T(6) = 120+(72-120)e^(-6x96/C) = 120-48e^(-576/C) and (T(6)-72)C = 18,432, so C = 384/(1-e^(-576/C). C = 384 on the right makes C = 494 on the left, and plugging that in on the right again makes C = 558,

596, 620, 635, 644, 649, 653, 655, 657, and 657 (whew!), so it looks like we can store overnight heat with 657/64 = 10.3 psf (about 2") of water above the ceiling, with T(6) = 120-48e^(-576/657) = 100 F.

The 8' cube needs 4x24(70-30)19.2 = 73,728 Btu for 4 more cloudy days. This might come from a "solar closet" (see paper on CD) inside a sunspace (the heat lost from the closet air heater glazing efficiently ends up in warm sunspace air that heats the cube :-) with about 73,728/(120-70) = 1475 lb or 184 gal. or 23 ft^3 of water cooling from 120 F to 70 F over 4 days.

Kallwall's "solar battery" was an early solar closet. Tom Hopper built an insulated box containing fiberglass water cylinders with double-glazed south wall and an insulating curtain (5 layers of aluminized Mylar) that automatically rolled down over the glazing at night. Bill Shurcliff (in his

1980 Brick House Thermal Shutters and Shades book) said this remarkable Mylar curtain (with spacers that unfurled when it deployed) was invented by the Insulating Shade Co. of Branford CN, and demonstrated at a 9/9/77 exhibition in Hartford, CN, and featured in the January 1979 issue of Popular Science Monthly.

With 1792 pounds of water in 56 10"x10"x13" 4-gallon ROPAK plastic tubs, we can supply 73,728 Btu as it cools from 120 to 120-73728/1792 = 78.9 F, stacking the tubs 7-high and 4-wide and 2-deep in 2'x4'x8' tall closet that's completely surrounded by insulation, with an air heater with its own closet vs sunspace glazing over the closet's insulated south wall and one-way dampers in that wall. With 56x4x10x13/144 = 202 ft^2 of tub surface and 300 Btu/h-F (buhfs) of water-air thermal conductance, we have diagram 1 again with a

78.9-70 = 8.9 F temperature difference and a (300+Q)/(300Q) resistor, so Q = 121 cfm.

With an 8' height difference and 121 cfm = 16.6Asqrt(8'(78.9-70)), we need 2 vents with A = 0.86 ft^2 for natural airflow into the cube.

As an alternative, the cloudy-day heat might come from 3072 pounds of water inside a 2'x4'x8' tall "shelfbox" with a 2'x4'x2' tall water tank below 18

2'x4' wire shelves on 4" centers, with 2" of water inside a $20 continuous piece of poly film duct folded to lay flat on each shelf and a small pump to circulate tank water up through the duct as needed. The tank might have a pressurized tank inside to make hot water for showers, with the help of an efficient external greywater heat exchanger, eg 300' of 1.17" OD plastic pipe pushed into two coils inside a 35"x23.5"" ID 55 gallon drum. The outer coil can be longer, with 27 turns and a 23.5-1.17 = 22.3" diameter and a 5.85' circumference and 27x5.85 = 157' of pipe. If the inner coil perfectly nested inside the turns of the outer one, its diameter would be 20.3", with a 5.31' circumference and 143' of pipe. Pushing the pipe around inside the drum is awkward but doable with two Keystone Kops trying to avoid kinks.

  1. A greywater heat exchanger

About 15 years ago, Eric Olsen at Earthstar manufactured a heat exchanger with a copper coil in a 55 gallon drum to recover heat in kidney dialysis centers. Drew says professor Jane Davidson at U Minnesota is an expert on plastic heat exchangers. She says "The efficiency you calculate is actually effectiveness which is very different from efficiency. A high effectiveness does not mean a good heat exchanger!" I wonder what she means by that.

I'm using a new $35 55 gallon lined steel drum with a strong removable lid (because the drum might end up under 2' of greywater head, with the inlet and outlet above the lid) and bolt ring and a 3/4" bung and a 2" bung with a 3/4" threaded knockout, with 100 psi/73.4 F pipe from PT Industries at (800) 44 ENDOT. Their PBJ10041010001 1"x300'100psi NSF-certified pipe is tested to 500 psi. The price is $59.99 from any True Value hardware store. Lowes sells the rest of the hardware. It's all installed through the lid, so the drum itself has no holes:

sales total # qty price description

25775 1 $5.73 24' of 1.25" sump pump hose (for greywater I/O) 105473 1 1.28 2 SS 1.75" hose clamps (for greywater hose) 54129 2 3.24 1.25" female adapter (greywater barb inlet and outlet) 23859 2 2.36 1.25x1.5" reducing male adapter (bulkhead fittings) 75912 1 0.51 2 1.25" conduit locknuts (bulkhead fittings) 28299 1 1.53 2 1.25" reducing conduit washers (") 22716 1 1.36 1.5" PVC street elbow (horizontal greywater inlet) 23830 1 2.98 10' 1.5" PVC pipe (for 3' greywater outlet dip tube)

The parts above are greywater plumbing ($18.99.)

23766 4 1.28 3/4" CPVC male adapter (for 1" pipe barbs) 23766 2 0.64 3/4" CPVC male adapter (fresh water I/O) 42000 2 3.84 3/4" FIP to 3/4" male hose adapter 23813 1 1.39 10' 3/4" CPVC pipe (for 1"x3/4" fresh water outlet) 23760 2 0.96 3/4" CPVC T (fresh water I/O) 22643 2 0.86 3/4" CPVC street elbow (fresh water I/O) 4 - 1" 3/4" CPVC pipes (fresh water I/0) 1 - 3' 3/4" CPVC pipe (fresh water inlet) 22667 2 2.56 2 SS 1.125" hose clamps (fresh water I/O) 219980 1 4.87 10.1 oz DAP silicone ultra caulk (bulkhead fittings) 150887 1 3.94 4 oz primer and 4 oz PVC cement

Parts above are fresh water plumbing. Subtotal $39.33.

26371 1 6.83 1500 W electric water heater element 22230 1 2.31 1" galvanized T ("nut" for heating element) 61294 1 11.76 single element thermostat with safety 136343 1 0.56 5 10-24x3/4" machine screws (mount thermostat with 3) 33368 1 0.37 5 #10 SS flat washers (mount thermostat with 3) 198806 1 1.38 10 #0 rubber faucet washers (mount thermostat with 3) 8763 1 0.67 5 10-24 SS nuts (mount thermostat with 3)

The above would make a standalone water heater, if needed. Grand total: $63.21.

For 4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm we might heat 75 gallons of 55 F water to 110 with 8x75(110-55) = 33K Btu with about 10 kWh worth about $1/day at 10 cents/kWh. If the "heat capacity flow rate" Cmin = Cmax = 75gx8/24h = 25 Btu/h-F and the pipe coil has A = 300Pi/12 = 78.5 ft^2 of surface with U = 10 Btu/h-F-ft^2 (for an HDPE pipe wall with slow-moving warm dirty water outside and 8x300Pi(1/2/12)^2 = 13 gallons of fresh water inside), the "Number of heat Transfer Units" for this counterflow heat exchanger NTU = AU/Cmin = 78.5ft^2x10Btu/h-F-ft^2/25Btu/h-F = 31.4, so the "efficiency" E = NTU/(NTU+1) = 97% for hot water usage in bursts of less than 13 gallons. This works best with equal greywater and cold water flows, with either a 110 F water heater setting (preferable), or the heat exchanger output feeding the cold water shower inlet as well as the water heater.

The Hazen-Williams equation says L' of d" smooth pipe with G gpm flow has a

0.0004227LG^1.852d^-4.871 psi loss. At 1.25 gpm, the pressure drop for 2 150' coils of 1" pipe might be 0.0004227x150x(1.25/2)^1.852x1^-4.871 = 0.03 psi.

If greywater leaves a shower drain and enters the heat exchanger at 100 F and fresh water enters at 50 F, the fresh water should leave at 50+0.97(100-50) = 98.5 F. Warming it further to 110 F would take 8x75(110-98.5) = 6.9K Btu/day with 2 kWh worth about 20 cents/at 10 cents/kWh, for a yearly savings of about ($1-0.20)365 = $292, or more, with a tighter shower enclosure and higher drain temperature. The 1500 W heater might operate 2kWh/1.5kW = 1.3 hours per day. We might wrap the drum with 3.5" of fiberglass and a 4'x8' piece of foil- foamboard with 7 4' kerfs (knife cuts partially through the board) to make an octagon and aluminum foil tape to cover the kerfs and hold it closed.

  1. Other promising solar house heating schemes

Harry Thomason's trickle collectors were used in hundreds of houses. Pump water up to the ridgeline of a metal roof and trickle it down under a glass cover to a gutter, then down to a large tank on the ground buried in rocks to help with water-air heat distribution. He also trickled water over bare asphalt shingle roofs at night for evaporative cooling. Dick Bourne at Davis Energy Systems also does this sort of thing, and lives in a roofpond house.

These days, a hydronic floor with a low water-air thermal resistance in an airtight house with lots of insulation might be simpler and use less power for heat distribution. This system needs lots of collector pump power, and it looks like the roof cover has to be polyethylene film or glass, since polycarbonate quickly degrades in warm water vapor. We might inflate a polyethylene film cover over a dark asphalt roof when water is flowing between them or it's very windy outdoors (>15 mph) or the roof is very hot (>150 F), with straps over the poly film to prevent outward ballooning.

Trickle collectors can only heat water to 80 or 90 F on cold winter days, so heat storage and distribution tend to be inefficient. An efficient solar heating system might keep a house warm on an average day with a hydronic floor and low temp water from a trickle collector and lots of insulation, and keep the house warm for 5 cloudy days with some higher temp water from a well-insulated and seldom used cloudy-day store.

Exercise 13.1: What's the min floor temp required to keep a house with a

200 Btu/h-F conductance and a 2400 ft^2 radiant floor 70 F on a 30 F day?

Zomeworks may soon have a more efficient "double-play" system with plastic tubes under closely-spaced metal roof standing seams, with a polycarbonate cover or a selective surface. Dawn Solar in New Hampshire is also working on something like this.

Zomeworks "Cool Cells" connect water pipes under the ceiling with roof ponds or radiators that lose heat by natural thermosypnoning at night, which stops during the day. A system with water in weaker flat poly ducts under the ceiling might use a 10 watt fountain pump to keep a roofpond full at night vs thermosyphoning. A roofpond that's dry during the day would consume less water and kill biological growths with solar heat.

Donald Wright's Habitat for Humanity houses near Safford Arizona use large hydronic solar collectors with fiberglass window screen between two Hypalon rubber layers for more uniform water flow. But they pump the warm water under a slab beneath lots of sand with a high thermal resistance, which seems like a big mistake, thermally-speaking. Adirondack houses also work this way, but they seem to be mostly wood-heated. Kachadorian's solar slab is a good idea, but I doubt it would work with thermosyphoning air, as he claims. And with no insulation above, it can't be much warmer than 70 F without overheating the occupants. Rich Komp calls his earlier slab a hypocaust, as in hollow Roman wood-fired floors.

A Thomason "pancake house" might have a draindown polyethylene film roof pond and an underfloor poly film pillow for heat distribution. A "concentrating solar attic" might have a transparent steep south roof and a north roof that approximates a parabola with 4 or 5 line segments (we want to avoid line foci) aimed slightly above the southern horizon to reflect 2-3 suns down into a

4'-wide water trough along the attic floor near the north wall. The trough might be 30" round polyethylene film greenhouse air duct (about 40 cents per linear foot) that lays flat to 48", with 1-2" of water inside during the day. This might lay on top of standard photovoltaic panels on the attic floor. We need careful layout or some sort of optical mixing for uniform illumination to avoid inefficient shadows and bright spots on the PVs. Cooled to 120 F, they should have a long lifetime under 2-3 suns. Your milage may vary.

Don Booth built a beadwall attic with waterbed heat storage in Concord, NH.

  1. Natural cooling

Less electrical usage helps, eg reflected sunlight at 200 lumens per watt instead of fluorescents at 50 lumens per watt. These days, the illuminance of a well-lit building is 50 footcandles (FC)...

To increase the "demand" for more power plants, the power and lighting industries were very effective in promoting ever-increasing illumination levels and incorporating them into codes. In some cases "required" light levels were so high that artificial lights had to be used all the time, regardless of the external climate and availability of natural light. Such lights often heated buildings to the extent that air conditioning was needed year-round. from Sunlighting as Formgiver for Architecture, by William M. C. Lam, Van Nostrand, 1986

...we might provide 50 FC = 50lumens/ft^2x64ft^2 = 3200 lumens with 3200/50 = 64 watts of fluorescents or 16 watts of sunlight. Bright sun is about 8000 FC, so we only need about 50/8000x64ft^2 = 0.4ft^2 of window for this, ie 0.6% of the floorspace as windows, with no transmission or reflection loss, if the sun could be distributed uniformly, or 2%, counting losses and nonuniformity. Or 4%, to have enough light on overcast days.

Our 8' cube might have a 1.28 ft^2 clerestory window 8' long and 2" tall on the south side of the roof near the peak, with a reflective roof underneath and an overhang that's reflective underneath and a reflective north ceiling. We might want taller windows with overhangs to admit all beam sun at noon on the winter solstice and exclude it all on the summer solstice.

At noon on 12/21 in the northern hemisphere, sun elevation Emin = 90-lat-23.5 degrees. At noon on 6/21, Emax = 90-lat+23.5 degrees. A horizontal overhang that projects p feet from a south wall d feet above the top of the glass of an h foot tall window can completely shade the window on 6/21 and admit all sun on 12/21 if tan(Emin)=d/p and tan(Emax)=(h+d)/p, so p=h/(tan(Emax)-tan(Emin)) and d = ptan(Emin). For example, at 40 N. lat, Emin = 26.5 degrees and Emax = 73.5, so an h = 8' tall window needs a p = 2.7' overhang d = 1.2' above the top of the glass.

Shading and light-colored walls help. Plants on trellises come to mind. Walls and roofs have more insulation than windows, but they also have much more surface. Some mobile homes have old tires on flat roofs. It pays ($1 each) to collect old tires where I live in PA. A simple green roof might have weeds growing in gravel over EPDM rubber. It might cool better and use less water without the weeds. Attics and sunspaces need venting.

Night ventilation can help, if a house has lots of internal thermal mass and lots of insulation. Ventilate with cool night air and button the house up during the day and let the thermal mass keep it cool. NREL says July is the warmest month in Boulder, with average 73.5 F days and daily lows and highs of 58.6 and 88.2 and an average humidity ratio w = 0.009. This is the number of pounds of water vapor per pound of dry air. It is more constant over a day than the relative humidity (RH), which depends how much water the air can hold, which depends on the air temperature.

"Comfort" depends on temperature, humidity, air velocity (faster is cooler), activity (sleeping vs wrestling) and clothing (three-piece suits vs shorts and a T-shirt.) The ASHRAE comfort zone relates temperature and humidity ratio. Experiments have found that people in developed countries are "comfortable" from about 67 to 81 F, with w = 0.0045 to 0.012. T = 89.4-1867w is a "constant comfort" line diagonally down through the zone. With w = 0.009, T = 72.6 is most comfortable. Can we keep our cube in the zone with night ventilation?

We might put N 2-liter water bottles inside the cube and remove the roof to make a "cold trap" at night with no lower temperature limit, or put a motorized damper near the top that lets warm air flow out of the cube every night until the inside air temp drops to 70.6 F.

If we model an average Boulder July day as a square wave with 12 hours at (58.6+73.5)/2 = 66 F followed by 12 hours at (73.5+88.2) = 81 F (not very close to the real sine wave, but maybe more useful than solar architects' rules of thumb), we have C = 4.4N Btu/F in series with its 1.83N thermal conductance to slow moving air and a 19.2 bhuf cube-to-outdoor conductance that's shunted at night with a Q cfm airflow conductance.

1/19.2 1/(1.83N) | | 81/66 ------------www--------------www-------| |---| | | | | | 1/Q | ---/ ---www------ 4.4N Btu/F

As the bottles warm, 74.6 = 81+(70.6-81)e^(-12/RCc) makes the charge time constant RCc = 24.7 h = 4.4N/19.2, approximately, so N = 108 (recall Kim Basinger's weight in "Batman"?) and C = 475 Btu/h. As the bottles cool,

70.6 = 66+(74.6-66)e^(-12/RCd) makes RCd = 19.2 h. Each bottle has about 1.2 ft^2 of surface with a 1.5x1.2 = 1.8 bhuf conductance, so 108 have a 198 bhuf conductance or a 5.06 millifhub thermal resistance. If RCd = 19.2, the total series resistance R to outdoors is 19.2/475 = 0.0404 F-h/Btu, and 1/Q = 0.0404-0.00506 = 0.035 fhub. Q = 28.6 cfm = 16.6Asqrt(8'(70.6-66)) makes A = 0.28 ft^2 for natural airflow. We could cool 2" of water above the ceiling with a gable vent or a thermal chimney above a flat roof.

We could also use a fan. In Pablo LaRoche and Murray Milne's tiny UCLA test house with some thermal mass, a "smart whole house fan controller" turned on a fan when outdoor air was cooler than indoor air. "Enthalpy economizers" do this for large buildings, but seldom for houses. With a humidity sensor, we might heat as well as cool a house by ventilation, and avoid condensation on mass surfaces, and bias the house temperature into the upper part of the comfort zone in wintertime and the lower part in summertime in order to store more heat or coolth in the mass of the house. Brand Electronics may soon sell fan controller like this.

We might also cool 2" of water above the ceiling with a roof pond, as in a Zomeworks thermosyphoning "architectural cool cell." Phil Niles says a T (F) pond in To (F) air loses 1.63x10^-9((T+460)^4-a(To+460)^4) Btu/h-ft^2 by radiation, where a = 0.002056Tdp+0.7378, with a dew point temp Tdp (F). With V mph of wind, it also loses Qc = (0.74+0.3V)(T-To) Btu/h-ft^2 by convection, and it evaporates Qe = b(T-Twb)-Qc, where b = 3.01(0.74+0.3V)((T+Twb)/65-1), and Twb (F) is the wet bulb temperature.

The pond radiates more heat to the sky if the air contains less moisture, with a higher dew point, since water vapor is a "greenhouse gas" that blocks radiation. Air at the dew point temperature is saturated with water vapor. The relative humidity is 100%. We can find the approximate dew point by first finding the vapor pressure of water in air on an average July day in Boulder with humidity ratio w = 0.009. Pa = 29.921/(1+0.62198/w) = 0.427 inches of mercury ("Hg--29.921 "Hg is 1 atmosphere.) Then we use a Clausius-Clapeyron approximation (don't ask) to find the temperature corresponding to that pressure, at 100% RH. If Pa = e^(17.863-9621/Tdp), ln(Pa) = 17.863-9621/Tdp, so Tdp = 9621/(17.863-ln(Pa)) = 514 R or 54 F in Boulder in July.

This makes a = 0.7399 in the formula above, so a 70.6 F pond in 58.6 F night air would lose 1.63x10^-9((70.6+460)^4-0.7399(58.6+460)^4) = 42 Btu/h-ft^2. NREL says the average July windspeed in Boulder is 8.1 mph, so a pond would lose (0.74+0.3x8.1)(70.6-58.6) = 38 Btu/h-ft^2 by convection. An 8'x8' roof pond would lose 8x8(42+38) = 5120 Btu/h by radiation and convection, more than a 5,000 Btu/h window air conditioner. Keeping the cube 72.6 F on an 81 F day in Boulder only requires 12h(81-72.6)19.2 = 1935 Btu, so we don't need to evaporate water from this roof pond. It might have a polyethylene film cover, since poly film is essentially transparent to radiation.

The dew point temperature only depends on the amount of water in an air sample, vs its temperature. A little water inside a perfectly-insulated cup might find itself at the dew point temp, as water evaporates from the surface and water vapor molecules diffuse to the top of the cup, while the air above the water acts as a good insulator (about R5 per inch for downward heatflow.) Which water depth will keep it coolest in an R5 3" diameter x 6" tall cup? As we fill the cup, the air layer insulates less, but more water evaporates, according to Fick's law, because the concentration gradient increases as the air layer thins...

When the water gets to the top of the cup, we'd expect to see it at the wet bulb temperature, when its heat loss by evaporation equals its heat gain by convection. A perfect swamp cooler would make air at this temperature. In

1926, I.S. Bowen said a pond's ratio of heat loss by evaporation to heat gain by convection equals 100(Pp-Pa)/(Tp-Ta), regardless of windspeed. This ratio is -1 at the wet bulb temp. With Pa = 0.009, and Ta = 58.6 F, and Tw (R), 100(e^(17.863-9621/Tw)-0.427) = 58.6+460-Tw, so Tw = 9621/(22.47-ln(561.3-Tw)).

The wet bulb temp is easy to find on a calculator. Plugging in Tw = 518.6 R (58.6 F) on the right of the equation above makes Tw = 514 on the left. Doing this again makes Tw = 517, 515, 516.2, 515.6, 515.9, 515.7, and 515.8 (55.8 F), between the dew point and dry bulb temperatures. So an uncovered roof pond would lose Qe = b(T-Twb)-Qc, where b=3.01(0.74+0.3x8.1)((70.6+55.8)/65-1) = 9.01 and Qe = 9.01(70.6-55.8)-38 = 95 more Btu/h-ft^2 by evaporation, like a 10K Btu/h window AC. This could be useful in Phoenix. One simple ASHRAE swimming pool formula says Q = 100(Pw-Pa) Btu/h-ft^2, regardless of air temp. Pw = e^(17.863-9621/(460+70.6)) = 0.764 "Hg and Pa = 0.427 "Hg makes Q = 34 Btu/h-ft^2.

Indoor misting with a vent fan for outdoor air works like a swamp cooler, when 4500(wi-wo)>To-Ti. The left side is the amount of latent cooling per cfm of vent air. The right is the amount of sensible heating per cfm of vent air. As they become equal, swamp cooling and misting no longer work.

The ASHRAE comfort zone has an efficient corner at Ti = 80.2 F and wi = 0.012 (at 56% RH), which makes To < 134.2-4500wo. NREL says Phoenix has an 88.2 F average temp in June, with an average daily min and max of

72.9 and 103.5 and average humidity ratio wo = 0.0056 pounds of water per pound of dry air in June, so misting should help until the outdoor temp rises to To = 134.2-4500(0.0056) = 109 F.

Evaporating a pound of water takes about 1000 Btu, and air weighs about

0.075 lb/ft^3, so a house with no internal heat gain or unshaded windows and a 200 Btu/h-F conductance that's 80.2 F indoors with wi = 0.012 when it's 88.2 F and wo = 0.0056 outdoors needs 1000x60C0.075(0.012-0.0056) = 28.8C = (88.2-80.2)(200+C) Btu/h of cooling, including cooling C cfm of air from 88.2 to 80.2, so C = 77 cfm (not much), with 4.5C(0.012-0.0056) = 2.2 pounds or 0.27 gph of water, which might come from a $5 0.5 gph Mister Mister nozzle and a solenoid valve scrounged from an old washing machine in series with a 80.2 F room temp thermostat and a $5 humidistat that turns on a $12 window box fan when the RH rises to 56%.

We can make 77 cfm move through the house with A ft^2 vents with 8' of height difference if 77 = 16.6Asqrt(8(88.2-80.2)), ie A = 0.58 ft^2, eg a 1 ft^2 vent to the attic, to keep it cooler. The 2.2 pounds of water per hour might come from plants in an indoor greywater wetland. With enough plants, The thermostat may never turn on the mister. In an air-leaky house, the humidistat may never turn on the fan.

Herbach and Rademan (800) 848-8001

formatting link
sell a nice brass $4.95 Navy surplus humidistat, item number TM89HVC5203, with a

20-80% range, a 3-6% differential, and a 7.5A 125V switch that can be wired to open or close on humidity rise.

Pw = 1.033 "Hg at 80.2 F and 100% RH, and Pi = 0.566 "Hg with wi = 0.012. ASHRAE says a square foot of pool evaporates 100(Pw-Pi) = 47 Btu/h, so we can evaporate 2.2 pounds of water per hour with 2200/47 = 47 ft^2 of wet surface, counting droplets in air. Masonry floors and walls or an indoor cylindrical rock gabion with a fountain pump (in a toilet tank?) with high thermal conductance and mass might allow more efficient cooling with cooler night air and keep the mass of the house cool with the fan off during the heat of the day. The gabion might build up a mineral encrustation that helps evaporate water.

In arid places like Phoenix, why not distill the water we drink in a greenhouse and filter the rest through a high-temp wetland?

NREL says 1020 Btu/ft^2 of sun falls on the ground and 1550 falls on a south wall on an average 53.6 F January (the worst-case month for solar heating) day in Phoenix. A 16'x16'x8' tall greenhouse would get 0.8(256x1020+128x1550) = 368K Btu of sun. Distilling 10 gallons of potable water at 1,000 Btu/lb (or a lot less, with Steve Baer's multi-effect still) would leave 368K-83K = 284K Btu. With 600 ft^2 of R1.2 surface, the greenhouse conductance would be 500 Btu/h-F, so 284K Btu could keep it 53.6+284K/(24hx500) = 80 F (27 C) on an average day.

We might put a outer layer of plastic film over 4 16' long x 8' tall outer bows and another layer over 4 inner bows, with a slot at the top to allow warm moist air to enter the spaces between the glazings and condense water into east and west troughs on the ground made from a fold in the outer layer, with a slot in the inner layer above the trough to allow air to recirculate.

If 80 F air enters the top at 100% RH with wt = 0.0226 and 53.6 F air leaves the bottom at 100% with wb = 0.0088, distilling 10 gallons in 24 hours means

24x60C0.075(wt-wb) = 1.48C = 10x8.33, so C = 56 cfm = 16.6Asqrt(8x(80-53.6)), for an upper vent area A = 0.23 ft^2, eg 3 4" holes. The inner films might have 3 2" holes near the ground. The troughs might also collect rainwater, if any. The greenhouse might have a gravel bed over an EPDM rubber layer to make an artificial wetland.

Metcalf and Eddy say an average family produces 400 gallons per day of wastewater with a 200 mg/L BOD concentration, ie about 0.7 lb/day, and it needs about 175 ft^2 of surface for treatment. Sherwood Reed says blackwater contains more than 75% of the nitrogen in only 40% of the flow. Eliminate that with a composting toilet or solar poop cooker, and we might only treat

240 gallons per day (0.908 m^3/day) with 0.175 lb of BOD, ie 86 mg/L.

Can we purify this to 10 mg/L (a tertiary sewage treatment standard) and recycle it for non-potable uses? PE Sherwood Reed's Natural Systems for Waste Management and Treatment book (McGraw Hill, 2nd edition, 1995) has a detailed design procedure for constructed wetlands, aka "attached- growth biological reactors." They work well at higher temperatures.

Equation 6.38 on page 226 of Reed's book says the rate constant for BOD removal in subsurface-flow wetlands at 20 C is K20 = 1.104d^-1. Equation 6.34 makes K27 = 1.104x1.06^(27-20) = 1.66d^-1 at 27 C. We need to reduce the BOD from Co = 86 to Ce = 10 mg/L with 256 ft^2 (23.8 m^2) of surface. If the bed is "medium gravel" (D10 = 32 mm) with a 0.38 porosity, it needs to be 0.908ln(Co/Ce)/(K27x23.8x0.38) = 0.13 meters deep, ie 5.1 inches. We might add gravel or divide the bed into 3 series cells for better plug flow and performance.

What will grow in this rainforest? Orchids? Mangos? Bananas? :-)

If we must use an air conditioner, why not mist the air around the hot coils or trickle rainwater over them? D. Y. Goswami at U. Florida measured a 20% gain in AC efficiency as a result of misting the air around the hot coil. I've measured 20% by trickling rainwater over a window unit.

A more sophisticated cooling system with no water consumption or outdoor air exchange might have a multi-effect LiCl solar still on the roof that absorbs water vapor at night from the pond below and distills water out of the LiCl solution during the day. This might also store cloudy day heat, with a wet basement floor to evaporate water in wintertime, and a LiCl pond on the first floor that acts as a "chemical heat pump."

Nick

Answers to exercises

0.1. If the current Gulf war costs $600 billion per year, and we consume 20millionx42gx365d = 307 billion gallons per year, a gallon of oil costs 800billion/307billion = $1.96 plus the pump price. I've just heard 38,000 people per year die prematurely because of pollution from oil burning. At $1 million per death, that would add $38 billion, ie another 12 cents... What's the cost of acid rain? Drew says low pump prices that only reflect part of the real costs send the wrong market message to consumers, who would conserve more energy if they paid the whole cost at the pump.

0.2. c) $20 worth of caulk might halve the air leaks and the heating bill.

1.1. C = 1/2 pound x 1 Btu/lb-F = 0.5Btu/F and dT = 200F-50F, so we need CdT = 75 Btu, if it's done quickly, so most of the heat stays in the water and little heat flows into the room.

1.2. 75x60m/h/(300x3.41) = 4.4 minutes.

2.1. 12V/24A = 0.5 ohms.

3.1. R = T/I = 12F/6Btu/h = 2 F-h/Btu (fhubs.)

3.2. 8'x16'/R10 = 12.8 Btu/h-F (buhfs.)

3.3. 20Cx2mx4mxU0.5 = 80 watts.

3.4. It would pass (70F-30F)3'x4'x0.25Btu/h-F-ft^2 = 120 Btu/h of heat. An R8 Hurd Heat Mirror window would only pass 60 Btu/h-ft^2. C-Sash makes low-e storm windows...

3.5. 40Fx24'x32'/R64 = 480 Btu/h.

4.1. 350 Btu/h = (70F-30F)6L^2/R10 makes L = sqrt(350/24) = 3.82 feet, max.

4.2. With 1 dog, L = sqrt(450/12) = 6.1'. With 2, L = sqrt(550/12) = 6.8' ...3 make L = 7.4'. With R50 strawbales, 650 Btu/h = (70-30)6L^2/R50 makes L = 11.6 feet. An LxLx8' tall "cube" has 650 = (70-30)(2L^2+4x8L)/R50, so L^2+16L-406.25 = 0, and L = (-16+sqrt(16^2-4(-406.25))/2 = 13.7'.

4.3. L = 24/13.2 = 1.8'

4.4. 774L^3/8/1024 = (70-30)6L^2/R20 makes L = 127' min, eg a 128'

16-story cube, something like the Borg, or a large anthill.

5.1 If 0.8x1000x4ft^2/24h = 133 = (T-30F)(4/2+(6x8'x8'-4)/R20) = 12(T-30), T = 30 + 133/12 = 41 F.

5.2 If 0.8x470 = 376 Btu/ft^2-day of solar energy comes in the window and 376A/24h = 15.7A = (70-40.2)(A/2+(6x64-A)/R20) = 13.4A + 572 Btu/h leaves, A = 249 ft^2, which is larger than the 64 ft^2 south wall of the cube, so we can't design a direct gain 100%-solar-heated house in Portland this way, without more insulation or mice or people or indoor electrical energy use.

6.1 T(1) = 30+(75-30)e^(-1) = 46.6 F.

6.2. C = 6x8x8/2 = 192 Btu/F. G = 6x8x8/20 = 19.2 Btu/h-F. RC = C/G = 10 hours. T(9) = 30+(70-30)e^(-9/10) = 46.3 F. Adding 1000x5 Btu/F makes C = 5019 and C/G = 261 h and T(9) = 30+(70-30)e^(-9/261) = 68.6 F, which is good for direct gain, but bad for night setback savings.

6.3. RC = 8x8x8x64/19.2 = 1707 hours makes T(9) = 69.8 F. T(5 days) = 67.3, and after 2 weeks, T = 67.0. This is a good direct gain house, for a fish.

7.1. On a cloudy day, with no sunspace airflow, the R20 cube in section 5 with an R20 wall between the cube and the sunspace would have a conductance of 6x64/R20 = 19.2 vs 34 Btu/h-F, with RC = 4.4N/19.2/24 = 0.00955N days. Starting at 75 F, T(d) = 65 = 30+(75-30)e^(-5/0.00955N) 5 days later, so

-523.6/N = ln((65-30)/(75-30)), and N = 2083.

8.1. The 0.08x2400 = 192 ft^2 of windows have 192ft^2/R4 = 48 Btu/h-F. The 2(30+40)8-192 = 928 ft^2 of walls add 928/30 = 31, the ceiling adds 2400/60 = 40, and 0.5x2400x8/60 = 160 cfm, or about 160 Btu/h-F, over 10X the 15 cfm/occupant ASHRAE ventilation standard. The total is 279, of which 57% comes from air leaks and 17% comes from windows. Why do houses need windows?

9.1. Ignoring sunspace vent and ceiling resistances, an 8' R32 cube with a ceiling temp T (F) and 8'x8' of R2 80% sunspace glazing might collect 0.8x470x64 = 24064 Btu and lose about 6h(T-40.2)64ft^2/R2 from the glazing during the day + 18h(65-40.2)64ft^2/R32 from glazing at night

  • 24h(T-40.2)64ft^2/R32 from the ceiling + 24h(65-40.2)3x64ft^2/R32 from the other walls at night, which makes T = 121.9 F, if gain equals loss on an average-day.

If the ceiling conductance to slow-moving air below is 1.5x64 = 96 Btu/h-F, and the cube needs (70-40.2)4x64/32 = 238 Btu/h of heat (less than 1 human), the min ceiling temp is 70+238/96 = 72.5 F. The average is (121.9+72.5)/2 = 97.2. If the cube loses 5x24h((65-40.2)4x64ft^2/R32+(97.2-40.2)64ft^2/R32) = 37.49K Btu over 5 days, we need 37.49K/(121.9-72.5) = 758.9 Btu/F, eg

758.9/64 = 11.9 pounds (2.22") of water per square foot of ceiling.

Or less, using the differential equation. At ceiling temperature T, I = (T-40.2)64ft^2/R32 + (65-40.2)4x64ft^2/R32 = 2T + 118 Btu/h flows from C, so dT/dt = -I/C = -2T/C - 118/C, and -118/2 = -59. After 5 days, T(t) = 72.5 = -59 + (121.9+59)e^(-2x120/C), so C = 240/ln(131.5/180.9) = 752 Btu/F, ie 752/64/64x12 = 2.20" of water.

Exercise 9.2. I = (T-40.2)64ft^2/R32 + 8/24(65-40.2)4x64ft^2/R32 = 2T-14.3 Btu/h flows from C, so dT/dt = -I/C = -2T/C + 14.3/C, and 14.3/2 = 7.1. T(t) = 72.5 = 7.1 + (121.9-7.1)e^(-2x120/C) makes C = 240/ln(65.4/114.8) = 427 Btu/F, ie 427/64/64x12 = 1.25" of water.

10.1. 300 Btu/h = 0.1714x10^-8x0.2(T^4-(460+20)^4) makes T = 982 R or 522 F.

13.1. I = (70-30)200 = 8K Btu/h. T = 70 + I/(2400x1.5) = 72.2 F.

Reply to
nicksanspam
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Kachadorian's air-core slab consists of hollow concrete blocks forming ducts under the floor. Kachadorian claims that air in these ducts would be warmed by the heat stored in the thermals mass of the hollow concrete blocks, create air movemnet and allow warmed air to float into the room above via duct openings in the floor.

I have always wondered would this design be improved if hydronic pipes were above the blocks in a cement screed. Any heat lost beneath the heated screed would heat the blocks and hence air in the ducts underneath. This must create air flow out of the ducts into the room above. Sort of re-claiming heat that may be lost.

Reply to
News

News wrote: ...

I'm not familiar with the details of Kachadorian's design but it seems to me that a good bit of insulation under the floor, hydronic pipes on top of that, and flooring on top of the pipes, would be the most sensible arrangement. Why let heat get lost beneath anything? If the flooring is more or less directly on top of the pipes then the floor itself would heat the air and create air flow directly in the room. Cleaning a floor certainly would be easier than cleaning air ducts in blocks under it.

Anthony

Reply to
Anthony Matonak

Problem is that the floor isnt a very good source of lower grade heat. Its got its real downsides even with electrical heating embedded in the concrete floor.

Even without carpet, its not ideal and worse with carpet.

And the problem is that how warm the room feels is mostly due to the surface temperature of the walls and the air temp, and its non trivial to get those up to reasonable levels with any form of heated floor. Even with an electrically heated floor, you can get a situation where the floor is hotter than is comfortable while the room still feels too cold.

Corse another way of looking at it is to not attempt to do all the heating using solar, but to use that to minimise the heating fuel costs.

Still got some real downsides with just a uniform floor surface that the solar heat is being supplied thru tho.

Reply to
Rod Speed

Wrong again Rod. A hydronic floor with lots of surface and a low water-air thermal resistance in an airtight house with lots of insulation works fine. Ignoring the R1 radiation conductance, the min floor temp required to keep a house with a 200 Btu/h-F conductance and a 2400 ft^2 floor 70 F on a 30 F day with a U1.5 slow-moving airfilm conductance is 70+I/(2400x1.5) = 72.2 F.

Nick

Reply to
nicksanspam

Ken how do you insulate under the floor piping-Radiant floor heating, here we use foamboard. What R value are your 4 pane windows, are they sold in the US What R value do you use in walls and ceilings

Reply to
m Ransley

Best ignore him.

Reply to
News

Best ignore him.

Reply to
News

Best ignore him.

Reply to
News

Best ignore him.

Reply to
News

Best ignore him.

Reply to
News

Mr Troll, you will find we are not all disciples of Nick. Do a Google and see the conflicts. ..err is it worth saying that to him? Probably not.

Reply to
News

I'll take a dump on him instead, thanks.

Reply to
Rod Speed

Master gutless f****it pom, I never said you were.

Not interested in gutless f****it pom shit thanks.

Reply to
Rod Speed

I know of a few installations of PVs where the underside of the PVs is ducting with air florced through. A heat pump collects the heat from the panels and house exhaust air. It wasn't that brilliant. I think it could have been better thought out.

How much heat do PV cells emit on the back and front of the cells?

Reply to
News

News wrote: ...

You could think of it this way. Typical PV panels are around 12% efficient at converting sunlight into electricity. This means that the other 88% is converted into heat. Clearly they are much better at making heat than electricity.

Anthony

Reply to
Anthony Matonak

I thought around 30% was reflected? This would leave about 50% as heat.

Reply to
Fred B. McGalliard

Nope, that would only be true if they were a perfect black body absorber of all energy that falls on them.

In practice quite a bit is just reflected off them.

But then so is say black roofing material.

Reply to
Rod Speed

Well, maybe 16 and 84%, eg 0.84x800 = 672 W/m^2 in AM2 sun.

Buth they are better than that. Rich Komp says the silicon is almost transparent to IR, and the aluminum contact beneath is shiny, so PVs are a selective surface.

Nick

Reply to
nicksanspam

I think that has a lot to do with whether there is an antireflective coating on the surface or not. Silicon is very shiny in the optical, (and I think the reflectivity is a function of doping), where most of the energy comes in, so, especially if the sun is at a substantial angle to the array, a lot of the energy is reflected from the top surface. What gets in and is not absorbed or reflected from the top surface metalization (around 10-20% I think for typical cells) passes through the active cell and what isn't absorbed there then passes through the rest of the bulk silicon used as a handle. The back side metalization would be under all this heavily doped handle. Very thin cells would be very different from the currently more common thick cells. I am not sure about the ones that are epi deposited on material other than silicon. If they are thrown down on a metal contact they are probably very reflective.

Reply to
Fred B. McGalliard

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