Subject: Re: Solar Heating, Empirical Measu(r)ements & Experience

From: [email-masked] (Nick Pine)
Date: 15 May 1996 13:01:12 -0400

William R Stewart  <[email-masked]> wrote:

>I would recommend talking to a number of different energy consultants.

Me too. I usually start by asking "Are you familiar with Ohm's law?"
That filters out more than half of the "energy consultants" I've met,
but asking this question is sometimes embarrassing all around...

Here's another litmus test for an energy expert: what will the average water
temperature be inside a 4' cube full of water surrounded by 5 R20 foam walls,
sitting outside in December, when the air has an average 24 hour temperature
of 36 F and the sun puts 1,000 Btu/ft^2/day of heat into the R1 glazed side
of the box? And how will that change over time if we shade the sunny wall?
Anyone care to answer that question? I'll offer a $10 reward to the first
person who answers it correctly.

>Nick does have a lot of theories, though he considers many of his untried
>pet theories to be far superior to anything anyone else has ever conceived.

It's hard to know how to dissect that sentence :-) It looks like an insult.
And untrue. Most of this stuff was conceived a long, long, long time ago.
New weather data, new arrangements of systems and new materials and controls
make it ever more practical and cost-effective and efficient now, but that
isn't the main problem...

Yes, I'm smarter than most people ("References please"--I belong to Mensa)
and I know a lot more about physics, and I've thought a lot more and in more
depth about solar heating and have more engineering degrees than many house
designers, backyard crackpots and armchair a.e.r posters who like to talk
about alt.energy but haven't done any at all, or PV hobbyists who have
learned how to screw a system together but don't know Ohm's law either, and
I do think _some_ of "my solar theories" are the cat's pajamas, especially
the ones that seem to make sense with my simple understanding of physics,
and the ones that really do work... the ones we've been trying out now in a
2' x 4' x 8' tall structure with an electronic data logger and modem inside,
in the real Pennsylvania winter outdoors on the roof of the physics building
at Ursinus college in Collegeville, PA since November 4... and physics prof
Paul Bashus and I have managed to hornswoggle our solar theories on paper
past the technical review committees of two international conferences in
the last year, as well as a number of PhDs, who have accepted our solar
closet paper for presentation at the Unesco-sponsored World Renewable Energy
Congress in Denver from June 16-21 ([email-masked] registration
fee increases from $340-740 after today, May 15), whose steering committee
alone has people from 130 countries, and I've talked and run sessions on solar
heating and cooling techniques at several US conferences, but. .  .
serious scientists and engineers consider solar closets and sunspaces
"trivial physics" and "unoriginal research."

Many scientists are out collecting funding to improve the solar collection
efficiency of some system from 16 to 17% by adding another $10/ft^2 to the
cost of something that already costs $30/ft^2. This kind of solar house
heating technology has been around since Edward Morse invented the "Trombe
Wall" in 1881, at least, and it's understood by many people who know what
they are talking about, and a lot more who don't. Solar house heating just
hasn't been done well yet, outside of the Southwest, on an significant scale,
partly because the field is peopled by ignorant hucksters, partly because
we are still waiting for the government to help make that happen... But
this is lumberyard and hardware store science, not stuff for theses and
Nobel prizes and NREL grants. It's waiting for bold entrepreneurs. And
more expensive energy prices, or people who care more about the environment,
or government regulations that make not caring more expensive...

Many heating and cooling engineers are working on heating and cooling systems
for skyscrapers, or heat pumps, where the money is today.

I've only been doing this full-time for 2 years now, after 25 years of
electrical engineering, and I'm still learning. Some things seem like good
ideas on paper but really do have to be tried out, and improved somehow for
some applications, or proved to be impractical for others, like bubblewalls.

Some things don't need to be tested much, like low-thermal-mass sunspaces,
and some things do, like bubblewalls. It's important to know the difference.

Bubblewalls are the subject of an old Swedish patent (I wish I had a copy.)
The original application was storefront windows. They have been tried in
this country over the years by people like John Groh, a New York grower (?)
and Dr. Merrill Jenson at U Arizona and Dr. Otho Wells at U New Hampshire,
who concluded in 1977 that bubblewalls were not practical night insulation
for commercial plastic film greenhouses simply because those greenhouses
need to be put together so roughly and quickly in the field by unskilled
people (so as not to raise their basic cost of 50 cents per square foot :-)
that leaks in such a system could not be avoided. Everyday leaks in plumbing,
plastic film connections, etc. These are real problems, and I don't mean to
minimize them, but they are also application-specific, and not hard to solve.

Dr. Wells says that windows in solar houses may be a different situation,
since they can be made more carefully... He also says he only got a US
R-value of 1.5 or so out of his 3"-6" thick poly film pillows, so it seems
we need to improve the bubbles themselves somehow, make them smaller and
longer lasting, with thicker walls perhaps, or go work on something else.
Dr. Aristid V. Gross of Temple U and the blind Belgian physicist Joseph
Plateau (1801-1883) both made bubbles that lasted over a year...

Dr. Wells was using an open system, more dustprone than a closed one, making
bubbles at the top of a 25' x 196' double poly film greenhouse to fill the
double poly film walls all round. He put slits in the plastic film, some
with sewn-in zippers (he said sewing zippers into poly film isn't easy),
to let some air out at the endwalls. He found he had to make more bubbles
every 2-4 hours, to keep the walls full of bubbles. Dr. Jenson experimented
with a photosensor to do this automatically, and also tried adding dyes to
the bubble solution, which were diluted so much in bubble from that they
had little effect. Another way to keep the glazing cavities full is to keep
a layer of bubbles moving slowly from bottom to top in a continuous fashion.
Another is to use temperature sensors to measure the R-value. Dr. Wells
said the small bubbles tended to settle out quickly and become liquid
again at the bottom...

Another problem: Dr. Wells mentions that the bubble distribution was not
uniform--big bubbles and open spots, thermal shunts with no bubbles, up to
1-2' in diameter would often develop. This might happen less if bubbles
move continuously up from the bottom instead of intermittently down from
the top of the cavity. Dr. Wells used a basic solution of 3% type E dust
reduction foam (a better choice might have been firefighting foam) from Mine
Safety Supply in Pittsburgh (now Mine Safety Appliances at (412) 776-7700?)
with 1% propylene glycol as antifreeze, because when the bubbles freeze
inside the outside layer of poly film, they break, and the R-value goes
down and their friction against passing bubbles goes up, and he didn't want
water frozen in pipes either. Can we make frozen bubbles that don't break?

Yes. That's been done, often, in public at the San Francisco Exploratorium
by Dr. Ilan Chabay, called "one of the world's foremost authorities on
frozen bubbles," in John Cassidy's _Unbelievable Bubble Book_ for children
(Klutz Press, 1987.) Has Dr. Wells ever talked with Dr. Chabray or Dr. Grosse?
I don't think any physicist would call bubbles trivial physics. Child's play
maybe, but not trivial :-) We need more child-like physicists.

There is at lot of work to do in this alt.energy field, and a lot of it,
like preventing leaks or finding the right soapy solution is not that
technically difficult. There isn't a lot of funding, but there's a lot of
work to do, a lot of opportunity for serious backyard builders and high
school science fair students to do interesting things that may later become
good, cost-effective, reliable, simple socially-useful techniques and
products, given some common sense and education and patience and a lack
of greed and shortsightedness.

Some things need to be tested and developed a lot, others are no-brainers,
if we start thinking from scratch. Not everyone wants to live in an
experimental bubblewall house, but lots of people like sunspaces. There's
a huge gap between commercial plastic film greenhouses costing for 50 cents
per square foot and residential sunspaces costing $50-150 per square foot.
Who will fill it?  NREL?  No...

There's another gap between serious scientists who say all this is trivial
well-understood physics, while they collect their salaries working on more
and more exotic research, and alt.energy.renewable experts who say it won't
work, it's been tried before, Trombe walls are the cat's pajamas, etc, etc.
Bullshit!

>I would have to say that distributing the energy evenly to where it is
>desired is yet the most important half.  Many houses have been built
>where some rooms overheated while others grew cold.  Distributing the
>heat to zones at specific times during the day is an interesting challenge.

Sure, that's important, but let's collect the heat to begin with, Will, and
then worry about that problem. Or wear a sweater in one room and take it off
in another. That's HVAC. Air pushers, ducts and fans and blowers and zone
controls, to move warm air out of our sunspace or solar closet around the
house. Not too hard in a new house, and harder in a retrofit, unless we can
mix the warm air into a big room or dump it into a return grate somewhere,
somehow. That's a lot better understood than solar heating. Straightforward
everyday cranking, for someone who knows how to do it.

Nick

It's a snap to save energy in this country. 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.

					  Tom Smith

---

From: William R Stewart <[email-masked]>
Date: Thu, 16 May 1996 00:29:41 -0400

> but asking this question is sometimes embarrassing all around...

Because that is an electrical term, as opposed to a thermodynamic term.
Appropriate if you are talking to a PV consultant, but confusing otherwise.
You are simply trying to make other people translate your way of thinking as
an EE.  Why not speak in thermodynamic terms, like the rest of the industry?

> person who answers it correctly.

You don't give enough data in order to answer your question, and your question
is not clearly stated.  However, for the sake of fun, I'll make some assumptions.

Assumptions;

1. The water starts out at 36oF.

2. Thermal conduction is the only heat transfer mechanism, aside from the energy
provided by the sun.

3. There are no losses incurred by the R1 glazing on the 1000 Btu/ft^2/day.

4. There are no data points for how much energy is provided on an hour-by-hour
basis, so I will assume an even amount of sun over a 10 hour period.

 ===========================

Weight of water:
4x4x4 = 64ft^3 of water

64 x 62.4 lb/ft^3 =  3993.6 lbs of water

Let's just call it 4000 lbs.

---------------------------

Thermal resistance of cube:
4x4 = 16 ft^2/side  value of R1 area

16x5 = 80 ft^2 of R20 surface

R1 total = 1/16 = 0.0625

R20 total  = 1/4 = 0.25

R total = 0.0625 x 0.25/(0.0625 + 0.25) = 0.05

U = 1/0.05 =20 Btu/hr-oF

--------------------------------

Thermal energy input

1000 Btus/day-ft^2 * 16 ft^2 / 10 hrs/day input
 = 1600 Btus/hr while the sun is shining.

------------------------------

The water in the cube would rise to a steady state temperature of
36 +80 = 116 oF if the energy input were continuous.
When shaded, the water in the cube would fall over time to 36oF.

A complete answer with specific times would require calculus, if specific
times was what you were looking for.  Approximations could be made, but
the answer would not be 'correct'.  Let's just approximate the time it takes
the water to rise 1 oF without considering the negligible loss through the
cube walls.

4000 lbs of water would require 2.5 hours of 1600 btu/hr energy input, again
without considering the loss through the cube walls.

Now, if you;
  -moved the water inside to the interior sunspace of a 68oF house,

  -made the glazing 7'x10 (4 in the entire house on the south side)

  -used a flatter 4"x4'x10' water wall (4 in the entire house), 4" away
   from the glazing inside the house,

  -used an R4 glazing when the sun was shining,
   and an extra R8 window cover when the sun wasn't shining,

then you would have a direct, passive solar design that retained
the sun's heat energy in the building interior and released it
slowly into the interior at night

Care to try the math?

assume another 60 ft^2 of window at R4 during the day, adding
   another R8 at the other 14 hours.
Assume R24 walls and R38 ceiling.
Assume a two story, 24'x 48' floorspace house with 8' ceilings.
Assume an outside temperature of 36 oF
Assume 0.25 air changes per hour (optional).
Assume 100 Btu/hr/ft^2 insolation over 10 hours every day.
Make an assumption about the thermal storage of the rest of the interior
  items/material.

> in another.

I try to look at the whole picture.  A solar closet might gather the energy,
but distributing it is another matter altogether.

>That's HVAC. Air pushers, ducts and fans and blowers and zone
> house.

You now no longer have a passive system, but now require temperature
regulation, ducting, electrical loads, etc.

Some people prefer the passive method, and it is not your perogative
to tell them that they are wrong in preferring that.

> Not too hard in a new house, and harder in a retrofit, unless we can

I can't wait until you have to balance your first air-handling system.  Keep
at it!

Cheers,

Will Stewart

---

From: [email-masked] (Nick Pine)
Date: 16 May 1996 04:52:30 -0400

David Hoadley <[email-masked]> wrote:
>[email-masked] (Nick Pine) writes:

Still no numerical answers, 16 hours after this posting. Perhaps $10 is
insufficient bait to catch an energy consultant, or they are out of season? 

Or perhaps it should be rephrased in international units:
 
Here's another litmus test for an energy expert: what will the average water
temperature be inside a 1 m cube full of water surrounded by 5 R5 foam walls,
sitting outside in December, when the air has an average 24 hour temperature
of 2 C and the sun puts 3 kWh/m^2 per day of heat into the R0.175 glazed side
of the box? And how will that change over time if we shade the sunny wall?
Anyone care to answer that question? I'll offer a $10 reward to the first
person who answers it correctly.

>This brings me to another point (thus the change of topic title). When I
>first started reading these groups I was rather taken aback by the high
>R-values people mention. Here in Australia, they are generally in the
>range R-1.5 to R-3 or 4 for (say) fibreglass insulation.

R30 house walls are pretty good walls in the US. Perhaps a good new
Australian or French or British house has R5 walls, made with
straw bales and mortar, 40 cm thick? A 1 bale wall?

>The units are, naturally, degC.m^2/W.

Of course Mother Nature thinks in meters and C, not feet and F :-) The
natural thing about a Btu (the amount of energy in a kitchen match) is that
it takes eggsactly 1 to raise 1 pound of water or 55 ft^3 of air 1 degree F.
It takes about 100 Btus to make a cup of coffee. Chickens make about
5 Btu/hr/lb of sensible heat. A typical 5.41 lb bunny makes 40. 1 cfm of
air flowing with a temperature difference of 1 F moves about 1 Btu/hour,
a single pane window is about R1, and 2 layers are R2. A pound of water
takes 144 (1 gross) Btus to freeze at 32 F, which is close to the average
air temperature in December where I live, and it takes about 1000 Btu to
boil away a pound of water at 212 F. A south wall in December receives
about 1,000 Btu/ft^2/day of sun where I live, 300 Btu/hour/ft^2 peak. The
thermal conductance of an airfilm with air moving at V mph is about
2 + V/2 Btu/hr-ft^2-F for rough surfaces. A black body at T degrees F
radiates 0.174x 10^-8(460+T)^4 Btu/hr-ft^2. US hardware stores sell rolls
of 6" fiberglass insulation stamped R19 in big letters (R10 with 2%
moisture, R0 in a good wind) for 25 cents/ft^2, and R10 foamboard 2"
thick costs about 50 cents/ft^2.   

What is natural in metric? Water freezes at 0 C and boils at 100 C, and
it takes about 1 kWh to heat 1 m^3 of water or 3,000 m^3 of air 1 C. A cup
of coffee takes about 30 Watt-hours. People make about 100 Watts, rabbits
5 W/kg. 1 m^3/sec of air flowing with a temperature difference of 1 C
moves about 1 kW of heat. It's a little harder to remember that 1 layer
of glass with an airspace has metric R0.175, and 2 layers have R0.35.
It takes about 100 kWh to freeze a cubic meter of water and 663 kWh to
evaporate one. The ratio of evaporative to convective power loss at a
wet surface with temperature Tp in Ta air is about (Tp-Ta)/(Pwp-Pa)/2,
independent of windspeed, where Pwp and Pa are vapor pressures in mm Hg.
A south facing wall in December receives about 3 kWh/m^2/day of sun where
I live, with a peak of 1 kW/m^2, and the thermal conductance of an airfilm 
with air moving at V m/s is about 10 + 2V W/m^2-C for rough surfaces, eg 
100 W/m^2 in a 5 m/s wind for a 25 C surface on a 20 C night. How much
more if the surface is wet, and the air has 50% RH? How much more if the
sky is clear? Do French hardware stores sell R2 foamboard 5 cm thick for
about about 30 FF/m^2? Why does the Lord permit such Suffering on Earth,
if He is Omnipotent?

>I was surprised to find that American ones are expressed in British Thermal
>Units (which I had thought went out with the ark)...

Now that Britain and Australia use Watts and meters, we are thinking of
calling these American thermal units, or perhaps Bunny thermal units.

>So I tried to work out a conversion factor. Can someone confirm it for me? 
>
>  Is it true that 1 watt = 3.412 BTU/hr?  If so, have I got it right that
>  R-1 (International) degC.m^2/W = R-5.678 (US) degF.hr.ft^2/BTU?

I think so.

Or would those be degrees K, in that particular alphabetical procession? 

Nick

   When we play tennis or walk downstairs we are actually solving whole
   pages of differential equations, quickly, easily and without thinking
   about it, using the analogue computer which we keep in our minds. What
   we find difficult about mathematics is the formal, symbolic presentation
   of the subject by pedagogues with a taste for dogma, sadism and
   incomprehensible squiggles.

          From _Structures: Why Things Don't Fall Down_, by J. E. Gordon

---

From: [email-masked] (David Hoadley)
Date: 16 May 1996 01:53:54 GMT

This brings me to another point (thus the change of topic title). When I
first started reading these groups I was rather taken aback by the high
R-values people mention. Here in Australia, they are generally in the
range R-1.5 to R-3 or 4 for (say) fibreglass insulation. The units are,
naturally, degC.m^2/W. I was surprised to find that American ones are
expressed in British Thermal Units (which I had thought went out with the 
ark), feet, and fahrenheit, but at least it explained the difference. So I
tried to work out a conversion factor. Can someone confirm it for me? i.e.

Thanks,
David.

--
----------------------------------------------------------------------------- 
David Hoadley                        Internet: [email-masked]
Electrical Engineering, RMIT
Melbourne, Australia                Ph: +61 3 9660-4847, Fax: +61 3 9660-2007

---

From: [email-masked] (Nick Pine)
Date: 16 May 1996 10:34:14 -0400

William R Stewart  <[email-masked]> wrote:
>> ...I usually start by asking "Are you familiar with Ohm's law?"

Many people learn Ohm's law in high school, Will...
I think an energy expert should be familiar with Ohm's law.
To far too many people these days, "energy" = "electrical energy."

>Appropriate if you are talking to a PV consultant, but confusing otherwise.  

Yes, Ohm's law is confusing to certain "energy consultants" :-) 

>an EE.  Why not speak in thermodynamic terms, like the rest of the industry?

Perhaps we should ask 'em about enthalpy? :-)

>is not clearly stated.

I think there is too much data above, and the question is reasonably clear. 

>However, for the sake of fun, I'll make some assumptions.

Good :-) This is the first numerical answer I've seen...

>2. Thermal conduction is the only heat transfer mechanism, aside from
>the energy provided by the sun.

Good. Although R-values include convection and radiation.

>3. There are no losses incurred by the R1 glazing on the 1000 Btu/ft^2/day.

The glazing is perfectly transparent? Fine.

>basis, so I will assume an even amount of sun over a 10 hour period.

OK. (I don't think that matters here, but...)

>Let's just call it 4000 lbs.

OK. 

>R total = 0.0625 x 0.25/(0.0625 + 0.25) = 0.05

Excellent! 

> = 1600 Btus/hr while the sun is shining.

Um, OK...

Perhaps, but recall the problem:

>>the sun puts 1,000 Btu/ft^2/day of heat into the R1 glazed side of the box.

>>And how will that change over time if we shade the sunny wall?

>When shaded, the water in the cube would fall over time to 36oF.

Well, yes...

>times was what you were looking for.

Yes, that was what I was looking for. More than the obvious... Finding the
form of the answer requires calculus, but Newton did that once, a long long
time ago. Finding the answer just requires plugging some numbers into a
simple formula, if you know the formula.

>Approximations could be made, but the answer would not be 'correct'. 

Approximations using arithmetic would be fine... That's easy to do.

>Let's just approximate the time it takes the water to rise 1 oF without
>considering the negligible loss through the cube walls.

OK (?)

>without considering the loss through the cube walls.

True, but why are you talking about heating rather than cooling?  

		    *     *     *

Sorry. No $10. Would you like to try again?

                    *     *     *

>  -made the glazing 7'x10 (4 in the entire house on the south side)

    ie 280 ft^2 of south glazing?

>   from the glazing inside the house, 

    ie about 400 pounds of water? 4" away from the glazing? Transparent
    water? Hmmm :-) I guess you won't be able to see very clearly out of
    those windows, but they will let in some light...
 
>  -used an R4 glazing when the sun was shining, 

With how much solar transmittance, at what cost, and what happens when the
argon leaks out? Some of these $40/ft^2 high-R windows have low solar
transmittance, on the order of 50%. Hmmm, 280 ft^2 x $40/ft^2 = $11,200.

>   and an extra R8 window cover when the sun wasn't shining, 

How would you do that? Movable insulation, at $10/ft^2, installed? Will you
move it twice a day religiously? Will it leak any air around the edges?

>then you would have a direct, passive solar design that retained the sun's
>heat energy in the building interior and released it slowly into the
>interior at night

Right. Another "direct loss" house :-)

>Care to try the math?

Sure. I like this arithmetic. Let me get a cup of coffee... Now I'm back,
200 Btu later. I've had this espresso machine for about 3 months now. I
guess if some people like me find it fun to wiggle the valves on an espresso
machine with all the steam and noise and watch the milk temp rise with a
thermometer twice a day, others might find it fun to move window insulation
all over a house twice a day, for three months... Or, maybe you only move
the window insulation on a cloudy winter day--but no, you want to do it at
night too... or you might leave some windows covered all winter, like Pat
Hennin or Malcolm Wells, in semi-hibernation, huddled in cold dark rooms. 

>   another R8 at the other 14 hours.

OK. That's a long winter day...

>Assume R24 walls and R38 ceiling.  

OK. Let's assume that includes the 2x6's, etc, that act as thermal shunts,
ie the equivalent thermal resistance of the entire wall is R24. This is not
just a wall with layers of insulation with R-values that add up to R24
(which might have a much lower R-value), and the insulation is installed
properly with no gaps, etc. Are you sure about all that?

>Assume a two story, 24'x 48' floorspace house with 8' ceilings.

OK. I guess you mean the footprint on the ground is 24 x 48?
So the house volume is 24 x 48 x 16 = 18432 ft^3.

>Assume an outside temperature of 36 oF

OK. (The average will be warmer during daylight, which is good.)

>Assume 0.25 air changes per hour (optional).

OK. That's 0.25 x 18432 ft^3/hr = 4608 ft^3/hr or 77 cfm :-) I'll assume
(I almost said "guess") this is leakage thru walls, etc, without an air-air
heat exchanger... Will the finished house have a blower door testing spec?

May we also assume you use a frugal 500 kWh/month of electricity, ie an
average of about 700 Watts? (Steve Baer only uses 80 kWh/mo :-) This is
equivalent to 700 x 3.41 = 2387 Btu/hr. Let's add the heat from two
300 Btu/hr people inside the house for 16 hours a day, and one 150 Btu/hr
dog and a 50 Btu/hour cat, full-time. Total internal energy generation:
2387(24)+600(16)+200(24) = 72K/day.

>Assume 100 Btu/hr/ft^2 insolation over 10 hours every day.

OK.

>Make an assumption about the thermal storage of the rest of the interior

How about 1 lb of water/ft^2 of walls and ceiling? (1/2" drywall has the 
equivalent of 1/2 lb of water/ft^2, so you may need some concrete furniture..)
I'll make some other assumptions, too.

I guess we'd want to know the thermal resistance of this house for starters.
So let's add up the thermal conductances U = Sum(Ai/Ri) and find
the reciprocal 1/U... 

We have 340 ft^2 of windows, R4 during a 10 hour day (Uday = 340/4 = 85)
and R8 during a 14 hour night (Unight = 340/8 = 43), with an average daily
Uwindow = (10xUday+14xUnight)/24 = 60.

Uwalls = (2(24+48)x16-340)/R24 = 82, and Uceiling = 24x48/R38 = 30.

And Uinf = 77 Btu/hr-F.

So U = 60+30+77 = 167 Btu/hr-F and R = 0.006, over 24 hours.

At night the U value would be lower, after you devotedly travel around
the house and painstakingly put up your night insulation on every window,
making sure the edges are ever so carefully sealed, akin to some little
twice-daily religious experience... Unight = 43+30+77 = 150 Btu/hr-F.

What will the steady state energy flow be for this house after a long
string of average December days, with an average amount of sun?

The energy Ein that flows into this house in a day might be 

  280 ft^2 windows x 50% transmission x 1000 Btu/ft^2/day = 140K 
                            +  internal energy generation =  72K
	                                        total Ein = 212K.

And the energy Eout that flows out of the house might be 

  24 hours x (68-36) x 167 = 128 K Btu/day.

So, after a long string of perfectly average December days, Ein-Eout
= 84K Btu/day, if you keep the house at an average of 68 F during the
day... This might make a fine New Mexico house.

If you let the house temperature float over this time, you might have
an average house temp T such that 24 (T-36) 167 = 212K, or T = 89 F.
Colder at night, warmer during the day. Very toasty.

Now, thermal mass: the house has about 3500 ft^2 of walls and ceilings with
a thermal mass equivalent to 3,500 pounds of water and another 400 pounds
of actual water. Let's call this 4,000 Btu/hr-F. So if you make the temp of
the house, say, 80 F at dusk, at the end of an average day, with some sun,
after a long string of December days, each with 1000 Btu/ft^2/day of sun,
during the night the house will lose approximately 14 hours (80-36) 150 =
100K Btu, which might come from the 4,000 Btu/F of thermal mass cooling plus
14/24 hours x 72K = 42K of internal energy generation, ie the temperature of
the house might drop to 80 - (100K-42K)/4K = 65 F at dawn. 

So yes, this 56%-electrically-heated house with all of the movable window
insulation looks marginally comfortable after a long string of average winter 
December days, each with an average amount of sun. But what do you do if the
next day is cloudy, or the day after that, or during a cloudy week in January,
when the temperature outside is -10 F? You will have to use backup heat...
Or, gasp, wear a sweater. With no sun and no backup heat, and with all the
windows boarded up on the inside, 24 hours a day, just using the internal
electrical energy consumption (and the people and livestock) after a day or
so, Ein = 72K Btu/day and Eout = 24(T-36) 150 ==> T = 56 F inside. Not bad.
Quite warm by Inuit standards. More dogs and cats would help.

How much backup heat will you need in an average year? One way to answer 
that question is to go find some TMY2 data somewhere on the web (references,
please :-) for a Typical Meteorological Year for this house location, and do
a very simple simulation using that data and formulas like the ones above.
Another is to do the same thing in more of a worst-case manner, ie more
reliably using hourly data for the last 30 years from an NREL/NOAA CD-ROM.
Then one might change the design until it is a "solar house," in the sense
that it would not have needed any other form of heat over the last 30 years,
according to the simulation, at which point, many people would opt to
eliminate the backup heating system, saving some space and money...

Here's an alternative design to try out in that simulation:

Eliminate half the south windows and all the movable window insulation, to
eliminate the daily labor and lower the price by $10K or so and lower the
overall U value to about 140 Btu/hr-F, and make a little more privacy if that
matters, and add on a 2-story, $5000, 16' wide x 8' deep lean-to sunspace
with an 10' tall x 13' long x 5' deep solar closet containing 36 55 gallon
drums, inside the sunspace, south of the house and adjoining the house wall.
Each square foot of sunspace would supply about 1000 Btu - 6hr (80-36)/R1
= 700 Btu/day of heat for the house, ie about 180K of heat, vs the 120K-72K
= 48K/day the improved house would require on an average day, via a $200
motorized damper at the top of the sunspace that uses 2 Watts of electrical
energy when moving (rarely) and 0 watts (mostly) in a fixed position to let
warm air flow into the house as needed, controlled by two thermostats. 

The solar closet would contain about 18,000 Btu/F of thermal mass, vs.
4,000 Btu/F, to keep the house at whatever temperature _you_ wanted,
independent of weather, overnight, or on a cloudy day. Assuming the water
starts out at 130 F after a long string of December days, with some sun,
the solar closet would store about 25K Btu of heat in each drum, enough
to keep the house at 68 F for about 25Kx36/48K = 19 days without any sun 
at 36 F, or 3.3 days without any sun at -10 F. We might improve that by
making the solar closet bigger. 

After 3 or 4 days without sun at -10 F, with all the windows boarded up on
the inside, and no backup heat, how can we estimate the temperature T inside
the house we started with, before the improvements? 

24(T-(-10))150 = 72K, so T = -10 + 72K/(24x150) = -10 + 20 = 10 F.

It might take longer than 3 or 4 days to get to that temp, since the pipes
and water walls would release about 400,000 Btu while freezing at 32 F, as
the house temperature plunged. On the other hand, the people might move out
sooner, depriving the house of their heat...

Were you planning to have hot water in this house? How were you planning
to do that? You might put a water heater on the second floor with some
Big Fins in the sunspace or 10-20' of fin-tube pipe near the ceiling of the
solar closet (less expensive), with a warm water thermosyphon loop through
the water heater, to supply a frugal hot water house load of 50K Btu/day...

About heat distribution...
    
>> ...wear a sweater in one room and take it off in another? 
>
>>That's HVAC. Air pushers, ducts and fans and blowers and zone controls,
>>to move warm air out of our sunspace or solar closet around the house. 
>

One might compare this to an oil fired hot air furnace that uses 800 watts
of electrical power when running, while consuming 38 kW (1 gph) of oil power
and delivering 35 kW to the house. That's a fossil COP of 44. Some old and
inefficient and expensive active solar systems have renewable COPs of 50.
For instance, CSI's air/rockbed system in New Hampshire, built in 1982, uses
"2% fan power, 98% solar power." A better way to compare heating systems
might be their yearly heating bills, including the electrical power to run
the heating system. I think one can build a solar house that consumes no more
than 50 watts average, for the heating system, with no backup heating fuel,
and no backup heating system.

A 560 cfm fan ("References please"--Dayton fan 4C688, page 2964, Grainger
catalog 386, 1650 rpm, 10" diameter, max temp rating 149F, $60.75) moving air
>from  a solar closet into a house with 0.05" static pressure (as measured in
our experimental structure) can move about 5600 Btu/hr into this house with a
temp difference of 10 F, ie 134K Btu/day, while dissipating at most 36 Watts
of electrical power. 

>to tell them that they are wrong in preferring that.

Oh sure it is, and it seems kind to tell people, if their passive houses are
such miserable and expensive performers. And as Newton Ellison and others
say, this passive/active dichotomy isn't very useful. I'd say what matters
more these days is money, comfort, reliability, cost and maintenance.

Sailboats are passive devices using powerful natural forces and often smart
low-power controls. Their captains care a lot about cost and performance... 

Would you hang an outboard motor on your America's Cup boat?

Nick