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re: need power to my land (now russian stove)
sun, 31 oct 1999

> it's a hot tub when the water feels the right temperature
> and a thermal mass when it is too hot or too cold.

and it's hard to change the tub temperature quickly, but
a fairly constant temperature hot tub might be a radiator
and a flue gas heat exchanger at the same time.

> ...a flue gas heat exchanger is a better idea, but isn't
> that more complicated?

in general, or at least larger. water and solids are better
heat conductors and transporters and stores than gases.
gas to air requires the most heat transfer surface. higher
gas/air velocity raises the surface conductance and
lowers the required area...

> do you build a passive flue gas heat exchanger
> to heat rooms 30 feet away from the fireplace?

tear down the walls and add a big ceiling fan? :-)
superinsulate the house, to minimize temp diffs
between rooms use ducts, or somehow capture
the flue gas heat in water and put fan coil units
(eg 1984 dodge omni automobile radiators)
in every room?

> i guess you would throw a cast iron radiator in the flue
> at a wide space in the flue (so as not to cut down on
> the draft too much) and hook it up to other radiators
> and hope for a thermal siphon or heat pipe action.
> as long as the radiator in the flue is a few feet below
> the radiators that heat distant rooms, it could work.

it could. that's rube goldberg, or mother earth news...
"me an charley, we drove all over creeation one afternoon
until we spotted this real big radiator buried in mud in a
cow pasture. we dug our crowbars outta the trunk, and
a few hours later, viola!" (these guys never mention
dimensions, and value their labor at $0.00001 per hour.)

> i can picture a brick hot tub hooked up to one side of
> a brick fireplace in my mind in a master bedroom.
> seems aesthetically pleasing in my mind. throw
> a log on the fire, go for a hot tub, then to sleep.

maybe. heating 500 gallons (4,000 pounds) of water
from say 75 to 105 f takes 30fx4000btu/f= 120k btu,
one 12 pound 10k btu/pound log, at 100% efficiency.

> > i've been thinking more about smoke-to-air heat
> > exchangers, eg a 2' cube on top of the stove,
> > a sandwich with 48 2x2' (galvanized?) metal sheets...

> i hate anything that has a noisy blower.
> i hate anything that breaks.

me too. my first electronic boss told me that if anyone ever
designed a box with a fan in it, he'd fire them. (he once fired
a guy who didn't even work for him because the guy smoked
a cigar, back when that was a matter of personal choice.)

> i don't like the idea suction fan in the fireplace chimney exhaust.
> it's so hot, it will break every year i bet.

i don't either, but this might be a good compromise.
grainger's 136 cfm 102 w blower (which might only
use about (50/136)^3x102 = 5 watts at 50 cfm) only
costs $75, and it can increase the draft and the flue gas
velocity and minimize chimney size and downdrafts, and
it probably has a guarantee, if the flue temp's less than
450 f, and we can guarantee that using water, and
we might use 2 for greater reliability.

so how about building up a flue-gas heat exchanger on
this 2'x3'  flat stovetop in layers: first a 1/2" thick cement
board perimeter layer and smokepipe ring to make a
a 1/2" vertical spacer, then 1/2" of vermiculite insulation
to keep the firebox warmer, then a solid layer of cement
board, then some 1/2" tall partitions to make a high-velocity
serpentine smokepath, then a layer of window screen,
then 50' of rectangular flat coiled soft copper pipe bent with
pipe bending springs, then an inch of cement (with stalactites
below the screenwire), another layer of screenwire, and another
inch of cement on top to make a nice flat temperature-controlled
cooking surface for simmering stews and teapots.

the smoke would exit from under the cooking surface through
a couple of 4" vent holes and smokepipes that go to the blower
attached to the chimney flue opening in the house wall.

the copper pipe might naturally thermosyphon warm water
to an insulated tank above, boiling as needed if using
soft water, eg rainwater to minimize scale, or we might
use a pump and a few 55 gallon plastic drums full of
130 f water. i'm inclined to go that way, with 300' of 5/8"
reinforced garden hose snaking in and out of the drums
to preheat water for showers.


from: "nick pine" 
subject: re: questions about groundsource heat pump...
date: sun, 24 oct 1999 03:53:50 -0400

rick solinsky wrote:

> > i own a 2700 sq ft home in the high sierra's of california...

nrel's solar radiation data manual for buildings says
reno, nv gets 1240 btu/ft^2 of sun on a south wall and
650 on a horizontal surface on an average december day,
ie sqrt(1240^2+650^2) = 1400 btu/ft^2 on a surface aimed
arctan(650/1240) = 28 degrees above the horizon. enw
walls receive 470, 180, and 480 btu/ft^2-day of sun.

the average december outdoor temperature is 32.7 f, and
the average daily high is 45.5, so a solar collector might lose
heat to 39 f air (the average of these two temperatures) in
the daytime. the average yearly temperature (deep soil or
groundwater) is 50.8...

a one-story 52' square home with 216ft^2 of r3 windows
(8% of the floorspace) and real r20 walls and an r30 ceiling
and 0.75 air changes per hour of air infiltration has a thermal
conductance of 216ft^2/r3 = 72 btu/h-f for the windows plus
2516ft^2/r20 = 126 for the walls plus 2700ft^2/r30 = 90 for
the ceiling plus 0.75x2700ft^2x8'/55 = 295 for air leaks, a total
of 583 btu/h-f, so it would need 24h(70f-32.7f)583btu/h-f
= 522 k btu to stay 70 f inside on an average december day.

with 60% solar transmission, 54ft^2 of windows in each compass
direction would gather 0.6x54ft^2(1240+470+180+480) = 77k btu
per day of solar heat, and 300 kwh/month of electrical energy
usage adds another 34k/day of internal heat gain, so the house
needs an additional 522k -77k-34k = 411k of heat to stay warm.

> > i was planning on hooking up four  4x10 solar hot water panels.
> > the output of the panels would go into an insulated 1000 gallon
> > septic tank (which would act as a heat exchanger...

the septic tank might not need insulation if it's indoors and used
with a heat pump. 70 f water can stay 70 f forever in a 70 f house.

anthony matonak  wrote:

> ...4 x 4x10 panels is about 160 square feet... you'll collect maybe
> 800 watts per sq meter for say 5 hours full sun a day.

maybe less. full sun (am2) is only 800 w/m^2, with a surface aimed
right at the sun, and water heating panels are typically 40-60%

>maybe 60,000 watt-hours per day. btu's are 3.4144 per watt-hour
>so call it 17,400 btu a day.

1 watt-hour is 3.41 btu, so 60kwh would be 205k btu...

with r1 polycarbonate glazing with 90% solar transmission, the
panels might collect 0.9x1400x160 = 202k btu/day. with 70 f
water inside and no heat loss from the back (eg the south wall
of the house), the panels might lose 6h(70f-39f)160ft^2/r1 = 30k
btu to the outdoors over a 6 hour solar collection day, for a net gain
of 202k-30k = 172k btu/day and a solar collection efficiency of
72k/(1400x160) = 77%. doubling the panel area might keep the
house warm, with additional heat gain from the heat pump's
electrical energy use.

>...1000 gallons of water is about 8330 pounds. each btu gives
> a 1 deg f rise per pound. 17,400 btu is going to raise that
> 8330 pounds about 2 degrees each day. assuming no
> losses through the insulation and that you aren't taking any
> heat out with the heat pump...

but why not calculate the performance on an average day,
with the heat pump removing heat at the same rate that the
sun replaces it? how long can the house go without sun before
the heat pump freezes the water? the house needs about
522k-34k = 488k of heat on a cloudy day. the heat pump's
electrical consumption might supply about 1/3 of that, with
another 325k btu from the water, or 1627k over 5 days,
enough to cool 1627k/(70f-40f) = 54k pounds or 6800
gallons of water from 70 to 40 f, eg 3 or 4 7' tall x 7' diameter
$900 2100 gallon polyethylene tanks. (flooding the basement
floor might help, as they reach 40 f.)

> > as i understand the way the groundsource heat pump works,
> > the water goes into the loop in the ground to heat it up to
> > about 45 degrees...

sounds reasonable.

> > and then it is run through the compressor of the heat pump
> > where through the process of compressing the water...

water is incompressible. the heat pump compresses a gas
like freon...

> >  it gives off about 120 degrees of heat to the heat exchanger
> >  for the in-floor hydronics. if by sending the loop through the
> > 1000 gallon sump, it heated the water to say, 60 degrees,
> > wouldn't the efficiency of the heat pump be in effect supercharged?


> >say, giving off 160 degrees in the ground source heat exchanger?

or maybe 120 f, with less electrical energy input, ie a higher "cop."

> > what would be the calc's on this?

i'd ask the heat pump manufacturer.


from: "foxwick farms" 
subject: re: need power to my land
date: sat, 13 nov 1999 20:46:46 -0500

sojourner  wrote

> sylvan butler wrote:

> > and we know that transportation costs for 11,000lbs of stonework is
> > significant compared to 1500lbs of steel.

and most steel stoves weigh about five times less, and 66% of the
steel used in the us is recycled, which lowers embodied energy.
the 2 20 pound steel drums in a kit stove (eg the $29.99 26 pound
16160-c141 barrel stove kit and $19.99 15 pound 16161-c141
top barrel adapter kit from northern tool (800) 533-5545) would
almost certainly be recycled (and free :-)

> ...not when you have a local brickworks.  there's no stone
> going into my masonry stove, it[']s all brick.

we are only discussing cement and brick (ignoring wire and rebar
and angle iron in masonry stoves, and transportation and food for
teams of builders, and so on) at 5 gj/tonne. brickworks tend to be
local, since bricks are so heavy and cheap.

> ...mining, smelting, refining and processing further (there are
> a bunch of different types of metals)...

really? but we are only discussing the ferrous ("mirror, mirror...")
at 40 gj/tonne (1000 kg.)

> the processing of metal takes a lot more energy and creates
> a lot more waste than me driving to the brickworks and picking up
> a pickup load of bricks.  even if i have to do this 10 times
> (which seems likely).

how much and what kind of metal? this may be another vague
article of faith (less politely, "bullshit") contradicted by several
collections of numbers. where are your numbers, sojourner?

let's see: your masonry stove weighs about 11klb/2.2/1000
= 5 tonnes, so creating one in your house requires at least 5x6
= 30 gj, about 8,300 kwh or 28 million btu or the equivalent of
220 gallons of oil, even without flying in builders from utah.

alternatively, you might create a drum stove with comparable
efficiency and pollution using 81 pounds of steel, 40 recycled,
with about 41/2.2/1000x40 = 0.75 gj, 40 times less embodied
energy, equivalent to about 5 gallons of oil, without those
10 trips to the brickworks. and your ancestors might recycle
the stove kit steel.


article: ...of misc.rural
from: "foxwick farms" 
subject: re: woodstove advice sought - help!
date: sun, 21 nov 1999 07:19:11 -0500

does ecnerwal  really doubt that

>> ...condensing  water in that heat exchanger further increases

>in thermogoddamnics, you don't get something for nothing.

unless you are a free energist :-).

>when you burn green wood, you have to boil the water in it,

agreed. or at least evaporate it. and heat the water vapor
up to some highish temperature, in most "woodstoves"
("composting stoves" excepted.) we may be discussing
different subjects, lawrence...

>which keeps the fire temperature in the stove down, and
>promotes incomplete combustion

yes, in principle, and in a vague qualitative way, but if we burn at
a higher rate and a higher temperature, with more air, that "promotes
more complete combustion," in the same vague and qualitative way.
(oh, you want to use numbers? :-) we have to evaporate water
in any case, but higher firebox temps make more complete combustion,
ie they burn more of the wood, so less unburned fuel goes
up the chimney, and the woodstove generates more heat per pound
of wood, in the firebox, whether the wood is green or not. the question
becomes how to recover that heat from the hotter fluegas...

>if you get the heat spent in boiling the water back by condensing
>the flue gas, you still haven't burned many components of the wood.

i disagree, altho this seems like a non-sequitur. that depends on the
temperature, no? higher firebox temps burn more of the wood.
they release more btu/pound of wood, whether it's green or not.
green wood can make achieving higher temps more difficult, eg
it's harder to light, and more draft air is required, but that's easy
to supply. one way to raise the firebox temp is to insulate the firebox.
>stovepipe and chimney maintenance leads me to believe that
>not much of that will burn properly if lead back to the stove -

there won't be much of "that" (unburned components?) in the
flue gas if the stove is "overfired" to begin with :-) that's what
i'm suggesting, and possibly doing it with green wood.

>if you dry out the wood first, the initial combustion is more complete
"the initial combustion?" i'm only talking about a single combustion
process here, vs "secondary combustion," or reburning unburned
components that run back down into the woodstove.

>and at a higher temperature in the stove...

sure, in principle, in the same vague qualitative way...

>and you still have the option of condensing out the (purer) water
>(mostly a combustion product) in the flue gas if you like.

sure. what's your point?

>dry wood is good. when using cellulose-stored solar enery combustibly,
>making use of some freely available direct solar energy over the summer
>to dry the heck out of your wood _is_ worthwhile.

maybe, maybe not:

1. it may be more convenient and economical to cut up a green tree
and burn it than to cut it up and wait a few months for it to dry, given
the time value of money and the cost of dry storage space. aged
products cost more.

2. removing heat from fluegas can be a lot easier if it contains more
water vapor. convection heat transfer typically has 1.5 btu/h-f-ft^2
of surface film heat transfer conductance, but condensation can have
a much higher  effective film conductance, on the order of 1800,
a thousand times higher (take a look at problem 9.15 in prof pitts
and sissom's 1998 schaum's outline on heat transfer, if you like)
which can make a fluegas heat exchanger much smaller.

3. interesting things can happen at higher temperatures, as i understand
this, and water can actually help make combustion more efficient.
for instance, at 1800 f, some water dissociates into hydrogen and
oxygen, which form methane and other combustible gases (take a look
at the 1996 us patent no. 5,589,599 on the ibm website, if you like.)
a little water vapor can improve your automobile gas mileage...


article: ...of
from: "foxwick farms" 
subject: re: heat loss & solar gain calculations
date: wed, 24 nov 1999 17:06:54 -0500

neil evensen  wrote

> i'm trying to see how well the sun could
> heat a 5x5x5 metre living space...

a 16' cube? let's use british units... :-)

> with a 5'c outside temperature and solar
> radiation of 350 w per square metre

that's power. the amount of energy depends
on the duration and direction. on an average
41 f december day in cloudy 44 n lat. eugene,
or, usa, 460 btu/ft^2 of solar energy falls on
a south wall, 330 falls on a flat roof, 210
falls on east and west walls, and 130 falls on
the north, according to nrel's solar radiation
data manual for buildings ("the blue book.)
that's a 24-hour average power of 11.2 btu/h
per square foot of cube surface. darkish
bricks might reflect 20% of that and absorb
80%, ie about 9 btu/h-ft^2.

> i'd like to show the difference in efficiency
> of a cube built from bricks, and a cube built
> from bricks with a layer of glass on the outside.

ok. if cb is the recurring cost of heating
the brick cube vs cs for the solar version,
we might define 1-cs/cb as the "efficiency."

>...i want an inside temperature of 15'c

59 f. coolish.

> heat loss = u value x s.area x dtemp.

looks familiar, as a rate of heat loss.

> if the walls and ceiling were made of
> solid brick this would be
> 2.1 x 125 x 10 = 2625 w

omitting the 25m^2 floor might be ok,
especially for a larger building. it's
likely to be warmer than outdoor air
in the dead of winter and account for
less heat loss than the walls and roof.

schaum's outline on heat transfer by pitts
and sissom (mcgraw hill, 1998, $14.95 :-) says
common bricks at 68 f have a 0.4 btu/h-f-ft
(=btu-ft/h-f-ft^2) thermal conductivity, so an
18" wall has u = 0.4/1.5' = 0.27 btu/h-f-ft^2
of thermal conductance and r = 1/u = 3.75, like
an inch of foam beadboard. not much. "face
bricks" are about twice as conductive...

let's add r1 for a slowly-moving inside air
film and r0.17 for windy outside air, for a
total us r-value of 4.92 ft^2-f-h/btu.

> thermal conductivity
> = ( heat loss? x thickness ) / ( s.area x dtemp. )

conductivity is a material property, vs a
conductance of a particular wall. both are
divided vs. multiplied by thickness, since
thinner walls have higher conductance. you'd
multiply resistivity by thickness to find
a wall's thermal resistance (r-value)...

the sun heats the brick cube, even with no
glass. modeling with an electrical analog
(viewed with a fixed font like courier)
with the sun as a current source,

   9 btu/h (sun)
    ---    |       i
           |     <---
     r0.17 |  4.92   0.67
    outdoor   wall  indoor
    air film        air film

here's a simplified circuit:


i = 2.86 btu/h-ft^2.

with a layer of us r1 90% transmissive glass,

  8.1 btu/h
    ---    |       i
           |     <---
     r1.17 |  4.92   0.67

which simplifies to


i = 1.27 btu/h-ft^2.

so we have 1-1.27/2.86 = 0.56, ie the
modified house is "56% efficient"
on an average winter day with
an average amount of sun.

here's another alternative:

add  insulation outside the brick and some
simple solar air heaters over the south
wall with low-power fans or passive dampers
that allow air to circulate between the house
and the heaters during the day, but not
at night. insulating the bricks avoids
inefficiently storing solar heat during
the day and losing most of it through the
low resistance of the air heater glazing
at night... 64ft^2 of 80f air heaters with
r1 90% transmissive glazing might collect
26.5k btu over about 6 hours while losing
6h(80f-41f)64ft^2/r1 = 15.7k, for an average
net gain of 11k btu day, or about 14k if
the surface to the south of the air heaters
is reflective.

add a gable roof or clear polycarbonate
greenhouse over the flat roof to make a
solar attic with an e-w ridgeline and a
transparent south side. line the north
wall with nielsen's aluminized mylar film
(, which is
90% reflective and costs 9 cents/ft^2 in
4'x100' rolls. it might be rolled on to
4 or 5 flat masonite panel segments using
grease for glue, so it can be replaced
easily if it wears out in 10 years.

aimed at the horizon, under a 1:1 roof pitch,
this linear parabolic concentrator might have
a broad line focus y^2/(4x) = 64ft^2/32ft
= 2 feet from the north wall. the insulated
attic floor could have a few simple flat
polycarbonate skylights with foamboard covers
hinged along the north edge that fold up
during the day, and a 4' shallow drain-down
trough or swimming-pool-type collector along
the north wall to collect concentrated
solar heat in water.

it's hard to collect hazy-day sun in places
like oregon or the uk, but howard reichmuth,
pe, says most of the solar energy that arrives
in a cloudy month comes during short clear
sunny periods, when we can collect it (howard
calls this beam sun energy "gift-wrapped :-)
at a high temperature, efficiently. higher-
temp heat stores require less thermal mass
with their larger temperature swings, and
make for easier water heating and room
temperature control.

nrel's solar radiation data manual for flat-
plate and concentrating collectors (the "red book")
says a south wall in eugene receives 1.5 kwh/m^2
of sun on an average december day, but a 1-axis
tracking collector with an east-west horizontal
axis receives 0.8 kwh of direct beam sun per
square meter, about 1 hour's worth, but more
than half the daily energy total...

a non-tracking trough with a low concentration
ratio might collect most of that in the winter,
and some of the hazy sun too. concentration can
make solar collection more efficient, with a
smaller "target area" and a smaller rate of
heat loss (proportional to that area) and a
shorter time over which the heat loss occurs.

with 8'x16' = 128ft^2, ie 11.9 m^2 of south
solar roof aperture and 90% transmissive roof
and trough glazing and a 90% reflector, we can
collect about 0.9x0.9x0.9x0.8kwh/m^2x11.9m^2
= 6.9 kwh or 23.5k btu per day of sun... and
64ft^2 of 130 f trough surface only loses
1h(130f-80f)64ft^2/r1 = 3.2k btu in an 80 f
attic during 1 hour of solar collection, for
a net gain of 20.3k btu. we might use all that
heat to maintain a large well-insulated
water tank at a high temperature.

meanwhile, on an average day, the attic
collects 0.9x460x8'x16' = 53k btu of sun
from the south and 0.9x330x8x16 = 38k of
overhead sun, ie 91k btu/day. if the north
wall is well-insulated and the attic temp
is 80 f for 6 hours, the attic loses about
6h(80f-41f)12'x16'/r1 = 45k btu, for a net
gain of 46k btu, and the house gains 14k
from the air heaters, for a total of 60k
btu per day, mostly from warm air that
circulates between the house and the attic
during the day. (we aren't double counting
here, if the large water tank is indoors.)

an r20 attic floor and r-value r sidewalls
means 60k = 24h(59-41f)(256ft^2/r20+4x256/r)
on an average day, so r = 8.1. let's use 2"
of beadboard covered by stucco, eg the dry-vit
process, developed in germany years ago. this
isn't a superinsulated house. the walls might
be r8 windows...

it needs 300k btu to stay warm for 5 cloudy
days in a row. electrical energy use of 300
kwh/mo contributes 130k, and 0.5 air changes
per hour (34 cfm) of air infiltration adds
about 5dx24hx34x(59f-41f) = 73k to the heat
requirement, so we need to store about 243k
in the water tank. if the water cools from
130 to 80 f over 5 days, 243k = (130f-80f)c,
so we need c = 4860 pounds or 600 gallons
of water, eg a $900 44" tall x 87" diameter
1000 gallon plastic tank under part of the
floor, which might be a hydronic slab.

keeping a 4x8x8' box with 256 ft^2 of surface
130 f for a day with 20.3k btu means 20.3k
= 24h(130f-59f)256ft^2/r, ie it needs an
r-value of 21, eg 4" styrofoam walls. there
isn't much heat left over for water heating
for showers, etc, even with a greywater heat
exchanger, but this is a small house. a 30x60'
house attic might gather 160k btu/day in water.

if attic air keeps the house warm for 6 hours
on an average december day, and 20k btu ends
up stored in the water tank, the house needs
to store 25k btu in 6 hours, ie about 4k btu/h
in its internal thermal mass. with capacitance
c btu/f and s ft^2 of air-heated surface, the
house would have a day-night temperature swing
of about 25k/c+3.7k/s. a 16' wall or column
containing 52 16'x4" sealed pvc pipes full of
water would make the day-night swing 10 f, so
we might not need any bricks.

as an alternative, we might store daily heat by
direct gain to the floorslab from r8 windows,
or optically pipe down concentrated attic heat
to a hot water-cooled section of masonry floor
with a reflective north wall channel that works
something like a right-angle feed microwave horn.


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