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re: graphite a heat storage materials useful with an air collector system
23 nov 2000
graham parkinson  wrote:

>...good idea to look up schaums outline on heat transfer.

a good bargain at $14.95 :-) mcgraw hill. isbn 0-07-050207-2 for the
second edition. most of the calculations are over my head, but it also
has lots of materials data. page 338 says "pyrolitic graphite" (yours?)
has conductivity k = 1,900 parallel to layers and 5.6 perpendicular. wow.
says it weighs 2200 kg/m^3, with c = 0.71 kj/kg-c. concrete with stone
has k = 1.37, rho = 1900-2300 kg/m^3, and c = 0.88 kj/kg-c.

>i have just bough enough used books on solar houses to heat a house
>(for a while) by simply burning books on solar designs.  however i am
>generating more heat by gradually reading them....

a lot of pre-1984 solar house books were poor and confused.
you might be better off burning them.

>from what i have read, people report that rock heat pit storage systems
>have problems with controlling temperatures in the living spaces. the
>rock pits function more as a temperature averaging system rather than
>as a on demand heat source.
 
this may have more to do with the poor designs or control systems than the
rocks. i like the idea of containers of water on grade, because they can
store about 3x more heat by volume, with less airflow resistance and large
slow fans using 10s of watts vs blowers with 100s of watts. they can even
charge and discharge by thermosyphoning with air. but the total container
surface needs to be at least 10x the solar glazing area.

>what i thought would help with this would be having a subdivided storage
>bin.  it would charge sequentially instead of diluting the heat down in
>temperature to an unusable level across the entire storage volume.

stratification is nice, especially if you plan to use significant backup
fuel, and as you say, it naturally happens if you charge and discharge the
store in opposite directions.
 
>the rocks r value is a clear starting point, the airs heat transfer ability
>is the mainly uncertain value, highly dependent on the turbulence...

you might plan on still-air, worst-case.

>is the real world charging time of a rock pit consistent with these low
>estimates for rc?  i understood that they took quite a while longer to
>charge/discharge.

an individual rock's time constant is a lot less than the time constant
for the entire store. you might charge up the whole store in 20 minutes
with lots of airflow, but it's usually more like a heat exchanger problem: 
the air cools as it flows through the store, and its heat capacity is small
compared to the heat capacity of the rocks. if the ratio is close to zero,
the heat exchanger effectiveness e = 1-exp(-ntu) = (thi-tho)/(thi-tci),
where ntu = au/cmin and thi is the air temp at the store inlet, tho is the 
air temp at the store outlet, tci is the rock temp, a is the total rock
heat transfer area, u is the conductance across that area (eg the still air
film contuctance, about 1.5 btu/h-f-ft^2) and cmin is the heat capacity
flow rate in btu/h-f (roughly equal to the cfm.) 
 
>there is also the temperature gradient along the airstream with rocks in the
>front of the pit getting the initial heat...

yes. warmer rocks near the inlet complicate the picture.

>plastic film would be great but in a high wind / seagull environment.
>= shredded plastic.  home glazed collectors, honeycombed maybe.

it might be ok in the wind if stretched over a curved surface...
 
>what would be ideal for the site would be a deck that functioned as a
>collector but thats a seperate design project!

the deck itself could be opaque, with a transparent south wall below.

>do beadwalls / low e argon windows really work (or justify expense?)

argon seems expensive, especially if it leaks out over 5-10 years,
with a 5-year guarantee. beadwalls work fine, but bubblewalls are
simpler and less expensive. 

>...a local smelter uses graphite rods of about 6"" by 3 feet as
>electrodes - if some fell off of a truck they would make a great
>conductive heat storage/supply to a floor slab if stacked on end
>about 4"" apart. 

sure. make the floorslab a very thick heat store, with the rods
protruding below to act as fins to gather heat from solar-warmed air
from a sunspace below and distribute it into the slab thickness. 
insulate around the slab perimeter down to the bottom of the rods.
when the sun goes down, the sunspace gets cold but the slab stays
warm, since warm air rises, as in an igloo heat trap. insulate the
top of the slab, and make a few holes in the floor with fans or
movable dampers to let some cool house air fall into the space below
the slab and let some warm air surrounding the rods flow up into
the house as needed.

nick




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