[Greenbuilding] Radiatore

Wed Oct 3 15:50:01 EDT 2007

Racheli Gai <racheli at sonoracohousing.com> writes:

> Part of our common house is a building which doesn't benefit
> from winter solar gain.

Is it shaded by firewood? :-)

> It has a kitchen, and when people congregate there to eat after a meal
> was cooked, it's warm enough.  However, there are times when people
> meet there (in the winter) and it's cold...

NREL data say January is the worst-case month for solar heating in 
Tucson, when 1090 Btu/ft^2 of sun falls on the ground and 1650 falls
on a south wall on an average 51.3 F day with a 63.9 daily max. This
is an excellent climate for solar heating, compared to Rochester NY
with 560 Btu/ft^2 on a south wall on an average 29.1 F day :-)

> What kind of heating would people recommend for a space which
> needs only intermittent heating, and not for long durations? 

Since there's a kitchen, how about a solar water heater in a nearby
building, eg a "solar shed" like Gary's at http://builditsolar.com?
Gary used flat-plate collectors, but Nathan Hurst successfully used
an auto radiator to heat water in a sunspace in Australia, measuring
an air-water conductance of about 2000 Btu/h-F. We ran a $25 used
Mitsubishi auto radiator with its 2 fans in series from a 20 watt PV 
panel at the PA Renewable Energy Festival. (David Delaney's passive 
car radiator scheme uses a kind of thermal chimney.)  

If the south wall of an equilateral A-frame shed or attic with 3 
$84 4'x12' U0.58 Thermaglas Plus twinwall polycarbonate panels with 
80% transmission collects 0.8x12'x12'cos(30)1650 = 164.6K Btu from 
the south plus 0.8x12x12sin(30)1090 = 62.8K from above (totaling 
227.4K over a 6-hour solar collection day) and we heat 140 F water
with a 2K Btu/h-F radiator on an average day, we have something
like this, viewed in a fixed font:

     227.4/6h = 37.9K Btu/h       
     ---                              1/2000
|---|-->|-------------------------------www------
     ---                         |               |
                                 |               |  140 F
       1/(12x12x0.58) = 1/83.5   |              ---
57.6 F ----------www-------------                -                                                 -
                                                 |
                                                 -
which is equivalent to:

                 1/83.5               1/2000
      -----------www--------------------www------
     |                                           |
     |                                           |  140 F
    --- 57.6+37.9K/83.5 = 511.4 F               ---
     -                                           -
     |                                           |
     -                                           -

so we might heat 6h(511.4-140)/(1/83.5+1/2000) 
= 178.6K Btu/day, enough for 5-6 families :-)

"Eli" <elitalking at hughes.net> writes:

> I am looking for information on a reversible fan to go into a 26"x26" chase
> that takes hot air from solar gains that stratify in the attic (heat chamber
> I call it) and pulls it to the crawl space, where Main Level Floor is a
> composite concrete slab for thermal mass.

I wonder if the heat chamber has a transparent south wall.

> I would like the fan to be reversible so I can also pull cool air in during 
> the summer night to vent out the cupola and cool the slab. 

Hot air rises... With a 10 F temp diff and a 16' height diff, 
16.6x26/12x26/12sqrt(10Fx16') = 986 cfm would naturally flow
up through the cupola... DC radiator fans can't work long in
both directions, because their brush timing is optimized for 
a single direction. 

> It would also open an automated vent that would close when 
> fan cuts off, when a low heat difference is reached, thereby 
> preventing the heat from rising back into the heat chamber.

We built one with a windshield wiper motor scrounged from 
a dead '97 Taurus with a little pulley and a string and 
2 limit switches, like this, electrically:

12 V -------------------------------
          |                         | 
          - open limit              - close limit
          |                         |
          X close to open           - close to close
          |         ------->        |
          |          -----          |
          |---------|motor|---------|  
          |          -----          | 
          |                         |
          - close to close          X close to open
          |                         |
    |-------------------------------

It drew about 1 amp with a 10 ohm resistor in series 
to reduce coasting.

A solar attic version might look like this, viewed 
in a fixed font:

|    | upper 
|    | attic          /
~    ~              /             
|    |            / mad  
|    |----------------------------- insulated attic floor
|    |a   h   r f        |h    |
|    |a   a   a a ==>    |a    |
|    |d   d   d n        |d    |
 ------------------------|     |     
                         |     |
                         ~     ~
                         |     |
                         |     | house floor

To heat water, the fan draws upper attic air down 
through a duct and through passive one-way plastic
film damper aad, then through the radiator, then
back up into the attic via raised motorized damper
mad. At night, with mad horizontal, the fan draws
house air out of the page through passive damper
had on the left, then through the radiator, then
through passive damper had on the right and down
to the house floor. The hads should be tilted so
they don't open when the mad is up.

> The slab works without the active components.

Hot air sinks? :-) It does in Kachadorian's book...

> I would be looking for ways of adding thermal mass...
> in a crawl space this could be racks of water jugs 
> supported by ground.

As in Anna Edey's Cape Cod Solviva greenhouse...

Given a radiator, you might store heat in an EPDM-lined plywood 
tank or an unpunched water-filled polyethylene film greenhouse 
ventilation duct on the ground, with both ends raised.

Nick