[Greenbuilding] Passive or Active Solar Heat

Tue Feb 10 05:02:17 CST 2009

Thanks Nick

You have to turn your globe upside down and I am near Mokolodi, Gaborone;
Botswana, 400km north of Johannesburg just inside the tropic of capricorn.
Climate seems similar to what you describe for Arizona although I struggle a
bit with units. RH is only above 50% in the summer mornings (=now - our
'rainy' season).

Given the rocks here we plan to soil-cover part of the sun (north) and E/W
side of the basement  so the window(s) would be narrow and wide max 50cm
high and 1m wide; one or two: They would have to be big enough in case I
need access to the basement - fan wiring etc.

>From the points you mention I see the main gap in my thinking is the use of
thermal mass. I kept using only air volumes. With the soil cover that will
have vegetation and benefit from reused watering there is extra mass.
Using design and insulation to lower the needs of the house has good
potential too.
Plenty to incorporate!

Cheers; Gi = Guy but pronounced à la française

On Mon, Feb 9, 2009 at 8:00 PM, Nick Pine <nick at early.com> wrote:

> Gi[l?] Broucke writes:
>
>  My environment is dry and semi-arid; day/night above 30?/20?C most of the
>> year up to 40/30, more than 300 days' sun. Only 2 months are 'chilly' but
>> that still means night temps between 5 and 10?C and day temps between 16 and
>> 20.
>>
>
> I wonder where this would be? Southern France, or Prescott, AZ, where 1570
> Btu/day of sun falls on 1 ft^2 of south wall on an average 36 F January day
> with a 50 F max. July is 73, with a 58 F min and humidity ratio wo = 0.0091
> pounds of water per pound of dry air.
>
>  ... most of the basement (sunny side) would be a 'sealed' room with a
>> window and shutters.
>>
>
> One window? How big?
>
>  In winter the shutters would be open and the heated air inside can be
>> released in the evening into the living room above.
>>
>
> This would be more efficient if the window didn't lose heat to the outdoors
> at night, eg if the window had an insulated wall to the north with a couple
> of holes with one-way plastic film dmpers at the top and bottom to allow
> airflow during the day that stops at night. A U0.25 window with 50% solar
> transmission could keep a large perfectly-insulated mass that supplies no
> heat to the house a constant temp T (F) in Prescott, where
> 24h(T-36)1ft^2xU0.25 = 0.5x1570 = 785 Btu/day, ie T = 36+785/(24hxU0.25) =
> 167 F, theoretically-speaking. A less-expensive R2 window with 80%
> transmission on a low-mass isolated sunspace could keep a mass
> 36+0.8x1570/(6hxU0.5) = 455 F, theoretically-speaking.
>
>  But cooling is my main objective. In summer the room will be cool (shutter
>> closed, but also the sun angle and design of the groundloor above will avoid
>> the wall warming).
>>
>
> You might cool basement ceiling mass by allowing outdoor air to flow
> naturally into the basement and out near the first floor ceiling at night.
> Shiny hot ceilng mass can store more heat than room temp mass in a
> Barra-like system, without overheating the bulk of the basement. This
> requires a passive vertical duct that allows hot ceiling air to return to
> the lower part of a sunspace or air heater without mixing with the bulk
> basement air.
>
>  Now I have two types of questions. First the mechanics to get this cool
>> air to rise and circulate in the room(s) above. Solar chimney?
>>
>
> A large slow low-power quiet fan. Solar chimneys are inefficient, since
> they draw in hot daytime air.
>
>  How do I ensure that the air sucked out of my living room is mostly
>> replaced by cool air from the basement? Fans installed in the floor of the
>> living room?
>>
>
> That could work. It would be nice to use the same fan for solar heat
> distribution in wintertime.
>
>  And second the quantitative issue, sizing the basement in function of air
>> temperature in the basement and the above room.
>>
>
> There are several quantitative issues. A C cfm fan can provide about
> (Tb-Tr)C Btu/h of heat from a Tb (F) basement to a Tr (F) room...
>
>  My own initial calculations are rather disappointing, using a 150m3
>> basement with air at under 20?C used for sustained cooling of a 35?/60m3
>> room would only be able to cool it by a few degrees
>>
>
> That depends on the volume of airflow and the insulation between the room
> and the outdoors.
>
>  or result in heating the basement very quickly...
>>
>
> That depends on the amount of thermal mass in the basement.
>
>  ...  if I use the fan system, can I channel the cool pumped air through
>> moist pads and cool it further?
>>
>
> Sure, if the outdoor air is dry enough. I like the idea of running an
> indoor swamp cooler when the indoor temp rises to 80 F, using a thermostat,
> and running an exhaust fan when the indoor RH rises to 50%, using a
> humidistat.
>
> Salut,
>
> Nick
>
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