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re: solar barn -- "first sun"
28 feb 2004
p.e. drew gillett"  writes:

> i have some suggestions you can forward...
>
> 1) include plastic dampers at bottom vent as well

this could be more reliable, if the heater channel can work with either
damper stuck open, and it might add thermal resistance when closed, but
it would also add airflow resistance, and it's more difficult to fit or
replace than the top damper. a barn with horizontal purlins vs vertical
studs might easily have one continuous damper strip vs 20, with less labor
and less leakage when closed... 

> 2) lower vents to floor level or provide ducting down to floor level so
> coldest air is scooped off floor during daytime. this could be done with a
> piece of plywood on the interior surface of the studs extending down to
> near the floor (note how the inlet air temp is rising during your tests while
> the floor remains cold.) even better for dayround heating and storage and
> overall perf would be to bring the air thru the floor (had you poured the
> slab on top of concreete blocks on there side this would have been easy.) 
> at this point just making some air flow across the surface of the floor (or
> letting the sun shine on it directly will work)

maybe this barn is better without heat storage...

> 3) flow thru the screening from outside to in is good  (keeps outer
> radiant temp low), but there should be several layers of screen since
> screen is only about 20% wire.   after i wrote this i saw your data on
> the extra screening.

i've been looking into that. see calc below, which doesn't account for
the first screen layer also keeping cool air near the glazing, and more
screen layers adding more sun-air conductance and airflow resistance...

> just looking at the photos its clear that painting the inside of the
> collector black everywhere would help performance as well.
>
> 4) the absorbers do nothing for daytime performance other than increase
> the temp near the glass increasing losses compared to simply having the south
> glazing act as a window. at night the glass would have a higher loss, but
> this is very dependant on how well the plastic dampers seal. this is why
> direct gain with night insulation works so well.
>
> 5) 24 hours of labor at 10 is 240 so my rule of labor equals materials for
> construction projects still applies.  $4/sg.ft installed is still a very
> good low price.  remember that at best such a system would deliver 1
> gallon of fuel oil per sq.ft/year so payback is over 5 years but still
> very good. more realistic performance is probably half that
>
> 6)southeast facing helps with early morning warmup and reduces afternoon
> overheating without too much overall output penalty.

simply rotate the barn :-)

> 7)peak output looks like about 150btu/sq.ft/hour which is about 50%
> efficiency. efficiency could be improved with higher flow rate, darker
> absorbers or direct gain.
>
> 8)a design suggestion would be to eliminate absorber and plywood for lower
> half or east and west panels to allow the barn to have some daylighting
> and direct passive gain.

maybe drew means "upper half," with a lightshelf to bounce some sun up
under a reflective ceiling...

> 9) on the deflection. these plastic panels have a much higher coeff of
> expanison than wood.  usually the panels are mounted so they can move a
> litle.  slotted holes, not fastened at bottom, mounted in channels so they
> can slide.  another trick is never let them stagnate. when you're
> removing the glazing to repaint the interiors and add a 3rd layer of screen,
> you can refasten with fewer screws.
>
> 9a) you might consider putting additional absorbers in the upper half and
> powering them with a pv fan down to the slab. the upper half already has
> windows, direct gain and heats easily as you have found.
>
> 9b) keep the snow in front of the wall. it is a great reflector 60% and
> increases performance significantly.  i simply laid down to 4x8 sheets of
> 1/2 inche refelctive aluminum coated celotex insulation board.  you can
> lean em up against the panels in summer to reduce over heat .
>
> 10) overall this is a very nice project well executed, and inspiring.
> that's why you get the free 10 suggestions of advice. all told they might
> double the useful gain.--drew gillett, p.e.

nick

10 sig=1.741e-09'stephan-boltzman constant
15 for t=.2 to .8 step .2'screen transmission
20 tg=40:ts=100:ta=200'initial values
40 gg30=4*sig*(460+(tg+30)/2)^3'radiation conductance from glass to outdoors
50 ggs=4*sig*(460+(tg+ts)/2)^3'radiation conductance from glass to screen
60 gga=4*sig*(460+(tg+ta)/2)^3'radiation conductance from glass to absorber
70 gsa=4*sig*(460+(ts+ta)/2)^3'radiation conductance from screen to absorber
80 num=180+30*gg30+ts*ggs(1-t)+ta*gga*t
90 den=4+gg30+ggs*(1-t)+gga*t
100 tg=num/den'glass temp (f)
110 num=240*(1-t)+tg*(1-t)*ggs+225*(1-t)+ta*(1-t)*gsa
120 den=4*(1-t)+(1-t)*ggs+(1-t)*ggs
130 ts=num/den'screen temp (f)
140 num=120+ts*(1-t)*gsa+tg*t*gga+225*t
150 den=2+(1-t)*gsa+t*gga
160 ta=num/den'absorber temp (f)
170 if abs((ta-tal)/ta)>.001 then tal=ta:goto 40'iterate to 0.1%
180 eff=(250-(tg-30)*2-(tg-30)*gg30)/250'solar collection efficiency
190 print t,tg,ts,ta,eff
200 next

screen        glass         screen        absorber      solar collection
xmsn (frac)   temp (f)      temp (f)      temp (f)      efficiency (frac)

.2            37.86897      99.60938      84.81229      .9106325
.4 (4 layers) 41.59563      102.5794      96.12272      .8678353
.6 (2 layers) 46.36161      105.8859      107.3936      .8127683
.8 (1 layer)  52.25522      109.4545      118.7363      .7439739




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