solar assisted interior lighting SOLAR.KnowledgePublications.com

solar assisted interior lighting
1 mar 2002
i'd like to put one of duane johnson's heliostats (www.redrok.com) into
our uu church art show near phila, opening 3/23. it would be about 20' from
the west wall of the sanctuary (which has no north windows.) it's a $300
collection of stuff including a 4'x4' flat mirror (mylar greased to plywood)
on a universal joint with 2 pv-powered $40 dc gearmotors with led sun-image
feedback sensing that constantly shine the sun (if any) onto the ceiling
through a quarter-cylindrical 3' radius window about 10' above the ground. 

the 30'x50' sanctuary has a grey carpet floor and white cathedral ceiling
and walls and 150 ft^2 of south glazing, with 50 ft^2 on east and west walls
and 16 75 w incandescent ceiling floods and another 8 150 w incandescents on
the walls, 2,400 watts altogether (perhaps the building commitee owns stock
in peco :-) these are controlled by 4 dimmers which are partially on, even
on sunny sunday mornings. so far i've been unable to find reasonably cheap
automatic light-sensitive dimmers that will fit in a standard wall box and
maintain constant illumination by dimming the incandescents in brighter sun, 
although i've been seeing this frugal function described in print for at
least 30 years, since nasa tech briefs in the 70s.

today i modified a lutron s-600ph-wh 600 watt dimmer ($19.98 at home depot)
by attaching an extra wire (a) to the junction of the 10k resistor and the
diac and the 0.25 uf cap to bring out the trigger signal along with the
existing common black wire (b) connected to the junction of the 3 0.25 uf
caps. then i modified an intermatic 1800 w k4221c photocontrol with swivel
mount ($9.76 at home depot) by soldering a wire across the 9.1k resistor
in series with the cds photoresistor which varies from 75k ohm when dark
to about 1k in 30 fc. 

with this mod, the photoresistor appears across the external red (r) and
w (white) wires, which i joined with wire nuts (respectively) to wires a
and b. i mounted the photocontrol in the top knockout of a rectangular
extension box which can be screwed to the original dimmer wall box and
aimed at the heliostat spot on the ceiling, with the modified dimmer on
the face of the extension box. this circuit works, although the effective
slider pot travel is reduced by about 50% and the regulation from darkness
to full sun can be improved, and it flickers slightly if the photosensor 
is aimed at the light controlled, or at low dimmer levels. the latter
improves with a 0.1 uf cap in series with the photoresistor. 

lutron makes a $2,000 "dac-pi" system to do this function, inter alia.
it would be very nice if lutron or leviton made an inexpensive product
like this. it seems time. they might even make and sell heliostats... 

duane figures 4'x4' is overkill here, at 1 kw incandescent per square foot
of mirror. he has in mind making smaller, cheaper self-powered heliostats
for home use with something like a 4" or 1' square mirror. what do you all
think of the prospects for selling them? they would compete with "solatubes"
which are less-efficient and more work to install, with holes in the roof
and attic floor, vs moving standalone lawn sculptures...

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                           (610) 489-1475/0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high performance, low cost, solar heating
and cogeneration system design. bsee, msee. senior member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick