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re: solar panel
14 may 1998
georg.ruppert@joanneum.ac.at wrote:

> i am thinking of installing air collectors to the southside of my house. is
> there anybody out there with experience in design and implementations and/or
> can my point somebody to a comprehensive internet server?

i've built 4 solar air heaters so far.

the first was about 1' deep x 2' wide x 8' tall, covered with 2 layers
of polycarbonate plastic, and it reached 157 f in the winter.

for the second, i covered a steep south attic roof with a single layer of
corrugated ("dynaglas") clear polycarbonate plastic to make a 600 ft^2
skylight over a 24x32' attic space with wide pine floorboards and insulation
beneath. a couple of years ago, the plastic roof survived golf-ball-sized
hail with nary a scratch. the attic peak, about 11' above the floor,
reaches a temperature of about 130 f in winter, with no heat removal
or north attic roof insulation.

the third was a quarter-cylindrical lean-to sunspace 8x24x8' tall, made
with 7 $5 double curved wooden bows on 4' centers, each bow being 2 12'
1x3s bent to an 8' radius, with small 1x3 spacers and deck screws every
2' to hold the 2 1x3s together. the foundation is a pressure-treated 2x4
on edge (the largest expense--i might use some old tires filled with dirt
the next time) staked to the ground every 4' with 3' or rebar. the floor
is astroturf over a poly film vapor barrier. it has a single layer of
polycarbonate over half and a layer of uv greenhouse polyethylene over
the other half. it reaches 125 f.

the fourth was a 12x32x16' tall lean-to greenhouse made with $500 worth
of 24' long curved galvanized pipe bows on 4' centers, standard commercial
greenhouse components slipped over ground stakes with a pressure-treated
perimeter board as a foundation and a $50 shredded wood playground mulch
floor over black plastic, and a layer of greenhouse poly stretched over
the pipes, with 1/4" nylon rope straps over the poly between the bows
to avoid wind fatigue. it reaches about 105 f. 

an air heater can be a simple glazed picture frame over a dark house wall
with no side insulation, with a couple of holes to let warm air circulate
through the wall. i'm building a 4" deep x 32' long x 20' tall pvc pipe
frame like that with $300 worth of materials for a local newspaper. it will
have a layer of barn-red greenhouse shadecloth inside (to match the barn-red
building, an old barn conversion) and greenhouse poly stretched over the
frame and held on with ripped sections of pipe and sheet metal screws. it
will be vented in summertime, and shaded with vines, perhaps, and should
reduce the air conditioning load. i prefer air heaters with enough depth
to be usable as sunspaces. this improves their economics.

glazing options include uv greenhouse polyethylene film that costs about
5 cents per square foot, comes in rolls up to 40' wide and has a 4 year
guarantee, although it should last longer if shaded in summertime. it is
recyclable, but shipping cost is a problem. bayer's new "dureflex"
polyurethane film (412-777-3837) is clearer than greenhouse poly, costs
about 35 cents/ft^2, comes in rolls up to 15' wide and has a 10 year
pro rata guarantee. a single layer of replex (800-726-5151) flat or
corrugated polycarbonate glazing costs about $1/ft^2, comes in 49" wide
x 0.020" thick rolls or sheets, is 200x stronger than glass against hail
or children with baseball bats, and has a 10 year guarantee. sliding glass
door replacement panels cost something like $4/ft^2, need firmer mounting
and last forever, unless attacked by children with baseball bats, etc.

the air heater box might have a dark screen inside (or dark paint), with
an air gap on each side, with room air flowing into the box from a hole
in the bottom, rising up between the screen and the glazing, passing
horizontally from south to north (vv. in oz) through the screen, and back
into the room through a hole in the house wall near the top and behind
the screen. for better airflow, the screen might tilt towards the south,
close to the glazing at the top and close to the wall at the bottom. 

a screen makes the collector more efficient at higher air temperatures,
or with soft plastic glazings which have little "greenhouse effect," by
a) keeping cooler room air near the cool glazing, to lower convective
thermal loss to the outdoors, b) raising solar absorbing surface-to-air
heat transfer area, which lowers its temperature, which reduces thermal
reradiation to the outdoors, c) increasing solar absorptance, vs. a simple 
dark wall, by absorbing some of the sun on the way to the wall, and some
of the solar radiation reflecting back from the wall, and d) blocking some
thermal reradiation from the wall to the outdoors. linear analysis using
r-values shows no difference between screen and non-screen collectors, but
it seems to me that screens do raise the solar collection efficiency with
poly film glazing, since poly is poor at blocking longwave ir, unlike
glass and polycarbonate plastic. 

air heaters can be passive, with warm air moving by natural convection,
or active, with fans or blowers moving the air. they can also use air-water
heat exchangers, eg fan-coil units or automobile radiators used in reverse,
as found with pv-powered fans in applebee restaurants. steve baer suggests
that the depth of the air heater needs to be at least 1/15th of the height,
for good natural convection. when using a dark screen ("transpired mesh
collector"), making the gap from screen to glazing large and screen to
back wall small increases thermal efficiency. 

the dimensions of the holes at the top and bottom of an air heater depend
on its size. some people say "make each hole 2% of the surface area..."

here's an empirical formula to predict airflow by natural convection in a
chimney with no significant airflow restrictions: cfm = 16.6 av sqrt(h dt),
where cfm is the airflow in cubic feet per minute, av is the area of each
hole in square feet, h is the distance between holes in feet, and dt is
the air temperature difference (f) between the holes. in metric, airflow
in liters/sec = 205 av sqrt(h dt), where av is in square meters, h is in
meters, and dt is in degrees c (k), if i did that right. this formula
can be used to help estimate solar collection efficiency vs hole size.

one cfm with a temperature difference of 1 f moves about 1 btu/h of heat.
one m^3/s with a temperature difference of 1 c(k) moves about 1 kw of heat.
full sun is about 300 btu/h-ft^2 or 1 kw/m^2, and a single layer of glazing
might transmit 90% of that, so an air heater with an airflow of 1 m^3/s 
per m^2 surface would have a temperature rise of about 1 c in full sun.

if 20 c air enters the air heater, and it's 0 c outdoors, and the thermal 
conductance from air to outdoors is, say 10 w/m^2c, then air leaves the
heater at 21 c, the average heater temp is 20.5 c, and it loses (20.5-0)10
= 205 w/m^2, leaving about 900-200 = 700 w/m^2 of useful heat output in
full sun, for a solar collection efficiency of about 700/1000 x 100 = 70%.
with 0.1 m^3/s of airflow per m^2 of air heater surface, the temperature
rise is about 10 c, the average air heater temp is 25 c, and the loss to
outdoors increases to 250 w/m^2, reducing full-sun collection efficiency
to (900-250)/1000 x 100 = 65%, and so on. 

one might move air with a fan or blower in series with 2 thermostats (eg
grainger's $15 2e158 spdt thermostat wired as a cooling thermostat in the
air heater, in series with another wired as a heating thermostat in the
house.) with fans, it's nice to keep every air passage in the air heater
large enough so the air velocity at any point (ft^3/m/ft^2 of passage cross 
section) is less than about 500 linear feet per minute (about 2.5m/s), and
maybe 1000 through the hole, in order to keep air friction and electrical
fan power low and keep the cop (heat power moved/electrical power consumed)
high. moving less but warmer air raises the cop, but lowers efficiency, 
since the air heater is warmer, and loses more heat to the outdoors. one 
wants to avoid exposing the fan or blower motor to air temperatures above 
its specified operating limit. 

my attic air heater receives about 180k btu/h (53 kw or 70 hp of heat) in
full sun. i could move that heat down to the house with a grainger's $380
4c861 2764 cfm 16" 275 watt multifan, rated at 311 f. this would involve
making a large duct from attic to basement, with a return duct up through
the attic floor, combined with a motorized damper/skylight. here's one
equivalent thermal circuit, if the fan actually moves 2,500 cfm: 

               sun current source   attic temp t  fan heat removal
                    -----                 |         -----
               |---| --> |----------------*--------| --> |----> to basement
                    -----                 |         -----
                  180k btu/h              |       2500(t-70)
                                          |
              attic thermal resistance    |       (or 3200(t-70) for 
 outdoor temp: 30 f --www---------------<-        air-water heat exchange)
                   r1/600ft^2                    

if 180k = (t-30)600 + 2500(t-70), the attic temp t would be 120 f, and
the solar collection efficiency would be 2500(t-70)/180k = 69%.

another option for getting the heat down to the basement is air-water 
heat exchangers, eg old auto radiators or $150 all-copper 2'x 2' shw 2347
horizontal duct heat exchangers made by magicaire (817-767-8333), which
transfer 45k btu/hour between 125 f water and 68 f air at 1400 cfm, with
a 0.1" h20 pressure drop. four of these under a ceiling fan with a couple
of graingers $109 2p079 100 watt pumps to circulate about 20 gpm up from an 
attic sump through the heat exchangers and back down through a few gravity-
pressurized plastic 55 gallon drums submerged in an unpressurized tank in
the basement, ie a insulated plywood box lined with a single folded piece
of epdm rubber?

heat exchanger thermal conductance would be about 4x45k/(125-68) = 3200
btu/h-f, so 180k = (t-30)600 + 3200(t-70), and t = (180k+18k+224k)/3800
= 111 f, and the solar collection efficiency would be 3200(111-70)/180k
= 73%. hmmm. fewer holes in the house, less thermal loss on the way to the
basement, a free gravity-feed solar water heater, and a way to avoid living 
inside the heat store, which can then have a higher temperature, but
more corrosion, leaks, and freezing problems...

fred mcgalliard  wrote:

>nick pine wrote:
>...
>> one of the things that's kept me from using the attic heat is the
>> requirement to keep house air from migrating up into the attic at night...

>nick, have you considered capturing the heat with a water exchange,
>copper tubing and thin aluminum sheets, and then you only have to punch
>small holes in the roof, and can store the hot water below house, and
>use a simple temperature controlled vent to bring up the heat when you
>need it, since convection alone will take care of the house heating step?

the copper tubing would need a good thermal connection to the aluminum
sheets, eg a couple of drywall corner beads and some bolts. a 5' square
of copper pipes on 3" centers draped with some 6" upside down aerodynamic
us made from thick aluminum roof flashing below one of grainger's $236
4f426 41k cfm 160 watt ceiling fans with a 135 f motor temp rating would
have a thermal conductance of about 21x5ft^2(2+24mph/2) = 1,470 btu/h-f.
fin-tube copper, with tight-fitting 3" aluminum squares, as in baseboard
radiators, would have a conductance of about 21x5x5.45x7 = 4,000 btu/h-f.

a air heater with no fan needs more height and larger holes to achieve the
same efficiency as one with a fan, and the larger holes are likely to be
more difficult to seal against air leaks at night (with thin plastic film
one-way dampers or foamboard dampers with honeywell's 2 watt ml6161a direct
coupled actuators and thermostats) and allow more conductive heatflow at
night, and cost more to make, and take up more space. some people just push
a foamboard plug into the upper air heater hole every night...

a 4x8 foot air heater with a 6" slot at the top and the bottom might have
16.6(2ft^2)sqrt(8dt) = 94 sqrt(dt) cfm of airflow, transferring 94(dt^1.5)
btu/h of useful heat. in full sun, when it's 32 f outdoors, with a single
us r1 layer of glazing, and incoming air at 70 f, it would receive about
32ft^2x90%x311 = 8957 btu/h of heat, and lose about (70+dt/2-32)32ft^2/r1
= (38+dt/2)32 btu/h to the outdoors, so 94dt^1.5 + 32(38+dt/2) = 8957, and 
dt is about 18 f (10 c) and the useful heat output is 7178 btu/h, for a 
solar collection efficiency of 7178/(32x311)x100= 72%. looks pretty good,
but sealing the holes at night isn't easy, and one needs to take care to
make the airpath large and smooth, and this doesn't count reradiation
loss, if the air heater surfaces are hot...

with a 560 cfm fan, 560dt + (70+dt/2-32)32ft^2/r1 = 8957, so dt = 13.4 f, 
the useful heat output is 7526 btu/h, the solar collection efficiency is 
7526/(32x311) = 75.6%, the collector can be horizontal or vertical, the
airpath can have more twists and turns, the holes in the wall and dampers
can be smaller and less lossy at night, and room temperature control can be
more precise, at a cost of 36 watts of electrical power, with a cop of 61,
for grainger's $70 4c688 electronic cooling fan, which has an upper temp 
limit of 149 f, or their $49 4c671 gable mounted attic fan, which includes 
an adjustable cooling thermostat and thermally protected motor, and moves
about 1200 cfm with a 110 vac 3.4 amp electrical rating.

i'd recommend fans, unless one is a "passive purist." large slow fans can
be very efficient. grainger's $140 4f424 56" ceiling fan in a shroud near
the top of a sunspace might move 27k cfm through a window with 110 watts,
with a motor temperature limit of 105 f, for a potential cop of about
27kcfm(105f-70f)/3.41btu/h/w/110w = 2500. above 105 f, a thermostat might
turn off the fan to protect the motor and let natural convection do the
job. or the fan might somehow move cool room air into the air heater, vs
moving warmer solar heated air out. or an efficient large low-speed blower
might move air with a motor that isn't exposed to the airstream.

solar heat is free, unlike electrical energy, and as much as engineers
like high solar collection efficiency, it tends to lower an air heater's
cop and raise recurring and non-recurring costs... 

nick

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

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




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