Subject: Tracking houses

From: [email-masked] (Nick Pine)
Date: 30 Mar 1996 08:18:57 -0500

A PV friend says: 

>I've been dreaming of suspending an array in between two trees
>at about the 90' level...

With ropes going east and west? That seems interesting, 3/4 up those tall
trees 300' to the south of your house, with some nylon ropes running north
and south to the ground from the bottom? Seems like this would usually lead
to shading by other trees, if not falling branches, etc. I would like to
build a treehouse, a large one for living in, even. I traded some letters 
with serious treehouse people years ago, one a prof in North Carolina.

Suppose the PV structure had some internal volume and a floor, with a rope
ladder for courageous children attached to safety ropes. We might insulate that
volume and put some thermal mass inside, eg a few batteries surrounded by an
air gap and insulation, to capture some of the other 90% of the sun's heat...

For two axis tracking, suppose it were supported from above at three points,
with a fixed rope at the top on the north side going up and north to a tree,
another at the north bottom center going to the ground, and two more going up
SE and SW from the EW upper south corners, said ropes being attached to two
small DC reversible winches or garage door openers inside the treehouse. A
rope ladder hanging from the south side deck would make this more stable. To
point east, crank in the west rope and let out the east rope, and vice versa
to point west. To tilt the array upwards, crank in both ropes. To tilt it down
towards the south horizon, let out both ropes. Picture a 4' x 4' x 8' shed,
with a reflective front porch and a 1:2 roof sloping up to the south and a
reflective overhang. It might look like this:

         from the east                from the south front

     \         8'                \             12'           /
       .                           .........................
       .   .   ^                   . .                   . .
       .       .                   .   .................   .
       .       .   .               .   .       8'      .   . 6'
       .       .       .           .   .             4'.   .
       .       .     PV. 2'        .   .      P V      .   .
       .................           .........................
           4'      4'    \                     l
       l                                       4'
                                           .........   
       l                                  .    PV   .
                                         .           .
       l                                .             .
                                       .       .       .    8'
       l                              .                 .
                                     .                   .
       l                            .                     .
                                   .........................
       l                         /                           \
                                          from the top

This would have a PV area of 8 ft^2 and a frontal area of 72 ft^2 with
a concentration ratio of 9, so the peak sun power would be 6.5 kW. How
would we cool it? Perhaps make the PVs the back of an thin aquarium tank,
with the sun shining through a little water, and a radiator above.

How big a radiator? Say the PVs collect 650 watts peak, with a solar energy
input of 5.5 kWh/m^2/day in July in Seattle, ie 36.8 kWh of solar energy in,
and 3.68 kWh/day of electrical energy out. The difference is 33.1 kWh/day.
That is the heat we need to deliberately waste. The average temperature in
Seattle in July is 18.4 C. Suppose we want to keep the average PV temp
at 28.4 C. The average windspeed is 3.5 m/s, or 7.8 mph. So assuming we
have enough water to store the heat for 24 hours, we have to waste 33.1 kWh
of heat, about the same as the heat in a gallon of oil, in 24 hours, with a
10 C temperature rise, or 138 watts/degree C, or 261 Btu/hr/degree F. A smooth
174 ft^2 surface or rough 130 Ft^2 surface would do that with no wind, as
would 44 ft^2 of rough surface exposed to the wind, or the wind from an
exhaust fan to the north on a calm day. Or we might try to evaporate about
0.005 gallons (0.7 oz.) per minute of water by slowly pumping it up to the
ridge peak and letting it trickle back down into the tank, or let some
rainwater flow thru the tank. 

How much water would we need to store that heat? 33.1 kWh will heat 6720
pounds of water 10 degrees C. Perhaps we should try to get rid of most of
the heat in real time, and cover the backs of the PVs with fins and such. 

It would be interesting if we could turn this structure around to the north,
swing the PVs out of the way with a linear actuator, say, and scoop up some
wind to flow through the exhaust fan at night, or on a cloudy day, to make
more electrical power.

With a couple of batteries inside and an inverter, we could run 110 or
240VAC wires up the north rope and back to the house. 

The rope ladder might not touch the ground. It might just end in a large
weight, say 3' above the ground, to make a large and interesting pendulum
with a period of about 5 seconds. If the rope ladder slanted up to the
treehouse from the south, we might add two more ropes to the ground to the
NW and NE and enclose the EW sides of the tetrahedron between the treehouse
and the ground, eg with dacron sailcloth, or burlap/cement, and put a wind
turbine on the ground at the south lee corner, just north of the base of
the rope ladder, to make a big north-facing windscoop (and snowscoop--we
might make the EW sides transparent, or enclose the north side too, and make
another house, barn, or solar closet and sunspace underneath, parking a car
or a tractor under the PV shed, or storing some hay or animals... 

A PV shed on top of a flat roof in a city might be suspended from the point
of a tetrahedral frame made with 6 poles, with some sandbags at the corners.

Suspending a real solar house from a 3-cornered manmade arch this way would be
interesting. It wouldn't have to be very high off the ground to let the house
track the sun, altho a 3-cornered St Louis-type arch would be nice :-), and
that might also serve as the house foundation... Electric and water might
enter the house with flexible lines from the top, with the house rotating no
more than 360 degrees, perhaps facing north on summer days. 
 
It might be less expensive to suspend the house from 3 telephone poles,
like this, seen from the top:
                                          s
                                          .
                                          .
                                          .
                                          p
                                          .
                                          .
                                          .
                                          H
                                        .   .
                                      .       .
                                    .           .
                                  p               p
                                .                   .
                              .                       . 
                            s                           s

and like this from the side:      p       p       p
                                . p   .   p   .   p .
                              .   p       H       p   .
                   .........s.....p.......p.......p.....s............
                            s                           s

The poles might each hold up about 30K lbs from the weight of the house
and another 30K pounds or so from the guy wires, so each pole would need
a 30 ft^2 footing, perhaps, and at 50 K psi, the steel cables would need
to be about 1" diameter. Big nylon or dacron ropes might work instead of
the steel cables. The wood of the poles would probably hold the weight in
compression parallel to the grain, but the poles should probably be cemented
on the outside, over wire reinforcement, to make them more durable and
increase their polar moment of inertia, so they aren't so inclined to buckle.
Tilting the poles out a bit would make the guy wires smaller. Tilting them in
so they touch would help too, but that doesn't leave much room for the house.

Althernatively, the house might be held up by a geodesic telephone pole
structure, 20 poles, each 30' long (connected by what, ropes thru holes in
the poles? Bolts, and steel plates. Lots of chicken wire and cement?) This
would be about 150' in circumference, about 50' in diameter (if the poles 
poles were curved and pi were 3) and about 40' tall at the central point.
Adding 5 more poles on the ground would solve the footing problem, and make
a nice berm for the EPDM rubber reflecting pond and wastewater treatment
facultative lagoon, with a small fountain. Enclosing the whole thing in
polyethylene film might be nice. Un gout special.

What would the building inspector say? Perhaps not much about safety,
if the house were only 6" above the ground.

What would the architect say ("THAT doesn't look like a house!" :-)

What would the health department say?

What would the neighbors say?

What would the bank say?

Or... the house could be floated on a shallow pool. A 30 x 30' two story
house might weigh 100 psf, requiring a 2' deep pool, taking some or all of
the load off the telephone poles. 

We could try this out with a prefab 8 x 12' shed, with PVs mounted on the
south wall, a reflective south deck and an overhang, and reflective wings,
sitting in a pool made with a 20' x 20' piece of EPDM rubber over a 2' tall
perimeter earth berm. The pond corners might point to compass directions,
and the shed might have a 12' rope from the center of the south wall to the
south pond corner. We might mount two winches or garage door openers at the
EW corners of the north wall of the shed, high up, connected to ropes to the
EW pond corners, for treehouse-type two axis tracking, tilting the shed back
to the north to point it up into the sky. The shed might have a waterproof
skirt coming 2' up the walls on the outside, made from a single piece of
EPDM rubber. So might a modular home... With all that water around it,
PV cooling should be easy. One might even want to solar heat the house.

>The sun hits the tops of the trees at 6 am or so, and doesn't hit the PVs
>I have now for several hours after that. It just becomes a question of how
>much it costs in money and aesthetics to put them up there, and how much
>more vulnerable they become.

A small PV tracking treehouse, 90' up, could be delightful. It might have
a few mirrors or a bit of stained glass to glint onto your house, 300' to
the north. Some wind chimes, perhaps, or some propellors and generators on
the guy wires. With the right combination of strong and elastic ropes and
mass and damping, vulnerability should not be a problem. It wouldn't be very
expensive either. Small DC winches cost less than $200. Some people use
$150 Sears radio-controlled garage door openers to haul out boats. 

Nick