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Posted by mike wilcox on August 1, 2005, 2:40 pm
 


Anthony Matonak wrote:

To hell with the desert, chuck the panels on top of existing real
estate, some of these shopping malls are a 1/2 mile square. The roofs
are already 120F during the day, make hot water as well as electricity ;~)

Posted by Jim Baber on August 1, 2005, 4:04 pm
 
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Jim Baber responding to Anthony's post:

Anthony Matonak wrote:


Anthony, I don't have any proof other than my own empirical results, but
when I installed over 1625 sq. ft. of solar PV panels on my own roof,
the attic temperature at a point 4 feet below the roof's ridge line
dropped from 145.8 to 125.5 degrees. The measurements were taken on two
different days a week apart when the actual outside temperature
registered the same 103.4 degrees at 5 PM.  I had been curious as to the
effect of the shading by the solar panels on my air conditioning power
load.  This was without the panels being turned on.  The next day the
panels had been turned on (testing) and the outside temperature @ 5 PM
only reached 102.7, but the attic temperature was even slightly lower
yet and that thermometer showed just 120.7 degrees. The assumptions I
made then, and I still believe them to be correct, are:

   1. The panels reflective surface reflected some infrared (heat)
      outward.  Solar panel manufacturers try to reject heat, since heat
      degrades the power production of the panels.  Solar panels are
      designed to convert certain wavelengths of light to electricity.
      They do not generate power from the heat of the sunlight.<>
   2. <>The 4 inch standoff from the roof by the panel mounting hardware
      allowed some airflow to cool the surface of the roof.<>
   3. <>The panels shaded the house just like a tree would.


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Posted by darthpup on August 1, 2005, 5:17 pm
 The Germans recently installed a one megawatt PV system in Bavaria.
I can imagine the engineering problems with clouds passing over such a
large array and causing surges.  A one gigawatt PV system may be a
major engineering problem.


Posted by Jim Baber on August 1, 2005, 6:47 pm
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Jim's comments:

darthpup wrote:


The whole system will not be constructed in a single circuit.  If they
follow the techniques of SMA, a major German supplier of PV inverters,
the system will be broken down into many 15 to 20 panel circuits that
supply 400 - 500 VDC to inverters that will convert that to appropriate
AC output voltages.  This means that broken cloud obscuration will be
localized to very small part of the total array's panels, and in fact a
single inverter could receive input DC from multiple panel groupings
separated by 200 to 300 hundred feet since, the line loss on the high
voltage DC is not nearly as significant as it is on the low voltage DC
that most typical solar PV systems utilize.  My own 10 kW system has
1600 sq. ft. of panels, divided into 4 groups of panels wired in series
each producing about 375 VDC at  4  inverter inputs.  1 group is almost
200 feet from the inverters, while the closest is about 75 feet from the
inverters, Yet there is less than a 3 VDC difference between any of the
circuits (that is actually less than the allowed tolerance for the
panels themselves). All are wired with 10 AWG solid copper wire, and
that alone is a lot cheaper and easier to work with that the much larger
wire needed for a typical 48 VDC system.  My panels frequently will have
clouds obscuring some part of them but the whole is not affected much.


They would just need transformers to gather multiple inverter outputs
and boost the voltage for transmission ease and efficiencies. You must
remember that the exact same transformers the utilities use to drop
their high voltage transmission lines down to your homes needs will also
step the output 120 / 240 from these inverters all the way back up to
the typical 200,000 to 500,000 VAC without any modifications needed.

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Posted by Ed Earl Ross on August 1, 2005, 6:04 pm
 Whenever power is generated by PV on the roof and used in the
house, the heat is moved from the PV via electricity into the
house. With refrigerated air conditioning, the heat is ejected to
the outside. The net heating from PV power should be about
zero--neither cooling nor heating.

Any overall temperature change must be due to reflection and
radiation that is different than reflection and radiation from the
original roof. All energy that is not reflected eventually becomes
heat, either at the PV site or wherever electricity it generates is
used.

Roof reflectivity varies, depending on coating, from about 20% to
80%. A PV system on a dark roof would help cool the house more than
one on a silver or white roof.

Jim Baber wrote:


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