[Darksky]monitoring light by PV power plants

[Jan Hollan]

Dear friends of night,

Thomas Posch got a wonderful idea during our discussions in Graz, where
he was my kind host during the Cancer and Rhythm conference (we had a
poster together there, on measuring faint light with PV panels): the most
sensitive and detailed data could be produced by solar power plants.

There are many stationary PV panels on roofs and facades, which are
monitored by computers all the day, for the power they produce at the
moment. Some are on the Internet online.

If the monitoring would be made at night as well, it would give excellent
numbers on the nighttime irradiation. (Millivolts would be a better
quantity than watts, but perhaps even those would be measurable.)

For getting illumination (photopic, scotopic, melatonin-suppressing) from
irradiation, the only need is to calibrate the system in a overcast night
without the moon (for urbanised, highly polluted sites) and a clear night
with the moon. PV panels voltage is proportional to illumination at low
light levels.

A crude assumption is, that the ratio of illumination/voltage is the same
for Moon and for man-made light pollution. The differences for HID and
fluorescent sources should be never over a factor of three (1 mV meant
some 3 mlx for moonlight and some 6 mlx of HPS light, for our small panel,
with 8 mono-Si wafers in a series).

Then no super-sensitive luxmeter (or a CCD camera) is needed for
calibration. My programme
  http://amper.ped.muni.cz/light/lun_illum.php
 can say the lunar contribution, if needed. Assume zenith extinction of 15
cmag for nights with rather clear air, to get numbers approximating the
global illumination by the Moon (direct and diffuse). Having a measurement
from a moonless night and a night with nearly full moon should be enough
(for places where moonless nights are still much darker than full moon
ones). (Scotopic is some 2.5 times the photopic illumination, for white
light as moonlight.)

If you know some people or institutions with panels for solar electricity,
try to move them to start such measurements.

In fact, it may be a good reason, why to install such a power plant at
home... the solar gain during the day would make it a scientific measuring
device which would pay for itself over decades... or at least produce the
electricity needed for computer monitoring.

happy All Saints,
  jenik