Posted by daestrom on August 26, 2006, 3:43 pm
Indeed it does. When you look at the I-V curve of a solar cell, the short
circuit current is almost perfectly linear with the light hitting the cell.
Keeping the circuit resistance low so that no voltage develops improves the
linearity. Feeding the cell to an analog meter movement requires that some
voltage develop across the cell output and that could reduce the linearity.
Some simple op-amp circuits can avoid this so that the cell 'sees' a zero
voltage across it's output at all times (similar to 'precision rectifier'
circuits, but not exactly).
Calibration may be a little tricky. That is, when the cell puts out X mA,
how much power is falling on a 1 m^2 surface? One can *assume* (and you
know what that makes U and ME :-) that full sunlight output on the cell is
equal to a full solar 'standard', and that solar heating would work linearly
with the cell output.
In other words, once you've established that X mA is output by the cell when
Y BTU/hr are falling on the collector, you have a calibration constant of
Y/X. And variations from that ratio can be reason to investigate
performance. But how to tell the initial ratio of Y/X is 'good', 'bad', or
'mediocre' is the problem.
But of course, installing a cheap 'solar meter' like this *before* a
complete solar installation can be very useful. Hourly logging of output
would provide great insight as to how the weather at your site might impact
system performance. Could be much better than 'averages' taken from NREL.
Heating season is coming on, I might just add one of these to my house
data-logging system and see where it leads me :-) One side of my house is
shaded by oak trees that lose their leaves in the fall. I've thought a
vertical panel there would work well. Plus the single story house to the
south would be covered with snow for more 'reflected' sunlight. I may have
to put a tiny 'solar cell meter' up on the second floor of that south face
to see what kind of performance I can get. The trick might be to compensate
for temperature affects on the electronics.
Posted by SJC on August 26, 2006, 3:56 am
solar radiation levels for checking solar hot air
available on a given day to compare panel output
Radioshack sells solar cells and small meters.
http://www.radioshack.com/family/index.jsp?categoryId 32305&cp 32058.2032235
Or you could buy an inexpensive light meter at a photography shop or online.
Posted by nicksanspam on August 26, 2006, 8:13 am
PE Howard Reichmuth warns they may not have an appropriate spectral response
for solar thermal applications.
Posted by schooner on August 26, 2006, 10:05 am
That is what I thought/read as well. A light meter is not the same as a
solar radiation meter correct?
Posted by nicksanspam on August 26, 2006, 11:58 am
A light meter might only respond to the visible part of the spectrum,
and the spectrum changes with weather. Howard recommends some sort of
calorimetric device, for thermal applications. Let the sun heat something,
and measure the temperature change. That doesn't seem to do electronically.
Steve Baer uses a Raytek IR thermometer to measures the temp of 3/4" of
water in a lightweight metal pan with 4 Btu/F-ft^2, which would initially
increase at 250/4 = 62.5 F degrees per minute in 250 Btu/h-ft^2 "full sun."