Measuring Efficiency of Photovoltaic Cells and Temperature

Name: Tara S.
Status: student
Grade: 6-8
Location: CO

Question: Hello, I am inquiring about the efficiency of a photovoltaic solar 
cell which is functioning in different temperatures for my science fair project. 
My question is: Is the efficiency of a PV solar panel relative to the temperature 
it is functioning in? I have found many sources that claim that it is 
'widely known that the efficiency of a solar cell decreases with a higher 
temperature environment' but lots of other sources state otherwise. What is 
the reasoning behind this? Since any hypothesis I seem to make is not supported 
by any valid background information, what should I use and say to support it? 
With what I have found, it seems like the efficiency of a solar panel does 
decrease with temperature, but how could I measure efficiency? It also seems 
like my hypothesis is already tested, but I am not sure. I was thinking of 
replicating different temperatures in a secure environment (like a glass box 
that is vacuum sealed) that cuts off all the surrounding environment. Then I 
would place the solar panel inside along with a flashlight of white light to 
replicate the sun's light. What kind of glass box would I need to make? Or 
anything of the sort- would it work?

REFERENCES I consulted:
1. "Abstract:  It is well known that efficiency of photovoltaic solar cells 
decreases with an increase of temperature, and cooling is necessary at high 
illumination conditions such as concentrated sunlight, or cosmic or tropical 
conditions. The purpose ..."( 

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V50-4CS26GT-3&_user=10&_
coverDate=02%2F28%2F2005&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_
version=1&_urlVersion=0&_userid=10&md5=b718520d30ec6971904405c63260d1ab )

2. "Photovoltaic solar cells convert sunlight to electricity and can be understood 
at the most basic level as a Carnot engine, driven by the temperature difference 
between the sun (6000K) and the cell (~300K). In the past, extensive studies have 
been performed testing the performance of solar cells at elevated temperature, but 
none have considered the possibility of operating at a lower temperature 
(below 300K). The low temperature regime is interesting as the efficiency of the 
cell increases, due to a combination of (1) a fundamental increase in the Carnot 
efficiency and (2) reduction in the activity of electronic..." ( 

http://www.physics.usyd.edu.au/app/solar/about/projects.html) 

There are a few more, and I can only find the web sites- 

www.simplesolarsolutions.com, www.crh.noaa.gov ..
. 
I have done lots of research on the overall topic, and in many sites, like IEEE, 
you have to be a member to look at the abstracts, and those abstracts looked 
similar to what I needed, and I found something close if you see the aforementioned. 
---------------------------------------
This is a good question for a science-fair experiment.  You do not have to
have the correct answer in advance, or even to know if there *is* a single
correct answer for all possible kinds of solar cells, over all temperature
ranges, etc.  I think your background research is enough to go on, because
it suggests that efficiency depends on temperature.  You are not sure in
what way, so you make some measurements.

Now, to your experiment.  What are you measuring?  Efficiency is all about
how the output compares to the input, so you would like to measure both, while
varying the temperature.  If you cannot measure both input and output, then
you would at least like to hold the one you cannot measure constant, during the
experiment.  I will guess it will be easier to hold the input (illumination)
constant.  This means you do not want to change anything about the
illumination source, or the light path, during the experiment.

How do you measure the output of a solar cell?  You could give it some work
to do, and measure how much work it accomplishes over some fixed time, while
the illumination is held constant.

The most important thing about a science-fair experiment is *your* thought
process while you are designing the experiment and performing it.  You have
to notice and think about everything that happens during your experiment,
and you have to decide which things affect your measurement.  For example,
if the flashlight battery runs down during the experiment, the measurement
is probably going to tell you more about batteries than it tells you about
solar-cell efficiency.
--
Tim Mooney
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Tara,

I am not a PV expert, so to answer this question, I would have to look up
journal articles and report my findings to you. It sounds like you are more
than capable of doing this yourself, you just need access permission to read
them. So, perhaps I can give you some suggestions that can help you continue
to pursue your research interests on your own.

As you seem to know, most abstracts are freely available to the public; it is
the full text that is not free. To see the full text (for free), I recommend
you go to the library of a local university. Most universities will have
access from their computers to read the abstract and full text, and the
public can often use computer terminals for free. Call the library first to
make sure, just in case. Your university may have a search tool to help find
articles; if not, Google scholar (scholar.google.com) is good.

A more ambitious idea is to contact professors at nearby institutions who do
research in PV. They would more than likely be very eager to engage a young,
bright student like yourself. Many schools have outreach programs for young
people like yourself (my institution does, but it is far away from Colorado).
Alternatively, you may be able to sign up for a one-credit-hour independent
study course (may cost a couple hundred dollars, and depends on the
university's admissions policies). At a minimum this would give you access
to their library resources (including off-campus access to journal
publications), and may also allow you to do research directly with a willing
professor. It does not hurt to ask!

PV is a hot research area, especially with novel multilayer PV cells and
nanotechnology entering the field. It is tempting to rely on journal
articles, but do not forget to consult textbooks. A science library at a
university will have great, recent PV texts that will help you cover the
fundamentals -- like how temperature affects efficiency. You may find that
there is not a single answer; for some types of cells, the effect may be very
different than it is for others.

Hope this helps,
Burr Zimmerman
====================================================================
Hi, Tara.  

You are correct in that the efficiency decreases with increasing temperature.  
You can get the hard data for specific cells by contacting a cell and panel 
manufacturer such as sharp, BP solar, or Kyocera.  
 
To do an experiment to demonstrate the effect, you are also on the right track, 
except that the light source should probably go outside the box (most sources 
generate heat by themselves, which could throw off your experiment).  You need 
to deliver a controlled amount and intensity of light to the cell, and control 
the cell temperature.  Place a thermocouple or other temperature probe on the 
back of the cell (make sure it is electrically insulated and does not damage 
the cell when you apply it) to measure the temperature, and you will have to 
have some sort of means to deliver heated and/or cooled air to the cell to control 
the temperature.  It would be a good idea to exclude ambient light as well. 
 
To measure the efficiency of the cell, you need to measure the power delivered to 
it in the form of light and the power it produces.  This can require expensive 
equipment, but there are shortcuts.
 
1) You can measure the power input indirectly by measuring the electrical power 
delivered to the artificial light source you are using.  This will make the cell 
seem quite inefficient (they are only about 15-20% efficient anyway), because you 
will be adding in the inefficiency of converting electricity to light in the first 
place.  This can be fairly low for incandescent bulbs, and they produce a lot of 
heat, but they also produce a bunch of light wavelengths.  There are more efficient 
light sources, such as compact fluorescents and LED's, but they can be more 
restricted in the wavelengths they produce.  You'll have to experiment to see 
what works best.
 
2) To measure the electrical power delivered and/or produced, you need to measure 
the voltage and current, and it may be helpful to measure the time at a certain 
set of conditions.  Power is measured in Watts, which is voltage multiplied by 
current.  At the levels of a typical desktop-sized experiment, with a single 
small cell (or group of small cells), you may have trouble getting enough 
resolution and accuracy in the instruments to see the temperature effect over 
a small temperature variation.
 
Although there are difficulties with such an experiment (on a budget, anyway), 
it is quite doable.  Most importantly, have fun with it.

David Brandt, P.E.
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