Calculating Efficiency of a Solar Panel

[ShannonE13]

Is there a section with step-by-step instructions on Science Buddies showing how to calculate the overall efficiency of a Solar Panel using a digital multimeter?

If not, could someone please explain how to do this.

The Solar Panel that is being used is rated at 6 volts, 50mA.
The motor that the panel will be running is rated 6 volts, 100mA max.

Shannon

[deleted-71588]

how to calculate the overall efficiency of a Solar Panel using a digital multimeter? ...If not, could someone please explain how to do this?

Your first two questions assume it is possible to do so. Lets evaluate whether that is possible.

Determining the efficiency of anything requires an evaluation criterion. For energy conversion devices, an evaluation in absolute terms ususally involves determining how much energy is utilized vs how much is wasted.

Given that your solar panel has an electrical ouput rating, the output is obiously electrical energy. By definition, solar implies that the input is the energy from the sun.

A typical digital multimeter can measure voltage, current, and resistance, so you should be able to devise a circuit that can use a DVM to measure the electrical output of the solar panel.

Expecting a DVM to directly measure solar energy arriving at the surface of the solar panel is wishful thinking because the units of measurement do not agree. You need some known "transducer" or "converter" that will convert solar energy into electrical power. But that is by definition an Electrical Solar Panel, so you have circular problem (which came first, the chicken or the egg?).

If you had two different solar panels, you could evaluate which is more efficient by comparing them.

So what are you really trying to do? You indicate you have a Solar Panel that can produce 6v @ 50 mA under some solar input. You indicate you want to power a DC motor that is rated at 6v @ 100mA max. Based upon my experience, If you just wire these two devices in series, I have no clue on whether the motor will turn or not. The starting current of a DC motor is typically the highest current demand so it will be close to 100 mA (I'm assuming the 100mA max means locked rotor current). Your Solar panel isn't rated to provide as much current as the motor probably needs to start.

[deleted-71882]

ShannonE13,

Craig_Bridge gave you a good discussion of the problems in measuring the efficiency of your panel. He's right that measuring the output of the panel is not easy, and so far as you have indicated, you have no way of measuring the solar input.
On the other hand you could get a very rough indication of the efficiency by searching for solar intensity measured by others. A quick search found http://replicator.phenix.bnl.gov/WWW/pu ... iation.htm for example. The intensity will depend on your latitude, weather conditions, time of year, the orientation of the panel, and maybe other factors. Your estimate (I won't call it an actual measurement) of the panel efficiency would be just the amount of power you can get out divided by the solar power input.
Let us know if you have any other specific questions.

[ShannonE13]

Thanks for all the help. My son tested the efficiency of a Fuel Cell and compared it to the efficiency of an ICE last year. This year he wanted to continue his project and compare the Efficiency of his Fuel Cell to that of a Solar Panel. So he puchased a Solar Panel and Battery (as listed above in the first post). Now he just needed to test the efficiency of the Solar Panel and compare those results with the results of his fuel cell from last year. Only, as mentioned above, it is not proving to be as simple as he thought it would be.

I will print off your comments and let him review it.

Plus, I am going to try and set up an appointment with someone in the field to help him.

Does anyone know of a good place to call that works with Solar Panels and might be able to assist my son, or maybe teach him more about Solar Cells/Panels?

[deleted-71588]

It is VERY hard to fairly compare the efficiencies of two systems if the inputs and outputs are not equivalent. Fuel cells and solar panels have considerably different inputs so it is like comparing apples to oranges. They are both fruit that grow on trees and both have vitamin C, but they grow in significantly different climates.

[ShannonE13]

Okay, any help would be great. My son, Craig took back his solor panel and motor. He purchased a Solar Kit that has several items that the solor panel included in the kit can power. This kit has a motor that can be powered by the solar panel, a light bulb, and a small radio.

His original hope was to determine the efficiency of the Solar Panel, but in reading the previous post, he now understands that he is not going to be able to do this without being able to calculate the Sun's Energy that is going into the Solar Panel (the input energy into the panel). There are ways to estimate this, but it is not exact. He wanted to continue his project from last year, but it seems this is not possible.

Craig is wanting to just stop, and do something else. He said that he should just pick something easy and get it done.

I personally feel like Craig has done much research on Solar Panels. He has written out notecards, a rough draft on the background informaiton and outline of his project.

Craig learned all about Energy, Input, Output, efficiency formulas, and circuits from last year. At this point he has spent way too much time on learning about Solar Panels to just drop this project completely. Could anyone help? Should he just put it all together and say that unfortunately he learned that he will not be able to do the calculations, because he is not going to be able to determine the Sun's Input Energy with his limited knowledge and equipment? And that comparing a Solar Panel to a Fuel Cell is not really realistic? Or should he take what he has learned and run a different experiment? Does anyone have any ideas? His complete project is due on November 9th. So he has this week and next weekend to complete everything and to write his final paper.

[deleted-71882]

I'm sorry to hear that this project has been discouraging, but I understand the feeling since I experienced it in trying various ideas when I was learning. It's disappointing, but it's also a good way to learn.
I don't have a really good idea for salvaging the project, but here's what comes to mind.

How about measuring the amount of power that you can draw out of the solar panel under varying load conditions? I am assuming that you have a small solar panel that puts out only a watt or so.
Get a variety of resistors (Radio Shack should have them).
Connect a resistor and a multimeter across the wires from the solar cell. Expose the solar cell to direct sunlight or to a very bright lamp (maybe a halogen lamp with a reflector).



For different resistors, record the resistance and the voltage across the resistor.
The power coming from the cell is given by two relations.

1) I = E / R, (this is Ohm's law)
where I is the current flowing through the resistor,
E is the voltage across the resistor, and
R is the resistance of the resistor.

2) P = E * I
where E and I are the voltage and current as in 1), and
P is the electrical power.

You should find that the amount of power is small for high resistance because very little current can flow and the power is low when the resistance is low because most of the voltage is lost inside the cell. The power should peak at an intermediate resistance.

Even this variation of the project has plenty of potholes to navigate. You will need to learn a bit about electric power to understand Ohm's law and the power relation. You will need to find resistors that have the appropriate resistance values for your solar cell. Perhaps you could read a bit about how the output of solar cells is handled so that it can eventually connect into the public power grid and make some comments on how your experiment relates to that issue.

Above all, choose a path that gets some project finished by the deadline. Even if it's very simple, a finished project is better than an unfinished but complex one.

Good luck.

[ShannonE13]

Thank you again for your reply. Craig is thinking about what you have said. He is going to show his science teacher everything he has so far. He took measurements of the Solar Panel during peak sunlight hours with clear skies, and cloudy skies. He created a circuit consisting of a 1.5 volt Solar Cell, a motor that came with the kit to be run by only 1.5 volts, and a .47 ohm resister. Then he connected the solar panel and took readings of the current/voltage and resistance (I am not sure how to say this, Craig understands circuits and how they work). I wrote down the numbers from each reading. Craig noticed that in direct sunlight the solar panel was able to run the motor, but with cloud cover the motor did not run; however, there was still an electrical current.

He is going to find out from his teacher if there is another way he can use his data. Craig really likes what WendelWiggins suggested and might go that route. He was also thinking of just comparing the Pros, Cons, and the data collected of both his Fuel Cell and his Solar Cell to determine which "Green Power" is more reliable. Either way, I think we got him lookng in the right direction.

The rough draft of everything is due on Nov. 10, but the actual final completed project is not due until Nov. 17. Craig has already completed a rough draft of the outline, and a rough draft of the background which would still pretty much be the same, so I am hoping that his teacher will give him an extension for the rough draft of the completed project. Plus, I am hoping that his teacher will help him this week. They were supposed to have after school labs to help out last week, but his teacher was sick and then Craig himself got the flu. At any rate, this was a learning experience for him.

[deleted-71588]

Your son's findings that the solar panel was unable to turn the DC motor in cloudy conditions points out some properties of both the motor and the solar cell that can be investigated.

Electrical engineers model things that output electrical power either as:
1) An ideal current source and a series resistor, or
2) An ideal voltage source and a series resistor

For things like solar cells, these internal resistance values may NOT be constant. To characterize these kinds of devices using the ideal voltage source model, you have to measure the open circuit voltage and using the previous external circuit diagram, plot the voltage across the load as a function of the load resistance. These plots are typically used for "load line" calculations.

If you have access to an incident light meter (like what some photographers use), you could measure the visual spectrum light intensity right next to the solar cell and show how the solar cell behavior changes with light intensity. In order to make this easier to have consistent lighting, you could utilize an adjustable high intensity lamp as your light source.

To make this a scientific experiment, you would have to come up with a hypothesis.

The DC electric motor is a dynamic load. To characterize it, you would need a lab style power supply where you could control the amount of current and voltage supplied, and a means of measuring the speed and torque produced. In other words a solar panel and motor circuit have two complex components that involve energy transfer.

[ShannonE13]

Dear Mr. Bridge and Mr. Wiggins,

On behalf of my son, Craig, I would like to say thank you for all your help. He only had a total of 9 weeks to complete his Intel ISEF Middle School Science Fair Project, from start to finish. Personally, I felt like the school was not giving them enough times to do the Intel ISEF. My feelings have been validated when I found that most students get 30 weeks to complete this Science Fair.

It was a very stressful process for him, with his advanced course work load, youth group activities, community services hours and science fair all at one time. He enjoys science and would love to do harder projects, which I cannot help with. Craig has stated that he plans on starting next years science fair in January giving him time if he finds himself stuck, and requiring expert assistance.

As for this year, Craig decided to only run the electrolysis process in the fuel cell using his solar panel. He collected the average input voltage, average input current, time it took to electrolyze the water into hydrogen and oxygen data, and compared those results with the results from last year using batteries. He found that the average input voltage was much lower with the solar panel, the average input current was a little lower, and the time to electrolyze water took much longer. It took the solar panel an average of 29 minutes, and the batteries an average of 2 minutes. He found that even though the solar panel was rated at 3 volts, it only put out 1.5 volts even during full sunlight. There is much more to it than that and he wrote it all out in his paper, but this is the part that I understood.

Anyway, thank you so much for your help and support. If you know anyone in the Pasco County Florida area who might be able to give Craig more guidance in this field, please let me know. This is just not my field of expertise.

Shannon