Measuring power. What resistor should we use?

[Toontron]

Greetings,

We are students from Belgium and are doing our final project. We have to examine a solar panel. We received a SDM-170/X-72M solar panel.

Electrical data:
Pmax: 170W
Vmp: 35.8
Imp: 4.76A
Voc: 43.6V
Isc: 5.25A

We like to measure the efficiency of this solar panel in function of the temperature, so we can see that the efficiency drops with increasing temperature.

We want to measure the output voltage but we are not sure what power resistors we have to use.

If we calculate Vmp/Imp we get 7,5ohms. This seems too few. Maybe we destroy our setup if we have 5A running through the system?

What kind of power resistors should we use? Is it safe to hook up a 10ohms power resistor for example? We don't want to destroy the solar panel ofcourse.

We will be using a 250W or 500W lamp, so the output will be much less than the datasheet describes.

Regards,

Toon and Benny

[Sunking]

Fist you cannot destroy the panel by opening or shoring the circuit. In fact shorting the output is one of the test called Isc.

As for the resistor size there is not a direct answer as it is dynamic and depends on the temperature and radiance. The optimum resistance is the value at which occurs on the knee of the IR curve. This is the whole concept of maximum power point tracking algorithm to find the optimum resistance. So the Vmp/Imp of 7.5 is close but only at STC conditions.

Look at the IR curves and get the range of resistance, then bet your hand on say a 0 to 16 ohm 100 watt resistor.

[Mike90250]

Look at the IR curves and get the range of resistance, then bet your hand on say a 0 to 16 ohm 100 watt resistor.

If you have a 170W panel, I would use at least a 200W resistor. The resistor may need a heat sink or fan to keep it cool.

[Sunking]

If you have a 170W panel,

Oops my bad, I thought the panel was 70 watts.

[Toontron]

We will test a range of power resistors and look up which resistor gives us the most power.

Sunking, your advice was to create a IR curve. With google I found something similar:

http://www.reuk.co.uk/Measuring-the-Power-of-A-Solar-Panel.htm

Here they create a VI curve and they also choose the knee as their operating point. I will use their data to create a IR curve and see how it shows up.

I will take mike's advices and use a 200W resistor just to be safe I guess.

Thank you very much for the fast replies. Now I don't have to be scared to undertake some steps.

Greetings

[Mike90250]

I think if you got 6, 50 ohm, 50Watt resistors, you could use those to build and alter, a resistor bank, from 8.3 ohms (6 in parallel) to 300 ohms (all 6 in series) using combos of series and parallel, you can get a good number of resistance values, and still have resistors of a managable size.