Nuts and Volts - August 2009

A/D converter receives a voltage of 100 millivolts that is well within the limits of a good A/D converter.

For our experiments, it was decided to save on the cost and complexity of a voltage amplifier and go, instead, with a one ohm sense resistor. If the actual load resistance is also low (like 10 ohms, or less, as in most of these experiments), there is ample voltage drop across the resistor — especially when applied to a 10- or 12-bit A/D converter which can measure down to 4. 88 and 1.22 millivolts, respectively, with a + 5 volt reference.

The downside to using a one ohm load resistor is that it adds to the load itself. So, for a 10 ohm load and a one ohm sense resistor, the total load is actually 11 ohms.

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Experiment #2: Determining the Maximum Power Point

This experiment demonstrates how solar panels in either series or parallel have a maximum power operating condition known as the Maximum Power Point or MPP. The maximum power point is where the solar panels can deliver the maximum power into a load. MPP is a dynamic condition that varies based on external influences such as light intensity, tilt angle, heat, and either a series or parallel arrangement of the panels. You are shown that the MPP is achieved when the resistance of the solar panels matches the load resistance. You will discover this when you vary the load resistance to produce maximum power with solar panels in series and parallel configurations. Satellites in space must constantly adjust their solar arrays to acquire the MPP, which is why this is such an important concept to understand.

Procedure:

Wire the solar panels in series and adjust the 100 ohm potentiometer from full resistance to a lower resistance until the maximum power is reached. Continue to adjust it for lower resistance to see what happens. Then, rotate it back to the maximum power level. A plot similar to the one in Figure 9 is displayed. Notice the large dip in voltage, current, and power as the heaver load resistance overwhelms the solar panel’s ability to source power.

Next, wire the panels in parallel and repeat the same procedure with the potentiometer. While the voltage is noticeably lower due to the parallel wiring configuration, the voltage does not dip as much and the output current and power remain much steadier under the varying load (Figure 10). Thus, solar panels in parallel have a better means to supply more stable power over varying loads as compared to an equivalent series arrangement.

Therefore, be aware of the fact that the resistance values displayed on the computer are the “sum” of the one ohm sense resistor plus the actual load resistance.

Conclusions

This first brief look into solar photovoltaic experiments gave you a glimpse into what desktop solar panels are capable of doing and how their performance is measured. Commercial solar panels behave in much the same way which is one of the main points of these experiments; that is, to make the connection between what you learn in model form in order to apply the same techniques to real-world systems.

Next time, we will investigate the effects of tilt angle, heat, and shading on solar panels which will give you even more background information. In the meantime, conserve energy and “stay green.” NV