Machinery uses power to do work. The type and amount of work performed dictates the amount of power required. Therefore, the solar cell array’s maximum power output is a critical piece of information to know. Recall from the online tutorial; analysis of permanent magnet DC motors, that it is also important to know when an electric motor operates at its most efficient speed. Therefore, an electric motor has a power curve that needs to match the solar cell array’s maximum power output.

So, we will first determine the solar cell array’s electrical power output curve, and then we will do the same for the motor’s mechanical power output curve.

Below is a diagram of the solar cell IV characterization test setup. The setup consists of four resistors connected in series at the output of the cell array. There are two voltage measurement points, volt meter M1 measures the voltage across R1 and volt meter M2 measures the voltage across three potentiometers (variable resistors). The potentiometers are used to increase or decrease the resistive load connected to the photovoltaic cells.

R1 is a current sensing resistor. Its value is precisely known. IV characterization of solar cells requires the measurement of the voltage and the current as the load is decreased. In order to know the current flow out of the array, the voltage across R1 is continuously measured. Since Ohms Law states V = I x R, then by algebraic manipulation this formula yields the current: I = V / R. So, measurement of the voltage across R1 and knowledge of the resistance of R1 gives us the current flowing through it.

The sum of the voltages measured with volt meter M1 and volt meter M2 yields the total voltage across the array. Hence, we now have the total current (I) that flows out of the array and the total voltage (V) across the array for the IV Characterization.

IV characterization requires 50 to 100 voltage and current measurements. These measurement points must then be entered into a computer to graph the IV Power Curve. A device that speeds up the voltage and current measurement process is an Analog to Digital Converter (ADC). The SESP ADC takes about 3 measurements per second and transfers them into the computer. Once the measurements are in the computer, Microsoft Excel spreadsheet is used to graph the measured points. Below is an Excel spreadsheet graph of the measured IV points.

The general procedure for characterizing the solar cell power curve is to start by applying maximum resistance to the solar array. Then direct a constant, evenly distributed light source to the solar cell array. Now with maximum resistance applied, the current out of the array is minimum or close to zero and the voltage output is at maximum or close to the cells specification. While the ADC measures the voltage and the current and transfers them to a computer, the potentiometers are used to slowly decrease the resistance. As the resistance decreases the current flow increases. As the current flow increases, the voltage decreases.

Each resistance value results in a different voltage and current value, or data point. A data point is a voltage and a current measurement corresponding to each resistance setting. The SESPADC measures and transfers to a PC the voltage and current data point. The SESP ADC is useful low cost alternative to the more expensive data acquisition systems available on the internet. The graph below was produced by hundreds of voltage and current data points acquired and processed by the SESPADC and its software.

The kind of light used to illuminate the solar cell array is critical. All eight solar cells of the array must be illuminated uniformly. Since the cells are wired in series, cells receiving less light produce less current. A single cell can affect the overall current production of the array. Below is the graph of three attempts to uniformly illuminate the solar cell array. As expected the more times you do something, the better you get at it. Notice the first attempt produced 0.18 Amps, the second attempt to distribute the light evenly across the array produced 0.2 Amps, and the third attempt produced almost 0.3 Amps. The curves on the bottom of this graph represent the power output of the array for the different light distribution efforts.