# Selecting a solar energy conversion method

July 3, 2012

Solar energy conversion with embedded maximum power point tracking (MPPT) –

Active power optimizers or micro-inverters

1. Introduction
A major problem facing solar energy system designers is determining the best, most cost effective method to extract power from a solar array and deliver it to the AC grid. Of equal importance is how to solve the problem of shading. A shaded panel can burn out and reduce functionality of an entire string of panels. Methods will be presented to solve this problem.

A literal bird’s eye view of a typical solar system is shown in Figure 1.

Figure 1

Solar panels are mounted on the roof of a building facing southwest. Southern exposure is obvious. Southwest exposure is frequently needed to capture the afternoon sun. A typical solar panel delivers 24VDC. Solar panels connected in series drive an inverter which connects to the grid. Grid voltage to a home or business is 115VAC or 230VAC. The peak value for a 230VAC system is 325V. The series connected panels form an array which typically provides 350VDC to the inverter to power the grid.

2. Voltage, current and power characteristics of a solar cell
The equivalent of a solar cell is shown in Figure 2.

Figure 2

The cell contains a PN junction and can be treated like a diode. The current through the diode is the same as a standard diode and is called the dark current. The current generator produces a current in the opposite direction proportional to the absorbed light. Series resistance Rs represents conduction losses where power loss is proportional to the square of the output current. Parallel resistance Rp is caused by leakage current due to poor insulation around the edge of the cell. The effect of Rs and Rp on a solar panel’s output characteristics will be shown later in this section.

From the basic diode representation, a solar cell’s current as a function of voltage and power as a function of voltage is developed. Figure 3 shows the I-V characteristic of a solar cell with no illumination.

Figure 3

Figure 4 shows the cell’s I-V characteristic with light applied.

Figure 4

Since the cell produces power, we are used to seeing a current vs. voltage curve flipped upside down as in Figure 5.

Figure 5
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