Selecting a solar energy conversion method

Ed Friedman, applications engineer, STMicroelectronics Inc. - July 03, 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
3. Evolving from a cell to a panel
Individual cells are connected in series and parallel to form a panel. Panels are connected in series and parallel to form a photovoltaic array. Connecting cells in series increases the voltage. Connecting them in parallel increases the current. If an individual cell has a forward drop of 0.5V and with a given illumination produces 100mA, connecting 50 cells in series would produce a 25V string. Connecting 60 of the strings in parallel would produce a 25V, 6A panel. If each panel could deliver 150W, connecting 50 panels on a rooftop would deliver 7.5kW.

The four key parameters of a solar panel are:
Voc, the open circuit voltage where Iout = 0, Pout = 0
Isc, the short circuit current where Vout = 0, Pout = 0
Vmp, the output voltage at when the power extracted is maximum
Imp, the output current when the power extracted is maximum.

In Figure 6 the red curve is the current as a function of voltage and the green curve is the power as a function of voltage along with the location of the maximum power point.

Figure 6

In the solar cell (or panel) equivalent circuit the parallel resistor Rp affects the slope of the current vs. voltage curve at Vout = 0. For an ideal panel, Rp = ∞ and the slope is zero. Series resistor Rs affects the slope of the power vs. voltage curve at Vout = Voc. Ideally Rs = 0 and the slope is infinite.

4. Transferring maximum power from a panel
The objective is to find the maximum power point (MPP) and always operate the panel voltage and current at that point. The MPP will vary with irradiance and temperature. Decreasing irradiance is represented by a lower Isc. As Isc reduces, the MPP moves to a lower voltage. As temperature increases, Vmp reduces and the maximum power gets less. Voc, Isc, Vmp, Imp and the effects of temperature are shown in the panel manufacturer’s datasheets. A method is needed to dynamically track these changes as the panel environment changes and always operate the panel at the maximum power point, regardless of external factors.

Since the equivalent circuit of a solar panel is represented by a current source with parallel and series resistances, the Thevenin equivalent circuit can be shown as a voltage source with a single series resistance. To transfer maximum power from the voltage source to the load, the load resistance must equal the source resistance. Figure 7 shows the I-V curve and the load line R2 with the proper slope that intersects the I-V curve at the MPP.

Figure 7

This is easily accomplished with the STMicroelectronics SPV1020 boost converter with embedded maximum power point tracking (MPPT).

The SPV1020 DC-DC boost converter with embedded MPPT is an active power optimizer. Its purpose is to increase the output voltage from a panel while simultaneously adjusting the panel’s output voltage to Vmp. This optimizes or maximizes the power extracted from the panel. The converter’s output voltage is set by the user. The converter’s duty cycle is determined by the Perturb and Observe MPPT algorithm. The converter’s input voltage (or panel’s output voltage) is the dependent variable and is set by the formula:

Vin = Vout * (1-duty cycle)

In the SPV1020, the duty cycle starts out at a low value of 5 percent. The input voltage and input current are measured and power calculated. Then the duty cycle is increased. The new input voltage is measured and input power calculated. If the new power is greater than the old power the duty cycle is increased again. This process continues until the new power does not change or is less than the old power. If the new power does not change, that is the maximum power point. If the new power is less than the old power, the duty cycle DECREASES and the process is repeated, until the new power equals the old power, and the maximum power point has been determined. In this case the converter will be operating at the top of the power vs. voltage curve as shown in Figure 8.

This Perturb and Observe algorithm runs continuously, at 256 times the switching period. The switching frequency is by default 100 kHz. The switching period is 10 microseconds and the MPPT algorithm is updated every 2.56 milliseconds.

Figure 8

5. Washing your car
Washing your car is an example of the need for maximum power transfer. Let’s say you’re washing your car with a garden hose without a nozzle and there’s a stubborn accumulation of mud on the fender. Placing your thumb over the hose opening acts as an impedance matching device to transfer maximum power from the high pressure water main to the mud on the fender.

Figure 9

There are major drawbacks to this approach. MPPT is performed by the central inverter on the entire array. The central inverter can also be known as a string inverter because the panels are connected in a series string.

If one or more of the panels is partially shaded, its power output will be less, its maximum power point will change and the single central (or string) inverter has no way of knowing this. The maximum available power from the entire array will not be extracted by the central inverter. MPPT needs to be performed on each panel individually.

7. Effects of shading
Shade on a solar panel has a major negative effect. Shade is due to the panel surface covered or partially covered with dust, dirt, leaves, a flock of birds flying overhead and the presence of clouds. The panel power is reduced. When the panel is part of a series string the entire string output is affected. To prevent a shaded panel from reducing the performance of the entire string, a bypass diode or a Cool Bypass Switch is connected across the panel as shown in Figure 10. Under normal conditions the output current from each string flows through the panels connected in series. If one panel is shaded its current source reduces. The full string current will then flow through the panel Rp as shown in Figure 2. Rp is a high resistance and the panel will severely overheat. To prevent this situation, bypass diodes placed in parallel with the panel offer a path for string current to flow, preventing a hotspot.

Figure 10

An alternative to Schottky diodes is the Cool Bypass Switch. The SPV1001 Cool Bypass Switch consists of a MOSFET that is switched on and off by a control circuit which provides a path for current to flow around the shaded panel. Compared to a Schottky, the Cool Bypass Switch has less leakage current during the OFF time and lower voltage drop during the ON time thus improving efficiency.
8. Providing MPPT for each panel
An improved method of solar system design is to use micro-inverters. A micro-inverter is a 250W inverter that is connected to each panel. MPPT is performed by the micro-inverter at the panel level. Figure 11 shows a system consisting of thirty micro-inverters, one per panel. The micro-inverters AC outputs are connected together and properly phased with the AC line.

Figure 11

Micro-inverters are fairly complex electronic products. A picture of one from STMicroelectronics for evaluation purposes, with its components clearly visible, is shown in Figure 12.

Figure 12

A simpler method of implementing a photovoltaic power system with MPPT is to use an active power optimizer such as the SPV1020. An overall system diagram is shown in Figure 13.

Figure 13

In this case one active power optimizer is connected to each panel. It boosts the panel output voltage and performs the MPPT function while doing so. Panel output voltage must be at least 6.5VDC. The SPV1020 output voltage can be as high as 40VDC. A typical value is 35VDC as shown in Figure 13. The active power optimizer utilizes the Perturb and Observe algorithm until it finds the maximum power point on the power vs. voltage curve as shown in Figure 7. This optimizer measures the input power to determine the panel’s Vmp. There are other types of MPPT converters available but they assume Vmp is a fixed percentage of Voc. This could be case under one specific operating condition and thermisitors are required to approximate the change in Vmp with temperature. The SPV1020 makes none of these assumptions. It measures input voltage and input current to determine the actual input power in setting the maximum power transfer operating point. Figure 12 shows the simple external connections. The panel is connected through the boost inductor at the Lx input and the load is connected to Vout. No other source of power is required. The resistor divider connected at the panel output senses the chip input voltage for MPPT purposes. Input power is determined by sensing the current through the main MOSFET switch and multiplying its value by the input voltage. The resistor divider connected at Vout sets the value of the output voltage.

Figure 14

The SPV1020 is an interleaved four channel converter. Figure 14 shows one of the four switching channels. Of particular interest is its 320W the power handling capability. This is accomplished in the small PowerSSO-36 package by splitting the power handling components into the four channels.

The 4-phase interleaved topology as shown in Figure 15.

There are four interleaved switching sections interleaved every 90 degrees. This diagram shows a single panel and single load, and how the four switching section are connected. Each section has its own inductor. The switch and diode pictured are both MOSFETs with low Rds on. At the default switching frequency of 100kHz, each section operates at 25kHz. The SPV1020 also integrates four zero crossing blocks, one for each branch. Their role is turn off the related synchronous rectifier to prevent reverse current flow from output to input.

In order to guarantee a correct power-up sequence, the converter initially operates in burst mode. When the input voltage is greater than 6.5 V, the converter sequentially activates each of the four phases. Initially, only phase 1 starts to work in burst mode, charging the inductor only for one cycle over 15 cycles. Then the duty cycle is progressively increased until phase 1 is switched on at every cycle at the default switching frequency of 100 kHz. After phase 1 has reached its steady-state condition, the other phases are progressively switched on in the following sequence: phase 3, phase 2 and, lastly, phase 4. If lower power than 320W is required, it may be possible to use only two of the four phases, eliminating the cost and space requirements of two inductors.

A major advantage of interleaved architecture is low ripple. Assuming a resistive load, the output voltage ripple is proportional to the output current ripple. In interleaved-4 phase architecture, total output current is the sum of the four currents flowing in each inductor. Since each phase carries one fourth the total current, for a given value of inductance, the peak to peak ripple would be one fourth the value of a single phase architecture system.
10. Implementing the active power optimizer
As with a micro-inverter, there is one active power optimizer for each panel.

Figure 16

When higher output voltage than one panel is needed, the optimizer outputs are connected in series.

Figure 17

With the final output driving a central inverter.

11. Comparison of active power optimizers with micro-inverters
In comparing active power optimizers with micro-inverters considerations must be given to cost, complexity of installation, number of components and ease of maintenance among other factors. Absolute comparisons are difficult. However, the simplicity of the active power optimizer has been discussed. A block diagram of a micro-inverter is shown in Figure 19 which shows its major internal functions. In comparing with power optimizers and a central inverter the MPPT, DC-DC portion are stripped out and the DC-AC portion is replaced with the central inverter.

The following table for a 30 panel, 7.5kW system puts these issues into perspective to help the solar system designer to make the best decision. Both features in common and differentiators are shown. Comparison is made with a typical Enphase M215, 215W micro-inverter. Other micro-inverter suppliers will have similar characteristics. In comparing these two systems the specifics of the installation are critical. This comparison offers guidelines the system designer to consider.

STMicroelectronics manufactures integrated circuits for power optimizers, micro-inverters, string inverters as well as the Schottky diodes and Cool Bypass switches which are located behind the panels. We are completely unbiased regarding technology choices for solar energy conversion.

12. Conclusion
This article has covered photovoltaic technology from the solar cell or panel being the first element in the power chain to the inverter as the last element driving the AC grid. It has shown how the panel’s voltage – current curve is derived from a standard diode characteristic and the need to operate the panel at its maximum power point. The principle of operation of the SPV1020 active power optimizer was mentioned as one method to achieve this goal. A comparison between active power optimizers with micro-inverters was presented, with the conclusion that the use of one method or the other is very dependent upon the specific application and physical location of the solar panels. Large installations where panels are easily configured in a grid fashion are better for active power optimizers where smaller installations when panels have to be strategically placed to avoid shading and other effects are better suited for micro-inverters.

About the author:
Ed Friedman, applications engineer, STMicroelectronics Inc.

Ed Friedman has been with STMicroelectronics for 15 years and now specializes in solar power conversion. He was previously responsible for the technical marketing of industrial products including motor control, off-line power supply and DC-DC converters. Prior to STMicroelectronics, Mr. Friedman held Marketing Management, Product Line Management and Applications positions at Unitrode, AT&T and Analog Devices.

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