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.
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.
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.
Very interesting MPPT control method. I like the micro-inverter idea as it is the most efficient way to dump energy to the grid. Managing multiple 200-300W power is much easier than a huge 10kW stuff. Great article!
Using a power optimizer is not complicated and system design is straightforward. For a low voltage lighting application, the power optimizer will allow the maximum amount of power available to be extracted from the panel.
The power output of a solar panel changes a slight amount with time and this is specified in the panel manufacturer’s data sheet. This case the power optimizer can be used to very good advantage because as the Vmp and Imp points change with time, the SPV1020 dynamically tracks these changes and enables the maximum power available to be extracted from the panel.
We are looking at the same thing. The article compared a microinverter delivering 115VAC, not necessarily Enphase which delivers a higher voltage, to a higher voltage, lower current DC system.
The SPV1020 has performance monitoring features available through its SPI bus that include input voltage, input current and duty cycle. Output voltage can be computed from input voltage and duty cycle.
Interestingly, a solar panel sales guy came by on the day that this article was published. I was astonished to find out that I could actually save money immediately. It turns out that we use double the "average" user and for anything beyond the norm, the 15 cents/kwh goes to about 30cents/kwh. Thank you PGE. This makes getting enough panels to knock our electricity useage in half has a quick payback period. (7-8 years).
If you are a do it yourselfer, it can be even less. see
for example pricing. And no, I am in no way affiliated with these folks. They popped out of a simple google search. YMMV.
Optimizers can be used for batteries as well as AC grid systems. STMicroelectronics has an evaluation board with P/N STEVAL-ISV005V1 that uses the SPV1020 and SEA05 constant voltage constant current controller to charge a 240W lead acid battery.
You would need to monitor the panel output voltage and input voltage.. This can be done through the SPI bus in the SPV1020. The SPI bus provides a direct measurement of input voltage, input current and duty cycle. Output voltage can be computed by Vout = Vin/(1-du).
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