EE6623 Chap.4 PV Cells, Modules and MPPT
PV Cells, Modules and MPPT
Read the operating curve
A photovoltaic device converts absorbed photons into mobile charge carriers that the junction separates. Photon energy must meet the semiconductor threshold, but energy above that threshold is not automatically converted into terminal electricity.
The electrical result is described by an I-V curve rather than one fixed voltage or current.
Use the three landmark operating points
Short-circuit current occurs at zero terminal voltage, so terminal power is zero. Open-circuit voltage occurs at zero current, so power is also zero. Maximum power lies between them near the knee of the curve.
Multiplying voltage and current at candidate points is more reliable than choosing the highest value of either variable alone.
The equivalent circuit diagnoses curve shape
The light-generated current source represents carrier production, the diode represents junction behaviour, series resistance captures conducting-path losses and parallel resistance represents leakage.
Higher series loss rounds the curve near the high-voltage region; lower parallel resistance harms the low-voltage region. Point to the changed part of the curve before naming the likely parameter.
Scale from module to array without mixing rules
Series modules add voltage while carrying a common current. Parallel strings add current while sharing voltage.
Draw one string, calculate its voltage and current, then count strings. Cross-check total array power against module power multiplied by the number of modules. That second route catches most series-parallel mistakes immediately.
Shading changes both current paths and tracking
A shaded series section can restrict the whole string or enter reverse stress.
Bypass diodes provide another path, protecting the section and recovering some string current while losing voltage. Partial shading can create steps in the I-V curve and several peaks in the P-V curve. Maximum-power-point tracking controls operating point; it does not create irradiance or remove converter losses.
What this chapter covers
- 01
Photon energy and band gap
- 02
Photovoltaic junction operation
- 03
The single-diode equivalent circuit
- 04
Series and parallel resistance
- 05
I-V and P-V curves
- 06
Module and array wiring
- 07
Shading and bypass diodes
- 08
Maximum-power-point tracking
Worked example · free
Wire a twelve-module array
- 2Add voltage along one string.
- 2Add current across the parallel strings.
- 4Calculate power and cross-check by module count.
Key terms
- Band Gap
- The semiconductor energy threshold relevant to creating mobile charge carriers from absorbed photons.
- Short-circuit Current
- The current at zero terminal voltage on the photovoltaic I-V curve.
- Open-circuit Voltage
- The terminal voltage at zero current on the photovoltaic I-V curve.
- Bypass Diode
- A path around a shaded series section that limits reverse stress and recovers string current.
- Maximum Power Point
- The operating voltage-current pair that maximises electrical power under current conditions.
PV Cells, Modules and MPPT FAQ
How does irradiance move the photovoltaic curve?
Higher irradiance mainly raises current, so short-circuit current and maximum power increase strongly while open-circuit voltage changes more modestly. Temperature has a different effect and commonly reduces voltage as the cell becomes hotter, so keep the two influences separate.
Why does one shaded module affect a series string?
Every series element carries the same current. A weak module can therefore restrict string current or enter damaging reverse bias. A bypass diode gives current another path around a section, sacrificing that section's voltage while limiting stress and recovering part of the output.
Does maximum-power-point tracking increase panel efficiency?
It adjusts the electrical operating point seen by the module so the array works near the maximum of its present P-V curve. It cannot create irradiance or remove device losses, and its own converter has losses. Partial shading can also create several local peaks.
Exam move
Sketch the I-V and P-V curves on the same voltage axis and mark short circuit, open circuit and maximum power. Explain why the first two points both have zero terminal power. Change irradiance on one sketch and temperature on another so their different effects remain visible.
Draw the single-diode equivalent circuit and connect each element to a curve symptom: photocurrent to current level, diode behaviour to junction shape, series resistance to the high-voltage region and leakage resistance to the low-voltage region. For array practice, draw one module, one series string and the parallel combination.
Calculate string voltage and current before scaling to the array, then verify total power by multiplying module power by module count. For shading, draw the bypass path and predict the loss of section voltage before looking at any curve. Compare a smooth single peak with a shaded multi-peak power curve and explain why a local tracking perturbation can stop at the wrong point.
End with a boundary chain from collector irradiance to DC module output, array output, converter output and AC delivery. Record mistakes where both voltage and current were multiplied by the module count, open-circuit voltage was treated as useful power, or maximum-power tracking was described as creating energy.
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