Lecture 10 Solar Energy : Part 7

Summary

The lecture explains the electrical performance of solar cells, how PV modules are connected to meet voltage and current requirements, the main types of PV panels, and the components of a solar module.

Solar-cell characteristics

• Current–voltage (I–V) curve: Shows the current a PV cell produces at different output voltages for a given sunlight intensity and temperature.

  • Short-circuit current ((I_{\text{sc}})): The maximum current, measured when the cell’s terminals are connected directly.
  • Open-circuit voltage ((V_{\text{oc}})): The maximum voltage, measured when the terminals are not connected to a load.
  • Maximum power point (MPP): The operating point where the product of voltage and current is greatest: [ P_{\max} = V_{\text{mp}} \times I_{\text{mp}} ] The MPP voltage and current are lower than (V_{\text{oc}}) and (I_{\text{sc}}), respectively.

• Power–voltage (P–V) curve: Created by multiplying each voltage value by its corresponding current. The curve peaks at the maximum power point.

• Effects of operating conditions:
  • Higher solar irradiance produces more charge carriers, substantially increasing current and power while raising voltage only slightly.
  • Higher temperature reduces voltage and therefore power, while current changes comparatively little. The lecture attributes the voltage reduction to increased diode saturation current and a reduced semiconductor band gap.

Fill factor and efficiency

Fill factor (FF) indicates how closely a real cell’s output approaches its theoretical maximum:

[ \text{FF} = \frac{P_{\max}}{V_{\text{oc}} \times I_{\text{sc}}} ]

It can also be visualized as the area of the MPP rectangle divided by the area of the rectangle formed by (V_{\text{oc}}) and (I_{\text{sc}}). FF is below 1; a higher value generally indicates a better-performing cell. The lecture gives a typical range of about 0.70–0.82 for commercial silicon cells.

Efficiency is the maximum electrical power output divided by the incident solar power on the cell:

[ \text{Efficiency} = \frac{P_{\max}}{\text{incident solar power}} ]

The lecture gives approximate commercial efficiency ranges of 20–25% for monocrystalline, 15–18% for polycrystalline, and 17–19% for thin-film panels. It notes that laboratory results may exceed commercial performance. Perovskite/tandem and multi-junction cells are mentioned as higher-efficiency approaches, though the lecture describes them as costly or not widely deployed commercially.

Connecting PV modules in series and parallel

• Series connections add voltage while current remains approximately the same as that of one module. They are used when a higher voltage is required.
• Parallel connections add current while voltage remains approximately the same. They are used when more current and power are required at the desired voltage.

For large PV systems, the usual design approach is to connect modules in series to reach the required voltage. Each series-connected group is called a PV string. The strings are then connected in parallel to reach the required current and power.

Wiring conventions:

• Series: Connect the positive terminal of one module to the negative terminal of the next.
• Parallel: Connect positive terminals together and negative terminals together.

Worked example

A system requires 240 V and 40 A and uses modules rated at 30 V and 10 A:

• Modules in series per string: (240 \div 30 = \mathbf{8})
• Parallel strings: (40 \div 10 = \mathbf{4})
• Total modules: (8 \times 4 = \mathbf{32})

Thus, four parallel strings of eight modules each provide the target voltage and current.

Types of solar PV panels

The lecture distinguishes three panel types mainly by their silicon structure, cost, efficiency, appearance, durability, and suitable applications:

• Monocrystalline: Made from single-crystal, relatively uniform silicon. Typically described as the most efficient and durable, but more difficult and costly to manufacture. Suited to rooftops, homes, and locations where space is limited.
• Polycrystalline: Made from multiple silicon fragments, giving the material a less uniform appearance. Presented as an intermediate option in performance and cost, and commonly suited to large ground-mounted plants.
• Amorphous thin-film: Uses a non-crystalline silicon layer. It is thin and can be flexible, making it useful for curved surfaces and building-integrated PV, but the lecture describes it as less efficient and shorter-lived than crystalline panels.

The lecture’s comparisons of efficiency and price per watt are not entirely consistent across the subtitles, so the figures should be treated as approximate rather than as a precise ranking.

Components of a solar PV module

A typical module includes several protective and electrical layers and parts:

• Tempered glass: A transparent front layer, typically described as 3–4 mm thick. It protects cells from weather, dirt, and dust while allowing sunlight through.
• Solar cells and interconnections: Cells are wired together in series and/or parallel to form the module’s electrical output.
• EVA encapsulant: Transparent ethylene-vinyl acetate layers above and below the cells that laminate and protect the cells and hold them in place.
• Backsheet: The rear layer, which provides mechanical protection and electrical insulation.
• Aluminium frame: Supports and protects the module assembly.
• Junction box: Usually mounted on the rear, providing the point where module wires connect to the rest of the PV system.
• Busbars and connectors: Thin conductive strips collect current from the cells, while cross-connectors link cells or modules. The lecture identifies aluminium or copper as materials used for these conductors.
• Silicone sealant or glue: Helps hold the assembly together and protects it from moisture, dust, chemicals, and weather.

The lecture notes that common panels have often used 60 cells, with 72-cell panels also used in commercial systems. It also mentions larger modern cell formats and higher cell power ratings.

Speakers and sources

• One lecturer presents the material; the subtitles do not provide the lecturer’s name.
• No additional speakers or specific external sources are identifiable in the subtitles.