Photovoltaics made simple

Operating Principle of Photovoltaic Systems

The operating principle of a photovoltaic (PV) system is based on the photovoltaic effect, where two materials with specific characteristics (semiconductors with ion dopants of different polarities) produce electric current when exposed to sunlight. These contacts form the photovoltaic cells, which are connected into frames, panels, and ultimately arrays.

The current produced (direct current, DC) is converted into alternating current (AC) using inverters for use in appliances for self-consumption or for sale to the electricity provider (DEI). In off-grid systems (where the electricity is used entirely for the needs of the installation’s electrical load), batteries are also used to meet the energy needs during nighttime or cloudy hours.

Types of Photovoltaic Panels

Silicon (Si) is the material that has dominated the market to date. The main types of photovoltaic cells are:

Monocrystalline Silicon (c-Si)

• Made from large crystals, the material thickness is around 300 μm and the color is dark blue.
• Efficiency is 13-16% and requires 7-8 m² for 1 kWp.
• Advantage: Higher efficiency in the same area compared to polycrystalline panels, typically used when space is limited.
• Disadvantage: No significant advantage over other types when space is not a concern.

Polycrystalline Silicon (m-Si)

• Blue in color with visible crystalline areas on the surface.
• Efficiency is approximately 12.5-15.5% and requires 8-9 m² for 1 kWp.
• Commonly used in rooftop installations.
• Disadvantage: Slightly lower efficiency than monocrystalline panels but more cost-effective.

Amorphous Silicon (a-Si)

• Has a much higher absorption coefficient for solar radiation, requiring only a few micrometers of material to form photovoltaic cells.
• Efficiency ranges between 6-10%.
• Disadvantage: Not commonly used for rooftops or terraces due to space limitations.

Hybrid – High-Efficiency Panels

• Made of monocrystalline silicon covered with a thin layer of amorphous silicon.
• Efficiency over 18%, allowing for greater power in the same area.
• Advantage: Lower temperature coefficient compared to other panels, resulting in higher energy production from the same system power.
• Disadvantage: Higher initial cost, but it pays off over time with greater returns in 25 years.

Photovoltaic System Performance

The performance of a photovoltaic system is closely linked to the meteorological and climatic conditions of the region, such as solar radiation, temperature, and geographical factors like latitude, longitude, and altitude.

• Solar Radiation:
  Greece enjoys some of the highest levels of sunlight in Europe. The annual energy output per kWp can range from 1,100 kWh in northern areas to 1,500 kWh in southern areas (approximately €600 to €750 per year).

• Orientation and Tilt:
  To maximize the energy efficiency of photovoltaic panels, optimal utilization of incoming solar radiation is necessary. Tracking the sun continuously is not cost-effective for roof-mounted installations, so an optimal tilt and orientation are chosen instead.

  • For the northern hemisphere, the optimal tilt angle is between 10° and 30° facing south.
  • In Greece, the best orientation is south with an angle of about 28°.

For off-grid systems, tilt and orientation are less about maximizing annual output and more about the specific use requirements, such as summer or winter use. For morning usage, east orientation is important, while west orientation is preferable for evening usage.

Shading

Shading is a crucial factor in the performance of a photovoltaic system. Even partial shading of a panel can significantly reduce its output and energy production. Moreover, the shaded panel acts as a bottleneck, limiting the current in the entire array, thus lowering the total output. Repeated shading during peak sunlight hours can also cause thermal stress, accelerating the aging of the shaded panels.

• Solutions for Shading:
  • Remove obstacles if possible (e.g., solar water heaters, antennas, wires).
  • Reduce the height of obstacles (e.g., chimneys, antennas).
  • Place panels far enough from obstacles, ensuring that the height difference between the obstruction and the panel is at least double the distance.
  • Group panels with similar shading conditions together for optimal performance and connection to separate inverter inputs.
  • For complex cases, modern inverter technologies that handle each panel individually without affecting others are used.

Environmental Benefits of Photovoltaics

A 10 kWp photovoltaic system can generate approximately €7,700 annually, with 50% of this benefit being direct (financial) and the other 50% being indirect, contributing to the environment and society.

• A typical 1 kWp system prevents the release of 1.3 tons of CO₂ annually, equivalent to the absorption of carbon by 2 hectares of forest or 100 trees.
• To generate the same amount of energy with oil, it would require 2.2 barrels, which corresponds to reducing 7,000 km of driving by a typical car annually.

For any further information concerning photovoltaic systems in Greece do not hesitate to call us: +302103217895

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