Irradiance

Irradiance

The sun is the primary source of energy that the earth can receive, it is made up of 80% hydrogen, 19% helium, and 1% are unknown elements.

The energy radiated by the sun comes from a thermonuclear reaction that converts mass into energy. The summary is in the famous relation: E = mc² (energy equal to mass times the speed of light squared).

So the sun radiates energy to the outside which propagates in the form of radiation and 99% of this has a wavelength between 0.4 and 0.74 µm.

Between irradiance and radiation

Irradiance is the power per unit of the surface coming from the sun (W / m²) which can be in the order of hundreds of W / m², in practice it expresses the density of solar radiation, and reaches a maximum value around 1,000 W / m² which is lower than the solar constant (1,350 kW / m²), due to the phenomena of absorption and reflection from the atmosphere.

Therefore the solar radiation or irradiance that reaches the earth's soil is divided into:

• Direct radiation: when it hits a single point
• Diffuse: when it hits multiple angles
• Reflected: when it is reflected from the ground, water, or other horizontal surfaces.

Conclusion: the Global Radiation is given by the sum of: Direct Radiation + Reflected Radiation + Diffuse Radiation.

RG = RD + RR + RD = 1000 W / m² (Theoretical)

Eg. With a clear sky the global radiation is 1000W / m²; Fog = 600W / m²; Cloudy = 500W / m²; Barely perceptible sun = 200 W / m²; Dense fog = 100 W / m²; Covered sky = 50 W / m².

Irradiance

Irradiation is the Energy per unit area from the sun (kWh / m²) which varies point by point.

Data on irradiation and, consequently, orientation is essential for designing a photovoltaic system whose cells use sunlight to generate electricity.

The solar radiation on the ground is affected by the following factors:

• From the distance from the Earth to the sun
• By the presence of the atmosphere surrounding the planet
• By the influence of the clouds
• To the spheroidal shape of the Earth.

Example:

• The annual irradiation in Milan is about 1405 kWh / m² (ENEA data on a 30 ° inclined plane); if I divide it by 365 days, I will have irradiation of 3.85 kWh / m² day, which correspond to 3.85 hours of sun day equivalents (hse / d).
• The annual irradiation in Rome is about 1653 kWh / m² (ENEA data on a 30 ° inclined plane). If I divide it by 365 days, I will have irradiation of 4.53 kWh / m² day, corresponding to 4.53 hours of sun equivalent day (hse / g).
• The annual irradiation in Palermo is about 1732 kWh / m² (ENEA data on a 30 ° inclined plane); if I divide it by 365 days, I will have irradiation of 4.75 kWh / m² day, which correspond to 4.75 hours of sun day equivalents (hse / d).

Below table reference of Italy Irradiation

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