Solar irradiance

[edit] Introduction

Solar Irradiance is the power being emitted by a nuclear fusion reaction when protons of hydrogen atoms create helium atoms by violently colliding within the core of the sun. Solar irradiance is measured in watts per square metre (W/m2).

The process occurring in the sun is known as a PP (proton-proton) chain reaction or the CNO cycle as it relies on carbon, nitrogen, and oxygen. The sun fuses about 620 million metric tons of hydrogen every second. This releases tremendous amounts of energy, heat, and light that flow from the sun as electromagnetic radiation (EMR).

Low frequency infrared waves and the colour spectrum of light are emitted as well as high-frequency waves of Gamma rays, X-rays, and ultraviolet radiation or UV rays, of which the most harmful are absorbed into the atmosphere whilst others fall on the surface of the Earth and can cause sunburn.

This power at any given point in time is measured relative to a unit area of the surface it is hitting, normally Watts per metre square (Wm2). Total solar irradiance is a measure of all possible wavelengths of solar power that enter the earths atmosphere at its upper edges.

For more information see: Radiation.

[edit] Solar irradiance and carbon dioxide

Whilst 30% of the solar energy coming from the sun is reflected back into space, the rest is absorbed by the atmosphere or warms the surface of the earth. Some infrared waves are radiated back into the atmosphere. These waves are intercepted by greenhouse gases, such as water vapour and carbon dioxide as they rise through the atmosphere. About half of that energy goes out into space, and about half of it returns to Earth as heat, contributing to the 'greenhouse effect'.

For more information see: Greenhouse effect.

[edit] Solar irradiance and energy

Solar irradiance to cell efficiency, refers to the percentage of solar irradiance that can be turned into useful electrical energy. A standard photovoltaic panel can normally achieve up to 20% (sometimes up to 23%). However in May of 2022 the Fraunhofer Institute for Solar Energy Systems developed the world's most efficient solar cell with 47.6 percent efficiency, under direct sunlight conditions.

Whilst efficiency at any given point is important, energy is used over longer periods and this is dependent on the amount of energy on a surface over time as well as how well a solar PV system can convert that to useful electricity for say a year, from both direct an indirect sunlight on cloudy days for instance.

[edit] Global Horizontal Irradiance (GHI)

Global Horizontal Irradiance is a measure of the total, direct and indirect irradiance of the sun on a flat surface of the earth measure, relative to the solar zenith angle. Also called solar exposure or insolation it is given as a measure of SI units in Watts per metre square (W/m2). It can be measured using a pyranometer.

[edit] Direct Normal Irradiance (DNI)

Direct Normal Irradiance is the amount of direct power hitting a surface, such as the Earth when it is perpendicular to the sun. It excludes the losses likely to occur within the earths atmosphere by absorption and the scattering of radiation via moisture, clouds, ozone etc. It can be measured using a pyrheliometers.

[edit] Direct Solar Irradiance (DSI)

Direct Irradiance (DSI) is the irradiance that hits the outmost edge of the earths atmosphere, excluding any diffused by elements in the atmosphere.

[edit] Diffuse Horizontal Irradiance (DHI)

Diffuse Horizontal Irradiance is a measure of radiation from all areas reflected within in the atmosphere and excludes radiation coming directly from the sun.

[edit] Global Tilted Irradiance (GTI)

Global Tilted Irradiance (GTI) Is the same as Global Horizontal Irradiance and measures the total, direct and indirect irradiance of the sun but on a surface that is at a specified angle, tilt or azimuth. It is often relevant for solar devices that track and move with the sun.

[edit] External references

• https://education.nationalgeographic.org/resource/solar-energy
• https://www.ise.fraunhofer.de/en/press-media/press-releases/2022/fraunhofer-ise-develops-the-worlds-most-efficient-solar-cell-with-47-comma-6-percent-efficiency.html

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