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Solar Radiation and Solar Energy

At its most basic, solar energy simply refers to the output of energy from the sun that comes in the form of electromagnetic radiation. In other words, solar energy is just the light (both visible and invisible) and heat from the sun.

Insolation and Irradiance

Insolation is a term that refers to the total amount of solar radiation that a given surface area receives in a given amount of time. The term is most often used to reference locations on Earth by how much sunlight they receive. The value is expressed as irradiance per time.

Irradiance is the amount of sunlight that a given area receives and is expressed as watts per square meter (W/m2). When irradiance is determined over a given timeframe, it becomes insolation and is expressed as W/m2/hour (or some other unit of time). The term “one Sun” of insolation is sometimes used. This is a standard that refers to 1000 W/m2 per time. One Sun per hour would be 1000 W/m2/hour.

The majority of insolation occurs at the equator and decreases as one moves toward the poles. Insolation is affected by the angle at which light hits it. This is because direct light is more concentrated than angled light. Thus, the Northern Hemisphere receives more insolation when it is tilted toward the sun, and experiences summer in June, July, and August. The opposite is true of the Southern Hemisphere. The angle of insolation is responsible for the seasonal changes that occur at latitudes distant to the equator.

Solar Radiation on Earth

The Earth receives about 250 W/m2 of solar radiation on average each day across its entire surface. This is an average though that accounts for changes in season and distance from the equator. Solar radiation ranges from a high of near 1100 W/m2 near the equator to a low of only a few watts per square meter at the poles. Antarctica receives the least amount of solar radiation on average.

The number above constitutes the total amount of solar radiation reaching the surface of the Earth. This is somewhat different from the amount that that hits the upper atmosphere, which is on the order of 1300 W/m2. The atmosphere reflects a great deal of the radiation before it strikes the surface.

The sun provides roughly 12,180,000,000,000,000 kilowatts (that is 12,180 trillion kilowatts) of power to the Earth’s surface in a year. By contrast the world consumes about 15,000,000,000,000 (15 trillion) kilowatts of power each year. Put another way, the sun provides enough energy in one hour to supply the needs of the whole of humanity for an entire year.

As pointed out in the insolation section, the problem with harvesting solar radiation is that the energy does not fall on all parts of the Earth equally. The poles can be discounted, which means most solar radiation falls on the other one half of the surface area of the Earth.

This is where the calculations above become confusing. It is often said that the total amount of sunlight reaching the surface of the Earth is 1000 W/m2 in a given year. If all surfaces were near perpendicular to the Sun’s rays at sea level and without clouds every day, this value would hold. At the latitudes where most humans live, the yearly average solar irradiance is 250 to 800 W/m2 in a day.
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