Solar Energy Basic Principles

Solar energy consists of the light and heat emitted by the sun, in the form of electromagnetic radiation.

With today's technology, we are able to capture this radiation and turn it into usable forms of solar energy - such as heating or electricity.

Although one could go into technical dissertations on the subject of electromagnetic radiation, how it is converted into solar energy, and the exact qualities of its electromagnetic rays, this is not something the average person needs or wants to know.

But in order to be able to benefit from the use of solar energy, there are a few facts you should know. Knowing these facts can assist you to make a sound decisions, when looking at the use of solar power as a clean energy source for your home, RV, or whatever the case may be.

The technical feasibility and economical viability of using solar energy depends on the amount of available sunlight (solar radiation) in the area where you intend to place solar heaters or solar panels.

This is sometimes referred to as the available solar resource.

Every part of Earth is provided with sunlight during at least one part of the year. (I say "part of the year" as the north and south polar caps are each in total darkness for a few months of the year.) The amount of sunlight available is one factor to take into account when considering using solar energy.

There are a few other factors, however, which need to be looked at when determining the viability of solar energy in any given location. These are as follows:

• Geographic location
• Time of day
• Season
• Local landscape
• Local weather

Because the Earth is round, the sun hits its surface at different angles, at different locations on the globe. This ranges from 0º (just above the horizon - a good example of this is the north pole during the winter) to 90º (directly overhead, at and near the equator).

When the sun's rays are vertical (directly overhead), the Earth's surface gets a maximum of solar energy. The more slanted the sun's rays are, the longer they have to travel through Earth's atmosphere before reaching the surface (becoming more scattered and diffuse as they go along).

The more scattered and diffuse the sun rays are, the less concentrated the solar energy is. Because the Earth is round, the polar regions never get direct sunlight, and they receive no sun at all during their respective winter months.

The Earth travels around the sun in an elliptical orbit. Because of its elliptical path, the northern hemisphere of the Earth is closer to the sun during one half of the year, and the southern hemisphere is closer during the other half of the year.

When one part of the Earth is closer to the sun, it receives more concentrated solar energy. This time of year is referred to as "summer."

But regardless of summer or winter, the 23.5º tilt of the Earth's axis plays a larger role in determining the amount of sunlight striking Earth at a particular location. The tilting of the earth results in longer days in the northern hemisphere during one half the year, and longer days in the southern hemisphere during the other half of the year.

Areas such as the United States and Europe receive more solar energy between May and September - not only because days are longer, but also because the sun is nearly overhead during this season. The sun's rays are far more slanted during the shorter days of the winter months. Cities such as Denver, Colorado, receive nearly three times more solar energy in June than they do in December.

As sunlight passes through Earth's atmosphere, some of it is absorbed, scattered, and reflected. Following general list of materials that cause the sunlight to be diffused:

• Air Molecules
• Water vapor
• Clouds
• Dust
• Pollutants

Sunlight affected in this way is referred to as diffuse solar radiation or diffuse sunlight.

Sunlight that reaches the Earth's surface without being diffused is called direct beam solar radiation or direct sunlight.

The sum total of all diffuse and direct solar radiation in a given location is called global solar radiation. It is the total amount of sunlight hitting the Earth at any specific spot, both direct and diffuse combined.

Pollution and other atmospheric conditions (such as weather patterns) can reduce direct sunlight by 10% on clear dry days. They can reduce direct beam radiation by 100% on thick, cloudy days.

Note that the absence of direct sunlight does not imply total darkness, as some diffuse light will still get through.

Scientists measure the amount of sunlight available in specific locations during the different times of year.

They are then able to estimate the amount of sunlight which falls on similar regions at the same latitude with similar climates and conditions.

Measurements of solar energy are normally expressed as "total radiation on a horizontal surface", or as "total amount of radiation on a surface tracking the sun".

In this last case, the assumption is that one is using a solar panel that automatically tracks the sun.

In other words, the solar panel would be mounted on a tracking device so that the panel would remain at right angles to the sun throughout the day.

This system is primarily used for industrial setups, when it is used at all.

Measurements

Radiation data (the amount of solar energy available at a given location) for solar electric (photovoltaic) systems is often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may be expressed as watts per square meter (W/m2).

Radiation data for solar water heating and space heating systems is usually represented in British thermal units per square foot (Btu/ft2).

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