Several photovoltaic solar cells. An individual PV cell is usually small, and can only produce about 1 or 2 watts of power. To boost the power output of these cells, they are connected together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, PV systems can be built to meet almost any electric power need, small or large.

By themselves, modules or arrays do not represent an entire PV system. Systems also include structures that point them toward the sun and components that take the direct-current electricity produced by modules and “condition” that electricity, usually by converting it to alternate-current electricity. PV systems may also include batteries. These items are referred to as the balance of system(BOS) components.

Combining modules with BOS components creates an entire PV system. This system is usually everything needed to meet a particular energy demand, such as powering a water pump, appliances and lights in a home. Also, the electrical requirements of a community can also be met if the PV system is large enough.

Solar Energy Resources

The electromagnetic radiation emitted by the sun is generally called solar radiation, and often referred to as the solar resource. As explained earlier, solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource.

Basic Principles:

Every location on earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one spot on the earth’s surface varies according to the following:

Geographic location

Time of day

Season

Local landscape

Local weather.

Because the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the sun’s rays are vertical, the Earth’s surface gets all the energy possible.

The more slanted the sun’s rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid Polar Regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year.

The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy. The Earth is nearer the sun when it is summer in the southern hemisphere and winter in the northern hemisphere. However, the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.

The 23.5° tilt in the Earth’s axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location.

The rotation of the Earth is also responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon, when the sun is at its highest point. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon.

Diffuse and Direct Solar Radiation

As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by:

Air molecules

Water vapor

Clouds

Dust

Pollutants

Forest fires

Volcanoes.

This is called diffuse solar radiation. The solar radiation that reaches the earth’s surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.

Measurement

Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface or as total radiation on a surface tracking the sun.

Radiation data for solar electric (photovoltaic) systems are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).

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