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Solar Energy: Background

What is Solar Energy?

Solar energy is the energy that comes to the Earth from the Sun. It is commonly known as sunlight. This energy arrives in the form of electromagnetic radiation, which is energy that travels in waves. A variety of modern technologies are able to capture and convert solar energy into other useful forms of energy, such as heat and electricity. The Sun is a gigantic nuclear fusion reactor with 330,000 times more mass than Earth. It is by far Earth's most available energy resource. If more solar energy could be harnessed properly, it could provide many times today's global demand for energy.

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The Nature of Electromagnetic Radiation

The atoms that make up all matter are composed of protons, neutrons, and electrons. A proton is a positively charged particle that occurs in the nucleus of an atom. Electrons are negatively charged particles that occur in a cloud surrounding the nucleus. Because these particles have opposite charges, they are attracted to each other. Electrons are always vibrating around protons. These movements produce invisible electric and magnetic forces that travel away from atoms in all directions. These forces move together in the form of waves called electromagnetic waves. Electromagnetic waves traveling through space are called radiation.

Electromagnetic radiation has a range of wavelengths. Wavelength is the distance between one crest of a wave and the next corresponding crest. The total range of wavelengths of radiation is called the electromagnetic spectrum. Waves in the electromagnetic spectrum are grouped according to smaller wavelength ranges. For example, long radio waves have wavelengths from several centimeters to thousands of kilometers. The wavelengths of short gamma rays, on the other hand, are less than 0.003 nanometers - shorter than the width of an atom. Though the wavelengths of radiation may differ, the speed at which they travel does not. All types of radiation travel with the speed of light, which is about 300,000 kilometers per second.

electromagnetic spectrum diagram
The electromagnetic spectrum. Source: NASA

Most of the wavelengths of the electromagnetic spectrum are invisible to the human eye. Certain wavelengths, however, are visible. This energy is called visible light. We see these waves as colors. Each color has a different wavelength. The longest wavelengths of visible light are red, and the shortest are violet. When all the waves are seen together, they make white light. Sometimes we can see the different colors in white light when it passes through a medium such as a piece of glass or ruler and slows down.

All objects contain atoms and emit radiation. All objects emit all forms of radiation, but the wavelength of most intense radiation an object emits depends on its temperature. As the temperature of an object rises, the wavelength of most intense radiation that it gives off decreases and becomes longer.

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Solar Radiation

spectrum of solar radiation diagram
The spectrum of solar radiation.
Source: NASA

The electromagnetic radiation reaching the Earth from the Sun is known as solar radiation. It is formed by the atoms that make up the Sun. The Sun is a gigantic ball of gas that produces energy by joining hydrogen atoms into helium in its core, a process known as nuclear fusion. These reactions make the Sun very hot, over 5,500° C at its surface, and release enormous amounts of electromagnetic radiation in all directions.

Solar radiation is mostly concentrated in the visible-light part of the electromagnetic spectrum. Visible light represents 43% of the total radiation emitted by the Sun. Solar radiation includes other wavelengths of radiation as well. Half of the energy emitted by the Sun is spread over wavelengths longer than those of visible light, with most of this radiation being infrared radiation. Wavelengths shorter than visible light, like ultraviolet radiation, account for 7% of the total radiation emitted by the Sun.

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Variations in Solar Radiation

Every location on Earth's surface receives solar radiation, at least part of the year. The rate at which a given area receives this energy depends on several factors, including latitude, season, time of day, landscape, and weather.

Solar radiation strikes the Earth at different angles. This occurs because the Earth's surface is curved. The latitude of a particular area determines the angle at which solar radiation is received. Solar radiation hits the area around the equator directly, at nearly a 90° angle. At this location, the Sun appears overhead. Near the poles, solar energy strikes the surface at a smaller angle than at the equator. This smaller angle spreads the same amount of energy over a larger area. At the poles, the Sun appears low in the sky.

Solar radiation varies with latitude diagram
Solar radiation varies with latitude.
Source: NASA

In addition to latitude, the solar radiation received by the Earth is also affected by the Earth's orbit of the Sun. The Earth's axis is presently at an angle of 23.5° to the plane of orbit around the Sun. This causes different parts of the Earth to be tilted toward the Sun over the course of a year. When the Northern Hemisphere tilts away from the Sun, the Sun is low in the sky and days are short. Energy from the Sun is more spread out during this time, causing the lower temperatures of winter. When the Northern Hemisphere tilts toward the Sun, the Sun is high and days are long. Solar radiation strikes the earth directly, causing the higher temperatures of summer. Between these two extremes lie spring and autumn. In the Southern Hemisphere, the effect is the opposite.

Earth's orbit diagram
Solar radiation received by the Earth is affected by the Earth's orbit.
Source: NOAA

The rotation of the Earth is also responsible for variations in solar radiation received at the Earth's surface. The Earth rotates on its axis once each day. At any point in time, solar radiation is lighting up half of the Earth (day) while the other half is dark (night). Near sunrise and sunset, solar energy hits the Earth at a very low angle because the Sun is low in the sky. At these times, solar energy is spread out, or diffuse. At noon, the Sun is at its highest point and an area receives the greatest concentration of solar energy.

Solar radiation diagram
The receipt of solar radiation varies over the course of a day.
Source: NASA

Landscape and weather are also factors that determine the rate at which an area receives solar radiation. Clouds scatter and absorb solar radiation and reduce the amount of direct solar radiation that reaches the Earth's surface. The slope of the land surface is also a determinant. Surfaces that face in the direction of the Sun receive more solar radiation than those that face away from the Sun. For example, north facing walls are much shadier and cooler than those facing south.

Due to the combined factors that influence the rate of solar radiation, some areas of Earth's surface receive greater amounts of solar radiation than others. In the United States, the southwest is the part of the country that receives the most solar radiation. This includes parts of California, Nevada, Utah, New Mexico and Texas. This region is closest to the equator, so the Sun is more directly overhead than other regions. In addition, for much of the year, atmospheric conditions in this part of the country are clear and without much cloud cover. As a result, the southwest has the greatest potential for using solar radiation as an energy resource.

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Solar Heating

Solar Heating System diagram
A passive solar heating system.
Source: U.S. Department of Energy

In order for solar radiation to be used as an energy resource, it must be converted into other types of energy. One way is to convert solar radiation into thermal, or heat energy. The resulting heat can be used for a variety for purposes, including heating homes, buildings, and water.

In passive solar heating systems, buildings are designed to let in large amounts of sunlight. The heat produced from the light is trapped inside. Buildings designed for passive solar heating usually have large, south-facing windows with overhangs. The overhang acts like an umbrella, shielding the interior. In winter, the sunlight shines directly through the large windows. The floors and walls heat up during the day and release the heat slowly at night. In summer, the high Sun is blocked by the overhang from shining directly into the building and making it too hot.

Solar heating system collecto
A solar heating system collector.
Source: The American Geological Institute,
EarthComm: Project-Based Space and Earth
System Science, 2nd Edition
, Armonk,
New York: It's About Time, 2012, pg. 783.

Active solar heating systems use special equipment to collect and distribute solar energy. These systems use a collector, usually a large flat panel, to absorb solar radiation. The panel is mounted on the roof of a building, facing the Sun. While the Sun is shining, the solar radiation that strikes the panel is converted to heat. Air or a liquid is then pumped through the panel and into a storage tank. As it passes through the panel, the air or liquid is warmed. The building is then heated from the storage tank. Sometimes, water is used in a solar heating system and the heated water is used for household purposes, such as washing dishes or clothing.

Solar Water Heater diagram
An active solar heating system used to heat water.
Source: U.S. Department of Energy

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Solar Electricity

Solar electricity is the conversion of solar radiation into electricity. Currently, there are two main ways by which this is accomplished: concentrated solar power and photovoltaic cells.

Solar panels
Concentrated solar power uses mirrors to reflect and concentrate solar radiation onto pipes containing a fluid.
Source: Left image: Wikipedia | Right image: Wikipedia

Concentrated solar power involves focusing the Sun's energy to boil water, which is then used to provide power. In these systems, mirrors reflect and concentrate solar radiation onto pipes containing a fluid. The fluid heats up and is circulated to a series of other pipes that allow for the heat to be transferred to water. Eventually the water boils and changes into steam. The steam in turn drives turbines and generators to make electricity. In this way, solar radiation is first converted to thermal energy in steam. Then, the turbine converts the thermal energy into mechanical energy. Finally, the mechanical energy of the turbine is converted into electrical energy through the generator.

Concentrated solar power technology is fairly new, and power plants that use concentrated solar power are starting to be built to generate electricity. These are known as solar thermal power plants. Today, there are around 10 such plants in the United States, including several in California and Arizona, one in Nevada, one in Florida, and one in Hawaii. A plant located in Harper Lake, CA is the largest solar thermal power plant in the world.

Liddell Power Station
Solar thermal power plants use the Sun's energy to boil water, which is then used to drives turbines and generators to make electricity.
Source: Wikipedia
Photovoltaic cells
Photovoltaic cells
Source: Wikipedia

The second way that solar radiation can be used to produce electricity is through photovoltaic cells, commonly known as solar cells. This technology uses special materials to convert solar energy directly into electrical energy. Photovoltaic cells are composed of thin, transparent layers of boron and phosphorous enriched silicon. When sunlight strikes the silicon, it dislodges electrons. The structure of the photovoltaic cell guides the electrons in such a way that their flow creates an electrical current.

A single photovoltaic cell produces only a small amount of electricity. However, many cells can be connected together into a panel to increase the amount of power produced. Likewise, panels can be wired together into very large arrays for even more energy.

Solar panel and solar array
Left image: Solar cell panel. Source: Wikipedia
Right image: Solar cell array. Source: Wikipedia

Simple photovoltaic systems are commonly used to provide power for small consumer items such as toys, calculators, and wristwatches. Larger systems provide electricity for other purposes, including water pumps, road and traffic signs, and communications satellites. These systems are sometimes installed on the roofs of houses and buildings to provide electricity for lighting and to run appliances. The most complex and largest systems are used in photovoltaic power plants. Today, there are less than 10 such plants in the United States, including ones in California, Nevada, New York, Arizona, and Florida. However, more than a dozen more have been proposed or are being built, and will be operating in the near future.

solar panels on a house and a photovoltaic power plant
Left image: Solar cell panels can provide electricity for a house. Source Wikipedia
Right image: Solar cell arrays at a photovoltaic power plant. Source Wikipedia

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Advantages and Disadvantages of Solar Energy

Solar energy has a number of advantages over other sources of energy. First, solar energy systems are very clean and do not produce air pollutants. This includes active and passive solar heating systems, concentrated solar power systems and photovoltaic systems. Second, despite the fact that solar energy systems may have set-up and maintenance costs, the solar radiation that subsequently powers them is free. Thus, the long term costs decrease each year and eventually the system pays for itself. An additional benefit is that solar radiation will never be depleted. Also, solar power is incredibly versatile and can be used to power a variety of technologies.

In terms of generating electricity, photovoltaic systems have several additional advantages. Photovoltaic panels and arrays can be installed quickly and in any size to meet the needs of a range of consumers. Furthermore, photovoltaic systems are relatively simple and have no moving parts. These systems require little maintenance. Also, photovoltaic cells are fairly durable and last a very long time before they need to be replaced. In remote locations, photovoltaic systems can be a more feasible option for local electricity generation than running long wires to connect to an electrical grid.

There are several disadvantages to solar power. The amount of solar radiation that arrives at the Earth's surface is not constant. It varies depending on the location, time of day, time of year, and weather conditions. This means that despite the continuous flow of energy from the Sun it is not continuously available everywhere. Also, in those places which receive relatively more solar radiation than others there are still peaks and troughs in energy receipt. When there is not enough solar radiation to power solar energy systems, then other energy sources must be used.

Photovoltaic systems have some added disadvantages. Currently, the cost of photovoltaic cells is high. In addition, they are not very efficient. The efficiency of most commercial photovoltaic panels and arrays in converting solar radiation to electricity ranges from 5% to 15%. However, photovoltaic technology is constantly improving and photovoltaic cell costs are decreasing. Researchers are also trying to achieve efficiencies up to 30%.

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