How Solar Energy Works
The Solar Resource
The amount of energy from the sun that falls on Earth’s surface is enormous. All the energy stored in Earth’s reserves of coal, oil, and natural gas is matched by the energy from just 20 days of sunshine. Outside Earth’s atmosphere, the sun’s energy contains about 1,300 watts per square meter. About one-third of this light is reflected back into space, and the Earths atmosphere absorbs some of it.
By the time it reaches Earth’s surface, the energy in sunlight has fallen to about 1,000 watts per square meter at noon on a cloudless day. Averaged over the entire surface of the planet, 24 hours per day for a year, each square meter collects the approximate energy equivalent of almost a barrel of oil each year, or 4.2 kilowatt-hours of energy every day. Deserts, with very dry air and little cloud cover, receive the most sun—more than six kilowatt-hours per day per square meter. Northern climates, such as Boston, get closer to 3.6 kilowatt-hours. Sunlight varies by season as well, with some areas receiving very little sunshine in the winter. Seattle in December, for example, gets only about 0.7 kilowatt-hours per day. It should also be noted that these figures represent the maximum available solar energy that can be captured and used, but solar collectors capture only a portion of this, depending on their efficiency. For example, a one square meter solar electric panel with an efficiency of 15 percent would produce about one kilowatt-hour of electricity per day in Arizona.
So you can now see that this crucial first step in solar power generation is an amazing universal wonder. The sheer power of the sun is so immense it would be a shame to waste it! Read on to learn how solar power is generated from this resource.
Solar power is produced by collecting sunlight with photovoltaic solar panels and converting its power into electricity. A photovoltaic (photo = light, voltaic = electricity) panel is formed by sandwiching thin layers of positive and negative silicon beneath a layer of non-reflective glass. Silicon is a semi conductive substance that is able to alter the sun’s energy into electricity. When the 2 sheets are sandwiched together there exists an energy field between them. When sunlight strikes the silicon sandwich this energy field becomes a hotbed of electrical activity. Photons from the sunlight collide with the atoms contained in the layers, freeing charged electrons and bumping them into the energy field. Like a waterfall, the electrons are able to move in only one direction causing a buildup on one side of the energy field.
To give those crowded electrons somewhere to go, an external circuit (wire leading out of the over full side and around to the other) is created and directs the charged electrons out of the panel and back in to the other side. When reaching that side they are in the form of raw electricity. The resulting electricity can be used in its natural form (DC or 12 volt) if you have appliances that use 12 volt energy. However DC is not useable for most common purposes. So, next the DC power is transformed through an inverter to alternating current, or AC at 120 Volt, a common-use voltage. A small amount of solar energy is lost in this DC to AC conversion but is now ready for distribution to the household.
Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.
Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.
Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules connected to the electricity grid through an inverter, often in combination with a net metering arrangement.
Solar cells are regarded as one of the key technologies towards a sustainable energy supply. They are constantly under research development to achieve maximum efficiency at minimum cost. We are now at the third generation of solar power development. This is an exciting time for solar science and solar power generation is at an all time peak. From here on solar power will only get cheaper and cheaper, empowering us to have energy independence.
Three generations of development
The first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of sunlight. These cells are typically made using a silicon wafer. First generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market.
The second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. Later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This contributed to a prediction of greatly reduced costs for thin film solar cells. Currently (2007) there are different technologies/semiconductor materials under investigation or in mass production, such as amorphous silicon, poly-crystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. Typically, the efficiencies of thin-film solar cells are lower compared with silicon (wafer-based) solar cells, but manufacturing costs are also lower, so that a lower price in terms of $/watt of electrical output can be achieved. Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light materials or flexible materials, even textiles.
Third generation photovoltaics are very different from the other two, broadly defined as semiconductor devices, which do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include photoelectrochemical cells, Polymer solar cells, and nanocrystal solar cells.
Companies working on third generation photovoltaics include Xsunx, Konarka Technologies, Inc., Nanosolar and Nanosys. Research is also being done in this area by the USA’s National Renewable Energy Laboratory (http://www.nrel.gov/).
How does solar power work at night?
Home Solar Power Systems require electricity 24 hours a day. In order for a solar energy system to provide all day and night power the panels must be tied to a utility grid or a battery bank.
- A utility grid provides power to your home during night hours and cloudy days when your panels don’t keep up with demand. Also, when your system is humming along making more power than your household needs the extra energy is sent to the grid rather than your home and the power company buys the energy from you and sells it to your neighbors.
- A battery bank stores excess energy for powering your household at night and on cloudy days or weeks. The addition of batteries is what makes a solar power system completely independent. The system is now capable of making energy and storing it for use later so that power is available to the home at night and on cloudy days.That’s how solar power works in your home when the sun isn’t shining.
Passive Solar Energy
There are more uses for solar energy than just generating electricity directly. Solar energy can also be used to heat everything including our homes, running water and even food. Solar energy can also be used to generate electricity in the “old fashioned” way by heating water to boiling pressure in order to spin turbines. So you see solar energy is indeed universal! (Pun intended)
Passive Solar Design for Buildings
One simple, obvious use of the sun is to light and heat our buildings. Residential and commercial buildings account for more than one-third of U.S. energy use. If properly designed, buildings can capture the sun’s heat in the winter and minimize it in the summer, while using daylight year-round. Buildings designed in such a way utilize passive solar energy—a resource that can be tapped without mechanical means to help heat, cool, or light a building. Simple design features such as properly orienting a house toward the south, putting most windows on the south side of the building, skylights, awnings, and shade trees are all techniques for exploiting passive solar energy. Buildings constructed with the sun in mind can be comfortable and beautiful places to live and work.
Solar Heat Collectors
Besides using design features to maximize their use of the sun, some buildings have systems that actively gather and store solar energy. Solar collectors, for example, sit on the rooftops of buildings to collect solar energy for space heating, water heating, and space cooling. Most are large, flat boxes painted black on the inside and covered with glass. In the most common design, pipes in the box carry liquids that transfer the heat from the box into the building. This heated liquid—usually a water-alcohol mixture to prevent freezing—is used to heat water in a tank or is passed through radiators that heat the air.
Oddly enough, solar heat can also power a cooling system. In desiccant evaporators, heat from a solar collector is used to pull moisture out of the air. When the air becomes drier, it also becomes cooler. The hot moist air is separated from the cooler air and vented to the outside. Another approach is an absorption chiller. Solar energy is used to heat a refrigerant under pressure; when the pressure is released, it expands, cooling the air around it. This is how conventional refrigerators and air conditioners work, and it’s a particularly efficient approach for home or office cooling since buildings need cooling during the hottest part of the day. These systems are currently at work in humid southeastern climates such as Florida.
Solar collectors were quite popular in the early 1980s, in the aftermath of the energy crisis. Federal tax credits for residential solar collectors also helped. In 1984, for example, 16 million square feet of collectors were sold in the United States, but when fossil fuel prices dropped and tax credits expired in the mid-1980s, demand for solar collectors plummeted. By 1987, sales were down to only four million square feet. Most of the more than one million solar collectors sold in the 1980s were used for heating hot tubs and swimming pools.
Today, a small number of U.S. homes and businesses use solar water heaters. In other countries, solar collectors are much more common; Israel requires all new homes and apartments to use solar water heating, and 92 percent of the existing homes in Cyprus already have solar water heaters. But the number of Americans choosing solar hot water could rise dramatically in the next few years as a result of federal tax incentives that can reduce their cost by as much as 30 percent.
According to the U.S. Department of Energy, water heating accounts for about 15 percent of the average household’s energy use. As natural gas and electricity prices rise, the costs of maintaining a constant hot water supply will increase as well. Homes and businesses that heat their water through solar collectors could end up saving as much as $250 to $500 per year depending on the type of system being replaced.
Solar Thermal Concentrating Systems
By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems can produce very high temperatures—as high as 5432 degrees Fahrenheit. This intense heat can be used in industrial applications or to produce electricity. One of the greatest benefits of large-scale solar thermal systems is the possibility of storing the sun’s heat energy for later use, which allows the production of electricity even when the sun is no longer shining. Properly sized storage systems, commonly consisting of molten salts, can transform a solar plant into a supplier of continuous base load electricity. Solar thermal systems now in development will be able to compete in output and reliability with large coal and nuclear plants.
Solar concentrators come in three main designs: parabolic troughs, parabolic dishes, and central receivers. The most common is parabolic troughs-long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. The liquid, at about 572 degrees Fahrenheit, runs to a central collector, where it produces steam that drives an electric turbine.
Parabolic dish concentrators are similar to trough concentrators, but focus the sunlight on a single point. Dishes can produce much higher temperatures, and so, in principle, should produce electricity more efficiently.
A promising variation on dish concentrating technology uses a stirling engine to produce power. Unlike a car’s internal combustion engine, in which gasoline exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. These dish-stirling generators produce about 30 kilowatts of power, and can be used to replace diesel generators in remote locations.
The third type of concentrator system is a central receiver. One such plant in California features a “power tower” design in which a 17-acre field of mirrors concentrates sunlight on the top of an 80-meter tower. The intense heat boils water, producing steam that drives a 10-megawatt generator at the base of the tower. The first version of this facility, Solar One, operated from 1982 to 1988 but had a number of problems. Reconfigured as Solar Two during the early to mid-1990s, the facility is successfully demonstrating the ability to collect and store solar energy efficiently. Solar Two’s success has opened the door for further development of this technology.
To date, the parabolic trough has had the greatest commercial success of the three solar concentrator designs, in large part due to the nine Solar Electric Generating Stations (SEGS) built in California’s Mojave Desert from 1985 to 1991. Ranging from 14 to 80 megawatts and with a total capacity of 354 megawatts, each of these plants is still operating effectively. Nevada Solar One, a 75 MW parabolic trough plant that was built near Boulder City, Nevada in 2007, offers another example of recent success in the burgeoning U.S. solar thermal industry.
More commercial-scale solar concentrator projects are under development in the United States, thanks mostly to various state policies and incentives. To help meet California’s 20 percent renewable electricity standard, for example, almost 5,000 MW of solar thermal capacity are under review by the state’s Energy Commission and Bureau of Land Management. Additionally, more than 3,500 MW of capacity have been announced or agreed to under power purchase agreements between major utilities and power-producing companies. As of 2009, the largest project awaiting approval is a 1,000 MW plant to be owned by Solar Millenium, LLC. Concentrating solar thermal is on its way to becoming a strong competitor in utility-scale energy production.
I hope this article has helped you to understand how solar power works for all its different uses. And I hope that it was not too difficult and technical to read through and learn from. Solar energy is really not too complicated but like any science it can get really complicated if you want to know every little detail. I tried to stay away from long technical math and physics with this article on how solar power works and believe I did a good job! Please comment and let me know what you think, and as always don’t forget to share the info with your friends! Thanks for reading.