Solar Energy Resources
Solar radiation, often called the solar resource, is a general term for the electromagnetic radiation emitted by the sun. 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:
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. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other 6 months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.
Countries such as the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun’s rays are far more slanted during the shorter days of the winter months. Cities such as Denver, Colorado, (near 40° latitude) receive nearly three times more solar energy in June than they do in December.
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).
Concentrating Solar Power
This solar concentrator has a fixed-focus faceted dish with a concentration of about 250 suns. This system can be used for large fields connected to the utility grid, hydrogen generation, or water pumping.
Credit: Science Applications International Corporation / PIX 13464
Concentrating solar power (CSP) technologies use mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine that drives a generator.
Concentrating solar power offers a utility-scale, firm, dispatchable renewable energy option that can help meet our nation’s demand for electricity. CSP plants produce power by first using mirrors to focus sunlight to heat a working fluid. Ultimately, this high-temperature fluid is used to spin a turbine or power an engine that drives a generator. The final product is electricity.
Smaller CSP systems can be located directly where power is needed. For example, single dish/engine systemscan produce 3 to 25 kilowatts of power and are well suited for distributed applications.
There are several varieties of CSP systems. Learn more about:
Linear Concentrator Systems
Dish/Engine Systems
Power Tower Systems
Thermal Storage Systems.
Or learn more about the latest CSP research from the EERE Solar Energy Technologies Program.
Energy 101: Concentrating Solar Power Basics
From towers to dishes to linear mirrors to troughs, concentrating solar power (CSP) technologies reflect and collect solar heat to generate electricity. A single CSP plant can generate enough power for about 90,000 homes. This video explains what CSP is, how it works, and how systems like parabolic troughs produce renewable power.
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Linear Concentrator Systems for Concentrating Solar Power
The Solar Electric Generating Station IV power plant in California consists of many parallel rows of parabolic trough collectors that track the sun. The cooling towers can be seen with the water plume rising into the air, and white water tanks are in the background.
Credit: Sandia National Laboratory / PIX 14955
Linear concentrating solar power (CSP) collectors capture the sun’s energy with large mirrors that reflect and focus the sunlight onto a linear receiver tube. The receiver contains a fluid that is heated by the sunlight and then used to create superheated steam that spins a turbine that drives a generator to produce electricity. Alternatively, steam can be generated directly in the solar field, which eliminates the need for costly heat exchangers.
Linear concentrating collector fields consist of a large number of collectors in parallel rows that are typically aligned in a north-south orientation to maximize annual and summer energy collection. With a single-axis sun-tracking system, this configuration enables the mirrors to track the sun from east to west during the day, which ensures that the sun reflects continuously onto the receiver tubes.
Parabolic Trough Systems
The most common CSP system in the United States is a linear concentrator that uses parabolic trough collectors. In such a system, the receiver tube is positioned along the focal line of each parabola-shaped reflector. The tube is fixed to the mirror structure, and the heated fluid—either a heat-transfer fluid or water/steam—flows through and out of the field of solar mirrors to where it is used to create steam (or, in the case of a water/steam receiver, it is sent directly to the turbine).
The largest individual trough systems generate 80 megawatts of electricity. However, individual systems being developed will generate 250 megawatts. In addition, individual systems can be co-located in power parks. Their capacity would be constrained only by the transmission capacity of nearby power lines and the availability of contiguous land.
A linear concentrator power plant using parabolic trough collectors.
Trough designs may incorporate thermal storage. In these systems, the collector field is oversized to heat a storage system during the day that can be used in the evening or during cloudy weather to generate additional steam to produce electricity. Parabolic trough plants can also be designed as hybrids, meaning that they use fossil fuel to supplement the solar output during periods of low solar radiation. In such a design, a natural gas-fired heater or gas-steam boiler/reheater is used. In the future, troughs may be integrated with existing or new combined-cycle natural-gas- and coal-fired plants.
Linear Fresnel Reflector Systems
A second linear concentrator technology is the linear Fresnel reflector system. Flat or slightly curved mirrors mounted on trackers on the ground are configured to reflect sunlight onto a receiver tube fixed in space above the mirrors. A small parabolic mirror is sometimes added atop the receiver to further focus the sunlight.
A linear Fresnel reflector power plant.
More Information
For more information about linear concentrator power plant technologies, visit the National Renewable Energy Laboratory’s TroughNet Web site or the EERE Solar Energy Technologies Program’s Linear Concentrators Research and Developmentpage.
To learn about other types of concentrating solar power systems, see:
Dish/Engine Systems for Concentrating Solar Power
Power Tower Systems for Concentrating Solar Power
Thermal Storage Systems for Concentrating Solar Power.
Dish/Engine Systems for Concentrating Solar Power
The dish/engine system is a concentrating solar power (CSP) technology that produces relatively small amounts of electricity compared to other CSP technologies—typically in the range of 3 to 25 kilowatts. Dish/engine systems use a parabolic dish of mirrors to direct and concentrate sunlight onto a central engine that produces electricity. The two major parts of the system are the solar concentrator and the power conversion unit.
