Let The Sun Shine!
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Can we use the sun to help us do work? Of course we can! The type of energy we get from the sun is called solar energy. There are two types of solar energy, passive solar energy and active solar energy.
Passive solar energy is when solar energy is used to light something or heat something without requiring electrical or mechanical components. Passive solar energy includes lighting a house using windows or skylights, or heating a house by positioning it to receive the most direct sunlight during the cold season and minimize the solar gain from the sun during the warm season. People have used passive solar design for a long time. Hundreds of years ago, Native Americans built their houses inside cliffs, which was an innovative passive solar energy design. The cliff dwellings at Colorado's Mesa Verde National Park kept their inhabitants cool in the summer and warm in the winter (see Figure 1). Active solar energy is when energy from the sun is transformed into electrical or mechanical energy, which is used to run fans, pumps and electrical controls. Engineers design the solar energy technologies to generate electricity from sunlight. One of the most successful uses of solar energy is to run an engine or a generator. Have you ever deliberately stood in the sun to warm up? Have you ever touched a piece of metal after it had been sitting in the sun? It's hot! Some solar power plants produce electric power by changing the sun's energy into high-temperature heat using mirrors or solar panels. The heat is channeled through a generator and changed into electricity that we can use in our homes to run radios and refrigerators. The most common type of solar power plant is a power tower system. The sun's energy is concentrated by a field of hundreds or even thousands of mirrors (called "heliostats") onto a solar panel receiver located on top of a tower. When the energy heats up molten salt flowing through the receiver, the salt's thermal energy generates electricity in a conventional steam generator. Use a browser for photographs of the 144 heliostats in the University of California-Davis's CACTUS solar power plant in the Mojave Desert. When is the best time of day for solar energy? Well, during the day, when the sun is out! Since people use electricity 24 hours a day, but solar power cannot be generated at night or when it is very cloudy, engineers design solar power plants to store energy, too. They use molten salt because it retains thermal energy well and can be stored for hours or even days until it is needed to generate electricity. Another type of solar concentrator designed by engineers is a trough system. In it, the sun's energy is concentrated with mirror reflectors shaped like a trough or a rain gutter (parabolic), onto a receiver pipe running along the middle of the inside of the curved surface (see Figure 2). The sun's energy heats oil flowing through the pipe and the heat energy generates electricity in a conventional steam generator. The southwestern region of the U.S. offers the best development opportunity for solar technology. Why do you think this is? Well, it is pretty sunny there! For example, if an area about one-tenth the size of the state of Nevada — a plot of land 100 miles (161 km) on a side — was covered with parabolic trough systems, enough electric power for the entire U.S. could be generated! This region is also where the most amount of electricity is used in the hot summer months when air conditioning is in heavy use. When do we use the most air conditioning? In the hottest part of the day — which is also the sunniest time of the day. These high amounts of energy demand (peak loads) happen at the same time that solar concentrators produce the most energy, so, solar power plants are well matched to meet summer electricity demand. When do you think is the best time of day to obtain the most solar energy? Best month? Best season? Best direction — north, south, east or west? Well, the sun changes its position every day of the year, and the best direction to orient a solar panel depends on the time of the day and the day of the year. Engineers need to understand the movement of the sun to determine when and where to get the most energy from the sun. The answer to all of these questions is: when the sun is directing most energy our way! Another way to capture solar energy is with photovoltaic cells (also called solar cells or PV cells). Have you ever used a solar calculator? Engineers designed a solar cell in the calculator to convert light energy into power. Photovoltaic cells are considered expensive and are usually made from silicon crystals. Many PV cells are often linked together to make solar panels. They use sunlight to power satellites, lights, road signs and household appliances. You may have seen them on roofs. The photovoltaic system for a home might require a dozen panels while a calculator may have only one PV cell. All over the world, engineers are investigating and designing solar energy systems because the sun is a renewable energy source that offers promise as an inexhaustible source of electricity. While sunny locations area good place to study how solar energy works, we need to figure out how we can use solar energy in cloudy or cooler places as well. Using the Sun to Regulate Building Temperature — Passive Solar Design Passive solar buildings are oriented in an east/west direction to allow the most sunlight to enter the building during the winter and keep it out during summer. In the temperate latitudes, the sun is low in the sky in winter and high in the sky in summer. Overhangs on windows reduce excess heating in summer while allowing in all the sunlight in during winter, thus warming the building. In the northern hemisphere, overhangs are most important on windows on the south and west sides of buildings. In parts of the world in which people can afford window glazing, passive solar design includes larger windows on the south side of the building in the northern hemisphere or on the north side in the southern hemisphere. Typically, fewer and smaller windows are used on the other sides of the building. Well-insulated, double- or triple-pane windows are best for solar applications, particularly with appropriate coatings, because they allow sunlight to transmit to the inside during the cold season and reflect heat during the warm season. The use of insulation is very important for successful passive solar design in temperate climates. Wall and roof insulation reduces the need for both heating and cooling because it minimizes heat transfer between the building interior and the outdoors. Insulation — whether fiberglass, straw, foam or a thick masonry wall — helps to maintain a building's temperature. The presence of thermal mass — such as adobe, water, concrete, bricks, tile, stone — helps to maintain a building's temperature. Floors and walls made of these materials keep buildings warm at night by releasing the energy they gained from sunlight during the day. In the summer, thermal mass keeps buildings from getting too hot if they are cooled at night (night ventilation). Good air exchange, or ventilation, is very important to efficiently cool a building in summer. In the northern hemisphere, deciduous trees on the south and west sides of a building provide shade on warm afternoons, reducing the need for cooling. In the winter, deciduous trees lose their leaves and no longer block the sunlight from entering the building to warm it. Passive solar design is used in most parts of the world, but it works best in places with clear skies. It is used most commonly for smaller buildings and homes, although some aspects (such as daylighting, shading, etc.) are incorporated into commercial structures. In cold climates, passive solar design maximizes heating from the sun; in hot climates, it minimizes heating from the sun. Passive solar buildings do not cost more than conventional buildings if they are designed by a knowledgeable engineer or architect. And, passive solar buildings save their occupants money over their lifetime with reduced energy bills. |
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Vocabulary/Definitions
Absorb: To be taken into a material without transmission or reflection.
Active solar: Solar energy generating systems that require electrical or mechanical components, such as fans, pumps and electrical controls. These systems can be used for heating water or heating/cooling buildings.
Convection: The transfer of thermal energy in a fluid (gas or liquid) by the circulation of currents in the heated fluid causing warmer packets to rise while cooler packets sink.
Energy: The ability to do work.
Heat exchanger: A device, such as an automobile radiator, that transfers heat from one liquid to another without allowing them to mix.
Heat-transfer fluid: A fluid circulated in a heat exchanger; this fluid gains energy from one region and transfers it to another region.
Insulation: A material used to prevent the passage of heat, electricity or sound; a non-conducting material.
Light energy: Visible light energy, such as from the sun, light bulb, fireflies, computer screens or stars, is one form of electromagnetic energy. Others forms include infrared, ultraviolet, radio and x-ray. Your eyes are detectors of visible light energy.
Passive solar: Solar energy generating systems that do not require electrical or mechanical components. These systems directly heat water or buildings, or reduce solar heat gain of buildings (for example, with window awnings to keep buildings cool).
Photovoltaic (PV) cells: See solar cells.
Photovoltaic system: A system that converts solar energy into electricity.
Reflect: To cause to bend back or return upon striking a surface.
Renewable energy: Energy that is made from sources that can be regenerated. Sources include solar, wind, geothermal, biomass, ocean and hydro (water).
Solar cells: Semiconductor devices that convert the energy of sunlight into electric energy. Also called photovoltaic cells or PV cells.
Solar energy: Energy from the sun.
Solar panel: A group of connected solar cells.
Thermal mass: Materials that store thermal energy, such as water, concrete or brick.
Transmit: To allow to pass through a material.
Ventilation: To admit fresh air into a space; to replace existing air.
Absorb: To be taken into a material without transmission or reflection.
Active solar: Solar energy generating systems that require electrical or mechanical components, such as fans, pumps and electrical controls. These systems can be used for heating water or heating/cooling buildings.
Convection: The transfer of thermal energy in a fluid (gas or liquid) by the circulation of currents in the heated fluid causing warmer packets to rise while cooler packets sink.
Energy: The ability to do work.
Heat exchanger: A device, such as an automobile radiator, that transfers heat from one liquid to another without allowing them to mix.
Heat-transfer fluid: A fluid circulated in a heat exchanger; this fluid gains energy from one region and transfers it to another region.
Insulation: A material used to prevent the passage of heat, electricity or sound; a non-conducting material.
Light energy: Visible light energy, such as from the sun, light bulb, fireflies, computer screens or stars, is one form of electromagnetic energy. Others forms include infrared, ultraviolet, radio and x-ray. Your eyes are detectors of visible light energy.
Passive solar: Solar energy generating systems that do not require electrical or mechanical components. These systems directly heat water or buildings, or reduce solar heat gain of buildings (for example, with window awnings to keep buildings cool).
Photovoltaic (PV) cells: See solar cells.
Photovoltaic system: A system that converts solar energy into electricity.
Reflect: To cause to bend back or return upon striking a surface.
Renewable energy: Energy that is made from sources that can be regenerated. Sources include solar, wind, geothermal, biomass, ocean and hydro (water).
Solar cells: Semiconductor devices that convert the energy of sunlight into electric energy. Also called photovoltaic cells or PV cells.
Solar energy: Energy from the sun.
Solar panel: A group of connected solar cells.
Thermal mass: Materials that store thermal energy, such as water, concrete or brick.
Transmit: To allow to pass through a material.
Ventilation: To admit fresh air into a space; to replace existing air.
Website Retrieved From: Teach Engineering http://www.teachengineering.org/view_lesson.php?url=collection/cub_/lessons/cub_energy2/cub_energy2_lesson09.xml
Date Retrieved: 11/1/12
Contributors (authors)
Xochitl Zamora-Thompson, Sabre Duren, Jeff Lyng, Malinda Schaefer Zarske, Denise Carlson
Copyright
© 2005 by the University of Colorado
Supporting Program
Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Last Modified: June 26, 2012
Date Retrieved: 11/1/12
Contributors (authors)
Xochitl Zamora-Thompson, Sabre Duren, Jeff Lyng, Malinda Schaefer Zarske, Denise Carlson
Copyright
© 2005 by the University of Colorado
Supporting Program
Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Last Modified: June 26, 2012