Solar
Everyone on this planet can benefit from solar energy, some more than others. Renewable energy such as energy from the sun, has been used by humans throughout history to do everyday chores and to maintain daily life. For example, the sun has always provided humans with warmth as well as food. Without the energy from the sun, trees and plants would not be able to photosynthesize, which would be devastating to life on Earth.
Solar energy is produced by the sun, which is a gigantic nuclear fusion reactor running on hydrogen fuel. The sun converts five million tons of matter into energy every second. Solar energy comes to Earth in the form of visible light and infrared radiation. Scientists expect that the sun will continue to provide light and heat energy for the next five billion years.
The amount of solar energy that strikes Earth's surface per year is about 29,000 times greater than all of the energy used in the United States. The solar energy falling on Wisconsin each year is roughly equal to 844 quadrillion Btu of energy, which is about 550 times the amount of energy used in Wisconsin. Although the amount of solar energy reaching Earth's surface is immense, it is spread out over a large area. There are also limits to how efficiently it can be collected and converted into electricity and stored. These factors affects the amount of solar energy that can actually be used.
Solar Energy for Heat/Light
Space Heating/Lighting - Passive
Systems
Through historical documents and artifacts, we can see that humans
have been utilizing solar energy for a long time. For example, ancient
Greeks oriented their homes for passive solar gain because it was
practical and useful. Let's explore passive solar and how we can
incorporate passive solar design into our homes. Observe the images
below.
Passive solar design uses a building's orientation, structure, and materials to capture the sun's energy and is a way to heat and light a building, such as the windows you see in the above photos. There normally aren't any fans, pumps, or controllers in a passive solar heating system. The windows allow the sun's heat to warm the building inside. Rooms called sunspaces, solariums, greenhouses, and sunrooms are also built on the south side of a home or building to collect solar energy. The building is often designed so that the warmed air from these spaces can naturally circulate to other rooms. Sometimes, a stone or brick floor or wall will allow more of the sun's heat to be absorbed, which will then be emitted over a longer period of time, most favorably at nighttime. The walls and floor of a building which absorbs the sun's heat becomes the storage component of the energy, also called thermal mass. To learn more about Passive Solar Design in Wisconsin, click here (.pdf).
Cool Daylighting is a part of passive solar design which uses natural light to reduce the need for electric lighting while reducing solar heat gain. Cool daylighting can include sun roofs, sun tubes, clerestory design, and windows.
Sun Tube
Photos courtesy of
Sun-Dome
Clerestory Design
A clerestory design allows the
winter sun to enter the room on cold winter days when the sun is lower
in the sky. During summer months when the sun is higher in the sky, an
overhang blocks the sunlight from directly entering the room, which
keeps the room cooler as well as bright.
Visit Wisconsin Solar Design for more information on skylights, greenhouses, and solariums. For more information on sun tubes, visit Sun-Dome. To learn more about Cool Daylighting, go to www.daylighting.org/.
Active solar space heating systems are located external to the
building design and have fans or pumps that transfer the heat to the
interior of the building. Solar energy used for heating is measured in
Btu (British thermal units).
Solar hot air is produced by a transpired solar collector, which warms the inside of a building using the sun's heat. A transpired solar collector is a large, perforated, dark piece of material mounted on the south side of a building. Once air enters the perforations and becomes warmed, the buildings existing ventilation or an added fan pulls the warm, fresh air inside the building. On a sunny winter day, a single panel (8-ft. by 2.5 ft.) can produce temperatures up to 50 degrees F higher than the outdoor air temperature and can heat up to 40 cubic feet of space per minute.
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Solar liquid space heating is another solar application which uses a flat plate collector usually mounted on a roof or wall of a building (or the ground) facing south. The flat plate collector is an insulated flat box 3-4 feet wide, 6-10 feet long and approximately 3-5 inches thick. These flat plate collectors have a glass cover and a series of tubing inside that is attached to the collector plate. The sun heats the collector and its tubing, which is filled with fluid that is circulated through the building transporting the heat. Many times the tubing goes into the floor of the building heating the room from the bottom.
Water Heating
Systems
Solar hot water
heating systems are designed to heat water, which are used
for domestic purposes such as bathing, washing dishes, laundry, or
heating swimming pools. Many times the water is pre-heated by the sun
before it goes into a water storage tank inside the house. An
advantage of having solar pre-heated water is that when you need to
use the warm water, it is already warmed naturally and may only need a
small amount of electricity or gas to raise the temperature to where
you need it. Many times, it is too hot and may need to be mixed with
cold water to bring it down to the right temperature.
Flat panel solar water heating systems are the most popular type of solar collector and consist of an insulated weatherproof enclosure with an absorber plate, flow tubes, and a transparent cover. The transparent cover allows solar energy to pass through and be absorbed by the absorber and flow tubes. The heat generated is then transferred to the fluid circulating through the flow tubes (usually a non-toxic propylene glycol-water antifreeze mixture) and pumped to a storage tank where heat exchangers transfer the heat from the fluid to the household water.
Click photos to learn more
Evacuated tube collectors heat water in residential applications that require higher temperatures. These collectors consist of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate in a flat plate collector) and is covered with a selective coating. Evacuated tube collectors are modular which means tubes can be added or removed as hot-water needs change. When evacuated tubes are manufactured, air is evacuated from the space between the two tubes, forming a vacuum. Conductive and convective heat losses are eliminated because there is no air to conduct heat or to circulate and cause convective losses. Sunlight enters through the outer glass tube and strikes the absorber tube, which is filled with liquid. Evacuated tube collectors are more efficient than flat plate collectors because they perform well in both direct and diffuse solar radiation, the vacuum minimizes heat loss to the outdoors (good for colder climates), and the round tubes allow sunlight to directly hit them throughout the day (compared to the flat plate design where directly sunlight hits the tubes only when directly above them (at noon). These are more expensive then flat plate collectors too.
Photovoltaic (PV) panels are
another way to collect sunlight actively to produce electricity, which
is measured in kilowatt-hours. A
photovoltaic cell is a semi-conductor device that is light sensitive
in that it produces an electric charge or voltage when light strikes
its surface.
Photo: Madison Gas &
Electric
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