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 affect 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Through historical documents and artifacts, we can see that humans have been utilizing solar energy for a long time.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.
(Mary Ann Cofrin Hall, University of Wisconsin-Green Bay; Photos by S. Windjue)
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, click here.
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
Sun tubes transfer solar light from the sun to the building without the heat gain of ordinary skylights. Sun tubes are smaller in size than skylights but have mirrors inside the tube that magnifies the light that enters producing bright, free, renewable power. View sun tubes in schools (.pdf).
Image courtesy of Cyber Woodworking Depot Image courtesy of Arizona Solar Center
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. Click here for more information on sun tubes vs skylights. To learn more about Cool Daylighting, go to www.seventhwave.org/daylighting.
Space Heating - Active Systems
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.
(Mary Ann Cofrin Hall, University of Wisconsin-Green Bay; Photos by S. Windjue)
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.
Photo: Alan Ford Photo: Christopher Drake
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.
A typical household can meet 50-80 percent of their hot water needs by installing a solar water heating system. In year-round warm and sunny climates, these systems can meet 100 percent of a household's hot water needs. Currently, there are more than 300,000 solar water heating systems installed across the United States, excluding swimming pool applications.
Solar Energy for Electricity
Photovoltaic (PV) panels are another way to collect sunlight actively to produce electricity, which is measured in kilowatt-hours. Solar cells provide an ideal electrical power source for satellites, outdoor lighting, navigational beacons, and water pumps in remote areas. 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. Most cells are made of silicon, a material that comprises 28 percent of the Earth's crust. One solar cell measuring four inches across can produce one watt of electricity on a clear, sunny day. To produce more electricity, many cells are wired together in a series to produce a solar panel, and panels are wired together to form arrays. The solar panel size (or number of panels) depends on how much electricity needs to be produced which can be determined by doing a load analysis (this will be covered in Unit 3).
Photo: Madison Gas & Electric
Solar thermal electric systems is when solar energy is used to heat a fluid to produce steam that spins a turbine connected to an electrical generator. One type of solar thermal electric system, the solar power tower, uses mirrors to track and focus sunlight onto the top of a heat collection tower. The Ivanpah Solar Power Facility in the Mojave Desert in Southern California is the largest concentrated solar power (CSP) facility in the world with a gross capacity of generating 392 megawatts of power. Another type of solar thermal electric system called the Parabolic Trough uses curved, mirrored collectors shaped like troughs that focuses the sun's heat on pipes running through the middle of the collectors. The largest system of this type in the United States (and the second largest in the world) is the Solar Energy Generating Systems CSP which consists of three separate locations also located in the Mojave Desert with a generating capacity of 354 megawatts, which is roughly equal to the capacity of a small-sized electric power plant in Wisconsin.
Of the total electricity production in the United States, solar energy still provides less than two percent. In Wisconsin, only about 0.12 percent of total electricity production is from solar energy.
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