Chapter 22: Uses of Solar Energy

Transcription

1 Chapter 22: Uses of Solar Energy Goals of Period 22 Section 22.1: To describe three forms of energy derived from solar energy water power, wind power, and biomass Section 22.2: To illustrate some uses of solar energy Section 22.3: To describe the features of a passive solar home Period 22 discusses the uses of solar energy for heating. As a practical application of solar energy, we discuss the design and operation of a solar home. The generation of electricity using solar cells is considered in Chapter How the Earth is Illuminated by the Sun Solar Insolation Since solar energy is spread out over an area, a convenient way to refer to a quantity of solar radiation is to specify the amount of solar energy passing through a given area in a given amount of time. This quantity of solar radiation is the solar power per unit area and is called solar insolation. Do not confuse insolation with insulation. Insulation is material that slows the flow of something such as electricity (insulation on wires) or thermal energy (insulation of walls). Just outside of the Earth's atmosphere, the value of the solar insolation is about 1340 watts/meter 2. As explained in Chapter 13, this solar radiation is reflected and absorbed by gases and particles in the atmosphere and clouds, with the result that only some of this radiation reaches the ground as visible light. The actual values may be significantly less than 1340 W/m 2, such as the Columbus winter average of 97 W/m 2. Table 22.1 on the next page lists the seasonal averages of the solar insolation of selected locations measured in watts/meter 2. Also included in the table are the seasonably averaged temperatures at these locations. 211

2 Table 22.1 Solar Insolation at Selected Locations Seattle, Washington (400 ft elevation, 48 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) Minneapolis, Minnesota (800 ft elevation, 45 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) New York, New York (200 ft elevation, 41 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) Columbus, Ohio (800 ft elevation, 40 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) Albuquerque, New Mexico (5300 ft elevation, 35 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) Houston, Texas (100 ft elevation, 30 o latitude) Temperature ( o C) Solar Insolation (W/m 2 ) Miami, Florida (see level, 26 o latitude) Temperature ( o C) Solar Insolation (W/m 2 )

3 Figure 22.1 illustrates how solar insolation is distributed over the United States. Source: Factors that Determine Solar Insolation The solar insolation map above exhibits a latitude dependence, as expected, since southern regions are generally warmer than northern regions. However, some areas at the same latitude have quite different values of insolation. This is because other factors such as cloud cover play an important role in determining insolation. For example, the desert southwest is relatively cloudless and dry and so the solar insolation is greater than for regions at the same latitude along the East Coast. Why is there a latitude dependence in the climate? One important factor is that sunlight must pass through the atmosphere, which absorbs and reflects solar energy. Although the atmosphere is rather uniform around the Earth, sunlight must pass through a thicker layer when the Sun is not high in the sky. The polar regions, typically much cooler than other areas, receive sunlight only at very oblique or grazing angles. The atmosphere through which the sunlight passes is also significantly greater near the poles. Contrast this with the equatorial region, which is typically warm. The seasons in these regions are not very pronounced; the poles have a winter-like climate while the equator has a summer-like climate. Elevation above sea level is also important in determining the climate. At relatively high altitudes, the temperature is typically cooler. This is partly because there is less air and clouds to retain the solar energy. Another factor is that since cold mountain tops might be covered with snow even near the equator, much of the incident solar radiation is simply reflected back into space. 213

4 Day-Night Cycle Only half of the globe is illuminated by the Sun at any time. The illuminated side is in day while the dark side is in night. Since the Earth is rotating on its axis, while it revolves around the sun, a given place on the earth alternately experiences day and night. This we call the day-night cycle. All places on the Earth have the same total number of day-lit hours for each year, however, not all places receive the same amount of solar energy. This is because when the Sun is high in the sky, the sunlight is generally more intense. When the Sun is low in the sky, that is, at dawn or dusk and in the high latitude regions, the sunlight is relatively less intense. The Seasons Besides spinning on its axis, the Earth is revolving in a nearly circular orbit around the Sun. It takes one year (about 364 days) for each revolution. The plane defined by the orbit of the Earth is called the ecliptic. The axis about which the Earth rotates is tilted relative to the ecliptic by a constant 23.5 degrees. Ordinary globes of the Earth usually illustrate this tilt by the way they are mounted on their stand. The Earth's axis is defined as the axis about which it rotates. On its journey around the Sun, the Earth's axis points in the same direction, that is, it maintains its orientation relative to the distant stars. The Earth s tilt results in uneven solar insolation on the northern and southern hemispheres, as shown in Figure This results in the seasons. Figure 22.2 Solar Insolation and the Earth s Tilt Source: Notice that the northern and southern hemispheres have seasons opposite to each other. For example, when the southern hemisphere is experiencing winter, the northern hemisphere is experiencing summer. The temperate regions have more pronounced seasons. Specifically, there is the familiar annual cycle of winter, spring, summer, and autumn. 214

5 A common misconception is that the seasons arise because of a varying distance between the Sun and the Earth. The orbit of the Earth is not exactly circular, meaning that there is a varying distance. However, this variation in distance is too small to cause the seasons. When the northern hemisphere is experiencing summer, the Earth is actually slightly farther from the Sun than during the winter six months later. Orientation of a Solar Collector When interpreting the tabulated values of the solar insolation, such as those in Table 22.1, it is important to know the direction that the solar collector is facing. For example, is the collector lying flat on the ground or is it aimed at the Sun? The convention used in the northern hemisphere is to face a collector south. If the collector angle is not changed during the year, it can be oriented at an angle equal to the latitude. With this convention, the solar collector will be aimed at the Sun only around noon when the sun is high in the sky. If the collector s angle can be adjusted, it is positioned at 15 O greater than the latitude in winter and at 10 O less than the latitude in summer. Columbus, Ohio, is at 40 o N latitude, so a collector would be oriented at an angle of 55 o in winter and at 30 o in summer Direct Conversions and Utilizations of Solar Energy Energy from the Sun is plentiful, renewable, and generally available to everyone. If we rely, even in part, on solar energy, we will reduce our heavy dependence on nonrenewable and polluting fossil fuels. In this section, we will discuss how we may effect the direct conversion of solar energy into other forms suitable for our use. Some of the conversions may actually be a series of conversions, however the intermediate steps will be explained. The final form of energy (electrical, thermal, etc.) will then be in a form ready for human consumption. Use of Solar Energy for Lighting We begin with an obvious or even trivial conversion, where solar energy is used for lighting or illumination. [This is not really a conversion but just a very direct utilization of solar energy.] The obvious way to use solar energy for lighting is to use windows that let sunlight into a room during the daylight hours. Besides windows, optical fibers, or light pipes, may be used to carry sunlight to interior rooms without windows. The sunlight provides lighting for the interior rooms which reduces the demand on artificial lighting. One practical economic device is daylight-saving-time whereby the clock is adjusted so that normal working hours coincide with sunlit hours. Among other things, this reduces the demand on artificial lighting (at least in rooms with windows) and thus saves money and energy. Conversion of Solar Energy to Thermal Energy Next we consider the conversion of solar energy into thermal energy. In this case, the radiant energy is converted into thermal energy when it is absorbed by an object. As already pointed out, this is a very common and familiar conversion: 215

6 Remember that solar energy (radiant electromagnetic energy) is not identical to thermal energy and that a conversion is actually taking place. The thermal energy may be used directly to heat air or water, or it may be used to power a heat engine. When an object is placed in the sunlight, it will warm up. However, the properties of the object will determine efficiency or effectiveness of the conversion of radiant energy to thermal energy. If the object is painted black, it will absorb more radiant energy than if it were painted white. This is because the lighter colored object reflects more radiant energy than the darker object. What color might you paint a collector used to absorb solar energy? Another practical use of the solar-to-thermal energy conversion is in heating water for home use. A popular design of a solar collector consists of dark panels placed on the roof. A fluid, such as water, is passed through pipes in contact with the panels. When the panels are heated by the Sun, the water passing through the pipes becomes hot. The heated water may then be used directly or stored for future use. An added summertime benefit of a roof-top collector is that thermal energy that normally would heat the house is now being used to heat water, thus reducing the air-conditioner demands. Conversion from Solar Energy to Kinetic Energy Next, we consider a multistep conversion of solar energy into kinetic energy. The intermediate energy form is thermal energy. The kinetic energy may be used directly to power a machine such as an electrical generator. In this energy conversion, solar energy is used to power a heat engine or more specifically an external combustion engine. Steam engines and Stirling engines are examples of external combustion engines. Instead of powering the engine by burning a fuel, the thermal energy is provided by solar energy. To be efficient, the temperatures must be high. This is achieved by focusing the light using curved mirrors or lenses. In practical designs, mirrors are used because they can be made much larger than lenses to collect more radiant energy Focusing Radiant Energy It is possible to obtain higher temperatures by focusing the sunlight using curved mirrors or lenses. When the light is focused, its intensity (energy per unit area) increases, however the total amount of available energy is not increased. The focused energy may be in a form which is useful and practical. When the focused light is absorbed by an object, the temperature of the object may rise very high. The focusing of sunlight is used in many types of solar collectors. Refraction of Light Regardless of their wavelength and frequency, all waves of electromagnetic radiation travel at the same speed in a vacuum, 3 x 10 8 meters per second, the speed of light. However, light travels at different speeds in different materials. When light enters 216

7 a transparent material, the speed of the wave changes and the light beam is refracted, or bent, as shown in Figure Figure 22.3 Refracted Light Air Water The ratio of the speed of light in a vacuum to the speed of light in a material is called the index of refraction. The index of refraction is a measure of the amount that a light beam is bent as it passes from one medium to another medium. The amount that light is refracted depends on the frequency of the light wave. When light passes through a prism, the waves with the highest frequency are refracted more than waves of lower frequency. This difference in refraction separates the light into a rainbow of colors. Figure 22.4 A Prism Refracts Light Source: The difference between the speed of red light and violet light is greatest for materials with the largest index of refraction. For this reason, a well-cut diamond is very effective in breaking light up into colors. The best cut for this purpose is known as a brilliant cut. 217

8 Focusing Radiant Energy For a variety of applications it is desirable to focus, or concentrate, radiant energy. This can be accomplished using curved mirrors or lenses. We will refer to two types of curvature. If a mirror or lens is curved inward, it is said to be concave. A mirror or lens that bulges outward is convex. When light strikes the surface of a mirror, it is reflected at an angle equal to the angle at which it struck the mirror. From Figure 22.5, you can see that this means a concave mirror can concentrate light. Curved mirrors are used in common devices such as flashlights and headlights to provide a beam of light. Figure 22.5 Radiant Energy Reflecting from Mirrors Focus A Concave Mirror Focuses Radiant Energy A Convex Mirror Spreads Radiant Energy As light travels from one medium to another, such as traveling from air into a glass lens, it changes speed. If the light does not enter the new medium perpendicular to the boundary, it will also change direction, or refract, just as a row of marchers in a band will change direction if the marchers at one end of the row slow down before the others do. As shown in Figure 22.6, this means that curved lenses can concentrate light. Figure 22.6 Radiant Energy Passing through Lenses Focus A Convex Lens Focuses Radiant Energy A Concave Lens Spreads Radiant Energy When using solar energy, it is often desirable to concentrate or focus the rays of light. In class you will see how focused light can be used to heat water. 218

9 22.4 Design of a Passive Solar Home We next consider how a home may be made more energy efficient by simple design features. Such a home is called a passive solar home or more simply as a solar home. The solar home need not rely entirely on solar energy: for example, the home may use electricity provided by local utilities. The important point is that the solar home reduces the demand on the more conventional and non-renewable energy sources. For the purposes of this discussion, we assume the solar home is in central Ohio. Embankment and Trees These two landscaping features, embankment and trees, may aid significantly in protecting a home from the weather. A few feet under the ground, the dirt remains at a relatively constant temperature of about 50 o F all year long. This constant temperature may help regulate the inside temperature as the outside temperature varies throughout the seasons. Trees strategically placed may help block a cold wind in the winter or shade the home during the hot summer. Of course, effective landscaping may add to the appearance of any home. Overhangs and South-Facing Windows One simple feature of the solar home is to place most of the windows on the south-facing side. This is the side which receives the most sunlight. Windows on the east and west walls receive a smaller amount of direct sunlight but windows on the north side never do. During the summer, it might be desirable to keep the direct sunlight out. Of course, shades do this but another way is to use an overhang or extension of the roof. When the sun is relatively high in the sky during the summer, it is blocked; but during the winter, when the sun is lower, direct sunlight is permitted to enter. Insulation The house should be sufficiently insulated. Insulation prevents the flow of thermal energy though the floors, walls, and roof. Modern windows are also available which are very effective in blocking infrared energy. Vents By strategically placing vents and other air passages, convective flows of air which aid in summertime cooling are obtained. For example, if there is an open vent in the ridge of the roof, hot attic air is allowed to escape. No fans are needed since the warm air naturally rises. Cooler air, perhaps air passing through a cool thermal mass, is drawn into the house as a result of the convective flow. During the winter, the vents may be closed to prevent the escape of warm air. Thermal Mass A solar home contains a large mass that is used to store thermal energy. Such a mass is popularly called a thermal mass. The thermal mass might be a pit filled with small stones through which air circulates. The air is either warmed or cooled (depending on the relative temperature of the stones and the air) and may be circulated 219

10 throughout the house. The thermal mass might work by warming up during a hot day and then supplying the stored thermal energy to the home during the cool night. Rooftop Solar Collectors A rooftop solar collector may be used to heat water for domestic use. The collector might be an integral part of the roof and thus not detract from its appearance. The collector should be on the south facing side of the roof with a tilt of about 40 degrees (the latitude of Columbus) although small variations are not important. An added summertime benefit of the rooftop solar collector is that it absorbs solar energy which might otherwise warm the home: This reduces air-conditioning costs. Hot-water solar collectors are relatively inexpensive and easy to incorporate into more conventional and reliable systems. Period 22 Summary 22.1: Solar energy shining through windows can be used directly for lighting. Optical fibers, or light pipes, may be used to carry sunlight to interior rooms without windows. The amount of solar insolation striking the Earth depends on 1) the amount of radiation that passes through the atmosphere and reaches the Earth s surface. The amount of cloud cover and pollution in the air affects the amount of radiation striking the surface. 2) the latitude. Higher latitudes near the poles receive less insolation, the equator receives more. 3) the season of the year. Seasons are caused by the tilt of the Earth s axis (23.5 degrees). Due to this tilt, the Sun shines more directly on the northern hemisphere in June and more directly on the southern hemisphere in December. Seasons are not caused by differences in the distance between the Earth and the Sun. In fact, the Earth is closest to the Sun during the winter (in the northern hemisphere). 22.2: In the northern hemisphere, solar collector face south. In the winter, a moveable collector is positioned at a angle 15 O greater than the latitude. In summer, the collector is positioned at 10 O less than the latitude. Columbus, Ohio, is at 40 o N latitude, so a collector would be oriented at an angle of 55 o in winter and at 30 o in summer. 220

11 Period 22 Summary, continued 22.3: As light passes from one medium to another it is refracted, or bent. Radiant energy can be focused by reflecting beams from a mirror or passing beams through a lens. As light travels from one medium to another, it changes speed. If the light does not enter the new medium perpendicular to the boundary, it will change direction as well. A concave mirror focuses beams of radiant energy, but a convex mirror spreads the beams. The opposite is true of lenses: a convex lens focuses radiant energy and a concave lens spreads the energy. 22.4: Features of a passive solar home include embankments and trees to protect the home from weather, overhangs to reduce solar heating in the summer and south-facing windows to maximize solar heating and light in the winter, insulation to prevent the flow of thermal energy into and out of the house, vents to exhaust hot air from the house in the summer, thermal masses that absorb heat during the day and release the heat at night, and rooftop solar collectors to heat water. 221