Solar Heating and Cooling

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Solar Heating and Cooling Heating and cooling of a space is needed when the temperature falls below 4 The seasonal variation of the lengths of days and nights can be explained by noting the relative position of the sun with respect to the earth. Twice a year on the vernal (first day of the spring) and autumnal (first day of the fall) equinoxes, day and night become equal in length. The vernal equinox has been celebrated throughout history as the time of rejuvenation and rebirth, thus marking the start of a New Year in the Zodiac calendar. The summer solstice occurs around June 22, and represents the longest day of the year. The shortest day of the year falls on winter solstice, at or around December 22 in the Northern Hemisphere. Solar Energy: The Facts Did You Know That ...? • Every second, about 4.3 million tons of solar mass is converted into massive amount of energy, totaling to equal to roughly one billion hydrogen bombs. • Photovoltaic modules covering 0.3% of the land in the United States could supply all its electrical demand. • The sun emits radiation at all possible wavelengths, from very short- wavelength, high-energy cosmic and gamma rays to very long-wavelength, low-energy radar and radio waves. Interestingly, a large fraction of solar radiation (48%) falls in the narrow band between 400-700 nm (4x10-7 to 7x10-7 m) which is visible to our eyes. Another 46% of the radiation is in the form of heat and falls within the infrared region. Figure 10-4 Earth travels around the sun along an elliptical orbit about once every 365 days. AutumnEquinox Vernal EquinoxSummer Solstice+23.5-23.595.9 million miles89.8 million miles WinterSolstice 223 Chapter 10 - Solar Energy or rises above a desired value. To determine how much heating or cooling is necessary, we must determine how much heat enters or leaks out of a building. A simple procedure for the rough calculation of thermal load is presented in the box below. The more detailed calculation is outside the scope of this book, but the interested reader is referred to texts and journals dedicated to the topic.5 Passive Heating Passive heating refers to the collection of solar energy without use of electrical or mechanical power. The simplest form of passive heating is the direct gain of solar energy as it passes through window glass (Figure 10-5a). In a more complex system, a medium with a large thermal mass (such as a concrete wall, or a drum of oil or water) stores the solar energy before releasing it at a later time when it is needed. Because they have fewer (if any) moving parts, passive designs are simpler, more reliable, more durable, and cost less than active systems. Passive solar systems can be designed in a variety of forms.6 Solar rooms are south-facing rooms with large windows, thick walls, and well-insulated roofs. Large windows allow maximum gain of direct solar radiation. Double and triple glazing of windows can significantly reduce heat losses from the room, while only moderately reducing the solar gains. Thick walls have large masses and high thermal storage capacity. Solar chimneys allow cool ambient air into the bottom of a glass collector where it is subsequently heated by incident sunlight and then rises by natural convection to the top before being released into a room. Solar chimneys are advantageous in that they can eliminate direct sunlight and glare and reduce heat losses during the night. More elaborate designs allow the amount of circulating air (and thus heat input) to be adjusted 5 See for example ASHRAE Journal of Heating, Ventilating, and Air Conditioning. 6 Anderson, B., and Wells, M., Passive Solar Energy, Brick House Publishing, New Hampshire, 1981. Figure 10-5 Passive and active solar systems Window Glass FYI ... Window glass is a simple and economical way of getting both light and heat conveniently into a living room or an office building. However, in many instances, such as during the hot summer days, we would like to block the sun’s heat while still allowing the light into the room. This is traditionally done by laminating window glass with ultra-thin layers of silver, or by adding infrared-absorbing dyes that are broken down by strong sunlight and scatter light, thus giving the glass a smoky haze. A new technique has recently been developed that blocks the infrared by an order of magnitude and, at the same time, allows visible light to pass through.i The key is to sandwich small particles of lanthanum hexaboride in the glass. These particles strongly absorb the near-infrared radiation, but are so small in size (smaller than the wavelength of the visible light) they cannot scatter sunlight, leaving the glass highly transparent. i Schelm, S. & Smith, G. B. Dilute LaB6 nanoparticles in polymer as optimized clear solar control glazing. Applied Physics Letters, 82, 4346 - 4348, (2003). 224 as necessary. Active Heating The main component of most active systems is rooftop solar collectors, where solar energy heats a working fluid like water and either stores it in a hot water reservoir or distributes it directly to interior spaces through pipes or ducts. For most applications, temperatures of about 100°C are sufficient and flat plate collectors are the most convenient. For applications that require higher temperatures, energy must be concentrated and solar concentrators such as lenses and mirrors are necessary. Flat plate collectors consist of a number of tubes through which a working fluid (such as water or air) is heated by solar energy. Tubes are arranged in parallel inside an airtight collector box that is covered by a sheet of glass or plastic and insulated on the back. To absorb the maximum amount of energy possible, the back-plate is painted black.7 The collectors are placed on rooftops or in open areas and tilted at an angle that maximizes solar insolation. Ideally, collector plates should be perpendicular to the sun’s rays at all times. This requires costly and complex tracking devices, however, so fixed flat plate collectors are often used instead. For optimal efficiency, collectors are faced south (north if in the southern hemisphere) and tilted at an angle equal to the latitude of their location. Evacuated tube collectors are similar to flat plate collectors except that the glass tubes are replaced with two concentrating tubes. Fluid flows inside the inner one while the outer tube is evacuated. This arrangement eliminates conduction and convection losses, which increases the collection efficiency. The most common uses of flat plate collectors are for domestic hot water systems, pools, spas, and space heating.8 Usually a gas or an electric-powered heater is added as a backup for periods when sufficient solar energy is not available. Evacuated-tube solar collectors can be used under cloudy conditions, so their year-round efficiency is higher. They are considerably more expensive and maintenance costs are higher. A typical domestic hot water system is shown in Figure 10-5b.


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