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The Rural Missouri NEWS Service January
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May 2009 Sun power for your home: Is passive solar for you? If you’ve ever built a solar oven with foil and cardboard, you know the power of concentrated solar energy to boil water or heat food. Humans have used solar energy for thousands of years. You, too, can apply some of this abundant “free” solar energy to your house. With an existing home, you can use solar to supplement your electric, gas or wood heat. Passive solar utilizes the sun’s energy without mechanical or electrical systems. It is not without cost, however. Depending on how much passive solar retrofitting your budget will allow, you may be looking at adding a sun room, changing windows and building thermal mass. Active solar, which we won’t discuss here in detail, requires more expensive pumps, fans or electrical controls to move solar heat. To understand how passive solar works, begin with a law of physics: heat moves from warmer to cooler materials until there is no longer a temperature difference. According to the U.S. Department of Energy (DOE), passive solar makes use of this law through conduction, convention and radiation to distribute free solar heat throughout the living space. Conduction is the way heat moves through materials from molecule to molecule. A spoon placed in hot water carries the heat directly to your hand. Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser sinks. Warmer air will accumulate on a second floor, leaving the basement cool. Some passive solar uses air convection to carry solar heat from a south wall into the building’s interior. Radiant heat moves through the air from warmer objects to cooler ones. Think about the heat radiating from a wood stove. In passive solar, walls, windows and foundations absorb solar radiation and reflect or transmit it towards cooler spaces. Opaque objects absorb 40 to 95 percent of incoming solar radiation, depending on their color. Darker colors typically absorb a greater percentage than lighter colors. Two critical components of passive solar technology are solar collectors and thermal mass. The most common form of collector is your basic window. Clear-glass windows transmit 80 to 90 percent of solar radiation. The solar radiation is absorbed by the house and radiated again from the interior surfaces as infrared radiation. Although the window glass allows solar radiation to pass through, it absorbs infrared radiation and traps solar heat from leaving the house. Thermal mass most commonly refers to concrete, stones, brick and tile in floors and walls but can include water – which holds twice as much heat as other materials – and it can include any surface in your house that absorbs and holds heat. Thermal mass absorbs the heat and then releases it, following the laws of physics, to cooler living space. Now may be the time to invest in passive solar. The new federal tax credit of 30 percent on some residential solar applications can make your passive solar retrofit more affordable. Go to www.energystar.gov/index.cfm?c=products.pr_tax_credits#c4 for details.
Let the Sun Shine In! Simple Solar Uses You don’t have to spend an arm and a leg to use passive solar. As the Missouri Department of Natural Resources (DNR) notes, the most direct way to use the sun’s energy is through skylights, windows and light tubes that bring sunlight into your house. A skylight lets light in through a glazed opening in the ceiling directly above the room to be lit. Light tubes allow sunlight to enter a room not directly beneath the glazed opening in the roof. They are flexible tubes lined with highly reflective foil that run from the roof to the ceiling of the room to be lit. Light tubes are used when you don’t want a skylight opening on the roof on the front of the house or when pipes or other elements in an attic space make construction of a skylight impossible.
Pay attention to heat transmittance, measured by U-factor. A smaller U-factor provides more insulating value than a larger one. The smaller the number, the better. A window is energy efficient if the U-factor is less than .40. The glass will be coated with a thick layer of material engineered to transmit or reject certain frequencies of radiation. This coated glass is called low-emissivity (low-e) glass. Also consider the solar heat gain coefficient (SHGC), which is a decimal number less than one. A number of .60 means that 60 percent of solar radiation passes into the house and 40 percent is reflected back into the environment. Passive solar requires a high SHGC. In general, DOE recommends:
Are You Ready For Prime-Time Passive Solar? Given the 30 percent federal tax credit in 2009 and 2010 on some residential solar applications (www.energystar.gov/index.cfm?c=products.pr_tax_credits#c4) the time may be right for you to tackle a larger passive solar project. The U.S. Department of Energy (DOE) recommends these tips for getting started:
South-Facing Glazed Windows: The Most Affordable Passive Solar Technique You can tap into passive solar energy in several ways, but the most affordable and most simple is using south-facing glazed windows. This approach is called direct gain.
In direct gain, sunlight enters south-facing windows and strikes masonry floors and/or walls (thermal mass) that in turn absorb and store the heat. Typically, these surfaces are a dark color, because dark absorbs more heat than light colors. At night, as the room cools, the heat in the thermal mass convects and radiates into the room. The amount of solar gain you get will depend on how much solar heat is collected through the windows and the amount of the thermal mass. The latter must be insulated from outside temperatures or the heat will rapidly drain away. Whichever passive solar design approach you follow, pay attention to summer comfort as well. Block summer heat with roof overhangs, awnings, shutters and trellises. More detailed guidance on overhangs, which can make or break the effectiveness of your passive solar system, is available at DOE’s site at www.doe.gov. Search for EERE Consumer’s Guide: Passive Solar Home Design – Roof Overhangs. If you’re an experienced do-it-yourselfer, passive solar retrofitting can be a snap. If not, consult with an experienced solar designer or builder, particularly when it comes to calculating proper overhang dimensions.
When washing clothes, use less water by running full loads. In addition, approximately 80% of the energy used by washing machines heats water, so by using cold water and cold-water detergents, you can cut a load's energy use in half. DOUG RYE SAYS...
Let me take this opportunity to say to every single person who worked so many hours to restore electric distribution systems that originally took many years to build – thank you, thank you, thank you. But, as always, life goes on and it is still important that you make your home more energy efficient, especially with summer on the way. In just a few weeks, the temperature will start rising and attics will be-come much hotter. Well, I say, “cool it.” The worst-case scenario for home energy efficiency is to have a roof without shade, inadequate attic insulation and ductwork in the attic. The attic could easily be 140 degrees on a hot summer day. How would you like to sit in that attic and try to make homemade ice cream? Most likely you would keel over before the ice cream was ready. Now you understand why it is better not to place any part of your cooling system in the attic. For years I have been teaching this but most still install the cooling system in the worst possible environment. So, let’s look at a solution. If you will install a radiant barrier on both the roof rafters and the west or southwest gable end of the attic, you can reduce the 140-degree temperature by about 30 degrees. From a cooling cost standpoint, this is huge. The attic is 30 degrees cooler, the insulation is 30 degrees cooler and the ductwork is 30 degrees cooler. Basically, this radiant barrier is a strong, reinforced aluminum foil and should be installed on the bottom of the rafters. It comes in 1,000-square-foot rolls. Check with your local home center for availability. For new construction, the radiant barrier can simply be a part of the roof decking. I look forward to writing this column every month. And I think that you can tell my heart is in it. Many of you have told me at my seminars, on my radio show and at my office, that you have implemented my suggestions and that they work. Actually, I never even think about them not working because they always do. They always have a payback and once again, they are good investments. About a week ago, my wife and I were at our favorite sandwich shop to order a toasted turkey sandwich. A family of four was in line ahead of us. The wife just kept looking at us and finally she just blurted out, “Are you Doug Rye?” To which I replied, “Does that scoundrel owe you money, too?” She grabbed her husband’s arm and said, “Honey, Doug Rye is here, right behind us.” I have to admit that I like it when this happens. He shook my hand and told me that he was so glad that a subcontractor suggested that he order the Doug Rye New House video before they started construction. He and his wife watched the video every night for several nights to be sure that their house was built correctly. Actually, his words were, “We did it and it worked.” He then thanked me. Isn’t that great? I was able to help another family, just like I help you with these columns. Now, I was hoping he would offer to buy me a sandwich with the money he saved. Oh well … I guess I better “cool it.” Doug Rye can be heard on KGOZ and KAAN locally. Every Saturday morning, Doug Rye hosts a live call-in show on several area radio stations. He can be heard from 9-10 a.m. on KGOZ, 101.7, Gallatin, and on KAAN, 95.5, Bethany. |
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Windows are the most common way to admit sunlight into your home. Their location, size and type will determine how efficiently you utilize passive solar energy. Buying the right window for your application can be quite complicated. The U.S. Department of Energy (DOE) offers this advice on windows for passive solar applications. Also go to
Since most homes have at least some windows on the south, virtually all homes have some passive solar heating potential. To use it to your best advantage, keep windows clean and use awnings, shutters or trellises to enhance passive solar heating, reduce nighttime heat loss and prevent summer overheating.
I wish I had the power to command no more ice storms. But, of course, I don’t. Well, I say, springtime, yes, ice storms, no. I am aware that many of our readers have been affected by the January ice storm. In fact, the Ice Storm of 2009 was the worst that I have ever seen. When this happened, it just made me
really aware of how much we depend on this wonderful product we call
electricity.