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The New Independent Home
by Michael Potts
from chapter 8 :
Building in Harmony with Sources |
By considering energy needs throughout the planning stages of a new, independent house, you will find many ways to integrate good energy practices and vastly improve your homestead's performance. The solar fraction is the percentage of a household's energy requirements that can reasonably be expected to be generated by harvesting solar energy. In some places, where cloud cover is rare and the sun beats down unimpeded, the fraction is easily 100 percent. An old government map of solar resources showed such spots in red, while less gifted sites ranged through the spectrum to blue where, at one time, it was thought that no solar capture was feasible. Vermont was all blue, but this estimate, like the report of Mark Twain's demise, was premature. Richard Gottlieb, perennial candidate for governor of Vermont, has been cheerfully harvesting solar electricity for almost two decades, and reports that in his part of the state (the south) the solar fraction could be as much as 70%. Deficits, of course, may be made up with more modules and a bigger battery bank, but it is more sensible to develop a second energy harvesting method if possible.
The solar energy falling on a south-facing roof can be either a curse (as an undesirable heat source for the space within, in which case we recommend the attic fan arrangement in chapter 2) or a blessed source of electricity and hot water. Low-voltage electricity does not travel long distances efficiently, but the area required for solar panels -- photovoltaic modules plus solar hot water panels -- often corresponds nicely to half of the house's roof area. This serendipity is helpful, however, only if the roof faces in the right direction. By dedicating roof space to capturing the sun, the solar panels are incorporated neatly into the house's profile, while protecting and helping to insulate the roof. As already noted, one progressive utility, the Sacramento Municipal Utility District, enlists customers in their "Give Us Your Roofs" program for precisely this purpose. Ratepayers have been rushing to sign up, willingly paying a 15% premium for the renewable energy they use. |
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Good maps show magnetic north (MN) and the direction to the North Star as well as Geographic North. This compass rose comes from the USGS map for Caspar.
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To prepare for this design economy, a house's roof must align with the site's solar window, so the sun's energy may be captured most effectively. The sun's path across the sky varies in two ways, by time of day and time of year, and reaches its highest point at noon on the summer solstice. At noon on the equinoxes, spring and autumn, the sun can be found due south at the angle of the site's latitude below the zenith -- the center of the sky directly overhead. On the summer solstice, the sun reaches its annual high point in the sky at noon: due south and at an angle below the zenith equal to the location's latitude minus 23 degrees. At noon on the winter solstice, the sun appears due south at an angle equal to the site's latitude plus 23 degrees. (Remember, the earth's 23-degree angle of precession causes the seasons.) Through the rest of the year, the sun commutes sinusoidally between these points. The proper angle of exposure for any solar device corresponds to the site's latitude plus or minus the seasonal fraction of the sun's annual 46-degree range. Some solar devices are racked or tracked so their angle can be adjusted, but seasonally adjusting a roof's angle is impractical, so a fair compromise must be achieved. Choosing the site's latitude as the roof's angle from the vertical varies from the optimum by approximately 23 degrees at the solstices, but is direct enough for most of the year. If maximum electrical loads are expected in the dead of winter, as is typical in the northern tier, the savings realized by mounting modules on the roof can be invested in additional modules to compensate for the inefficiency of slightly misaligned modules during part of the year.
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| Compass corrections reflect the earth's magnetic field. For the lower 48, approximate corrections can be found using this map, but for final alignment, a large scale local map should be consulted.
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Please note that compasses point to magnetic north, which coincides with true north in only a very few locations on the planet. Get the local correction before using a compass to orient the house, then verify your measurement by making careful observations using the time-honored method: With a tall stick (the gnomon) and pegs mark the shadow's end over several hours; a line from the end of the shortest shadow through the gnomon points due south.
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| Point your face south and envision this image as a quarter sphere going into the page and large enough for your head to fit inside. East will be over your left ear, south right in front of your nose, the zenith right overhead. The sun will move along the paths shown on the equinoxes and solstices. |
Carpenters express roof pitch as the inches of rise to twelve inches of horizontal run; a typical commodity house has a rise of three in twelve. To get the best out of Caspar's winter sun, I designated a section of roof for solar harvesting, and angled the roof perpendicular to the sun about a month before and after the winter solstice, or eighteen degrees higher than my 40° latitude, resulting in a precipitous pitch of fourteen in twelve. My best solar aperture is slightly west of due south because of trees I do not care to cut and typical wintertime cloud patterns. Solar energy experts have special tools for predicting the solar aperture and its changes over the solar year, and can help get the roof oriented precisely. Getting the array angle wrong can easily subtract twenty percent efficiency from your array.
Coincidentally, increasing the roof pitch with latitude serves the need to employ steeper roofs in snowier lands. Since snow will cling to a glass surface at or below a 50-degree pitch (particularly if the surface is dirty and the snow is wet), some thought must be given, if the homestead relies on PV for snow-season power, to convenient ways to clear the snow. If you live in snowy parts or very far north, you might consider mounting your modules vertically in winter. When the air is cold and there is snow on the ground, vertically mounted modules perform better than under any other conditions.
[[photo: view through PV glass roof]]
In 1997, photovoltaic shingles and panels became widely available at very reasonable cost when considered as energy sources as well as roofing material. Architects have integrated solar modules into roofs for more than a decade, cleverly solving the problems of sealing delicate electrical connections away from the elements, but owner-builders can employ these new roofing materials very cost effectively.
Photovoltaic panels become less productive when they get hot. This perverse behavior cannot be corrected by the usual means -- you do not want to shade your array -- so they must be mounted in a way that keeps them as cool as possible. Unventilated, the temperature behind modules will be as much as thirty degrees above the ambient on a sunny day, decreasing module performance by about ten percent. Mounting modules on vertical rails creates a chimney effect, drawing cool air in at the bottom and allowing heated air to escape at the top.
When strong winds blow, modules are expensive, fragile, sharp-edged kites waiting to fly. Aficionados of the electrical code don't worry as much about this as I do; they worry instead about another unlikely but potentially devastating event, lightning, and insist that every module and metal part be individually grounded.
There is a brisk argument in the solar community about the efficacy of racking and tracking as compared to solid installation. By seasonally tinkering with the angle of the modules, small gains in efficiency can be made; by keeping modules pointed at the sun as it makes its daily transit from east to west, productivity may be improved by as much as 50%. Active trackers using electronics to follow the sun are accurate to within half a degree, and get on the job first thing in the morning, but they are also subject to the failures and costs of any high-tech equipment. Passive trackers that use freon (a CFC, but safely encapsulated, we hope, and not prone to escape) are less accurate, and take half an hour to wake up in the morning. I hasten to warn that any use of CFC must account for eventual decommissioning, when it is important that the equipment be relieved of its ozone-eating molecules without further damaging the stratosphere. Racks and trackers are typically rated to withstand 120-mile-per-hour winds; in Caspar, or wherever the weather gets intense, that is barely adequate. As a veteran of winter's lusty gusts, and having seen the effects of serious storms and weathering, I value the advice of those who favor fixed mounting of the solar array. Having lived now with a tracker in the front yard for several years, and having repaired it only twice, I also see merit in arguments favoring trackers.
In 1993 I wrote, "To my eye, a tracker full of photovoltaic modules in the yard is just slightly less offensive than a powerline swooping in from the street: it bespeaks a lack of forethought and a hint of impermanence." I now admit I was wrong. The powerline is a blight, but the tracker and its modules are a sort of rectilinear sunflower productively following the sun's great eye across the sky. If we wish to make a sculptural statement about the source of our energy, a tracker does so very nicely.
[[photo: a tracker in the garden]]
Economic factors may help resolve the controversy in many cases. Trackers usually cost about 30% of the modules that populate them. At 40 degrees latitude, trackers can add more than 40% yield to a PV module's summertime power harvest assuming the whole day-long solar path is available. In winter, when the sun shines from a lower angle, a tracker's improvement to the daily yield drops to less than 20%. If year-around power production is important, then adding one-third more fixed modules eliminates moving parts, costs the same, and harvests the same energy. If excess summertime energy can be used to power fans, pumps, or sell energy back to the grid, the tracker makes sense.
Many homesteaders are attracted to photovoltaics because of their elegant passivity. If it is impossible to orient the home's roof correctly, or if a better solar window is available at a distance from the home site, it may be better to incorporate the array into the appropriately sloped roof of an outbuilding than to plant a large tracker in the yard. This shed may become the power house, sheltering batteries and power-conditioning equipment. Take care to account for energy losses due to distance between source and loads.
Typical single-crystal cell modules convert about 15% of the sun's energy to electricity. Since the photovoltaic effect has a theoretical maximum efficiency of less than 30%, and the most efficient cells are more expensive to manufacture, another obvious way to improve energy yield is by using lenses and mirrors to concentrate sunlight from a large area onto smaller high-yield photovoltaic cells. New modules employing this strategy have been in the field for a few years now, and work satisfactorily where available space is at a premium. But modules with optical concentrators must be kept pointed directly (to within a quarter of a degree) at the sun, and existing active trackers are barely capable of pointing this precisely over long periods of time. Concentrator modules are likely to be a high-maintenance solution perfect for folks who like to tinker. In order to harvest the most summertime energy, perhaps for agricultural pumping, trackers improve yields in a benign climate. For most households, over the twenty or thirty years of life we expect from our modules, I still believe it is more cost-effective to buy a third more modules and mount them solidly on the properly oriented roof.
One photovoltaic module may provide enough energy to keep the battery in an Emergency Callbox charged, but most homes need several modules. (How many modules do you need for your lifestyle? You will find a worksheet in the appendix.) When mounting modules, remember that they can be expected to operate for thirty or more years, and so the mounting structures and hardware should be correspondingly durable. Since "up" for an electron is away from a negative charge, modules can be aligned in any way that is convenient so long as their faces are unshaded and perpendicular to the best sun. We have already noted that modules perform better if kept cool, and so this also suggests mounting strategies that enhance air flow.
Experienced installers arrange an array of PV modules into conveniently wired subarrays. This arrangement permits the use of smaller-gauge wire for connecting the arrays, which is cheaper and easier to handle, and also simplifies troubleshooting. If you suspect that your array is not producing as it should, you can test each subarray with a multimeter until you determine which subarray is performing poorly, and then start unwiring modules and testing them individually. Carrying the subarray notion on into the balance of system equipment, using two or more small charge controllers is often cheaper and more reliable than using a single large controller. |
| By arranging the PV array into subarrays, smaller-gauge wire and lower-capacity charge controllers can be used. Redundancy is good.
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Roofs capture another precious commodity: water. In most of the world, roof catchment is the primary source of domestic water, but in conventional American homebuilding, roof run-off is considered more a nuisance than a resource. Roof-captured water requires treatment, particularly in a suburban or urban setting, but as our shared water resources approach their limits, we ought to put these pennies from heaven to good use. Thanks to gravity, it takes very little effort to divert roof run-off to a holding tank; it takes even less when this intent is incorporated into your plan early on. If the run-off is to be used for drinking or to water edible plants, care should be taken to select a roof surface that does not spoil the water with leachates or unfilterable impurities. The manufacturers of the roofing material should be able to provide reliable information.
click here to download the Exceltm Sizing worksheet we use
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