Understanding how much sunlight falls on your solar system is the key to success. Positioning your solar collectors correctly in the solar window will ensure you get the most from your investment.
Isogonic map of the United States.
Illustration courtesy New Society Publishers
Of all the renewable energy options open to us, solar hot water heaters have been around the longest. There are several designs to choose from, and selecting the best solar collector can be confusing. Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems, by Bob Ramlow and Benjamin Nusz (New Society Publishers, 2010), is the definitive guide to this clean reliable technology. In this excerpt, the authors explain how to site your solar energy system properly by determining the solar window of your location using tools like the Solar Pathfinder and an isogonic chart.
You can purchase this book from the MOTHER EARTH NEWS store: Solar Water Heating.
Sometimes it seems silly to state the obvious, but here goes: solar collectors must be in the full, direct sun if they are going to work properly. This may seem obvious, but you wouldn’t believe how many times we’ve had prospective customers tell us there is lots of sun where they are thinking of placing the solar collectors, only to find the spot in considerable shade. So, what is the bottom line for choosing a location for solar collectors? Read on.
First, let’s review how the sun shines on the Earth. The relative movement of the sun across the sky determines our days as well as our seasons. The path of the sun changes every day. Here in Wisconsin, on the first day of spring (March 21), the sun rises directly in the east; at noon it is directly to the south and is above the horizon at about a 45 degree angle, and it sets almost directly west. On that day, the sun is up for 12 hours and down for 12 hours. On the longest day of the year, the first day of summer (June 21), the sun rises in the northeastern sky, is directly overhead at noon (68 degrees from horizontal) and sets in the northwest. The first day of fall (September 21) is exactly like the first day of spring. The shortest day of the year is the first day of winter (December 21), and the sun rises in the southeastern sky, is directly to the south at noon and is about 23 degrees above horizontal (low in the sky) and then sets in the southwest. The height of the sun, in degrees, will vary depending on your geographical location. The further north you go, the lower the sun will be during the winter months.
Generally, solar collectors should face within 30 degrees of south, be mounted at an angle to the sun that will maximize their performance and be in the direct sun (no shading at all) from 9 a.m. until 3 p.m. It is between these hours that a fixed point will receive 80–90 percent of all the solar radiation it receives over the whole day. Some solar installers advocate for full sun only from 10 a.m. until 2 p.m. In some cases, this will be suitable, but for optimal collection, you should try to have full sun between 9 a.m. and 3 p.m.
To get the maximum performance from your solar energy system, the solar collectors must face the sun at noon. Both early and later in the day, the energy coming from the sun must pass through more of the atmosphere than during the middle of the day. This atmosphere is full of dust, water vapor and moving molecules. All this stuff in the air interrupts and weakens the energy flow from the sun to the Earth’s surface. The less atmosphere the sun’s energy has to pass through, the stronger it will be. It may seem elementary, but solar collectors need to be in the full sun for most of the day. They will not work if the direct sun must pass through trees or other vegetation, even if the leaves are gone. A tree without leaves will block up to 75 percent of the sun’s energy.
Solar collectors should face as close to true south as possible, but a variation of up to 30 degrees is generally acceptable and will not significantly reduce panel performance. To find south, you first have to find north and then look the other way. However, compared to a compass reading of north, true north is not the same everywhere. To find true north at your particular location, drive a stake vertically into the ground and watch its shadow. When the shadow reaches its shortest length, it is pointing exactly north. Unless you have time to sit around staring at a stake in the ground, you probably want an easier way to find true south. You can also find the declination of the compass reading for your area from your local weather service, or by consulting an isogonic chart. An isogonic chart demonstrates how the compass reading of north will vary depending on global location. To use the isogonic, you simply find your location on the map and note the nearest line. Those on the western half of the United States will have to subtract the given number of degrees from their compass reading and those in the eastern half will have to add them. For instance, if you live in southern California, you will need to subtract between 14–16 degrees from what your compass reads as south. Many compasses have a dial for you to make this adjustment.
One way to determine whether a proposed site is good is to imagine a big window in the sky directly to the south of your solar collector array. Imagine that all the sun’s energy that will fall on your solar collectors must pass through this window. The solar window is defined by the path of the sun across the sky throughout the year.
On June 21, the summer solstice, the sun takes its highest path of the year across the sky. The lowest path throughout the year is on the winter solstice, December 21. These paths of the sun define the top and the bottom of the solar window. The sides of the window are defined by where the sun is at 9 a.m. and 3 p.m. You want this solar window to be completely open and free of any obstructions.
Professionals may wish to purchase an instrument that can be set up at any location to show the solar window. There are several on the market today that work successfully.
The first, the Solar Pathfinder, uses a plastic dome to reflect the solar window onto a sun chart. The sun path chart is selected based on your latitude and will have the appropriate sun angle for the different months of the year. You trace the shadows reflected on the dome onto the sun chart, giving you a concrete analysis of any obstructions and where they are. The Solar Pathfinder will also allow you to rotate the surface to account for magnetic declination, which is a handy feature. This tool also has software in which you can upload a digital photograph of the reading in order to determine the percentage of total annual solar radiation that the proposed site will receive. There are two very important considerations when using site assessment software to analyze solar thermal performance: first, are you able to set the hours that will be analyzed, and second, is all shading considered 100 percent or can it be adjusted?
All site assessment tools currently on the market were originally designed to assess a site for PV. As such, they typically analyze the dawn-to-dusk solar window because PV is typically put on a tracker where the solar collectors move to face the sun directly at all times. Because solar thermal solar collectors never track, we do not need to analyze dawn-to-dusk but need to analyze only the three hours before and after solar noon. Analyzing anything more than that will give a false impression of the output of a solar thermal collector. The other big consideration is the way shading is considered in the analysis. As you may know, any shading on a PV panel can completely turn off the solar collector. However, with a solar thermal collector, a little shading will not turn off the whole collector; only the shaded part will lose productivity. Most software analyses consider any shading at 100 percent, but for solar thermal it probably is not 100 percent in most cases. Consider deciduous trees when they lose their leaves during the winter. On average, 50 percent of the sunlight is blocked and refracted, and the other 50 percent shines through, so shading behind a tree with no leaves is typically 50 percent shading, not 100 percent shading.
The Solar Site Selector is another option. It is composed of a semicircular base in which you attach a transparent sun path chart. This sun chart will vary based on your geographical location. To use the Solar Site Selector, point the instrument straight south, keep it level and then look through an eyepiece located on the base. When you look through the eyepiece, you will see the sun chart transposed over the actual solar window in the sky. The advantage of this instrument over the Solar Pathfinder is that you can actually see any potential obstruction instead of just its reflection. However, you do not end up with a concrete recording of the solar window.
If you are not willing to purchase one of these instruments, or you just want a quick way to determine obstructions, you just need to find out the angle of the sun during the winter solstice for your area. Next, take a simple plastic protractor and a piece of straight wire. Bend the end of the wire and insert the bent end into the hole in the base of the protractor. Hold the protractor so that the flat edge is up, allowing the wire to point down. Stand where you plan to locate your solar collectors and tilt the protractor so that the wire is pointing to the appropriate angle for your location for the appropriate time.
Protractors typically have the 90 degree mark at the tip of the circle, so you will have to subtract your sun angle from the 90 degrees to get the “protractor angle.” Next, point the protractor to where the sun will be during that time of day. At noon, the sun will be straight south. At 9 a.m. the sun will be just under 45 degrees east, and at 10 a.m. the sun will be just under 30 degrees east. Look along the straight edge of the protractor. If you see anything other than sky, you will have obstruction at that location during the winter solstice. Determining 2 p.m. or 3 p.m. should be done using the same angles. Now, this is a rather crude method for determining obstructions, but it is simple enough that you can do it without specialized tools.
If using any of the solar window assessment tools above reveals that more than 10 percent of the solar window between 9 a.m. and 3 p.m. has obstructions of any kind, you might want to re-evaluate the location. If the obstructions are tree branches, you may be able to remove them. However, remember that trees grow, solar collectors don’t. If the obstructions are only at the very bottom of the solar window, then the shadows will be cast only when the sun is lowest and the days are the shortest. This is the time of year when the solar energy system will produce the fewest Btus per day, so you may be willing to sacrifice some performance. This is a judgment call you will have to make. We tend to go for no shading. If the location where you would like to have solar collectors has just a few obstructions, it doesn’t mean that solar is not for you. Sometimes you may be able to increase the size of your solar collector array to accommodate for shading. We always want the most out of every solar collector we install, so we tend to be kind of sticklers.
Many locations can be suitable for mounting solar collectors. Collectors do not always need to be mounted on a south-facing roof or even on the roof at all. You also have the option of mounting the solar collectors on a rack on the ground. However, these are not the only options: any place will work as long as it has a clear solar window and you adhere to the appropriate mounting methods.
Roof-mounted collectors are more likely to be in a sunny location than ground-mounted collectors because they are higher in the air. As you evaluate your site for the best location for the solar collectors, you will see that the higher you go, the less shading you will encounter. Roof-mounted collectors also take up less room in your yard than ground-mounted collectors. Another advantage of roof mounting is that the length of piping required is often less than for ground mounts. Roof mounts are also less likely to be affected by vandalism than ground-mounted collectors. On the other hand, sometimes the dwelling is located in a shady spot and you would rather not cut trees around the house just for the solar collectors. In this case, you may be able to install a ground mount near the dwelling, in a better location to collect direct solar radiation.
This was the case at Bob’s first home. It has several large maple trees in the yard directly south of the house. Those trees keep the yard and home nice and cool all summer and are also a haven for many birds. Consequently, the solar collectors were ground-mounted about 75 feet from the house and away from the trees. A ground-mounted system often costs slightly more than a roof-mounted system because of the added cost of the rack that holds the solar collectors, as well as the added costs involved in trenching for the pipe runs. You will want to find the best location for your site. If there are multiple locations, all with clear solar windows, then you should consider other variables, such as aesthetics, cost, ease of installation, length of pipe runs and/or weight load.
Learn more about how to get the most from solar energy in Pick the Best Solar Water Heater for Your Home.
This excerpt has been reprinted with permission from Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems by Bob Ramlow and Benjamin Nusz, published by New Society Publishers, 2010. Buy this book from our store: Solar Water Heating.
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