Solar Siting

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The knowledge of your latitude, the solar path for that latitude and some basic geometry will give you an idea of how much open land you need between your solar home and tall trees or buildings to insure solar access.

Your Latitude

Start with your latitude. If you don't know what it is, wikipedia frequently has this information. Look up your city or your town. The information should be on the right of the page.

The Solar Path

Understand the solar path for your location. Go to Sustainable By Design and use the Sol Path tool.

http://www.susdesign.com/solpath/index.php Your output will look very similar to the image below. All times are solar times, not clock times. This example is done for Detroit, Mi.

You can see that on Dec. 21^st, at 42 deg latitude north, the Winter Solstice, the sun rises 55 degrees east of south and never gets over 30 degrees off the horizon. This is the shortest day of the year. If you had windows facing SSE or SSW you can experience significant solar heat gains during this month from those windows. The amount of solar insolation does not peak until solar noon but the Btu's available before that can add up and given how low the sun is off the horizon, those Btu's are travelling in nearly a straight line to your SSE and SSW windows. When the sun is low off the horizon, windows facing normal to the sun take a direct hit. The more normal or perpendicular the suns rays are to a window, the higher the heat gains. At large angles more solar radiation is reflected. Light rays travel in a straight line and when they encounter a reflective surface they reflect off of it at the same angle they hit it. You would experience the maximum gain from SSE or SSW windows at about 9 am and again at about 3 pm.

The temperature of the Earth's atmosphere and the soil does not follow directly with the path of the sun over the course of a year. You would think that December would be the coldest month given it has the least amount of sunlight per day when compared to the other months. The coldest month is January, the average daily minimum temperature is 15.62 deg F. In December, the average daily low temperature is higher at 21.38 deg F. The average daily high in January is 30.38 deg F. and in December its 35.24 deg F. February is very close to January. The average daily low is 17.6 deg F and the average daily high is only 33.26 deg. F.

During peak solar collection hours, south glazing during these cold months has the highest solar gain. That's because the sun has a tendency to rise closer to the south than in the summer, when it rises to the north of east and sets to the north of west. Given how low the sun is in the sky, solar radiation will go directly underneath a properly sized overhang and heat the south side of a building.

On March 21^st , the day of the Spring Equinox, the sun is rising exactly due east, sets exactly due west and climbs to almost 50 degrees off the horizon. The solar path in March is identical to the solar path in September, on the 21^st, or the Fall Equinox. The length of day is equal on the Equinoxes and the length of day and night are also equal.

Eventhough the sun follows the same path ambient conditions are not the same in March and September. The average daily low in March is 26.96 deg F and in September its 52.52 deg F. The average daily high in March is 44.42 and in September its 73.94.

This lag is due to the thermal mass of the Earth. Solar radiation heats up the Earth. It cannot radiate back out everything it gains at the same rate. The result is an increase in temperature. That increase is felt several months after the actual peak of the solar path.

By the time June 21^st rolls around, the sun is rising 30 degrees North of East, and climbing to over 70 degrees off the horizon. Due to the low altitude of the sun early in the morning and late in the afternoon, significant heat gain can occur from east and west windows. June 21^st is the Summer Solstice and its the longest day of the year. A properly sized overhang will prevent the sunlight from making a direct hit on the south facade of the building. Even if the south windows and facade of the house do get inundated with sunlight, the sun is so high in the sky at solar noon that the angle that it hits at is nothing close to a direct hit so much of the solar radiation at that time of the year is actually reflected off the building instead of entering into it. The angle of reflection is the same angle as the transmission angle. Think about light bouncing off a mirror. If a laser beam is shined at a mirror head on, it will come right back to you. If you shine it at a highly oblique angle, it will bounce off the mirror at an equally oblique angle. The same thing is happening with the sunlight. You can think of it as billions of directed laser beams emanating from the sun in all directions. Each one travelling in a straight line from its source.

Many mature trees do not exceed about 40' in height and many places have zoning laws that do not allow a building height to exceed 35'.

If on Dec. 21^st, the sun peaks at 30 deg off the horizon, due south, you want to make sure a tree or building at 35' in height is not going to obstruct your solar access.

Recall from geometry, SOHCAHTOA, pronounced – sew-cah-tow-ah. Being interpreted, it means, sin = opposite/hypotenuse, cosine = adjacent/hypotenuse and tangent = opposite/adjacent.

We know the angle and the length of the leg opposite that angle. We are looking for the length of the leg adjacent to the angle. We need the tangent function to solve this problem. Tan(30 deg) = 35/x. We solve for 'x', with x = 35/0.577. Therefore, x = 60.65'. You need about 61 feet due south without any obstructions that exceed 35' in height.

At 9 am there is significant solar insolaton to be harvested. At that time, the sun is only about 15 degrees off the horizon. We repeat the calculation with 15 degrees. The distance x = 35/(tan(15)) . Solving for x we obtain 131 feet. At that time of day, the sun is about 45 degrees due East of South.

On March 21^st, we are still in a heating dominated month. On September 21^st, we are in a transition month. In March we may want solar gain and in September, we may not. East and West windows can significantly contribute to the total solar gain per day.

In March, or in September, the sun rises to almost 50 degrees off the horizon. Checking that against a 35' tall obstruction, the distance we need clear is given by x = 35/(tan(50)). Solving for x we obtain a distance of 29.36 feet. This is well within the distance from December. Solar collection can start as early as 8 am in March. At that time the sun is just over 20 degrees off the horizon and is at about 70 degrees due East of South.

We compute the distance we need for solar access with x = 35 / (tan(20)). We obtain 96 feet. In this case if solar access is satisfied for December, it will also work for other heating dominated months. The other measures of interest can be found using the same geometrical principles.

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