Lesson 1: Solar Basics

The Sun

The sun, which is 93 million miles away from the Earth, is actually a middle-aged star. The sun’s light can be used as a source of energy for the Earth. It is capable of producing light because of nuclear fusion of hydrogen atoms into helium atoms, and can supply approximately 10 billion years worth of light energy. It’s no wonder that solar power is so appealing as a potential energy source for planet Earth.

Solar Radiation

Insolation means solar radiation striking a certain location at a particular time. It is measured in kilowatt-hours per square meter, represented by the abbreviation kWh/m^2.  It can also be expressed using other units such as BTU, Joules, or Peak Sun Hours. The phrase “peak sun hours”  means the number of hours per day that insolation is 1 kW/meter^2. This means that if peak sun hours is equal to five, the amount of energy being described is equivalent to the sun shining for 5 hours  at a concentration of 1kW per meter squared. The following can be used to convert between different units of insolation.

1 peak sun hour  =  1 kWh/m^2  =  317.1 BTU/foot^2  =  3,600 Joules/m^2

The Sun’s Orientation

Did you know that what a compass shows as north or south isn’t necessarily true north or true south? A compass aligns with the Earth’s magnetic field, but may not actually represent the true direction. The term magnetic declination represents the number of degrees that magnetic south or north deviates from true south or north. The degree of deviation is specific to the location on Earth where the measurement is taken. Similarly, the sun’s position relative to true north or south is called the azimuth. In the northern hemisphere, the optimal amount of energy can be obtained if the solar array is facing true south, or an azimuth of zero degrees. See this Magnetic Declination Map of the North America as of 2010.

Altitude is the sun’s angle relative to the horizon.  When the sun is at it’s highest altitude of the day, which is called solar noon, the azimuth of the sun is zero degrees or true south. This can be difficult to visualize when reading it in words. The following figure shows the relationship between azimuth, horizon, and altitude.

Figure 1. The relationship between azimuth, altitude, and horizon.

Site Analysis and System Design

Solar panels produce their maximum amount of energy when the sun is facing directly at them. In other words, performance is optimal when the sun’s rays hit the panels at 90 degrees or perpendicular to the panel surface. It is important to keep this in mind when determining the tilt of your solar array.

Criteria to consider when designing a new solar system is optimizing the tilt angle of the panels for the month in which the average  insolation is the lowest for that year, the availability and amount of sunshine for the location where the system will be installed, as well as any potential shading obstacles — anything that may block sun from reaching the panels.

While you can determine potential instances of shading by following the path of the sun across the horizon from east to west to find a site with good year-round exposure, this can be difficult and sometimes impractical to do because it requires observation over a long period of time. The sun’s path can be predicted in the short term by time zones, a sun chart, or solar site analysis devices that have built-in sun charts.

References

  1. Figure 1 Image Credit: Image credit: http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Azimuth-Altitude_schematic.svg/436px-Azimuth-Altitude_schematic.svg.png
  2. Solar Energy International. Photovoltaics Design and Installation Manual. April 2010.
  3. NASA: http://imagine.gsfc.nasa.gov/docs/science/know_l1/sun.html
  4. University of Hawaii Institute for Astronomy: http://www.ifa.hawaii.edu/~ger/ASTRO-110_sp08/Lecture17_TheSun

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