Understanding solar radiation is vital to the design and installation of solar energy components. Solar radiation varies greatly over time and seasons, as well as location and weather conditions. With this in mind solar arrays can be optimized for their available solar energy by aligning them and locating them properly. Also new technology in solar tracking devices can increase efficiencies. Solar radiation resource data is collected by government weather agencies and this information is used to determine proper orientation and provide an estimate of energy output that can also be extrapolated to include financial projections.
The Sun is a vaporous body made up of hydrogen, some helium, and other elements. The atmosphere of gas swirls and flows with the forces of gravity, magnetic influences, and heat energy. Gravity creates intense heat at the core and causes nuclear fusion reactions. The fusion combines atoms from lighter elements into heavier elements, while releasing gigantic quantities of energy. Hydrogen fuses to helium at the core then energy radiates outward. This energy propagates to the photosphere, and escapes into outer space as visible light and other frequencies of radiation. Radiation travels outward from the core of the sun. The scientific community has two theories of how radiation travels. One way is in waves and the other as particles. The photosphere is the visible portion we can see as the sun’s surface.
The sun is approximately one AU unit away from earth, which is 93 million miles and is the primary unit of measure for other distances in the solar system. Visible light travels at 186,000 miles per ssolarpowerinstallation.orgch our earth. The Earth receives approximately 170 million gigawatts of energy (power) from the sun. This amount of energy seems like a lot but actually it is only a small amount of the sun’s total output. 170 million gigawatts is millions of times more than the whole earth’s population would require.
A photon is another primary unit of electromagnetic radiation. Photons created in fusion reactions at the core of the sun travel outward to the surface but take an extremely long time to finally reach the surface. As they move outward to the surface they lose energy. The travel time from core to surface for a photon is estimated at tens of thousands of years. Once at the surface they escape the forces of the sun as visible light.
There are a number of factors effecting the amount and type of solar energy that makes it to Earth’s surface that can be harnessed by solar cells.
Solar Irradiance (Solar Power)is a term used to describe solar power per unit area and is measured in watts per square meter. (KW/m2). Solar irradiance is an instantaneous measurement. Solar irradiance is the reference used to evaluate the abilities of a solar panel or solar cells grouped and measured to a specific size compared to the one square meter. As the sun goes through cycles of fusion activity solar irradiance varies. Depending what distance you are from the sun you would receive more or less radiation based on the inverse square law that states the amount of radiation is proportional to the inverse of the square of the distance from the source. This would mean that if you are twice the distance (2 x one AU unit or186 million miles) you would receive one forth of the energy the Earth receives. Moving 3 times the distance away would mean 9 times less radiation.
Most radiation from the sun is waves that travel as electromagnetic radiation. A smaller amount is mad up of subatomic particles called corpuscular radiation. Charged protons, neutrons, helium nuclei and electrons all make up this smaller amount called the solar wind. When they arrive at the earths magnetic field they gather at the poles effecting the atmosphere and are known as the northern or southern lights or aurora borealis. In extreme solar activity these particles can cause havoc with radio and power transmission.
Solar Irradiation (Solar Energy) is measured is a measurement of accumulated energy per area per time. The period of time may be an hour, day, month or year. Solar irradiation is expressed in watt hours per square meter or kilo watt hours per square meter. This measurement is of great importance for designing and estimating the performance as well as the size of a photovoltaic system.
It is important to understand the terms solar irradiance and solar irradiation and how they relate.
Solar irradiance is comparable to the speed of an object, while solar irradiation is comparable to the distance the object travels over time.
Solar irradiance varies daily at the Earth’s surface. From zero at night, peaking around noon hour, and decreasing as the sun sets.
Solar irradiation can be calculated by using the formula H= E x t
t = time in hours
H = solar irradiation in watt hours per meters squared Wh/m2
E = average solar irradiance in watts per meters squared W/m2
Example: If an average solar irradiance is 800 W/m2 over 8 hours then calculate the total solar irradiation for this period.
H = E x t
H = 800 x 8
H = 6400 Wh/m2 or 6.4 KWh/m2 ( m2 = meters squared or m x m not m x 2)
Extraterrestrial Solar Radiation is solar radiation just outside of the Earth’s atmosphere and is referred to as TOA or top of atmosphere radiation. When designing solar capturing systems for space craft or satellites you must design them differently than on the earth’s surface. On Earth there is a solar constant which takes into account reflected and scatter light. If you are one AU from the sun on earth you can be assured of an approximate value of 1366 W/m2. (m2 = meters squared) The solar constant is essentially stable but does vary slightly over the course of a year. For the design of interstellar craft you would use the inverse square law mentioned earlier.
E = Ee/D2 (D2 = D squared)
E= solar irradiance at location in W/m2 (m2 = m squared)
Ee = solar constant for Earth (1366 W/m2) (m2 = m squared)
D = ratio of distance of location from sun to one AU
For example:
Let’s calculate the solar irradiance for the planet Mars, which orbits the sun at an average distance of 1.52 AU
Using the fomula from above E=Ee/Dsquared
1366/1.52 x 1.52 = 591 Watts per meters squared (W/m2)
Therefore solar irradiance at the planet Mars is approximately 591 W/m2 which is 43% of the Earth’s because Mars is further from the sun than Earth. The Sojourner rover was designed keeping this in mind so that it captures enough power.
Electromagnetic radiation is the primary or most abundant radiation coming at us on Earth in the form of waves with electric and magnetic properties. These waves vary in wavelength depending on their frequency. The wavelength frequency determines the type of radiation. Really short wavelengths in the order of trillionths of meters is considered gamma rays. Long wave lengths are radio waves, and everything in between you find xrays, ultraviolet, visible light, and infared rays. The electromagnetic spectrum encompases all of these rays. Our sun gives off all of these various rays fairly constantly for different wavelengths and has it’s unique spectral signature in the universe.
The Atmospheric Effects on the solar radiation we receive here on Earth varies daily because of absorption, scattering, and the reflecting of light by the atmosphere including our ozone layer, pollutants we have exhausted over many years, clouds, and other particles floating around. Volcanic erruptions and large scale forest fires have been known to reduce radiation substantially. The two most prevalent forms of radiation that reach surfaces on Earth are direct radiation and diffused radiation. The sum of all of the radiation hitting the Earth is the sum of both types of these two radiations. Direct radiation is what creates shadows and is sometimes called beam radiation. This type of radiation travels directly through our atmosphere with little impedance from it. Diffused radiation is scattered radiation that is dispersed in many directions when it hits an impeding atmospheric condition, or reflected from a surface on earth such as a body of water, snow or even a man made object. The regular solar panel captures both types of radiation while a solar concentrating sytem relies only on direct radiation. Radiation that is reflected back out into space is called albedo radiation or reflectance radiation. It is for the most part unuseable for PV systems unless it somehow bounces back off the atmoshpere and again mixes in with the diffused radiation.
The Air Mass that solar radiation must move through scatters and reflects it. When the sun is at its Zenith or directly overhead for a defined location it is at this point that the maximum solar radiation hits Earth. The Zenith Angle is the angle between where the sun is at a given time and the zenith. As the zenith angle increases the sun is moving to the horizon. As the sun approaches the horizon the sun’s rays must pass through a larger amount of atmosphere to reach Earth’s surface.
Air mass (AM) is a measurement of the thickness of the atmosphere. AM = 1 when the sun is at it’s zenith at sea level. Larger measurements than 1 AM are only relative to this sea level measurement. AM 0 is only outside of the Earth’s atmosphere.
For example AM2 is usually measured when the sun and zenith are 60 degrees or 30 degrees above the horizon. In this instance the length the sun’s rays must travel through the atmosphere to hit the Earth is twice that of zenith. Air mass is also effected by the time of year, the altitude and latitude of the place on earth you target. Air mass calculations are also effected by zenith angles greater than 60 degrees due to atmospheric refraction.
The Zenith Angle can easily be calculated at any hour by utilizing a vertical stake driven in the ground so to create a specific height. The ruler shadow along with the height of the stake are the adjacent and opposite sides of a triangle. Using trigonometry the length of the ruler / over the length of the shadow and taking the arctan of that ratio will provide the angle.
Air Mass (AM) calculations can be made once the zenith angle and other parameters are known. Air mass at sea level is approximately 1013 mbars. You need to find the local AM for your location. Generally the United States has a reletive air mass of 1.5 mbars.
Am = Plocal/1013 x cosine of the zenith angle.
Example: For a local atmospheric pressure of 836 what will the air mass be if the zenith angle is 53 degrees
AM = Plocal/ 1013 x cos53 degrees
AM = 836/ 1013 x .062
AM = 1.37
Peak Sun is an estimate that has been evaluated over time by the scientific community. It is the maximum terrestrial solar irradiance during solar noon at sea level. This value is 1000 watts per square meter. Slightly greater values have been measured at higher altitudes in a clear dry climate, but 1000 watts per square meter is the basis that all solar cells, and panels are rated by for their efficiency. Typical solar panels as of 2010 are around 14 % efficient or create about 140 watts of electrical energy per square meter.
Peak Sun hours is the number of hours in a day is the total amount of solar irradiation that accumulates at peak sun conditions. For example during a typical day there may be only two hours that recieve peak sun conditions, however the total irradiation for the day is expressed as units of peak sun hours which is the accumulation of the whole day. A day with an average irradiance of 600 watts per meters squared over an 8 hour period may only have 1 peak sun hour would have 4800 Wh/meters squared or 4.8 peak sun hours. There are peak sun hour charts kept for all areas of the Earth so that PV solar arrays can be designed and have an expected performance.
Insolation is another term you will hear in the solar pv industry.Insolation is the solar irradiation received over a period of usually one day. It is expressed as kWh per square meter per day. Insolation is typically used to give a rating to a particular area and how much energy potential there is there.