THE APPLICATION OF SOLAR PANELS IN A PV SYSTEM
In forgoing explanation of solar items, silicon and solar cells were brought into focus.
Now we move the focus to solar panels, arrays and PV installations.
However, before this can be set in practice the necessary background information will be given. So we start with some relevant basic solar facts and figures.
Moreover some essential theory is needed for the understanding of PV technologies for “gaining: solar energy.
4.1 FACTS AND FIGURES OF SOLAR ENERGY
The centre of our universe is the sun. The big mass of the sun defines the orbits of our planets, planetoids and other celestial bodies. The sun is the source of all kinds of live in our universe. The sun consists of about 80 per cent hydrogen, 20 per cent helium and only 0.1 percent of other elements.
The sun is the source of insolation due to the behaviour of this star.
The sun is a huge star with a diameter of 1.4 million kilometres and act as a thermonuclear furnace fusing hydrogen atoms into helium.
The resulting loss of mass is converted into about 38 x 1020 MW of electromagnetic energy that radiates outward from the surface into space
The temperature of the sun kernel is 15 million degrees Celsius and the pressure in that kernel is 200 billiard bars. The sun continuous radiates energy into space. This energy is partly collected by the various celestial bodies such as our planet earth, the blue planet.
That’s why live is possible on earth. Without sun irradiation, there is no live.
The radiant intensity outside the earth atmosphere varies slightly depending on the distance between earth and sun. The variation of this flux density is about 1.7 per cent. The average value of the radiant intensity is called the solar constant E0, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU).
The AU is roughly the mean distance from the sun to the earth. The "solar constant" includes all types of solar radiation, not just the visible light. It has an average value of1.367 kW/m² and varying slightly with solar activity. This value is not reached on earth surface level, because the atmosphere obstructs the passage of the sun irradiation. The irradiation is partly reflected, absorbed or scattered.
Finally only 14.4 per cent is irradiated on the continents.
33 per cent irradiates the ocean surfaces,
31 per cent is reflected by the atmosphere,
17.4 per cent is absorbed by the atmosphere and
4.2 per cent is reflected by the earth surface.
Hardly independent of the place on earth, a flux density of 1000 W/m2 is reached around noon and with an unclouded sky.
The radiation of the sun delivers over the year an average amount of energy of 220.000 billion kWh. That is 2500 times the total energy use of the complete world population. For Germany and The Netherlands the yearly sun insolation amount to respectively 900 and 1200 kWh/m2. That means that a substantial part of the sun insolation is delivered in more sunny areas on earth like Central America, The Sahara, South Africa, Saudi Arabia and the north of Australia. Another point of interest for e.g. the Netherlands is that gaining of sun energy primarily takes place in the summer months (80 per cent). But in spite of that, it is still certainly worth to gain sun energy in The Netherlands.
The spectrum of sunlight is partly visible for human eyes.
The spectrum of sunlight spans from 240 nm to about 1700 nm.
Visible light overlaps a range between 400 and 800 nm. The reach of a solar cell is on both sides somewhat more. So, not the whole area of the sunlight spectrum is directly used.
The actual solar spectrum corresponds to wavelengths within the ultraviolet UV (7 %), visible radiation (47%), and infrared radiation IR (46%).
The visible spectrum, which lies between the UV and IR, ranges from 0.38 μm (violet) to 0.78 μm (red).
As solar radiation makes its way toward the earth’s surface, some of it is absorbed by various constituents in the atmosphere, giving the terrestrial spectrum an irregular, bumpy shape. The terrestrial spectrum also depends on how much atmosphere the radiation has to pass through to reach the surface.
4.2 THE AM VALUE
As we know now, the atmosphere is an obstructing medium for sunlight. The thicker the air layer the greater the obstruction. In order to take this effect in account, a so called AM factor is introduced.
AM stands for Air Mass. The AM value is obtained from the relation between the measured values for the radiation amount in an environment without air (AM 0) and the environment around the equator.
The irradiation angle with the insolating surface is supposed to be 90 degrees.
The AM factor can be calculated with next expression.
where h1 = path length through the atmosphere with the sun directly overhead,
h2 = path length through the atmosphere to reach a spot on the surface, and
β = the altitude angle of the sun
AM = 1.5 is the situation for irradiation of the earth surface in an angle of 41.5 degrees.
So, Path h2 is 1.5 longer than Path h1. The air mass layer is 1.5 times thicker.
AM = 1.5 is become an important value because it’s now a reference unit used as the norm for characterizing of solar cells.
The geographic position is also important. The equator’s yearly insolation is 1.8 times bigger as the average insolation value in Middle Europe.
4.3 THE INSOLATION OF SUNLIGHT
On a clear day, sunlight irradiation on earth has an average of 1000 kW / m2.
Though the indication kW / m2 is mostly used, several other unities are applied.
As there are:
BTU = British Thermal Unit = (The amount of energy that become free during the burning process of a match).
The conversion factor of this doubtful unity is 1 kW / m2 = 317.1 BTU / ft2.
The insolation can also be expressed in Joules (J) or Langleys.
1 kW / m2 = 317.1 BTU / ft2 = 3.6 MJ / m2 = 1 Langley / 85.93 cm2 = 1 peak sun hour.
4.4 PEAK SUN HOURS
The day hours with a sun insolation equal or greater than1000 kW / m2 are peak sun hours. For instance 5 peak sun hours delivers 5 kWh / m2. In practice tables are made on different locations on earth for the average daily energy yield in a particular month of the year expressed in peak sun hours. So, a conversion calculation is made.
In reality, the average energy yield during daylight is of course lower then 1000 kW / m2. This average then is translated to an amount of standard peak sun hours. The figure shows a number of tables as they used in America for PV installation design. By now there are complete works for the insolation values in a lot of locations on earth.