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Technical Information Solar Energy Solar energy is free and basically unlimited. Using it produces no greenhouse gases or other adverse environmental impact. To directly convert this outstanding energy source, however, more efficient photovoltaic (PV) systems must be developed. Whether the development effort involves new semiconductor technology or new concentrator designs, solar simulation systems will be play integral part. Solar simulation is much more than just choosing a lighting source with a spectrum that "looks good". The whole premise behind artificial solar simulation is to replicate, as accurately as possible, the effects of actual sunlight on products or PV material. Whether you are trying to measure the detrimental effects of UV or the beneficial output of PV panels, this data must parallel the results from actual sunlight and be repeatable. For this reason, international standards have been developed to benchmark solar simulation lighting. The current ASTM G173-03 and IEC 60904-3 international standards (see below) quantify the solar radiation energy level across the spectrum from 280nm to 4000nm.
Solar Spectrum The insolation, the amount of solar radiation over a given surface area and a specified time, at the surface of the earth may be reduced up to 45% by our atmosphere, primarily due to reflection and absorption. About half of the insolation finally reaching the earth's surface is in the visible portion of the electromagnetic spectrum. Even considering this, the global potential for solar energy is huge. The amount of energy that reaches the earth's surface every year exceeds the total energy consumption by roughly a factor of 10,000.A significant percentage of the total losses in a PV cell (more than 50%) are associated with spectral mismatch (the inability of the semiconductor material badgap to absorb energy across the full solar spectrum). This illustrates the importance for solar simulation systems to match, as close as possible, the spectral distribution of solar radiation incident on the surface of the earth.
PV Energy Conversion Efficiency When the sun is at a right angle or "normal", to the face of a solar panel, the panel will receive the maximum available direct sunlight. If the sun is off to the side of the panel, and possibly casting a shadow across the face of the solar panel, it will not receive direct sunlight and minimal power will be produced. The amount that the direct irradiation is reduced is a function of the cosine of the incidence angle. The efficiency of many PV panels is reduced significantly as the incidence angle is increased. Because of this, useable sunlight is very restricted and maximum output can only be maintained for a few hours each day, even in clear weather. For this reason, solar simulation systems are in great demand both for development and production verification. A solar cell's energy conversion efficiency (η, "eta"), is the percentage of power converted from absorbed light, when a solar cell is connected to an electrical circuit. This is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/m²) under Standard Test Conditions (STC; see above) and the surface area of the solar cell (Ac in m²).
As
an example, a solar cell of 12% efficiency with a 100 cm2 (0.01 m2)
surface area can produce approximately 1.2 watts of power. Clearly, the
need to improve the efficiency of PV material is critical to achieving
cost competitive energy conversion.
International Standards for Solar Simulation Spectral Distribution The
reference spectral distribution of sunlight at global Air Mass 1.5
is defined in ASTM G173-03 and IEC 60904-3. Although these tables include
the spectrum from 280nm to 4000nm, for classification of solar simulation
systems, this is restricted to the wavelengths from 400nm
to 1,100nm. This spectral bandwidth is defined in reference IEC 60904-9; ASTM
E927-05. The spectral match of a
simulation system is classified with respect to these standards (see
section on ASTM / IEC
Standards). Heat Energy Output from Continuous Full Spectrum Arc Tube Lamps
Our sun is a very efficient heat source. It is obvious that
a large solar
simulation array will also generate a significant amount of heat.
Regardless of whether the simulation array is in an enclosed chamber, or
whether it is an open array in a large room, this heat must be managed. A
chamber often requires an external chiller and sufficient airflow to
maintain the desired operating, or device-under-test, temperature. If you
are operating an open array in a large room, the HVAC system for the area
must be sized to ensure an acceptable room ambient temperature is
maintained. Below is a quick calculator to assist in sizing your air
handling system. After you know the lamp wattage and number of lamps that
will be used in your solar simulation array, simply type in the total
amount of lamp wattage and click on "calculate". The resultant answer will
be the amount of heat in BTU/hr that the simulation array will generate.
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