Characteristics of Electrical Solar Panels – Photovoltaic Panels

Standard Test Conditions

Standard Test Conditions is an outlined set of lab test conditions which approximate conditions in which solar panels, or Photovoltaic modules, could be used. The same standard is also used to judge potential installation locations. Since it is used to calculate the following 3 values:

Irradiance – measured in in Watts per square meter falling on a flat surface. The measurement standard is one kW per sq m.

Air Mass pertains to “thickness” and lucidity of the air thru that the daylight passes to get to the modules, remember the sun angle is affecting this value.

Cell temperature that may differ from ambient air temperature. The graph is known as an “I-V curve”, and it refers back to the module’s output relationship between current ( I ) and voltage ( V ) under prevailing conditions of daylight and temperature.

The curve appears like a seated person’s leg: Theoretically, each solar panel has multiple I-V curves ( many of which are shown above for one actual module ) – one each for all of the different combinations of conditions that will affect the Standard Test Conditions rating parameters above : temperature, air mass, irradiance that is a large amount of possible graphs. A solar panel module loses voltage as the cell temperature increases; that effect is not unusual to all crystalline modules. Due to Ohm’s Law ( and the equation Power = Voltage x Current ), the results of reduced voltage is reduced power output.

The best position on any I-V curve-the sweet spot where we are able to collect the most power from the module-is at the “knee”. That is the maximum power point ( MPP ), and you can see that its position changes with temperature and irradiance. In battery-based Photovoltaic systems, an MPPT, or maximum power point tracking charge controller monitors the array continually to find the ever-changing MPP and therefore capture the most power from the array. In straight grid-tied systems, MPPT technology is built into all of the inverters, so these systems have a tendency to have awfully high. 2 groups of conditions which can turbo-charge voltage-and change the MPP-in a Photovoltaic or solar electrical system include over-irradiance and temperature effects. Over-irradiance is simply a fancy way of pronouncing daylight with a power above the standardized Standard Test Conditions price of one thousand Watts per square meter.

This suggests less energy-robbing atmosphere for daylight to pass thru.

This condition could happen at high altitudes, for instance.

This effect happens as a cloud shade passes out of the incoming daylight’s pathway to the solar panels. Refraction can concentrate the daylight while the fringe of the shadow passes by. The result’s a lift in module voltage output. Powerful reflections from nearby bodies of water and even a surrounding carpet of snow on a bright, winter day can produce a lift of solar power that will affect voltage.

Temperature effects are the result of an inherent characteristic of crystalline silicon cell-based modules. They have a tendency to produce higher voltage as the temperature drops and, inversely, to lose voltage in high temperatures. Any solar panel or system derating calculation must include adjustment for this temperature effect. Customarily, this derating is performed when figuring out the sizes of related system parts, for example charge controllers or grid-tied inverters, as these components must be sized to deal with the possible current spikes from the Photovoltaic array due to over-irradiance and temperature effects. Grid-tied inverter sizing, especially, depends on identifying the lowest recorded temperature at a suggested site. Inverter manufacturers include this variable in their tables or online Photovoltaic string-sizing tools. They, in turn, get factoring info. If a low temperature is even remotely possible you may plan for it or you risk frying your charge controller or inverter. Since module voltage could spike under bright daylight and lower temperatures, you want to make sure that your charge controller or inverter can handle the highest possible voltage and current. For site temperature info, I counsel consulting the weather info available at weather base.

In planning a Photovoltaic system, Standard Test Conditions is your first guide for sizing and planning. But Standard Test Conditions relies on lab conditions. Following acceptable derating procedures will make sure an effective and safe Photovoltaic system. Click here to read more on how to build a solar panel.


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