Understanding Module De-Rating: Concerns and Issues

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Module De-rating accounts for consistency and accuracy of the rating given by manufacturer. Since not all modules in a system will have the same specification (manufacturing defect) and there would be degradation over the years in panel, so to have a measure of working of panels over a period of time, de-rating is given by manufacturer at the time of selling of the product.

  • Manufacturer output Tolerance:  The output of a PV module specified in Watts, with a manufacturing tolerance and based on cell temperature of 25 C

Example: A 230 W module has a manufacturer’s tolerance of ±5%. The “worst case” adjusted output of the PV module is

Therefore, 230W x 0.95 = 218.5W

  •  De-rating due to dirt: Output of PV module can be reduced as a result of accumulation of dust on the surface of the panel. The value of this De-rating would depend upon the actual location, but it can be high due to pollution level in the air. An acceptable de-rating would be ±5%.

             Example continued: The de-rated module of 220W would be de-rated by a further 5% due to dirt: 218.5W x 0.95 = 207.6 W

  • De-rating due to temperature: the average temperature of the cell within the PV module can be estimated by the following formula: Tcell.eff = Ta.day + 25C

Where,

Tcell.eff = average daily effective cell temperature, in degrees C

Ta.day = daytime average ambient temperature (for the month of interest), in degrees C.

Array frames in stand-alone power systems are typically tilted at higher angles and the modules have good airflow. With rooftop grid-connected systems, higher temperatures have been observed.

  • For grid-connect systems the effective cell temperature is determined by the following formula:

Tcell.eff = Ta.day + Tr

where

Tr = effective temperature rise for specific type of installation.

It is recommended that the following temperature rise (Tr) be applied for different array frames:

  • Parallel to roof (<150mm standoff): +35°C
  • Rack-type mount (>150mm standoff): +30°C
  • Top-of-pole mount, free standing frame and frame on roof with tilt angle of around+ 20 degrees to slope of roof: +25°C.

Solar Modules have different temperature co-efficient depending on module technology. Typically ranging from +0.2%/°C to -0.5%/°C.

De-rating of array will depend on the type of module installed and average ambient temperature for the location.

The temperature de-rating factor is calculated as follows:

ftemp = 1 + (Y*( Tcell.eff – Tstc))

where,

ftemp = temperature de-rating factor, dimensionless

Y = value of power temperature coefficient per degrees C

Tcell.eff = average daily cell temperature, in degrees C

Tstc = cell temperature at standard test conditions, in degrees C.

Example continued:

Assume the average ambient temperature is 25°C (Ta.day) and the module is polycrystalline and frame is parallel to roof but less than 150mm off roof. The average daily effective cell temperature is:

Tcell.eff = Ta.day +35 = 25 + 35 = 60°C

In the above formula the absolute value of the temperature coefficient (Y) is applied, this is -0.5%/°C and cell temperature at standard test conditions is 25 °C (Tstc)

Therefore the effective de-rating factor due to temperature is: 1 +( -0.5% / (60 – 25)) = 1 -17.5% = 0.825

The temperature de-rating becomes 82.5% of 207.6 W or 171.3 W.

Posted by:

Prateek Sikka

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