Understanding Module De-Rating: Concerns and Issues
Green Energy

Understanding Module De-Rating: Concerns and Issues


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 0 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 + 250 C
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: Omega Green Energy




- Noaa Reduces 1997 Global Temp (by Over 3 Degrees Farenheit) In Report Claiming 2015 Was Hottest Year Ever
From MRCTV: In a 1997 report discovered by Thomas Nelson, NOAA said the global average temperature for the year was 62.45 degrees Fahrenheit. As Blogger Wattsupwiththat explains:  In that 1997 report, they say clearly that the Global Average Temperature...

- Noaa: Us Cooling …just Sayin’
Someone please show me an accurate climate model?Responding to widespread criticism that its temperature station readings were corrupted by poor siting issues and suspect adjustments, NOAA established a network of 114 pristinely sited temperature...

- How To Choose A Solar Panel
Solar panel is one of the important elements in solar photovoltaic system. Choosing the correct solar panel is crucial factor. You need to determine the right wattage of a solar panel. The size of the solar panel in watts directly affect the cost which...

- Estimation Of Solar Pv System Output
Factors Affecting Output Standard Test Conditions Solar modules produce DC electricity. The dc output of solar modules is rated by manufacturers under Standard Test Conditions (STC). These conditions are easily recreated in a factory, and allow for consistent...

- The Celsius, Fahrenheit, And Kelvin Temperature Scales
Tc = T - 273.15 This Equation shows that the Celsius temperature Tc  is shifted from the absolute  (Kelvin) temperature T by 273.15°. Because the size of a degree is the same on the two scales, a temperature difference of 5°C is equal...



Green Energy








.