Light-Induced Degradation (LID)

Module output power degrades by a few percent from its initial maximum in the first few hours of exposure to sunlight, a phenomenon known as Light-Induced Degradation (LID). Manufacturer's data for modules normally reflect the pre-LID performance of the modules, and assuming the module performance model was fitted to the manufacturer's data, the results will reflect the pre-LID performance of the modules. In order to obtain a more realistic estimate of the performance of modules during the majority of their lifetime, the user can enter an LID effect \(\Delta_{\text{lid,mtype}}\) for each type of module included in the SolarFarmer workbook.

As \(\Delta_{\text{lid,mtype}}\) is constant over time, the instantaneous and time-averaged values of this effect are all simply equal to \(\Delta_{\text{lid,mtype}}\). An exception to this would be where an array consists of several types of modules with differing \(\Delta_{\text{lid,mtype}}\) values and the contributions to the total power output by modules of each type varies with time.

Assuming the plant array is composed entirely of a single type of module, plant array power after the power is reduced by LID is simply:

$$P_{Parray,\ npmc,LID}\left( t \right) = \left( 1 + \frac{\Delta_{lid,mtype}}{100%} \right) \bullet P_{Parray,npmc,binning}(t)$$

For plant arrays consisting of a mixture of module types with differing LID effects:

$$P_{Parray,\ npmc,LID}\left( t \right) = \sum_{j = 1}^{N_{\text{submodules}}}{P_{submod,npmc,LID}\left( t \right)}$$

Where

$$P_{submod,npmc,LID}\left( t \right) = \left( 1 + \frac{\Delta_{lid,mtype}}{100%} \right) \bullet P_{submod,npmc,binning}\left( t \right)$$

The plant LID effect at time t is:

$$\Delta_{\text{LID}}\left( t \right) = \left( \frac{P_{Parray,npmc,LID}\left( t \right)}{P_{Parray,npmc,binning}\left( t \right)} - 1 \right) \bullet 100%$$

Powers can be summed over desired periods to get monthly or annual LID Effects. As for other effects, it will be necessary to de-season if the input data is not a single complete year.

To consider the LID Effect alongside other conversion effects in the mismatch calculation, an adjustment to the current from each submodule is calculated as:

$$\eta_{submod,LID} = \left( 1 + \frac{\Delta_{LID,mtype}}{100%} \right)$$