# Degradation and Plant Construction Schedule

Large power plants are not constructed and commissioned at one point in time, but come on line on a Block-by-Block basis. Therefore, different sections of the plant will start contributing energy staggered in time as each comes on line, and accounting for energization times lasts until the last section is energized and the full capacity of the plant is realized.

In addition, the long term power plant production reduces continuously over time due to environmental exposure, component aging, and accrued module defects. Three mutually-exclusive models are supported, and can be selected by the user on simulation setup:

# DC Linear

Linear DC degradation function reduces the DC power input to the inverter as a linear function of time by the specified rate from the user.

## Algorithm

1.) Loop for all simulation steps for each Block

a.) The continuous-time function in hours is as follows:

b.) Find the system power due to energization time and degradation for time t.

# AC Linear

This function will reduce the AC power output as a linear function of time.

## Algorithm

1.) Loop for all simulation steps for each Block.

a.) The continuous-time function in hours is as follows:

b.) Find the system power due to energization time and degradation for time t.

# AC Stepped

This function will reduce the AC power output as a stepped function of time. The AC output of the block is reduced by the linear annual degradation rate at a discrete point in time.

## Algorithm

1.) Loop for all simulation steps for each Block

a.) The discrete-time function in hours is as follows:

b.) Find the system power due to energization time and degradation for time t.

# Light and elevated Temperature Induced Degradation (LeTID)

Light and elevated temperature induced degradation (LeTID) is another forms of power output degradation that affects crystalline silicon (c-Si) solar cell technology.

This degradation effect can be seen in both an immediate drop in module power and a sustained degradation rate over many years. LeTID is present in both p-type and n-type silicon cells and universally affects today’s most widely used c-Si module types.

LeTID occurs on a longer time scale on the order of multiple years. This blind spot requires additional treatment in the energy prediction to ensure an accurate performance prediction based on best available data for a given module technology type, specific manufacturer and cell process. The algorithm used is the same as in DC Linear degradation.

If manufacturer or independent test data is obtained, it should serve as a basis for incorporated loss factors. In absence of this module specific data, the table below summarizes the default settings for the energy prediction loss factors in Plant Predict for each module technology type based on the studies surveyed and referenced below.

Category Parameter Value or Setting
P-Type c-Si,
MonoPERC,
Cast-Mono
(Mono and Bifacial)
N-Type c-Si,
Ga Doped P-Type
(Mono and Bifacial)
LeTID First Year LeTID degradation rate 1% 0.5%
Custom long term degradation rate Y2-Y5 Multi-Year Power Degradation rate (in addition to default degradation rate) 0.5%/yr. 0.3%/yr.
Custom long term degradation rate Y6+ Multi-Year Power Degradation rate (in addition to default degradation rate) -0.3%/yr (=50% recovery) or demonstrated rate -0.17%/yr (=50% recovery) or demonstrated rate

Figure 40: List of LeTID default values

Reference:

First Solar Application Note TSD-00536 Rev 1.0 : LID & LeTID Behavior of Industry Leading PV Module Technologies