Solar PV Performance Analysis: 3.4. Soiling

Module soiling is also a major influencer of underperformance, and soiling can be caused by many factors generally classified between:

1. Human, e.g. factory exhaust fumes (the worst of those being cement dust), kerosene from airplanes etc

2. Natural, such as sand, leaves, lichen, rocks, snow, minerals from rain or bird droppings.

Soiling management is a critical part of a solar power plant’s operational strategy, as dirty modules lead to (1) site underperformance and importantly (2) long term module damaged caused by hot spots.

Contrary to certain people’s ideas, solar modules are not sufficiently cleaned by rain, even in tropical zones, and regular cleaning anywhere between 6-monthly to weekly frequency is needed. This can be combined with drone infrared thermography to pinpoint area with specific soiling like leaves or bird droppings.

Whilst soiling can generally be detected through performance monitoring by "less than expected output", the accurate performance impact of soiling on a solar site is very difficult to determine by (1) SCADA or performance data or (2) visual inspections as the soiling is generally very widespread and there are no reference strings or inverters that have no soiling at all.

The best method to accurately calculate the soiling losses is to perform a module level test on a sample of modules, where some modules are cleaned e.g. weekly and some modules are not cleaned at all. Then module-level I/V curve measurements between the cleaned vs non cleaned modules will determine the soiling losses. We will publish a post on the methodology for this shortly.

In addition to determining the performance losses from soiling, it’s important to look at the source(s) of soiling and how to minimise that.

As a few examples:

• On solar floating sites, it’s common to have bird nesting as a floating site provides the ideal sheltered habitat away from predators which in turns leads to module soiling from bird droppings. In such cases it’s important to look at bird deterrents such as removing nests or trying to scare the birds away.   

• For solar rooftops affected by factory exhausts fumes, it’s good to discuss with the factory owner/operator about minimising the fumes, either by adding filters to chimneys or running certain machines less often.

• On low tilt projects, it can be helpful to have solar “clips” placed on the downward sloping edge of the module, that help to drain the water and prevent the stains often caused by minerals in the rain

When simulating soiling losses in PV Syst, it’s important to have visited the site before construction to have a general idea of fumes and dust at the project.

And also it’s important to take into account seasonality; I’ve been involved in one project where the engineering team neglected to consider snow in the soiling losses, which led to dramatic underperformance (against budget) in the winter months!