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Andrew Gong
Author

Andrew Gong

Andrew Gong is a Research Engineer at Aurora. He previously worked on two Solar Decathlon projects, and he received his M.S. from Stanford and B.S. from Caltech.

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Understanding PV System Losses, Part 3: Soiling, Snow, System Degradation

Andrew GongAndrew Gong

Being able to give your solar customers accurate estimates of how much their solar installation will produce is essential. But there are many factors that impact how much the PV system will produce–from physical characteristics of the components and design to environmental factors like shade and dust.

In order to deliver accurate production estimates, it is crucial to understand what factors reduce the energy production of your installation (PV system losses)–and by how much. In today’s article, the latest installment of Aurora’s PV System Losses Series in which we explain specific causes of solar production loss, we explore soiling, snow, and system degradation losses.

About This Series

In this series, we provide an overview of various causes of energy production loss in solar PV systems. Each article will explain specific types of system losses, drawing from Aurora’s Performance Simulation Settings, and discuss why they affect system performance.

System losses refer to effects that simulation engines do not explicitly model; these linear loss factors are applied as percentage reductions to the estimated system production calculated by the simulation engine. (For the purposes of this article, we assume the simulations are run using the Aurora Simulation Engine; however, PVWatts will also use these settings if selected.)

For Aurora users, this series will provide tips for improving the accuracy of your performance simulations by sharing research-backed recommendations for what values to input in your Simulation Settings for different loss types. While Aurora provides default values for these fields that fit most use cases, this series will also highlight cases in which you might want to use different values depending on the specifics of your design. (For a quick summary of system losses, and how to configure your account settings in Aurora, see the Aurora Help Center.)

This guide for picking better loss values will help you give your customers the most accurate estimate of how much their system will produce and how much they can save by going solar.

See how your solar contracting business can work smarter with Aurora.

Environmental Losses: Soiling

Suggested Values:
5% for regions with long dry seasons
      -Add 1-2% if region experiences frequent dust deposits
      -Add 1% if near major vehicular traffic areas
      -Remove 0.5% if the system will be cleaned in the summer
2% for regions with year-round rain

Commercial installers may want to consult with ground monitoring companies to create an on-site estimation of soiling, since O&M costs are in part dictated by the frequency of cleaning required.

solar panel cleaning can be important in reducing PV system losses

A PV system’s performance is directly tied to how much sunlight reaches the panels to be transformed into electric energy. When we think about things that block the sun, we often think of trees and cloud cover but, on a smaller scale, we have to worry about the dust and grime that accumulates on solar panels and all other surfaces over time. This is referred to as soiling.

Soiling losses capture the amount of sunlight that is blocked by dirt and debris that accumulate on solar panels over time. Just like a car’s windshield, small amounts of dust and dirt can accumulate and scatter light. Soiling differs drastically based on the region that a system is located - everything from the amount of wind, exposed dirt, and local traffic, to the frequency and strength of rain and the number of birds in an area can have an effect.

Let’s start with the industry standard for PV losses, NREL’s Performance Parameters Paper. The document suggests a 5% typical soiling loss, but has a range of 2% to 25% which is incredibly wide. Let’s dig into why there’s such a large range:

Variation Across the United States

An ongoing study from the National Center for Photovoltaics tracks a number of systems across the United States with estimated soiling values. []. Some notable effects are that the systems scattered across central California see 4-5% loss compared with Texas plains around 1%.

Rainfall Is a Major Factor

Locus Energy has a disaggregation algorithm that lets them estimate soiling losses based on production data, and they found that areas around Southern California saw very high soiling losses during summer months with a drop off in the winter. Additionally, areas in the Northeast maintained a very low soiling rate year-round. They attributed the seasonal variation in California and the consistent low soiling in the Northeast to rainfall keeping panels clean.

Soiling Loss Builds Up Until Cleaned Manually, or by Heavy Rain

Dust does not build up all at once–it accumulates over time, ranging from 0.01%/day up to 0.5%/day in extremely dusty areas. This means that panels should be washed periodically to prevent excessive buildup. An NREL model that uses a steady soiling increase found that, for a system that would build up soil that blocks 1.9% of sunlight over the course of a year, having one annual cleaning would maintain the loss around 1.5%, two cleanings per year could drop the average loss to 1.3%, and three cleanings per year would reduce that further to 1.2% average annual loss.

A Note on Birds

Bird droppings are typically counted under soiling losses since they represent a temporary environmental blockage in the panel, but they have some distinct behavior. First off, rain is often not sufficient to wash away the mess, and it requires manual cleaning. Unlike dirt particulate matter, it only affects one or two cells at a time but the cells it does impact are substantially blocked. Due to the series connection of cells within a panel, the one impacted cell could knock out the cell string or the entire module if bypass diodes are not present. The best advice is to clean off the panel quickly. 

solar panel cleaning is needed to reduce energy losses from bird droppings and other soiling

Tips for Reducing Soiling Losses

Owners in drier climates should use spare water to gently clean panels between rainfall. In rainier climates, rain will handle most dirt. Bird droppings will have to be cleaned manually and should be addressed quickly since just a little shade can knock out an entire string.

Resources for Further Reading:

PV Soiling Rate Variation over Long Periods–Reports a soiling loss of about 0.5%/day in a desert environment.

Time Series Analysis of Photovoltaic Soiling Station Data: Version 1.0, August 2017–Reviews soiling at various U.S. locations, finding that these values range 0.03 to 0.15%/day.

Impact & Consequences of Soiling and Cleaning of PV Modules–Finds soiling losses are highly dependant on the local environment and explores the impact of a variety of cleaning methods. 

Environmental Losses: Snow Cover

Suggested Values:
Snow loss is tricky since it is very dependent on the site. We suggest using monthly snow loss factors rather than annual. Snow loss can reach upward of 20% in some regions during winter months. An NREL study that quantified snow losses with a snow-removal model calculated losses ranging from 10-30% for fixed-tilt systems.

Snow cover blocks sunlight from reaching the solar panels. In snowy climates, the amount of snow loss will be dependent on several factors, including the tilt of the panels, duration and intensity of snowfall, ambient temperature, and possibly wind. Similar to how inverter shutdowns will entirely shut down a system (as discussed in Part 2), snow cover will block production until it either slides off the panels or melts away.

NREL studies suggest that sliding is the primary mechanism for snow clearing, although the application of this also requires some knowledge of how the panels will be spaced relative to the roof and whether there will be space for the snow to fall. 

snow is another source of PV system losses; solar panel cleaning to remove snow can help

 Time Losses: System Degradation

Suggested Values:
0.3%/year for high-end modules
0.5%/year for monocrystalline
0.6%/year for polycrystalline

You can also use manufacturer production guarantees for a conservative estimate - for example, if a manufacturer guarantees that their panels will have 80% production after 25 years, that is 20%/25 years = 0.8%/year degradation.

The materials in solar cells lose efficiency over time, and to account for this Aurora models the reduced power output over time when analyzing the lifetime value of a project.

A standard silicon solar panel consists of silicon wafers, connecting busbars, encapsulant material, glass, and polymer backing sheets, and other electrical connection components. Over time, these components will be worn down and damaged by the effects of thermal expansion and contraction, UV light, and damage from windblown particles. Weaker electrical connections cause resistive losses within a cell, decreased shunt resistance allows current to “leak” within a cell instead of being used to power loads, and discoloration and damage to the layers in front of the cells reduces the available light.

Studies on this topic are numerous since the lifetime degradation of a module and an array determines how profitable a project can be. Degradation of silicon panels depends on the type of wafer (monocrystalline or polycrystalline), the temperature of the installation site, and the materials used in the construction of the panels. Individual manufacturers also recommend certain settings for their own products.

Resources for Further Reading:
Photovoltaic Degradation Rates–An Analytical Review
Potential-induced Degradation in Photovoltaic Modules: A Critical Review

Note: In Aurora, system degradation losses are accounted for at the financing stage (under Systems and Maintenance, as shown below), rather than under Performance Settings as for other loss types.

Aurora accounts for PV system losses from degradation in its financing toolsIn Aurora, system degradation losses are accounted for at the financing stage. To see how to add a financing option to your project, see this video (0:58 shows the system degradation setting).

About Our PV System Losses Series

This article is part of Aurora’s PV System Losses Series. Each article explains specific types of system losses, drawing from Aurora’s Performance Simulation Settings, and discusses why they affect system performance.

Understanding PV System Losses, Part 1: Nameplate, Mismatch, and LID Losses
Understanding PV System Losses, Part 2: Wiring, Connections, and System Availability
Understanding PV System Losses, Part 3: Soiling, Snow, System Degradation

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Andrew Gong
Author

Andrew Gong

Andrew Gong is a Research Engineer at Aurora. He previously worked on two Solar Decathlon projects, and he received his M.S. from Stanford and B.S. from Caltech.

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