Delta T (ΔT), in the realm of drone thermography, refers to the temperature difference between two measured points. This seemingly simple metric plays a crucial role in detecting and diagnosing anomalies in a variety of systems. When applied to solar drone inspections, Delta T becomes a powerful diagnostic tool, enabling operators to identify thermal irregularities that could impact the efficiency and safety of solar installations.
Table of Contents
Being able to detect, to identify and to quantify the severity of defects that appear within photovoltaic modules is essential to constitute a reliable, efficient and safety system, avoiding energy losses, mismatches and safety issues.
In solar drone thermography, the ability to measure Delta T allows for precise detection of issues such as hotspots, diode failures, and cracking within photovoltaic modules. These anomalies often manifest as temperature variations that, if left unaddressed, can lead to reduced energy output, increased maintenance costs, or even fire hazards. By using the data provided by Delta T measurements, drone inspections provide actionable insights that help maintain the health of solar assets and optimize their performance over time.
Understanding Delta T in Drone Thermography
Definition and Context
Delta T (ΔT) is the temperature difference between an anomalous component and its surrounding nominal components. In solar thermography, this metric is critical for identifying irregularities in photovoltaic modules.
Related Article – What is an IEC Compliant Drone Solar Inspection?
Hot Spots indicate a defect at cell level, where one or several cells have a higher temperature than the neighboring ones. Depending on the temperature difference (temperature delta) between the heated and normal cell, a Hot Spot may indicate a defect of varying levels of severity.
Common anomalies that exhibit Delta T differences include overheating junction boxes, cell hotspots, bypass diodes, and severe cracking. These issues can cause uneven temperature distributions, leading to energy inefficiencies and potential long-term damage to solar modules if not promptly addressed.
Calculating Delta T of Bypass Diode (Photo Credit - SiteMark)
Measuring Delta T (ΔT)
The calculation of Delta T in relies on analyzing thermal imagery, often in the form of radiometric JPEGs (RJPEGs). These images encode radiometric data, which can be converted into precise temperature measurements.
Hot Spots indicate a defect at cell level, where one or several cells have a higher temperature than the neighboring ones. Depending on the temperature difference (temperature delta) between the heated and normal cell, a Hot Spot may indicate a defect of varying levels of severity.
To measure Delta T, thermal imaging algorithms compare the average temperature within a defined measurement field of view (MFOV) of an anomaly to the average temperature of nearby nominal components. This process ensures a conservative and accurate estimation of temperature deltas, even for anomalies with complex patterns, such as cracked modules.
For a single-cell anomaly, Delta T is determined by subtracting the nominal average temperature of nearby cells from the average temperature of a 4×4 pixel MFOV within the anomalous cell. This approach ensures that the measurement captures a representative temperature of the anomaly, avoiding inaccuracies caused by relying on singular pixel readings. In cases of multi-cell anomalies, Delta T is calculated by comparing the average temperature of the largest affected region, measured using an MFOV, to the average temperature of nearby nominal cells. This method accounts for the complexity of multi-cell issues and provides a precise assessment of the anomaly’s severity.
Advanced algorithms may also incorporate irradiance levels and environmental factors to normalize Delta T measurements, ensuring consistency across varying conditions. The choice of MFOV, which can vary depending on the camera model and specifications (e.g., 4×4 or 10×10 pixel groupings), plays a critical role in delivering accurate and reliable temperature measurements for different types of equipment.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
Examples in Practice
Consider two real-world scenarios:
- Cell anomaly: A bad solder joint causes a single cell in a module to overheat. The maximum temperature of the anomalous cell is 58.2°C, while the average temperature of the surrounding nominal cells is 54.4°C, resulting in a ΔT of 3.8°C. This ΔT indicates that the anomalous cell is 3.8°C hotter than the rest of the module, providing a clear basis for targeted corrective action.
- Cracking anomaly: Severe cracking across multiple cells within a module created uneven temperature patterns. In this case, the hottest region of the cracked area is measured at 65°C, while the surrounding nominal cells average 35°C. This results in a ΔT of 30°C, clearly highlighting the severity of the issue. Such a large temperature difference signals a critical anomaly that requires immediate attention to prevent further damage and energy loss.
In addition, irradiance normalization plays a pivotal role in ensuring accurate Delta T assessments. By factoring in irradiance measurements from plane-of-array (POA) pyranometers, drone inspections can adjust for environmental variables, such as changes in sunlight intensity, which could otherwise distort temperature differences.
Irradiance normalization is a process that can involve adjusting the value of direct beam solar spectral irradiance… this is often done for photovoltaic cell performance calculations and measurements.
This robust understanding of Delta T enables solar operators to identify, categorize, and prioritize anomalies efficiently, laying the groundwork for effective maintenance and optimized solar park performance.
HAVE QUESTIONS ABOUT droneS? START HERE.
How Important is Delta T for IR Inspections of PV Modules?
Detection and Diagnosis
Delta T (ΔT) serves as a cornerstone for identifying and diagnosing thermal anomalies in solar modules. By highlighting temperature differences, ΔT pinpoints irregularities such as hotspots, diode issues, and bypassed substrings that could lead to significant performance degradation if left unaddressed.
Anomalies detected through ΔT analysis are often categorized by severity levels such as mild, serious, or critical, giving operators the ability to prioritize repairs effectively.
Energy Loss and Cost Implications
The magnitude of a ΔT anomaly is directly correlated with energy efficiency. Higher ΔT values typically indicate significant energy yield loss, translating to financial impacts for solar site operators.
For instance, a ΔT anomaly with a 10°C difference might result in a measurable reduction in output, potentially causing annual energy losses worth thousands of dollars depending on the scale of the site. By quantifying ΔT, operators can estimate the economic impact of underperformance and prioritize remedial actions that deliver the greatest return on investment.
Proactive Maintenance and Decision-Making
Delta T data is a valuable tool for proactive maintenance, enabling operators to optimize inspection schedules and reduce downtime. By integrating ΔT measurements into long-term asset management strategies, solar site owners can extend the lifespan of their modules and improve overall performance.
For example, identifying a hotspot early can prevent it from escalating into a more severe issue, reducing repair costs and operational disruptions. Additionally, by addressing ΔT anomalies before they cause significant damage, operators can enhance the reliability and profitability of their solar assets.
Delta T Minimums for Warranty Claims
Module manufacturers often specify ΔT minimum thresholds for warranty claims. For instance, a hotspot with a ΔT exceeding a predefined value might qualify as a manufacturing defect, enabling owners to file a warranty claim. Understanding these thresholds ensures that owners can leverage ΔT data effectively to protect their investment and secure timely replacements or repairs when eligible.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
How to Determine the Severity of Delta T
Delta T (ΔT) is a critical metric in thermographic inspections, but interpreting its severity requires careful consideration of multiple factors. Just because a temperature difference exists doesn’t always indicate a problem—there are numerous variables at play that can influence ΔT readings. Understanding the context and severity of thermal anomalies ensures that maintenance actions are both appropriate and efficient.
Factors Influencing Delta T Measurements
Several factors can contribute to differences in ΔT between components, many of which are outside an inspector’s control. These include:
- Load During the Measurement: The load on the system at the time of the inspection significantly impacts ΔT readings. Measurements taken under peak load conditions tend to produce more distinct ΔT values, helping to identify anomalies with greater accuracy.
- Load Consistency: Evenly distributed loads across components yield more reliable comparisons, while varying loads may lead to misleading ΔT differences.
- Future Load Expectations: Anticipated increases in load can exacerbate existing thermal issues, elevating the urgency of corrective actions.
Other factors include:
Measurement Technique:
- Direct Readings: Viewing bare connections provides the most accurate ΔT data.
- Indirect Readings: Viewing through insulation or an apparatus that contains the faulty equipment may result in reduced accuracy due to heat transfer effects.
Environmental Conditions:
- Emissivity and Reflectivity: Surfaces with low emissivity or high reflectivity (e.g., shiny or polished) can distort readings.
- Weather Exposure: Components exposed to prolonged sunlight, wind, or precipitation may show temperature variations unrelated to faults.
- Surface Characteristics: Factors such as rust, corrosion, paint color, sheen, texture, or contamination (e.g., grease or dirt) can affect the thermal signature of a component.
- Wet vs. Dry Conditions: Wet surfaces can mask thermal anomalies due to evaporative cooling.
These variables highlight the importance of understanding the context in which ΔT measurements are taken to accurately assess the severity of anomalies.
Severity Levels of Delta T
According to the NETA Severity Criteria guidelines, thermal anomalies are typically classified into severity levels based on ΔT measurements and the criticality of the affected component:
1. Mild:
- Minor ΔT differences could be possible deficiency; warrants investigation.
- Example: A hotspot with a ΔT of 1°C to 3°C under similar loads.
- Temp Difference: ΔT 1°C to 10°C between component and ambient air.
- Could be considered within the range of normal operation when environmental factors or load conditions explain the difference.
2. Moderate:
- ΔT differences that indicate probable deficiency; repairs needed as time permits, possible intervention during routine maintenance.
- Example: ΔT of 4° to 15°C on a critical component under similar loads
- Temp Difference: ΔT 11°C to 20°C between component and ambient air.
- Indicates a developing issue that may escalate if not addressed.
3. Elevated:
- ΔT differences indicate monitoring needs to take place until corrective measures are taken.
- Example: ΔT of 1°C to 15°C on a critical component under similar loads
- Temp Difference: ΔT 21°C to 40°C between component and ambient air.
- Indicates an issue that has escalated and should be risk assessed and corrected.
4. Severe:
- Significant ΔT differences indicating immediate corrective action; repair immediately
- Example: ΔT exceeding 15°C on a vital component such as a diode or bypass substring.
- Temp Difference: ΔT that exceeds 40°C between component and ambient air.
- Represents a high risk of failure, energy loss, or safety hazards if left unresolved.
Prioritizing Maintenance Actions
The degree of temperature rise (ΔT) and the criticality of the affected equipment are key factors in prioritizing maintenance efforts. For example:
- Components critical to the system’s operation or safety should be addressed first, even if the ΔT is only moderate.
- Less critical components with mild ΔT differences may be monitored over time to assess changes in severity.
It’s important to recognize that ΔT alone doesn’t always signify a problem. Even identical components side by side may exhibit temperature differences due to inherent variability or environmental factors. Inspectors must combine ΔT data with contextual information to make informed decisions about maintenance priorities.
Best Practices for Severity Assessment
To ensure accurate and actionable assessments:
- Conduct thermal inspections under maximum load conditions to capture the most significant ΔT anomalies.
- Normalize ΔT measurements for environmental factors such as weather conditions, surface properties, reflectivity, and emissivity.
- Cross-reference ΔT data with historical inspection records to track changes in severity over time.
By considering all contributing factors and evaluating the severity of ΔT in context, operators can ensure that maintenance resources are directed where they are most needed, reducing downtime, minimizing costs, and safeguarding system performance.
Getting Started with Delta T (ΔT) Inspections
Delta T (ΔT) is a cornerstone of thermographic inspections, providing valuable insights into the health and performance of solar PV systems. By identifying and assessing thermal anomalies, ΔT measurements help pinpoint issues such as hotspots, overheating fuses, and diode failures, which can degrade system efficiency and lead to costly energy losses. Understanding the severity of ΔT and its contributing factors enables proactive maintenance, optimized decision-making, and extended equipment lifespans, ensuring the long-term success of solar installations.
Thermal anomalies don’t have to compromise your solar energy production. With The Drone Life, you gain a trusted partner equipped to identify and analyze Delta T variations with precision. Our expert pilots and thermography specialists use state-of-the-art drone technology to deliver actionable insights that protect your assets and optimize performance. Schedule a free consultation with us today to learn how our services can enhance your solar panel inspections and ensure maximum energy output for years to come.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
