Why (didn't) we use drones for inspections?

Why do we use piloted aircraft for our inspections?

As Heliolytics has grown over the past six years, there is one question which has been consistent all the way through: “Why don’t you use drones for inspections?”. While we do use UAV technology for some of our services, it is true that we primarily rely on aircraft and satellite services for our imagery capture. The answer -in short- is that we started Heliolytics as a company to provide the highest quality services to the PV sector, not to sell drones to the PV sector.

Because of this, we have had a fundamentally different approach to building out our hardware, software and business models. This has been focused on providing the best possible product to the industry in new and innovative ways, always with an eye towards making the best possible technical decision for our customers.

We started Heliolytics as a company to provide the highest quality services to the PV sector, not to sell drones to the PV sector.

We do actually use UAV’s for some inspection jobs, and they can be a useful tool for some applications. Overall, however, we have found that we are able to provide a much more accurate, faster, cheaper and repeatable process using an aircraft based system.

One of the key reasons that using an aircraft allows us to optimize inspections is because we can use the best possible sensors for inspections of solar PV, not for the sensors that fit on a drone.

Let’s talk about sensors

We can start with a quick review of available IR camera technologies:

Microbolometer:

This is a space and power efficient module used to measure spatially resolved temperatures. If you have used a handheld or drone based IR imager, it is using one of these devices.

A microbolometer is a set of small (micro) temperature sensors (bolometers) which are set into an imaging array. By measuring the temperature of each bolometer, the device can provide an image by converting the temperatures to pixel values.

Because this unit can operate at room temperature, it is very versatile and one of the most widely used IR imaging technologies due to its space and power efficiency.

What it gives up here are a few key attributes:

• Pixel averaging: Because the microbolometer is an array of temperature sensors, each measurement is an average of the 10 nearest pixels
• High NEDT: NEDT is a measure of the noise in an image, visually this looks like “grainyness” in a photo. Because the unit is not cooled, it has a higher NEDT
• High integration time: Because the measurement is a temperature measurement, the thermal constant (how long something takes to change temperatures) matters in the measurement, and the amount of time to resolve an image is longer. This is especially important when the camera is in motion.

Cooled Photonic Sensor:

This sensor type is similar in a lot of ways to the cameras that we use everyday. Instead of measuring temperature with bolometers, the sensor is a semiconductor, which counts the number of photons hitting each pixel. Literally, it is an array of microscopic solar panels, instead of generating power they generate a voltage signal which is encoded as a pixel value.

For a visible light camera, the material used is silicon, just like a normal silicon solar cell, which is reactive to visible light bands. For infrared imagery, the substrate is more exotic and measures photons in the infrared bands.

Because the camera is measuring photons in the infrared, it needs to be cryogenically cooled. Imagine trying to take a photo when the camera itself is glowing brightly - this is what would happen to a photonic thermal sensor if it wasn’t cooled close to absolute zero.

These cameras therefore have a lot of advantages, as shown below, but a big disadvantage in that they are big, power hungry and expensive. They simply do not work on the current generation of standard imaging drones.

This is offset by a few key advantages:

• Integration time - the time it takes to resolve an image is fast - between 2-5 milliseconds - and this makes the camera able to operate in a fast moving aircraft
• Resolution - these cameras are able to achieved much higher resolutions than a microbolometer, and our cameras are 2-4X higher resolution than a standard drone mounted camera
• NEDT - Simply put, the images are less “grainy”, this means that when we are trying to pick up on subtle thermal differences we are more likely to find real issues and not noise.
• Pixel accurate measurements - Because we are measuring photons not heat, there is no pixel averaging, meaning that our true measurement spot - the area we are able to resolve accurate temperatures - is up to 3X higher than a microbolometer.

These differences in sensors lead to a few practical benefits in how we are able to offer services:

Inspection times

Because we are operating on an aircraft at (relatively) high speeds and with a much higher resolution camera, our inspection times are significantly different. We are able to inspect at a rate of up to 150MW/hr at high resolution, compared to a UAV inspection at 2.5MW/hr - 7MW/hr for a UAV inspection, depending on the site size. This can lead to a substantial difference in total time to inspect.

Long inspection times can lead to a few concerns related to data quality:

• Varying irradiance profiles throughout the day - Because of the variance in the amount of light hitting panels, there will not be consistency in the temperature delta of faults, and we have seen that there are some faults which do not appear at specific irradiance levels. Therefore, inspection over a large site over a large period of time can lead to misrepresentation of patterns and fault evolution on site.
• “Pushing the envelope” - especially for larger sites, there can be a significant financial motivation to capture data close to or under the limits of irradiance (600 W/m^2) as required by IEC 62446-3
• Commissioning and time sensitive work - The long time needed by drones may not be compatible with required timelines. Of course, it is possible to deploy multiple drones for a project, and we have seen this done successfully, however it does add additional data normalization concerns.

As an important note, we are also able to scale to large rooftop portfolios - our aircraft record to date is 74 commercial rooftops inspected in a day - and the aircraft platform is able to scan these sites efficiently and unobtrusively, without needing to involve the building manager.

No technology lock-in

Another key consideration we looked at when first selecting our technology path was how we would see the industry evolving over time. Because we have built and manage all of our camera systems in house, we are able to ensure that they are properly maintained and calibrated on a regular basis without needing to rely on third party drone operators.

The biggest advantage, however, is that we are able to regularly upgrade and update our camera systems as new technologies come online. Because we manage a small number of very high capacity cameras, we can rapidly deploy new sensors, software and tools to make sure that we are using the latest available technology.

Enables condition based services

The difference in acquisition modes also allows us to offer our industry leading condition based services, where we are able to very efficiently capture data for a large portfolio and process it to identify regions where there is the highest return on investment for the portfolio.

No site access required

For some of our customers, this is one of the most tangible benefits, as they do not need to spend time and money ensuring that a UAV operator can access a remote site. For the aircraft, we just need to know the site is online, and we can do the rest.

Scaleable and enterprise grade

Finally, this platform allows us to scale rapidly while maintaining flexibility with our customers. In 2020, we were able to inspect over 25GW of projects in a year across three continents. Our unique deployment strategy ensures that we can provide fully managed acquisitions for large and global portfolios effectively, and using traceable cameras and operational procedures.

If you're interested to hear more about how Heliolytics advanced aerial inspections can help with your portfolio needs, feel free to reach out.