Everything you need to know before inspecting a solar plant with drones and thermal cameras (2023)

The use of drones and thermal cameras in the field of photovoltaics increased the efficiency in which an inspection is performed, the results obtained and consequently an improvement in energy production.

Without drones, the inspection of solar plants would be manual and would take not only many days to complete, but more physical energy of technicians, many more employees in the field and much more money.

To carry out a photovoltaic inspection with drones and thermal cameras we must take into account environmental factors, have certain technical knowledge, as well as know and choose the ideal equipment for the type of inspection we want to carry out. In this article we share everything you need to know to carry out this task and obtain the desired results.

Step 1. Define which camera and drone to use depending on the type of inspection.

THERMAL IMAGING

Step 2. Define flight altitude

First of all, to define the flight height we will need to know what type of inspection we will do based on what results the client expects to obtain. Once we know this we proceed to calculate the GSD and in this way define our flight height. If you don't know how to do it, in this article you can learn how to calculate the GSD for your flights.

Step 3. Define overlaps

This refers to the overlap that we leave between one image and another, that is, how much information from the previous image appears in the next.

We have two types of overlaps, vertical and horizontal. The "horizontal overlap" type refers to the direction of the panels and the "vertical overlap" is perpendicular to the flight, i.e. the overlapping of the images between one row and the next one vertically.

If the overlap is too large we could end up with too many images and too much post-processing. But if the overlap is too small we might miss out on necessary plant information.

Thus, through experience and trial-and-error, we have defined an ideal overlap of Front Overlap 80% which would be for horizontal overlap and Side Overlap 20% which we refer to as vertical overlap.

In case you want to make a thermal orthomosaic, you have to change the overlap to 85%-85% respectively.

Step 4. Camera angle

Ideally the camera should always be tilted perpendicular to the ground. However, depending on the time of day, it may happen that sunlight causes reflections on the panels and in this case the camera angle should be 20 degrees.

_{Image 3. Angle of the camera with respect to the solar panels.}

Step 5. Time of day and weather for capture the images

The ideal time of day to carry out the inspection is when the sun is high enough but not perpendicular to the plant, as this could cause too many reflections, although in this case, tilting the camera to an angle of 20 degrees would help to avoid these reflections, as previously mentioned.

With regard to weather, the following is indicative: clear sky or few clouds, preferably 60% or less humidity. The ideal wind speed would be below 6.5 m/s and an irradiance of 600 W/m2 or more.

Irradiance is the amount of solar irradiance that is incident on a specific surface at a specific time. If you don't know how to measure it, you can learn how to measure solar irradiance here.

Step 5. How to capture images

The images should always be taken horizontally, in the direction of the rows. To visualise this better we leave two images with the correct and incorrect way to do it:

Step 6. Flight speed

In order to know at what speed to fly we must take into account several factors, such as the storage size of the memory card we are using, the camera and the percentage of overlap. Starting with the overlaps, if the percentage is high, as in the case of an orthomosaic, the flight time will be longer to capture all the images, compared to flights with smaller overlaps.

With regard to the type of camera, the shutter speed of the camera will also have an influence on the speed of flight .

And then there are the memory cards, which it is important to choose correctly as the type of memory and its compatibility with the camera we use will guarantee the speed of flight we are looking for.

So, first of all we must know the specifications of our camera to find out what type of memory card it uses and the type of data transfer interface (known as the "bus" of both the camera and the memory). This can be found in the technical specifications of the equipment.

The following are the three types of "bus" that exist:

Camera data transfer rate

Once we know the data of our camera we must choose a compatible memory card. On memory cards the data transfer speed or "bus" is expressed in Roman numerals I, II, III. This is compatible with the camera's transfer speed equal to or lower, e.g. the memory card specifying II is compatible with a camera interface type UHS-I and UHS-II.

Then we have to evaluate the storage capacity of the memory card and the write (trigger) and read (transfer) speed . The read speed as you will see below is specified on the memory card, the write speed is usually less than that number.

All the characteristics of the card can be found in the symbols on the memory card. Below is a diagram showing the meaning of these symbols and the types of memory cards.

TYPES OF MEMORY CARDS

CHARACTERISTICS OF MEMORY CARDS

"Speed classes" indicates the minimum write speed of a card. There are different write classes, from class 2 to class 10. For example; class 2 indicates 2MB/s, it is very slow and can record very low resolution photos. While class 6 indicates that it records 6MB/s and can record higher resolution photos in RAW format. Class 10 is the fastest of them all, guaranteeing a write speed of 10 MB/s, and allows you to record videos and take high-resolution photos without any problems.	"UHS-Class Speed". Within the Class 10 cards there are two subcategories, U1 and U3. These cards are designed for UHS-compliant equipment. U1: guarantees a constant minimum write speed of 10 MB/s U3: guarantees a constant minimum write speed of 30 MB/s.	This nomenclature indicates the speed of the video. This classification is known as "video speed class" and can be: V6, V10, V30, V60 and V90. In this case, V10 = 10MB/s. From V10 onwards, it is class 10 writing speed. And category U1. From V30 onwards it is class 10 write speed and category U3.

The V10 which in turn is class 10 and U1 is the minimum rating we should opt for if our work involves video, RAW or burst shooting.

And from the V30 rating onwards, which in turn is Class 10 and U3, we are assured that the card will be able to record Full HD photos, 4K videos and burst images at a guaranteed write (trigger) speed of 30 MB/s without any problems.

Remember, before choosing a memory card, make sure that it is compatible with your photographic equipment. In addition to this, remember to take a laptop with you to keep a backup of the work you are doing. Equipment can fail and we don't want to lose all the work we have done.

Also, we recommend that you opt for reputable brands when purchasing your memory card to avoid any mishaps.

Some examples:

The Zenmuse XT2 camera specifies in its data sheet that it supports UHS-3 class memory with a maximum capacity of 128 GB. It recommends the following memory cards: Sandisk Extreme 16/32 GB UHS-3 microSDHC and Sandisk Extreme 64/128 GB UHS-3 microSDXC. Whereas, the data sheet of the Flir Vue Pro R camera recommends using a microSD memory card with a maximum capacity of 32GB .

Step 7. Flight route

The flight path should be flown from left to right, clockwise. To visualise this better we have highlighted the flight path in the following two demos we made in the DJI GS PRO software,

And in the UgCS software,

Once we have all this we can plan the flight in a dedicated software for this task. In the following step we provide a comparison of the three most famous software and their main advantages and disadvantages.

Step 8. Different software options for flight planning.

UgCS	DJI Ground Station Pro	Pix4DCapture
Higher level of technical knowledge Paid software Not very intuitive Android IOS	User-friendly (simple and very easy to use) Free version (enough tools to perform the flight) IOS - IPAD	Higher level of technical knowledge Paid software Not very intuitive Android IOS

And finally, we have made a demo mission on how to perform the flight planning to inspect a solar plant using the DJI GS Pro software and the FLIR Zenmuse XT2 thermal camera, you can find it in this link or in this post.