PV Array (Solar Panel) Thermography

 Solar energy is a clean and inexhaustible energy source. According to the World Energy Outlook 2014, solar power has contributed significantly (18%) to the growth of renewable energy technologies in the recent decade, after the wind power (34%) and hydropower (30%). Solar energy is gaining popularity in many countries because the cost of solar energy is getting cheaper making it more competitively priced against generating electricity using fossil fuels. Many countries have achieved Grid Parity (when solar or other renewable power sources can generate electricity at costs equal or less than the conventional fossil fuel sources).  In some remote areas, solar energy has become one of the substantial alternative energy sources where the conventional electrical grid is hard to reach. 

The solar photovoltaic (PV) system converts sunlight into electrical energy using the photoelectric effect.  With continuous technology innovations and cost reduction aided by global subsidies, solar PV is becoming a fast growing industry.  
A solar PV system consists of several main components:
- Solar panels to absorb and convert the solar power to electricity
- Solar inverter to change the electrical current from DC to AC
- Mounting and cabling accessories that make up the system

A solar panel consists of a matrix of solar cells. The failure of any solar cell may lead to a drop in power generation causing output yield losses. A solar farm may consist of a few thousand solar panels. Testing each individual solar panel at the installation site using the direct wire connection for checking output is time consuming and cumbersome. A more effective method is using the thermography scan to detect if the solar cells is overheating due to shade or defective cells. When a cell is shaded or not working, the cell consumes power from the adjacent series of solar cells instead of generating power.  This causes the cell to overheat as seen in the thermal images indicated by the hotspots as shown in the following figures. To minimize the shading effect, the manufacturer normally installs bypass diodes to the solar panel. However, the bypass diodes can degrade or become defective, which will also create the similar problem of hotspots.  If the affected cells continue to heat up the adjacent cells, the power generated will be greatly reduced. 

The anomalies detected in the thermal image should be compared with a normal solar cell. It is recommended to further confirm the anomalies detected with relevant electrical testing. 

Figure 1 Thermography scan with thermal-visual side-by-side images using Keysight TrueIR Analysis and Reporting Tool showing multiple hotspots indicated in red detected on one of the solar panel

Figure 2 Picture-in-picture (fusion) mode in Keysight U5855A Thermal Imager enables user to identify the location of the abnormalities easily with a combination of IR and visible images

Quickly Identify and Characterize Thermal Measurement Points

Being an R&D electronics engineer, have you ever wondered if your first prototype works as designed? Using your notes or experience, you can theoretically derive where the most power is dissipated and identify the potential problem areas, but a flaw in the design where power is being consumed at an unexpected rate might go undetected. A thermal imager can quickly help to identify these problem areas. Then, you can characterize your design in different scenarios using DAQ system and thermocouples.

Quickly identify thermal measurement points with a thermal imager or a thermal camera

First, you need to identify the area that you want to monitor. In traditional electronics design, this means finding hotspots or areas where you have poor air flow. In other applications, such as building inspection, hot or cold spots may be area of concern. Using a thermal imager will quickly allow you to determine where to focus your efforts. Below are some samples of images and its respective thermal images that highlight areas that are relatively hotter. 

Figure 1: Picture of a printed circuit assembly (PCA) under test

Figure 2: Two thermal pictures of a PCA. Right image is a close-up portion of the left-hand portion of the PCA. 

Most thermal camera in the market will highlight the maximum and minimum temperature on the display, and some comes with the option to add spot measurements. The thermal images above shows some hotspots, allowing us to determine where to focus our efforts. To ensure you get an accurate measurement, remember to set the emissivity setting at the thermal imager to match your printed circuit board, or the material you are measuring. Emissivity of a material is its relative ability to emit infrared energy. As an example, the emissivity of normal FR4 PCB is 0.91. One other option is to spray your board with a spray-on high emissivity coating, such as boron nitride lubricant, that has an emissivity value of 1.

Making data acquisition temperature measurements

Once the points have been determined, a DAQ system can be used to further characterize the heat profile of your design. One of the first steps to characterizing your temperature is to choose the right temperature sensor. Common temperature sensors include thermocouples, Resistance Temperature Detectors (RTDs), thermistors, and IC sensors. Each has its own particular advantages for different applications.

Once you have decided on the type of device to use for temperature monitoring, you will need to mount the devices onto your board or structure. Once your system has been wired and mounted, you can do a long term monitoring of your design in various environmental conditions, under real-world conditions or in an environmental chamber.

Using a thermal imager, you can quickly identify thermal points that you want to monitor. With DAQ system and temperature sensors, you can make reliable, accurate and long-term temperature measurements to fully characterize your designs. With a thermal imager and a DAQ system, performing temperature measurements on your designs has never been easier. 

For more information on this application, click here to read on. 

HV cable insulation resistance test for hybrid vehicle

Figure 1 Toyota Prius, one of the most recognized hybrid cars on the road

Toyota Prius is among the first hybrid cars in mass production. The Toyota Prius is the world's bestselling hybrid car, with cumulative global sales of over 3 million units.  It was designed for fuel efficiency and ultra-low emissions. A hybrid electric vehicle (HEV) is a type of hybrid vehicle and electric vehicle which combines a conventional internal combustion engine (ICE) propulsion system with an electric propulsion system. The presence of the electric power train is intended to achieve either better fuel economy than a conventional vehicle or better performance.

The hybrid control system combines the best operating characteristics of the combustion engine and electric motor depending to the driving condition.  Prius adopts the sophisticated Toyota Hybrid System (THS/THS II*). The system essentially is an energy recovery mechanism which slows down a vehicle or object by converting its kinetic energy to supplement the power of fuel burning. This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted.  This system therefore helps to achieve superior fuel efficiency and reduction of CO₂ emission.

* The THS II developed under Toyota’s “Hybrid Synergy Drive” concept, the refinement of the original Toyota Hybrid System. 

The hybrid system consists of the following main components:

• Gasoline engine – engine runs to drive the wheels during normal driving and at acceleration
• Motor generators (MG) – generate electrical power and recharge HV battery
• Power Split Device – split the torque between the motor generators and engine.
• Inverter system – converts high DC voltage (HV battery) to AC (MG) and vice versa
• HV Batteries – supply electric power to Motor generator during start-off, low speed, acceleration and uphill driving.

The hybrid vehicle operates at high-voltage system up to a few hundred volts. The high voltage system includes the HV battery, inverter assembly and the motor generators. They are connected with the high voltage power cable in orange color. Any leakage in high voltage insulation system may lead to shorter HV battery life. It may be harmful to the human body if accidentally touched. To check the HV cable insulation integrity, the hybrid vehicle service technicians use insulation resistance tester to measure the insulation resistance between the power cable and body ground, and compare the test result with the manufacturer recommended limit. Sometimes, contamination or moisture may lead to low insulation resistance reading. 

Figure 2 Testing the insulation resistance value between the HV power cable and the body ground.  The Keysight U1461A insulation multimeter shows >260 GΩ, indicates the insulation is in good condition.

Tips: Capturing crisp and clear infrared thermal image

Happy New Year, readers! I wanted to start the year by sharing some useful tips on how you can capture crisp and clear infrared thermal image. Thermal imagers or thermal cameras available on the shelf today are quite intuitive and easy to use. However, getting the best thermal image and then interpreting it later requires knowledge and experience. Here are some tips that can help you be better prepared when you are out in the field.

1. Get the right focus on the target area for more accurate temperature readings. There are several types of focusing mechanism available – fixed-focus, manual focus or auto focus, with the latter being the most advanced. Take your time to get it right at this stage because this cannot be edited later!
2. Perform a quick scan on the targeted area that you are inspecting. Set both the temperature range and scale to auto mode, to quickly get the temperature range of the area. Once done, you can fix the temperature scale by enabling the manual mode, to capture a more stable image. Change the mode quickly with the Auto/Man quick access- button (press-and-hold) on the U5855A TrueIR thermal imager.
3. Once a suspected hotspot is found, manually refocus on the hot spot area. Always validate if it is a real hotspot by checking for any possible reflective heat sources or solar loading effect, if the inspection is done in an open area or under the sunlight. Move from side to side to eliminate possible external or reflective heat sources.
4. Check the surface condition and material of the hotspot area and apply a suitable emissivity coefficient factor, Ɛ. Note: If the object’s surface is polished or shiny in nature, both the Ɛ and emitted IR energy will be low. These surfaces can also reflect IR energy from other sources. In such cases, the low Ɛ and external reflected IR energy typically produces an inaccurate temperature measurement. Ensure these values are compensated by setting the right reflected temperature, ambient temperature and humidity of the area of inspection.
5. Capture and save the IR image for reporting purposes. Extra identification and information on the hotspot area can be done by note/photo tagging (with TrueIR thermal imager, user can capture and tag up to 3 visible photos to each of the infrared images captured) or with simply writing it down in notebook.

Sample infrared thermal image of electronics PCB board
taken with U5855A TrueIR thermal imager

Sample infrared thermal image of solar panels
taken with U5855A TrueIR thermal imager

Perform post-IR thermography analysis, such as adding additional temperature spot measurements, box measurements,line or histogram analysis to further support the findings. Correction parameters, such as emissivity, reflected & ambient temperatures as well as color alarm setting can also be done during post-analysis. TrueIR thermal imager comes with free downloadble software  (www.keysight.com/find/TrueIR_ART) that can be used for analysis and reporting purposes.