In order to guarantee the quality of PV panels, manufacturers use test methods which include exposing panels to physical pressure and temperature changes. However, according to researchers at the University of Florida, these tests are not sufficient to ensure long-term retention of efficiency. Using an adapted test protocol, they exposed four types of panels to various stresses, with remarkable results.
According to Eric Schneller, researcher at the University of Central Florida's Florida Solar Energy Center, PV panels are exposed to a variety of physical influences that can cause cracks. "This is due to human actions, such as during transport and installation. But weather influences also play a role, such as snow, wind and extreme temperature changes." According to him, cracks have serious consequences: "They degrade the panel's performance, and they can create dead areas. These lead to voltage differences which further reduce performance. Cracks sometimes also cause 'hot spots' which, in the worst-case scenario, pose a safety risk - in the form of a fire hazard."
To prevent cracks forming, manufacturers carry out tests to determine how and when cracks are formed. However, according to Schneller, these tests are not sufficient. In particular, they ignore the fact that cracks formed at 'moment 1' increase the problems caused at 'moment 2' by other circumstances. To illustrate, Schneller gives an example: the weight of a snowpack on a panel can cause microcracks which can close automatically as soon as the snow melts away and the weight pressure decreases. If the same panel is subsequently exposed to a high peak temperature or vibrations due to strong winds, i.e. at 'moment 2', the same microcracks can open again and develop further, which can result in a loss of power.
4-step test protocol
In order to map out the consequences of consecutive moments with physical and thermal pressure, Schneller developed a new test protocol. This Mechanical Evaluation Protocol consists of four steps that a panel has to undergo in succession. In the first step, the front of the panel is exposed to a physical pressure of 5,400 Pa for one hour. This simulates a situation in which the panel is covered with a thick layer of snow. This can cause microcracks, but as mentioned before, these cracks tend to close again as soon as the physical pressure decreases.
In the second test within the protocol, the same panel is briefly exposed to 1,000 Pa a thousand times in a row. In a field situation, this pressure can be caused by prolonged exposure to gusts of wind. One possible consequence of this is that cells become electrically isolated, which leads to a loss of capacity. In the third test, the panel is exposed to 50 cycles with significant temperature increases and 10 cycles with frost and humidity to simulate highly variable weather conditions. In addition to creating new microcracks, existing microcracks can be 'pulled open' to become real cracks, and there is also a chance that the various layers that make up the panel will come loose. The second and third tests in the protocol are also part of commonly applied test procedures, but not in a cycle that is preceded by a physical pressure test and followed by another test. That subsequent test is the final and fourth step in Schneller's protocol, and a repetition of the second test: the panel is again subjected to 1,000 Pa a thousand times.
Results per panel type
Schneller's team used the new protocol to expose four well-known module designs to various stresses: Photovoltaic module HIT®, Mono-PERC, Multi-PERC and Mono-PERT (see box). After each step, the panels were examined to assess the damage and loss of power. It is worth noting that each module type reacts differently to each of the four stress tests. After completion of the first step (5,400 Pa pressure) it turned out that the Photovoltaic module HIT® had not suffered any damage whatsoever. Damage to the Mono-PERC panel was limited to four cracks, the Mono-PERT panel had seven, and the Multi-PERC panel had no fewer than 37.
In the second test (1,000 x 1,000 Pa), the Photovoltaic module HIT®, Mono-PERC and Mono-PERT panels did not suffer any new damage. The Multi-PERC panel was in a significantly worse condition. As a result of the long series of mechanical pressure moments, microcracks created in test 1 were found to have been opened again and further torn open, resulting in heavy damage. The third step (peak temperatures, frost and humidity) caused hardly any additional damage, except for the Mono-PERT panel which suffered several new cracks. The final test (1,000 x 1,000 Pa) resulted in 6 new cracks being found in the Mono-PERC panel, and 'countless' new cracks in both the Mono-PERT and Multi-PERC panels. Remarkably, the Photovoltaic module HIT® came through this test completely unscathed. According to Schneller, this is probably due to the specific design of the interconnects, and the foil material used for this panel.
Final conclusions for each panel
After completion of the full protocol, the final 'total damage' to the four panel types was assessed and the number of cracks incurred was counted. The largest number - 54 - was found in the Multi-PERC panel. This was followed closely by the Mono-PERT panel, with 45 cracks. The Mono-PERC panel did much better in the tests, with 11 cracks, and the absolute winner turned out to be the Photovoltaic module HIT®. Just one crack was found on it - but according to the researchers it should not be counted because it was caused by incorrect transport of the panel.
Of course, the most interesting outcome of the series of tests is what consequences they have for the power delivered by the panels. Here, too, the research team found major differences. The Multi-PERC panel came out of it the worst, ultimately losing almost 10%. The Mono-PERT panel delivered 3.5% less power after the four cycles, and the Mono-PERC panel about 2.5%. The big 'winner' of the test turned out to be the Photovoltaic module HIT®, which showed no loss of power after going through the entire test procedure.
The secret behind Photovoltaic module HIT®
The most important question remaining is why Photovoltaic module HIT® is much better at withstanding a long series of tests with physical and thermal pressure. According to Panasonic, the company that invented Photovoltaic module HIT® and also the only manufacturer of them, the Florida study shows in any case that strict internal quality procedures are bearing fruit. The manufacturer uses its own test protocol which, like Schneller's protocol, goes far beyond what is customary in the market and which are described in guidelines from the IEC (International Electrotechnical Commission). For tests, for example, the IEC prescribes a 'cold / heat' shock test in a climatic chamber with temperature changes from +85 to -40 °C. According to the guidelines, such a temperature change must take place 200 times in a row, but Panasonic applies 600 cycles, in order to build in extra certainty about the reliability of its panels. "We also put our competitors' panels through our tests," says Andreas Thoma, business development at Panasonic Germany. "It turned out that after those 600 cycles the Photovoltaic module HIT® showed only a few percent power loss, while panels of our competitors lost up to 23%".
According to Thoma, these better results are the result of a production process that among other things uses a different method of securing the tabs in the panel. "Because we use different tabbing technology from soldering, there is less tension on the tabs which makes them more resistant to large temperature differences and they do not come loose. This does sometimes happen with other panels, resulting in a considerable loss of power." According to Thoma, the fact that the Photovoltaic module HIT® withstood not only the temperature cycles but also the physical stress test much better during the tests in Florida is due to an optimised frame design, the use of smaller, more flexible cells, and the application of special low-temperature technology to connect the cells.
PERC, PERT and HIT®
- In PERC technology (Passivated Emitter and Rear Cell), sometimes also referred to as Passivated Emitter and Rear Contact) one or more extra layers are added to the back of the cells. This extra layer or layers reflect back sunlight that has not been absorbed by the cells, so there is a chance that it will still be 'captured'.
- PERT (Passivated Emitter Rear Totally Diffused) cells have an efficiency-enhancing diffuse surface on the back. Production requires special techniques, which is why PERT involves higher production costs than PERC.
- HIT® (Heterojunction with Intrinsic Thin layer) is registered as Panasonic trade mark. They are composed of a monocrystalline silicon wafer that is coated with an ultra-thin silicon layer. This combines the advantages of crystalline technology (high production) with those of amorphous technology (reduced electron loss).