PV cleaning robots: new testing methods for a new technology

Heading: A robotic cleaning system in operation. Credit: QEERI.

As PV cleaning robots have significant advantages over solar O&M, Ben Figgis of Qatar Environment & Energy Research Institute at Hamad bin Khalifa University is reviewing the latest developments in industry standards..

Robots are coming. And their standard tests as well.

It is now common in the desert regions to use machines to clean large-scale PV plants. Soil losses can be as high as 1% per day, and it is impossible to manually clean the mileage of the PV matrices. Meanwhile, robotics technology has improved while costs have dropped. But PV cleaning robots raise new questions such as how often they should be run, whether or not to use water, and the violation of the anti-reflective coating (ARC) of the modules.

ARC abrasion is a special concern. The coating increases the light transmission – and therefore the power of the module – by about 2-3%, and up to 5% at large angles of radiation. However, he has a limited life without contact cleaning, according to a recent estimate of between one and 15 years (arxiv.org/abs/2101.05446v1). Some commercial cleaning robots use water to reduce the potential for abrasion, while others use a dry brush for simplicity and to prevent water use in desert environments. The question is how quickly the ARC brush degrades depending on the type of brush, the frequency of cleaning and the wet / dry option.

To predict the life expectancy of ARC, laboratory and real-world tests must be combined. Accelerated laboratory tests provide results quickly and under controlled conditions. Real-world tests validate simulations of accelerated tests, and allow us to study other effects in addition to abrasion, such as micro-cracks in modules. Of course, it is advisable to have repeatable test results and have “apples apples” between different labs; therefore, abrasion test standards have been developed specifically for PV coatings (both anti-reflective and anti-fouling).

The abrasion test standards were the main topics of the PV Robot Cleaning Workshop organized by the Qatar Environment & Energy Research Institute at Hamad bin Khalifa University. Two open standards have been published this year, and at least one PV module manufacturer is developing its own internal test protocol.

Standard development

The U.S. National Renewable Energy Laboratory led the development of IEC 62788-7-3, published in February 2022, because it did not properly simulate PV abrasions and cleaning in other industries. By default, the standard uses Arizona medium test powder as an abrasive for brush test methods, although other compositions may be used for local testing.

It is primarily designed for testing coupons – samples can be as small as 7.5 cm square – although in principle full-size photovoltaic modules or glass cover can also be used. The rule also covers a number of abrasion scenarios: linear and / or rotating brushes, wet or dry operation with test powder, as well as falling or swollen sand. The closest simulation of cleaning large-scale photovoltaic installations is a rotary linear motion brush, although as the standard states, “there is no commercial test equipment” for this configuration.

The other standard launched this year, DIN SPEC 4867: 2022-04, came from a consortium led by the German Fraunhofer CSP. It is aimed at simulating the cleaning of PV modules in the real world. Samples must be in “original format” (full size) modules or glass samples to achieve industrial-grade ARC manufacturing and coating properties. The cleaning mechanism is a linearly rotating brush with the usual tools and cleaning characteristics of commercial products. For consistency, the default abrasive is feldspar, which is applied only as a wet slurry. The standard has two “modes”: comparing the durability of different ARCs and comparing abrasion using different cleaning parameters, such as brushes. Unlike the IEC standard, the DIN determines in detail how the resulting ARC abrasion should be measured and calculated using a reflection photometer.

These rules will help to achieve repeatable and comparable laboratory abrasion tests. They are especially useful for “A vs. B” tests, such as comparing the durability of ARC candidates or different types of brushes. Another possible use may be to speed up the acceptance of a particular robot with a particular module. For example, if a robot is approved for use with the X module and a standard test shows that the Y module has the same abrasion resistance as the X module, then the robot could be safely accepted for the Y module.

Reflectometer in use. Credit: QEERI.

The rise of robots

Comparative tests of robots were also discussed in the workshop. Jinko Solar presented a test project for dust-cleaning robots; the company says it is the first module supplier to perform abrasion testing. The program tests robots proposed for a specific PV project in the planning phase, with results validated by Jinko, EPC, and “third parties”. The tests inspect not only the ARC abrasion, but also the movement of the robot: crossing followers, obstacle resilience, and emergency stop.

Many tests have been performed on Jinko’s Tiger-Neo n type modules to confirm that it has the least power to the modules. The robots run on an accelerated 10,000-pass program, simulating 30-year operation. So far Jinko has tested robots from five manufacturers. Finally, it aims to test other PV manufacturers and release the protocol as an industry standard.

The key question is how the laboratory tests simulate the mechanics of real-world module abrasion in the long run, and whether a brush cycle at the laboratory stand is the same as a brush cycle in the field. The main difference in the field is that dust accumulates for one or more days between washes, during which dew, humidity, and high temperatures in the PV modules can affect the properties of the dust. Therefore, it is useful to validate the results of the internal tests with the external field tests. These internal / external comparisons will also be supported by accelerated testing for local dust types and adhesive mechanisms.

During the workshop, QEERI presented its external robot testing program as part of its Solar Consortium industry team. The project has been in continuous use since 2020 of a dry brush cleaning robot with the same photovoltaic modules and coupons in the Outdoor Test Facility. In particular, the tests are not accelerated – the samples are cleaned once a day, once a week or not. at all. Although this approach will take years to produce results (final measurements are due in late 2022), it will provide very realistic data on photovoltaic robot abrasion in desert conditions.

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