Words: Erik Lohse, Michael Fufl
The quality of solar modules is not always as expected. However, a cost-efficient way to prove the quality on-site is by using mobile test centres – under the condition that measurements are performed and test results are implemented by well-trained and knowledgeable solar experts. Testing modules on-site can establish a basis for operating them at maximum yield and profit over long periods of time. PES presents an illuminating discussion.
Installing and operating a PV plant is not always going to go according to plan. The quality of the delivered module might be not as good as expected. Meanwhile, lightning or hailstorms may strike existing plants and cause breakdowns that must be quickly taken care of to minimise service interruptions. Similarly, the operator of a solar plant may develop doubts about the continued and agreed performance of the installed modules shortly before the warranty period runs out.
In light of this, he might ask himself how to claim damages from the vendor (if it’s still in business). Other operators might just want to increase string performance by occasionally recombining the modules according to their actual performance. All this will drive the manufacturers of PV generators to offer products with tested quality to ensure the maximum benefit to customers.
For all these situations there now is a simple, cost-efficient solution: mobile PV test centres contracted and used on-site. Before, faulty or suspicious PV modules had to be shipped to a stationary test laboratory, which involved a certain logistic effort. If the modules had been already mounted they had to be taken out, cleaned, packed and trucked to the test facility. Upon arrival they would enter a queue of similarly disqualified products, and then they would undergo the test procedure, would be packed again, shipped back to the plant and re-mounted.
This is generally a time consuming and costly process involving its own risk – especially during transport – due to inappropriate packing and handling.
The fact is, the quality of a module is known and well documented only at the time of its leaving the test station. What remains of this when the tested module arrives back at its field location is not known, since the customer cannot verify the module’s actual quality just by visual inspection. This, by the way, also applies to modules that are taken out of storage, transported, tested, shipped back and stored again.
A welcome alternative therefore is to reverse the traditional test procedure: instead of moving the PV modules to the test lab, the test lab is brought to the modules’ location. This optimised test procedure reduces transport cost and risk of damage. Also, the logistics of getting a mobile PV test centre, which is mounted on the trailer of a passenger car, to the location of the PV plant is much easier than transporting hundreds of modules to the next available stationary test lab.
Ensuring project quality – on-site
The mobile on-site test facility is a perfect tool to simplify the implementation of project-related quality assurance measures and to maintain the high quality standards that customers have come to value and expect.
The most important criteria to achieve customer satisfaction is to know the qualification of all PV modules right at the place of the solar plant. This can uncover any impairment of the desired quality due to transport problems and can be compensated for by appropriate measures.
Of special significance is measuring the power output of a module, plus carrying out a detailed electroluminescence analysis. Measured performance helps decide which modules should be combined to strings to yield maximum efficiency. Electroluminescence testing delivers valuable information on the modules’ behaviour in regard to long-term performance. All this, however, needs the trained eye of a PV module specialist with a broad educational background and knowledge.
This level of knowledge and experience is offered by MBJ Services and their partners. 14 trailers are already in operation in Germany, Greek, Denmark and Japan. “While we already have in Germany a good network with trailers, we expect that there is a need for a couple more in Europe. We recently sold one to the Netherlands, France and a second unit to Japan. We are expecting another order form the US within a short time”, so Erik Lohse says.
By utilising the latest advances in technology, the MBJ mobile PV-testcenter fits comfortably in a passenger car trailer (Figure 1), which is easily moved to any required location. To initiate the measurement procedure, the trailer’s backdoor is opened and a support frame is pulled out, which can accommodate a PV module up to 1,050 x 1,970 mm in size.
After mechanically securing the PV module and connecting it to the measurement circuitry, it is shoved into the lightproof measurement chamber. The power output is measured by a newly-developed highly-efficient large-area LED flasher. For capturing the near-infrared electroluminescent radiation, NIR cameras are used. Their images are fused by stitching software to a summary view. The cameras are positioned quite close to the module so that the entire set-up is held to a minimum size to accommodate the tight interior of the trailer.
More space is needed for the third measurement task that the mobile PV test centre can perform. This is the detection of possible hot spots, also done in the near infrared. For the hot-spot test the support frame carrying the PV module is pulled out again and an IR camera, which is integrated in the system, is activated. The camera takes a thermal snapshot of the appropriately biased module, and documents the results. The daylight present during this procedure is of no concern.
The entire functionality of the test centre is coordinated by a built-in computer. Its control board is seen in next to the pulled-out module. All measurement results are presented right away in a 24-inch screen for a more detailed analysis (Figure 2). Power measurement and electroluminescence imaging combined take up just three minutes. Thus, the mobile PV test centre can reach a throughput of 200 modules per day or more.
Precision power measurement
The LED flasher’s rapid firing sequence and high efficiency enable a series of multiple measurements. Power measurement is done according to IEC 60861, Procedure 2. Since the module’s temperature will rarely be at the exact STC value of 25 ∫C; the actual module temperature is measured by an area sensor and the STC power value is then calculated by means of the two coefficients, Alpha and Beta.
This is the crucial point of the procedure where the diligence and knowledge of the test personnel come in. The more accurate the two coefficients are known and the closer to 25 ∞C the measurement temperature is, the more accurate the power measurement. To optimise this procedure, the module under test or the entire trailer can be heated accordingly. Air conditioned units are also already in operation.
To satisfy very high demands in power measurement accuracy, it is recommended to use a reference module, whose exact specifications have been checked and determined in a lab environment. The reference module is occasionally inserted in a regular module test sequence to develop a series of reference data points and use them to evaluate the results of the PV modules under test.
As in the largely automated power measurement procedure, the trained eye of an experienced PV professional is equally important when it comes to correctly evaluate the electroluminescence images captured by NIR cameras. They depict the module’s current state and may reveal problematic areas. The electroluminescence technology is provided by MBJ Solutions, which is one of the leading companies in this field of technology.
Summary
In conclusion, Erik Lohse, General Manager of MBJ Services, explains: “Testing with mobile labs has enabled operators, investors and contractors of PV plants to facilitate a quick expert evaluation of a variety of modules. Additionally, mobile PV test centres can identify error sources that, left untreated, might induce future power degradations. And all this is done directly at the location of the PV plant.”
Erik Lohse is General Manager at MBJ Services, Hamburg.
Michael Fufl is General Manager at MBJ Solutions, Hamburg.
