How to measure operating conditions in PV power plants? Which sensors to use? What are the advantages and disadvantages? What precautions to take? In this article we review common practices and give advice on how to get the most out of your data in an aspect that, although well known, offers possibilities for improvement when developing photovoltaic power plants.
The evaluation procedures for photovoltaic power plants are based on determining their performance or, in other words, on comparing the input and output of the system. The input is given by the effective irradiance incident on the generator and by the operating temperature of its cells, both known as the operating conditions. In turn, the output of the system is given by the energy produced. Correct measurement of the operating conditions is therefore essential to accurately assess the performance and quality of the installations.
To measure operating conditions, pyranometers were initially used to measure irradiance (both horizontally and in the plane of the generator) and thermocouples or PT-100s to measure cell temperature. Later, photovoltaic devices, such as modules and reference cells, were also used.
On the other hand, the development of advanced data acquisition systems for monitoring has promoted the simultaneous installation of several sensors in the same plant . The positive side of this redundancy is greater robustness: for example, if communication with one sensor fails, there are more to fall back on.However, the negative side is that the use of different devices often results in different results, which adds considerable uncertainty when calculating the performance indices of the installation, which vary depending on the sensor selected .
Given this situation, how do we know which sensors are preferable? What are their main advantages and disadvantages? In short, how to measure the operating conditions in power plants? In the following points we will review the state of the art and see how, despite being a known aspect, the measurement of operating conditions hides possibilities for improvement that are worth paying attention to when developing photovoltaic projects.
THE DIFFERENT MEASUREMENT OPTIONS
To ensure correct measurement in the field, attention must be paid to the measuring device itself, but some additional precautions must also be taken. On the one hand, it is necessary to ensure the calibration not only of the sensor but also of the associated electronics. For example, voltage/current converters, transducers, data loggers… must ensure adequate performance for the wide temperature and humidity ranges that will be encountered in the field. On the other hand, the effect of dirt must be measured, especially in arid climates or when there are sources of dirt nearby (crops, unpaved roads, quarries…).
The measurement of irradiance can be done, fundamentally, with pyranometers, cells or reference modules. The difference between the former and the latter is that pyranometers measure the global irradiance while cells and modules measure the effective irradiance, which is the result of correcting the global irradiance with the angular and spectral responses characteristic of the photovoltaic devices.
What is relevant in this context is that pyranometers are a suitable device for measuring horizontal irradiance, the purpose of which is the comparison with the irradiance data used in the design of the installation, since also those data sources (satellites, weather stations…) measure global irradiance. However, pyranometers are no longer suitable for measuring the effective irradiance actually incident on the generator, which is the variable to be used when calculating the performance indexes of the installation. In this case, angular and spectral corrections must be made, which generally implies an increase in uncertainty of up to 3%.
Measuring with photovoltaic devices
This increased uncertainty can be avoided by directly measuring the effective irradiance with photovoltaic cells or modules. These sensors should be of the same technology as the generator to ensure equal spectral and angular responses, and to obtain more accurate and repeatable results. Here are some differences between cells and modules that make the latter more recommendable as reference sensors:
- Robustness against dirt: On the one hand, the dirt distribution in the reference modules can be expected to be similar to that of any other module, making it the sensor that best represents the dirt distribution patterns occurring in the generator. On the other hand, localized dirt (dust accumulation, bird droppings, mud…) has a limited effect on the module. However, neither of these two statements is assured for a reference cell: it is directly affected by any dirt, localized or not, and its smaller size makes its dirt distribution patterns different from those of PV modules.
- Repeatability: the response of a module is more stable and repeatable than that of any other sensor.
- Temperature correction: Many reference cells include the measurement of the operating temperature, usually by means of thermocouples, and correct its effect on the irradiance measurement. However, this temperature measurement is not representative of the module behavior, since the cell has different heat dissipation mechanisms. For example, the surface/volume ratio is higher in cells, which makes them cooler than modules.
- Manufacturing and calibration: Reference modules are manufactured according to strict quality standards (such as IEC-61215) that guarantee both calibration and long-term durability in the field, which may not be the case for reference cells. For example, intercomparisons performed in European laboratories have shown a calibration accuracy of better than 2% for crystalline silicon modules.
Figure 1 shows the comparison between the irradiance measured by a module and three reference cells in a PV plant. One of the reference cells is a secondary standard recently calibrated in a European intercomparison of irradiance sensors. The other two are commercial devices with two years of sun exposure. Table 1 summarizes the irradiance measured during the test: the difference between the secondary cell and the module was as low as 0.6%, while the other two reference cells recorded 4.6% and 5.1% less irradiance than the PV module. This result may be due to a calibration problem, possibly increased by premature degradation of the reference cells.
Figure 1: Comparison between the effective irradiance measured by a module and three reference cells (in blue, the secondary standard cell).
|Cell – secondary pattern
Calibration experiments carried out on reference modules from 10 Spanish PV plants showed similar results after 5 years of operation. The maximum degradation found in the modules was 1.2%, which is below the measurement uncertainty. Furthermore, in 8 of the cases, the degradation was less than 0.2%, which gives an idea of the high stability of the reference modules and their calibration over time.
The most common way to measure the operating temperature of the cells is to use thermocouples or PT-100 connected to the back of the module. However, there is also the possibility to take advantage of the open circuit voltage dependence of the modules with the temperature to use a reference module as a sensor. This option is a more recommendable alternative for the following reasons:
- Representativeness: the value given by a reference module is the equivalent temperature of the entire device, thus representing the average temperature of all cells. Even under normal operating conditions and depending on the size of the module, it is common to find temperature differences between cells ranging from 4°C to 10°C. According to our experience, this temperature difference can be divided into 4°C due to variations in operating point voltage and 6°C due to dissipation differences between cells, and is not related to any faulty performance. Therefore, an equivalent temperature is more representative of the behavior of the module as a whole than any point measurement (as is the case with thermocouples).
- Measuring point: thermocouples measure the temperature of the back surface of the module, which is not necessarily the same as the internal working temperature of its cells. A correction coefficient can be applied to obtain the internal temperature from the external one, usually based on specific experimental cases. However, this process adds additional uncertainty. This is avoided when using reference modules, which directly provide the operating temperature of the cells.
- Stability:The stability of thermocouple measurements in the field is questionable. It is common to find devices detached after a few months of installation. Even if they remain properly attached, the thermal variations to which the modules are subjected can lead to a deterioration of the contact at the module-thermocouple interface, resulting in a faulty measurement. On the other hand, the stability of photovoltaic modules over time is guaranteed by their manufacturing process.
- Dispersion: previous studies have shown that the dispersion in temperature measurement is much lower when using reference modules than when using thermocouples
- Signal characteristics: the larger signal amplitude in the case of reference modules, which makes it more robust to noise associated with signal transmission.
Figure 2 presents the comparison of the cell temperature measured by two reference modules and three thermocouples (perfectly bonded) in a photovoltaic plant, while Table 2 shows the corresponding results in terms of equivalent daily temperature. A high agreement is observed between modules (difference of 0.3%) while significant differences appear with thermocouples (between 4.0% and 8.7%).
Figure 2: Comparison between the cell temperature measured by two reference modules (gray dots) and three thermocouples (red, green and blue dots).
|Equivalent temperature (°C)
Table 2: Equivalent daily temperature measured by two reference PV modules and three thermocouples.
Again, calibration tests of reference modules carried out in 10 Spanish photovoltaic installations after 5 years of operation showed maximum degradations of 0.9% and an average degradation of less than 0.2%, which is an indication of the high stability of these devices .
REFERENCE MODULES AS SENSORS
As we have seen, reference modules are the best alternative for measuring operating conditions in the field, not only because of the accuracy and repeatability of their measurement but also because of their durability and long-term stability. At the constituent level, they must be modules of the same technology (generally of the same manufacturer and type) as those constituting the PV generator, but previously stabilized (exposed to more than 60 kWh/m2) and carefully calibrated in an independent laboratory or in the field. Moreover, when they are supplied as part of the batch of modules of the power plant, their availability and warranty are assured. Last but not least, they are a fully economically competitive alternative.
Figure 3 shows examples of reference modules installed in plants with static structures, single-axis and dual-axis tracking, as well as with c-Si, CIGS and CdTe technology modules.
Figure 3: Example of reference modules installed in operating plants, with static structures, single and dual-axis tracking, as well as c-Si, CIGS and CdTe technology modules.
 Fuentes, M. et al., 2014. Design of an accurate, low-cost autonomous data logger for PV system monitoring using ArduinoTM that complies with IEC standards. Solar Energy Materials and Solar Cells, 130, pp.529–543.
 Caamaño-Martín, E., Lorenzo, E. & Lastres, C., 2002. Crystalline silicon photovoltaic modules: characterization in the field of rural electrification. Progress in Photovoltaics: Research and Applications, 10(7), pp.481–493.