Monitoring systems for solar systems
Systems for monitoring solar systems serve several purposes. They are used to track trends in a single photovoltaic system , identify faults in or damage to solar cells , compare a system's performance against design specifications, or compare PV systems in different locations. This area of application requires various sensors and monitoring systems adapted to the intended purpose.
Sensors and monitoring systems are standardized in IEC 61724-1 and divided into three levels of accuracy, which are identified by the letters "A", "B" or "C" or by the terms "high accuracy", "medium accuracy" and "basic accuracy" are. For example, Class A is suitable for large commercial solar systems, while Class C is more suitable for small residential systems.
Measurement of solar radiation
On- site solar radiation measurements are an important part of PV performance monitoring systems. The irradiance can be measured in the same orientation as the solar cell, so-called “plane of array” (POA) measurements, or horizontally, so-called horizontal global radiation measurements . Typical sensors used for such irradiance measurements include thermopile pyranometers , PV reference devices, and photo-diode sensors . In order to meet a certain accuracy class, each type of sensor must meet certain specifications. These specifications are listed in the table below.
| Sensor type | Class a
High accuracy / High precision |
class B
Medium accuracy / Medium accuracy |
Class C
Basic accuracy / Basic accuracy |
|---|---|---|---|
| Thermosaul pyranometer |
ISO 9060 "Secondary standard"
or WMO Guide "High quality" (measurement uncertainty ≤ 3% for hourly sums) |
ISO 9060 "First class"
or WMO Guide "Good quality" (measurement uncertainty ≤ 8% for hourly sums) |
- |
| PV reference devices | Measurement uncertainty ≤ 3%
from 100 W / m 2 to 1500 W / m 2 |
Measurement uncertainty ≤ 8%
from 100 W / m 2 to 1500 W / m 2 |
- |
| Photodiode sensors | not suitable | not suitable | - |
When an irradiance sensor is placed in POA, it must be placed at the same tilt angle as the solar cell, either by attaching it to the module itself or to an additional platform or arm with the same tilt. Checking whether the sensor is correctly aligned can be carried out with portable inclination sensors or with an integrated inclination sensor.
maintenance
The standard also specifies a required maintenance plan for each accuracy class. Class C sensors require little to no additional maintenance. Class B sensors need to be recalibrated every two years and require heating to prevent precipitation or condensation. Class A sensors need to be recalibrated once a year, need to be cleaned once a week, require heating, and require ventilation (for Thermosaul pyranometers).
Remote satellite measurement of irradiance
The irradiance can also be estimated from satellites . These measurements are indirect because the satellites measure the solar radiation reflected from the earth's surface. In addition, the radiation is filtered by the spectral absorption of the earth's atmosphere . This method is typically used in non-instrumented Class B and C monitoring systems to avoid the expense and maintenance of on-site sensors. If the satellite-based data is not corrected for local conditions, an error in radiance of up to 10% is possible.