PV power plant efficiency measurements – Solar Integrator

When evaluating the efficiency of a PV plant it is essential that the values of solar irradiance and produced power are recorded. Using irradiance (W/m2) and net panel area we acquire maximum available solar power which the plant may process. The ratio between the actual produced electric power and the irradiance power which the panels are receiving gives the instanteous efficiency of the entire system.

However, the efficiency depends on irradiation intensity, the elevation of the Sun, dispersion of light, possible shading of some panels, and other factors. The instanteous efficiency thus is not the best economic-technical parameter; it is more suitable for fault diagnosis, and if evaluated properly, it may help to optimize the operation of the plant. But do we have a real parameter which indicates how efficient the plant is as a whole, when the instanteous efficiency varies in time very fast?

It may be the total efficiency, which is based not on power (which is momentary value) but on energy, which is power integrated in time. To be able to calculate this efficiency we need to know the system output – the total production of the plant over a certain time period (day, week, year) – and input, which is energy supplied to the panels by sun. This energy (e.g. in kWh/m2/a) may be read from public sources, such as meteo servers (but not always for a particular location where the plant is installed), or calculated by integration of the irradiation in time and multiplication of the integrated value by the area of the panels. According to IEC 61724, for sensors with short reaction time, integrating sensors should be used rather than sampling and following calculation based on trend data.

Measured energy with irradiation sampling and calculated integration. The energy is equal to the coloured area. For illustration, a very long integrating interval of 1 hour is used. The error decreases with decreasing interval of integration.

Continuous integration of solar irradiation, or calculation of cummulated solar irradiation in kWh/m2 based on instanteous irradiation value, is the function of the solar integrator UI415.

The solar radiation integrator reads input voltage (proportional to the solar irradiation) from an external 0..10 V sensor. (The sensor must be ordered separately.) The high impedance input allows using existing sensors connected to the already installed monitoring system. It is only necessary to beware correct power supply and grounding connections. The actual value of solar irradiation is displayed on the LCD display of UI415. This value is integrated once per second and a potential-free output (solid state relay) provides pulses indicating energy per m2 of the measured plane. These pulses may be connected to any counting module which cumulates the solar irradiation. The cumulated values are used toghether with the electrical production of the plant to calculate both short and long-term plant efficiency. Both actual and cumulated values are available at the optically separated RS485 bus (protocol Modbus RTU). The solar integrator is then easy to integrate in many control and monitoring systems. The register description is part of the data sheet.

The solar integrator provides basic values as follows:

Daily energy cumulated

The integration starts at midnight (00:00) and its value increases throughout the day. At next midnight (24:00) it is copied to the Last day cumulated energy and then reset to zero.

Energy integrated using the solar integator (orange area). The integration interval is 1 s which makes possible to measure with 1 Ws resolution. The accuracy is determined mainly by the irradiation sensor or pyranometer used.

Last day cumulated energy

For the next 24 hours it contains the accumulated energy of yesterday. This is for comfortable readout of the daily cumulated energy.

Total energy

This value integrates the total cumulated energy since commissioning of the device. The value is backupped by a battery.

In the registers, another set of values is calculated:

Cumulated energy above the irradiation limit

This register integrates only if the actual irradiation exceeds a predefined limit (settable over the bus as an analogue value, default 300 W/m2). This cumulated value is useful because the inverters start operation only above certain irradiation level, and the measuring errors caused by night residual light cumulation are avoided.

Cumulated energy when inverters active

This register integrates only if the integration is enabled by a binary value written over the bus from a PLC or another Modbus master. The binary value indicates operating inverters and thus the comparison of the real produced energy (read at the inverters or at the main meter) and this value gives an image of efficiency of the inverters and distribution components.

Example of energy integrated only when inverters active (yellow area), 9:00 to 11:30 dropout due to maintenance, morning startup time and evening switch-off time increased for better legibility

At plants where it is not possible to read out the values over the bus, the pulse output may be used. The pulses are weighted 10 Wh/m2 and they can be acquired by any pulse to M-Bus converter for reading of power meters, water meters, and other cumulative meters.

The integrator contains real time clock backupped by a battery. The device is powered by 24 V AC or DC, usually from the same power source as the irradiation sensor. There is also integrated thermometer on the PCB, the value of which is available on the bus. The temperature may be used as informative value for monitoring of enviromental conditions at the installation place (switchboard, transformer room etc.).

Find more details including data sheet with Modbus table and connection example at Domat-int.com.