Solar Irradiance Monitoring in Solar Energy Projects

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Kipp & Zonen Products

Site Prospecting and Energy Forecast

Where to construct a solar power plant? Which technology will have the lowest levelized cost of energy (LCOE) and bring maximum return on investment (ROI)? What is the optimal size of the power plant on a given location? These questions can be answered by knowing the amount and distribution of solar irradiance with low uncertainties.

A key input for selecting the optimal location, size, and technology of a solar energy project is the amount of solar radiation available over time under local environmental conditions. Feasibility studies and technical due diligence calculations of a solar energy project invariably begin with energy resource evaluation.

High precision on-site measurements of solar radiation offer the lowest uncertainty for bankable data regarding the energy resource and the potential energy yield. Such measurements are carried out by a high quality solar radiation monitoring station that determines all three components of solar radiation which include diffuse horizontal irradiance (DHI), global horizontal irradiance (GHI), and direct normal irradiance (DNI). Additionally, other meteorological parameters applicable to the project, for example, air temperature, precipitation, wind speed, humidity, and direction have to be monitored by an exclusive weather station.

A complete solar monitoring station comprises of a pyrheliometer fixed on an automatic sun tracker for measuring DNI, a shaded pyranometer for measuring DHI, and an unshaded pyranometer for measuring GHI, and also a high performance data logger to obtain and store the measured values.

Solar monitoring stations can be expanded by spectral instruments and/or a sky radiometer for investigating the aerosol concentrations in the atmosphere, which is particularly significant in arid desert areas with high concentrations of dust and sand, in the air.

Recommended System

SOLYS2 automatic sun tracker, with sun sensor (for active tracking) and shading ball assembly, equipped with:

  • ISO 9060 First Class pyrheliometer for DNI measurement (SHP1 or CHP1)
  • Two ISO 9060 Secondary Standard pyranometers, of which one shaded for DHI measurement (CMP10 or SMP10) and one unshaded for GHI measurement
  • Weather station with air temperature, precipitation, wind speed, humidity, and direction sensors
  • Ventilation units for pyranometers, based upon the site environment (CVF4)
  • Optional POM-02 or POM-01 sky radiometer
  • High quality meteorological data logger

Automatic sun tracker SOLYS2 with a pyrheliometer and with two ventilated pyranometers. The ventilation units help to keep the pyranometer domes clean from dust and dew.

Fixed Photovoltaic (PV) Installations

Rooftop Installations, Building Integrated Photovoltaic (BIPV) Systems and Small Ground-Mounted PV Plants (kW Range)

For both ground-mounted and roof installations, solar radiation measurement considerably enhances the monitoring of the performance of the solar energy system.

In many smaller PV installations, if any monitoring is performed, it is generally a comparison of the output from one PV panel to another PV panel. One then monitors the relative efficiency and can spot a defective connection or panel. Although useful, it really does not tell whether one will get the maximum out of their system. As long as one does not measure the input of solar radiance into to the PV installation, they will not know whether they should be content with the output of their PV installation.

For monitoring the energy yield of the PV installation, a tilted pyranometer is fixed at the same angle as the panels to measure plane of array (POA) irradiance. A horizontal pyranometer can be included for measurements of global horizontal irradiance (GHI) – this enables comparison of the on-site data to other sites and to data obtained from meteorological stations.

Low-cost low-maintenance smart pyranometers SMP3 (ISO 9060 Second Class) and SMP10 (ISO 9060 Secondary Standard) are perfect for small scale installations. Smart pyranometers have an amplified analogue output (4-20 mA or 0-1 V), internal digital signal processing, and a RS-485 serial communication with Modbus® protocol. They can be directly linked to latest inverters and effortlessly incorporated into digital supervisory control and data acquisition (SCADA) systems.

Fixed Photovoltaic (PV) Installations

Recommended System

  • SMP10 or SMP3 pyranometer for POA measurements, LOGBOX datalogger
  • An extra pyranometer for GHI is optional

Fixed Photovoltaic (PV) Installations

Medium and Large Commercial PV Installations (MW Range)

In large and medium installations (greater than 1 MW), the uncertainties in the energy generation efficiency have significant effect on the profitability of the project. A measurement uncertainty of as low as 2% for a plant of nominal capacity of several MW can mean a considerable difference in energy production predictions and can hence directly affect profit or loss!

POA and GHI irradiance measurements are critical for determining performance ratios and tracking the efficiency of energy yield. For performance ratio monitoring, reliable data collection and high quality measurement instruments are required. It is advised to use ISO 9060 Secondary Standard pyranometers for the highest quality of the data, for instance, the SMP10 or CMP10 low maintenance pyranometers.

In the case of large scale plants, a high quality solar monitoring station is suggested for measuring all three components of solar radiation (DNI, DHI, and GHI). This data can be complimented by tracking POA irradiance at various locations on the plant, usually feeding into the array inverters.

Smart pyranometers with RS-485 Modbus® are addressable. They can be connected to a single network loop and can be incorporated into the SCADA system of a solar plant for simple and practical monitoring as well as reduced cable costs.

Large projects usually stretch an area where meteorological conditions may differ owing to differences in microclimate. In such instances, weather stations and solar monitoring stations may need to be installed in various sections of the plant for close monitoring of the local conditions.

Redundancy of the measurements is assured with the help of two or more solar monitoring points. The data collection will remain continuous when some of the instruments need to be substituted or sent for calibration.

Recommended Basic System

  • For larger plants, extra pyranometers should be placed, dispersed over the location
  • A minimum of two ISO 9060 Secondary Standard pyranometers, one for GHI measurement (SMP10 or CMP10) and one for POA

Recommended Advanced System

SOLYS2 automatic sun tracker, with sun sensor (for active tracking) and shading ball assembly, equipped with:

  • ISO 9060 First Class pyrheliometer for DNI measurement (SHP1 or CHP1)
  • High quality meteorological data logger
  • Ventilation units for pyranometers, based on the site environment (CVF4)
  • Two ISO 9060 Secondary Standard pyranometers, of which one shaded for DHI measurement (SMP10 or CMP10) and one unshaded for GHI measurement
  • Weather station with air temperature, precipitation, wind speed, humidity and direction sensors

Fixed Photovoltaic (PV) Installations

Tracking Photovoltaic (PV) Systems

PV systems with one- or two-axis tracking can significantly increase the output of solar panels by guaranteeing higher received irradiance during the day. Using high quality pyranometers (ISO 9060 Secondary Standard), the POA irradiance should be measured, together with the GHI, to monitor the system’s performance. The pyranometers can be fixed on a dedicated high precision sun tracker or on the solar panel tracker.

Recommended System

  • For larger plants, extra pyranometers should be placed and distributed over the location
  • Two ISO 9060 Secondary Standard pyranometers (SMP10 or CMP10), one fixed on panel tracker for the measurement of POA and one for the measurement of GHI

Concentrating Photovoltaic (CPV) Systems

CPV systems employ optics for concentrating a large section of sunlight onto a small solar cell and are either reflective (with mirrors) or refractive (with lenses). In order to realize high concentration ratios, the optics has a narrow field of view and only uses direct normal irradiance (DNI) from the sun.

To provide reliable data about the solar radiation input, DNI is most accurately determined by a high quality pyrheliometer fixed on an accurate automatic sun tracker. A state-of-the art system with pyranometers measuring global and diffuse irradiance (GHI and DHI) can provide a quality check of the measurements of DNI.

Concentrating Photovoltaic (CPV) Systems

Recommended Basic System

SOLYS2 automatic sun tracker with sun sensor for active tracking and small top plate, equipped with:

  • ISO 9060 First Class pyrheliometer for measurement of DNI (SHP1 or CHP1)
  • High quality meteorological data logger
  • Ventilation unit for pyranometer, based on the site environment (CVF4)
  • ISO 9060 Secondary Standard pyranometer for the measurement of GHI (SMP10, CMP10, CMP21, or CMP22)

Recommended Advanced System

As the basic system, plus:

  • Shading assembly
  • Second pyranometer for the measurement of DHI (SMP10, CMP21, CMP22, or CMP10)
  • Weather station with air temperature, precipitation, wind speed, humidity, and direction sensors

Concentrating Solar Power (CSP) Systems

The direct normal irradiance (DNI) from the sun is used by the thermal systems for generating heat. This heat can be used as the energy source for steam turbine electricity generators. These systems concentrate solar radiation by using mirrors. They are different from PV cells and can exploit the whole spectrum of solar radiation, including near infrared and ultraviolet light, resulting in high efficiencies. For such types of systems, it is highly significant to track the broadband solar radiation with high precision, because sky conditions have a powerful impact on the performance of a CSP plant.

It is critical to determine solar radiation locally so as to forecast the energy yield of a CSP system with a minimum of uncertainty. Satellite measurements and associated models do not consider the impact of local climatic conditions, for example, clouds, nor do they include the impact of local aerosols (sand, dust, and other particles). Two CSP plants in different sites with equal direct irradiance totals, based on satellite data, may have extremely different energy outputs, because of differences in aerosols and clouds in the specific sites, which influence the incoming radiation.

To guarantee the data’s redundancy and reliability, a standard CSP solar monitoring station employs high precision instruments with low uncertainty for diffuse, direct, and global irradiance measurements. In this manner, the direct radiation measurement can be compared with values obtained from the global and diffuse radiation. This makes it possible to detect issues with a specific instrument, for instance, due to soiling.

Although the overall impact of aerosols on the incoming radiation can be easily measured by solar irradiance sensors, one should use a sky radiometer, such as the POM-02 or POM-01, to investigate the type of aerosols to enhance forecasts of solar energy available over time.

Recommended System

SOLYS2 automatic sun tracker, with sun sensor (for active tracking) and shading ball assembly, equipped with:

  • Two ISO 9060 Secondary Standard pyranometers, of which one shaded for DHI measurement (CMP10, SMP10, CMP21, or CMP22) and one unshaded for GHI measurement
  • ISO 9060 First Class pyrheliometer, for the measurement of DNI (SHP1 or CHP1)
  • Weather station with air temperature, wind speed, precipitation, humidity, and direction sensors
  • Ventilation units for pyranometers, based on the site environment (CVF4)
  • Optional POM-02 or POM-01 sky radiometer
  • High-quality meteorological data logger

SOLYS2

Choosing the Right System

Application Measurement Parameters Measurements Instruments
Basic Advanced
Prospecting DNI, GHI, DHI, weather, data logger Automatic sun tracker: SOLYS2
ISO 9060 First Class pyrheliometer: CHP1 or SHP1
ISO 9060 Secondary Standard pyranometers:
CMP10 or SMP10
Weather station
Fixed PV Small POA, GHI optional Smart pyranometers SMP3 or SMP10 integrated into monitoring system
Fixed PV Medium POA, GHI POA, GHI, at least 2 locations, weather,
data logger
ISO 9060 Secondary Standard pyranometers: CMP10 or SMP10
Weather station
Fixed PV Large GHI, at least 2 locations, distributed POA POA, GHI, DNI, DHI, distributed POA, weather, data logger Automatic sun tracker: SOLYS2
ISO 9060 First Class pyrheliometer: CHP1 or SHP1
ISO 9060 Secondary Standard pyranometers: CMP10 or SMP10
Weather station
Tracking PV POA, GHI DNI, GHI, DHI, weather, data logger ISO 9060 First Class pyrheliometer: CHP1 or SHP1
CPV DNI, GHI, data logger DNI, GHI, DHI, weather, data logger Automatic sun tracker: SOLYS2
ISO 9060 First Class pyrheliometer: CHP1 or SHP1
ISO 9060 Secondary Standard pyranometers: CMP10 or SMP10
Weather station
CSP DNI, GHI, DHI, weather, data logger Automatic sun tracker: SOLYS2
ISO 9060 First Class pyrheliometer: CHP1 or SHP1
ISO 9060 Secondary Standard pyranometers: CMP10, SMP10, CMP21 or CMP22
Weather station

Kipp & Zonen and its local representatives can also design and offer complete solutions for solar monitoring at solar energy projects including:

  • Dataloggers for data acquisition
  • Communication systems for data transmission
  • PV module temperature sensors
  • Weather stations

This information has been sourced, reviewed and adapted from materials provided by Kipp & Zonen.

For more information on this source, please visit Kipp & Zonen.

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