Words: Ruud Ringoir, Product Manager at Kipp & Zonen
Instruments for monitoring solar radiation have been used in meteorology and climatology for almost a hundred years, but today they are also a common sight in solar energy. The data required differs for PV (Photovoltaic), CPV (Concentrated PV) and CSP (Concentrated Solar Power) applications, but they all benefit from high accuracy solar monitoring with radiometers that are easy to use and easy to integrate.
For PV panel applications, two pyranometers with hemispherical views are normally used to measure global horizontal irradiance (GHI) and tilted global irradiance from the sun and sky. For concentrating systems that focus the beam from the sun, a pyrheliometer with a narrow view is fitted to an automatic sun tracker to measure direct normal irradiance (DNI).
Most radiometers that fulfil the performance requirements of ISO 9060:1990 “Solar energy – specification and classification of instruments for measuring hemispherical solar and direct solar radiation” are based on thermopile detectors, or a Peltier element that functions similarly. A black coating absorbs incoming energy and heats up the top surface of the detector, which is mounted into a metal housing, and the flow of heat generates a small voltage in the detector.
Depending upon the material of the instrument dome or window, these thermoelectric instruments respond equally to all the ultraviolet, visible and near infrared radiation from the sun and sky. Low cost radiometers are based on a silicon photoelectric sensor, but due to their limited spectral response they do not meet the requirements of ISO 9060.
Traditional pyranometers and pyrheliometers do not require any power; the incoming solar radiation is converted to a small, but linear, output voltage. These small signals are only a few millivolts, even on a bright sunny day, and can be easily distorted. High quality cable, proper shielding and a good (low offset and high resolution) amplifier or data logger are needed to handle these signals properly.
This is all well understood in meteorology and climatology, but in solar energy ‘industrial’ amplifiers or data loggers are often not suitable and substantially affect the accuracy of the measurement data. The use of lower grade, or unshielded, cables and poor grounding will affect these small signals.
The smarter generation
Commercial solar energy plants are industrial sites that typically use interactive Supervisory Control and Data Acquisition (SCADA) systems for the plant sensors, controls and actuators. They operate using 4-20 mA analogue signals and/or serial digital communication. An industrial standard, low power, bi-directional data interface for solar radiometers was one of the major requests coming from the solar industry. This market need resulted in a new generation of smart pyranometers and pyrheliometers from Kipp & Zonen, with a state of the art, custom-designed, Smart interface built into the instruments.
Smart radiometers, such as the SMP3, SMP11 and SHP1 models, have intelligence. The detector and internal temperature sensor signals go to a very accurate analogue to digital converter and are then processed digitally by a micro-controller to enhance performance. The interface connects to the plant systems by RS-485 serial communication using the industry standard Modbus protocol. In addition, digital to analogue conversion provides a 0-1 Volt output (-V versions) or a 4-20 mA output (-A versions).
This gives the customer two options; either use the amplified analogue output, which eliminates the need for a separate amplifier or a sensitive data logger input, or use the digital interface. In both cases the critical low-level signal handling is done inside the instrument and expensive cables are not needed, although good grounding is always advisable.
Smart radiometers have many more advantages thanks to their on-board processing power, firmware and memory.
Increased accuracy
Because radiation data is used to check bankability, plant performance and efficiency, accurate data is crucial. The Smart interface increases accuracy by using an internal temperature sensor, mounted close to the detector, and a polynomial function to correct for the non-linear change of detector sensitivity over the range from -40∞C to +70∞C. This minimises one of the largest sources of errors in solar radiation measurements.
Faster response
A further advantage of the Smart interface is the shorter response time of the instruments due to digital signal processing. SMP model pyranometers are typically 2.5 times faster than their passive CMP model equivalents. When fast response is used it gives a better match between the output of PV modules and the radiation signal. The SMP 11 has a response time of less than two seconds to 95% of the final value for a step-change of radiation input.
More information
The analogue outputs include the temperature correction and faster response, but much more information is available using the digital communication. Real-time corrected or raw radiation data, power supply voltage, and internal temperature are available. The calibration history, instrument type, serial number and status can be retrieved over the Modbus interface. This makes it much easier for QA/QC departments to maintain traceability and to know when the next recalibration is due.
Easily interchangeable
The fact that all Smart radiometers have the same standardised digital and analogue outputs make the re-adjustment of a data logger or amplifier unnecessary after a recalibration. This feature, and the weatherproof cable connector on the housing, means that instruments are easily interchanged. This not only saves time but prevents possible errors, as reconfiguration of linked equipment is not needed.
Low power
There is not always mains AC electricity available at the site where the sensors are located, particularly when solar prospecting in remote areas, and often power comes from solar panels and batteries. This means that low power consumption of equipment is important.
Smart radiometers are designed to consume extremely low power. One reason is to minimise the dissipation of energy inside the radiometer; being thermal detector, this is critical to maintain accuracy. A further benefit is that it allows the use of a small and low cost PV panels as power sources. Smart radiometers accept a wide input voltage range between 5 and 30 VDC and can work from the 12 or 24 VDC supplies used by meteorological sensors, data loggers and data acquisition systems.
Going forward digitally
In solar energy, more parameters than just solar radiation are usually measured; such as wind speed and direction, and PV panel- and environmental temperatures. Often there is a complete automatic weather station (AWS). Using the Modbus RTU communication interface allows up to 247 compatible sensors to be given individual addresses and connected to a single 3-wire RS-485 bus cable back to the control room. The cable can be up to 1km long, depending on the baud rate used.
Smart software
In solar power plants there are often existing Modbus networks available linking inverters and other devices, directly or via a portal. This means that Smart radiometers can be linked to the network, but need communications settings and a unique Modbus address. The easy to use software supplied as standard allows the Smart radiometer to be configured from a PC and also offers the possibility to visualise real-time measurements with a numerical display or a graph, so that an instrument can be checked before adding it to the network.
Finally, there is a logging function that can be used to store the data from up to 10 Smart radiometers on a PC. The storage interval can be set and the software can produce a csv text file per measurement or per day. This is especially useful when laboratory checking or testing of PV panels is being done. Communication between a PC and Smart radiometers requires a RS-485 to USB converter and, ideally, this should be isolated. In the field Smart radiometers connect directly to the existing network and/or other Modbus devices.
Smarter solar radiation monitoring
With Smart radiometers there is no requirement for any additional equipment to translate incoming solar radiation into a highly accurate measurement of irradiance that is enhanced by digital data processing and can be used directly by analogue or digital industrial control and measurement systems.
The digital Modbus interface provides easy connection and integration with solar power plant network systems and provides access to a wide range of instrument information. For solar energy applications, Smart radiometers are the future – and the smarter solution.
Modbus(r) is a Registered Trademark.
About the company
Kipp & Zonen provides class-leading instruments for measuring solar radiation and atmospheric properties in meteorology, climatology, hydrology, industry, renewable energy, agriculture and public health.
Kipp & Zonen is the specialist in the measurement of solar and sky radiation, from the ultraviolet to the far infrared. The company offers a complete range of high quality instrumentation and accessories, from reliable cost-effective products to the best performance available. Kipp & Zonen pyranometers, for example, are used in meteorological networks around the world.
Furthermore, the company’s expertise and close links with the scientific community have led to high-end solutions for the measurement of atmospheric properties such as stratospheric Ozone, UV Spectra and evapo-transpiration.
Its mission is to satisfy the fundamental need to monitor atmospheric properties related to climate change, classical meteorology, agriculture, renewable energy and the available water budget.
