Specifications
SR20-D2
| Measurand | hemispherical solar radiation |
| ISO 9060:2018 classification | spectrally flat Class A pyranometer |
| ISO 9060:1990 classification | secondary standard pyranometer |
| IEC 61724-1:2021 compliance | Class A (when used with VU01) |
| Application | PV monitoring commercial and utility scale, PV monitoring small sized, meteorology / climatology, other |
| Heater | no |
| Output | 4-20 mA current loop, Modbus RS-485 |
| Calibration uncertainty | < 1.2 % (k = 2) |
| Zero offset a | 5 W/m² unventilated and 2.5 W/m² ventilated |
| Calibration traceability | to WRR |
| Calibration registers | accessible to users |
| Spectral range | 285 to 3000 x 10⁻⁹ m |
| Rated operating temperature range | -40 to +80 °C |
| Heater | no heating |
| Temperature response test of individual instrument | report included |
| Directional response test of individual instrument | report included to 95 ° |
| Standard cable length | 5 m |
| Backwards compatibility | SR20-D1 |
| Digital output | |
| Output | irradiance in W/m² and instrument body temperature in °C |
| Communication protocol | Modbus |
| Hardware interface | 2-wire (half duplex) RS-485 |
| Transmission mode | RTU |
| Rated operating voltage range | 5 to 30 VDC |
| Power consumption | < 75 x 10⁻³ W at 12 VDC |
| Heater | 1.5 W at 12 VDC |
| 4-20 mA output | |
| Output | irradiance in W/m² |
| Transmitted range of 4-20 mA output | 0 to 1600 W/m² |
| Rated operating voltage range of 4-20 mA output | 5.5 to 40 VDC |
| Power consumption | |
| main supply | < 75 x 10⁻³ W |
| 4-20 mA current loop | < 40 x 10⁻³ W at 12 VDC |
Options
- longer cable, in multiples of 5 metres
- adapted transmitted range for 4-20 mA output
- ventilated with VU01 ventilation unit
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Best pyranometer for the PV industry
SR20-D2 is the most accurate digital secondary standard pyranometer available. Its benefits:
- Digital output: easy implementation & servicing
- Best-in-class temperature response, < ± 0.4 % (-30 to +50 °C), best “zero offset a” and best calibration uncertainty
- Test certificates for temperature response and directional response included as required by ISO 9060
- Re-calibration registers fully accessible to users
In order to improve overall measurement accuracy, Hukseflux effectively targeted two major sources of measurement uncertainty: calibration and “zero offset a”.
In addition, SR20-D2 has a negligible temperature response. All are best in class. The temperature response of every individual instrument is tested and corrected onboard by the instrument electronics, using a second degree polynomial. SR20-D2’s low temperature dependence makes it the ideal candidate for use under very cold and very hot conditions.
SR20-D2 pyranometer design
The pyranometer employs a state-of-the-art thermopile sensor with black coated surface, two domes and an anodised aluminium body. The connector, desiccant holder and sun screen fixation are very robust and designed for long term industrial use. SR20-D2 uses a high-end 24-bit A/D converter. All parts are specified for use across SR20-D2’s entire rated operating temperature range. SR20-D2 offers two types of outputs commonly used in the solar PV industry: digital output via Modbus RTU over 2-wire RS-485 and analogue 4-20 mA output (current loop).
Application of this instrument
Suggested use
- PV system performance monitoring
- all networks with regular instrument exchange
- scientific meteorological observations
- reference instrument for comparison
- extreme climates (tropical / polar)
Areas of Application
How does a pyranometer work?
A pyranometer measures the solar radiation received by a plane surface from a 180 ° field of view angle. This quantity, expressed in W/m², is called “hemispherical” solar radiation. The solar radiation spectrum extends roughly from 285 to 3000 x 10⁻⁹ m. By definition a pyranometer should cover that spectral range with a spectral selectivity that is as “flat” as possible.
In an irradiance measurement by definition the response to “beam” radiation varies with the cosine of the angle of incidence; i.e. it should have full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 ° angle of incidence), zero response when the sun is at the horizon (90 ° angle of incidence, 90 ° zenith angle), and 50 % of full response at 60 ° angle of incidence. A pyranometer should have a so-called “directional response” (older documents mention “cosine response”) that is as close as possible to the ideal cosine characteristic.
In order to attain the proper directional and spectral characteristics, a pyranometer’s main components are:
• a thermal sensor with black coating. It has a flat spectrum covering the 200 to 50000 x 10⁻⁹ m range, and has a near-perfect directional response. The coating absorbs all solar radiation and, at the moment of absorption, converts it to heat. The heat flows through the sensor to the sensor body. The thermopile sensor generates a voltage output signal that is proportional to the solar irradiance.
• a glass dome. This dome limits the spectral range from 285 to 3000 x 10⁻⁹ m (cutting off the part above 3000 x 10⁻⁹ m), while preserving the 180 ° field of view angle. Another function of the dome is that it shields the thermopile sensor from the environment (convection, rain).
• a second (inner) glass dome: For secondary standard and first class pyranometers, two domes are used, and not one single dome. This construction provides an additional “radiation shield”, resulting in a better thermal equilibrium between the sensor and inner dome, compared to using a single dome. The effect of having a second dome is a strong reduction of instrument offsets.
• a heater: in order to reduce the effect of dew deposition and frost on the outer dome surface, most advanced pyranometers have a built-in heater. The heater is coupled to the sensor body. Heating a pyranometer can generate additional irradiance offset signals, therefore it is recommended to activate the heater only during night-time. Combining a heater with external ventilation makes these heating offsets very low.
Why use a pyranometer?
There are good reasons why pyranometers are the standard for solar radiation measurement in outdoor PV system performance monitoring.
The purpose of outdoor PV testing is to compare the available resource to system output and thus to determine efficiency. The efficiency estimate serves as an indication of overall performance and stability. It also serves as a reference for remote diagnostics and need for servicing.
The irradiance measurement for outdoor PV performance monitoring is usually carried out with pyranometers. Some standards suggest using PV reference cells. Reference cells are (with some minor exceptions) unsuitable for proof in bankability and in proof of PV system efficiency. Pyranometers are and will remain the standard for outdoor solar energy monitoring.
From a fundamental point of view:
- Pyranometers measure truly available solar irradiance (so the amount of available resource). This is the parameter you need to have for a true efficiency calculation.
- Reference cells measure only that part of solar radiation that can be used by cells of identical material and identical packaging (flat window), so the yield of a certain PV cell type. This is not a measurement that can be used in an efficiency calculation and in fact leads to several percentage points error in efficiency estimates.
The International Energy Agency (IEA) and ASTM standards for PV monitoring recommend pyranometers for outdoor PV monitoring. PV reference cells do not meet IEC 61724-1 class A requirements for irradiance measurement uncertainty: their directional response makes them systematically overestimate daily radiant exposure in J/m2 (or W·hr/m2 ) by more than 2 %, larger on hourly basis.
How do I choose a pyranometer?
- are there standards for my application?
- what level of accuracy do I need?
- what will be the instrument maintenance level?
- what are the interfacing possibilities?
- recommended pyranometer class
- recommended maintenance level
- estimate of the measurement accuracy
- recommended calibration policy
- recommended interface
What is the difference between a pyrheliometer and a pyranometer?
A pyranometer measures hemispherical solar radiation. When measuring in the horizontal plane this is called Global Horizontal Irradiance (GHI). When measuring in “plane of array”, next to PV panels, this is called plane of array POA irradiance.
A pyrheliometer is used to measure Direct Normal Irradiance (DNI). DNI is defined as the solar radiant flux collected by a plane unit surface normal to the axis pointing towards the centre of the sun, within an optical angular aperture. DNI is composed of the solar irradiance within the extent of the solar disk (half-angle 0.266 ° ± 1.7 %) plus some circumsolar radiation.
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SR20-D2 pyranometer
- Digital output: easy implementation & servicing
- Best-in-class temperature response, < ± 0.4 % (-30 to +50 °C), best “zero offset a” and best calibration uncertainty
- Test certificates for temperature response and directional response included as required by ISO 9060
- Re-calibration registers fully accessible to users
