what will be the instrument maintenance level?.are there standards for my application?.But first, you should ask yourself the following questions: How do I choose a pyranometer?Ĭhoosing the right pyranometer for your application is not an easy task. hr/m2 ) by more than 2 %, larger on hourly basis.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 The International Energy Agency (IEA) and ASTM standards for PV monitoring recommend pyranometers for outdoor PV monitoring. 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. 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 the parameter you need to have for a true efficiency calculation. Pyranometers measure truly available solar irradiance (so the amount of available resource).Pyranometers are and will remain the standard for outdoor solar energy monitoring. Reference cells are (with some minor exceptions) unsuitable for proof in bankability and in proof of PV system efficiency. Some standards suggest using PV reference cells. The irradiance measurement for outdoor PV performance monitoring is usually carried out with pyranometers. It also serves as a reference for remote diagnostics and need for servicing. The efficiency estimate serves as an indication of overall performance and stability. The purpose of outdoor PV testing is to compare the available resource to system output and thus to determine efficiency. There are good reasons why pyranometers are the standard for solar radiation measurement in outdoor PV system performance monitoring. Combining a heater with external ventilation makes these heating offsets very low. Heating a pyranometer can generate additional irradiance offset signals, therefore it is recommended to activate the heater only during night-time. The heater is coupled to the sensor body. 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 effect of having a second dome is a strong reduction of instrument offsets. 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. a second (inner) glass dome: For secondary standard and first class pyranometers, two domes are used, and not one single dome.Another function of the dome is that it shields the thermopile sensor from the environment (convection, rain). 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. The thermopile sensor generates a voltage output signal that is proportional to the solar irradiance. The heat flows through the sensor to the sensor body. The coating absorbs all solar radiation and, at the moment of absorption, converts it to heat. It has a flat spectrum covering the 200 to 50000 x 10⁻⁹ m range, and has a near-perfect directional response. In order to attain the proper directional and spectral characteristics, a pyranometer's main components are: 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. 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. In an irradiance measurement by definition the response to “beam” radiation varies with the cosine of the angle of incidence i.e. By definition a pyranometer should cover that spectral range with a spectral selectivity that is as “flat” as possible. The solar radiation spectrum extends roughly from 285 to 3000 x 10⁻⁹ m. This quantity, expressed in W/m², is called “hemispherical” solar radiation. A pyranometer measures the solar radiation received by a plane surface from a 180 ° field of view angle.
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