Measuring Solar Radiation

Accurate measurements of the incoming irradiance are essential for solar power plant project design, implementation, and operations. Large and medium solar energy systems require ground-measured irradiance data for:
• Site resource analysis, yield assessment, financing
• System design
• Plant operation and performance control.
Ground-based irradiance measurements are also essential for developing, testing, and enhancing models to estimate and forecast solar radiation resources. This includes performing a site adaptation of long-term resource datasets and applying recent measurements for the creation of a forecast. Solar irradiance measurements are only available at a limited number of stations, so a specific measurement campaign must typically be performed at (or around) the location of each specific new project. Depending on the solar technology that will be used in the project, different radiometers are of interest. Pyranometers are used to measure global horizontal irradiance (GHI), global tilted irradiance (GTI) (also referred to as plane-of-array [POA] irradiance when measured on a photovoltaic [PV] array), diffuse horizontal irradiance (DHI), rear plane of array (RPOA), or reflected horizontal irradiance (RHI). Pyranometers can measure the resource used by PV and flat-plate solar systems. RPOA, DHI, and RHI are particularly relevant for bifacial PV (BPV). As an alternative or in addition to measuring RPOA with a pyranometer directly, this RPOA can also be estimated based on RHI, GHI, and DHI. In this approach, RHI is divided by GHI to calculate the local surface albedo that is then used to derive the ground-reflected contribution to RPOA. DHI is utilized along with so-called view factors to obtain the diffuse radiation’s contribution to RPOA. In the case of PV systems, PV reference cells are also of interest, as they might replace GTI and RPOA pyranometers for monitoring applications. To measure DHI, a sun-shading device such as a tracked shading ball for the pyranometer is often used, which complicates the measurements compared to the other above-mentioned radiation components, because a solar tracker is needed. For systems based on concentrating collectors, direct normal irradiance (DNI) measurements are required. DNI is also of interest for large and tracked PV installations. Such measurements can be collected with pyrheliometers that are mounted on solar trackers. The protective windows of these instruments are prone to soiling, and their signal is affected more by soiling than that of pyranometers. Thus, pyrheliometers require more frequent maintenance. To avoid expensive maintenance-intensive and failure-prone solar trackers for DNI and/or DHI measurements, alternative instruments are available. Such instruments are comparably cheaper and require less maintenance, but cannot reach the same high accuracy as well-maintained tracker systems equipped with a pyrheliometer and two pyranometers of the highest accuracy class, even if their operations and maintenance (O&M) is optimum.
To generate reliable irradiance measurement data, it is important not only to select the most appropriate sensors, but also to ensure their correct installation, operation, and maintenance. The station’s design must avoid unwanted effects caused by shading or other influences from, for example, traffic, industrial activity, insects, animals, strong winds, or vandalism. The frequent periodic inspection of the station by trained personnel, including required corrections and cleaning of the sensors, is typically a simple task, but essential for measurement accuracy. In operating solar power plants, conflicts of interest resulting from the expected difficulty for a single contractor to reconcile two different objectives (high solar plant efficiency and high irradiance data quality) should be avoided. This is because, for instance, measurement errors caused by soiled radiometers or wrong radiometer installation can lead to an overestimation of solar system efficiency. The calibration of radiometers also contributes significantly to overall uncertainty and must be repeated periodically according to the instrument specifications and international standards. The provided recommendations are based on the relevant international standards and are complemented by exemplary station plans and checklists for the various steps needed for accurate solar irradiance measurements.