Outdoor PV performance monitoring: Pyranometers versus reference cells

What instrument to use for the measurement of solar radiation in a PV plant? A thermopile pyranometer or a reference cell? This paper explains why thermopile pyranometers are the de-facto standard sensors for the measurement of solar radiation.

Outdoor PV performance monitoring: Pyranometers versus reference cells

Read the first chapter below or view the paper in PDF format: outdoor pv performance monitoring - pyranometers versus reference cells v1211.pdf

1. Solar irradiance monitoring for PV systems

The irradiance measurement for outdoor PV performance monitoring is usually carried out with pyranometers. Some standards suggest using PV reference cells. This paper explains why this is a mistake. 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.

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.

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 false impression that reference cells can be used outdoor is caused by the fact that reference cells are excellent for use as indoor references (so why not in outdoor experiments?) and by the fact that yield of reference cells is closely correlated to available resource.

Some added remarks:

In rare cases the monitoring purpose is not to measure efficiency but rather stability of a system. In that case a matched solar reference cell (of exactly the same type as the system) might be used, but a pyranometer is equally applicable.

In monitoring with very high accuracy requirements a combination is used of a shaded pyranometer and a pyrheliometer. The shaded pyranometer performs the measurement of diffuse global radiation and the pyrheliometer measures the direct solar radiation.

In tilted installation, when assessing PV system efficiency, the irradiance measurement, which is local, is not necessarily representative of the total irradiance received by the system; reflected solar radiation and local obstruction of the field of view may be large sources of measurement uncertainty.

1.1 Reference cell fundamental shortcoming: bankability and legal position / standards

Figure 1.1.1 The bankability issue: process of designing and financing solar power plants, proving to investors that basic performance is met in practice. Solar prospecting and solar atlases provide measurements in true available resource in W/m2. The reference cell does not provide legally sustainable proof of performance.

The pyranometer is the de facto standard for outdoor PV studies, and will remain the standard. This is acknowledged in the outdoor PV monitoring standard of IEC 61724 (Photovoltaic system performance monitoring – guidelines for measurement, data exchange and analysis) and the ASTM E2848C (Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance standards), as well as the CAISO Business Practice Manual for Direct Telemetry (Version 1, 8/2/2011). The IEC and ASTM standards do allow use of reference cells but under severe restrictions only. CAISO does not allow the use of reference cells at all. The fundamental reason for preferring pyranometers is that reference cells measuring daily totals will systematically over-estimate system efficiency. The reference cell can therefore not be used as a tool in “bankability” and efficiency discussions. A pyranometer can. See figure 1.1.1.

A less fundamental, but more practical reason for preferring pyranometers is their universal applicability. The resulting logistics is relatively simple. This is illustrated in figure 1.1.2. Contrary to reference cells, pyranometers do not need to be “matched” to the PV panel type nor do they need post-processing. The post processing of PV reference cell data also contains subjective elements, which reduces their status in bankability analysis and legal position. Another consequence of using reference cells is that using local weather as a correction, data obtained at different sites become incomparable. Another major restriction when using a reference cell is that technology, construction, packaging and surface textures should be identical to technology and construction of the solar panels. If this is not the case, the reference cell output will not have a linear correlation to energy production. There are dozens of different cell technologies and dozens of different reference cells.


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pyranometer pyranometers SR25 by Hukseflux
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