outdoor pv monitoring: pyranometers versus reference cells ... pv performance... · outdoor pv...

Outdoor PV performance monitoring: Pyranometers versus reference cells outdoor pv performance pyranometers versus reference cells v1211

http://www.hukseflux.com/�

outdoor pv performance pyranometers versus reference cel ls v1211 2/16

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 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.

. The reference cell does not provide legally sustainable proof of performance.

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.

Pyranometer measurement leads

to proof of bankability

Pyranometer: measures:

true available radiation

Reference cell: measures: radiation that can be used

Reference cell measurement leads to overestimation

of system performance

Solar prospecting: Solar atlas or survey

with pyranometer PV

installation design

Financing System verification



As a result measurements of solar irradiance by using different reference cells may vary within a bandwidth of approximately 10% *)

. In practice installers apply a variety of panel technologies. If the installer uses a pyranometer for the measurement of solar irradiance, measurements at different locations are comparable.

Figure 1.1.2 The logistics using reference cells is extremely complicated. It involves matching reference cells to local systems, cell dependent calibration and local weather corrections. As indicated in figure 1.1.1, a measurement with a reference cell does not provide legally sustainable proof of performance. Solar irradiance measured with a pyranometer is logistically simple, serves as an objective value with which efficiencies of all systems can be determined and compared.

(*)Source: “One year round robin testing of irradiation sensors measurement results and analyses”, Mike Zehner et al., University of Applied Sciences Munich, Department of Electrical Engineering, Fraunhofer IWES, 2009, 24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, p. 3804, Fig 10.)

Monitoring solar irradiance with: PV reference cell

Monitoring solar irradiance with: thermopile pyranometers

PV panel technology

Silicon - amorphous

Silicon – single crystal

Silicon – multicrystallin

Variations on mentioned

technologies

single crystal reference cell

Amorphous reference cell

Multi-crystalline

reference cell CIGS – CIS

based

Reference cell technology X

Reference cell technology A

Post processing

Post processing

Post processing

Post processing

Post processing

choosing reference cell with least

possible output mismatch with

panel technology

Local weather data (for spectral

corrections)

Pyranometer

Result (not bankable)





Multiple recalibrations

Result (bankable)

Causes of increasing errors

recalibration



1.2 Comparing pyranometer and PV reference cell

A reference cell does not measure incoming irradiance, but it measures the part of the irradiance which can be used (converted to electricity) by a flat window solar cell. Because of reflecting properties of the flat window a reference cell will, even when ideally calibrated, always measure a daily total irradiance level that is several percent points lower than the irradiance that is actually available. This is illustrated in figure 1.2.1 and in more detail in appendix 2.1. It measures yield of a similar solar cell and not the truly available resource. Reference cell measurements can therefore not be used in comparison to data offered by “solar atlases” and site surveys that are used for planning and bankability studies. These data are based on available resources and based by definition on pyranometer measurements. Measurements with reference cells will give efficiency figures that are systematically too high; in other words, the reference cell does not provide legally sustainable proof of PV system performance.

The limitations of outdoor performance were clear to the original designers of reference cells; reference cells were originally designed for indoor use only. The IEC 60904 standard covering reference cells indicates that outdoor measurements can only be done with reference cells that are identical to the cell used in the PV system, under severe angle-of-incidence restrictions and applying spectral corrections based on atmospheric conditions of that moment. In the appendices we discuss these issues in detail.

A reference cell is designed for indoor irradiance measurements using a known and constant source at normal incidence and comparing to a cell of exactly the same material composition and housing. When used outdoors, all these boundary conditions change, and a reference cell is no longer a practical tool. Corrections for temperature, cell mismatch, weather type (clear sun / overcast) and air-mass must always be applied. This creates a situation with complicated logistics, an element of subjectivity in data analysis, and data that are not suitable for site comparison.

A pyranometer is designed for outdoor application. It is subject to a formal system of classification under ISO 9060. It is compensated for the above effects by having quantified limits to temperature dependence, directional errors and spectral response.

A pyranometer can therefore be universally applied, independent of cell type, weather type and instrument temperature. It provides data that are suitable for site comparison and can also be used with thermal solar applications.



Figure 1.2.1 Measurement error due to directional error only, pyranometer compared to PV reference cell, last three hours before sunset (and first three after sunrise), the directional error is causing large misreading’s. Reference cell errors are particularly large at sunset and sunrise (up to 50 W/m2

). See also appendix 2.1.

0

50

100

150

200

16:00 18:00 20:00

Wat

t

Time [minuts]

Measurement error, due to directional error (Watt) (Würzbrug, 21/6, SUMMER, sensors mounted in tilted position)

Measurement error (W/m^2) PV ref cell with airmass

Measurement error (W/m^2) pyranometer with airmass

Incoming watt/m^2 with airmass(global)

ISO limit



Table 1.2.1 PV reference cell compared to pyranometer

PV REFERENCE CELL PYRANOMETER

Goal

OVERALL JUDGEMENT

Measures cell yield: irradiance that can be converted to electrical power by a PV panel of the same cell type

Measures available resource: total available solar radiation

Directional error Error > 5 % above 55 degrees angle of incidence, > 2% above 50 degrees.

Error within quantified limits, typically <5% up to 80 degrees angle of incidence

Reference cells systematically underestimate irradiance & overestimate system performance / efficiency.

Global horizontal irradiance

Not suitable: Directional response error limits the measurement time frame to 4 hrs. per day.

Suitable Pyranometer is suitable for both tilted and horizontal measurements

Spectral mismatch Strongly variable, depending on weather: Must be compensated with real time data. Effective sensor sensitivity changes in the 5% range have been reported.

Estimated uncertainty limit in 1% range.

Reference cell spectral mismatch compensation requires correction with real time weather data and remains subjective.

Intercomparison between sites / adaptation to cell type

Reference cell must be identical to monitored cell. Measurements at different sites are incomparable.

Independent of cell type, flat spectral response. Measurements at different sites can be compared.

Users with mixed cell type PV plants would need a lot of reference cells, only one pyranometer. Only pyranometer results allow intercomparisons between all sites.

Other applications Can only be used with PV cells, preferably “matched”. Perfect for indoor comparison under stable lamps.

Can be used with all solar applications, including thermal solar

Pyranometers are universal and the de facto standard for outdoor, reference cells for indoor measurement

Cost Low cost models (EUR 400 end user price) are used in case cost of pyranometers is prohibitive

Legal position Standardisation Bankability

Required only by IEC 1829; 1995 (Future 61829) Crystalline silicon PV array- on site measurement of IV characteristics Not recommended at all by CAISO.

Accepted by all IEC standards except IEC 1829. CAISO: pyranometer is mentioned in recommended practices ASME 2848: advertised as most common choice in outdoor PV testing

A pyranometer is the only possible standard for outdoor PV performance monitoring



2. Appendices

2.1 Appendix: reference cell directional error

The most fundamental drawback of PV reference cells is their very large directional error, which always leads to significant (>5%) underestimation of the available irradiance in W/m2

at angles of incidence (θ) higher than 55 degrees. This leads to efficiency estimates that are several percentage-points too high. Pyranometers on the other hand are constructed with a glass dome, resulting in superior transmission of irradiance, also at larger angles of incidence.

Figure 2.1.1 Directional response error for reference cells is caused mainly by reflection of the glass surface; In general these reflections follow Fresnel equation for transmission.

Directional error of PV reference cell compared to a pyranometer Figure 2.1.2 represents the measurement error of a silicon reference cell and pyranometer as a function of the angle of incidence θ of the direct radiation. Also added are maximum and minimum comparison and error limits of a first-class pyranometer according to ISO 9060. Silicon reference cell error estimates are solely based on the reflective properties of uncoated glass (front and backside) and air, according to Fresnel equations. It should be noted that this directional error is fundamentally unavoidable using a detector with a flat glass or plastic plate on top. Properties of the PV cell material (typically silicon) and adhesive layers are not taken into account. At around 45 degrees the effective transmission of glass decreases significantly. Above 50 degrees there is a marked difference between PV reference cells and pyranometers. The measurement error is displayed along the vertical axis.



Figure 2.1.2 Directional error vs. angle of incidence for PV reference cells and pyranometers.

Figure 2.1.3 Because of the directional error, a reasonably accurate irradiance measurement by PV reference cell is only possible in a surprisingly short time frame.

0%

10%

20%

30%

0 10 20 30 40 50 60 70 80

Dir

ecti

onal

err

or

Angle of incidence [degrees]

Typical PV reference cell

Typical pyranometer

Allowable deviation first class pyranometer (ISO 9060)

15%

5%

2%



2.2 Appendix: reference cell directional error analysis

At Hukseflux we modelled the solar position and PV reference cell behaviour.

Input for the model:

• sensor directional behaviour • sensor sun position parameters • Positioning of the sensor on the spot

Output of the model:

• Global and directional irradiation, Also airmass taken into account • Sensor response, directional errors taken into account • Total solar energy, and total solar energy measured calculations

Following studies and formulas are applied:

Calculation of Airmass: Kasten, F., and A. T. Young, wata"Revised optical air mass tables and approximation formula" Applied Optics, vol. 28, issue 22, no. 22: OSA, pp. 4735–4738, 11/1989.

Calculation of incoming radiation: Meinel, A. B. and Meinel, M. P. (1976). Applied Solar Energy Addison Wesley Publishing Co.; Direct: column N, global = direct* 1,1 (diffuse radiation constant)* cos (theta)

Calculation of solar declination and hour angle: Solar position algorithm of Michalsky J.J. Michalsky, 1988, The astronomical almanac’s algorithm for approximate solar position (1950-2050), Solar Energy, Vol. 40, No. 3, pp 227-235

On the next page an example of the output is displayed. Wurzburg, 21 of June and Wurzburg 21 September.



2.3 Appendix: PV reference cell spectral mismatch error

In addition to their directional error, a second drawback of reference cells is their spectral selectivity. Reference cells are calibrated using an ideal solar spectrum. The compensation of reference cell for deviations of the solar spectrum from the ideal spectrum is extremely complicated. Outdoor conditions are never ideal. Procedures are given in IEC 60904-7: 2008 (PV devices – part 7: computation of the spectral mismatch correction for measurements of PV devices) and IEC 60891 (Procedures for temperature and irradiance corrections to measured IV characteristics of crystalline silicon PV devices). Corrections involve spectral data on a real-time basis. It is not realistic to assume that these data are available. Correction using real-time weather data is another option, but this is highly subjective, and therefore not desirable when estimating system efficiencies.

When comparing uncorrected reference cells, spectral mismatch between different cell types has reportedly led to an error band of 10%. This illustrates the order of magnitude of the problem. (Source: “One year round robin testing of irradiation sensors measurement results and analyses”, Mike Zehner et al., University of Applied Sciences Munich, Department of Electrical Engineering, Fraunhofer IWES, 2009, 24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, p. 3804, Fig 10.)

Figure 2.3.1 spectral response of reference cells, triple junction cell and pyranometer



2.4 Appendix: definition of measurement uncertainty

There is no standard for uncertainty estimates of solar irradiance measurements. Among experts there is consensus that a standard must be made; present expressions of measurement uncertainty are mostly unrealistic (for example +/- 2%, or like IEC 61724: “the accuracy of irradiance sensors, including signal conditioning, shall be better than 5 % of the reading”), and tend to take the uncertainty of calibration equal to the measurement uncertainty in the field. This is not at all realistic. ISO 9060 (standard title is “Solar energy - Specification and classification of instruments for measuring hemispherical solar and direct solar radiation”) states more realistically in note 5:

“The accuracy of solar radiation data measured by pyranometers depends not only on the category of the instrument but also on:

a) the calibration procedure, b) the measurement conditions and maintenance, and c) the environmental conditions.

Therefore statements about the overall measurement uncertainty can only be made on an individual basis, taking all relevant factors and the category of the instrument into account.”

This means that at a certain location, the user of instruments should make an individual assessment of uncertainty. In general the uncertainty will be lower at conditions of high maintenance, and conditions close to reference conditions at calibration (temperatures close to 20 degrees C, 1000 W/m2

In tilted installation, when assessing PV system efficiency, the irradiance measurement is not necessarily representative of the total system input; reflected solar radiation and local obstruction of the field of view may be large error sources. Many PV systems are continuously monitored; a redundant sensor may be used to detect irradiance-sensor fouling / malfunction.

irradiance and 0 degrees angle of incidence).

Some sources claim that the calibration uncertainty of a pyranometer is larger than that of a reference cell. This is only true for calibration across the full sky at various angles of incidence. The calibration of pyranometers at normal incidence is as accurate as that of reference cells; in the order of 1.5%.

2.5 Appendix: “lost sales” and system stability estimates

In discussions about the estimate of and system stability and of “lost sales” in case the grid cannot accept the power (this is a factor in contracts between the PV plant owner and the grid owner), the identical (matched) reference cell might seem to offer a better method of comparison, because of its identical directional and spectral response. However, in estimation of lost sales the correlation of irradiance readings (with pyranometer or reference cell) with PV system output over a comparable day (season / panel temperature) is the main factor of importance. The choice of weather type / conditions is the main source of uncertainty; the measurement with a pyranometer is equally acceptable and has the advantage of being independent of cell type.



In case of system stability analysis the assumption is that the calibration uncertainty of a pyranometer is larger than that of a reference cell. This is only true for calibration across the full sky at various angles of incidence. The calibration of pyranometers at normal incidence is as accurate as that of reference cells; in the order of 1.5%. Stability analysis of the PV system can also be made using measurement data obtained around solar noon only; the solar noon pyranometer data are more accurate than those at sunrise or sunset, because directional errors and offset errors play a smaller role at low angles of incidence.

2.6 Appendix: the specification measurement of solar irradiance in PV applications

The specification of the measurement should include:

• instrument classification, • maintenance and quality assurance (recalibration) program • measurement uncertainty aim (possibly limiting the conditions). • redundancy

Instrument classification: ISO 9060 specifies the following instruments for the measurement of solar radiation: based on measurement uncertainty requirements:

• Secondary standard pyranometer • First class pyranometer • Second class pyranometer • First class pyrheliometer

Hukseflux’ idea is that a realistic scenario for larger PV installations in moderate climates would be: one first class pyranometer plus one redundant second class pyranometer.

Maintenance schedule and quality assurance:

• 2-weekly cleaning of instruments • 1-yearly re-calibration • weekly checks against theoretical maximum levels / other local measurements

Measurement uncertainty aim: Assessment of system performance with daily totals can best be made with pyranometers, and for very large systems or systems with high accuracy demands with a pyrheliometer / tracker / pyranometer combination. Stability analysis of the PV system can also be made using measurement data obtained around solar noon only; the solar noon pyranometer data are more accurate than those at sunrise or sunset because directional errors and offset errors play a smaller role at low angles of incidence. Redundancy: Use of addition measurements, entering redundancy into the measurement, may serve to reduce uncertainty, in particular at low maintenance conditions or when analysing remote systems.



3. Hukseflux company background Hukseflux Thermal Sensors, founded in 1993, aims to advance thermal measurement. Our present activities are centered on design, assembly, calibration and quality assurance of the highest quality sensors and systems for measurement of heat flux, (solar) radiation and thermal conductivity. Hukseflux’ main source of income is sales of heat flux sensors, thermal conductivity measurement systems, pyranometers and net radiometers. We also provide consultancy and related services such as performing measurements and installations. Hukseflux aims to become a market leader in the field of solar radiation measurement. At present, solar energy related sensors represent around 50% of Hukseflux’ turnover.

Figure 3.1 Manufacturing of radiation sensors


© Hukseflux Thermal Sensors B.V., 2011 www.hukseflux.com

Hukseflux Thermal Sensors B.V. reserves the right to change and/or alter specifications without notice.

outdoor pv monitoring: pyranometers versus reference cells ... pv performance... · outdoor pv...

Documents