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Performance Measurements of Photovoltaics and Distributed Generation Systems Project

Summary:

Distributed generation, especially as provided by renewable sources like solar photovoltaics (PV), is required to achieve net-zero energy buildings.  Computer simulation tools and testing and rating methods for evaluating and comparing distributed generation devices allow fair competition among manufacturers and informed decisions by consumers. Standards organizations and select government laboratories, including NIST, are developing and improving methods for testing and rating the power/energy production of PV and micro-cogeneration devices.  NIST is advancing the measurement science associated with PV spectral response measurements, is collecting quality field data for PV system model validations, and is leading efforts to develop a consensus rating standard for micro-cogeneration devices.

Description:

Objective:  By the end of FY 2013, to advance the measurement science needed to better quantify the power generation capabilities of solar PV modules and micro-cogeneration devices by focusing on PV spectral response measurements and the collection of high quality data from grid-tied PV systems to evaluate and improve PV simulation models.

What is the new technical idea?  Improved methods for accurately measuring and predicting the electrical output of PV modules and micro-cogeneration devices will bolster the deployment of these technologies.[1],[2],[3]  NIST will investigate improvements and alternatives to current methods for characterizing PV cells and modules indoors, with particular focus on the measurement of spectral response.  Spectral response measurements are important for creating reference devices and to allow corrections to standard solar spectra.

NIST will explore enhancements and the merits of combining features of two solar cell spectral response measurement facilities that were commissioned in FY12.  The three primary pursuits are to expand from relative to absolute spectral response measurements, increase the size of the PV specimen that can be tested, and reduce the measurement uncertainty.  The project will capitalize on access to NIST world-class metrology capabilities for spectral irradiance measurements. At the conclusion of this study, NIST will be able to judge whether to pursue member status in the select group that maintains the World Photovoltaic Scale (WPVS).NIST will continuously monitor three medium-size, newly constructed grid-tied PV systems.  The NIST datasets, which will be used internally for PV model validation and SmartGrid modeling work, will be added to a central depository that is managed by Sandia National Laboratories (SNL).  SNL has a framework for allowing the datasets to be used to evaluate and improve the capabilities of PV modelers and the modeling algorithms, especially those used in established computer programs like EnergyPlus and the Solar Advisor Model.NIST will also continue to advance the establishment of standard laboratory testing and modeling methods for characterizing and predicting the installed performance of micro-cogeneration systems.  Leading the development of a consensus standard is the current focus.

What is the research plan?  NIST is improving upon the test methods used for characterizing PV modules and cells. The main focus in FY13 will be to identify ways to accurately measure the spectral response of PV cells.  Trade-offs between different levels and sources of light bias, between underfilling and overfilling, from introducing an integrating sphere into the optics path, and from using different chopping frequencies are among the factors that will be investigated.  Different mechanisms and light sources for making absolute measurements will be explored.  Efforts will be made to increase the allowable PV cell test size to areas greater than 4 cm2 and to allow spectral response and current versus voltage measurements using the same test setup.  The impacts on uncertainty will be an important determinant in identifying the ultimate test configuration(s) and protocol(s).  NIST primary standards for spectral irradiance measurements will be used as part of the calibration of the spectral response measurement test facilities. Spectral response and current versus voltage (IV) curve measurements conducted on the same PV samples by NIST and other leading laboratories will be compared.

Two grid-tied PV systems will be monitored for the entire 12-month period.  Monitoring of a third PV system will commence in the Fall with instrumenting of a fourth and final grid-tied system being completed and monitoring initiated thereafter.  An in-house database for archiving the collected data will be developed and maintained.  The completeness and quality of the collected data, along with the overall operation of the four PV arrays, will be tracked.  Instrumentation and the PV array components will be regularly inspected; cameras will be deployed to augment these inspections and to aid efforts to study the effects of intermittent clouds, morning/evening shading, and snow accumulation.  A computer model of the first 2 field sites will be developed, with predicted performance being compared with measured data.

The Building 226 rooftop metrology station will be completely renovated and expanded so that it may better complement the data collected at the PV field sites and offer capabilities for testing and calibrating solar radiation sensors.  Additions to the rooftop facility will allow the monitoring of individual PV modules of the same type and varied orientations as used for the campus PV sites. The station and its weather data will be available to other NIST researchers.

Expertise gained and performance data collected and generated at NIST from laboratory testing and modeling different micro-cogeneration devices will be used in developing ASHRAE Standard 204P, “Method of Test for Rating Micro Combined Heat and Power Devices.”

 

[1] National Institute of Standards and Technology, “Measurement Science Roadmap for Net-Zero Energy Buildings:  Workshop Summary Report”, March 2010.

[2] Rummel S., et al, “Results from the Second International Module Inter-comparison,” Proc. IEEE 4th World Conference on Photovoltaic Energy Conversion, May 2006.

[3] Emery, K., “State-of-the-Art Measurements for PV,” NIST Grand Challenges for Advanced Photovoltaic Technologies and Measurements, May 11-12, 2010, Denver, Colorado.

 

Major Accomplishments:

Recent Results:

Outcomes:

  • Organized, explained, and archived data from the NIST BIPV roofing measurements so that the data may be used by outside researchers for evaluating and improving PV system models. (FY12)
  • Completed installation and commissioning of a new monochrometer-based test facility for measuring the relative spectral response of small solar cells. (FY12)
  • Lead technical contributors to the successful construction of four solar photovoltaic arrays that were erected on the NIST campus and collectively have a rated power output of 600 kW. (FY12)

Outputs:

Standards and Codes:

NIST personnel serve on ASTM E44.09, “Photovoltaic Electric Power Conversion”, and the US Technical Advisory Group for IEC Technical Committee 82, “Solar Photovoltaic Energy Systems” for the purpose of tracking and commenting on PV standards aligned with this NIST project. NIST recently accepted the role of chairing the subcommittee that revises ASTM Standard E1021-06, “Standard Test Method for Spectral Responsivity Measurements of Photovoltaic Devices.” NIST personnel also chair ASHRAE Standards Project Committee 204 which is developing a method of test for micro-cogeneration devices.