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Field Test Best Practices: A Resource for Practical Residential Building Science

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Photovoltaic (PV) Systems

The actual performance of a residential photovoltaic (PV) system depends on many factors. Some are related to the PV modules themselves: module specification, variations in the manufacturing of the modules, damage to the modules that may occur during transportation or installation, and interactions among the modules in the array (e.g., dissimilar modules in parallel).  The wiring configuration is important; for example, the spatial arrangement of modules in series can cause shading or snow cover to drastically reduce array output.  The inverter also affects performance - the inverter must be properly matched to the array, the rated performance of the inverter may be different than the actual performance, and there are interactions between the inverter and the array (e.g., MPPT). The installation also affects performance, though some impacts can be mitigated. The quality of installation (e.g., wire sizes and connections; damage to components), configurations of the array (azimuth and tilt), blown fuses, and normal electrical losses in the wiring all can play a role in performance. Environmental conditions also have an impact, including the temperature of the array during operation, the local microclimate (e.g., fog in a valley or near a coast), optical losses at high angle-of-incidence, variations in the spectrum of the sunlight, based on airmass, humidity, haze, clouds, etc., dirt or snow accumulation on the array surface, hail damage, and natural degradation over time. All these factors play a role in how much power produced by the PV panel can be used in the home. 

Some of these effects can be mitigated, but most cannot, so PV performance tests are generally aimed at determining the expected performance after taking into account all the inherent impacts. Three types of field test may be conducted, with different degrees of performance verification:

1. Verification of Good Installation.

The purpose of this initial, short-term test is to detect any problems with damaged equipment or incorrect installation, such as bad wiring, blown fuses, etc., and to verify that the electrical performance is within reasonable limits.  Tests may include:

  • Visual inspection of the equipment for any obvious damage
  • A series of electrical tests outlined in the form of a flow chart: "Checking that the Array is Operating Properly" [1].

2. Short Term Performance Test / Long-Term Modeling

The purpose of this short-term test, developed by Barker and Norton [1] is to measure the electrical performance of the array over a range of irradiance levels, voltages, currents, and module temperatures, in the form of a family of I-V curves.  These measurements are then used in a simulation model with typical weather data (TMY) to predict the annual energy production of the PV array.

3. Long Term Performance Test

The most comprehensive test procedure, which captures all of the factors listed above that affect system performance (with degradation over a limited time), is a long term field test of the complete PV system.  Using a datalogger to record the measurements, the recommended measurements include:

These measurements can be used to calculate:

  • PV array output
  • Inverter efficiency
  • Total system energy production.

It is recommended that the total system energy production be compared to model predictions, and that any large differences be explored.

Reference

  1. Barker, G.; Norton, P. (2003).  Building America System Performance Test Practices: Part 1 – Photovoltaic Systems.  NREL TP-550-30301.