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Page • Sandia National Laboratories is facilitating a collaborative group of photovoltaic (PV) professionals (PV Performance Modeling Collaborative or PVPMC).  This group is interested in improving the accuracy and technical rigor of PV performance models and analyses.  Such models are used to evaluate current performance (performance index) and determine the future value...

Page • Sandia National Laboratories facilitates this collaborative group of photovoltaic (PV) professionals (PV Performance Modeling Collaborative or PVPMC).  This group is interested in improving the accuracy and technical rigor of PV performance models and analyses.  Such models are used to evaluate current performance (performance index) and determine the future value of...

Page • Energy losses on the AC side of the inverter can be significant when the AC power is raised to distribution or transmission level voltages and is transferred any distance before the utility meter.  The value of the energy produced by the plant needs to include all losses before the utility...

Page • Energy losses on the AC side of the inverter can be significant when the AC power is raised to distribution or transmission level voltages and is transferred any distance before the utility meter.  The value of the energy produced by the plant needs to include all losses before the utility...

Page • As more PV systems are added to the electrical grid, the requirements for interconnection are changing.  New PV inverters are being required to provide advanced functions to help support the robust operation of the electrical system.  These functions include: Volt-VAr (voltage regulation)Commanded Power Factor (voltage regulation)Frequency-watt (frequency regulation)Commanded Maximum Power...

Page • Air mass is a relative measure of the optical length of the atmosphere.  At sea level, when the sun is directly overhead (zenith angle = 0),  the air mass is equal to 1.  As the zenith angle becomes larger, the path of direct sunlight through the atmosphere grows longer and air mass increases. "Relative"...

Page • Air pressure is measured with a barometer.  Air pressure measurements are used in PV performance modeling to help adjust air mass estimates.  Fluctuations in air pressure are correlated with changes in the thickness of the atmosphere and can also lead to systematic changes to the solar spectrum on the ground.

Page • Air temperature (also known as dry-bulb temperature) is the temperature of the ambient air when the measurement device is shielded from radiation and moisture. The most common method of air temperature measurement uses a resistive temperature device (RTD) or thermocouple within a radiation shield. The shield blocks sunlight from reaching...

Page • Albedo is the fraction of the Global Horizontal Irradiance that is reflected. $$albedo\approx 0$$ when the surface is very dark and $$albedo\approx 1$$ when the surface is bright white or metallic. The PVsyst modeling software provides the following guidance for estimating an appropriate value for albedo: Urban environment 0.14 – 0.22Grass 0.15 –...

Page • The AM 1.5 Standard Spectrum refers to exactly two standard terrestrial solar spectral irradiance spectra. The two spectra define a standard direct normal spectral irradiance and a standard total (global, hemispherical, within $$2\pi$$ steradian field of view of the tilted plane at $$38^{\circ}$$ from horizontal) spectral irradiance. The direct normal spectrum is the direct...

Page • The angle of incidence ($$AOI$$) between the Sun’s rays and the PV array can be determined as: $$AOI=\cos^{-1}[\cos\left(\Theta_z\right)\cos\left(\theta_T\right)+\sin\left(\theta_Z\right)\sin\left(\theta_T\right)\cos\left(\theta_A-\theta_{A},_{array}\right)]$$ where $$\theta_A$$ and $$\theta_Z$$ are the solar azimuth and zenith angles, respectively.  $$\theta_T$$ and $$\theta_{A},_{array}$$ are the tilt and azimuth angles of the array, respectively. Azimuth angle convention is defined as degrees east of north (e.g. North = 0, East = 90, West =...

Page • The annual yield or energy yield is simply the number of kilowatt hours produced in a year divided by the STC DC rating of the PV system.  It will change from year to year depending on the weather.  It is expressed in units of kWh/kWp, where kWp is “kW peak”...

Page • Array orientation is an important input to any PV performance model calculation.  Array orientation is usually described as either fixed tilt or tracked. A fixed tilt orientation is one that does not move. A tracked array’s orientation changes with time as the tracker moves the array in an attempt to reduce the...

Page • One of the most significant inputs to PV performance models is the measured irradiance on the plane of the array.  If the irradiance sensor is misaligned relative to the array or the array orientation is out of specification, the prediction accuracy will suffer.  This mini-study is aimed at discovering the...

Page • The performance of an illuminated PV array or system is characterized by the system’s I-V curve.  To harvest power from the system the voltage must be adjusted in order to maximize the power.  This function is called Maximum Power Point Tracking (MPPT).  MPPT is typically performed by the inverter or...

Page • The simplest approach to estimating $$IAM_B\left (\theta_{AOI} \right)$$  was developed by Souka and Safat (1966) and later adopted by the American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE), and is known as the “ASHRAE incidence modifier.” $$IAM_B= 1-b_{0} \left (\frac{1}{\cos \left (\theta_{AOI} \right)} -1\right)$$ This function has the benefit of...

Page • Determining the relative position of the sun from any location on Earth at any time of the year involves a series of calculations, many of which are approximations.  The first step to understanding these is to summarize a few basic quantities. Declination of the Sun – This is the angle between...

Page • A standard module rating condition for bifacial PV modules would be a boon to the PV community as it would provide a common, accepted basis for measurement and nameplate rating of bifacial PV products. In particular, conditions that also harmonize with existing PV module rating standards are desired.Through simulation and...

Page • The main challenge for modeling the performance of bifacial PV arrays and systems is the prediction of the irradiance hitting the array from the back side. Unlike the front side irradiance, which is usually assumed to vary little with position on the array, backside irradiance varies quite a lot with...

Page • Traditional AL-BSF PV cells are made to convert light hitting the front side of the cell to electricity. However newer PV cell designs (e.g., PERC, PERL, PERT, and HIT) with some minor modifications can be made so that light hitting from both sides contributes to the current produced by the...

Page • There exist many models to estimate POA irradiance from the standard components (GHI, DNI, and DHI).  We describe a few of the more popular models in the next several pages.

Page • Inverter efficiency is the ratio of the usable AC output power to the sum of the DC input power and any AC input power. Typical grid-tied inverter efficiencies exceed 95% under most operating conditions Efficiency changes as a function of AC output power, DC voltage, and sometimes inverter temperature. Sandia...

Page • Cell temperature is required to calculate the IV curve of the module.  Cell temperature is affected by the incident irradiance, weather conditions (such as air temperature and wind speed), and module construction and material properties.

Page • Ray Tracing Work at the University of Iowa COMSOL is a finite element method (FEM) solver and the Ray Optics Module can be used to model electromagnetic wave propagation in systems in which the wavelength is much smaller than the smallest geometric detail in the model. The electromagnetic waves are...

Page • This project is led by Joshua S. Stein, a Senior Scientist at Sandia National Laboratories, Albuquerque, NM. If you would like to contribute content to this site, please email Dr. Stein at jsstein@sandia.gov with your content or ideas. All contributed content will be reviewed and edited for accuracy and consistency. Contributors will...