Energy Modeling in PVSketch Mega

Simplified Model

The Simplified model is based on the PVWatts energy model from NREL. (https://pvwatts.nrel.gov/) PVWatts consists of a set of component models to represent the different parts of a photovoltaic system. PVWatts performs hourly simulations to calculate the electricity produced by the system over a single year. PVWatts assumes that there are 8,760 hours in one year.

The following is a high-level description of the algorithm PVWatts uses to calculate the photovoltaic system’s hourly electrical output:

• Calculate the hourly plane-of-array (POA) solar irradiance from the horizontal irradiance, latitude, longitude, and time in the solar resource data, and from the array type, tilt and azimuth inputs.

• Calculate the effective POA irradiance to account for reflective losses from the module cover depending on the solar incidence angle.

• Calculate the cell temperature based on the array type, POA irradiance, wind speed, and ambient temperature. The cell temperature model assumes a module height of 5 meters above the ground and an installed nominal operating cell temperature (INOCT) of 49°C for the fixed roof mount option (appropriate for approximately 4 inch standoffs), and of 45°C for the other array type options.

• Calculate the array’s DC output from DC system size at a reference POA irradiance of 1,000 W/m², and the calculated cell temperature, assuming a reference cell temperature of 25°C, and temperature coefficient of power of -0.47%/°C for the standard module type, -0.35%/°C for the premium type, or -0.20%/°C for the thin film type.

• Calculate the system’s AC output from the calculated DC output and system losses and nominal inverter efficiency input (96% by default) with a part-load inverter efficiency adjustment derived from empirical measurements of inverter performance.

• The Simplified Model allows for the following system loss inputs:

  • PVWatts does not factor in the specific I-V curve of the solar module or inverter efficiency curve. This simplification means that the results are not as accurate for specific equipment as our Advanced energy model.  However, the Simplified model has the advantage of not requiring .PAN or .OND files to run and therefore is a good solution for early stage project design where final equipment has not yet been specified.

Advanced Model

The Advanced model is based on Canadian Solar’s System Simulator (CASSYS), an open source energy model developed by the module manufacturer. The Advanced model considers a solar project details description such as arrays, inverters, and modules, site location, and weather conditions on a sub-hourly interval to calculate the state of the system at each step and provide a detailed estimate of energy flows and losses in the system.

The Advanced model is a more accurate model than is PVWATTS, allowing, for example, to consider the specific IV curve of the solar module and it’s interactions with the inverter input. Both energy simulations use the same transposition models Hays and Perez.

Comparison with PVSYST

The Advanced model is organized to function similar PVSYST using the same underlying equations, and users can translate between these two models with relatively low effort and to obtain similar results, usually with 1% of each other.

The standard test condition (STC) parameters for the module are obtained from a .PAN file. Module behavior is calculated for several non-STC operating conditions such as open circuit, fixed voltage, and maximum point tracking. Values are then converted from module to array level and losses are applied in accordance with user input values.

Shading factors on the beam, diffuse, and ground-reflected components of incident irradiance, based on the sun position throughout the day resulting from a near shading model, are available for panels arranged in an unlimited rows or a fixed tilt configuration.  In the scenario of an unlimited row model, the Advanced energy model neglects edge effects because it assumes such rows are large enough that edge effects are not significant. This assumption reduces the calculation of the shading factor at different times of the day to a simple geometrical construct, as does PVSYST.

In a paper by Canadian Solar introducing CASSYS, they validated the model through cross-validation between CASSYS and PVSYST, as well as comparisons against measured data. Real world comparisons between measured and simulated values (once all post-construction conditions and parameters are reflected in the system definition) show that CASSYS provides a reliable basis to estimate the energy production by a defined system on a sub-hourly basis. In the same paper it is referred that a more thorough study is required to further understand the sources of error, and the fine-tuning steps for the model inputs.  However, simulations that fall within a couple of percent of the actual values are usually considered excellent, as the accuracy of the various instruments used in the measurements themselves rarely falls below that threshold. The agreement between the tools is found to be -0.35% for the energy predicted over an entire year (CASSYS being the more conservative tool) when the inputs to all models are closest to each other.

PVSketch Mega applies a hierarchy to available weather data sources to run the Advanced model. First the Advanced model will try to download data from NSRDB first (https://nsrdb.nrel.gov/), and if it fails for a given location, the model will default to download data from the PVGIS database (https://ec.europa.eu/jrc/en/pvgis).

Key parameters:

• Latitude and longitude

• Inverter model name and performance parameters

• Module model name and performance parameters

• Number of modules per series string

• Number of series strings per array

• Tilt () and azimuth of the array (or tracking angle algorithm for tracked arrays)

• Albedo of the ground (or roof) surface)

• Horizon map showing potential for shading from obstructions

• Irradiance data is reported as three components: direct normal irradiance (DNI), global horizontal irradiance (GHI), and diffuse horizontal irradiance (DHI).