Aurora

Aurora's simulation engine is the preferred method of simulating system performance in Aurora. It employs a detailed, circuit-level simulation in order to simulate the operation of the system and to determine its energy production. When using Aurora's shading engine, the simulation will also account for the impact of shading from buildings, trees, and obstructions.

Methodology

Aurora's simulation engine is based on the "Five Parameter Model" proposed by De Soto et al., and simulates the performance of a solar module using an equivalent circuit based on parameters solved for from datasheet values.

Based on the equivalent circuit of each module, the engine constructs a circuit for each string in the system and simulates the MPPT tracker of the inverter to find its operating point. In the case of a DC optimizer or microinverter design, the MPPT tracking happens on a module-by-module basis.

When the Shading Engine option is checked (as it is by default), Aurora determines the incident irradiance on each module, taking into account shading from trees, buildings and roof obstructions in the site. By varying the IV characteristics of each module based on its incident irradiance, Aurora can accurately simulate the effect of shading on the system.

Simulation Parameters

• Weather Dataset: Specifies which dataset you would like to use for the simulation. Currently NREL's Typical Meterological Year 3 (TMY3) is available for the US and ASHRAE's International Weather for Energy Calculations 2 (IWEC2) is available for international locations. CWEC and RMY files are available for Canada and Australia, respectively.

• Weather Station: Specifies which weather station (from the selected weather dataset) will be used for the performance simulation. Aurora presents you with a list of the five closest weather stations and will, by default, select the one that is closest to the project location.

• Irradiance Model: Specifies which irradiance model is used for the simulations. Available options are 'Perez' and 'Hay Davies'. The Perez model is commonly regarded as the most accurate model and therefore is the default.

• Shading Engine: Indicates whether the shading engine should be used during the simulation. If checked, Aurora will determine shading and irradiance in the system based on its irradiance engine. If not, you need to specify a shading system loss factor to account for the effect of shading on system energy production.

• Submodule Simulation: Specifies whether or not to perform the simulation at a submodule, or cell string, level. When this option is checked, Aurora will create an equivalent circuit for every cell string in the design, where a cell string is a series connection of solar cells inside a module that has a parallel bypass diode. This is the finest level of granularity available for solar energy production simulation, offering the best simulation accuracy. Submodule simulation is only supported for modules where Aurora has verified the internal cell string configuration and bypass diode connections with the manufacturer and for inverters that support global maximum power point tracking. You can verify the components in your design have these properties by opening them in your Database section.

Simulation Logs

Aurora will automatically generate a log of the performance simulation details that clearly specifies relevant simulation settings and any assumptions made to generate the results. The log is divided into three categories:

• Simulation Settings: Specifies whether the simulation was run at the module- or submodule-level, if the shading engine was used, if inverter clipping was enabled, and which weather station and dataset were used in the simulation.
• Bypass Diodes: For each inverter in the design, Aurora will log if that inverter supports global maximum power point tracking. If the inverter does have this functionality, Aurora will also log the assumption that each bypass diode in the design has a voltage drop of approximately 0.7 V. For module-level simulations, where each "diode" across the module-level equivalent circuit usually represents three internal bypass diodes in series, Aurora will assume the voltage drop is approximately 2.1 V and the log message will reflect this. If an inverter does not support global MPPT, the log will state as such and not simulate any bypass diodes connected to the inverter.
• Inverter Efficiency Models: For each inverter in the design, Aurora will log if a fixed efficiency was used to simulate that inverter's performance, or if measured test data from the California Energy Commission (CEC) was available to construct a full efficiency curve as a function of DC input power and voltage.

Simulation Warnings

Aurora will output warnings that may have an impact on how the user chooses to configure their design. For example, Aurora will indicate the percentage of daylight hours for which the string voltage fell above or below the inverter's stated operating voltage window. If this occurs for a significant percentage of hours (where what constitutes "significant" is up to the user), it may be a good idea to change the design to ensure the string voltage is more compatible with the selected inverter. Aurora will also create a warning stating how much energy was lost to inverter clipping, which can also be seen in the system loss diagram. If a significant amount of energy is clipped, it may be a good idea to re-size the design.

Simulation Errors

If a problem precludes Aurora from running the simulation, an error will be reported. For most errors, a specific message is displayed indicating what the problem was. If you are unsure how to fix a simulation error, please contact Aurora support.

Validation

In addition to several successful internal simulation studies, NREL validated Aurora's simulation engine and confirmed that its output is comparable (and, in fact, is often better) than the production estimates generated by other simulation software for the systems studied.