HelioScope - Version 101 - Photovoltaic (PV) Solar Array Software
Software Overview
The core mathematics is based on industry-standard approaches for modeling each step in the system performance. None of the steps described here are proprietary or secret, instead, they are all based on research and analysis published by academic research institutions and national labs.
By understanding the drivers of performance in more detail, system engineers can make better decisions about the components and designs they deploy. The approach described in this guide is the exact math used in HelioScope.
The Modeling 101 Guide describes the high-level relationships between the drivers of system performance modeling. For a more quantitative view of the exact equations, see the Mathematical Formulation available in HelioScope.
System Design
Module Orientation
- Azimuth defines the direction on a compass that the module is oriented. A zero degree azimuth corresponds to due North, 90 degrees will face East, 180 degree azimuth corresponds to due South.
- Tilt defines the angle of incline of the module, with zero corresponding to completely flat, and 90 degrees corresponding to completely vertical.
The most common orientation for a solar array woul...
Row-to-Row Spacing and Ground Coverage Ratio
System Sizing
Environmental Assumptions
- Global Horizontal Irradiance, GHI (W/m2): the total amount of sunlight available at a given moment, based on a collector that is oriented perfectly flat on the ground. This includes both the direct and diffuse components of the sunlight.
- Diffuse Horizontal Irradiance, DHI (W/m2): the indirect sunlight that is available to a collector that is oriented flat on the ground. This includes the general brightness of the sky, as well as reflected light. The best way to visualize this irradiance is to picture a person’s shadow on a sunny day. The ground under the shadow is not totally dark, even though the direct component has been removed. The remaining sunlight is the diffuse irradiance.
- Ambient temperature (°C): the air temperature at a given point in time. Note that this is not the same as the cell temperature – that is calculated separately based on a few inputs.
- Wind speed (m/s): The speed of the wind. This is used in the cell temperature calculations.
Weather Files
At each step, the system size increases, adding mo...
Diffuse Irradiance and Transposition Modeling
Once the POA irradiance is calculated, we next mov...
- The diffuse irradiance for a module is based on the area of sky that is above the module – and so that will be reduced based on the share of the sky that is covered by the obstructing modules. [Add sentence about integrating up the face of the module?] Note that this diffuse loss will persist year-round, since the blocking module racks will always be blocking that share of the sky.
- The direct irradiance loss from row-to-row shading is based on the exact sun angle, and the shade pattern created from the front row of modules. As the shade line moves up the shaded modules, there is a non-linear effect on the shaded module’s direct irradiance. With modules mounted in landscape, the first 1/6th of the module will reduce the DNI by 1/3, since that effectively shades the first cell-string entirely. Then the second 1/6th of shading has no additional power loss, since that cell-string has already had the DNI reduced to zero. This same logic is applied to the second and third cell-strings as well.
Module Behavior
System Electrical Effects
- Over-voltage: If the optimal string voltage is too high for the inverter, the inverter will draw its maximum voltage (though the voltage is below the modules’ maximum power voltage). As a result, the system will still produce some power, but it will be less than the maximum of what it could be. The resulting power loss will be bucketed as mismatch loss.
- Under-voltage: If string voltage is too low, there are two potential effects. If the under-voltage is only moderate, then the inverter will still be able to draw some current from the array, but it will just be lower than the array’s maximum power. However, if the voltage imbalance is more severe, then the strings will not be able to drive the inverter, and can collapse entirely and produce no power. This is why under-voltage (particularly as string voltages sag in hot weather) can be particularly detrimental to a system’s production.
- Over-power: Every inverter has a maximum power rating (defined as the output power on the AC side of the inverter), which it will not exceed. If the input power increases such that the inverter starts to exceed its output limit, the inverter will increase its operating voltage, to pull the modules off of their maximum power point, until the system output power is below the inverter’s maximum. This is commonly known as clipping. This mode is happening more often, as PV system designs increasingly use higher inverter loading ratios to optimize the system economics.
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