GTI simulator overview

In this document

This document introduces the GTI simulator used in Solargis Evaluate for GTI energy systems. It describes the purpose and scope of the GTI simulator, the inputs and mounting configurations it supports, how the optical simulation calculates GTI, and which outputs it produces.

The GTI simulator is a fast optical simulator used in Solargis Evaluate to simulate GTI energy systems. It converts time series of global horizontal irradiance (GHI) and direct normal irradiance (DNI), together with sun position, into global tilted irradiance (GTI) on the front surface of a PV module for one mounting configuration. The GTI simulator performs no electrical simulation, so it does not produce photovoltaic power output (PVOUT).

GTI simulator purpose and scope

In Solargis Evaluate, the GTI simulator calculates GTI for energy systems created in the GTI Energy system designer. Solargis Evaluate simulates PV energy systems with the S2 PV simulator instead. To set up a GTI energy system, see Creating GTI energy system.

Aspect	How the GTI simulator handles it
Simulation type	Optical simulation only, on the PV module front side. No electrical simulation, so the GTI simulator does not produce PVOUT.
Shading	The GTI simulator models far horizon shading only. It does not model near (row-to-row) shading or self-shading.
Terrain	Terrain in the GTI simulator is flat and horizontal. GTI energy systems in Solargis Evaluate do not use terrain data.
Mounting configurations per simulation	The GTI simulator simulates one mounting configuration per simulation.

How the GTI simulator calculates GTI

GTI simulator inputs

The inputs the GTI simulator needs depend on the outputs selected for the simulation. Location and sun position are always required. The other inputs are required only for specific outputs or settings.

Input	Format	Required when
Location and sun position	Latitude, longitude, and time series of sun azimuth and sun elevation. The length of the sun position time series defines the number of time slots.	Always
GHI and DNI	Time series. The GTI simulator derives DIF from GHI and DNI.	For any irradiance or GTI output
Albedo	One value per time slot, one constant value, or 12 monthly values	[SME to confirm]
Far horizon	Horizon profile defined by azimuth and height	For every horizon-shaded output
Date and time	ISO 8601 strings with an explicit UTC offset, one per time slot, for example `2020-11-18T00:30:00+02:00`, `2020-11-18T01:30:00+02:00`	For monthly or seasonal tilt, and for monthly albedo

Mounting configurations in the GTI simulator

The GTI simulator supports the following mounting configurations:

Fixed mounting — One constant tilt, 12 monthly tilts, or seasonal tilt changes on given days of the year.
1-axis horizontal tracker and 1-axis inclined tracker — Axis tilt (inclined tracker), east and west rotation limits, backtracking, and an optional night stow angle.
1-axis vertical tracker — Tilt, east and west azimuth limits, and backtracking.
2-axis astronomical tracker — Tilt limits, azimuth limits, and backtracking.

In the GTI simulator, the azimuth setting applies to fixed mountings and to 1-axis horizontal and inclined trackers. The default azimuth depends on the hemisphere.

In the GTI simulator, relative spacing is required when backtracking is enabled and is not set otherwise. For 1-axis horizontal and inclined trackers, relative spacing can be one value or separate east and west values.

Optical simulation in the GTI simulator

The optical simulation calculates GTI on the PV module front surface from horizontal irradiance and sun position.

Sky irradiance model — The GTI simulator models the sky with the anisotropic Perez diffuse model. For a description of the sky irradiance models used in Solargis simulators, see Incident irradiance — Sky irradiance.
Far horizon shading — The GTI simulator models far horizon shading with a view factor model, using the far horizon profile from the simulation inputs.
Angular reflection losses — The GTI simulator can apply angular (reflection) losses on top of GTI. GTI with angular losses is reported as effective GTI. For a description of the angular loss model, see Incident irradiance — Angular losses.

GTI simulator outputs

The GTI simulator calculates only the outputs selected for a simulation and runs only the computation those outputs need.

GTI simulator output	Description
Theoretical GTI	GTI on the PV module front surface without horizon shading.
Horizon-shaded GTI	GTI on the PV module front surface with far horizon shading.
Effective GTI	GTI with angular losses, available with and without far horizon shading.
GTI components	Direct, circumsolar, isotropic and horizon belt, and reflected components of GTI, available with and without far horizon shading. The horizon-shaded components also split the diffuse part into sky, terrain, and horizon belt.
Horizontal irradiance	GHI, DNI, and DIF, available with and without far horizon shading.
Panel orientation and angle of incidence	Panel azimuth, panel tilt, and angle of incidence on the panel. Panel tilt is also valid in night time slots.
Copied inputs	Date and time, sun azimuth, sun elevation, and surface albedo, copied from the inputs.