Introduction
PVGIS estimates photovoltaic (PV) power output from irradiance and weather inputs. The process can be understood as a simple pipeline: first compute solar geometry, then compute irradiance on the panel, and finally convert irradiance into power.
-
Inputs
- Location, orientation, tilt
- Time period
- Irradiance data
- Ambient temperature and wind speed
-
Solar Geometry
- Solar altitude and azimuth
- Incidence angle on the panel
- Horizon masks (sun above/below horizon)
-
Irradiance Components
- Direct irradiance
- Diffuse sky-reflected irradiance
- Ground-reflected irradiance
- Total irradiance in plane
-
PV Conversion
- Efficiency coefficients
- Module and system effects
- Instantaneous PV power output
- Energy yield over the selected period
flowchart TD
A[Define period, location, orientation, and tilt] --> B[Compute solar altitude and azimuth]
A --> H[Read ambient temperature and wind speed]
B --> C[Compute incidence angle and horizon masks]
C --> D[Direct irradiance component]
C --> E[Diffuse sky-reflected component]
C --> F[Ground-reflected component]
D --> G[Total in-plane irradiance]
E --> G
F --> G
H --> I[Derive efficiency coefficients]
G --> I
I --> J[Estimate PV power output]
J --> K[Aggregate over time to obtain energy yield]In short, PVGIS combines solar position, irradiance components, and weather-dependent efficiency effects to produce accurate, time-resolved PV power estimates for user-defined conditions and surface orientations.
Introduction in the commmand line
Photovoltaic power is the electrical
power generated by a photovoltaic (PV) system when solar irradiance
strikes its surface and is converted into electricity.
This process relies on the photovoltaic effect, where
solar cells convert sunlight into direct current (DC) electricity, which
can then be used, stored, or fed into the electrical grid. The amount of
power produced by a PV system depends on various factors such as the
angle of sunlight, the efficiency of the
solar panels, environmental conditions, and the available solar irradiance.
╭─────────────────────────────────── Note ───────────────────────────────────╮
│ │
│ PVGIS can estimate the photovoltaic power output for a series of │
│ technologies using either broadband or │
│ spectrally resolved irradiance data. │
│ │
│ │
╰────────────────────────────────────────────────────────────────────────────╯
A key metric for evaluating the overall performance of a photovoltaic (PV)
system is the cumulative Energy produced over a time period
(e.g., daily, monthly, or annually). In other words, energy production is an
aggregate of the instantaneous power estimations over a time series.
Instantaneous power values reflect the current output of the PV
system at each moment in time, which in turn depends on the effective irradiance.
How does PVGIS calculate photovoltaic power output ?
First, it calculates the position of the sun relative to the
solar surface over the user requested period of time. This boils down to one
key trigonometric parameter : the solar incidence angle. The
incidence angle depends on the solar altitude and
azimuth angles at any given time, combined with the
location, orientation, and tilt of the
solar surface.
Important consideration is the various conventions or angular measurements of
orientation. PVGIS 6 follows the North-Up = 0 deg definition. See also the
following diagram :
Origin of Azimuth
Solar azimuth is an important element in solar positioning. PVGIS follows the
North-Up = 0° convention for azimuth angular measurements.
In overview :
- North-Based System (N=0): Default in PVGIS.
- East-Based System (E=0): Common in some literature and solar
positioning models.
- South-Based System (S=0): Alternative system in specific
applications.
Below is a visual representation of various azimuth angle conventions and
conversions, i.e. from a North-based system to either East- or South-based
systems :
┌─────────────┐ ┌────────────┐ ┌────────────┐
│ N=0 │ │ N │ │ N │
│ ▲ │ │ ▲ │ │ ▲ │
Origin │ W ◄┼► E │ │ W ◄┼► E=0 │ │ W ◄┼► E │
│ ▼ │ │ ▼ │ │ ▼ │
│ S │ │ S │ │ S=0 │
└─────────────┘ └────────────┘ └────────────┘
┌─────────────┐ ┌────────────┐ ┌────────────┐
│ │ │ │ │ │
│ │ │ │ │ │
Input South │ 180 │ │ 90 │ │ 0 │
(IS) │ │ │ │ │ │
│ │ │ │ │ │
└─────────────┘ └────────────┘ └────────────┘
┌─────────────┐ ┌────────────┐ ┌────────────┐
│ │ │ │ │ │
Internal │ │ │ │ │ │
│ = │ │ IS - 90 │ │ IS - 180 │
Conversion │ │ │ │ │ │
│ │ │ │ │ │
└─────────────┘ └────────────┘ └────────────┘
Second and based on the incidence angle, it estimates the direct
and diffuse irradiance components.
Following is a step by step analysis of the calculations.
### Step by Step
Analytically the algorithm performs the following steps :
1. Define an arbitrary period of time
The user selects a time period over which the energy production will be
evaluated.
2. Calculate the solar altitude angle series
- The solar altitude angle is the elevation of the sun above the horizon.
- The default algorithm for solar time is
solar_time_model is set to SolarTimeModel.milne
(see: pvgis.constants.SOLAR_TIME_ALGORITHM_DEFAULT).
This calculates the apparent solar time based on the Equation of Time (Milne, 1921).
- The default solar positioning algorithm is
solar_position_model is set to SolarPositionModel.noaa
(see: pvgis.constants.SOLAR_POSITION_ALGORITHM_DEFAULT).
- The default model to calculate the solar incidence angle is
solar_incidence_model is set to SolarIncidenceModel.iqbal
(see: SolarIncidenceModel.iqbal).
3. Calculate the solar azimuth angle series
- The solar azimuth is the compass direction from which the sunlight is
coming, calculated using the NOAA solar position algorithm.
4. Derive masks for solar position
- Create masks for different solar positions
i. Above the horizon and not in shade.
ii. Low sun angles (for example, near sunrise or sunset).
iii. Below the horizon (nighttime).
5. Calculate the direct horizontal irradiance component
- Compute the direct irradiance component that reaches the PV surface from
the sun without scattering.
6. Calculate the diffuse and ground-reflected irradiance components
- For times when the sun is above the horizon, calculate the diffuse
irradiance (scattered light) and reflected irradiance (from nearby
surfaces).
7. Sum the individual irradiance components
- Combine the direct, diffuse, and reflected components to derive the global
inclined irradiance on the solar panel.
8. Read time series of ambient conditions
- Input data such as ambient temperature, wind speed, and spectral factors
are read from time series, which affect panel performance.
9. Derive the conversion efficiency coefficients
- Conversion efficiency is calculated based on the temperature, wind speed,
and other factors influencing the PV panel's ability to convert irradiance
into power.
10. Estimate the photovoltaic power output
- Photovoltaic power is estimated as the product of the global inclined
irradiance and the conversion efficiency coefficients. This gives the
instantaneous power output, which is then integrated over time to
calculate the total energy produced.