Introduction

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Photovoltaic power is the electrical energy 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 transform sunlight into direct current (DC) electricity, which can then be used immediately, stored, or fed into the electrical grid. The power output of a PV system depends on multiple factors such as the angle of sunlight incidence, the efficiency of the solar panels, environmental conditions, and the amount of solar irradiance available.

PVGIS estimates photovoltaic power output for various technologies using either broadband or spectrally resolved irradiance data (the latter not implemnted!). A crucial metric for evaluating PV system performance is the cumulative energy produced over a given time period -- daily, monthly, or annually -- which aggregates instantaneous power outputs over that timeframe.

PVGIS calculates power output in two key phases. First, it determines the sun’s position relative to the solar surface during the selected period, focusing on the solar incidence angle, which depends on solar altitude and azimuth angles combined with the panel’s location, orientation, and tilt. PVGIS follows the North-Up = 0° convention for azimuth angular measurements, an important consideration in solar positioning and incidence angle calculations.

Second, based on the incidence angle, PVGIS estimates the direct, diffuse, and ground-reflected irradiance components reaching the panel surface. It then integrates environmental inputs like ambient temperature and wind speed to derive conversion efficiency coefficients that reflect the PV panel’s ability to convert irradiance into electrical power.

The algorithm includes :

  1. Define an arbitrary period of time

  2. Calculate the solar altitude angle series

  3. Calculate the solar azimuth angle series

  4. Derive masks for the position of the sun in the horizon

  5. Calculate the direct horizontal irradiance component

  6. Calculate the diffuse sky-reflected and ground-reflected irradiance components

  7. Sum the individual irradiance components

  8. Read time series of ambient temperature and wind speed conditions

  9. Derive the photovoltaic conversion efficiency coefficients

  10. Estimate the photovoltaic power output

This approach enables PVGIS to provide accurate, time-resolved estimations of photovoltaic power outputs tailored to user-defined conditions and surface orientations.

Introduction in the commmand line 
pvgis-prototype power introduction

    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: pvgisprototype.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: pvgisprototype.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.