Plane-of-Array Solar Resource Estimation

Plane-of-Array Solar Resource Estimation refers to the process of calculating the amount of solar irradiance incident on a surface that is oriented at a specific tilt and azimuth angle, corresponding to the plane in which a solar array (such as photovoltaic panels) is installed. Unlike horizontal or global irradiance measurements, this estimation transforms the solar resource data into the frame of reference of the solar collector surface, providing a more accurate representation of the available solar energy for power generation or thermal capture.

Components of Plane-of-Array Solar Resource Estimation

The estimation process involves breaking down the total solar irradiance into three primary components that contribute to the total irradiance received on the tilted plane:

Direct Beam Component

This is the portion of solar radiation that travels in a straight line from the sun to the surface without being scattered. For the plane-of-array, the direct beam irradiance is calculated by projecting the direct normal irradiance onto the tilted surface using the angle of incidence between the sun’s rays and the array surface normal.

Diffuse Component

Diffuse irradiance consists of solar radiation scattered by molecules, aerosols, and clouds in the atmosphere, arriving at the surface from all directions of the sky dome. Estimating the diffuse component on a tilted surface requires modeling the angular distribution of the diffuse sky radiation and integrating it over the hemisphere visible to the tilted plane. Common diffuse irradiance models used include isotropic, anisotropic, and volumetric sky models.

Ground-Reflected Component

Also called albedo irradiance, this component accounts for solar radiation that reaches the ground, reflects off it, and then impinges on the tilted surface. The magnitude depends on the ground reflectance (albedo), the global horizontal irradiance, and the tilt angle of the array, as the visible portion of the ground changes with tilt.

Calculation of Total Plane-of-Array Irradiance

The total plane-of-array irradiance is the sum of the direct, diffuse, and ground-reflected components:

Where:

• IPOA is the total irradiance on the plane of the array,
• Ibeam is the direct beam component on the tilted surface,
• Idiffuse is the diffuse sky component on the tilted surface,
• Iground is the ground-reflected component.

Array Orientation and Tilt Inputs

To perform plane-of-array estimation, the orientation of the solar array must be specified by two angles:

• Tilt angle (β): The angle between the array surface and the horizontal plane. A tilt of 0° corresponds to a horizontal array, while 90° corresponds to a vertical surface.
• Azimuth angle (γ): The compass direction that the array faces, measured in degrees from south (in the northern hemisphere) or north (in the southern hemisphere). Due south is typically 0°, with east and west represented as negative and positive deviations respectively.

These inputs are essential for determining the solar incidence angle and the projection of various irradiance components onto the array surface.

Time Series Estimation

Plane-of-array solar resource estimation is often performed for time series data (e.g., hourly or sub-hourly) to capture temporal variations in solar irradiance due to changing solar position and atmospheric conditions. This enables accurate simulation of energy production for solar power systems and facilitates performance assessment, system design optimization, and financial modeling.

Summary of the Estimation Process

1. Input solar resource data: Typically includes global horizontal irradiance (GHI), direct normal irradiance (DNI), and diffuse horizontal irradiance (DHI).
2. Calculate solar geometry: Determine solar zenith and azimuth angles for each time step.
3. Compute angle of incidence: Calculate the angle between the sun's rays and the array normal based on orientation.
4. Project direct beam radiation: Calculate direct beam irradiance on the tilted surface using angle of incidence.
5. Estimate diffuse irradiance on the tilted plane: Apply an appropriate sky model to redistribute diffuse irradiance.
6. Calculate ground-reflected irradiance: Use albedo and array tilt to estimate reflected radiation.
7. Sum all components: Combine direct, diffuse, and ground-reflected components to obtain total plane-of-array irradiance.

Diagram of Solar Irradiance Components on a Tilted Surface

Importance of Plane-of-Array Solar Resource Estimation

Accurate plane-of-array solar resource estimation is fundamental to the design, simulation, and optimization of residential solar power systems. It enables precise prediction of energy yield by accounting for the actual orientation of solar modules rather than relying solely on horizontal irradiance measurements. This improves system sizing, financial forecasting, and performance evaluation, ultimately leading to better system reliability and return on investment.

Summary Table of Key Parameters and Definitions

This comprehensive estimation method ensures that solar energy systems are designed to maximize energy capture by appropriately accounting for the spatial and temporal variability of solar irradiance on surfaces oriented as actual solar arrays are installed.