Q14Wind and Solar Energy Systems

Estimation of Solar Energy Availability

Estimating solar energy availability at a given location and for a proposed solar installation requires combining measured or modeled solar irradiance data with the site's specific geographic and orientation parameters. The fundamental input data is the site's solar resource, characterized by Global Horizontal Irradiance (GHI, total solar radiation received on a horizontal surface, combining direct beam and diffuse sky components), Direct Normal Irradiance (DNI, the beam radiation component received on a surface always kept perpendicular to the sun's rays, most relevant for concentrating solar technologies), and Diffuse Horizontal Irradiance (DHI, the scattered sky radiation component received on a horizontal surface) - these quantities are typically obtained from ground-based pyranometer/pyrheliometer measurement stations where available, or from satellite-derived solar resource databases and typical meteorological year (TMY) datasets for locations lacking long-term ground measurement.

Once the horizontal irradiance components are known, they must be transposed (converted) to the actual tilted and oriented plane of the proposed solar collector or PV array using solar geometry relationships (accounting for the site's latitude, the collector's tilt angle and azimuth orientation, and the sun's position at each hour of the year, determined by solar declination and hour angle) - this transposition calculation combines the beam component (using the direct geometric relationship between the sun's position and the collector's orientation), the diffuse component (typically using an isotropic or more sophisticated anisotropic sky model), and the ground-reflected component (dependent on the ground's albedo/reflectance and the collector's tilt) to arrive at the total irradiance actually incident on the tilted collector surface.

This tilted-surface irradiance is then combined with the specific solar technology's conversion efficiency (accounting for PV cell/module efficiency and temperature-dependent derating for photovoltaic systems, or optical and thermal efficiency for solar thermal systems) and system losses (soiling, wiring, inverter conversion losses, shading) to estimate the expected energy yield of the proposed installation, typically expressed as expected annual energy production (kWh/year) or capacity factor (the ratio of actual average output to rated peak capacity) - this complete solar energy availability estimation process, combining resource data, geometric transposition, and system performance modeling, is the standard methodology used by solar project developers, financiers, and utilities during the planning, feasibility assessment, and financing stages of any solar energy project.