Solar Energy Fundamentals and PV Basics

• Irradiance: $1000\ W/m^2$
• Cell Temperature: $25^\circ\text{C}$
• Atmospheric Mass (AM): $1.5$

Atmospheric Mass (AM) represents the thickness of the atmosphere the sunlight must penetrate to reach the cell. An AM of 1.5 simulates the path of the sun at a typical angle.

Exam Tip: Real-world performance rarely matches STC. While STC uses $25^\circ\text{C}$, solar cells actually get much hotter in the field. As cell temperature increases, PV voltage decreases, which can impact overall efficiency.

Types of Solar Radiation

Not all sunlight reaches a PV module in the same way. To accurately predict production, we must understand the components of Global Radiation:

1. Direct Radiation: Parallel rays coming straight from the solar disk.
2. Diffuse Radiation: Sunlight that has been scattered by molecules, dust, or clouds in the atmosphere.
3. Albedo: Radiation reflected off the ground or surrounding surfaces, such as a white roof or snow.

To measure these components, professionals use specialized tools: a pyranometer measures total global radiation, while a pyranoheliometer can specifically measure direct beam radiation.

Celestial Mechanics and Sun Path Analysis

The position of the sun in the sky changes constantly due to the Earth's rotation and its $23.5^\circ$ tilt. Understanding this movement is vital for predicting solar availability.

Latitude and the Tropics

A location's latitude is the primary factor determining its solar path. In the tropics (the area between $23.5^\circ\text{N}$ and $23.5^\circ\text{S}$), the sun can actually pass directly overhead. Outside of this zone, the sun will always appear at an angle in the sky.

Solar Noon, Azimuth, and Elevation

• Solar Noon: The moment when the sun reaches its highest point in the sky for that day.
• Solar Azimuth: The horizontal angle of the sun, measured in degrees from a reference direction (usually North or South).
• Solar Elevation: The vertical angle of the sun above the horizon.

To track these variables, installers use sun charts or tools like the Solar Pathfinder to predict the sun's path and identify potential shading throughout the year. Note that when using a compass for solar orientation, one must account for magnetic declination, which is the difference between magnetic north and true north.

Optimization: Tilt, Tracking, and Shading

To maximize the energy captured, the PV array must be oriented to face the sun as directly as possible.

Tilt Angles

A common rule of thumb for setting the tilt of a fixed solar array is based on the site's latitude:

• For maximum annual production: Tilt $\approx$ Latitude.
• To optimize for Summer/Spring: $\text{Latitude} - 15^\circ$.
• To optimize for Winter/Fall: $\text{Latitude} + 15^\circ$.

Tracking Systems

While fixed tilts are common, trackers can significantly increase energy yield:

• Single-axis trackers: Follow the sun from east to west.
• Dual-axis trackers: Follow the sun's daily movement and seasonal altitude changes, providing maximum exposure. These are often used in conjunction with Concentrating PV (CPV) systems to focus intense light onto small, highly efficient cells.

The Impact of Shading

Shading is the enemy of PV performance. Because cells in a module are often connected in series, shading a single cell can create a bottleneck for the entire string—a phenomenon known as the "Christmas light effect".

Critical Design Note: Shading is most damaging during the peak production hours of 9 am to 3 pm. Furthermore, in ground-mounted arrays, installers must consider inter-row shading, where one row of panels casts a shadow on the row behind it.