Solar Radiation

Insolation is the term used to describe the amount of solar radiation that strikes a surface at a specific time and location. When insolation is referred to as power, it is represented in watts per square meter and is typically reported as an average daily value for each month. The total insolation striking the planet on a clear day is around 1,000 watts per square meter. However, several factors, including as air conditions, the earth’s position in regard to the sun, and barriers at the site, influence how much sunlight is available at a given site.

Solar Geometry

The amount of available solar energy is affected by the Earth’s distance from the sun and its tilt. From June to August, the earth’s northern latitudes are inclined towards the sun, bringing summer to the northern hemisphere. Summer days have much more accessible energy than winter days due to longer days and a more favorable tilt of the earth’s axis.

To harvest solar energy in the northern hemisphere, when the sun is primarily in the southern sky, photovoltaic modules should face south. Designers should optimize solar collection by arranging the array to capture the highest amount of sunlight available at a given area. Fortunately, the sun follows an orderly and predictable course through the sky.

The latitude of the place (the distance north or south of the equator) impacts whether the sun seems to travel in the northern or southern sky. California, United States, for example, is positioned at around 36 degrees north latitude, and the sun sweeps across the southern sky. The sun is exactly true south at midday.

Orientation

The apparent location of the sun east and west of true south is known as azimuth, and it is measured in degrees east or west of true south. Because a circle has 360 degrees and a day has 24 hours, the sun appears to travel 15 degrees in azimuth every hour (360 degrees divided by 24 hours).

If fixed modules are oriented true south, daily performance will be optimized. On a daily basis, an array that deviates 15 degrees from true south will collect 90 percent of the sun’s available energy.

The features of the local climate should be thoroughly analyzed and taken into account. You can, for example, correct for early morning fog by shifting the photovoltaic array west of south to acquire more late afternoon insolation.

Tilt Angle

The altitude of the sun above the horizon is measured in degrees above the horizon. The sun’s height is 0 degrees when it appears to be just rising or setting. The sun will be at its highest altitude for the day when it is true south in the sky at 0 degrees azimuth. This is known as solar noon.

Throughout the year, the latitude of a location impacts how high the sun shines over the horizon at solar noon. Because of the Earth’s skewed axis in its orbit around the sun, the sun is at different altitudes above the horizon during solar noon throughout the year.

The collector with the greatest average insolation will have a tilt angle equal to the latitude. To maximize a system’s performance, certain seasonal use factors must be considered.

·         Loads that are present all year: Tilt angle equals latitude.

·         Tilt angle equals latitude plus 15 degrees for winter loads.

·         Tilt angle equals latitude minus 15 degrees for summer loads.

Adjusting the tilt angle of the PV array seasonally can greatly boost power production for year-round loads. When the sun’s rays strike perpendicular (90 degrees) to the cells, photovoltaic arrays perform optimally. The angle of incidence is "normal" when the cells are directly facing the sun in both azimuth and altitude.

Solar Radiation Types

There are two forms of solar radiation that reach the earth’s surface.

Direct Solar Radiation

Direct solar radiation is solar radiation that reaches the earth’s surface without being dispersed.

Diffused Solar Radiation

Some sunlight is absorbed, scattered, and reflected by clouds (moisture and ice particles), particulate matter (dust, smoke, haze, and smog), and different gases as it moves through the atmosphere. Finally, solar energy that reaches the earth’s surface is referred to as dispersed solar radiation. It has no specific direction but comes from multiple sources at once. As a result, diffuse radiation does not cast shadows like direct radiation does when barriers are in its path.

When direct solar radiation strikes PV modules, it produces more electricity than diffused solar radiation because direct solar radiation is far more powerful than diffused solar radiation. In cloudy weather, dispersed solar radiation will still generate power when it strikes PV modules, but the amount of power will be lower.

Important Points

·         The total insolation striking the planet on a clear day is around 1,000 watts per square meter.

·         Designers should optimize solar gathering by arranging the array to maximize the amount of sunlight available at a given place.

·         The daily performance of fixed modules is improved if they are oriented true south, which is the optimal generic orientation for Northern hemisphere sites.

How to test the influence of PV panel orientation and tilt angle on output.

1.      Using an inclinometer and a compass facing true south or north, tilt a PV panel 15 degrees (latitude near the equator).

2.      Using a volt-ohm millimetre (VOM), measure the PV panel output, including open circuit voltage and short circuit current. VOM settings must be customized to your needs. For example, if you are testing open circuit voltage, the VOM must be set for DC volts greater than the PV panel’s open circuit voltage.

3.      Take notes on the readings.

4.      Now, alter the tilt angle from 33 to 70 degrees and record the results again.

5.      In the following step, keep the tilt angle at 33 degrees but change the orientation from true south to true east and record the measurements again.

6.      Compare and contrast the three readings. The highest output of a PV panel was found to be at a tilt angle equal to latitude, i.e. 33 degrees and facing true south.

Therefore, orientation and tilt angle are critical for maximizing PV panel production.

Next Chapter: PHOTOVOLTAIC MODULES