Understanding Solar Radiation – The Root of All Solar Energy

Understanding solar radiation is key to knowing how solar energy systems work to create energy of various types – including both solar photovoltaics (PV) and solar thermal. Solar radiation, which can be captured in various forms, is the important to comprehending how solar energy is generated and harnessed.

Fundamental Solar Radiation Concepts

At the very basic level, solar radiation (or E-M radiation) is the wave-like energy that emanates from the sun, which can be captured and converted into electricity or heat energy. Every location on earth receives a different quantity of solar radiation throughout the year, which impacts where solar PV and solar thermal can be used to produce energy. Determining the available solar radiation, often called solar resource, is therefore required to know whether a solar technology is technically feasible.

 

How much solar energy reaches the surface of the planet is impacted in part by the interaction of sunlight with the local atmosphere. Because of things like water vapour and clouds, pollution and particulate matter, dust (including that from volcanoes and forest fires) and air molecules, solar energy is either absorbed, scattered, or reflected. These factors can diffuse solar radiation anywhere from 10% to 100%.
The solar energy that passes through these obstructions is called diffuse solar radiation, whereas that which passes to the earth’s surface undiffused and unobstructed is called direct beam solar radiation. The total of both makes up the complete global solar radiation picture.

 

Measuring the solar resource in a particular region at any given time depends on many factors, including geographic location and landscape features, current season and time of day, as well as local weather patterns. Well go over each one in turn.

The geographic region and local landscape

The earth, of course, is round, and that means the rays of the sun strike different regions at different angles throughout the day and over various seasons. The angles of these solar rays can range anywhere from 0º at nearly horizontal angles to 90º at nearly completely vertical angles. The greater the angle of entry, the more the solar energy has to filter through the atmosphere, which means less solar radiation reaches the surface of the planet.

 

As a result, completely vertical solar radiation provides the most energy possible, while vertical solar radiation provides much less. Consider that regions on both poles of the planet receive little to no high sun, making them less ideal for solar energy. In fact, these regions can experience periods with no solar radiation during certain parts of the year.

The season and time of day

The rotation of the planet also impacts how much solar radiation the earth receives. The earth’s rotations create what we know as day and night as well as different seasons. As the earth rotates, the amount of sunlight received by the earth changes hourly, with the most energy received at high noon when the sun is high in the sky. Revolving on an axis of 23.5º, certain portions of the planet are physically closer to the sun during half the year and further way during the other half of the year, like this:

 

• The northern hemisphere regions are closer to the sun from spring (approximately April) to fall (approximately September), resulting in longer, warmer days. The remaining six months have shorter, colder days.
• The southern hemisphere regions are closer to the sun from fall (October) to spring (March) and farther away in the opposite six months, resulting in opposing climatic patterns to those in the northern hemisphere.
• On the central latitudes, days are close to 12 hours long throughout the year, resulting in much more even amounts of solar radiation and therefore even temperatures throughout the day and over the course of the year.

Local weather patterns

Despite the changes in solar radiation from morning to night and season to season, the effects of differing solar resources are mitigated by the oceans. Because of their vast size and ability to absorb and slowly release solar energy, oceans moderate extremes in climate, especially in coastal regions. This creates varying weather patterns throughout most continents. Landscape features also play a role in the types of weather patterns experienced in different regions.

 

Measuring Solar Radiation for Solar Energy Systems

It should be obvious by now that the solar radiation varies significantly from one part of the planet to another. Scientists use several methods to arrive at an estimate of the total solar energy received in any given location. These estimates are useful for determining how feasible a solar energy installation will be.
Solar resources are generally expressed in terms of total radiation on a horizontal surface, which is a usually a measurement of the solar radiation received in a specific region taken at different times throughout the year. The solar resource is expressed in two different ways depending on the solar energy technology being considered:

• For solar PV systems, the solar radiation is expressed in kilowatt-hours per square meter, or kWh/m2.
• For solar thermal systems, the solar radiation potential is expressed in British thermal units per square foot, or Btu/ft2.

Determining a site’s feasibility for solar PV

Much of the US is suitable for solar PV systems, though some possess higher electricity generating potential than others. Nevertheless, it is always a good idea to have a solar professional perform a solar site analysis before deciding to install a solar PV system at your location.

 

Once that has been completed, you’ll need to determine the ideal location of your PV system on your property to generate as much electricity as possible. There are two main siting factors that impact how well a solar PV system will collect solar radiation: the orientation and the tilt of your solar panels.

 

When it comes to orientation, most PV modules in the northern hemisphere are oriented true south to achieve the optimum solar radiation collection. That said, sometimes structures prevent this ideal orientation, including roof orientation, local landscape and obstructions, as well as local weather conditions. As such, if true south is not possible, you can orient your PV panels up to 45º east or west of that without a significant impact on performance.

 

Tilt is the second factor to consider when determining the ideal siting for PV systems. For rooftop installations, the angle of the roofing is often a reasonable tilt for achieving good solar results. Nevertheless, an ideal tilt is usually calculated as that equal to your local latitude.

Determining a site’s feasibility for solar thermal

Sunny, warm locations are obviously good locations for solar thermal installations for water heating or space heating. The good news is that even if you live in a climate that goes through periods of colder weather, solar thermal may still work very well for you, with the right siting.

 

In most cases, a south-facing roof is recommended for solar thermal. In addition, a thermal system should be sited in a location with little to no shading to ensure adequate solar resources throughout the day. Once again, a solar professional can provide a solar site analysis to determine the ideal location for a new system.

 

As you can see, understanding solar radiation is pivotal to any solar training program. Techniques and technologies used for evaluating and siting a solar energy system based on available solar resources is absolutely necessary for achieving the most efficient results.