Although considerable, the solar resource is not constant resulting in presence of solar variants. It varies throughout the day and year, and by location.
For a large part, these variations result directly from the earth’s geography and its astronomical movements (its rotation towards the East, and its orbiting the sun).
But these variations are accentuated and made somewhat less foreseeable from day to day by the interplay between geography, ocean and land masses, and the ever-changing composition of the atmosphere, starting with cloud formation.
All places on earth have the same 4380 hours of daylight hours per (non-leap) year, i.e. half the total duration of a year.
However, they receive varying yearly average amounts of energy from the sun. The earth’s axis of rotation is tilted 23.45° with respect to the ecliptic – the plane containing the orbit of the earth around the sun.
The tilting is the driver of seasons and the existence of solar variants.
It results in longer days, and the sun being higher in the sky, from the March equinox to the September equinox in the northern hemisphere, and from the September equinox to the March equinox in the southern hemisphere.
Direct / Diffuse Light- Solar Variants Continues
As we have noted, the Earth’s atmosphere and cloud cover absorb, reflect, and scatter some of the solar radiation entering the atmosphere.
Nonetheless, an enormous amount of the sun’s energy reaches the Earth’s surface and can therefore be used to produce Photovoltaic or PV electricity.
Some of this radiation is direct and some is diffuse, and the distinction is important because some PV systems (flat-plate systems) can use both forms of light, but concentrator systems can only use direct light.
Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.
- Direct light consists of radiation that comes straight from the sun, without reflecting off the clouds, dust, the ground, or other objects. Scientists also talk about direct normal radiation, referring to the portion of sunlight that comes directly from the sun and strikes the plane of a PV module at a 90-degree angle.
- Diffuse light is sunlight that is reflected off clouds, the ground, or other objects. It obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot be focused by the optics of a concentrator PV system.
- Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight is composed of direct-normal and diffuses components of sunlight. Additionally, diffuse and direct-normal sunlight generally have different energy spectra or distributions of color.