One of the most confusing things about solar energy is the mysterious link between the installed power and the actual energy output. To put it another way, if you put a 200-watt solar panel on your roof how many watt-hours of electricity would you produce in a day or throughout the whole year? It is very important to understand that the performance of solar panels is location specific. The same 200-watt panel will produce very different amounts of energy if we install it in Seattle, WA and then take it down to Phoenix, AZ. But the real question is: what causes this difference and how can we set realistic expectations about our solar energy potential?
A gentle introduction to solar radiation
Solar radiation, or insolation, is the “fuel” of all solar energy systems. The performance of solar photovoltaic systems which generate electricity and solar thermal systems which produce hot water all depend on the availability and intensity of solar radiation.
For all practical purposes, solar radiation at the top of the atmosphere can be considered a constant. However, the precise location of a site on Earth and atmospheric effects drastically change the level of radiation at the surface of Earth.
Now let’s have a quick look at the top five factors that affect your solar energy potential.
Due to Earth’s spherical shape, the solar rays have more intensity around the equatorial regions. As we move further away from the equator, the energy density decreases as the solar rays are distributed around a larger geographical region. When it comes to solar energy, it’s a definite advantage to be close to the equator.
2. Cloud cover
Clouds have a big impact on the amount of solar radiation reaching the surface of Earth. They reflect and absorb a significant part of the incoming solar radiation. Therefore, two sites at the same latitude may have drastically different solar radiation levels depending on the cloud cover. On average, clouds absorb or scatter about 20% of the incoming solar radiation.
Aerosols are small particles that float in the atmosphere. By absorbing or diffracting solar radiation, they can act as a filter and decrease the level of solar radiation reaching the surface. Atmospheric pollution and sand storms also have similar impact. While aerosols degrade the performance of photovoltaic systems, their impact is more severe on the performance of concentrating solar applications and solar thermal systems.
The distance solar rays have to travel through the atmosphere is less at higher altitudes. Therefore there is less atmospheric absorption, and consequently, more solar radiation as the elevation increases.
The immediate surroundings of a project site also has a big impact on the level of solar radiation reaching solar energy systems. Vegetation, buildings, even certain landscape features such as hills can block incoming radiation and cast a shadow, drastically reducing the usable solar radiation.
If your solar energy project happens to be located near the equator, in a relatively arid region away from urban pollution, and if it’s quite a bit above sea level with little or no shade around it, you would be the winner of the global solar radiation lottery. Why not Discover solar energy where you live?
Image credits: geography.wa.gov, climatesciences.jpl.nasa.gov, nasa.gov, usgs.gov, epa.gov