YKES THAT’S HOT! The sun bathes the earth with more energy each minute than the world consumes in one year. But, except in the tropics, the sun is never directly overhead and its intensity varies by season. For example, at a latitude of 45°, solar radiation may vary from 92% (early summer) to 38% (early winter) of theoretical maximum insolation. The average intensity at this latitude is 71% (early spring and fall) of maximum.
At higher latitudes, solar radiation also follows a longer path through the earth’s atmosphere. Scattering and absorption of incident and reflected radiation by gases such as CO2, methane, chlorofluorocarbons, and particulates further influence the solar resource available. These global conditions– plus local variations such as cloud cover, topography, and altitude– cause solar to be a variable resource.
The evolution of PV modules has paralleled the successes of PV cells. Module is the term used to identify a grouping of interconnected PV cells into an enclosed, environmentally sealed package. Modules utilize a transparent front material (or lens for a concentrator module), a cell and cell encapsulant, and a back cover material. Today’s PV module is considerably improved compared to versions produced only four or five years ago. This rugged package is available with several different front materials, low-iron or conventional glass being the most prevalent, but polymer materials also are being used successfully by a few manufacturers.
Modules can be characterized as flat plate or concentrator systems. The advantages of flat plate collectors are that they are simpler to design than concentrator systems. They can use all the sunlight, both direct and indirect. There is complete flexibility in tracking systems (fixed, one-axis, two-axis). The disadvantage is that a large number of PV cells are needed to cover enough area to produce power comparable to that of other types of solar energy generation systems including concentrator PV systems. However, the disadvantages of solar concentrator systems are their need for direct sunlight.
In general, concentrator systems with one- or two-axis tracking are designed for semiarid and desert regions and are geared toward utility scale applications. Homeowners living under arid conditions will use flat plate systems with your choices being either CdTe or silicon based. CdTe panels provide power on overcast days due to the fact that they create energy from ambient daylight. These panels continue to churn out power through all types of temperature changes ranging from freezing to desert conditions.
A small solar electric or photovoltaic (PV) system can be a reliable and pollution-free producer of electricity for your home or office. Small PV systems also provide a cost-effective power supply in locations where it is expensive or impossible to send electricity through conventional power lines. However, the amount of power generated by a solar system at a particular site depends on how much of the sun’s energy reaches it. Before you buy a PV system, you will want to be sure your site has enough solar energy to meet your electricity needs efficiently and economically. Your local system supplier can perform a solar site analysis for you or show you how to do so on your own.
See You Next Time! Dr. Stripling