Let the Sunshine In

Aug 29, 2007 4:44 PM
By Lou Pechi, President, STRATA-Strategic Advisors, San Diego, Calif.

The most common method of harvesting the sun’s energy is to use photovoltaic cells that convert light energy into electrical energy. The small solar cells, producing low voltages, are typically interconnected into photovoltaic (PV) modules, which in turn are connected to a large PV array that add all the low voltages to produce higher voltages of up to 360 V. Connected to the PV arrays are dc-ac inverters that convert the dc power produced by the modules into alternating current that in turn are connected to the grid.

The main challenge for designers is first to maximize the efficiency of the solar cells and secondly to optimize the conversion efficiency. The output of the PVs varies with intensity, angle of sunlight rays and cell temperatures. To optimize the conversion efficiency of the solar cells, it is necessary to operate at a voltage that will produce the maximum power. This point is commonly called maximum power point (MPP), and tracking this point is referred to as maximum power point tracking (MPPT).

MPPT is not a mechanical tracking system that “physically” moves the modules to make them point more directly at the sun. Rather, it is an electronic system that varies the electrical conversion point of the modules, so that the modules are able to deliver maximum available power.

MPPT varies the ratio between the voltage and current delivered to the system to deliver maximum power. As the voltage of the PV array varies with temperature and other conditions, it "tracks" this variance and adjusts the ratio accordingly. Such a tracking system is usually incorporated into a very efficient dc-ac converter that generates sine waves of 600 V with less than 2.5% total harmonic distortion at efficiencies in the upper 90 percentile.

This energy, transformed into ac by the inverter, is directly fed to the power distribution grid. If the energy of the PV system is not enough, to ensure standard operation, energy is drawn from the grid. When the produced energy exceeds the energy used, the difference is directly fed to the grid, thus becoming available to other users. According to national and local standards and regulations, the produced energy can also be sold to the grid or credited to the user against future consumption.

The cost of solar cells and the conversion equipment is gradually decreasing, while coal and oil prices continue to rise. So as solar and wind power generation becomes cost effective, adoption rates will increase.

In my next article, I hope to examine some of the challenges of wind-power generation which in essence is solar power converted by nature to mechanical force. I would greatly appreciate your comments, which you can send to me at pechil@sbcglobal.net.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.