Current Density

The electric field at ESP collection surfaces is not uniform due to both the electrode-collector geometry and to irregular resistance resulting from uneven dust layer thickness. The parameter determined in studying the situation at the collection surface is current density, j (nAcm~2). With poor distribution of current over the dust layer, collection efficiency is diminished. Re-entrainment can occur in regions of low current density, and high local currents can lead to back corona even with a low average current. Land-ham et al. measured the current density distribution in a pilot scale ESP by inserting a segmented copper electrode sensor board into a collector electrode. Conventional, intermittent, and pulse energization with both wire and barbed-strip discharge electrodes were investigated. (Barbed-strip electrodes are used by some manufacturers to force a more uniform current distribution at the collector.)

Intermittent energization (IE) is achieved by interrupting the rectifier circuit output for one to twenty half cycles of the supply power line. A baseline voltage is maintained, with relatively broad pulses superimposed. In pulse energization (PE), pulses of 1 |is to 1 ms width and up to —75 kV are superimposed on a DC voltage set just below the 'spark limit'. Both techniques have been found to save energy and to reduce the incidence of back corona. PE is more effective in countering problems with low resistivity dust. Dubard and Nichols set IE to one full AC cycle on and four off (duty cycle 0.2), and PE base voltage

— 25 kV. Conventional full-wave rectified energization (CE) was set at about —35 kV.

These researchers found that CE and PE give Gaussian current density profiles across the sensor plate surface, while IE gives a more even, but lower magnitude distribution. Under good operating conditions (clean collector surface), PE had less collector area above dust layer breakdown current than CE or IE, with 96% useful area as compared to 83% and 86%, respectively. Under severe back corona conditions, PE maintained 84% useful area, CE 12%, and IE 54%. Similar results were obtained with both barbed-strip and wire electrodes. The results indicate a linear relationship between the increase in collector area receiving useful values of current density and collection efficiency.

Among alternative discharge electrode designs, barbed plate discharge electrodes appear more effective in producing uniform current distributions (see McKinney and Davidson). This design may also have some advantage with respect to flow turbulence.

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Solar Panel Basics

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