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Tuesday 21 April 2015

Direct current (DC) for off-grid photovoltaics?  

A little historical reminder of the current standards

More than a century has passed since the end of the war of current. But what was it all about? And why did alternate (AC) won over direct current (DC) generation?  Well simply put, it was just about current production and distribution! For more info on this check out the Wikepedia page here. Well it makes perfect sense that AC currents won, given that at the time of Thomas Edison and Nikola Tesla solid state electronics did not exist and that power generation from sustainable (local) sources wasn't at all considered an important criteria! Indeed,  DC current distribution implied that, to avoid losses in transmission through the wires, current should have been generated in close proximity of the consumers households since there was no convenient method to elevate the tension for long distances current transport, and subsequently lower the tension at the point where it were to be consumed. AC current was much better suited to the latter task using power transformers, which had been developed at the end of the nineteenth century (remember Maxwell-Faraday low of induction, transformer can only work with an alternating current). Then AC won and household appliances were developed specifically to function on AC current standards (mainly 2 standards ACV110 and ACV230) (Check this video from the "Post Apocalyptic Inventor" for a very good explanation on current generation and transport and transformers; I strongly recommend you check his Youtube channel if you are interested in electricity related subject).
Since the times of Edison and Tesla things have evolved quite a bit, especially in the area of power electronics, which at that time did not exist. With the invention of power transistors (and especially MOSFETs and IGBTs) lowering a DC voltage source has become much easier. The principle still relies on feeding an alternate source to the primary winding of a transformer, except that now the alternate current is fabricated by chopping a DC voltage source at a given frequency using a power transistor. This principle, most commonly known as  "inverter" is used nowadays in virtually all domestic appliances power supplies (so-called "switched mode power supplies", see here for an outstanding tutorial video series) which supplanted the old fashion primary transformer based power supplies. In other words, instead of using the properties of the 50/60Hz AC mains supply to reduce the voltage in our house hold appliances, the AC input is actually rectified to a positive DC voltage internally and subsequently chopped by means of a power transistor at a higher frequency (typically a few tenth of KHz) and fed through a transformer out of which the voltage comes out reduced to a given value. This method has many advantages, the first one being the extremely high achievable transformation yields (>90%), the very significant reduction of the transformer size (and therefore inducing a reduction in weight), and the ease of output voltage control.There are many other appliances which work on a similar principle, like variable frequency drives, air conditioners, welders....
So I'm getting to the point: One may ask himself, why would we question a current standard which has been established more than century ago? Taken out of context, there is no real motivation for that, but in the case of photovoltaic solar cells, where the current generated is DC type, and that power is generated in close proximity to where it is being consumed, there is a clear advantage if the current could be used straight out of the solar panels, rather than having to feed it to an expensive sine wave inverter which converts the DC output from the solar panels to a usable AC power source. In the following blog we describe the development of an off-grid photovoltaic system which takes advantage of DC power generation to provide for:

-A cheaper alternative to photovoltaic installations
-A more efficient way to use energy, using it when it is available, and storing it into different forms (electricity, heat, cold, potential energy).
-Automatize consumption according to the DC voltage output.

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