The Gross Inefficieny and Ridiculous Cost of Solar Power

PV is not the most promising form of solar energy conversion for large powerplants. CSP is building up more steam than silicon-based modules in that application.

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No. PV solar is usually a red herring employed - as in the OP here - to distract political attention from the realistic and attractive forms of large scale solar power generation.

 

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You've got it backwards. DC is subject to the same resistive loss as AC is, at a given voltage, but is less susceptible to losses from reactance and corona discharge than AC.

There are factors that we generally ignore in simple, everyday electrical circuits, such as the inductance of a straight piece of wire and capacitance between parallel conductors in a line. A piece of wire carrying AC is surrounded by an oscillating electromagnetic field. This electromagnetic field induces eddy currents in anything around it even remotely conductive, including conduit, support towers, vehicles, people, saltwater, and minerals within the earth itself. As these objects and materials all have their own electrical resistances, some of the circulating current within the transmission line is lost to these parasitic eddy currents along the way. Likewise, there is a capacitance between adjacent conductors in a line, and between the lines and ground. This capacitive reactance imposes its own loss. Not even superconducting cable can escape these phenomena. These losses can become very large when we start making the wires hundreds or thousands of miles long. There are photos of lit fluorescent tubes near transmissions lines that serve as a neat example. Obviously, dumping all that energy into the surrounding environs hurts the efficiency of the lines. In particular, the inductance loss in seawater (a medium with very low electrical resistance) is so obscene that DC transmission is cost effective over much shorter distances than it is for dry cabling.

DC transmission lines also suffer lower losses from corona discharge than AC lines do. A DC corona point is a continuous ion source, whereas an AC corona point is a momentary ion source anytime its antinode potential rises above the breakdown voltage of the surrounding fluid. The steady ion stream of a DC corona point builds up a space charge around it that acts as a sort of buffer zone, effectively keeping the surrounding air from touching it and draining more energy away. When voltage first comes up on a DC line, there is a transient load on the line as the space charges stabilize, but after that the losses to corona discharge are negligible. The example that best illustrates this phenomenon is the sound that is audible while standing near AC transmission lines on a wet day. A DC line of equal peak voltage would be nearly silent; only audible when its surrounding fluid is disturbed and the space charge leaks away, such as from a strong gust of wind or incident rainfall. The low pitch buzzing/hissing characteristic of AC lines is caused by the space charges building up and collapsing 120 times each second.

More than one, actually. Off the top of my head, in no particular order:

• Alternators that produce AC are simpler, cheaper, and more reliable than DC generators
• AC can pass through transformers; DC can't
• AC is easier to produce a revolving magnetic field with, and thus works great with motors
• DC transmission is only economical in very long-haul scenarios, or submarine cables through seawater, due to the complexity, expense, and reliability issues with rectifying and reinverting it
• AC produced by an inverter generally has a dirty waveform as compared to AC produced by an alternator, on account of being synthesized from a PWM square wave rather than a rotating magnetic field
• Until relatively recently, thyristors could not be scaled thick enough to handle the necessary voltage (500+ kVDC) without breaking down
• Tesla + Westinghouse > Edison

Note that the power transmitted in modern HVDC links isn't quite "true DC" in the way most people think of it as having a constant voltage, like a battery. Modern HVDC is supplied from three phase AC power that gets rectified, but not filtered. With 60* between each phase antinode, the rectified output is rich in harmonics. There can still be some reactive losses from the power contained in the harmonics, though it won't be anything like what exists in current AC systems. This is really just an academic point though.

 

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