Alan,
losses in HVDC are purely resistive, whereas in AC circuits there are also reactive losses, especially corona effects in which the electric fields induced by the AC interact with the surrounding environment.

Resistive losses are proportional to the square of the current, so increasing the voltage for a given amount of power reduces the losses since it reduces the current. this is true for both AC and DC circuits. Inductive losses, on the other hand, are related to the electric field flux density, which increases with voltage. The reason that HVAC lines are mounted so high is to minimise corona losses due to interaction of the electric field with the ground.

There are a couple of reasons for preferring AC for local distribution, including the cost of terminal equipment and the large existing installed base of AC powered equipment. However, HVDC makes a lot of sense for distribution lines.

Especially those laid directly on the sea floor surrounded by conductive salt sea water!

Has anyone else noticed how opponents of renewable energy almost invariably treat every technical challenge as if it were an insurmountable obstacle?

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Alan B,

Getting energy from where it's generated (and has a low value) to where it's needed (and has a high value) is entirely rational, and the energy required to do so is an order of magnitude less than you think!

What's irrational is your droning on about thorium fission power. It certainly has lots of potential, but we know neither the build cost nor the running cost.

And have you actual evidence that the processed mineral sands from NZ actually have any thorium in them?

India, China and other countries are building an additional 2500 coal fired power plants. This suggests that our sacrificing of our economy will do little to reduce overall CO2 emissions. Further, our atmosphere and oceans contain 38,000,000,000 tonnes of CO2 and this gas moves between the two depending on whether the planet is in the warming part or the cooling part of the climate cycle - we know, for instance, that 96% of CO2 emissions are natural. NASA scientist studying the sunspot cycles say that the current spotless sun means we are now entering a cooling phase of the cycle - as was the case during the Little Ice Age - although the severity of the current cooling phase is still unknown.

Perhaps government funds and efforts would be better directed at coping with the coming cold.

Hi Gerard,
I think fossil fuels (including nuclear) are going to be a vital source of electrical power for perhaps the next 20-30 years, which is of the order of the lifetime of a power plant built today. The transition to renewable sources is going to take some time and as they come on stream we'll be able to retire older and less efficient fossil plants progressively.

There may well be a need for some fossil generation in perpetuity - it's difficult to predict what new technologies will be developed over the next few decades.

On the subject of planetary cooling, I tend to the view that the climatic regime over the past several hundred years is something of an anomaly, but I don't think we know enough to be confident in saying its due to atmospheric carbon increase as a result of human activity. It may well be, but we may have it all wrong. In the meantime, surely it's prudent to keep our powder dry, as it were, by keeping our planetary stores of highly reduced carbon in that form? If it turns out that the climate does enter a cooling phase as seems likely, then all of that would be available to help ameliorate the worst effects. We know that we are past due for a new major cooling cycle that has lead to extended ice age conditions in the past.

It's nice to see some interesting questions being asked, thanks.

I should have also mentioned that I don't see any "sacrificing" of the economy to transition to renewables. Quite the contrary in fact, since there will be new industries developed, which will require new skill sets and new workforce needs.

Our present economy is failing to meet the needs of a large part of the populace. The ABC reported on the weekend that some 13+% of the population is living in precarious economic circumstances. We can't keep pretending that all is golden.