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must flow in the cable to charge th --Apple-Mail-5-840182924 Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Content-Transfer-Encoding: 7bit Hi all: I would highly recommend this book: http://www.amazon.com/Energy-Problems-Technical-Society-Kraushaar/dp/0471573108 When my son was taking engineering it was the text for one of his courses. I then read it and passed it on to a co-worker. It has lots of worked examples with real-life numbers but you don't need a degree in math to follow them. One of the best parts was the diagram that showed where all the energy in gas-powered car went when driving at highway speeds. If I recall correctly, out of the 100% of energy possible with complete combustion, less than 5% of that actually is used to move the car. The rest is lost as heat, friction, and aerodynamic drag. The latter is the main reason milegage starts going down after 85-90 km/h at high speed The book I read was the first edition. The second edition is now out but I have not seen it yet. Pat On Aug 30, 2012, at 10:14 AM, Christopher Majka wrote: > Hi Steve, > > Perhaps my understanding of High Voltage DC (HVDC) transmission > lines is flawed. It was some time ago that I was looking at this and > I can't quite recall what lead me to this conclusion. Looking at the > Wikipedia entry on high voltage DC I see it says: > > "Long undersea / underground high voltage cables have a high > electrical capacitance compared with overhead transmission lines, > since the live conductors within the cable are surrounded by a > relatively thin layer of insulation (the dielectric), and a metal > sheath. The geometry is that of a long co-axial capacitor. The total > capacitance increases with the length of the cable. This capacitance > appears in parallel with the load." > > Does this indicate that the capacitance of cable itself can store > electricity or am I completely off-base? ;~> > > One interesting thing that Monbiot highlights in relation to HVDC > is, "But most importantly, though the initial electricity loss on a > DC line is higher, it does not increase with distance. On AC > systems, by contrast, the longer the line the more you lose." > > The wikipedia entry elaborates this further by saying, "Depending on > voltage level and construction details, losses are quoted as about > 3% per 1,000 km. ... Where alternating current is used for cable > transmission, additional current must flow in the cable to charge > the cable capacitance. This current flow causes energy loss via > dissipation of heat in the conductors of the cable. Additional > energy losses occur as a result of dielectric loss in the cable > insulation. However, when direct current is used, the cable > capacitance is charged only when the cable is first energized or > when the voltage is changed; there is no steady-state additional > current required. For a long AC undersea cable, the entire current- > carrying capacity of the conductor could be used to supply the > charging current alone. The cable capacitance issue limits the > length and power carrying capacity of AC cables. DC cables have no > such limitation, and are essentially bound by only Ohm's Law." > > In relation to the development of offshore wind energy -- a very > promising area, I gather, since wind blow more strongly and > consistently over the ocean (not encountering terrestrial obstacles) > and there is little NIMBY effect -- Monbiot argues that HVDC lines > could therefore efficiently open up pretty much any area on the > continental shelf (irrespective of how far it is from land) to the > installation of wind turbines. > > Cheers! > > Chris > > On 30-Aug-12, at 2:21 AM, Stephen R. Shaw wrote: > >> Hi Chris, >> Very interesting, never heard of this before. But how is it >> supposed to work as some sort of "battery"? >> >> Suppose you have a 'perfectly' insulated DC high voltage conductor >> line made of copper (except perfect insulation is not possible), >> that is suspended in air and goes for 1700 km or more but doesn't >> connect to anything. When you connect this line to a power source, >> no current will flow into it from the power source if there is no >> external load and no leakage. I don't see how you can store any >> charge/energy at all in a network like this no matter how large. >> Actually the network would lose some charge (DC current) from the >> source through the small amount of inevitable leakage that would >> occur, especially operating at very high voltage. >> >> Something therefore seems to be missing from the argument -- have >> the proponents of this approach found some practical way of making, >> attaching and insulating huge capacitors that can store large >> amounts of charge at high voltages? Recollecting bygone times with >> tube circuits that operated up to fairly high voltages (say 250 >> volts DC rating before electrolytic breakdown): a practical >> electrolytic capacitor of ~20,000 microfarads capacity (not that >> much) already had reached the size of ~4 inches square by ~1 inch >> thick. Scaling this up to a 100,000 volts DC rating would probably >> make it have to be ~400-fold thicker, which sounds impractical. I >> thought that this was the long-recognized problem with storing >> electricity, the impracticability of constructing capacitors with >> enough storage at non-colossal sizes. >> >> So what's the actual storage mechanism, and is there a useful place >> to look this up on-line? >> Steve (Halifax) > > > > Christopher Majka - columnist, Rabble.ca > Halifax, Nova Scotia, Canada > Email: c.majka@ns.sympatico.ca > http://rabble.ca/blog/26142 > "The significant problems of our time cannot be solved by the same > level of thinking that created them." - Albert Einstein > > > > > = = ======================================================================== Patrick Kelly Director of Computer Facilities = = ======================================================================== Faculty of Architecture and Planning Dalhousie University = = ======================================================================== MAIL COURIER PO Box 15000 5410 Spring Garden Road Halifax, Nova Sco