[NatureNS] wind power storage

From: Patrick Kelly <patrick.kelly@dal.ca>
To: naturens@chebucto.ns.ca
Date: Thu, 30 Aug 2012 19:51:26 -0300
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must flow in the cable to charge th

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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
>
>
>
>
>


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Patrick Kelly
Director of Computer Facilities
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Faculty of Architecture and Planning
Dalhousie University
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