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Small scale electricity distribution

 
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adam2
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PostPosted: Sun Feb 17, 2013 1:51 pm    Post subject: Small scale electricity distribution Reply with quote

Doubts exist as to the continued availability of mains electricity, certainly wont be available after TSHTF.
Many threads on these forums refer to making ones own electricity, often from solar or wind power.
Such equipment usually works at 12 or 24 volts DC, it is the accepted wisdom that such low voltages cant be economicly distributed over significant distances, but that use is confined to one building, or very nearby outbuildings.

This may not be the case with a bit of modern technology, and for limited lighting, distribution for 1000M or more is now economic.

Consider the following examples.
Presume a 24 volt system and a basic lighting requirement of 3 lamps each 25 watts or equivalent.
With 24 volt, 25 watt filament lamps, the load will be about 3 amps.
2.5mm cable has a resistance of about 1.5 ohms per 100M, therefore a 50M length will be about 0.75 ohms, and at 3 amps will give a voltage drop of about 2.25 volts which is about the upper limit for a decent light output from filament lamps.
A total circuit length (not distance as the crow flies) of 50M clearly limits the application of such voltages to a single building, or very nearby and small buildings.
Going up to large and expensive 10mm cable gives in theory up to 200M but less in practice because such large cable wont fit light switches or light fittings, therefore part of the run will be in smaller cable.

Now consider the use of the latest type of multi voltage 3 watt LED lamp, these give a similar light to a 25 watt filament lamp and a lot more light than a 24 volt filament lamp run at 21.75 volts.
Not only is the current far less, but full light output is achieved on a reduced voltage.
Presuming 18 volts at the lamps, the total current would be about 0.5 amp, and the maximum permissable length of 2.5mm cable is about 800M ! By increasing the voltage at the supply end to an admitedly non standard 32 volts, then either the load could be increased to 7 lamps, or the distance increased to about 2KM.

For a system under central control, the added complexity of 3 wire DC is worthwhile and gives additional gains.
Since you cant get a center tapped 50 volt battery, presume rounding up to a 52 volt battery with a center tap so as to give a 3 wire system at 26/52 volts.
Again presume the use of about 800m of 2.5mm cable and multi voltage 3 watt LED lamps. Starting with 26 volts and assuming 18 volts at the lamps that gives a maximum permissable voltage drop of 8 volts, or say about 8 lamps connected to between each outer and the neutral, for a total of 16 lamps.

The calculations for different permutations of number of lamps and length of cable runs become tedious, but in general it might be said that "cable runs of hundreds of meters each supplying a dozen or 2 dozen lamps are reasonable"

This certainly suggests that if only modest lighting is required, that a private network operating at 24 volts or at 3 wire 26/52 volts is feasible to supply a fair sized village.
Presuming that the power source is located centraly, then a village up to a few KM accross could be lit thus.
Despite the significant losses in distribution, such a system would probably compare well with a PV module and battery for each house.
And of course a wind turbine or steam engine are viable for a central system.
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Last edited by adam2 on Tue Feb 19, 2013 10:59 pm; edited 1 time in total
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clv101
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PostPosted: Sun Feb 17, 2013 2:21 pm    Post subject: Reply with quote

A good observation! Reducing the current, reduces the voltage drop over the transmission line. Typically this is done by upping the voltage (maintaining the same delivered power at the end). As you quite rightly point out, reducing the amount of power delivered also reduces the current and therefore the voltage drop - effectively extending range.

The point to remember is that although the range has been significantly extended, less power is being transmitted. Of course this doesn't matter because as we all know, we don't demand power, we demand energy services. In this case a well lit room. Switching from filament bulbs to LEDs does allow the utility of a lit room, at much greater distance that was previously possible (and with less total energy use). Just don't be tempted to plug the kettle in!
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Little John



Joined: 08 Mar 2008
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PostPosted: Sun Feb 17, 2013 2:56 pm    Post subject: Reply with quote

Maybe someone needs to reinvent a simple mechanical AC inverter. If they did that, couldn't 240 DC be simply mechanically inverted to 240 AC and then sent over long distances with the added advantage of being suitable for all existing AC devices. The reason i make mention of an inverter being mechanical is that, if simple enough in design, it could then be produced and maintained at a local level with fairly low tech tools.

http://www.youtube.com/watch?v=k46Z8Pg0I4g
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adam2
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PostPosted: Sun Feb 17, 2013 3:41 pm    Post subject: Reply with quote

I rather doubt that mechanical inverters will ever be mass produced again.
They are less efficient, and make a noise, and though simpler to maintain do require regular maintenance unlike an electronic inverter which is fit and forget, until it goes bang at some inoportune moment!

If 240 volts DC is available, then that implies a relatively large system, and I would be inclined in such circumstances to use it "as is" rather than convert to AC.
Many modern appliances that have switched mode power supplies suited for 90 to 270 volts AC will also work fine on 240 volts DC.
Many LED lamps are also suitable for 240 volts DC.

If a lower voltage DC supply is available then I would use it "as is" for lighting.
In a future planned community or the like I would anticipate that 24 volts DC or 3 wire DC at 26/52 volts would be distributed for up to a KM or two as described, for basic lighting.
Given a large enough low voltage DC supply then substantial loads can be powered including grain mills, sawbenches, weaving looms, large water pumps, machine tools and the like. Such loads would have to be located close to the battery bank and power source, perhaps in a shared workshop or other facility.

I would prefer to avoid inverters if at all possible, but if they cant be avoided would want at least 2 complete spares plus a repair manual and parts.
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Little John



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PostPosted: Sun Feb 17, 2013 4:14 pm    Post subject: Reply with quote

adam2 wrote:
I rather doubt that mechanical inverters will ever be mass produced again.
They are less efficient, and make a noise, and though simpler to maintain do require regular maintenance unlike an electronic inverter which is fit and forget, until it goes bang at some inoportune moment!

If 240 volts DC is available, then that implies a relatively large system, and I would be inclined in such circumstances to use it "as is" rather than convert to AC.
Many modern appliances that have switched mode power supplies suited for 90 to 270 volts AC will also work fine on 240 volts DC.
Many LED lamps are also suitable for 240 volts DC.

If a lower voltage DC supply is available then I would use it "as is" for lighting.
In a future planned community or the like I would anticipate that 24 volts DC or 3 wire DC at 26/52 volts would be distributed for up to a KM or two as described, for basic lighting.
Given a large enough low voltage DC supply then substantial loads can be powered including grain mills, sawbenches, weaving looms, large water pumps, machine tools and the like. Such loads would have to be located close to the battery bank and power source, perhaps in a shared workshop or other facility.

I would prefer to avoid inverters if at all possible, but if they cant be avoided would want at least 2 complete spares plus a repair manual and parts.
Would another option be to have 12 volt battery banks placed on the 12 volt "mains supply" at various positions acting as power "reservoirs"? Would such banks effectively drag the power back up to 12 volts? Or am I talking bollocks?
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adam2
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PostPosted: Mon Feb 18, 2013 10:12 am    Post subject: Reply with quote

The use of 12 volt batteries spaced around a system and connected together by relatively thin wires wont work very well if regularly loaded, for example by lighting used every evening.
The battery will partialy discharge every evening and is unlikely to fully charge before the next evening.
Take for example a 200M length of 2.5mm cable with a central battery and charging source at one end, and a smaller battery and a lighting load at the far end.
Presume that every night 10AH is used for lighting, spread over perhaps 6 hours. A fair gues is that 9 AH will come from the local battery and 1AH from the remote battery.
Therefore during the 18 hours of no load, the 9AH consumed the night before has to be replaced, an average charging current of 0.5a being required.
If the central battery is being charged at 13.8 volts and the remote battery is at 13 volts, then with 0.8 volts difference, only a very small current will flow through the 3 ohms cable resistance, not enough to charge the battery. And it is being VERY optimistic to presume that the main battery will be at 13.8 volts ALL DAY, rather than for a few hours.

Batteries distributed around the system work fairly well for emergency or standby purposes as they will eventually fully charge and be available for any emergency need.
Wont work well if a regular load is to be supplied.
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PS_RalphW



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PostPosted: Mon Feb 18, 2013 11:34 am    Post subject: Reply with quote

Another problem I see with all these different voltage and AC/DC options will be the loss of standardisation leading to extra expense and confusion and consumer errors leading to accidents. That was a huge advantage of moving to the 240/V AC supply in the first place. Plug and forget. You won't be able to buy this stuff at Argos.

It is going to a huge shock to most people.
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adam2
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PostPosted: Mon Feb 18, 2013 12:59 pm    Post subject: Reply with quote

RalphW wrote:
Another problem I see with all these different voltage and AC/DC options will be the loss of standardisation leading to extra expense and confusion and consumer errors leading to accidents. That was a huge advantage of moving to the 240/V AC supply in the first place. Plug and forget. You won't be able to buy this stuff at Argos.

It is going to a huge shock to most people.


Yes, for a LARGE backup or off grid power system I would normally recomend 240 volts AC, and for a very large system 3 phase, 4 wire at 240/415 volts so as to permit of standard appliances being used.
This introduces the risk of inverter failure, but on a large system I would expect the inverter(s) to be at least duplicated. At times of heavy load a standard off the shelf generator can also be used.

However for small systems low voltage DC is almost certainly better, both 12 volts DC and 24 volts DC are already fairly standardised for vehicles, boats and off grid homes.
There is little point in use of 240 volts AC if the system is too small to supply typical standard household appliances.

Unless there is a good reason to do otherwise I would normally suggest 12 volts for up to a few hundred watts, and 24 volts for up to a a few KW.
Beyond that, which is a very large off grid system 240 volts AC might be best. Even if an inverter is used for some appliances, lighting and refirgeration are often best supplied at 24 volts from a battery.

In the type of system I proposed in the O/P I was anticipating a load of less than 10 watts per dwelling, for very basic lighting and possibly a radio receiver. A relatively cheap way of providing minimal electric light for a village or community, and certainly not for the modern electric lifestyle. 10 watts is minute by todays standards, but still gives more light than most of our ancestors enjoyed.
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Last edited by adam2 on Sat Mar 09, 2013 5:52 pm; edited 1 time in total
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RenewableCandy



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PostPosted: Mon Feb 18, 2013 10:41 pm    Post subject: Reply with quote

RalphW wrote:
It is going to a huge shock to most people.
Oh-oh. Bad Metaphor Day...
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vtsnowedin



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PostPosted: Mon Feb 18, 2013 11:54 pm    Post subject: Reply with quote

RenewableCandy wrote:
RalphW wrote:
It is going to a huge shock to most people.
Oh-oh. Bad Metaphor Day...
Only one day? What day wasn't?
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adam2
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PostPosted: Tue Feb 19, 2013 10:23 am    Post subject: Reply with quote

RenewableCandy wrote:
RalphW wrote:
It is going to a huge shock to most people.
Oh-oh. Bad Metaphor Day...


Only a small and hardly noticable shock from 12 or 24 volts Very Happy

Seriously though, I would avoid badly non standard voltages for off grid power unless some exceptional circumstance requires this.
For typical "owner installed and owner operated" systems I would go for

12 volts DC, negative earth for small systems
24 volts DC, negative earth for medium systems
240 volt, 50 cycles, single phase AC with earthed neutral for large systems.

For systems under central control and feeding large loads or large areas, I would also consider
3 phase, 4 wire, 50 cycle AC at 240/415 volts with an earthed neutral
Single phase, 3 wire AC at 240/480 volts with an earthed neutral.
3 wire DC at 26/52 volts with an earthed center conductor.
240/250 volts DC
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Last edited by adam2 on Sat Apr 20, 2013 6:21 pm; edited 1 time in total
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