Unusuall PV installation

[adam2]

Thought others might be interested in a rather unusuall PV system, at present in the planning stage.

The customer owns a large, formerly energy intensive home, they are well off but still wish to reduce FF use, and to prepare for disasters in general.

Previous preps have included, better insulation, new oil burning heating boiler, larger heating oil tanks, solar thermal hot water, keeping a riding horse, 3 ! wood burning stoves, many tones of wood and coal stored, food stores, rainwater capture etc.

They are now contemplating a large battery/wind/PV system for essiential lighting, and limited power.
The maximum load is estimated at about 2KW, with an average of 500 watts.

A 12 volt system wont be practicle owing owing to the size of the system and the many outbuildings requiring lighting.

A 24 volt system would be better, but still require extremly large cables to keep voltage drop to an aceptable figure.

A 240 volt AC system useing a low voltage battery and an inverter would appear suitable, but the owner does not want to be reliant on a inverter (despite the fact that they could well afford to keep a spare, or even several)

The choice therefore comes down to

1) 120 volts DC from a 120 volt battery bank, this is simple, and for a large system, cheap. No inverter to fail, standard size cables can be used, and standard light fittings with 120 volt lamps can be used.
120 volt power tools can be used provided that they dont use induction motors.
The drawback is a relativly dangerous system, potentialy more dangerous than 230/240 mains.
(definatly a case of "back to the past", as the premises used to have a 120 volt DC system, useing batteries and a steam engine)

2) A 48/50/52 volt DC system with basic lighting worked at the battery voltage, and an inverter being tolerated for other loads.
The drawback being extra complexity, and requireing 50 volt incandescent lamps., or special flourescent fittings.

3) A three wire DC system operating at either 24/0/24 volts or 26/0/26 volts.
This would still require relativly large cable sizes, but more manageable than a straight 24 volt system.
Standard light fittings with 24 volt lamps could be used, and standard light switches also rather than the specials needed for 120 volts DC.
This system could work high efficiency 24 volt refrigeration as well as lighting.
If an inverter was required later, these are readily available for 48/52 volt input.

I would favour option 3, though the customer is more keen on option 1, matter still under discussion.

[sam_uk]

Not really an answer to your question but a slightly different approach;

Get a 240V CHP plant. One that can run on gas or wood chip.

Plant an ash coppice and run the system on gas until the coppice is ready for harvesting. Then use electric tools and chippers to harvest the Ash..

That way you can use the heat energy for heating the house, and perhaps sell a couple of those wood burners..

just an idea..

Cheers

Sam

[adam2]

CHP plant is being actively considered, though it would be oil burning, not biomass.
Diesel CHP is much simpler than biomass, but does need an external fuel source.
In a perfect world we would burn no oil, if however one accepts oil use, then CHP plant makes better use of the oil than a boiler or a standard generator.

They would still require battery lighting though, for extensive premises. The grid supply is already unreliable in the area, and any single engine CHP plant, though useful, cannot be considered a secure source of power.

The three woodstoves dont see a great deal of use at present, they were installed a few years ago as backup in case heating oil was unavailable.
Oil is stored for two years normal use, and fire wood for an additional few years.
In the long term, numerous trees could be felled for fire wood, but at present they prefer to look at the trees and buy heating oil.

[kenneal - lagger]

How about a CHP system running for a limited number of hours a day charging a 12 or 24V battery bank, with possible backup from PV and/or wind, and giving power for larger appliances. The battery bank can then power lights and smaller appliances, 24/7. The genny then runs less but more efficiently at a higher power rating.

[adam2]

The initial requirement is for a relativly large battery backup system.

In the longer term, diesel CHP plant is being activly considered.

The anticipated operating regime would be
HEATING SEASON- Grid supply to be used during off peak hours, with CHP plant at other times.
NON HEATING SEASON- grid supply to be used 24/7, with the CHP unit run only in case of power cut, or for testing.

At any time, lighting would be available from the battery in case of loss of other power sources.

I believe that a large (2,000 A/H +) 24 volt battery and an inverter is the best solution, however the customer is against inverters.

I would install two consumer units, essiential supplies and non essiential.
The essiential circuits would be most lighting, refrigeration, IT, and radio, TV and phone equipment, together with heating pumps.
The esiential loads would be supplied by the inverter/grid/CHP with seamless changover. The load being limited to 5,500 watts.

With battery charging from both grid and CHP any PV input would be a needless complication and best avoided.
A standard grid tied PV system being more efficient.

The battery and later the CHP unit would give good protection against short or medium term power cuts.
In the event of TEOTWAWKI then the PV modules could be re-purposed for battery charging.

This however means convincing the customer that an inverter is the way forward.
At present they would prefer lighting to be supplied direct from a battery, which would have to be at least 120 volts.
I hope to convince them that pair of 5.5 KW inverters (duty and standby) is the way forward.

[sam_uk]

I agree that inverters seem to be the way forward.

"The battery and later the CHP unit would give good protection against short or medium term power cuts."

Why not get a true multifuel Stirling engine?

If your customer thinks that TEOTWAWKI is a possibility then having a system that will generate power from biomass would seem to be a easy sell?

Use oil rather than gas to power it in the meantime, but to me it seems a bit strange to go to all the trouble and expense of installing a CHP/Stirling engine and then not get one that will run on biomass.

2p

Sam

[scott]

modern pv inverters have 5 to 20 years factory backed warranties
for repair or swap out
tested for huge MTBF rates these days
or run 2 redundant inverters through two consumer units
with shunt or motor operators for dc/mains failure fitted to mcbs
chances of both failing are virtually zero
weak part of a off grid system is always the battery
unless using advanced deep cycle long life gel/agm or lifepo4 types
with up to 9000 cycles made these days to 9000ah
2kw pv grid inverters are well under £2k with 20 year warranty
with built in diagnostics via pc/gsm and remote functions in event of failures

[Bandidoz]

I'd favour the first option, or perhaps even a dual system of (1) and (2) where (1) is less accessible. I don't trust inverters for reliability either.

120VDC should be fine if handled wearing shoes. Even hand-to-hand the current across the body (20k) would only be 6mA.

http://en.wikipedia.org/wiki/Electric_shock

Are there devices such as RCDs for DC safety? Regulator with foldback current limiting?


Possibly even have a 3-wire system, grounded in the middle, i.e. +/-60V - so a short to ground is from a lesser potential. You could then run DC or AC through the cabling - if the lights can run from both AC and DC then you could normally use the battery system, and hook up a 120VAC genny to cover a failure?

[emordnilap]

What's going on here:

http://www.power4home.com/index.php?hop=ttinc1

??

[scott]

solar rcd for dc
www.abb.com
rcds to power stations
if i can help scott
pv 20watts to 30,000kw

[adam2]

The matter is still being discussed, but a 120 volt battery appears the likely solution.
Charging normally from a 120 volt nominal solar array, with grid charging as backup.
The proposed diesel CHP plant would also charge the batteries if required.

The owner is not keen on inverters, despite the increased reliability of modern units.
An inverter of several KW would be required, in duplicate.
An inverter would not be very efficient running a single lamp, as might be the case at 3 am.
An inverter for 120 volt input could be considered for power, but lighting is desired direct from a battery.
The premises are very extensive, with cable runs of well over 100M in the house, and 800M to outbuildings.

A friend of the owner posseses a restored showmans engine, as used at fairgrounds, and this could perhaps be used for battery charging if all else fails, it is fitted with a 120 volt dynamo of about 250 amp capacity.

110/120 volt CFLs are readily available, though doubt exists as to suitability for DC.
Testing required.

[kenneal - lagger]

Adam2

I've got a problem with a connection to a wind turbine. The turbine is a 12V 600W machine so I've got about 50A to move about 50m which gives an horrendous cable size. I was wondering if it would be possible to transform the 12V DC to 120 or even 240 v DC at the turbine and then back again at the battery connection.

Feasible? If it would work, any ideas on where to get the transformers and what the cost would be?

[adam2]

True transformers are only for AC , however the term DC transformer is in common use for what should more correctly be termed a DC to DC converter.

Converter are readily available to change from 12 volts to 24 volts, and vice versa.
Other voltages exist but are less common.

The better converters have isolation between the input and the output, therefore by useing a 12 volt to 24 volt converter, one may obtain 36 volts by connecting the input in series with the output.

All this entails considerable expense and complication, and additional losses , and I fear that the wind turbine may "dislike" being connected to an electronic converter rather than a 12 volt battery.

If the load at 12 volts is a steady say 5 amps, then put the battery near the turbine.
The long cable will then carry the 5 amp load current rather than the variable but up to 50 amps turbine output.

If the load is at line voltage AC then place the battery and inverter close to the turbine, the long wires then only carry a few amps at line volts.

If neither of the above are possible, then I would look for 50 M of very thick cable, cheap.
Scrap merchants often have secondhand cable very cheap, or offcuts of new cable.
Even if you need cable as big as 100mm, that may prove no more costly than DC to DC converters, and would certainly be simpler.

Such large cables would almost certainly have to be jointed to something smaller to fit terminals.
Ensure that this smaller cable is still as large as possible, and as short as possible.
Remember that just 10cm of 10mm cable has added as much resistance as another metre (2% !) of the 100mm cable.

The current is probaly about 40 amps for a 600 watt turbine, since the output will be nearer 15 volts when charging a 12 volt battery.

Do you know what the output voltage of the turbine is ?
Some types generate at relativly high voltages, and step down to battery charging voltage in the control unit.
If this is the case, then place the control unit close to the battery, and run 50M of thinner cable between turbine and control box.
(the turbine output may be three phase in this case, in which case you will need three core cable)

Alternatively could you use 24 volts instead ? cable sizes, though still substantial will be only 25% of those needed at 12 volts.

If you loads are all 12 volt, then consider a three wire DC system from the battery.

[kenneal - lagger]

The set up is a bit complicated because I've got a 20m long cable run from a 30A PV array on the house, in the opposite direction to the wind turbine, to a Powermaster 3kW inverter/charger/solar controller which is sited next to the batteries. The batteries are connected to the Powermaster by a 1.5m long 100 sq mm cable.

Also I want the dump load for the turbine to be an immersion in the HW cylinder in the house so that I don't waste any power produced. The HW cylinder is almost next to the PV array about 15m cable length away from the Powermaster. The turbine controller would be near the Powermaster.

Another electrician has suggested that I run a 25 sq mm armoured cable on posts direct from the turbine to the battery shed using the armouring as the negative. This would get the cable into the air and shorten the distance to about 25 metres. I can coppice enough poles to get the cable up to 5m or so high. I don't really want a cable floating about in the air but if needs must...

Hope all that makes sense, Adam.

[adam2]

Well since the PV array cant reasonably be moved, nor the batteries, nor the inverter, it looks as though a very thick cable will be required.

In view of the large cable size required, I would certainly go for the shortest possible route.

I can not recomend useing the armouring of steel wire armoured cable as the negative.
There is nothing electricly unsafe in so doing at such a low voltage, but the armouring will probably have a greater resistance than one of the cores, since it is made of steel not copper.

Large armoured cables are commonly 4 core (intended for 3 phase and neutral mains) and could be used with two cores for positive and the other two for negative.

At such a low voltage, armoured cable is not required IMHO, and is very heavy unsightly stuff to put on posts.
If you can obtain it cheaply from a scrap merchant, then welding cable is worth considering.
Alternativly bare conductors could be considered if out of reach, even at 12/15 volts they should be out of easy reach if bare.

11 KV lines erected by power companies increasingly use copper conductors with a lightweight green plastic covering.
For 11 KV use these must be mounted on insulaters and high up, for 12/15 volts they may be simply nailed to posts, the lightweight covering being ample insulation at such a low voltage.
Scrap merchants sometimes have offcuts of such cable.