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A simplified guide to backup power sources.
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adam2
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PostPosted: Mon Jul 11, 2016 12:22 pm    Post subject: A simplified guide to backup power sources. Reply with quote

This refers only to backup sources of electricity and not to non-electric alternatives.

Likely sources of backup electricity include primary or single use batteries, secondary or rechargeable batteries, or fuel burning generators of various sorts.

Factors to consider include the amount of power needed, for how long it is needed, and how often, the initial cost, the ongoing cost of consumables, the shelf life of the equipment, and whether AC or DC is preferred or required.

Here are a few suggestions.

Up to 1 watt for up to a few hundred hours. OR up to 10 watts for a few dozen hours.
Disposable batteries are often best. 6 good quality alkaline D cells cost under 5 and will supply 1 watt for about 100 hours. That is every winter evening from dusk till bedtime for 2 or 3 weeks.
Or a transistor radio for a couple of hundred hours.
20 alkaline D cells will supply 10 watts for well over a dozen hours, maybe 20 hours.
Shelf life is many years and no servicing or other attention is required. Very safe.
Alkaline D cells are usually the cheapest readily available type of battery and should therefore be used if possible. Air/alkaline cells of THOUSANDS of amp hour capacity do exist but are a bit specialised and not much used.

Small household type rechargeable cells are useful for day to day use but cant be recommended for most emergencies due to limited capacity, short shelf life, and concerns regarding availability of charging.

Up to about 10 watts long term or up to about 100 watts short term.
A deep cycle lead acid battery is often the best choice, charged from grid power, renewables or a generator.
A relatively cheap battery of about 110 AH capacity will supply 10 watts for over 100 hours, or 100 watts for about 8 hours.
AC current may be produced via an inverter, but remember that this adds cost, losses, complication, and a single point of failure.
When not in use regular charging is very important and even well looked after it is unlikely to last more than 5 years.
Charging may be from the mains if available, or from a vehicle if fuel be available, or from PV. You will need an astonishingly large PV array to charge the battery in a reasonable time in UK winter conditions.

Up to about 100 watts long term or up to about 1KW short term.
A battery as above but of about 10 times the capacity is possible, but quite an expense and very large and heavy. Battery banks of that size are a significant risk in case of accident or misuse.
A basic petrol driven generator will be cheaper in initial cost and need less maintenance, and have a longer shelf life.
Consider the risks of handling petrol and remember that only 30L may be lawfully stored.
The noise of a generator might attract most unwelcome attention.

Over about 1KW batteries are only viable for short term use and a generator almost essential. (loads of a lot more than 1KW are routinely supplied by large battery banks with ample PV and/or wind turbine input, and such schemes have much to commend them, especially for off grid premises. Much too costly though just for standby or backup power)

A generator over a few KW should be diesel driven not petrol, due to the fuel cost of petrol and the storage limitations.
If properly serviced a diesel generator should last decades, I know of a couple that are 50+ years old and are as good as new.
A portable(ish) diesel generator is very vulnerable to theft and makes an undesirable noise.
A permanently installed one is less of a risk.
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fuzzy



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PostPosted: Mon Jul 11, 2016 1:03 pm    Post subject: Reply with quote

I suppose I have never done much digging, but I am suprised no one has modified or sold a small domestic generator to run on methane. There is no way they will be turning the domestic gas off unless it is armegeddon. I suppose we are all scared witless of the endless government regs re gas and electricity.
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adam2
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PostPosted: Mon Jul 11, 2016 2:10 pm    Post subject: Reply with quote

fuzzy wrote:
I suppose I have never done much digging, but I am suprised no one has modified or sold a small domestic generator to run on methane. There is no way they will be turning the domestic gas off unless it is armegeddon. I suppose we are all scared witless of the endless government regs re gas and electricity.


Small and larger generators are available for methane, but as you have found they are not exactly common.
As you have surmised, a number of regulations apply and are enforced with some vigour.
I suspect that there may be an unofficial policy of discouraging natural gas powered generators since any widespread use of same could result in extra demand at times of shortage.

A portable petrol or diesel generator may be purchased for cash, taken home and put to use without TPTB knowing. A natural gas powered unit requires a gasafe engineer, who will no doubt insist on a part pee approved electrician, who will notify the local authority who no doubt will have other requirements.
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PS_RalphW



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PostPosted: Mon Jul 11, 2016 3:18 pm    Post subject: Reply with quote

In an ideal world, how about

1. off grid PV systems with batteries to provide power in summer. Excess
production could be used to heat water, etc.

2. A wood pyroliser system to produce wood gas to drive a Stirling engine generator and waste heat used to heat your house and/or hot water, etc.

3. An intelligent electronic control to balance all parts of the system.


Not a new idea, just mixing the best ideas I have seen on the web for my local needs.

Now all we need to do is design, finance, build and market the system

Very Happy
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kenneal - lagger
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PostPosted: Mon Jul 11, 2016 6:18 pm    Post subject: Reply with quote

A deep cycle battery should not be discharged more than 50% if you want it to last five years. So, a 110Ahr battery should only have 55Ahrs taken out of it. If you need 110Ahrs you need two batteries connected in parallel. I know because of personal experience with my system.
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adam2
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PostPosted: Mon Jul 11, 2016 8:15 pm    Post subject: Reply with quote

kenneal - lagger wrote:
A deep cycle battery should not be discharged more than 50% if you want it to last five years. So, a 110Ahr battery should only have 55Ahrs taken out of it. If you need 110Ahrs you need two batteries connected in parallel. I know because of personal experience with my system.


Agree entirely if for regular use as in an off grid home. opinions differ as to the optimum level of regular discharge, but definatly 50% or less.

For standby, emergency or backup use the position is rather different. A FEW deep discharges wont have much influence on battery life.
Even a cheap "deep cycle" battery should survive several dozen deep discharges, maybe as many as 100 deep discharges.

If a fully discharged battery fails after 50 cycles, that is only about 7 weeks of daily cycling and the need for a larger battery is clear beyond all doubt.

For standby use, 50 cycles is 4 years of monthly power cuts, and that is making the very pessimistic assumption that EVERY power cut is long enough to fully discharge the battery.
In practice most of the power cuts will be only a few hours and the battery not deep discharged.
I would expect the battery to die of old age before it is killed by deep discharge in such cases.

For domestic standby use I would be inclined to size a battery for about 25% discharge in a 3 hour rota cut. That will give an excellent cycle life in the event of frequent or numerous rota cuts.
In the hopefully rare event of a much longer power cut then I would accept the odd 100% discharge.

A 120 AH battery loaded to 10 amps for a 3 hour rota cut will be about 25% discharged. The worst reasonably foreseeable rota might have 3 such cuts on the same day.
First cut from 06-00 until 09-00, probably not a full 30AH used as one might be either at work or en route for part of it, or if not working might stay in bed for part of it.
Second cut from 12-00 until 15-00, again not likely to be the full 30AH used as it would be daylight.
Third cut from 18-00 until 21-00, use of the full 30AH is probable.

Since 120AH batteries are widely sold and reasonably affordable, that to me suggests designing for a sustained load not exceeding 10 amps.
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woodburner



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PostPosted: Mon Jul 11, 2016 11:29 pm    Post subject: Reply with quote

Forget the wood pyroliser unless you live in or next to a wood/forest. It needs a substantial piece of hardware, and is very inefficient. It gets through large amounts of wood, which could provide home heating, but it is not as effective as a woodburner for that purpose. Sweden used pyrolisers during WW2, but they had no oil and loads of forest, it was not a sustainable exercise.
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adam2
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PostPosted: Tue Jul 12, 2016 11:53 am    Post subject: Reply with quote

woodburner wrote:
Forget the wood pyroliser unless you live in or next to a wood/forest. It needs a substantial piece of hardware, and is very inefficient. It gets through large amounts of wood, which could provide home heating, but it is not as effective as a woodburner for that purpose. Sweden used pyrolisers during WW2, but they had no oil and loads of forest, it was not a sustainable exercise.


Agree, MIGHT be worth considering if you have a great deal of wood, but much too large, costly and complicated if just for emergency or standby use.
Likewise a wood burning steam boiler and a steam engine might perhaps be justified in some exceptional cases, but not normally for domestic standby power. I have seen one that produced about 25KW at 120 volts DC but it needed about a ton of wood an hour.
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vtsnowedin



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PostPosted: Wed Jul 13, 2016 2:51 am    Post subject: Reply with quote

adam2 wrote:
woodburner wrote:
Forget the wood pyroliser unless you live in or next to a wood/forest. It needs a substantial piece of hardware, and is very inefficient. It gets through large amounts of wood, which could provide home heating, but it is not as effective as a woodburner for that purpose. Sweden used pyrolisers during WW2, but they had no oil and loads of forest, it was not a sustainable exercise.


Agree, MIGHT be worth considering if you have a great deal of wood, but much too large, costly and complicated if just for emergency or standby use.
Likewise a wood burning steam boiler and a steam engine might perhaps be justified in some exceptional cases, but not normally for domestic standby power. I have seen one that produced about 25KW at 120 volts DC but it needed about a ton of wood an hour.

I'm not following your math here. Or as you would say maths. (I don't know why you Brits think every math problem is plural). 25KW at the USA standard of 120 volts comes to 208 KWHs per hour and my house uses just over 320 KWHs per month ? Could you straighten that out for the rest of us that are confused?
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kenneal - lagger
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PostPosted: Wed Jul 13, 2016 5:05 am    Post subject: Reply with quote

vtsnowedin wrote:
....... I'm not following your math here. Or as you would say maths. (I don't know why you Brits think every math problem is plural).


We usually speak of mathematics rather than mathematic when talking of even one calculation. I've never heard anyone, even a 'Merican, talk of mathematic.

Quote:
25KW at the USA standard of 120 volts comes to 208 KWHs per hour and my house uses just over 320 KWHs per month ? Could you straighten that out for the rest of us that are confused?


There is the efficiency of conversion of the wood to gas and then the efficiency of the conversion of gas to electricity to consider. The latter is usually about 33% efficient using an ICE and added to whatever the former is the overall efficiency is going to be less than 30% efficient so you would need over 75kW, maybe over 100kW, of energy from the wood.
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woodburner



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PostPosted: Wed Jul 13, 2016 8:20 am    Post subject: Reply with quote

It's much worse than that (and I would expect it to be very low efficiency) if your plant was using a ton an hour. That would represent at least 3,000kWh. So 25/3000 = 0.8%. I understood that small scale conversion was about 2% at best. We haven't accounted for the felling and processing of the wood yet. Electricity production is, basically, diabolical.

Figures for biomass fuel. I am surprised that there is 3+MW in a ton of wood chip though.
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adam2
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PostPosted: Wed Jul 13, 2016 8:36 am    Post subject: Reply with quote

Care should be taken not to confuse kilowatts and kilowatt hours.

A kilowatt is a rate of producing or consuming electricity, examples include "this is a 3 kilowatt water heater" or "in good sunlight my solar array can produce 4 kilowatts"

A kilowatt hour is a quantity of electrical energy. Examples of proper use include "I normally use 600 kilowatt hours a month" or "my local power company charge me 20 cents per kilowatt hour" or "my solar array produced 4,560 kilowatt hours in the last 12 months"

There is no direct relationship between kilowatts and kilowatt hours, it depends on for how long the electricity is produced or consumed.

Voltage is not relevant, a kilowatt is a kilowatt at any voltage. Electrical equipment has of course to be correctly selected for the voltage.
A 1KW heater designed for and used on 120 volts will produce exactly the same heat as a 1KW heater designed for and used on 240 volts. And presuming a similar tariff, it will cost the same to run.

The electrical service to a home is usually rated by the number of kilowatts available. A common figure in the UK is 24KW , this means that up to 24 kilowatts may be used.
The electricity bill will be largely based on the number of KWH consumed.
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PostPosted: Wed Jul 13, 2016 9:25 am    Post subject: Reply with quote

kW and kWh are not a problem when the time is taken as 1hr and we are running ar the stated output for that time.
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adam2
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PostPosted: Wed Jul 13, 2016 10:19 am    Post subject: Reply with quote

woodburner wrote:
It's much worse than that (and I would expect it to be very low efficiency) if your plant was using a ton an hour. That would represent at least 3,000kWh. So 25/3000 = 0.8%. I understood that small scale conversion was about 2% at best. We haven't accounted for the felling and processing of the wood yet. Electricity production is, basically, diabolical.

Figures for biomass fuel. I am surprised that there is 3+MW in a ton of wood chip though.


Yes, the figures that I quoted are only approximate, we did not weigh the wood, and the output was rather variable with 25KW being a fair estimate of the average load.
But yes, wood into electricity on a small scale has a very low efficiency, in the low single figures.
Probably only justified in special cases such as a saw mill utilising waste wood to generate electricity to light their premises, and/or power machinery.

Dry wood in chips or logs contains about 4KWH per kilo, or 4MWH per ton, perhaps a bit more if well dried.
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vtsnowedin



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PostPosted: Wed Jul 13, 2016 11:21 am    Post subject: Reply with quote

Well Americans say do the math not do the maths. It's all arithmetic to me. Smile
American house services are rated by Amperage. My house has a 200 Amp entrance which is large enough for me to switch to electric heat in my dotage. Unlike the UK most of the wall receptacles are 120 volt with a 20 amp circuit breaker (ground fault type) at the breaker panel. Only the clothes dryer and my arc welder are on 240 volt circuits.
An appliance plugged into a 120 volt circuit that draws 10 amps would use 1200 watts or 1.2 KWH in a hour of use.
The efficiency of a wood fired generator system could be greatly improved in the winter if the heat being wasted up the smoke stack was recovered and used to heat the building. I remember a factoid that a Chevy V-8 gives off enough heat through the radiator and tail pipe to heat five houses.
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