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Join Date: Aug 2008
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Green Power and Amp Hours

03/15/2010 2:34 PM

Considering our efforts to go GREEN and the sources of power through wind and PV, unless we are very rich, we may not be able to configure a 15KW power source and have to rely on a more modest configuration that can supply a more modest but a steady supply of power during the day for extended use at consumption peak times.

So how does one size a reasonable battery storage array to meet the needs of a household presuming one lives in Southwest Florida when using Wind, PV or both?

Prolly would be best to configure and discuss the array in AMP HOURS vs specific battery configurations.

--Duck

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#1

Re: Green Power and Amp Hours

03/15/2010 4:11 PM

Use your highest monthly electrical power bills kilowatt hour number and divide that out into how many days there are in that month. That will be your realistic minimum kilowatt hours of safe working reserve power that you would want for one single day.

My average monthly power consumption is around 1000 KWH per month. So 1000/31 = 32.26 KWH per day.

Amps times volts equals watts. That would mean that I would need 32,260 watts divided by my battery voltage plus at least 30% extra for system losses so that I am not draining my batteries flat on that type of load in 24 hours. 32260 x 1.30 = 41,938 watt hours. If I used a 120 volt DC battery bank that would mean I would need 42,000/120 = 350 Amp hours of battery capacity. Which works out to be about 35 12 volt 100Ah deep cycle batteries. I would need 40 if it was a 120 volt system, four banks of ten in series to make 120 volts, for one days reserve run time without concerns for over draining the batteries to the point of doing damage.

On top of that you also have to figure out how many days you could realistically end up without sufficient wind or sun to recharge the batteries. So if its ten days then you would need ten times more battery's to cover it. At this point you also have to consider how fast you need to recharge those battery's as well.

For me if I chose to have the capacity to recharge all of that ten days worth of battery power in 48 hours I would need a a combined continuous power source capable of 480 KW h's (system and charging losses factored in) or around 10 KW's per hour for two days non stop.

Around here all factory built equipment would run you around $100+ a battery times 400 batteries and a 20 Kw factory built wind generator is around $40,000+ installed. Being the wind does not always blow at the speed that a wind generator's power is rated at I need to at least double what power it needs to be rated for in order to meet the 10 Kw capacity that I do need. Also the inverters and other auxiliary equipment will likely put anther $10,000 or more on that. So for me it would take around $90,000 for a system that could produce and store about 10 days worth of electricity which would only cost me about $30 off the grid. And I have yet to purchase a single solar panel or any or the equipment that goes with them either.

If you go with having third world country ghetto living standards then a single 12 volt 100 Ah battery and a 150 watt solar panel would be sufficient. still your out around $250 or more for that alone and you still cant run anything off of it that does not work in a car and the battery might live for a year or two. Then your out another $100+ assuming the weather or other people don't wreck your solar panel by then as well.

Self sufficient off grid is not cost effective by any real and honest standard of living or application. You can get by with less but then you also live without a whole lot more as well.

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#2
In reply to #1

Re: Green Power and Amp Hours

03/15/2010 7:41 PM

Thanks for a promt and thoughtful reply.

Your numbers and observation are very similar to my own. Was just hoping that I was missing something.

One point that I missed was the depreciation aspect of the batteries and their replacement over time as well as the weekly maintenance to ensure that one is working at peak capacity.

Regardless, I'll make out my check to Florida Power and Light now..

--Duck

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#4
In reply to #2

Re: Green Power and Amp Hours

03/15/2010 9:14 PM

I have been playing around with the wind power stuff since my early teens which was 20+ years ago now.

Home built can be cost effective to a point and done for about 10% - 15% of factory cost for most stuff but good mechanical and engineering skills plus an eye for good deals on surplus and used equipment are very necessary to make any of it work and be at least part way cost effective enough to justify as anything more than a hobby.

As far as factory built and installed from what I have ever understood is the break even on anything anyone could afford privately is about at the end of the most idealistic working life time period as possible. If they rate something for 20 years of service life then its purchase and install cost is about the same as what the power could produce would be worth over that same 20 years. That is assuming absolutely nothing breaks down or needs any service for those 20 years which is highly unlikely.

Forget battery life. I have yet to ever find anyone who has honestly ever gotten close to what they say their battery systems will ever last. Very high end and expensive batteries will last longer but still the cost per KW averaged out over their life is not any better. Some batteries may cost four times as much and last four times along but still the life operating costs are about the same more or less.

For basic power a smaller more practical sized wind generator and or solar panel system set up with a grid tied co-generation system is likely going to be the most cost effective. Its what I use when I have a wind generator up and running that can produce enough power to justify connecting it. That being if it can't realistically co generate enough power to offset its build and install costs by saving power over a year or two of normal run time it is not worth the effort. Any time line longer than that and the likelihood that breakdowns and the related costs to repair or replace something will make it a never win situation money wise.

Spend $1000 setting up a system then get $600 worth of avoid costs out of it and then have a $500 breakdown then hope it doesn't happen again before the additional $500 cost is paid off plus the remaining $400 you still had to go and at some point you will realize you where better off having never started the project.

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#3
In reply to #1

Re: Green Power and Amp Hours

03/15/2010 7:59 PM

Nice job.

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#5
In reply to #1

Re: Green Power and Amp Hours

03/15/2010 11:14 PM

Could have written the same thing myself, and in fact have a hundred times over the last ten years.

Heres a few more caveats. Your 100Ah batteries are beasts and weigh about 35kg. 400 units is 14 tonnes + steel cabinets to hold them. Thats not the sort of load you stack on a domestic floor. Concrete strength is measured in kPa; ask a local builder how much a 10kPa floor is.

Bear in mind that batteries aren't the safest of devices. Although rare, they can explode. Having 400 lying below your house is not something I would be confortable with. So think about another outhouse building.

My friends beach-house (called a 'bach' here in NZ) was 'off-grid'. We just ran 1000m of under-sized cable and put in a voltage compensator. Now the electricity works whenever he wants!

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#6
In reply to #1

Re: Green Power and Amp Hours

03/16/2010 12:58 AM

tcmtech-

You have basically hit the nail on the head here in your summary, but I would make slight adjustments in some of your basic assumptions:

A) Based on actual performance data from installed wind generators, it is unlikely you will have wind for more than 30% of the time- more likely, you can count on something on the order of 20-25% availability in some of the best locations.

B) You will significantly reduce the life of your batteries if you consistently draw them down below 50% of rated capacity- it is best to design a system that does not regularly discharge the batteries more than 70%. Which means more batteries.

C) It is unlikely that using $100 automobile batteries is going to provide adequate performance- these batteries are designed for short-duration heavy loads, not continuous loads as would be experienced in a typical residential application. What would be better would be deep cycle batteries. Batteries are the one area where scrimping on price will ultimately cost you more. Pay more for your batteries, and they will last longer (assuming they are properly maintained...)

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#7

Re: Green Power and Amp Hours

03/16/2010 2:27 AM

Wow! post #1 really parts the air...but he knows what he's about too.

Depending upon how far southy x westy the location and what the needs are may temper the result a bit. As you say...

meet the needs of a household presuming one lives in Southwest Florida when using Wind, PV or both?

Meeting the needs, taken as not emergency needs but continuous use. Some of the older sw Florida housing was designed prior to a/c being common and friends and family have never had a/c in their homes there. If without a/c and use of solar water heating your needs may not create a high maintenance or prohibitive system. SW Florida is pleasant most of the time right...

Soon tcmtech will pull his meat out of the snow bank and into the freezer again. But could you only open the freezer when the sun is shining? Maybe having the grid too is good...

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#8
In reply to #7

Re: Green Power and Amp Hours

03/16/2010 9:53 AM

bwire-

I'm not sure aboput Southwest Florida, but here in Panama, AC typically consumes about 60% of the typical power used, for those that need AC. I live in a building that was constructed prior to AC being common, and it was built for the climate- high ceilings, open ventilation (augmented with ceiling fans), well-shaded, no insulation (but I live on a lower floor). Quite comfortable, temperature inside (measured) never varies more than about 4 degrees Fahrenheit night or day, all year round. Other older buildings with which I am familiar are quite similar. Newer construction is unlivable without air conditioning- seems a lot of developers in this part of the world like to import design and construction techniques from more temperate climates, rather than build to the climate. The difference is phenomenal. My monthly electrical bill runs in the $20 to $35 range (depending on how much I use the computer), while a neighbor with air conditioning in a very similar (but newer building) pays in excess of $200 per month, no matter how much he uses his computer...

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#9

Re: Green Power and Amp Hours

03/16/2010 6:56 PM

Hi HMcDuck,

Do you have grid power available? If so, as tmtech says, it really isn't viable to go green unless there are some good governmental tax or subsidy reasons.

However if you don't have the grid available then that's another story. Batteries, as has been said, are expensive and have limited lifespan. The best compromise, in my opinion and experience, is to use solar and wind as suits your location but back-up/ top up with a generator set. Generator sets are cheap in terms of capital cost verses power output compared to any other means but they do need fuel and maintenance so keeping the run-hours to a minimum is imperative.

If you intend to use aircon then you will need a big solar/ wind system. If not then you can should be able to live quite comfortably on 1,000Ah/24V. This will give you between 12kWh and 7 kWh/ day, assuming you don't discharge below 50% total capacity and oft times it will not be practical to recharge to the full100% i.e. it is not worth charging to 100% with a generator set as the last 20% will be a law of diminishing returns because the charge current will taper off rapidly after the VMAX has been reached. Use you major loads while the generator set is running so that you can use your 110V directly without double conversion. The key to success is power management unless you have a large wallet. I have a neighbour with a 35kW generator and about a 2.4kW solar array and he is still struggling. I have a 750W array and a 7kW generator and I run two houses from this with statistically a better supply availability than the local grid. (the last power out was about 2 years ago for a planned battery exchange)

Regards Chas

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#10
In reply to #9

Re: Green Power and Amp Hours

03/18/2010 3:26 PM

Hey Cap,

This is interesting stuff. I was thinking the same thing about just powering from generators verses the batteries. Without wasting a lot of time pulling out my physics books do any of you know if there is any advantage by adding a huge flywheel to the system?

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#11

Re: Green Power and Amp Hours

03/18/2010 4:02 PM

Hi Duck,

It's all about power management and compromise. Big budget and a whole load of solar panels, BIG battery bank and two inverters means no need for a generator but to cover those cloudy days you will need a big bank of batteries.

No one likes to pull into a gas station but who wants to lug around a 100gal fuel tank?

With enough battery power to last for a full day you will cover most of your needs, add a generator big enough to cover peak loads and charge the batteries.

Don't try to go fully generator, even at US diesel prices you will find it expensive, and a pain to keep serviced.

Look at all your loads, and see where you can chip away any dross. Low energy lights, switch off standby appliances (TV, etc.) Use a low power water pump that runs for longer as it's kinder on the batteries, Fit a digital Ammeter to the battery so that you become aware of how much power you are using.

(eg http://www.mastervolt.com/view_product.php?lang=2&section=marine&prggr_id=1027&prg_id=1138&pro_id=5906) there are plenty of others also.

It is difficult to advise you well from here because US usage trends differ quite markedly from European use. It is not uncommon to see a US boat with twice the installed power of its equivalent European yacht.

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#12
In reply to #11

Re: Green Power and Amp Hours

03/18/2010 7:06 PM

Capblanc-

You hit a very critical issue when it comes to trying to live off grid- in any part of the world. Reduce consumption. Do you really need all those lights at night? One or two might do. Why a water pump? Why not a hand pump- reduces water consumption as well. Open up the house, and plant a lot of shade trees around- use natural cooling instead of air conditioning or artificial breezes. Well, maybe up north you need to think more about heating- in which case, wind breaks, proper insulation, sealed windows. Keep the family together in a single room- body heat can do a lot, if not allowed to escape...

Trying to maintain the current wasteful energy use practices with alternative energy sources is going to cost a LOT of money. The less you use, the more viable alternatives become.

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#13
In reply to #12

Re: Green Power and Amp Hours

03/18/2010 8:41 PM

Hi Charlie,

There is no need to go 3rd world. Pretty much all the mod cons can be accommodated, just carefully. Air-co, normal electric type is probably the only no-go, but as you say, if the house is treated sympathetically little cooling is needed, ceiling fans will suffice.

I am quite comfortable with "off grid" but I will pay the tens of thousands needed to get mains when I decide to sell because it will add anything from 20% to 50% to the house's value.

saludos

Chas

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