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re: system sizing
28 mar 2000
george ghio wrote:
>>>>>>>>>the panels put out 100ah per day and we use 100ah per day.
>>>>>>>>>the first day we have no sun but we use 100ah
>>>>>>>>>the next 4 days we get 100ah per day.
>>>>>>on an average day, you used 100 ah, but you only collected
>>>>>>6cycles(4daysx100ah+1dayx0ah)/30days = 80 ah on an average day...
>>>>in the question the panels put out 100amp hours per day. no average in it.
>>>>on sunny days. however, the average output was 80 ah/day, according
>>>>to webster. statistical fairies would say the "mode" was 100 ah/day.
>>>so your saying that 2+2=2 and 1+1+1+1=1 cause thats how the formula for
>>>averaging works.
>>i never said 2+2=2 or 1+1+1+1=1. i am finding it difficult to follow your
>>logic here, george. a solar power consultant like you might correctly say
>>that the average of 1 and 1 and 1 and 1 is 1, ie (1+1+1+1)/4 = 1.
>and i never said average sun hours.
but you described a situation with an average 80 ah/day.
>did you rewrite the questions in your school exams to suit you
>lack of undrestandind of the question?
no, i just tried to understandind the question itself :-)
you are acting with ignorance, arrogance, and considerable stupidity.
you are the one who lacks understanding here, george. accusing others
of misunderstanding won't change that.
>>>>and how, exactly, is "battery lifetime" defined?...
>>>a battery is considered worn out when it will no longer hold more than
>>>80% of its original capacity.
>if the battery does not deliver 80% of its rated amp hours after
>charging it is considered dead.
but what do you mean by that, exactly?
>>>>>taken from battery energy's data;
>>>>>10% daily depth of discharge - 12-14 years life...
>>>>>thanks, george. do they have a lifetime vs temperature curve too?
>>>yes they do. give me a good reason for desigining a system that will
>>>cause you to buy batteries more often than you should have to and i
>>>will chase up the data.
>>cold batteries last longer, so they need replacing less often.
>>otoh, they are harder to charge and they have less apparent
>>discharge capacity (which reappears when they are warmed :-)
there's your good reason. your turn to chase up the data.
>you see you can learn new things. a battery will not deliver 100 amp
>hours at 5 degrees if the rating for the battery is 100 amp hours @ 25
>degrees.
of course, but we knew that already. you aren't reading carefully, george.
>if you want to keep your batteries at 5 degrees you will need
>much larger batteries. i say much larger because a refrigerator uses
>around 600 to 1000 watt hours a day on top of the reduced capacity of
>the cold batteries.
wrong scenario. i suggested keeping the batteries as cool as possible,
naturally, in an insulated box, outdoors, in wintertime, unless and
until the extra capacity gained from reheating them to 25 c is needed.
>>>>>or lifetime vs time spent in various degrees of discharge?
>what advantage do you ascribe to leaving batteries in a low state of
>gharge?
i've never ascribed such an advantage.
>>>>still waiting.
>>>as above
>>well, we know that batteries left in a state of deep discharge sulfate
>>quickly, and one trojan battery guru told me that keeping them on float
>>charge wears them out too. he said he believed the best way to prolong
>>the life of an idle t105 was to let it self-discharge to about 70%, then
>>recharge it. was he right? why 70%? is there a better strategy? can we
>>predict this from models with measured data?
there's your second good reason, george. your turn to act, as promised.
>traction batteries are designed to be discharged to a low state and
>then recharged over night. in other words they are designed for a
>specific job. that job is not home power.
but they can be the most cost-effective batteries for home power.
>>>>>how do we combine these factors to predict lifetime?
>>>>still waiting.
>>>as above
>>pv batteries aren't going to be constantly charged or discharged or
>>sitting on shelves, but with more models and data and patterns of use,
>>we might figure out how to use them more economically.
there's your third good reason, george. how do we combine these factors?
>>>>...for my hard earned money i would settle for a 20% dod withe the
>>>>batteries kept as close to 25 degrees c as possable
>>>>keeping them 25 c when it's 5 c outdoors shortens their lifetime.
>>>does it? why?
>>cold preserves things, eg walt disney. most chemical processes like
>>sulfation and corrosion happen more quickly at higher temperatures.
>>people have known for over a century now that many things fail faster
>>at higher temperatures. if you believe in battery "discharge life,"
>>ie that they fail after a certain number of discharge cycles, consider
>>that they self-discharge more quickly at higher temperatures, and
>>these extra cycles shorten the discharge life.
that's why.
>>>real example:
>>>s.d. bought second hand telecom batteries which were taken out of
>>>service after 10 years. s.d. has replaced them this year after 15 years
>>>use. they were kept at 20 - 25 degrees c for the whole time. we are
>>>talking about 25 years total use in ideal conditions. how much longer
>>>would they have lasted if they were kept at 5 degrees.
>>i don't know. that seems more like a story and a question than an
>>example. shark repellent works pretty well where i live, 100 miles
>>from the ocean. telecom batteries used for electronic system backup
>>are only discharged during grid power failures, ie almost never.
interesting story, but what does it prove?
>>>battery energy considers that a battery is considered worn out when it
>>>will no longer hold more than 80% of its original capacity. i consider
>>>a battery worn out when it ceases to do the job i chose it for.
>>>the two are not the same.
>>seems like a reasonable tradeoff. invest in more pv panel power to decrease
>>ongoing battery expense. so what's your end-of-life capacity fraction
>>threshold for a 25 year life at 20% daily dod? got any data to prove it?
so what's your capacity threshold, george?
>>>days of autonomy required is the number of days you want the system to
>>>run without/or low sun. 4 - 7 days is common. we will use 5 here.
>>why 5?
why 5?
>>>allowance for low temperatures of battery enclosure...
>>>here is a rough and ready temperture conversion chart
>>> temp.degrees c conversion factor [motive power]
>>> -10 .85 0.62
>>> -5 .88
>>> 0 .91 0.77
>>> 5 .94
>>> 10 .96 0.88
>>> 15 .97
>>> 20 .99 0.95
>>> 25 1.00
>>>the capacity of batteries is influenced by the temp of the inclosure in
>>>which they are installed.
>>the apparent capacity. i added a few golf-cart points.
>>>as the temp goes down the capacity goes down.
>>just the apparent capacity, as i understand this: if you charge a
>>battery to 100% at 25 c, then store and discharge it at 0 c, you
>>only get 77% back, but you can get back the other 23% if you reheat
>>it to 25 c, as needed. meanwhile, the time it spends at 0 c makes
>>it last longer and self-discharge more slowly. a smart inverter or
>>homeowner might control the reheating to keep the batteries as cool
>>as possible, depending on the capacity needed.
>>>assume that the lowest daily average temp is 10 degrees c this means
>>>that we must determine the battery capacity on that day to allow for
>>>the worst case.
>>or warm up the battery. this warming energy can "release" more battery
>>capacity than it consumes, even with a battery powered heater. as an
>>alternative, we might turn on a small fan to blow some warm house air
>>through an outdoor insulated battery box. some plastic water jugs inside
>>the box could keep it from dipping much below 0 c in a cold climate
>>and allow it to store coolth from summer night air, as in a cool cell.
let's try that again, with an example. suppose it's 0 c outdoors for a
month, and your maximum dod is 80%, one time during that month. let's
say our battery stores 200 ah at 6 v, ie 1200 wh, and it weighs 50 lb.
it only takes 20 wh to heat 50 pounds of lead 25 c, but that heating
"releases" 23% more capacity, ie 276 wh, for a net gain of 256 wh. but
there's more: consider these two scenarios:
1. keep the battery 25 c for the whole month.
at 25 c, the self-discharge rate is about 12%/week, according to
fig. 5.9 on page 93 of dr. komp's book, so by the end of the
month, it's down to 0.88^4 = 60% of capacity, ie it only contains
0.6x1200 = 720 wh. so it goes dead before the end of the month,
and it's lost some lifetime as well, compared to scenario
2. keep the battery 0 c initially, and warm it to 25 c at the end.
at 0 c, the self-discharge rate is about 2% per week, so by the
end of the month, it's only down to 0.98^4 = 92% of its capacity,
and removing another 20 wh to heat it to 25 c makes this 1087 wh,
ie 90.5% of the initial charge...
in scenario 2, the battery stores 1087/720 = 51% more energy, no?
and in scenario 2, the battery lasts longer.
>>>available energy from the sun
>>>this is expressed in peak sun hours(psh). radiation data for your area
>>>may be available in mj/m2/day or kwh/m2/day. to convert mj/m2/day to
>>>psh divide by 3.6. the figure forkwh/m2/day is the same as psh.
>>>in our example we will assume that the site of the installation is in
>>>an area where the psh is 4.5
and where does that 4.5 come from, exactly? if somebody gave you
30 years of actual hourly radiation data, what would you do with it?
>>>the output current of the mdule should be the current at normal
>>>operating cell temperture (noct) at 14 v.
>>and 6.3 c? why not add 3:1 reflecting fins to silicone-potted panels
>>and trickle some water over their faces in summertime, when more sun
>>is available?
why not add fins and use 6 vs 16 panels? and evaluate their output at 6.3 c?
>>>if we choose a module with inoct@14v = 3.96 a its guaranteed current
>>>will be 3.6 a - 5%. this is because the manufacturers state the output
>>>of their modules to be + or - 5%
>>>guarantied current = 3.96 x .95 = 3.76
>>a random assortment of 16 panels would have a "guarantied current"
>>that's closer to average, ie 3.96 a x (1-0.05/sqrt(16)) = 3.91 a.
that's simple statistics, george.
>...the truth is that thousands of people around the world
>use solar power every day with out drama.
so what? they also use toothbrushes without drama :-)
>this calculation is a common one for the sizing of solar power systems.
might as well do it correctly.
>the fact that you don't seem to be able to make solar power work...
is that like the fact that you are 5'3" tall? :-)
>it does not matter how long or loud you tell people that solar dosn't
>work or costs too much or is a cargo cult.
i've never said it doesn't work, but it is very expensive, and
from what i can tell, it's mostly a cargo cult. why else would
a "solar power consultant" like you want to keep batteries warm?
>my old dad once told me not to argue with women, children or congenetal
>idiots as none of them make sense and all for different reasons.
he might have added "solar power consultants" :-)
>goodby nick.
i'll doubtless respond if you continue to post idiocy, george.
and do my best to warn potential customers to avoid hiring you,
unless you change, which seems unlikely, but miracles happen.
nick
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