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re: system sizing
18 mar 2000
george ghio wrote:
>>>>>assume a 100% efficient system, 100ah of panels
>>>>>and 100ah of load and 500ah batteries.
>>charged to start with?
you seem to have finally answered this question "yes."
>>and how many ah do these panels collect on an average day?
the answer is 80 ah, in your scenario.
>>>>>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.
>>>>>what is left in the battery bank if this pattern of days is repeated
>>>>>for 30 days?
>>on an average day, you used 100 ah, but you only collected
>>6cycles(4daysx100ah+1dayx0ah)/30days = 80 ah on an average day...
>as the out put from the panels was stated at 100ah it would be fair to
>say that this is the average days output.
i disagree. that's peak output, with "100 ah of panels" (your words),
according to most panel specs. the _average_ day's output above is
simply the sum of the outputs over the 30 days divided by 30, ie
80 ah/day vs 100 ah/day of average use. no wonder you fell behind.
let's try this again in baby steps, george. if bob gives you 100 apples
per day for 4 days, and none on the 5th, what is the average number of
apples per day he has given you?
if you start out with 500 apples, and you only eat 80 per day,
will you ever run out of apples?
>>>>>>>....if you collect as many kwh as you use on an average day,
>>>>>>>with an average amount of sun, accounting for inverter and
>>>>>>>battery inefficiencies, you'll never run short...
this is like 2+2 = 4. how can it be untrue (with a large enough
pile of batteries)?
>>is it better to a) spend another $100 on panels or b) spend another
>>$100 on batteries, in order to minimize the average amount of time
>>that power is unavailable because the batteries are discharged?
here's a little 50 year simulation:
10 ahpeak=200'peak charge from pv panels
20 battcap=500'battery capacity (ah)
30 gosub 110
40 battcap=600'battery capacity (ah)
50 gosub 110
60 battcap=500'battery capacity (ah)
70 for ahpeak=204 to 212 step 4'peak charge from pv panels
80 gosub 110
90 next ahpeak
100 end
110 outage=0'initialize outage days
120 for year = 1 to 50
130 ahstored=battcap'initialize battery charge
140 for day = 1 to 365
150 ahin=ahpeak*rnd'amp hours collected
160 ahout=200*rnd'amp hours used (average, 100)
170 ahstored=ahstored+ahin'cumulative stored charge
180 if ahstored>how, exactly, is battery lifetime related to the discharge depth
>>and the number of cycles?
>taken from battery energy's data;
>10% daily depth of discharge - 12-14 years life
>25% daily depth of discharge - 9-12 years life
>50% daily depth of discharge - 6-8 years life
thanks, george. do they have a lifetime vs temperature curve too?
or lifetime vs time spent in various degrees of discharge?
how do we combine these factors to predict lifetime?
which dod scheme is most cost effective, of these 3? should we use
10 batteries with a 10% daily dod, or 2 with a 50% dod? looks like
we replace 10 batteries every 13 years in the former case, ie 0.77
batteries per year, or 2 every 7 years in the latter, ie 0.29 per
year. and 4 every 11 years at 25% daily dod, ie 0.36 per year. so
fewer batteries with higher dod is best, even if the battery money
earns no interest, right?
>this amateur science as you call it is really quite precise
>when you do it right.
maybe, but all i see here is a cargo cult.
some alt.energy.homepower readers say we need to keep batteries warm
to make them last a long time, and many (including you, sometimes)
don't even seem to know the difference between power and energy.
let's try to change that.
nick
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