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re: a simple starter pv system
9 may 1997
don kulha  wrote:
>nick@ufo.ee.vill.edu (nick pine) wrote:
>> >what don meant, was...
>> ...different from what he posted? does this remind you of ronald reagan? :-)
>
>i'll remind you that the original post was direct from home power...

that's why it says "posted," vs "wrote," don :-)

>i even pointed out in it that richard had changed his viewpoint in the
>interim and that we had differing opinions about a few things.

i wonder which things. for one thing, it seems to me that people are shifting
more from 12v systems to 110v ones, with uniform 110v wiring and inverters
(plural, one hopes.)

>>>...in remote or "stand alone" power systems, pv panels are ultra-reliable
>>>compared to the alternative of gas or diesel genset power...

>>agreed, but don's posting also talked about city pvs, and said pv _systems_ 
>>were "ultra-reliable," not just panels, and those systems include inverters
>>and batteries. he mentioned a 10,000 hour mtbf for a trace inverter,
>>which implies that they fail about once a year...

>my fault, nick i assumed too much. the c30a mentioned is a relay type
>charge controller. at the introduction of their sw series sine wave
>inverters trace did have a few problems but overall considering all of
>their inverter lines i'm sure the mbtf is a lot higher.

i wouldn't bet on it. i'm an ee by training, and i've helped design and
analyze "ultrareliable" systems. it's very difficult to make an electronic
box of any complexity with more than a 10k hour mtbf, especially one that
handles significant power. running it 100% of the time doesn't help either.

>and of course no one would call anything that failed once a year reliable.

i wonder if powerstar makes an "ultrareliable" inverter? :-)

---

for the eighth time, my powerstar 1300 has failed. woke up tuesday
morning to the smell of fried electrical components :-(. since it had
failed under warranty previously, i've always sent it to powerstar for
repair. this time, out of warranty, (returned from the last repair
12-29-96), i decided to attempt the repair myself...

in a related note, my trusty old powerstar 200 also failed a few weeks ago.
...a trip to walmart yielded a power to go, chinese made 140 watt inverter
for $34.99, that has worked great since...

--- 
q myers
qmyers@teleport.com

redundancy can make a system ultra-reliable. more than one inverter can make
an inexpensive standalone system much more reliable, as can running a trace
in grid-tie-mode, mostly, with just enough batteries to cover the most likely
power outage durations. phone companies aim for "availability" in networks,
and design and estimate and try to guarantee that their systems will have a 
maximum "unavailability" of something like 10 seconds per year, at most.
q has found a new bargain in small inverters, which i'll put at the top of
my list from the last jade-mountain catalog:

                watts   cost   cost/watt   efficiency    warranty

power to go     140     $35     0.25           ?         "works great"
porta power     140      75     0.54       "over 90%"    90 days
powerstar       200     119     0.60       "over 90%"    1 year
ac genius       150      95     0.63       0.24a stby
prowatt         250     195     0.78       "over 90%"     
trace 812       800     550     0.69       0.02a stby
powerstar       400     389     0.97       0.06a stby    2 years

it makes sense to use the more efficient inverter most of the time, and
use the cheaper and less efficient one only when the first one breaks,
while it is being repaired...
  
if an inverter fails once a year and takes a week to get fixed, this markov
model might predict its availability:

                        where l = is the failure rate, 1/52 week,
	   l                  r = is the repair rate, 1/1 week,  
 ----  --------> ----         p1 is the probability that the inverter works
| p1 |          | p0 |    and p0 is the probability that it doesn't.
 ---- <--------  ----
	   r 
since it works or it doesn't, but not both, p1 + p0 = 1, and p0 = l/r p1, so
r/l p0 + p0 = 1, or p0 = 1/(1+r/l) = 1/(1+1/1/(1/52)) = 1/53, so we might
expect the inverter to be out of service an average of 1/53 of each week,
165 hours per year, a bit less than one week per year.

add another inverter (or a grid that fails once a year, for a week),
and this becomes
                        
	  2l              l             p2 <--> both work
 ----  --------> ----  -------> ----    p1 <--> one works
| p2 |          | p1 |         | p0 |   p0 <--> none work
 ---- <--------  ---- <-------  ----
	  2r              r

again, p2 + p1 + p0 = 1, p1 = 2l/2r p2 and p0 = l/r p1, so
2r/2l p1 + p1 + p0 = 1, or r^2/l^2 p0 + r/l p0 + p0 = 1, or 
p0 = 1/(1+r/l + r^2/l^2) = 1/(1+ 52 + 52x52) = 0.00036, so we
might expect both inverters to be out of service an average of
0.00036x8760 = 3 hours per year. (reducing the repair time to
4 hours would decrease the unavailability to 1/2 hour per year.)

add another inverter and this becomes
                        
	 3l            2l             l           p3 <--> 3 work
 ----  ------> ----  ------>  ---- ------> ----   p1 <--> 2 work
| p3 |        | p2 |         | p1 |       | p0 |  p1 <--> 1 works
 ---- <------  ---- <-------  ---- <-----  ----   p0 <--> 0 work
	 3r            2r             r

again, p3 + p2 + p1 + p0 = 1, p2 = 3l/3r p3, p1 = 2l/2r p2 and p0 = l/r p1,
so p0 = 1/(1+r/l+ r^2/l^2+r^3/l^3) = 1/143,365, so might expect all three
inverters to be out of service an average of 8760/143,365 = 0.061 hours or
3.7 minutes per year. this is starting to look like the phone company :-)

>> ...how will these people heat their houses, and provide water and septic
>> systems? and where can we find real "independence" these days? financial
>> independence seems more empowering than electrical "independence." can
>> anyone stop paying taxes for 2 or 3 years these days? a lot of those taxes
>> go to ensure cheap oil by bombing iraq and interfering with other countries
>> in many parts of the world. some estimates put the true cost of oil at about
>> $7/gallon...

>granted much of the above is true. i don't want to pay to bomb iraq either.

nice we agree on that :-) and i suppose you'd agree that the true cost of oil 
is higher than the selling price, after counting armies and air pollution.

>> >think about ten or twelve $300 pv panels replacing several diesel
>> >generators, and several thousand gallons of fuel.  much safer, quiter,
>> >nicer smelling, less offensive.

"independent" country dwellers need cars more than anyone else, i suppose,
and gasoline, and perhaps oil or wood to heat their houses, so for the tiny
amounts of electrical power we are talking about, it seems to me they would be
almost as "independent" if they made their own electricity (and hot water?)
by hooking their cars up to batteries inside their houses with jacks on the
house walls. an alternator making 16 volts at 30 amps only has to run about
1/2 hour per day to make 0.25 kwh/day, and the car might consume a half-gallon
or so of gasoline, for electrical power at $2/kwh, ie $180 per year, and we
don't have to buy any pv panels, just batteries and inverters. at the least,
this might be a backup system for the pv panels themselves.

>> ...why not put the panels in sunspaces, with mirrors above, and trickle
>> some water over their faces, to help heat houses? 
 
>well, the light energy going through the two interfaces (air-to-glazing
>and glazing-to-air) are going to lose at least 7% of it's punch and likely
>worse (given that the incident angle is going to be other than "normal"
>for most cases). i don't recall snell's law (not sure i want to) but if
>you want to calc it run it for refractive indicies of 1.00->1.52 and
>1.52->1.00.

i like sunspaces with plastic glazing, which has a lower index than glass and
passes about 90% of the sun's energy. the reflection coefficient at normal
incidence is ((n1-n2)/(n1+n2))^2, about 8.7% for a layer of glass with air
on both sides, but an extra layer of glass on a pv panel might have a thin
layer of water underneath, which has an index of about 1.33, reducing both
the reflection from the underside of the new glazing and from the top side 
of the original pv panel glazing. all this may not matter much, if the mirror
above increases the incident sun by 50 or 100%. 

>pv's will do much better outdoors.

that sounds ok to me too, but they seem hard to service on a roof.

>i don't have a sunspace yet (i will) but i want to put people, plants and
>living space in it myself...

sounds good.

>nick, i very much appreciate the fact that solar thermal is a much more
>beneficial way to use the energy in sunlight for more people than pv is at
>the present...

good...  

>why not write a letter to the editors of home power for publication
>expressing your feelings and consider writing an article about the
>benefits of solar thermal.

i haven't subscribed to hp in a few years now, but it seemed to me back then
that they were only interested in pv, and the few solar thermal articles i saw
lacked performance arithmetic, unlike the pv articles. i think "the benefits
of solar thermal" are best addressed in specifics. lots of people think they
know how to heat houses with the sun, but i've seen few houses in which that
is done predictably or efficiently. a lot can be done here, but oddly enough,
few people seem to care. solar ovens are fun, but won't make much of a dent
in our national oil consumption.

i like the idea of helping invent an inexpensive modular plastic sunspace kit
to put on the south side of a house, with options for heating controls and
thermal storage and water heating and maybe summer dehumidification. there's
a huge gap between commercially available sunspace kits at $50-150/ft^2 and
commercial plastic film greehouses at less than $1/ft^2. so far, the cloudy
polyethylene used in commercial greenhouses doesn't have much aesthetic appeal
to conventional homeowners, but tastes can change, eg in response to values,
like cost and privacy, and so can materials. bayer has announced a new
polyurethane greenhouse film that is very clear, comes in sheets 15' wide,
and should last at least 10 years.

i'm thinking of writing a book about solar house heating, with lots of simple
and useful arithmetic. i don't know if hp would be interested in that. mother
earth news and soft technology say this arithmetic is above the level of their
readers, or perhaps their editors :-)

>in part based on discussions we've had they've been running much more thermal
>stuff recently and want to do more.

good :-)

>as one of my favorite radio folks used to say "if you don't like the news
>go out and make some of your own."

easier said than done, sometimes :-)

nick


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