wmbjk wrote:

On Thu, 16 Jun 2005 21:55:15 GMT, "Pete C."
wrote:

compressed air storage Should be comparable or better efficiency than a wind driven generator
charging batteries.

Perhaps in some niches... but in any event a practical home power
setup needs some batteries, and charging them with wind, assuming
there's wind to harvest, is highly recommended. So you're talking
about *adding* systems because you believe it's worth the trouble, but
you haven't supplied any numbers or examples to back up your position.

See my notes below saving four steps of energy conversion for
applications where you are able to use air power directly such as nail
guns and die grinders. Four fewer conversion steps *has* to be more
efficient.


They also do this
with pumped hydro, but CAS is far more practical than pumped hydro in a
homepower environment.

But both are less practical than batteries.

Not by a long shot, especially for a large scale installation as a power
utility would use. It is far cheaper and lower maintenance to use a
large air or water reservoir than to use a huge string of batteries with
limited life spans and hazardous lead and acid to dispose of properly at
every battery replacement.

The batteries in your string may last what, perhaps 8 years? A large air
tank that is drained regularly and kept in an area not exposed to
weather should easily last 80 years. When it finally is failing all you
have is scrap steel to dispose of and you can actually get a few dollars
for it.


No, not mechanically driven. The refrigeration compressor would be belt
driven from an air motor. The thermostat simply opens the air valve when
it needs to spin up the compressor. Again the ultimate source of power
does not have to be wind, and in fact with CAS it's even easier to
combine energy captured from multiple sources. No need to worry about
charge controllers when you're simply pumping air into a big tank.

How big a tank? I think you're going to find a fly in the ointment
once you run some numbers on air consumption. And if air power could
be so efficient and practical, why do you believe it is that
off-gridders, often known to be innovative and unafraid of breaking
with convention, haven't flocked to the concept?

Perhaps because I haven't seen the concept even proposed in any
alternate energy books. As far as I know CAS and PH are both fairly new
concepts that originated from electric utilities need for a way to store
excess generating capacity during off peak times for use in peak shaving
later.


The point is that batteries can only accept a charge at a certain rate,
potentially wasting captured energy during peaks.

There's isn't any peak power wasting problem that I'm aware of with
home power systems, since the cost of generating prevents people from
buying excess capacity. Can you give an example of the problem you're
citing?

This would primarily apply to wind generation where peak gusts could
produce power faster than the batteries could accept it, causing that
power to be dissipated by the charge controller.

Hydro could have a similar case where excess capacity could go
uncaptured after heavy rains.

Solar PV is inherently pretty immune from generating peaks.


The efficiency of directly utilizing the energy of the compressed air
for mechanical applications is also higher. Instead of capturing wind
energy, converting to electricity, storing in a battery, converting to
AC, converting to mechanical energy with a motor, converting to
compressed air with a compressor coupled to the motor and then utilizing
the compresses air to fire your nail gun, you eliminate four conversion
steps.

You left out the AC to DC conversion of the turbine, and assumed that
energy used must be stored in a battery first. It's true that
compressed air for tools is a very inefficient process, on-grid or
off. Yet I've managed quite well with the just the same sort of
compressor that grid-connected folks use. I could do wind-powered shop
air more easily than most, but I wouldn't dream of adding another
system to cure an inefficiency that's such a small part of the big
picture.

Ok, so five less conversion stages.

It's all relative. The more use your shop has, the more potential
savings. If the shop demand is such that you'll require more generating
capacity, more storage capacity and more inverter capacity then there
may well be cost savings since components for an air system are cheaper.

If you an over-the-top green type, perhaps the ability to eliminate or
keep to an absolute minimum the need for big ol' lead acid hydrogen and
corrosive fume belching batteries might be a positive.


Coincidentally, I have a neighbor who plans a Bowjon type installation
(low tower, bulky rotor, single-stage compressor) for shop air using
multiple surplus storage tanks. I've suggested that since he hasn't
any wind power at present and could really use some, that the time and
money he's going to put into the new setup would be better spent on a
conventional wind genny and a tall tower.

Any reason not to combine both and put up your tall tower with the
electric gen up top and hang a compressor at a lower point on the same
tower?


This is where you really need the hybrid system. You run the inverter to
power your conventional appliances. When you are not running the
appliances you turn the inverter off.

That's one of those convenience/practicalities tradeoffs. Many
appliances don't like being de-powered, and it's a nuisance to fight
it. IMO, biting the bullet for full time capacity is one of those
things that goes a long way to making off-grid living palatable for
the average person. After a hard day of pining over the dearth of
rural ballet, the last thing you want is to have to reprogram the
clock on the microwave. :-)

True, maximizing efficiency does require active management of the
system. Some things can be automated, and I've even seen a few
microwaves where you can disable the clock though.


You run your lighting and TV and
whatnot that are your much higher duty cycle items from DC and avoid the
conversion.

The conversion losses are lamentable, but not generally worth working
around. As Scott mentioned, after you've fought that battle for a
while, you're ready for straight AC in order to eliminate the
diddling.

Guess you just need to setup a heat exchanger from your inverter(s) to
capture the waste heat for your DHW.


Perhaps your home welding is less than mine. I've got a Miller
Syncrowave 250 that I love and it can see quite a bit of use on project
weekends. I'm thinking your inverters would gag at the 240v 100a gulps
the Syncrowave takes, even if the typical gulp is only about 10 seconds
duration. On a big project those 10 second gulps add up to quite a few
minutes.

The fuses definitely couldn't handle it - 400A limit (24V system).
What kind of *home* welding are you doing that takes 24k Watts input?

Pretty run of the mill stuff actually. The last moderate welding project
was building the CNC router which is framed mostly from 14ga square
steel tubing (http://wpnet.us/cnc_router.htm).

While the welder is rated at 250a and the full output at reduced duty
cycle is 310a, most of the time I'm running in the 150-175a range.


People can size for whatever they can afford, but if I had the need
for more indoor stick/TIG, I'd be after one of these
http://cgi.ebay.com/ws/eBayISAPI.dll...m=7524207 277,
which should be a comfortable fit with our setup. But I find I rarely
stick weld indoors above about 120A, although I use the Powemig 255 up
to its max more often. The only really heavy stuff I have to work on
are the tractors, and that's only occasionally. Since they don't fit
inside, and neither does the smoke and dust of heavy work, I roll an
engine-driven unit outdoors.

I primarily use TIG, if for no other reason than I'm most comfortable
with it. I also do aluminum from time to time.

The last time I did stick was a quick repair to my snow blower in the
middle of a storm. Even for that I would have used TIG if I was going to
take the time to do it right vs. just weld through the snow pack. I
won't be doing much of that anymore either since I'm in TX now and don't
get much snow.


I didn't really intend the welder / generator to be used for backup to
the regular power system. I really meant it more as an option for
powering larger shop tools.

The auto-idle feature of a good unit will make that bearable, if the
tools' idle use is compatible. Still, the generator is going to be
either idling or roaring in between power tool use, at an average of
about $2 per hour in fuel. When we first moved onto our site, but
before we had the power system set up temporarily, I was stuck with
the welder generator. The running hours add up quickly, and I wouldn't
recommend it to anyone else except for occasional or temporary use, or
because there was no alternative, or if it's for a job that pays
enough to cover the expenses and aggravation.

Any generator is going to be most efficient at or near full rated
output. Load management is always difficult.


Modifying a DC inverter welder which are pretty inexpensive these days
is likely the most efficient way to get quality welding capacity from a
home power system. No line shaft required, and no need for oversized
inverters or load shedding.

Everyone has different needs, wants, and budget, but I think you'll
find that more and more people have a potential combination of house
loads that need such capacity that shop use isn't a leap. Around here
for instance are many who need to power the surge of a 2 hp well pump,

Soft start inverter drive to a three phase well pump with an unloader
valve?

along with other use concurrent. It can be done with a smaller
inverter and a generator, but it's sure nice to get that generator
time down if you can. We used to have a couple nearby who had a
generator/battery/inverter setup, over 10 hours generator time per
day. That's about 4000 hours and 2000 gallons of fuel per year. I
think the fuel cost, repair costs, and eventual generator replacement
cost were big factors in their pulling out after a few years. Even a
modest amount of PV could have cut that generator time in half, and
would have been far cheaper in the long run. Better still, the cost of
that (very nice) generator and fuel could have bought a combination of
hardware including a much smaller generator needing only a few hours
per week run time.

The only way to get reasonable efficiency out of a gen/bat/inv setup is
to size the generator to just barely above the average load and run it
24x7, and that requires a pretty specialized generator to handle that
duty. Not something I'd consider unless I had my own nat gas well, or
bio-gas generator.


Somehow it seems to cost
more to live self sufficient off-grid than it does to just pay the
utilities...

Not necessarily. Cashing out of a grid-connected place allowed us to
retire, start with a clean slate, and as the yuppies say, "leverage"
the advantages of home power to help keep the big picture cost down.
Cheap land, lower taxes, fewer utilities (still need the phone
company) are some of the benefits.

Looks like you had to go to some extremes to get your Internet
connectivity. When I was moving to TX I had to base my house choice on
the availability of high speed connectivity. I investigated satellite,
but unfortunately it doesn't work well with VPN. I ended up on cable
which works well.

In talking to off-gridders, I find
that the main factor affecting success isn't so much the power issues,
but whether the folks can afford and are comfortable with truly rural
living. For most, that usually means retirement or telecommuting, and
precludes having children at home. For those who need to commute or be
close to school busses etc., they're usually stuck with paying the
premium for grid access. Then again, when they want to generate their
own power, they can have cheaper and more efficient systems, and use
the grid for storage.

Well, I telecommute now. Trading a cubicle in a windowless concrete
dungeon for a sunny window office with a purring cat on the windowsill
is a wonderful thing.

Since I'm also single and also quite mechanically and technically
inclined, in theory I could do the off-grid in the middle of nowhere
thing. In the event my job changed and I was no longer able to
telecommute I'd be f'd though.

Unfortunately I don't really expect I'll ever be able to retire. I'm
theoretically making good money these days, at least it looks good on
paper. Unfortunately the economy is such that unless you are a DINK
you're still barely covering living expenses.

Perhaps as I'm able to build up some AE projects I'll be able to cut
operating expenses enough to start to get ahead.

Pete C.