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wind generator design question - Page 2

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Posted by daestrom on March 29, 2006, 10:11 pm

It isn't so much that a particular type of turbine 'give disappointint
results...', it's just how much power is available.  If a given diameter
wind turbine can develop 500 watts in a 10 mph wind, then it can't do better
than 500/8 = 62.5 watts in a 5 mph.  Just because there is that much less
energy in the wind, and that much less air moving past the turbine.

Yes, it is possible to design a turbine that generates more power at lower
wind speeds.  Just make the diameter larger.  But there comes a trade off of
how much a large turbine costs versus how much power it can generate.
Designing for very light winds pushes the $/watt up pretty quickly.  And a
turbine that generates max output at 7 mph ends up *throwing away* an awful
lot of energy when the wind speed is up to 14 mph.

Large, multiple blades as you're suggesting can develop a lot more torque.
But the lower revolution speed ends up taking over and the horsepower output
goes down.  And larger blades means bigger/stronger tower to hold them up
against that wind.  Large bladed units are also somewhat self-limiting as
wind speed increases because although the available power increases, the
turbine can't speed up much because of all the drag associated with spinning
those large blades through the air.


Posted by brian mitchell on March 30, 2006, 7:49 pm

"daestrom" wrote:

Can't argue with that! Although a turbine which could develop that much
power in 10mph winds and still be considered 'domestic' would be a
desirable object.

I can see that, but any turbine system is likely to end up throwing away
energy. What happens when you're batteries are fully charged and it
decides to blow a gale? And don't most small-scale systems use shunt
voltage regulation to protect batteries?

I'm trying to see the whole system as a unity, and that has to take
actual conditions as the base point. I've learnt that the annual mean
wind speed for my area is 8mph. That's at 10 metres above ground level,
so probably less at roof level. This suggests to me that for *most* of
the year one would be getting 8mph winds as a maximum, and one has to
consider viability, cost/benefit, etc, over a whole year.

It doesn't seem beyond possibility to me that wind as a wide-application
alternative energy source is a bit of a chimera. The fastest I've ever
seen those large wind turbines on the nearby hill turning is about one
revolution in five seconds. Maybe that's enough to generate usefully
large amounts of power, I don't know. But I do wonder about the economic
viability of them.

I'm working on the basis that a greater number of smaller systems which
can utilise battery storage might be a better bet. Although in the UK
you're allowed to connect really quite small systems into the national
grid and sell power back, that isn't storage in any sense, and they pay
you nearly nothing for it.

At the moment, for a system that might be called 'domestic' (ie.,
rooftop, easily made and installed) my mind is yawing towards vertical
axis, low windspeed jobs. Quite a lot of people seem to be looking in
that direction I now discover.

brian mitchell

Posted by Solar Flare on March 31, 2006, 12:28 am

I am thinking the same thing about vertical but the size seems to be
too huge and the bearings look like they would be a maintenance
problem, at first glance. Only 30-40% of the windside face actually
catches wind and the rest opposes it.


Posted by brian mitchell on March 31, 2006, 2:30 am

"Solar Flare" wrote:

Taking the Savonius-type rotor as an example, once one frees one's mind
of the "55 gallon drum" imprint there are many modifications possible to
address the problems and raise the overall efficiency. One instance of
this I've seen is a shroud or cowl which leaves only the working side
exposed and dramatically cuts down the drag of the returning 'cups'.
This design sacrifices the omni-directionality but increases the
efficiency, and it's only a lightweight cowl seeking the wind so there's
no gyroscopic forces involved.

Other designs have two tiers with staggered cups, or make them wider
than they are tall to take advantage of increased leverage to turn a
bigger load. And there's no reason why this kind of rotor shouldn't have
a top bearing as well.

Many developments, too, on the aerofoil, Darrieus-type vertical axis
rotors. Different configurations of blades, variable pitch as they
revolve, and so on. I don't know how tunable these are to low-wind
conditions, though.

brian mitchell

Posted by Mic on March 30, 2006, 3:52 am

On Wed, 29 Mar 2006 04:34:55 +0100, brian mitchell

Wind generator Links

Home built boat wind genny



From a seller on Ebay
Wind turbine generator PMA Permanent Magnet Alternator
If these voltages seem high just remember that standard automotive
alternators are usually putting out 40 to 50 volts into the 12 volt
battery in your car. That's right! It won't harm the battery AS long
as the battery is not FULL it can take high levels of charging voltage
no problem. A lead acid battery will easily buffer high input
(You will never be able to see voltages this high because the battery
pulls all power inputs DOWN to its own voltage level AND that's why
you NEVER pull off the battery cables when an engine is running
because your fuses, dash board lights and regulator blow up !!!!!!!
(Especially if you rev the engine! ,,,, NO battery---- NO buffer!!)
Have you every seen a battery charger that had a BOOST/START mode???
How could this work since you can't force more amperage into an
electrical circuit. Amperage has to be "Demanded" by an electrical
device so the BOOST/START switch on your battery charge is actually
increasing the VOLTAGE to about 18 to 20 volts thereby allowing you to
extract more amperage. Relax, it is normal to charge batteries with
much more power than their output ratings. Only start worrying when
the battery is fully charged and THEN any additional power must stop
coming into it or must be dumped to keep the battery cool and from
producing explosive hydrogen gas.
To make a long story short a 12 volt battery that is being charged
with a wind turbine putting out 50 volts will still only read "12
volts" to a volt meter proving that the high voltage power is easily
being swallowed up by the big lead plates of your battery.

The Chispito Wind Generator is a 100W machine. It outputs 14volts at
275 rpm, so it starts putting power into your batteries in low wind
speeds (5-7mph).
Assembly instructions
http://www.surpluscenter.com/item.asp?UID  05041919300147&catname=electric&qty=1&item-2167

"When modified, auto alternators can provide variable direct current
at 0 to 120 volts for battery charging, etc...."

A 20 amp charge controller that is installed between a solar panel and
a battery. Use for general purpose solar regulation. For 12 Volt

CirKits Links to Alternative Energy Web Sites  Large
Wind Turbine Power Output Calculator
The Basic Chemistry of Gas Recombination in Lead-Acid Batteries

Lots of good projects in including a CNC router???
Building your own wind turbine can be both a very fulfilling
experience or a disappointing exercise.
Wireless cycle computer as an RPM monitor

This is the WindGenZen SAIL model

#  The motor should produce 12 volts at 200-300 RPM.
# To find out if a motor will do this, take the rated voltage and
divide it by the rated RPM. I.E. a 50 volt motor rated for 1000 RPM
will make 12 volts at about 250 RPM. Use a linear scale to calculate
the voltage of any given RPM.
#  If the RPM required to make 12 volts is higher than 300 it probably
will not work unless you have a big blade and screaming winds.

# If the RPM required to make 12 volts is higher than 400 it will not
work and you should consider a different generator.

# For example, a motor that makes 72 volts DC at 1850 RPM will make 12
volts DC at about 300 RPM. That will work.

# A 50 volt motor rated for 1000 RPM would make 12.5 volts at 250 RPM.
That will work.

# A 36 volt motor rated for 800 RPM would make 12 volts at about 266
RPM. That will work.

# A 24 volt motor rated for 700 RPM would make 12 volts at about 350
RPM. That will work if you have good wind.

# A 36 volt motor rated for 1500 RPM would make 12 volts at about 500
RPM. That will NOT work and you should reconsider the generator.

# Do you see the relationship and ratio of RPM and voltage you need?
Avoid motors that require too many RPM for making 12 volts unless you
are charging 6 volt batteries: they won't work.

# Large AC motors can be rectified for less than $0.00 and often
produce tons of good DC power at low RPMs...lots, and often do it for
only a little more than ameteks.

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