T&T: AC genset and fuel burn discussion

[Arild Jensen]

LRZeitlin at aol.com wrote:
> Using a small DC generator to charge a big bank of batteries which power a
> pair of large inverters seems to be a complicated process if what you want is
> 60 cycle AC power. And I'm not sure if the price will be any lower or the
> efficiency any higher than installing a conventional AC genset. 
REPLY
Larry makes some excellent points about relative efficiencies.  
While delving into this  subject I found  that almost all manufacturers 
give fuel burn for 50% load and 100% load.
Thinking they might be fudging something I  checked deeper.  I 
discovered that the fuel burn  curve is practically linear from half to 
ful l oad.
In other words  fuel consumption will be  directly proportional to 
actual connected load. 
Below that  load level you get into problems like wet stacking  and what 
you might save in fuel burn is lost in higher maintenance.
Slowing down the genset  will not achieve as much in the way of  savings 
as might seem possible at first glance.  Not  mention which you no 
longer have the benefiit of  directly generating 60Hz  power.  Now you 
need an inverter.
Where a DC genset does look attractive is  when you need  charging 
currents  above 100 amps. 

Most inverter chargers  and even stand alone chargers  only provide 
about 100A.  There are a few exceptions at 120 amps.
Cost of a 100 A charger runs  around $1500  - 2000 installed.    
So if you have a large  battery  bank that can  take advantage of 200  - 
300 amps or more charging current then a DC genset  driving a huge 
alternators  makes some sense. 
Examples of  well known boats using such a setup (Electrodyne)  is Steve 
Dashew's "WINDHORSE"   and his line of  64 foot Patrol boats  recently 
featured in Professional Boatbuilder and other magazines.

I have access to  some really huge alternators ( Surplus Leece-Neville)  
and have one in my shop but let me tell you  the construction of  a 
decent support and belt drive is not a trivial  exercise.  These 
monsters  can deliver up to 300 amps @ 24V for short periods of time and 
have a continuous rating of over 200 Amps when run 24/7. 

regards
Arild
.