Efficient inverters vs. Load Segregation [RE-wrenches]

[Jim Easton]

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Depending on constraints, you might consider dividing up your loads, so that 
the large load(s) are on large inverter(s) de-energized when not needed, 
while loads that need to be constantly energized are on smaller inverter(s).

Jim Easton, PE
 (E 11974)
4364 Bonita Road, No. 166
Bonita, CA 91902-1421
Tel: 858-527-0240
Fax: 619-470-8616
----- Original Message ----- 
From: "troy" <taharvey at heliocentric.org>
To: <RE-wrenches at topica.com>
Sent: Wednesday, August 08, 2007 12:34 AM
Subject: Efficient inverters for stand alone power systems? [RE-wrenches]


>
> I'm trying to find an efficient inverter for stand alone power systems.
>
> It seems that in the grid-tied boom, that nobody (?)  has really 
> addressed the fundamental problem with stand-alone inverters:  efficiency 
> at low power draw.  Analyzing a couple of stand alone  residences & small 
> office applications, the *typical* power draw is  quite small. You still 
> need peaking performance for the occasional  surge requirement, but 80% of 
> the time the base load may be 100-200  watts or even less.
>
> Given the Outback Inverters as an example (a favorite of many off- grid 
> designers), the weighted average efficiency is only around 75%  or so for 
> these applications. The most insidious thing is, as you  improve the 
> efficiency of the base load appliances, efficiency drops  further 
> (approaching zero). So for example if only one 40 watt  computer is 
> running the efficiency is only 40%.
>
> Obviously the problem has to do with the fact you are running against  the 
> quiescent current requirements of the inverter controller (which  should 
> be only a small fraction of a watt) and switching losses (the  dominant 
> issue I'd guess). But this has been addressed in switched  converters long 
> ago with by reducing switch frequency  and/or  transitioning to PFM at low 
> load. We've designed power converters  that are 95% efficient down to 
> fractions of a watt. The point is, it  should be feasible to design an 
> inverter that is efficient down into  the few watt range (and lower).
>
> I'd like to hear if there are any multi kW inverters (ie 2-5 kW)  out 
> there that are efficient down to low power draws (tens of watts) .  And I 
> wouldn't mind hearing any commentary from inverter  manufacturers on the 
> subject.
>
> Thanks,
>
> Troy Harvey
> -------------------
> Heliocentric, LLC
>
>
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