Efficient inverters for stand alone power systems? [RE-wrenches]

John Berdner jberdner at sma-america.com
Wed Aug 8 11:34:20 PDT 2007


Troy/wrenches:
 
You identified several of the loss mechanisms but missed one of the
larger ones.
Most of the sine wave off grid inverters out there today use 60 Hz
transformers on the outputs.
In order to generate the ac output the inverter needs to provide the
magnetizing current for the transformer.
It is this magnetizing current which is responsible for a lot of the
"tare" loss.
You are correct that the efficiency at very low power levels is poor
due primarily to the essentially fixed power required just to keep the
inverter running.
This is why you see a near vertical slope on the efficiency curves at
very low power.
 
When designing an inverter you can make some trade offs to change the
shape of the efficiency curve.
For a grid tied inverter you want the efficiency peak to be between 50
and 80% of full power.
For an off grid inverter you want the peak to be much lower since the
inverter typically spends the majority of the time operating at lower
power levels.  
In the case of the Sunny Island 5048 the efficiency is > 90% at
anything above about 200 Watts.
The peak efficiency is 95.5% at about 1000 Watts and is > 95% from
about 500 Watts to 1500 Watts.
 
I hope this helps.
If you have further questions feel free to contact me.
 
Best Regards,
 
John Berdner 
 
 
 
 


>>> taharvey at heliocentric.org 8/8/2007 12:34:28 AM >>>

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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