Grid tie with Diesels? [RE-wrenches]

Darryl Thayer daryl_solar at yahoo.com
Wed Apr 11 16:55:59 PDT 2007


Hi Steve
I have not delt with a diesel-solar grid this big, so
take what I say with a grain of salt. I have delt with
solar this big and battery systems up to only 3000 AH,
you will be bigger than this. 

But it seems why have a 500kW genset and a 225 genset,
and only a solar 60 kW?  How does the 60 kW carry the
load? At night only?  A better description of the load
would be helpful.  Please note if the solar during the
day is directly grid tied, there is a reduction in
diesel fuel usuage.  BSFC (brake specific fuel
consumption) on a Diesel is rather flat over a large
power range.  (means the fuel consumption is dependent
upon the power delivered mostly, this is unlike SI
engines)

If the 225kW can carry the load why not have a second
225 kW as a backup?  If the Diesel is the core for the
3 phase power, several PV inverters can support the
Core, but it seems supporting the core is very
different from creating the core with the inverters.  

Battery life:
For battery cycling 50% might be too deep for daily
cycles, I think that 70% or 80% SOC or 30 or 20%DOD
would give longer Battery life.  Further as the
batteries age, that 50% DOD is going to get hardeer
and harder to reach. The batteries will recieve a lot
of heat in this process. 

I think that a repetitive discharge rate of up to C/10
will lead to long battery life.   Furthermore
batteries need to be charged at a slower rate.  Where
C/10 from 50% DOD to 30% DOD is ok and C/20 from 30 to
10% the last 10% should be charged at about C/50.  The
batteries need to be taper charged and as they reach 
full charge. This means the charge rate must be slow
near full charge.  At 50% DOD it might take 12 hours
to recharge and let the batteries cool.  

Darryl
--- Steve Johnson <stevejohnson at comcast.net> wrote:

> 
> Geof,
> I'm budgeting a system like this too, but with a 500
> kw diesel genset 
> (proposed by others).  This genset would be transfer
> switched to another 
> system backed up with a 225 kw diesel which is in
> tandem with some 
> inverters with batteries and some grid-tied and AC
> coupled with PV.  
> Maybe 60 kw PV.
> When the 225 diesel generator is on it would run
> close to full bore, say 
> 80% or so, charging batteries and running the grid. 
> When batteries get 
> charged it shuts off.
> The grid tied PV and battery/inverter systems would
> operate the system 
> quietly as long as capable.  When batteries get to
> 50% the 225 diesel 
> kicks in and supplies grid while charging batteries,
> then shuts off 
> again.  This is just a preliminary design and no I
> haven't done a 
> successful (or unsuccessful) project like this
> before.
> 
> The battery inverters would have to set the "tone"
> for the grid as they 
> are the only thing always available.  But if they
> set the grid standard 
> then how can the genset match it?  This is where I
> am stuck. But then it 
> is late . . .
> 
> 
> Steve Johnson
> LightWave Solar Electric
>


       
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