Off-grid with Diesels? [RE-wrenches]

[Darryl Thayer]

Hi Steve,
Diesels fuels run from 130,000 to about 150,000
BTU/gal (depending upon the weight).  a ususally
accepted number is 140,000 BTU/Gal.  Most modern
Diesel generators run about 40 to 45% eff. when
loaded. (Very large engines will run up to 50%)  The
cat is probibly 45%.  (with 3410 Btu/kWh) This gives
about 40 kWh/gal.  at 50% efficency you get about 20
kWh/gal from the genset.  You can see this is close
but it depends upon fuels and engines.  For midrange
to upper power range operation of the engine BSFC says
about the same, so efficiency says about the same. 
(At light load the efficiency decreases, striclty form
memory I think the efficiency holds with in 10% (45%
to 40%)(constant RPM) of best efficiency down to 10 %
of load. I have not worked on Diesel efficiency since
1975 (oil embargo)

In summary, 20kWh of solar saves about 1 gal of fuel) 
In most diesel operation O & M cost add a lot to total
cost.

Sorry for such a long answer.  Yes you calculated
corrected in your response below.
Darryl     

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

> 
> Darryl,
> 
> Thanks for the good info.
> I don't have info to develop load profile yet.  The
> peak load I referred 
> to is basically a worst case situation, i.e. air
> conditioners starting 
> at the same time etc. and I believe it would be very
> short duration, a 
> minute or two.  However I am not privy to this data
> yet.  Until load 
> profile is developed it is hard to go further.
> 
> I am out of town and don't have the Cat genset specs
> but am interested 
> in the data you present.  
> 
> <This means if you added solar to a loaded Diesel,
> the solar would 
> <reduce the fuel
> <consumption by (kWh fuel saved)= 0.98(kWh
> <solar)/(engine eff). Engine eff is about 40 to 45%.
> 
> <The same article showed only slight reduction in
> wear
> <at lower load.
> 
> Not sure how to interpret this.  If we call .98, 1,
> and call 0.45,0.5
> - does that mean that each kwh of solar generated
> would save generation 
> of 2kwh of diesel?
> 
> If I could equate kwh of solar generated to gallons
> of diesel saved this 
> would be really useful.
> I guess I need the load profile to get diesel usage
> in a 24 hr period. 
> The 500kw genset could use 1/2 to $1m of diesel per
> year.  It seems a 
> shame to throw that kind of money away when a good
> portion of it could 
> be spent in a much more benficial manner.
> 
> The solar will not be large enough to run
> everything.  Maybe in peak 
> hours, but they want 24/7 availabilty.  I'm thinking
> over a 24 hr period 
> one third solar, one third batteries, one third
> diesel.  Of course the 
> batteries would actually be diesel and maybe some
> solar.  Their function 
> would be to soak up excess gneset while it is
> running and provide quiet 
> time, reduce runtime on genset.
> The genrator has to be able to charge batteries
> early am and evening 
> while it runs grid.  Solar handles about six hours
> in middle of day, and 
> batteries pitch in inbetween to hold gen runtime
> down to 8 hours a day.
> 
> Steve 
> 
> 
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