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This question is based on the premise that using "cheap" biofuel in a diesel engine to power a generator (and not using batteries) is better than the alternatives. My understanding is that if you must grow crops to generate electricity that burning more of the plant is better - the cogen/trigen units I've seen mostly run of primarily-cellulose pellets rather than plant oil.

A 25kW generator will use about 2 gallons an hour at full power, perhaps 0.5 at idle, so I'm guessing 20 gallons a day. I suspect using a battery instead of idling could drop that but exact numbers are hard to find because RAPS using backup generators dominate the search results. Oilseed crops run as high as 240 gallons/acre/year (gross yield, not net!) and at a societal level we can't both eat and drive biodiesels.

20 gallons of fuel a day is about 7000 gallons/year requiring somewhere between 30 and 300 acres of arable land to grow. Alternatively, the average deep-fryer shop should need to dispose of about 5 gallons of oil a week but in practice it seems to be less than half that. So 140 gallons a week will mean visiting about 30 fast food outlets, plus a few extra to supply the vehicle fuel for those pickups.

Is it really better to burn plant oil than the whole plant?

As secondary questions:

  1. is it more efficient to use a battery pack instead of idling the generator at low-demand times?
  2. are my numbers for fuel consumption and DIY oil cropping right?
  3. is it practical to run a generator off scrounged vege oil like this?
  • In some areas this waste oil is already all used up for commercial biofuel. Certainly in most UK cities there are multiple forms collecting it. Some suppliers are involved in the collection making it efficient and invisible. – Chris H Dec 9 '16 at 7:42
  • I suspect the same is true in Sydney, Australia, based on the collection tanks and signage I see outside a few such places. – Móż Dec 9 '16 at 10:12
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    Purely from the standpoint of energy contained in the material you are right. But all kinds of practical issues come into play. For instance, it is physically easier to transport (pump) fluids than plants. And that fuel will have a much higher energy density than the rest of the plant, so you have to transport less mass and volume for equal amounts of energy. (I'm limiting myself to a comment here, because a quality answer would have to describe many more of those practical issues. Sherwood's diesel engine behavior is one). – Jan Doggen Dec 16 '16 at 11:13
  • I'm curious about the cogen units running on pellets, can you provide a link? thx! – mart Mar 21 '17 at 8:28
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Burning more of the plant makes better use of the plant, but requires a much larger setup to be efficient. I suspect that at the scale of us mere mortals, your best bet is to extract and burn the oil, and burn the rest of the plant for heating.

Problem is you will likely get enough waste heat from the power plant to provide most of the rest of your heating needs.

Diesel plants do not like to idle. They get cold and you have problems with carbon build up. You want to have thermally controlled shutters to keep the engine warm.

It would make more sense to have several generators of varying capacity along with a smart controller. E.g. You have a 25 KW a 10 KW and a 4 KW. This will mean that at any given time it will be unusual for any unit to be running at much below half of its rated output. Ideally for this scenario, you have a high current battery/inverter system that can handle brief spike loads so you don't cycle the 10 KW unit for 1 minute because the well pump and the dishwasher came on at the same time.

A better unit than the battery pack would be something like the Velkess flywheel. Alas, it's on hold due to low oil prices.

Also, unless you are bound and determined to be off grid, selling your oil and buying natural gas may be a lower total footprint.

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If burning the whole plant means it comes raw and moist, then it clearly is not worth the point to do it.

If you are in a very sunny part of the world, then yes your plant easily comes in dried, so it should be worth thinking of it.

Then it also comes to the waste you get. Burning the plant totally would probably yield to ashes only, whereas oil production byproduct would be more easily turned into useful composting medium.

I would say burn the oil and do something else with the byproduct of its extraction.

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