generator heat recovery

A few months ago I found a good deal on a 25 kw diesel generator, brand is "WhisperGen".  It is good for 14.4 kw single phase and runs about 3 qt. of fuel per hour.  It will run my whole spread and two 1500 watt heaters for making biodiesel.  The engine is 37 HP Isuzu diesel and is water cooled.  The radiator and exhaust heat discharge is on top of the trailer mounted unit.  I parked it next to the greenhouse and removed the wheels and leveled it.  I closed in one corner of the greenhouse to heat waste veggie oil for fuel, then I built a plywood duct to the space and added a centrifigul blower.  I extended the stack thru the duct to avoid exhaust fumes in the space.  In summer a door will open to exhaust heat to outdoors.
I found that the enclosed space for heating oil would pressurize, and resist further air movement.  Rather than overheating the gennie, I put a vent into the main greenhouse.  In the greenhouse I run two aquaponic systems with cold water fish.  Even bluegill and gold fish like water temp above 50 deg. to eat and grow fast.  With supplemental heat I can have winter tomatoes.  I heat at night with a homemade oil stove burning biodiesel but it burns almost as much fuel as the gennie and will not run on straight veggie oil.
The reason for heating the oil is so that it will flow thru sock filters and give up moisture to the bottom of the drum.  Heating further, to 160 deg. F. will permit running the engine.  Start the engine on diesel fuel or biodiesel and run it untill the WVO (waste veggie oil) is hot, switch over to WVO untill a few minutes before shut down.  Switch back to fuel to purge the lines.  This saves money and resources in making biodiesel, and yields bigger fish and tomatoes.

Right on.  Have you thought about taking heat recovery a step further and putting a thermo-syphon hot water exchange on the exhaust pipe or engine cowling?

I had to extend the exhaust pipe to get it above the ductwork.  By time it raises that extra three feet the stack is cool enough to touch...about 140 deg.F.  Any further cooling at that point is bound to result in condesation running down to the muffler.  Below that point, in the duct,  it is cooled by the added blower.  I could not use copper in the heat exchange because that kills fish and the fish water is the only goal nearby.  As it is I only need heat to the greenhouse in winter so will open the duct in summer.  I will continue to recover coolant heat for WVO year round.

The duct work is complete and going to the oil heating room in the rear of the greenhouse.  I put in a 12x16" vent into the greenhouse from there.  The oil heat room gets very warm and the greenhouse is noticably warmer after running the genny a couple of hours.  The coolant diverting to the coil in the WVO tank wasnt getting the tank hot so I had to put a metering valve on the inlety to the heated WVO filter and force some circulation to the smaller coil in the tank.  STill the oil is not hot enough to reliable run the engine...so I have to build and add a shell in tube heater.  The blower that I added at the end of the ductwork is not necessary.  The stock blower has enough power to push thru the new duct without raising engine temp.