Even the most efficient diesel generator turns 2/3 of the fuel input into heat.
By capturing this heat and putting it to use one is practicing co-generation. The obvious advantage of co-generation is less energy is wasted. Economically this means that less money is wasted. Environmentally this means less pollution is created. Politically this means we are just a little less dependent upon the rest of the world. These are all good things.
There are some other less obvious advantages to co-generation as well. Much more public funding is available for co-generation plants that use renewable energy than for plants that just produce electricity. Even for individuals hoping to tie into the existing electrical grid and sell back electricity they produce themselves this is much easier if the plant producing that electricity also produces heat that is captured and put to productive use.
The energy potential of the fuel used in a diesel generator is mostly turned into heat in the effort to turn it into electricity. Capturing as much that heat as possible is the primary goal the any co-generation project. The majority of this heat is a result of the combustion of that fuel and is lost along with the exhaust gases to the atmosphere normally. Capturing this heat is relatively simple using a coolant jacket exhaust pipe. As exhaust gases pass through this liquid cooled tube heat is absorbed by the coolant surrounding the hot exhaust pipe. These are relatively simple and inexpensive to fabricate using common steel exhaust pipe and black iron fittings but there are a few rules one needs to follow to keep costs low and efficiency high.
1) The pipe through which the exhaust passes the roughly twice the diameter of the exhaust outlet on the engine to avoid creating excessive back pressure.
2) A 50/50 coolant/distilled water mixture should be used as the heat transfer medium just as in the engine itself.
3) Insulation should be used liberally to avoid loss of the heat captured in this heat exchanger.
4) The heat exchanger itself should be large enough that the coolant exits at a temperature within 30° of the exhaust gases as they exit the heat exchanger.
5) The interior pipe of the heat exchanger must be easy to clean since the process of removing heat from exhaust gases will cause particulates to form and deposit there. Even a very thin build up of particulates will lower the efficiency of heat transfer.
6) Provisions must also be made for liquids that will condense out due to the cooling of the exhaust gases.
All of the above is actually fairly simple to provide for if you know what you're doing.
Most of the remaining heat of combustion is absorbed by the cooling jacket around the cylinder itself. For maximum efficiency of combustion the coolant in this jacket should be kept in the 180°F -200°F range. Is best accomplished by the use of a common automotive thermostat which will assure that once the engine is up to operating temperature it will remain there. The engine cooling jacket can be linked to the coolant jacket and heat exchanger to the coolest coolant returning is available to cool the engine whenever the thermostat opens. This will allow capture nearly 80% of the needed combustion which ends up in the coolant jacket of the engine itself. Higher efficiency is possible by providing an insulated enclosure for the engine from which combustion air is drawn by the air intake of the engine.
It is possible to attain even higher efficiencies by capturing the heat of friction from transfer of the engine's power to the generator as well as the heat produced by the generator it self.
Just as properly sizing the exhaust heat exchanger is critical for efficiency sizing the engine and the generator itself are just as critical. For maximum efficiency only the amount of electricity actually needed should be generated and the generator should be size to produce that with a minimum amount of fuel. Diesel engines run most efficiently and specific loads and rpm and matching the horsepower and torque of any diesel engine to a generator head is an art based on science. For longevity listeroids should be run at approximately 650 rpm. This allows them to be sufficiently de-rated to run 24/7 with minimum but regular maintenance for decades. This also allows the generator head it is connected to be a slightly larger Kw rating than the engine itself if an exact match cannot be found. Never choose the generator head smaller than the engine that powers that. This smoke that comes out of a listeroid being temporarily overloaded by a too large electrical draw on the generator head indicates only temporary fuel inefficiency. Not so the smoke from an overloaded generator head.
Converting a listeroid to vegetable oil is much simpler than converting a diesel engine in a vehicle. More on that next.