Nov 08 note: Just before we get into this after almost 8 years of living with alternative power my thinking has altered a little. If you need power for a heavy workshop or some thing else that requires a large number of kilowatt hours you may well need a grunty genset and a diesel is probably the way to go.
If we are talking about a house that has been well set up and the usual handyman type workshop stuff, I advise investing in as much PV (solar panels) as you can lay your hands on and then, if your design is correct, a small petrol will suffice for the few hours a year you need the back up. Perhaps you can get away with just hiring one when necessary? If you can afford a diesel for this type of duty, Great! go for it.
I still prefer diesels for the reasons outlined below but the capital investment for a good quality diesel is way higher than a small petrol and I think the money is better spent on PV for a number of reasons. The main two being, once installed correctly you can forget about them apart from some minor cleaning occasionally and even this may not be necessary. The other is they generate power silently. We have just invested in some more PV to cut down on our generator run time (Dec 08)
The same cannot be said for a generator. It will require your attention even if its just to fuel it, and if this is all you do i.e. no maintenance, then there will be grief long term
OK, back to why I like diesel engines
Why I think this:
- Diesel is a far safer fuel than petrol because is nowhere near as FLAMMABLE or EXPLOSIVE (important when storing/transporting quantities of fuel).
- Diesel is cheaper as it is not taxed so heavily at the pump.
- Diesel has a better shelf life especially if diesel bug killer is added.
- Diesel generators, from a reliable maker, have a design life up to 5 times or more greater than a good petrol machine.
- There is the possibility bio diesel will become commercially available, and some engines will run on home made fuel.
- Fuels from renewable resources are possible.
- Diesels don't drink as much fuel as equivalent sized petrol's (less volume of fuel to cart around).
- Diesels don't need sparkplugs, ignition coils and leads.
Ok so that's some arguments for a diesel Generator. If the machine is to be the primary source of power for the installation I would suggest that the generator should be water cooled especially if we are talking about larger capacity machines running long hours.
A water cooled machine will generally be quieter and last longer than its air cooled counterpart. HOWEVER there is a even more compelling reason to consider the water cooled machine;
It is generally accepted in engineering circles that the internal combustion engine is about 30% efficient with diesel being slightly more efficient than the petrol engine.
So what happens to the other 70% of the chemical potential energy we pour into the machine in the form of fuel? It is lost to us as heat and noise! BUT if the machine is water cooled we can tap into cooling water of the machine and recover some of the heat. With the addition of a simple heat exchange system we could harness this some of this energy. It takes a bit of engineering to build heat exchangers etc but if you are chasing maximum efficiency......... Its probably only practical on large capacity machines that are reasonably well loaded but its something to think about. I have found with my 6HP Listeroid that there is only a modest amount of heat to be had off the water jacket. I suspect could gain much more from a heat exchanger on the exhaust system but as yet have not tried this. It could also generate a few hassles? I would love to hear from anyone who has tried this especially if it has been successful :-)
Potential uses for recovered heat
- For space heating (This is how the heater in our cars work. so why not use it for our home?)
- We could use the hot water created to:
Heat an insulated concrete slab eg. under floor heating.
Preheat/heat our hot water.
Ironically, the waste heat could also be used to refrigerate!but there is some pretty trick engineering required to achieve this.
At this time I don't have figures as to how much of that 70% we can recover but anything we can claw back rather than losing it to atmosphere has to be good for us our families and the planet, not to mention the wallet!
If you have spent money for your fuel you might as well capture as many of the BTU's you have paid for as possible.