In my work, I manage several different sites with backup power schemes. About half are diesel and the others are propane. Here are my observations:
Diesels.
1. Quieter, all ours run at 1800 RPM(never seen a 1500 RPM genset, perhaps 50HZ?).
2. No issues with fuel storage. Our sites are checked for water accumulation and bio-bore added annually and that is it. We store between 250 and 1000 gallons and they are only topped off every other year, and only a few hundred gallons at the most. If you share it with a furnace fuel oil storage tank, it will get regular refueling and useage and fresh infusions of commercially treated fuels. If you have a diesel tractor, you can also use this fuel to keep a steady use of it so it is re-fueled on a regular basis.
3. These are the most efficient due to their higher torque, lower RPM and direct drive, around .125 gallon per KW per hour.
Ultrarunner, I would guess that that 10GPH is probably closer to what you are actually using at 30% load. a 150KW diesel generator is going to need closer to 20GPH at full load. That is at or near peak efficiency(close to 80% load), The efficiency falls off with a light load and 30% is pretty light for a diesel.
Propane:
1. Good fuel storage characteristics, but I have lost 2 full tanks of propane to the atmosphere due to regulator failures over the last 2 years. It dosn't take much of a leak to vent a whole tank away on an unattended tank. One of these was a recently filled 2500 gallon tank(2000 gallon lost) at nearly $3 per gallon delivered to the remote site. No real environmental concerns if you do have a spill though, unless there is a fire
.
2. louder in operation. Due to lower available torque, they either need to run at 3600 RPM or have a reduction gearbox so engine can run at peak HP RPM.
3. Fuel efficiency is less because of lower BTU content in the fuel, lower available torque and higher required RPM(more bangs per minute). I have an oversized 100KW genset that is only powering a 21KW load. At 25% load, it consumes around 7.6 gallons of propane per hour. That gives me about 11 days runtime on that 2000 gallons of propane. Since it is basically BTU to KWH, I would expect a 40KW load to use better than 10GPH of propane, so a 500 gallon tank would last 50 hours.
Cost wise, our diesels are cheaper to operate and easier to get fueled as we have more diesel suppliers than propane suppliers in our area.
Can your utility guarantee natural gas delivery during a power outtage(most can), but depending on your location, other disasters may shutdown that distribution system. A backup isn't much good in an emergency if it can't get fuel. This was a big issue for many last winter here in the northwest. They had the portable generator, but couldn't get gas for it as few gas stations within 50 miles had power.
Backfeeding and making a downed line hot is a possibility with a improperly wired installation, but it is also very unlikley that it could happen, as most likley the commercial line break is not between you and your neighbors. As soon as you backfeed your neighborhood circuit, all your neighbors homes are gong to attempt to draw power and your little generator is going to promptly stand on it's ear trying to feed that massive load. Not even an issue with a properly installed transfer switch or an interlocked backfed main panel.
Are you sure you need 40KW? You didn't say what your exact situation is, but that is a lot for a residence. I would not size a backup generator based on service capacity. Every KW you generate is horrendously expensive once you factor in all the aqusition, maintenance and fuel costs. A generator that is way oversized for your average electrical load is just wastefull, particularly for backup use as big gens cost bigger bucks. It is also hard on the generator, particularly a diesel as they do not live as long when run under a low load.
As an example: Take this with a grain of salt, as I tend to be a minimalist by nature(except when I post
). I have a 200 amp singe phase service to my home. My backup generator is 3KW. Yes, you read that correctly as THREE KW. This powers well pump, reefer, freezer, pellet stove, as many lights as I want to turn on(most all CFL's), TV, 2 small stove burners and microwave(not all at the same time of course
). Everything except the electric hot water heater, oven and electric cloths dryer, and I havn't really experimented with the oven yet but it is pretty close to my limit. My average load is well below this, and I will turn on some electric heat in the house(1000-1500W) when not cooking, to keep the generator load up(diesels don't live as long when under-loaded). I can hit my max capacity when all the auto appliances run at once, but this is rare, unless the power has been off for a while, in which case I can let them catch up one at a time when I switch in the generator, before I start to turn on other loads.
It is a water cooled single cylinder slow speed diesel and I am in the process of plumbing it for recovering the waste heat to maintain my hot water tank at 120F. When this is done, I have everything to keep my family comfortable when the local PUD fails to meet it's obligations(usually due to circumstances beyond their control). Running 12 hours a day at a 2KW typical load, That is 1/4 gallon average per hour or 3 gallons per day. A 55 gallon drum of fuel will last 18 days at that rate. Since I will use this tank as a fuel supply for my tractor as well, I will continue to buy it 5 gallons at a time on my way home from work as I have for the tractor over the past few years. This keeps the fuel rotated and fresh.
I am also a diesel fan as they seem to have the edge on efficiency and supply in my area.
