A Couple of generator questions.

solorone · February 2013

What is the preferred LP generator, in the 6500/7000 watt size, that people are using, I am just tired of Gas and diesel and do not see it getting any cheaper. Propane, today at 1.95, may be about the same price after considering the lower BTUs, but the convenience and the ability to sotre large amounts of fuel,,is a big plus.

Also in a conversation to a friend who has gensets and is going to grid tie solar. I mentioned the de-rating of wattage output per fuel type, and he was unfamiliar with it, saying that the genset just used more fuel to compensate for lower power output from the tri fuels that are available. I had a quick look, and sure enough, I can not find the tri fuel gensets with the previously listed varied outputs, i.e., 6500 gas 6000 LP, 5500 NatGas.
So what has changed while I have been snoozing.

BB. · February 2013

Re: A Couple of generator questions.

The generators use more fuel per kWH generated based on the heat content of the fuel (diesel, then gasoline, finally propane on the BTU per gallon ratings). Here is a handy chart (numbers are approximate and will change based on source/additives, etc.):

http://alternativefuels.about.com/od/resources/a/gge.htm

Gasoline Gallon Equivalents

Fuel Type
Unit of Measure
BTUs/Unit
Gallon Equivalent

Gasoline (regular)
gallon
114,100
1.00 gallon

Diesel #2
gallon
129,500
0.88 gallons

Biodiesel (B100)
gallon
118,300
0.96 gallons

Biodiesel (B20)
gallon
127,250
0.90 gallons

Compressed Natural Gas (CNG)
cubic foot
900
126.67 cu. ft.

Liquid Natural Gas (LNG)
gallon
75,000
1.52 gallons

Propane (LPG)
gallon
84,300
1.35 gallons

Ethanol (E100)
gallon
76,100
1.50 gallons

Ethanol (E85)
gallon
81,800
1.39 gallons

Methanol (M100)
gallon
56,800
2.01 gallons

Methanol (M85)
gallon
65,400
1.74 gallons

Electricity
kilowatt hour (Kwh)
3,400
33.56 Kwhs

Different fuels also burn with different efficiencies (i.e., does natural gas burn cooler, therefore engine needs to be derated?). And some fuels (like alcohol) can burn at higher efficiency, but when used in mixed fuel engines (with lower compression ratios) are less efficient in the application.

There are several issues to watch for... Propane seems to have quite variable pricing (both by location and over time). Right now, a small amount of propane costs ~$3.90 per gallon in our area.

And at least one person here found that tank rental/delivery charges were escalating dramatically. The more he conserved fuel usage, the higher the vendor seemed to be raising the "fixed costs" (other threads here have discussed the same issue with electricity, natural gas, telephone, city water, and other ways companies get around their fixed infrastructure costs vs a public that is dramatically conserving).

So--I get back to the basics first--Is the kW rating of your genset appropriate for your needs (i.e., try to run most of the time near 50% of rated output minimum for optimum kWH/Gallon or kWH/$$$ of fuel usage.

Another issue which may, or may not apply, to your situation--Very cold weather and problems starting the genset and keeping pressure high enough in the propane tank as the fuel vaporizes.

Long term storage, fuel stability, and large volume tanks certainly may propane a nice fuel.

-Bill

solorone · February 2013

Re: A Couple of generator questions.

Thanks for the chart, been awhile since I have seen that, let me put the question another was, yeaaaaars ago Winco one the few Tri fuel units, would list different outputs from their genset per type of fuel used. Is this NOT the case now. Do all multi fuel units put out rated power regardless of fuel type??

What are the starting issues and vaporising problems with propane? I would have thought that pressure would be constant once the LP boils off.
I had read, had it inferred that hand cranking an LP genset was a bit of a problem, but my friend does not report any issue with hand cranking LP, did I read this wrong elsewhere?

BB. · February 2013

Re: A Couple of generator questions.

I hesitate to say much more--I am getting out of my depth. And you have some people here like Chris Olsen who's living for many years revolved around these very questions for large industrial generators.

Both LP and Natural gas have much higher octane rating (something like 110 Octane for natural gas?) than gasoline, so you can get much better efficiency if you can convert the engine to higher compression ratios.

And I know for sure, that natural gas does require significant derating when used in otherwise unmodified gasoline engines.

Regarding cold weather and LP... If you have a vapor draw from your tank, the fuel must "boil off" to feed the genset. The boiling of the LP lowers the temperature of the gas and requires migration of heat from outside the tank to the liquid in the tank to keep the pressure up (over atmospheric pressure+pressure drop of system--something like >10-15 PSI).

For example, this website has several charts that relate generator size to BTU per hour to a tank's ability to source XXXX BTU per hour at XX temperature.

A 7.5 kW generator uses around 155,000 BTU/H at full power.

This Generac sizing document (PDF) shows that at 0F, a person would need a 250 gallon or larger tank. Note that as the tank empties, there is less surface area between the fuel and the outside atmosphere, so the vaporization rate will fall as the tank contains less liquid fuel (and buried tanks will have their own pluses and minuses around this issue). I have read about people building a fire under a propane tank to raise the pressure during a cold snap--but I do not know if that was real or just an urban legend.

There are, of course, engines that will use liquid propane and have a vaporizer mounted on the engine block and use the coolant water for vaporizing the fuel.

The other issue is the temperature of the engine itself when starting. There is some sort of heater required to keep the engine above ~freezing (as I understand, not an issue where I live) for easy starting.

Again, back to Chris Olsen, he has posted a lot about his trials of off grid living and, for his ice box location in northern Wisconsin--And between diesel, propane, and gasoline--He found gasoline to the best solution for his home generator (with a very good quality generator).

Dairyland Windpower - Chris & Kristin Olson's off-grid home

BB. wrote: »

Thank you Vtmaps for the original source of the battery voltage curves.

Poster ChrisOlson (and others) has some good advice here for "swimspam" :

Choosing a good generator
genset alternator failure (and if your generator is not generating AC? What to look for/do--Thread started by "Unicornio" from Spain)
Demonstration of Generator Support (A couple inverters that will share AC loads with the generator--smaller generator with large load support)

End result--you get what you pay for.

-Bill

Add 12/6/2012:

ChrisOlson wrote: »

BB. wrote: »

The smallest genset I have seen here so far that can be (more easily) configured to auto-start (automatic choke) is probably the Honda EM4000SX that Chris Olson is using. The choke/mixture is controlled by the electronic engine controller.

I'd like to point out that the little 46538/46539 Champion generators also have an auto-choke and I have mine configured for auto-start with the Trace/Xantrex GSM on the SW Plus inverter. When our Generac would fail to start in cold weather I used the Champion as a backup with the auto-start and it works fine.

It is basically a three-wire start generator and you only have to tap into the wires on the back of the Combination Switch to do it, which is quite easy to do.
--
Chris

-Bill

ChrisOlson · February 2013

Re: A Couple of generator questions.

solorone wrote: »

Do all multi fuel units put out rated power regardless of fuel type??

No, they do not. Any genset designed for gasoline will have a significant de-rate factor on LPG, and more de-rate factor yet on natural gas. Any manufacturer that doesn't show the de-rate factor is using smoke and mirrors. You can't simply shove more of a less energy-dense fuel into an Otto Cycle engine and achieve the same power output when you get a drop in volumetric and thermal efficiency because of it.

You can only shove in fuel up the limit of how many oxygen molecules the cylinder can hold to burn it. Unless the engine is supercharged. But I haven't seen too many Roots-blown gas gensets lately.
--
Chris

techntrek · February 2013

Re: A Couple of generator questions.

First, you'll want to load a genset up to about 80% of its rating to get the best kw per gallon of fuel. The more an engine is loaded up the more efficient it runs, but you also need to leave that last 20% for startup surges and to keep from killing the genset by loading it up too much.

Consider using two gensets. If I had to do it all over again that's what I would do. I'd get a cheap large one sized for my occasional large loads, then a small inverter genset to handle the usual loads and overnight base loads (most houses only need a few hundred watts then).

Sun Dog · February 2013

Re: A Couple of generator questions.

I believe propane boils up to -42C so not likely an issue in Georgia. How full your tank is and the size of your lines can also cause issues. We see temps in the mid to low -40s up here and like to keep the tank above 40% full at all times. At one point we only had a propane range and ran it off of a 20lbs BBQ tank. At temps as low as -32C we had issues, likely because the tank wasn't full enough. Now we have a 500 gallon tank with a 1/2" high pressure line going from the tank to the house. Had -47C this winter and no problems.

I would certainly never light a fire under a tank but you can throw a tarp over the tank and just put a 60W or 100W incandescent light bulb under the tank or use one of those magnetic heaters (200W+ range). Most propane companies will not condone either of the above but will offer a heated blanket that goes around the middle of the tank, just like a big battery blanket if you are familar with those.

Each to their own, but my diesel generator costs about half as much to run as my gas generator. I pay ~$1/L for diesel and ~$1.40L for gasoline.

ChrisOlson · February 2013

Re: A Couple of generator questions.

Sun Dog wrote: »

Each to their own, but my diesel generator costs about half as much to run as my gas generator. I pay ~$1/L for diesel and ~$1.40L for gasoline.

IMHO, diesel power is usually the best all-around option as long as you can come up with a way to reliably start it in cold weather situations. The reason I decided against diesel power here is because the energy input to keep a diesel warm enough to start reliably in the winter time more than offsets the slightly lower efficiency of a gasoline genset that starts with very minimal energy input even in extreme cold.

And we hardly use the genset in the summer time. So that cold weather trade-off in energy input for a diesel makes it not practical for us.
--
Chris

mike95490 · February 2013

Re: A Couple of generator questions.

techntrek wrote: »

...
Consider using two gensets. If I had to do it all over again that's what I would do. I'd get a cheap large one sized for my occasional large loads, then a small inverter genset to handle the usual loads and overnight base loads (most houses only need a few hundred watts then).

My house idle load for 20 hrs / day, is less than 300W The Fridge starting up, toaster, microwave and blow dryer are the high amp loads. (and the well pump)

Sun Dog · February 2013

Re: A Couple of generator questions.

Chris, are you familiar with propane catalytic heaters? I broke one of the cardinal rules and have my generator, batteries and inverter/charger all together in a 10' x 10' shed with one of these heaters. I have the 12" x 12" unit and even at -47C it never got colder than 12C in the shed. I run it for about 6 months of the year (most have shorter winters) and it has worked flawlessly. Only need power to initially start the unit and then it runs 100% electricity free. In hindsite a smaller unit would have been a better match. I'm sure there are other manufacturer's but I got mine from here:

http://www.scottcan.com/thermcat-catalytic-heaters.php

They have a small footprint, are pretty much 100% efficient, explosion proof (haven't tested that yet :roll:) and use no power. Of course if you don't already have a propane system it may not be for you, but a couple of 100lbs cylinders would likely last 4 months if the heater was run 24/7 at 1500BTU/hour.

ChrisOlson · February 2013

Re: A Couple of generator questions.

Sun Dog wrote: »

Chris, are you familiar with propane catalytic heaters?

Yes, I'm familiar with them.

We have our primary generator in an enclosure and it is kept heated by a 75 watt heat lamp - the lamp shines on the cylinder head and carb area and it has started this winter at -37 with no problem at all:

Since the engine is air-cooled having the proper ambient temperature for it is quite important too - running an air-cooled engine at full continuous load in -37 degree weather will destroy it quite rapidly. Our generator enclosure has a thermostatically controlled ventilation system that maintains the internal temperature inside the enclosure at 70 degrees F once the heat from the engine warms it up in there.

This works the best for us. I like to do everything with electric because my off-grid power system generates electricity, not propane. We rarely use the generator for battery charging. It's primary duty is for load support. And when the inverter calls for gen power because it's overloaded the generator has to start and come online at full rated load within 30 seconds - duty that's simply too severe for a diesel generator unless it was kept pre-heated at full operating temperature at all times. The gas genset handles it in stride with no problems at all, even in the coldest weather.
--
Chris

Mangas · February 2013

Re: A Couple of generator questions.

Attachment not found.

I am on my second propane fired 1800 rpm genset. The new unit a Cummins Onan RS 20000 20 kw genset is derated for our altitiude (5,100 ft) to about 18 Kw. For some reason it seems more fuel efficient and quieter than the 15 Kw unit it replaced. Well built.

Next month, we're installing another underground 1,000 propane tank in addition to our existing 500 gallon underground tank. We decided on more fuel storage to better manage (time) fuel prices, provide more delivery flexibility and maintain higher reserves when and if shortages occur. The house runs two propane furnaces in addition to the genset so we are sensitive to winter fuel usage in our remote area. We have a lot of experience with fuel types on the ranch and for many reasons prefer infrastructure propane for non vehicle use.

My opinion is if you can swing it build out your genset 30% larger than you actually need. Not sure where the break point is for a 1800 rpm water cooled unit but personally I would be willing to sacrifice some fuel consumption in exchange for durability and longevity. I realize that ratio is uneconomic but am one of those people that doesn't like to size everything too tight and am willing to spend a little more to have some headroom when the chips are down.

ChrisOlson · February 2013

Re: A Couple of generator questions.

Cummins has always built some of the more top-end gaseous fueled generator sets, and they also come with a more premium price. Cummins also builds the QD (Quiet Diesel) series premium commercial generators up to 12 kVA. The smaller ones up to to the QD8000 are inverter gensets:
http://www.cumminsonan.com/cm/products/diesel
--
Chris

NorthGuy · February 2013

Re: A Couple of generator questions.

ChrisOlson wrote: »

Our generator enclosure has a thermostatically controlled ventilation system that maintains the internal temperature inside the enclosure at 70 degrees F once the heat from the engine warms it up in there.

Chris, could you explain in more details how the ventilation system works?

ChrisOlson · February 2013

Re: A Couple of generator questions.

This photo shows the 20" window fan that forces air in the top:

There is also a 6" diameter exhaust duct fan in the top warm air outlet on the end of the enclosure. Both fans are turned on and off by a Sun Court duct stat inside the enclosure, and the duct stat is set to turn the fans on when the internal temperature rises above 77 and turn off at 70 degrees F. In the first photo that I posted with the end of the enclosure off you can see the orange duct stat to the upper left of the generator.

In really cold weather the fans hardly run at all - probably about 20-30 seconds for every 4-5 minutes of generator run time when the generator is running at full load. In more normal temps above zero F the fans will cycle on and off about 3-4 minutes running and a couple minutes off. Once the weather gets above freezing they will run all the time when the generator is at full load.

An air-cooled engine doesn't give off as much heat as you would suspect to keep it warm in there - my rough guess is that 75% of all the waste heat ends up going out the exhaust, and with the generator only at about 55,000 BTU input, 23% of that is converted to electricity, that leaves roughly 42,350 BTU of waste heat. With 75% of that going out the exhaust, that leaves only 10,500 BTU to heat the inside of the enclosure in cold weather. It can get it up to 70 degrees in there when it's -35. But 10,500 BTU don't go far when you introduce a blast of -35 degree air from that 20" fan on the top.

But it works well. The Honda engine runs at the right temp after it gets warmed up in there so it doesn't hurt it by running it too cold in cold weather. It's been quite happy in there all winter, and starts every single time with less than 2 seconds of crank time.
--
Chris

NorthGuy · February 2013

Re: A Couple of generator questions.

ChrisOlson wrote: »

This photo shows the 20" window fan that forces air in the top

Thank you Chris. How about the exhaust? Have you routed it to outside of the enclosure?

ChrisOlson · February 2013

Re: A Couple of generator questions.

Yes, there is a double wall stainless steel vent pipe that comes directly out the side of the enclosure for the exhaust.

--
Chris

NorthGuy · February 2013

Re: A Couple of generator questions.

ChrisOlson wrote: »

Yes, there is a double wall stainless steel vent pipe that comes directly out the side of the enclosure for the exhaust.

Thanks Chris. This is a very nice setup. Something to copy ...

Is the light bulb that heats the generator constantly on, or do you only turn it on before you need to start the generator?

ChrisOlson · March 2013

Re: A Couple of generator questions.

It is controlled by this thing:

I have the timer set to turn the light on at 3:30 in the morning and turn it off at 6:30 in the afternoon. That gives the generator at least three hours of pre-heat before it might be needed for peak loads (range for breakfast, or whatever) in the morning. And it never auto-starts after 7:00 at night because our heavy loads are either on and running (like clothes dryer going) at that time, or done by that time for the day.

It only turns the light on if the temperature in the enclosure is below 14° F.
--
Chris

solorone · March 2013

Re: A Couple of generator questions.

Well thank everyone for all the input in this thread, the workings of LP have been interesting, amazing at what I thought I knew.

There some great setups here, and I am glad I do not have to deal with such low temps.

I garther LP will not a problem at this latitude and elevation, I had forgotten about the de-rating for gensets per elevation.

I have 1000 and 500 gallon LP tanks, and plan to add either another 1000 or 500. As has been mentioned this makes dickering over price e lot easier, I paid $ 1.95 in August 2012, and a good bit extra on hand is a good feeling, but with more storage, you do run the risk of having a tank failure and a large economic hit, that is if you own your own tanks, which I would guess everyone does, or else you pay THEIR demand price.

ChrisOlson · March 2013

Re: A Couple of generator questions.

When we had our LP genset we had a 500 gallon tank. When we decided LP was too much hassle I had no problem selling the tank. Advertised it on craigslist and sold it for almost new price the first day after I posted the ad.
--
Chris

techntrek · March 2013

Re: A Couple of generator questions.

mike95490 wrote: »

My house idle load for 20 hrs / day, is less than 300W The Fridge starting up, toaster, microwave and blow dryer are the high amp loads. (and the well pump)

Exactly the situation many people have. No reason to run a big LPG genset to keep 200-300 watts going most of the time. I only run my 12 kw genset for my water heater and well pump (during an outage). Cheap screamer will do fine for that.

solorone · March 2013

Re: A Couple of generator questions.

techntrek wrote: »

Exactly the situation many people have. No reason to run a big LPG genset to keep 200-300 watts going most of the time. I only run my 12 kw genset for my water heater and well pump (during an outage). Cheap screamer will do fine for that.

I am considering a smaller one also, but I use the 6500/7000 watt to charge the batteries. These small quite inverter gensets are something to look into and surely have their use in various setups.

techntrek · March 2013

Re: A Couple of generator questions.

Sun Dog wrote: »

...
Each to their own, but my diesel generator costs about half as much to run as my gas generator. I pay ~$1/L for diesel and ~$1.40L for gasoline.

One big problem with using diesels for household loads, is keeping them loaded up high enough to prevent coking. Usually not a problem for commercial outfits which have known - and steady - loads all the time. However houses that need 6 kw for big loads for an hour, then 1500 watts for a few hours, then 200 watts for many hours will have problems. Not true for everyone of course, like if you mostly need a genset for battery charging which is a known, somewhat steady load.

Mangas · March 2013

Re: A Couple of generator questions.

Sidebar, we buried our owned propane tanks and fuel supply lines well away from structures to avoid catastrophic fire danger.

Two years ago we were surrounded by a 220,000 acre desert mountain wildfire. We would have lost the propane and 5,000 gallons of potable water supply tanks if they had been set above ground.

Defensive planning paid off.

icarus · March 2013

Re: A Couple of generator questions.

Do you have to use direct burial tanks? How can you test for corrosion etc?

Tony

solorone · March 2013

Re: A Couple of generator questions.

So back to my original query, what unit in the 6500/7000 watt range would be a smart purchase, I wish I could afford a Cummings LP but doubt that i not a possibility. My Kubota experience, I have been of grid since 1983, has led me to believe I am better off buying 2 gensets over 8/10 years,l than one 4/5 K unit, it is a great motor, but a rascal to work on. I can get a refurb Generac for 1000$, the refurb makes me a bit nervous, but they can be returned for a variety of reasons, a lot of it newbie inexperience/expectations.

ChrisOlson · March 2013

Re: A Couple of generator questions.

The Generac EcoGen is probably pretty close to what you're looking for:
http://www.generac.com/Residential/EcoGen_Series/EcoGen_Series/

The EcoGen is kind of a mid-price range, they're a fairly good unit, and designed for off-grid power. You can buy cheaper, and you can buy more expensive ones. When it comes to generators you get what you pay for, and if you're looking to get one cheap that's what you'll get.
--
Chris

Mangas · March 2013

Re: A Couple of generator questions.

Tony,

The underground propane tanks are specially built and prepped for underground use. Since we have such little rainfall and soil perks well this minimizes the risk. We also shade them in good rock free soil. Lockable riser caps protect the gauge and valve which are above grade.

The 1700 gallon Norwesco water tanks are also specially built for underground use with two manhole and covers each. One receives the grunfos pump and the other is plumbed for the float rod.

Rngr275 · March 2013

Re: A Couple of generator questions.

I purchased my Ecogen last fall and so far (knocking on wood) it has worked great. I really like the oversized oil tank except it requires synthetic oil which is expensive but luckily it doesn't use much. I paid ~$3000 US delivered.

ChrisOlson · March 2013

Re: A Couple of generator questions.

Some of the early EcoGen's had some problems, and they had poor documentation for the 240V kit for them. That's been sorted out and I haven't heard of any significant problems with any of them in the last 8-10 months.

One thing about the EcoGen anybody should be aware of, if considering one, is that the battery maintainer in them requires 120V power fed back to the genset from your inverter. I eliminated that when we had ours and put a solar panel on the genset to keep the battery charged, which worked fine. I don't know why Generac did that for a generator advertised for off-grid use, but they did. And it's stupid.
--
Chris

A Couple of generator questions.

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