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  • Sunking
    replied
    Chris I am not going to debate the merrits of Sandia, IEEE, or John with you. Every pro in the biz knows who they are and stake their careers on them and their collective research. Kind of like a college student arguing with NASA on rocket engine design. Nor am I going to debate your method with you if you are satisfied with it. I agree with some of your points just not all of them. I am not saying using a generator is a bad thing OK?

    My only question, and I hope I read it wrong, is are you saying the XW series inverters sync with a generator? That cannot be and I hope I misunderstood. The XW does have generator input, but does not sync with the generator to my knowledge, it uses the built in AC charger to rectify to DC power. It would be almost impossible to sync a small generator to an inverter. Generators outputs are way to unstable to do that on a small scale. Can be done with Prime Movers like dual 2 mw units, but not 2 kw units. A small change in load will change frequency and voltage by a significant magnitude the inverter could not follow. Basically works like any hybrid EV, UPS, or battery plant.

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  • ChrisOlson
    replied
    Originally posted by Sunking View Post
    Sandia National Labs, John Wiles (the father of off-grid solar and who writes NEC 690), and IEEE all recommend 5 day minimum reserve capacity, and C/8 gen backup. I prefer to stick with that group and use it in my practice.
    Sunking - I am going to submit that Sandia and John Wiles have neither the off-grid living experience, nor the equipment that I have, to develop and prove that there are more efficient methods. For instance, there are lots of off-grid homes up here and I know of nobody that has a large generator sized at C/8 that doesn't check their pockets first to see if they got enough left to be able to afford to even stick the key in the switch. The end result is that these off-grid folks avoid running the fuel sucking thing at all costs and the battery ends up be abused instead of managed.

    You might want to check into a "study" done by Sandia back when they developed what they called their ACONF generator management system for remote off-grid hybrid solar/generator installations. They used a DC genset and came up with a "revelation" on what I had been doing for years before they did their one year "pilot study" on it.

    Basically Sandia's ACONF used the genset for prime power to reduce cycling on the battery and keep it in its bulk charging range, and let the solar do the actual charging. They ran two systems side by side, the reference system designed in the "conventional" terms, and the ACONF system designed identical to mine (except I use AC gensets these days because they are more efficient than DC when you have AC loads).

    The results of their "pilot study" was that the managed system used the generator 17% more frequently than the reference system did (same here with prime power). However, the total generator run time was reduced by 38% vs the reference system. Fuel consumption reduced by 25% vs the reference system. Pretty much identical to what I had come up with years before, and have been doing all along. Only difference is that when the government does it they proclaim they just discovered something new!

    I fined tuned it because real off-grid homes have peak loads that requires a genset of larger capacity, and prime loads that require a genset of lesser capacity, further increasing the system efficiency over using one genset. The systems they tested had a simple static DC load with a resistor bank, and is not near as complicated as a real off-grid home as far as managing demand from batteries with wildly varying loads. Like I said, they don't have the off-grid experience I got, and IIRC they were testing all this for the USCG.

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  • Robert1234
    replied
    SunKing,

    Thanks for the clarification. I can see the benefits of both ways (high and low DOD implementation). Like you say... Anyway you slice it there ain't no free lunch.


    Chris,

    Your thought pattern and setup is along the same line as what I was originally assembling for our retirement cabin (we have since gotten grid) except I was using a Magnum with a NiFe bank. The batts were really only used for light loads. When I ran the bank down or pulled amps hard (and got voltage sag) the generator would kick in.

    A huge difference (make that potential improvement) in your setup is that the generator and battery bank add together. That gives you a continuous kW capability higher than either of the two systems alone. I didn't know there was an inverter capable of synching with a genny. Very slick.

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  • Sunking
    replied
    Originally posted by Robert1234 View Post
    If I take the typical battery curve & multiply the DOD & the number of expected cycles, it seems that the total kwh available over the life of the battery bank divided by the total investment is pretty close to a constant. Is this true? (I realize my "typical curve" may be not be totally accurate, but it looks fairly close to what SunKing displayed earlier.)

    My point in asking is that in a higher DOD setup the initial fixed investment is lower and the total (long term) investment with regards to battery consumption may be essentially the same as that of a low DOD design. (Battery replacement less often at 20% DOD, but it will also be at a much higher cost due to the increased bank sizing).
    You got it and has always been my point from the start. You can pay me now or pay me more later. In addition to fuel cost of a genny, there is maintenance and replacement cost to factor in. It is not easy to weigh in all the factors.

    Sandia National Labs, John Wiles (the father of off-grid solar and who writes NEC 690), and IEEE all recommend 5 day minimum reserve capacity, and C/8 gen backup. I prefer to stick with that group and use it in my practice.

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  • ChrisOlson
    replied
    Originally posted by Robert1234 View Post
    If I take the typical battery curve & multiply the DOD & the number of expected cycles, it seems that the total kwh available over the life of the battery bank divided by the total investment is pretty close to a constant. Is this true?
    I hope Mike doesn't see this as off-topic because it as the basis for how I propose an off-grid setup should size the battery bank, and how we have sized ours.

    Correct me if I'm wrong, but I don't think these charts take into account charging efficiency. I developed a spreadsheet about 5 1/2 years ago to figure this out and I can't find it. I have switched computers in the mean time and it may have gotten deleted or lost along the way.

    But the basis of my design is that lead-acid batteries are more efficient on charging during bulk stage than they are during absorb, i.e. if you run long deep cycles you will waste less of your RE energy you produce in heat in the battery. Versus cycling shallow and absorbing more often, you put energy into the battery that you never get back out. So the net result is that when you cycle on the 20 - 80% DoD part of the curve the battery's efficiency is higher, gaining you more kWh out for what you put in. When you cycle on the 0 - 20% DoD part of the curve the efficiency is horrible and 10-15% of your harvested energy goes up in smoke in heat loss in the battery.

    The other factor on life expectancy is that batteries don't like heat. Our logging system logs battery temp once per minute, 24 hours a day. My logged data shows that overall average operating temp of our batteries is 5-7° C cooler cycling deep vs shallow cycling with frequent absorbs.

    So I have become a firm believer in that what GB Industrial says is true - do not charge that battery and cycle it unless it absolutely needs it. Our logged data shows that we are cycling our battery once every 3.26 days on average. Some cycles go 7-10 days, some are cycled every day when our loading is too light and RE production is high. But it comes out to 112 cycles per year with our management regime. My belief is that the major design flaw in most off-grid systems is to cycle the batteries too shallow and cycle them every day, and the resultant efficiency is horrible.
    Last edited by ChrisOlson; 03-18-2014, 10:53 AM. Reason: addtional info on battery cycle frequency

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  • Robert1234
    replied
    Following up on the DOD discussion.... (Mike if after this is answered you feel it should be deleted from the sticky as being off-topic, by all means do so but this has been running around in my head and it's as good a time as any to ask about it.)

    If I take the typical battery curve & multiply the DOD & the number of expected cycles, it seems that the total kwh available over the life of the battery bank divided by the total investment is pretty close to a constant. Is this true? (I realize my "typical curve" may be not be totally accurate, but it looks fairly close to what SunKing displayed earlier.)

    My point in asking is that in a higher DOD setup the initial fixed investment is lower and the total (long term) investment with regards to battery consumption may be essentially the same as that of a low DOD design. (Battery replacement less often at 20% DOD, but it will also be at a much higher cost due to the increased bank sizing).

    Cycles.jpg

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  • ChrisOlson
    replied
    Originally posted by Sunking View Post
    Well a 5 day reserve only works out to 2.5 days of autonomy so as not to go below 50% DOD. Once below 50% lead sulfate crystals begin to harden and you will not be able to dissolve all of them with a full charge.
    I want to add some information to this thread to show folks how this all works in a real-world installation that does not use theories and instead uses logged data to prove it. They say pictures are worth a thousand words.

    We started the day's logging at 12:00AM with our battery at 88% SOC. We went all night using very little power and woke up this morning to snow and freezing rain coming down. The weather forecast is not good for the next 4 days at least. We never seen the sun all day and never got much from wind either.

    What do most off-grid people do when this happens? They continue on battery power until the point comes where they have to use a genset and inverter/charger to charge the battery back up. We don't. We went until noon and then decided to start and run our little prime genset all afternoon at 1.8 kVA generator support setting. The little generator will carry loads up to 1.8 kVA, then the inverter starts helping it from battery power if the load goes above that. When the load drops below 1.8 kVA, no energy is used from the battery and the load is carried by the generator. The generator ran for 4 hours, 58 minutes and burned .87 gallons of gas, which cost us $3.08

    So what did it achieve? This is the graphed data showing loads vs battery discharge for the day up to the 7:00 PM hour. If the graph is blown up the battery discharge line is very slightly above the load line thru the night time hours, even though they look the same on this small graph pulled from the tablet computer:



    Just before lunch time we made the decision to start the little genset and run it for about 7 kWh today. This is the loads vs genset input to the system. The kVA (actual load) setting for the generator is always higher than kW shown on the graph because of lagging Power Factor.



    This is the prime genset output curve for the day, and gen stats showing we used 6.9 kWh from it for just about 5 hours. Our cost for this power today was about 44 cents/kWh as we had abnormally low loads all day. Now, 44 cents/kWh is expensive by utility power standards. But it is pretty cheap by off-grid standards:



    The net result:
    Our battery has dropped from 88% SOC at midnight to 79% SOC tonight as I write this. If we wouldn't have used the little prime genset today, instead of the battery being at 79% SOC like it is right now, it would be at only 64% SOC. Tomorrow would be another poor day and sometime during the day we would be running a genset to charge batteries. And I got data that shows you how much that costs per kWh - and we don't even want to go there.

    Are you starting to see how it all works? Instead of cycling our battery down and recharging with a genset, we use that little genset to manage how fast the battery discharges during poor conditions. We can go 7-10 days doing this without ever using a genset to charge batteries at a cost of 35 - 45 cents/kWh, depending on how heavily we can load that little generator. Today, due to the lighter loads, it is a poor example of cost effectiveness because the prime power is more expensive when the genset is lightly loaded. But it is still a good example of how it all works.

    We can do this for a week to 10 days, starting with a full battery, and only run one discharge/charge cycle on those batteries for the whole period of poor weather. Whether you agree with it or not, it works. Been doing it for years - many years with a DC genset and fewer with AC genset. But it still works. Many folks can't get their head wrapped around the concept because they've been taught that "the way you do it" is to charge batteries with a genset and cycle them several times, instead of managing their discharge rate with the genset and only cycling them once.

    So I hope this helps to visualize how it works, or gives a better understanding of it. It goes against all convention in design of off-grid power systems. But just because all the books say "this is how you do it" doesn't mean the books cover all ways to do it. As an engineer I spent a good portion of my career being expected to think outside the box.
    Last edited by ChrisOlson; 03-17-2014, 10:45 PM. Reason: spelling, extra attachment

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  • ChrisOlson
    replied
    I'm getting questions from folks who have seen the video on auto-starting gensets and what inverters to use. This is a completely different topic but since it does involve integration with the system for automatic handling of peak load events on off-grid systems, I'll provide a brief rundown of what I know about.

    Both Schneider and Magnum Energy build programmable AGS that integrates with the system and will start a genset based on system loading. These controllers can can be used with three-wire or two-wire generators. Of the two, Schneider has 120/240V split-phase inverters (both XW and Conext SW series), Magnum Energy has a MSH4024RE 120V single-phase inverter available with generator support. They are both fairly simple to set up with three-wire gensets, or two-wire, and they are both proven to work.

    In older inverters, the Xantrex SW/SW Plus both have generator support, and are proven over many years to work flawlessly with all types of gensets using either the auto-start relays inside the SW, or the external GSM (Generator Start Module) used with the SW Plus.

    Outback does not build a programmable AGS that integrates with their systems. With an Outback all you have to auto-start a genset is a dry contact Aux port. Since the vast majority of generators used on off-grid power are three-wire, with an Outback you have to buy a pretty expensive Atkinson generator controller to convert three-wire generators to two-wire start.

    The Outback GS8048 Radian is advertised with generator support. However the Radian is primarily designed as a grid-tie inverter. I spent most of one afternoon trying to get one to work off-grid with a 4.0 kVA Perkins diesel genset and it did not work. It would qualify the genset, attempt to load it, then immediately spit it off and repeat. We tried setting allowable voltage and freq range down as low as they would go, to no avail. We were told the genset is the problem so we loaded it up in the back of my Dodge Cummins and hauled it to my place, hooked it up our system to try it on the XW. It worked perfectly fine on the XW. We never did get it to work on the Radian.

    We opened the Radian up and looked at it and they forgot one critical hardware component for gen support. Instead of a center-tapped transformer to assist in balancing genset legs during initial loading they have two paralleled split-phase modules that are basically FX units. If you recall, the FX-series was supposed to have gen support way back when, but it never got done. Gen support on 120V single-phase is pretty easy to do. On split-phase it isn't. Xantrex has years of experience building split-phase inverters. The Radian is Outback's first attempt at one. So it may work on the Radian with an excessively over-sized genset or with a PSX-240 transformer on the AC2 input. We did not have either handy for testing on that particular day. And the owner got disgusted with it and sold it and bought a XW6048. So I am not able to work with it anymore to see what it takes. I was hoping to test it by taking our PSX-240 to the neighbor's place but there was a lot of swearing, screwdrivers and pliers flying the air and wires being ripped out to put in a different inverter before I got there, so it never happened.

    The Outback GVFX-series have Grid Support and some folks have fiddled with them and gotten a rudimentary form of Gen Support to work using inverter type gensets. However, Outback does not even recommend using a generator on a GVFX at all, and they are not recommended for off-grid use. They are grid-tie inverters.

    Supposedly, SMA inverters have generator support in them. They are not as common in the US as Schneider, Magnum or Outback and I have never tested one, or heard of anybody using one with generator support. If somebody owns own and has tested it, it would be nice to hear about it.

    You can use a manually started genset for peak load off-grid power. But it requires user intervention in the form of first manually starting the genset and bringing it online before turning on your high-draw load. And it requires remembering to shut the genset off when your high-draw load is done.

    So at the present, the list of off-grid inverters that have gen support is pretty small:
    In 120V single-phase:
    Xantrex SW or SW Plus (proven over 20+ years)
    Magnum Energy MSH4024RE (fairly new but I have two reports of them being used successfully with generator support)
    SMA Sunny Island (is supposed to have it but I have never heard of anybody using it or testing it)

    In 120/240V split-phase:
    Schneider Conext XW-series (proven over many years)
    Schneider Conext SW-series (fairly new and I have one report of being used successfully with generator support off-grid with a CPE generator)
    Outback GS8048 Radian (fairly new and may require a balancing transformer, and may work with an excessively over-sized genset that can handle high split-phase leg imbalances).

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  • ChrisOlson
    replied
    Originally posted by Sunking View Post
    Chris don't get your shorts in a knot, I agree with a lot of what you are saying, just not on the batteries, and might have a few minor issues on generator sizing.
    Sunking - no worries there. I don't even wear shorts

    I should qualify and quantify what I am saying. If you buy Trojan T-105's and cycle them to 80% DoD they're going to be shot in 2 years. Of that there is little doubt. But we do not have light duty batteries here. We bought The Best There Is when we bought batteries. And that is why I said in my original post "real deep cycles". I have seen time after time where somebody says if cycle your batteries below 50% SOC they will be ruined for life. This is not true. Maybe for some real light duty hybrid deep cycles you buy off the shelf at Walmart. But not for Rolls 5000's.

    I went thru all the math on this when I originally figured it out (I had a spreadsheet that I designed to figure it out and if I can find that I'll post it), and how many kWh you get into and out of the batteries vs cost when it comes time to replace them. And that's why we manage our system the way we do. It has been working well here because the health certificate is issued when you have to discharge them to 80% DoD and see what they actually got in them. And at 5 years ours pass.

    As inetdog mentioned, each of our cycles does not go to 80% DoD either. Some are shallow cycles and I hate those because they seem to be hard on the battery. If we go for a couple weeks in the summer and the batteries get shallow cycled like to 20-30% DoD they lose capacity on the first discharge to 80% DoD. We don't get that capacity back until we pull 'em almost dead and then fully recharge. My logging data shows it plain as day. These big tall suckers need to be boiled to keep them mixed up or the dense electrolyte starts to settle to the bottom and they go "flat". Only way to do that is pull them down until the charge cycle can actually do something about keeping the battery mixed up and healthy.

    So I see it as a multifaceted issue that has a lot more to do with it than how many (theoretical) years you can get by shallow cycling vs deep cycling. When I figured it out I went with cost/kWh over the expected life.

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  • Sunking
    replied
    Well a 5 day reserve only works out to 2.5 days of autonomy so as not to go below 50% DOD. Once below 50% lead sulfate crystals begin to harden and you will not be able to dissolve all of them with a full charge. The deeper you discharge, and the longer you leave them discharge accelerates the process. 95% of all battery failures are sulfated batteries. Every well cared for battery will die from sulfation. The question is how long before that happens and how to delay it.

    Chris don't get your shorts in a knot, I agree with a lot of what you are saying, just not on the batteries, and might have a few minor issues on generator sizing.

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  • Mike90250
    replied
    1) I'm making this a stickie. [done]

    2) because it's a stickie, and of issues we've had before, all replies are going to be closely monitored, and the "me too's" will be purged to allow focus to the meat of the issue.

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  • inetdog
    replied
    Originally posted by Sunking View Post

    Thanks for providing a "typical" battery cycle life graph that we can look at.

    The first thing that you see is that at 20% DOD, the number of cycles is about ~2500.
    At 80% DOD, the number of cycles drops drastically to only ~500.
    The first reaction to that is "Oh my, the life in cycles is only one fifth of what it would be with the lower discharge level."
    And that would be perfectly correct if what you are looking at is a system that is designed to draw the batteries down to 80% DOD every day.
    There are two other ways to look at the lesson that this graph is teaching us:

    1. If you plan for 20% DOD and accept four days of no sun before being forced to start the generator or conserve drastically, then you are at 80% DOD and find that you have used up the battery life equivalent to five single day 20% cycles. But that battery life increment has carried you through four days without running the generator.
    No longer the simple factor of five loss of battery life that we took from the first look.

    2. If instead of designing for 20% DOD and getting ~2500 daily cycles, we design for 80% daily DOD, we will be replacing out batteries five times as often. But we are only buying a battery bank that is one fourth the size. Still not a good deal and probably a bad way to design if you want reserve capacity and autonomy, but not the factor of five misjudgement that we thought we saw at first glance.

    So, if we assume that we are just talking about batteries which follow that sample curve exactly, we find that higher discharge depth does cost system life, but is still withing the range of an engineering tradeoff rather than a factor of five mistake.

    What I have not seen so far is a graph of the effect of say, 10% of the time using 80%DOD cycles mixed with 90% of the time using 20%DOD cycles. That mix sounds more like what Chris is discussing in his last point. He is still recommending as design DOD of only 20%.

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  • ChrisOlson
    replied
    Originally posted by Sunking View Post
    If folks go discharging to 80% DOD are asking for very short battery life, It will take that 5 year battery and turn it into 2 years or less. Just about every DOD vs Cycle life graph published looks like this.
    Been doing it to ours for five years come March 21 and not a single problem with it. I have very sophisticated equipment on our system that keeps track of every kWh into and out of the batteries and they still have very close to rated capacity on discharge.

    It's all about cost/kWh. You can cycle a battery shallow and get more more years from it. But that doesn't mean you got more kWh into and out of that battery. Or you can buy less batteries and work the snot out of them and replace them more often, getting better charging efficiency and more kWh into and out of them over their life. I prefer the latter method. Getting 7-8 years down the road and dealing with batteries that only have 50% original capacity left and trying to squeeze another couple years out of them when they're half dead doesn't save you any money because their charging efficiency at that age is going up in smoke. You're throwing your energy you put into them right out the window.

    We also use quite sophisticated methods to reduce cycling. We don't recharge every day and waste a cycle in the battery's life. During periods of poor RE production our batteries will sometimes go thru a gradual 7-10 day decline to a very low SOC before being recharged again. That is only one cycle over that 7-10 day period.

    Not every manufacturer recommends cycling below 50% SOC. OTOH, some specifically recommend it, like GB Industrial, who doesn't recommend recharging their forklift batteries at ALL until they reach 80% DoD. Rolls specifically recommends deeply discharging shallow cycled batteries at least once per month to keep them healthy.

    As usual, check with your battery manufacturer.

    References:
    2. Over Charging and Opportunity Charging
    Industrial batteries are typically designed to last at least 1,500 charge cycles, over a five to fifteen year period. Each time you charge a battery, regardless of how long, it constitutes one cycle.

    Consistently charging a battery twice per day, during lunch breaks for example, is known as Opportunity Charging, and reduces the useful life of a battery by 50%.

    The additional heat generated by opportunity charging a battery usually reduces the run time equal or greater in proportion to the amount of charge it actually received, making the practice completely ineffective and costly.

    Routinely charging the battery before it is 80% discharged is another common form of over charging. For example, if you only use the battery a few hours a day, it’s best to use it until it is truly in need of charging before actually plugging it in. Remember, each charge constitutes one cycle, so try not to charge unnecessarily.

    http://gbbattery.com/FAQ.html

    And here is Rolls' cycle depth vs cycles for the 5000-series. At 80% DoD you are over 5 years life assuming you would cycle that battery every single day. Our batteries, according to our logging data, get cycled about 112 times per year with our present management regime. At their published number of cycles to 80% DoD we should expect 17 years from them. In reality our goal is between 7-10:
    http://support.rollsbattery.com/supp...discharge-5000

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  • Sunking
    replied
    Originally posted by ChrisOlson View Post
    There is a common misconception that you cannot cycle batteries below 50% DoD or it will ruin them. This is totally and blatantly false for real deep-cycle batteries. They can regularly be discharged to 80% DoD and it doesn't hurt them one bit. And in fact, it makes them more efficient on charging.
    Sorry got to bust you on this one as being absolutely false. There is not one bit of credible data to support that statement and contrary to every bit of published data by the manufactures and third party testing. If folks go discharging to 80% DOD are asking for very short battery life, It will take that 5 year battery and turn it into 2 years or less. Just about every DOD vs Cycle life graph published looks like this.

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  • ChrisOlson
    replied
    Originally posted by pleppik View Post
    Have you explored trying to recover waste heat from the generator to supplement your home heating?
    No. We heat 100% with wood in a central forced air Daka furnace (made in Pine City, MN) that has a catalytic recombustor on it to burn the particulates in the wood smoke. We have a virtually endless wood supply here that will last us for over 1,500 years just doing select cutting of mature hardwoods on our own property. There is no way we can keep up cutting the mature trees for firewood to open up sunlight area for the new undergrowth before they die. I have probably 3 years worth of harvested logs that I haven't processed, but got out of the woods because if I don't they will start to rot and go bad. I sell processed firewood and chips to about 30 customers in the area and still can't keep up with the rate that hardwoods reach maturity and are ready for harvest on our land.

    It is not worth it to mess with CHP when we have a fuel source like that.

    We do not burn any propane at all, nor will my wife allow a propane line into the house. Years ago we had an explosion in a rented house we lived in that blew one wall and all the windows out and lifted the house off its foundation and shifted it about 2 feet. It was due to a valve on the water heater that malfunctioned and we lost most of our personal belongings from it. We had a propane genset for a couple years and my wife made me hide the tank for that behind a rock wall and bunch of pine trees where she couldn't see it. But it bothered her to no end that it was there. And the propane genset didn't work worth a crap anyway in cold weather so we got rid of it.

    IMO, propane is one of the worst choices for off-grid fuel. You couldn't even buy the stuff around here this winter because all the suppliers ran out. And the last they had when they were rationing to people for fillups, they were selling for over $7/gallon. Plus there is no way in hell a LP truck can get in here. Our 500 gallon tank for the genset used to be mounted on a wagon running gear and I towed it 25 miles to where the truck could come and fill it up, then towed it home. The only upside to propane is its shelf-life. But it has so many downsides that it's not even worth considering unless you live in a tropical climate like Missouri. I can haul a 500 gallon tank of diesel fuel in here and it's got one hell of a lot more energy in that 500 gallons of diesel fuel than 500 gallons of propane has, for less than half the cost per BTU.

    The only thing we use the waste heat from the gensets for is to heat the powerhouse in the winter. The powerhouse has thermostatically controlled ventilation that keeps it at 75F in there in the winter for the air-cooled engines so they don't run too cold. Never did get into using liquid-cooled genset because it takes too damn much time to warm them up properly before they can accept full rated load.

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