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Author Topic: FYI - Real-life A/C on inverter power consumption numbers  (Read 2440 times)
Lin
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« Reply #15 on: June 23, 2014, 04:21:05 PM »

It's funny, the numbers work opposite for me.  We need a good onboard generator anyway, so using it to run the AC's while traveling does not entail any extra cost.  Since driving the bus most be about $.75/mile, the added generator fuel is not significant.  I would like to be able to run one AC through the inverter, but I would need a bigger alternator to start with.  Is there any extra wear issue for the AC unit running on modified sine wave?
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« Reply #16 on: June 23, 2014, 04:46:56 PM »

It's funny, the numbers work opposite for me.  We need a good onboard generator anyway, so using it to run the AC's while traveling does not entail any extra cost.  Since driving the bus most be about $.75/mile, the added generator fuel is not significant.  I would like to be able to run one AC through the inverter, but I would need a bigger alternator to start with.  Is there any extra wear issue for the AC unit running on modified sine wave?

I don't understand. You say running the generator to power the AC unit(s) does not add significantly to the fuel cost, but then you ask about running an AC on inverter?

I doubt if any AC unit knows if it is on shore, generator or inverter power, but if you are running it on inverter you are adding heat to the alternator which has to generator the power, and the engine is working harder to run the alternator.

I fully support the use of generators to run the house AC systems while driving in a coach not equipped with engine powered full coach AC.
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Jon

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Knoxville, TN
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« Reply #17 on: June 23, 2014, 05:11:04 PM »

Debo, thanks for the numbers. Our 4024 Magnum inverter is non-hybrid, but it is, none the less, a phenomenal machine. Reading about your setup helps me think about mine.

We full time in our 40' Gillig, and we're midway through our second year of doing so off grid. Four 255w Samung panels are our primary power, but we have an old 4500w Kohler gen set for backup. Four 8D AGM batteries give us 490 amp hours at 24v.

The reason I'm so interested in your experience is that I'm about to add AC to the mix. We survived last summer and this one so far with fans only, but we'd like to have an AC option. I'm planning to use the smallest, cheap window unit I can find, cut a hole for it high in the bulkhead wall above our headboard, and run it most afternoons and especially hot nights on the battery bank. We removed the original air handler early in the conversion, and I've always planned to put a small AC there. I do not have autostart for the generator, but I have the auto shutoff feature on the inverter set to its highest voltage. I think we can have air con when we need it most without damaging batteries.

Obviously, I will not try to cool the entire bus with such a small unit. Our bedroom/bathroom will soon be isolable from the rest of the living space and well insulated. We can retreat there on especially hot days. We are trying to live on a "solar budget", and we will not install larger capacity AC.

My plan for charging house batteries while the engine is running is to add a high capacity, 24v alternator to our 40 Series Detroit and drive it with a separate belt. That step is not motivated by a desire for AC, but it could easily run such a small room cooler while we're driving. When we're traveling, the solar panels alone should keep up with all our electric needs, but I like having charging options. Generators do break down. I have shore power capacity but seldom use it.

I do appreciate the time you spent measuring loads and writing about your setup.

Jim in NC
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Jim Huskins
Marion, NC
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« Reply #18 on: June 23, 2014, 05:13:24 PM »

Is there any extra wear issue for the AC unit running on modified sine wave?

The inverter he's using is pure sine wave.

Jim in NC
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Jim Huskins
Marion, NC
1999 Gillig H2000LF
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« Reply #19 on: June 23, 2014, 05:56:00 PM »

Jon-- There's no contradiction.  I said I am fine with using the generator for OTR AC.  However, doing it off the engine alone is also okay but not worth the investment for me.  I also asked if running an AC unit on a modified sine wave inverter would effect the life of the unit out of interest in the answer since I presently use (notice I did not say currently) a modified sine wave inverter.
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« Reply #20 on: June 23, 2014, 06:02:15 PM »

You have to do the math at some point.  At $450 that I paid for a new 15K btu Briskaire, not sure how many mini-splits you can buy.  In my area, about a quarter of one, then you have to install it, get it charged, all of that.  Numbers do not work for me.  As far as the mechanically driven system, it might work great for you forever, but mine needed around $2500 to recommission, and an estimated $1,500 a year to maintain, and finding someone to do the work is literally impossible.  Numbers do not work for me.  As far as running the generator, I need to pull 2.4 hp from my main engine.  That is around .75% of it's capacity, and the fuel usage is literally in the noise, as is the impact on performance.  It takes around 20 times that to simply make my engine spin at 1800 rpm, and a 2 mph difference in headwind overcomes that drag cost handily.  The cost to run an AC from the main engine is essentially free once the infrastructure is in place.  The cost of a typical 6 KW generator that can run OTR is at least 3 times the cost of my total infrastructure.  Once again the numbers do not work for me.  I do have a gas generator that I carry, the Yamaha, but I need that for emergency backup at home anyway.

Everyone has their reasons for doing what they do, but I am Scottish, and I spent the least amount of money possible to do what I am doing.  Pennies scream when I pinch them!  It really is a good solution. 

Brian

Thanks for layout of your math Brian.  I run similar numbers myself to try to find efficiency.  I didn't realize you weren't running a generator.
Doing a little bargain shopping one can probably retrofit a generator head to the main engine.  I have considered it, but in my case I'm told the DD 6-71 shouldn't idle when avoidable...and that kinda killed the idea for me.

As for the mechanical system, well my engine driven A/C would not keep this whole rig cold but it was never designed to carry passengers underway, so I've kind of had a mental short circuit on the idea of running electrical A/C's on a moving bus...wondering if there's something wrong because at some point wasn't the factory system supposed to keep the passengers cool?
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« Reply #21 on: June 23, 2014, 08:52:02 PM »

    Also, don't forget that you can run an engine-driven compressor A/C (takes some elec power to run fans, usually 12 or 24V) and also one or more roof wart or mini split systems at the same time.  You can do an engine compressor from an auto junkyard (usually a gamble, however), rig up a bracket and run a V-belt and then do the "other end" (condenser, evaporator, blowers, etc.) using "Red Dot" components -- just be sitting down when you read the prices in the catalog.  It would probably take a very unusual $$$ profile for it to make sense, but it can be done.

BH  NC   USA
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

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« Reply #22 on: June 24, 2014, 02:48:54 AM »

It's important to note in my math that I have the original 50DN belt driven alternator on the bus - if you didn't have such a beast you'd have a different equation to work with.  Similarly if your bus came to you with a generator that you can run over the road - no extra investment on your part.  My generator can't be run OTR, it's a little air cooled inverter-generator and it would overheat.  Lots of ways to  win with this deal.

Brian
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« Reply #23 on: June 24, 2014, 04:06:09 AM »

Sorry, this got a bit long...

Cool! This has turned into a pretty interesting thread! For me, the design of my system was really about two things: a.) simplicity and maintenance requirements, and b.) future cash outlay. I think it's important to disclose those, because how we all outfit our buses is a very personal thing. We all have different requirements and experiences. I'm going to retire in a couple of years and be on a fixed income, so I wanted things I could repair or replace myself for modest cost anywhere in the country. No special order parts, no bank-breaking single item. I also come from an aviation background, so I appreciate simplicity and low parts count. There's just less that can go wrong. Just for discussion, I'll give you some of my reasons for making the choices I did:

1. The battery inter-tie was a no brainer. It was clear to me that to run A/C off batteries for any real length of time would be unsustainable, so I'd have to run off the alternator while driving over the road. I also like the idea of being able to manually activate it and jump the chassis batteries in a pinch, or deactivate it completely. I made it automatic, because sooner or later I knew I'd forget to switch it over and kill my house batteries. I have to admit, I scratch my head a little when people with that big 50DN 24 volt alternator like I have (lucky) don't want to use it. When the coach was in service, it was running practically the same load placed on it by my system with the factory OTR blowers and air. I know it places a load on the engine, but I just don't see this as significant and apparently the original designers didn't either. Personal choice. Yours may be different and that's cool.

2. I chose a smaller generator for less maintenance, less noise, and lower replacement cost. Also, I just never could get comfortable with the idea of a generator running down below while I wasn't able to give it my attention (driving.) It's fine if that's an unfounded opinion. Based on my experience it's the right decision for me though. In my eyes, machinery running + lack of attention = trouble.

3. It's a pure sine, because it will run everything efficiently. Probably puts out cleaner power than the power company.

4. My OTR air was going to be horrendously expensive to fix and maintain, and I wasn't about to run the bus engine every time I wanted the air on. I probably pulled 2500 pounds worth of stuff out of there removing it. Talk about fuel savings...

5. I can buy replacement rooftop A/C's for relatively low cash outlay and install them the same day in the same hole. I don't think they're gorgeous either, but simplicity, ease of maintenance and replacement, and replacement cost trump that in my opinion.

6. As far as any extra fuel consumption from running the alternator with a load on it, that's just the cost of doing business to me. If I want the air on (and I do) it might cost me a MPG or two. If I don't have enough money for fuel with the A/C on, I don't have enough money for fuel. Plus, I can always turn the air off and it's the same load as always on the alternator.

I really don't see this as a "who's right or wrong" thing. We all have different needs and backgrounds, and how you outfit your coach is going to be different from how I outfit mine. My decisions have all been based on literally years of thinking about it and my personal experiences. If anyone wants to PM me to know more, feel free. This is an interesting topic to me. I'm no expert, but I have blown a lot of S#&t up over the years.  Grin
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« Reply #24 on: June 24, 2014, 06:13:38 AM »

...  I really don't see this as a "who's right or wrong" thing. We all have different needs and backgrounds, and how you outfit your coach is going to be different from how I outfit mine. My decisions have all been based on literally years of thinking about it and my personal experiences. If anyone wants to PM me to know more, feel free. This is an interesting topic to me. I'm no expert, but I have blown a lot of S#&t up over the years.  Grin   

     Couldn't agree more. 
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

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« Reply #25 on: June 24, 2014, 07:26:52 AM »

It's how we learn!  I liken this forum to a campfire - we all have different experiences and backgrounds, and we teach each other just by talking about things.  One time we were talking about pure sine vs modified sine, and a guy said "pure sine is just a fancy square wave inverter" or something like that.  "No it ain't" I said to myself, and then spent an hour finding out how pure sine wave inverters work.  Turns out they are fancy square wave inverters that put out a string of square wave pulses at around 20 KHz, push that through a filter and out pops a pure sine wave...   A little later I found out that a Vanner voltage equalizer is basically the same thing as a pure sine inverter, it just does something else with the output.  Whoda thunk?

Brian
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« Reply #26 on: June 24, 2014, 10:04:03 AM »

I really don't see this as a "who's right or wrong" thing. We all have different needs and backgrounds, and how you outfit your coach is going to be different from how I outfit mine. My decisions have all been based on literally years of thinking about it and my personal experiences. If anyone wants to PM me to know more, feel free. This is an interesting topic to me. I'm no expert, but I have blown a lot of S#&t up over the years.  Grin

No, the saying goes "no rules, only guidelines".

I bumble in here and make matters worse because I know a fair amount about energy, living off-grid, and mobile systems, but I don't know bus specifics, nor whose bus (or bus model) has what.

There's also huge differences in how people need to use their energy systems, and what their budgets are.
For me, I'm dead without my generator.  It's third in line after the engine and transmission.
My life circumstance is that I could never go down the road running the genset for the sake of A/C because I need to hear if something is going wrong back there just in case.
For me, even shopping on ebay or on Craigslist for bargains, I am limited to only a couple models that would fit, and they might not be available used...which means about $8k new, or more realistically, no generator if it blows and I go back to the stone age.
On top of the pricey generator, I also have a used inverter system which cost about $1500 to put together so far without even having another $1000 worth of needed battery bank.
I feel all this stuff is pretty fragile compared to the things that could go wrong in an engine driven A/C system.   I feel a lot more comfortable with the idea of blowing an A/C compressor than the gen/inverter system because in theory all I should lose is the air conditioning, not the whole game.

So not knowing that a coach doesn't have a engine driven A/C system, or a generator definitely changes the equation.  If it did, I would wonder why someone would put the hours on the electric system vs the mechanical.

I've also blown a lot of crap up in my days, and I tell you a story about one such instance...

One day I was rushing home from work in my turbocharged car because I had to pee...real bad.
I pulled in to my driveway, but someone had their car parked there so I pull off into the grass and ran in to relieve myself.
Granted, this was a reasonably long p*ss, but when I came back outside to get my things, the entire car was engulfed in flames.
Seems there was some tall dry grass where I pulled off and it interacted with the turbocharger.

Need to pee = bonfire.  It's hard to say what can happen, but you can usually narrow it down to what is operating at the time.

The point is that another angle on the cost equation is to factor in what needs to be functioning for that A/C to operate, and what is at risk during that operational time, and what could the loss of that system result in (including incidental losses).
A turbocharger and a piece of grass lost me a whole car.   Simply, if it had not been running, that car would have driven me to work the next day.

For for my particular engine, I'm not supposed to let it idle.  Your engine may be just fine with this, but even if I had an idle-friendly power plant, I'd be nervous about letting that system run just to make myself comfortable.  There are instances when A/C might be a matter of life and death, but excluding those, I am the type who can't relax while an engine is running mostly unattended.

So you can see, for me at least, the idea of the prime mover online, spinning an alternator, going to a inverter, to a charger, to a big hydrogen producing battery bank, to a pair of electric motors...and having the whole thing bouncing down the road while concentrating on bad drivers is a little scary.  Running the generator if you have one isn't much better.

Sure, if you're going down the road, the prime mover has to be operating. There is no avoiding it, but during that time my preference would be that the luxury of A/C was a cold and isolated system who's failure would only result in sweating a lot.

If that's not possible, I understand, but that was the basis of me suggesting to tackle the mechanical system first to see if it could be made to cool the coach while underway.

Cheers!
Sean
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« Reply #27 on: June 24, 2014, 10:08:00 AM »

I'm going to retire in a couple of years and be on a fixed income....

I'm jealous of people with fixed incomes. We have to get by on a broken one.
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Jim Huskins
Marion, NC
1999 Gillig H2000LF
Yes Virginia,
You CAN convert a low floor.
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« Reply #28 on: June 24, 2014, 04:15:59 PM »

  I'm jealous of people with fixed incomes. We have to get by on a broken one. 

    Hey, Jim, speaking of fixed, do you have any photos of your solar system?  Do you have a meter to tell you what kind of power you're making?  If so, what do you see at the peak sunshine times?  And what engine is in your Gillig?    Best wishes,   BH
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

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« Reply #29 on: June 25, 2014, 06:33:08 AM »

Hey Bruce. Good to hear from you.

The only metering I currently have on the solar is the volt meter on the remote for the Magnum ME4024 inverter and the LED state of charge lights on the MorningStar 60 amp MPPT charge controller. Ammeter and battery level meter are in the plans but not near the top of the priority list today. What I currently have shows that the batteries get full charge even on the cloudy days we're having lately and in spite of the fact that we're in shade half the day. I think the MorningStar gets a lot of credit, but I'm also pleased with the panels.

Until April of this year, I was using a bank of six group 27 AGM batteries. I was unwilling to use more than six because my research convinces me that it's a bad idea to use more than three strings of batteries in one bank. Two strings is apparently even better. I don't actually KNOW this, but it's what people who apparently do know tell me.

I realized early on that the group 27s were inadequate, but I thought we would get by for a couple years. The coldest winter in memory convinced us otherwise, and in April I replaced them with 4 8D AGMs. That was a marvelous move. Here is what they look like with their new 2/0 copper cables still in kind of a tangle:



The batteries sit on the original floor level, on the passenger side of the bus, just to the left of the door as one enters the bus. The gray-colored steel channel in the photo is the floor frame for the five foot extension I built next to the higher floor in the rear of the bus. Our full-size front load washer and dryer sit directly above the batteries with a hand sink between them. I have to move the washer and dryer to get to the batteries, but with AGMs, that does not happen often.

I still only have two of my four 255w panels mounted on the bus. We use all four panels at home and disconnect the two that are not mounted when we travel:



This industrial cart and some 2x4s works amazingly well as a non automated solar tracking ensemble. We got along last summer with only these two panels. We were still living in the truck, and our power needs were small. In September (I think) I got the other two panels on the roof:







These panels are mounted just aft of the rear roof vent, and that puts them over the bedroom. The other pair will mount just fore of that same vent.

I'm pleased with the mounting system except that I don't like the looks of the 12" sections of aluminum channel that bolt to the sides of the bus. I have since acquired a sheet of 3/8" tempered aluminum sheet from which I will cut side mounts for the other panels and to replace these.

That same channel does a wonderful job of spanning the roof and holding the panels. One of my design parameters was no holes in the roof. This mount is stable and secure at any speed we travel. The actual connection to the bus is via 3/8" SS bolts through both outer and inner aluminum bus wall segments. The nuts show on the inside, but they will all be covered by cabinets.

Don't know if this is what you wanted, Bruce, but I hope it's helpful. My original plan was to buy an 8 or 10 KW generator head and put it on one of the Isuzu/Thermoking diesels I have on hand. I was planning to put that where the original air handler was. With our solar panels and the fact that we decided we can live without two or more large AC units, the big generator would be silly. We could get by with 2000 watts of Honda or Yamaha for backup, but since we already have a good 4500 watt Kohler out of our old motorhome, I have spruced it up and am currently working on getting it installed above the engine. Run what ya brung, as they say.

I believe you also asked about our prime mover. This is my first experience with a DD Series 40. It's the larger one, 8.7L. Lots of opinion floating about what this motor is, but this is what I believe to be true. It is not exactly the same as the motor sold under the International brand. IH builds the long block, but apparently Caterpillar makes the fuel delivery system and DD ties it all together with proprietary management software.

All I know for sure is that this thing is a hoss! It's rated at 330 HP. That's the same figure as our old 6V92TA, but this bus will run circles around the Flxible Metro. Plenty of power and speed. No Jake, but I've become a fan of that Allison retarder.

Jim in NC
« Last Edit: June 25, 2014, 12:17:12 PM by Lostranger » Logged

Jim Huskins
Marion, NC
1999 Gillig H2000LF
Yes Virginia,
You CAN convert a low floor.
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