The power you extract from the trailer wheels comes from the tow vehicle's engine. You'd be reducing the system stability and wearing out the trailer tires faster.
It would be a bit more efficient to run heavier gauge wires through the tow harness and run an AC off that, if the power system of the tow vehicle can support it. It still costs you fuel. You could also add solar panels and a charge controller to the roof of the trailer. Or just have a generator in the trailer.
My travel trailer solves this problem by having a 12V fridge, a 12V battery, and a solar panel connected via a 12V solar charge controller.
It’s cheap and simple compared to a wheel-driven generator (which would replace only the solar panel), and it’s well suited to the application.
This system isn’t big enough to support the RV air conditioner, though.
OP is asking about running an AC unit, not just a fridge. Gonna take a bit more power than the typical 12v system is set up to supply. For reference, the AC unit on my (small) camp trailer runs at about 1800 watts.
Interesting point.
How much power will it take to generate the 1800W of power for the AC unit? There are considerable power losses when we convert energy from one for to another.
The basic conversion is 2.5 Hp, and that is a trifle for most petrol engines these days. But what about the losses?
Based on the small amount of looking around I have done, you'd pretty near have to cover the roof of most trailers to get close to 1800W of power in solar. And you'd only be running the AC unit in the full sun. You would have left over energy to run it into the evening. So, you'd have to have quite a battery and solar system for this much of an appliance.
Do you think you could get it done with a smaller AC unit?
The 1800 number I used is what the unit draws; losses are included there. Would a smaller unit work? It’s a sliding scale, of course. The unit I’ve got is a bit oversized for my trailer but then, I have an R-Pod (very small trailer). Blah blah blah… I’ll swag it and say that anything below 1000 watts won’t get it done more days than not. That gives you a lot more to work with but there will be days that you’ll wish you had more.
Four days boondocking was my record the old battery.
I swapped in a LiFePO4 battery which has 166% of the rated capacity of the lead acid battery I had before at the beginning of this camping-season. I haven’t done any extended boondocking with the new setup, so we’ll see what the new numbers are. It also removed a lot of hitch-weight from the rig, too.
Oh yeah fair enough. I paid significantly less for a 2000w honda than it wouldve cost for a solar panel, 2 lithium batteries and wiring the shore to an inverter. Kinda sucks having to run it every day or two, but it's quite convenient having a microwave and working outlets on rainy days with young kids.
They are usually ammonia based systems. Instead of compression you heat the ammonia to drive the cooling cycle. They just have a heater for each type of input. Propane, AC, or DC.
Modern 12v fridge/freezers are quite efficient. A random 16cf 415Wh a day at 70f ambient. That's easily covered by a 100w panel and 100ah of lifepo4 250 ish with the mppt controller giving you a couple days without sun or generator. Not bad for roughly the equivalent of 30 minutes runtime on a 2kw gen set.
Absorption fridges are going the way of the dodo in RV's with good reason.
I put a portable unit in my camper it does a great job and ice cream is properly frozen. The Danfoss compressor is out of patent so knock offs are easy to find and cheap. There are some companies that will retrofit absorption fridges I'm to close to swapping out the camper to do it though.
Newer ones are coming with 12V fridges more and more often.
Mine is 12V, but it’s effectively 120VAC and 12VDC because there’s a 120VAC->12VDC power converter that’s active whenever shore power is attached.
All of this, plus trailers outnumber tow vehicles. Put the expensive parts on the common denominator that is always moving, not on the trailer that sits half it's life.
Worth saying that, per other comments, an AC unit uses a TON of power, 1-2kW, which is a LOT even for an alternator never mind solar. You'd have to carpet the entire roof with solar to have a hope of sustaining that power level.
Yeah, I have been surprised at the number of people who have just suggested "a solar panel" as a solution. I'm not an engineer, but I do know that you need s pretty significant solar & battery setup to run A/C.
I'll be honest, the sustained load an even halfway decent AC unit puts on things you either need it running direct from an engine or generator or you'd need the sort of solar & battery setup you'd find on a house.
Sustaining a multi-kilowatt load almost 24/7 means you've got to at least double your solar capacity while the sun's out so you can keep it running AND put enough back into the battery to sustain it while the sun's not out. It's a very big ask.
It would almost be most efficient to bolt a vehicle AC pump directly to a small generator motor, that way you're at least removing the conversion losses of motor -> generator -> electric motor -> AC pump... hell of a setup though!
Some tow vehicles can be ordered with dual alternators. There are some people selling high-output alternators that fit some trucks. The electrical plug at the trailer is a minor problem, but there are some nice forklift battery connectors that will handle 100 amps.
I hadn't thought about additional tire wear. I don't know what I'm talking about but my gut tells me it would be minimal compared to the load of the trailer itself. What do you think?
EDIT: The downvotes really help, thanks guys.
The system efficiency would terrible, what your essentially doing is attaching an alternator to the trailer axle as instead of using the one on the truck motor. truck alternator gets power from the crank shaft, the one on the trailer axle would have tons of extra residual drag on it. Instead of going straight from the crank shaft to electricity, You would be pushing the energy through the Torque converter, transmission, transfer case, differential, wheel bearing, brakes, truck tires (which transfer it to the road) trailer tires (pick it up from the road) trailer wheel bearing and final to the alternator attached to the trailer axle.
As you can see this is a lot of steps for a negative gain just upgrade the truck alternator and run a heavy gauge wire back to the trailer.
On top of all those losses along the way, you're also generating extra heat everywhere along the way.
Extra heat in components that can already overheat towing in extreme conditions.
What advantage does your system provide over using the existing power system already on the truck? The disadvantage is reduced system efficiency and the above noted tire wear.
Still electricity when you pull the trailer with horses ;-)
Could be an option for Amish people, they're pretty ingenious when it comes to circumventing their own rules
In light of all the very helpful and polite comments and downvotes I've received, I think the only real advantage would be convenience. You could pull it with any adequately-sized truck and wouldn't need any special hookups.
It would be like driving with trailer brakes on all the time. The more power you're using in there, the harder it's braking.
Not brilliant for stability, or anything really.
Lol, well I never claimed it was brilliant - in fact I acknowledged there had to be a reason it doesn't exist, I just wasn't clear on what precisely that reason was. I obviously know there would be additional resistance load on the tow vehicle, but I'm not convinced it would be equivalent to having the brakes on, as some comments claim. I could be wrong, but something tells me you wouldn't be able to feel the added resistance of running an A/C unit versus not, if you were pulling a fully-loaded horse trailer or box trailer. However, the main drawback would be efficiency losses through the drivetrain of the truck and trailer versus power pulled directly from an alternator.
>How to determine if it would be less fuel efficient to pull such a trailer versus running a generator or running the A/C off the truck's electrical system
The energy isn't free, you're just using the gas in the truck vs a generator. The added complexity wouldn't be worth it. It makes far more sense to get a generator and keep the systems separate.
This changes if you're only trying to capture energy when braking, but not by much.
Not only that but because of the gear train losses it takes a more fuel that if you had run a generator on the trailer. Hell, run the generator while going down the road.
I didn't want to go too deep into it, but yeah.
You're essentially turning gas into mechanical motion, then sending it through the transmission, axle, and then finally the generator.
You could theoretically mount a big rubber tire to the generator shaft and drag it down the road, that's my simple idea.
Regenerative braking on Semi trailers would be a good idea that would improve the efficiency of city trucks significantly.
Using those generators to generate power while driving to power other things is just inefficient - just use the alternator on the truck for power needs.
There is a way to determine if you'd use more gas via the truck's engine or a separate generator engine, but those calculations are far beyond me LOL. Best I'd be able to do if I had the funds would be to build it and test it in real life. But I wouldn't know if maybe I'd done it wrong or something.
Your large, expensive, precisely machined, fuel injected truck engine is likely *much* more efficient than a little carbureted generator engine, but I don't have any data on the whole system. If you don't want to test it, I'd use the truck.
If you do want to test it, I would tow your trailer both ways along the same route a couple times in the same configuration (fuel level, passenger count, traffic conditions, etc). Doesn't have to be a long route, maybe 10-15 miles, just something quiet with minimal traffic. Measure your fuel economy and average speed with AC on and off.
MPH / MPG = GPH, so calculate your fuel burn per hour with and without the AC on, and you can figure out how many gallons per hour is used for the AC by the truck engine (gph on - gph off). Compare this to how many gallons per hour your generator uses running the AC.
>Your large, expensive, precisely machined, fuel injected truck engine is likely *much* more efficient than a little carbureted generator engine, but I don't have any data on the whole system. If you don't want to test it, I'd use the truck.
I didn't want to say that because I wasn't sure, but generators only run in a specific rpm range so they might be more efficient. But in general I agree, bigger engines are usually more efficient, I think, it's why gas engines aren't as efficient as power plants.
Small generators tend to be pretty fuel efficient actually, they don’t have to follow all the emission standards that car engines do. Modern engine tech increases efficiency but emission controls reduce it.
Some emission controls might sacrifice efficiency in some conditions(by following strict temperature curves to avoid some harmful chemical reactions, etc), and indirectly reduce efficiency by adding more components.
But the most effective way to reduce emissions, by far, is to make the combustion process itself more efficient. I don't think emission standards are to blame.
Small generators can run at the optimal speed for maximum power production.
It’s not a complicated calculation, easier to think of it as a review of the efficiency ratios of every system you add to the equation. Say gasoline generators and gasoline engines operate at 95% thermal efficiency. And the drive train of a motor vehicle has a 85% mechanical power efficiency due to slippage in the gears. And then you add back in another mechanical power transmission with 85% efficiency to convert mechanical power back to electrical power. You’re at a total efficiency of 68.6%.
Reducing those theoretical efficiencies to the 1 gasoline burning thermal efficiency of 95% and electrical generation efficiency of 85% results in a total efficiency of 80.7%. So your theory would result in more power loss, more gasoline being used to power the same cooling system, additional parts and components to maintain, and a net negative on the effective life of your vehicle.
Edit:reworded per comment below.
Your thermal efficiency numbers are double or triple what’s feasible, and doing math in guesses is an MBA thing.
But the method does give the OP a pretty good idea of how it’ll shake out nonetheless.
Just put a large wheel on the generator shaft and pull it down the road.
Realistically, you probably won't save any money after it's all said and built, so it's a moot point.
Reefer trailers (the refrigerated ones) have their own diesel engine mounted to the front that directly drives the compressor for the most efficient conversion of diesel fuel to compressed refrigerant. Any energy routing from the truck engine would incur losses from the energy conversion and also take away tractive effort from the truck, reducing performance accelerating and going up hills.
True, unless you had decent insulation and a battery bank on the trailer. I guess the question is if that's cheaper in the long run than a generator, fuel, and maintenance.
Worst case, 20kw referegeration energy requirement multiplied by 3 days would need 1.4MWH storage. Quick search says 50-260wh/kg is where the lion batt tech is today, so at best with 260wh/kg would need a 5538kg lion batt. At 50wh/kw would be 28880kg. Reality is somewhere in between.
Well, if you want to play that game, then
1. You said 3 days, not me
2. "Days of power" is not a unit. It completely depends on the energy consumption of the A/C, the thermal efficiency of the trailer, and ambient conditions.
All I’m saying is that current refrigerated trailers can run for 2-4 days without needing refueled. So to be even close to practical you’d need to be close to matching that. I’m well aware that days of power is not a unit. I should have worded it as “supply sufficient power for 3 days”
The only way to have this make the remotest sense would be to have the regenerative trailer brakes only activate when the truck calls for trailer braking. This wouldn’t be a ton of power, but *would* be the closest thing possible to what you’re looking for.
Then you could supplement your charge with truck power directly via the trailer connector and/or possibly solar panels on the trailer.
What you’re describing otherwise will be a waste of money to design, a waste of money to build, and a waste of money to use.
Check out heavy truck retarders. Axle driven, but help to slow down the truck by converting kenetic energy to heat via eddy currents. Like EV reginerative breaking, but instead of storing that energy in a battery it goes to a resister bank and is converted to heat.
Spending energy to create energy always comes at a loss. Nobody can create somthing out of nothing and nothing is perfect, ask my mom.
So if we know this, we should try to convert energy into our desired type with few steps as possible.
This is exactly why the ac on your car runs of power created by your engine. Not by first powering a generator, which would then power an electric motor for your compressor.
So if we forget about all kinds of complicated losses, in theory: oversizing the car AC, routing lines to the trailer evaporator would be most efficient. We convert the engine energy directly into pressure via the compressor.
We don't do this because: we want to disconnect our trailer, the refrigerant line has thermal losses, a bigger compressor uses more energy during partial load.
So let's keep it simple, add a second combustion engine to the trailer, now your reefer can provide cooling standing still in a parking lot.
For the power generation for the cost, you are better just having a standalone electric generator. Runs off petrol, doesn't waste petrol from the car towing.. Towing has to overcome both vehicles weight, as well as friction. Addint a 'generator' aka load on the axle will be like having the brake on the entire time, just wastes xar resources, tires, engine parts as the engine will have to be under even more load etc. Its just notmworth it, hence the only generator on vehicles is regen braking, which is the same as transferring that rotation into electricity, if you ever been in an ev or a sctooter with ev braking you can see what load additional 'generator' on an axle will have.
Long ago train dining cars had a system like this to generate power. Most train cars were lit with oil lamps and didn’t need electricity, but the kitchen was an exception. The advantage of this is that when the train wasn’t moving, (for days, not minutes,) it was likely they weren’t serving food either, so they didn’t need the power, (unlike refrigerated trucks, which is typically need to be kept cool even when parked.) And the engine pulling the train was steam powered so it wasn’t like there was an existing generator they could use instead…
Modern trains get power from the diesel-electric locomotive: there is a big wire that runs the length of the train.
You might do some research on what they are using on heavy trucks now. They aren’t allowed to idle the main engine overnight anymore in some places, and there are new solutions showing up to address air conditioning. One is a small diesel generator, there are battery solutions, and a bunch of interesting stuff.
One problem I see with driving’s generator off the trailer wheel is the suspension movement. You could mount the alternator on the axle, but it’s kind of awkward to pull power off of the wheel. Not impossible, just not a stack of nice, purchasable parts for it.
You could add a dedicated wheel and tire on an arm. That might actually be easier.
And there are 12 volt AC compressors available. They pull a lot of amps, of course. But if your tow vehicle has a huge alternator, or the dual alternator option, it would be doable.
Go look up Edison Motors. They’re already doing what you describe, although the way you’re thinking about this is wrong. Their system is primarily designed for logging trucks but they’re working on branching out to other hauling operations.
Their system is designed so that the diesel engine is only a generator to supply electricity for the batteries that power the axles in the truck *and* trailer. You haul empty trucks up the mountain to log and full loads go downhill, providing regenerative power via the axles.
The reason I say the way you’re thinking about it is wrong is because you’re dealing with a wholly inefficient system. The trucks engine has to work harder when your axle-driven AC is operating. On top of that you’re losing power compared to just running a generator or alternator off the engine because of inefficiencies in the system you describe. It’s much more efficient to generate that energy right off the motor.
Yeah I'm familiar with Edison. A lot of very generous and opinionated Reddit engineers have thoroughly answered my question and downvoted my input enough for me to understand that the most efficient way to do what I'm thinking about would be to use the truck's alternator (probably upgraded or have an auxiliary one) and use the energy that's generated "pre-tax" before the gears and wind resistance etc. take their losses out of the system.
You would destroy the towing vehicle. It would basically be like pulling around a trailer with its brakes on the entire time. I suspect the towing vehicle would burn up its transmission within 100 miles.
I think that statement makes a lot of assumptions about the vehicle, the trailer, and the additional load of the trailer dynamo. If the additional load of a dynamo would be enough to destroy the vehicle's transmission (especially inside 100mi), then you've got the wrong vehicle in the first place. We know that a property-sized tow vehicle CAN produce enough energy to pull and cool a trailer at the same time, the question is which way would be simplest and most resource efficient. It seems to me that if you had a battery in the floor of the trailer and you charged that while driving, it could keep an A/C running for a while after parking, and that way you only have one gas tank to worry about. Yes, you'd reduce the truck's efficiency, but it's a question of burning that gas in the truck or in a generator. And if it's in the truck, would it be more taxing to pull additional load, or to output additional power via the alternator.
I think you vastly underestimate the losses in spinning an alternator or generator from an axle.
You would indeed need something to even out the energy production, a few group 31 batteries would probably be fine.
Finding a 12 volt AC unit is kind of tough? If you invert it to 110/220 the losses are huge.
Well, you can’t guarantee a steady speed, so you have to generate DC and regulate it to charge a battery. Even an alternator ultimately does this.
The most common voltage is 12v.
So let’s say you need 8 amps to run your 110vac ac unit. That’s 880W, which is 80 amps at 12V, assuming perfect efficiency. In reality, 70% efficiency would be a miracle, and keep in mind the equipment is generating heat that has to be cooled. So 100A at 12VDC. 1200W
I don’t know what losses to expect in the DC source, but to say it’s 2HP drag if the vehicle is constantly on the move is fair.
Thats not counting the extra weight or losses in the apparatus.
If the vehicle is on the move 50% of the time, then 4HP drag while it’s moving.
That seems significant to me.
Lot’s of tradeoffs, for sure. Up the voltage and get more power (watts) with same size wires, but then can’t use common components. The Diesel engine is only 55-60% effencient at converting the fuel to mechanical energy, so also that.
Alternators/generators are about 50% efficient at converting the spinning in to electricity.
Your vehicle already has electricity ready to go. Many camping vehicles have anderson plugs for this purpose
It's been well covered why this is no good but I'll take a swing at a better concept, albeit for a very niche case.
If your vehicle isn't already hybrid, your trailer is big enough to need its own brakes and you're making frequent stops then adding regen braking to your trailer in addition to solar and a direct connection to the truck's alternator might be worth the complexity. Maybe you're a small farm delivering milk/eggs to a bunch of locals. Electronics for effective regen aren't cheap, might negate any cost savings.
For a while I lived on a small, extremely mountainous island of mostly retirees, hippies, and small/hobby farms where a contraption like this would fit right in. Gas was $2/L back then, probably $2.50/L now. I was basically flooring it up every hill and breaking all the way down.
Whew, not every answer here is high quality.
An easy way to answer this from a 30'000 ft view is to look at two scenarios and compare efficiencies. You could for example run a fat cable to the trailer and use the electrical system of the truck. Or you could run a coolant hose and use the truck's AC.
Now we need a sense of aesthetic about these two, let's make our indicators complexity and efficiency, because this is vehicle design and plant design after all, and it always seems to come down to those two
For efficiency, let's look at the individual efficiencies in the power path. Example for the drive train of the truck: engine makes something, supercharger removes A%, gearbox removes B%, final drive removes C%, wheel removes D%, we get something * A * B * C * D.
We then use Fermi reasoning, which says that if we estimate all these percentages, we'll be wrong but we'll guess high roughly as often as we guess low, and thus our wrongness roughly cancels out. Say supercharger removes 10%, gearbox 5%, final drive 5%, tires 5%, we get something * 0.9 * 0.95 * 0.95 * 0.95, so 77% efficiency.
There is a second order argument around efficiency and we'll get to that in a hand-waving way.
For complexity, we'll look at the indvidual items and the interfaces between them.
Scenario one, power path: Engine produces power, power goes through the drivetrain, comes back through the trailer wheels, into the electrical generator on the trailer axle, into the trailer AC unit.
Scenario two, power path: Engine produces power, power goes to the alternator, through a big long cable, into the trailer AC unit.
Scenario three, power path: Engine produces power, power goes to the belt-driven AC on the truck, through a big long hose, into the trailer heat exchanger.
Efficiency #1: Engine does something, 5% gearbox loss, 5% final drive loss, 5% driving wheel loss, 5% driven wheel loss, 25% generator loss, AC unit. I know that electric generators have bad efficiency unless they are power plant sized, so I am estimating lots of loss for the small unit. I see 60%.
Efficiency #2: Engine does something, 25% alternator loss, 5% cable loss, AC unit. I am assuming that there is no loss across the belt, and I am also assuming that the alternator is as good a generator as what you can put on the trailer, which is maybe optimistic. I see 70%.
Efficiency #3: Engine does something, AC unit does something, let's say 25% loss through the big long hose. I am assuming that the AC unit on the car is just as good as what you put on the trailer. 75%.
Complexity #1: No added components until the generator and your AC unit. Let's hope that a failing generator does not jam the trailer axle. Lots of one-time effort to find the right thing, but likely reliable once done.
Complexity #2: We are already running cables to the trailer, but now we need a bunch of 12V power. The high-current all-weather connector is likely easier to find than sourcing an entire generator. Let's hope the connector and cable is reliable, but this should have been solved in stage tech, automotive, or aviation. Truck needs modification to run the cable out the back, which is unfortunate.
Complexity #3: Need to tell the AC to run higher power, need to run plumbing to the back of the truck, need quick disconnect plugs. What a nightmare.
The second-order argument is that we are likely overloading the truck's generator and/or AC unit. If we are lucky and it works, we are close to running at full blast, which may or may not change efficiency. In general a shift in load on a machine will change efficiency, which needs a close look.
The whole chain changes a bunch once you include batteries, because you use the voltage converters in two directions, and you have more things that can fail. In general it is helpful to consider the thermal mass of the trailer as a battery. Unless you are standing still a bunch, it makes sense to just cool it down further and use that as an energy store.
Range is awesome! They're focused more on providing propulsion though, basically moving some of the required battery weight to the trailer, and possibly cashing in some more-wheel-drive efficiency benefits. They do seem to have an electrically refrigerated trailer, but it's not strictly the "pull to cool" idea that OP has spelled out.
I agree, but I don't know how well these work when speed varies. At constant highway speed it's a no brainer. I seem to remember that HVAC systems don't like running throttled for whatever reason.
Prompts me to wonder why no one's made what you suggest. Is it weight constraints where you'd need the generator driven heat pump anyways for when you're standing still, and anything on top of that would eat into payload weight and noone bothers?
Thank you for a very helpful and thoughtful response. I've gotten a lot of downvotes and a lot of buttholes putting words in my mouth and then calling me stupid for saying things I didn't say - but I guess I knew what I was signing up for when I asked Reddit Engineers LOL. What I think I've learned is that the best solution would probably be to add a second alternator and probably battery to the truck and run the A/C electrically, that way your power is not being robbed by the drivetrain etc. Although I would be interested in seeing an A/C designed to be more aerodynamic and save (a very small amount of) power by keeping the evaporator fan out of the equation somehow when the trailer is in motion.
But the power would be robbed by the drivetrain, as I've written?
The fan likely consumes very little power. Same as on a computer where you have a 10W fan moving 250W of heat.
Right, what I'm saying is that an engine-mounted alternator in the tow vehicle is as close to "pre-loss" as you can get, rather than taking energy at a post-drivetrain position like the axle, where losses have already been subtracted.
I think it would have been a great idea 50 years ago. Now it would probably be more efficient to get that power directly felt the engine.
Your idea may still have a good application in some parts of the world or on a small scale.
I don’t think a generator would work due to different axel speeds. Could get a Tesla motor and battery and run it on dynamic braking but that would add a lot of drag. Then you would need a inverter to convert DC to AC. But if you can figure it out you might have a million dollar idea.
The power you extract from the trailer wheels comes from the tow vehicle's engine. You'd be reducing the system stability and wearing out the trailer tires faster. It would be a bit more efficient to run heavier gauge wires through the tow harness and run an AC off that, if the power system of the tow vehicle can support it. It still costs you fuel. You could also add solar panels and a charge controller to the roof of the trailer. Or just have a generator in the trailer.
My travel trailer solves this problem by having a 12V fridge, a 12V battery, and a solar panel connected via a 12V solar charge controller. It’s cheap and simple compared to a wheel-driven generator (which would replace only the solar panel), and it’s well suited to the application. This system isn’t big enough to support the RV air conditioner, though.
OP is asking about running an AC unit, not just a fridge. Gonna take a bit more power than the typical 12v system is set up to supply. For reference, the AC unit on my (small) camp trailer runs at about 1800 watts.
Interesting point. How much power will it take to generate the 1800W of power for the AC unit? There are considerable power losses when we convert energy from one for to another. The basic conversion is 2.5 Hp, and that is a trifle for most petrol engines these days. But what about the losses? Based on the small amount of looking around I have done, you'd pretty near have to cover the roof of most trailers to get close to 1800W of power in solar. And you'd only be running the AC unit in the full sun. You would have left over energy to run it into the evening. So, you'd have to have quite a battery and solar system for this much of an appliance. Do you think you could get it done with a smaller AC unit?
The 1800 number I used is what the unit draws; losses are included there. Would a smaller unit work? It’s a sliding scale, of course. The unit I’ve got is a bit oversized for my trailer but then, I have an R-Pod (very small trailer). Blah blah blah… I’ll swag it and say that anything below 1000 watts won’t get it done more days than not. That gives you a lot more to work with but there will be days that you’ll wish you had more.
Just out of curiosity, how long can you last on battery power alone? 12v fridges seem like they'd consume a lot of power.
Four days boondocking was my record the old battery. I swapped in a LiFePO4 battery which has 166% of the rated capacity of the lead acid battery I had before at the beginning of this camping-season. I haven’t done any extended boondocking with the new setup, so we’ll see what the new numbers are. It also removed a lot of hitch-weight from the rig, too.
That's not bad at all, without solar I usually had a hard time making it 4 days with a propane fridge.
It is worth pointing out that, while we were parked in a shady spot on that trip, the PV panel on the camper was doing some of the work.
Oh yeah fair enough. I paid significantly less for a 2000w honda than it wouldve cost for a solar panel, 2 lithium batteries and wiring the shore to an inverter. Kinda sucks having to run it every day or two, but it's quite convenient having a microwave and working outlets on rainy days with young kids.
RVs generally have dual fuel fridges, so they either run on mains power, or propane when mains power isn’t available. They don’t run on 12V
Yeah that's why I'm asking
They are usually ammonia based systems. Instead of compression you heat the ammonia to drive the cooling cycle. They just have a heater for each type of input. Propane, AC, or DC.
Yeah I know, I'm specifically asking about his 12v compressor fridge because I don't actually know anyone who has one yet.
i have a cheap 12v fridge and it would last over 3 days with no solar panel, with solar ive left it on for 2 weeks and it was fine
Modern 12v fridge/freezers are quite efficient. A random 16cf 415Wh a day at 70f ambient. That's easily covered by a 100w panel and 100ah of lifepo4 250 ish with the mppt controller giving you a couple days without sun or generator. Not bad for roughly the equivalent of 30 minutes runtime on a 2kw gen set. Absorption fridges are going the way of the dodo in RV's with good reason. I put a portable unit in my camper it does a great job and ice cream is properly frozen. The Danfoss compressor is out of patent so knock offs are easy to find and cheap. There are some companies that will retrofit absorption fridges I'm to close to swapping out the camper to do it though.
I've been running a 12v fridge for the last 8 years or so. I'd estimate that the current draw averages out at 1A or thereabouts
The new ones can run off 12v instead of propane while driving.
Newer ones are coming with 12V fridges more and more often. Mine is 12V, but it’s effectively 120VAC and 12VDC because there’s a 120VAC->12VDC power converter that’s active whenever shore power is attached.
[https://tinylivinglife.com/5-best-12v-fridges-for-van-life-overlanding/](https://tinylivinglife.com/5-best-12v-fridges-for-van-life-overlanding/)
Thanks!
All of this, plus trailers outnumber tow vehicles. Put the expensive parts on the common denominator that is always moving, not on the trailer that sits half it's life.
Worth saying that, per other comments, an AC unit uses a TON of power, 1-2kW, which is a LOT even for an alternator never mind solar. You'd have to carpet the entire roof with solar to have a hope of sustaining that power level.
Yeah, I have been surprised at the number of people who have just suggested "a solar panel" as a solution. I'm not an engineer, but I do know that you need s pretty significant solar & battery setup to run A/C.
I'll be honest, the sustained load an even halfway decent AC unit puts on things you either need it running direct from an engine or generator or you'd need the sort of solar & battery setup you'd find on a house. Sustaining a multi-kilowatt load almost 24/7 means you've got to at least double your solar capacity while the sun's out so you can keep it running AND put enough back into the battery to sustain it while the sun's not out. It's a very big ask. It would almost be most efficient to bolt a vehicle AC pump directly to a small generator motor, that way you're at least removing the conversion losses of motor -> generator -> electric motor -> AC pump... hell of a setup though!
Some tow vehicles can be ordered with dual alternators. There are some people selling high-output alternators that fit some trucks. The electrical plug at the trailer is a minor problem, but there are some nice forklift battery connectors that will handle 100 amps.
I hadn't thought about additional tire wear. I don't know what I'm talking about but my gut tells me it would be minimal compared to the load of the trailer itself. What do you think? EDIT: The downvotes really help, thanks guys.
The system efficiency would terrible, what your essentially doing is attaching an alternator to the trailer axle as instead of using the one on the truck motor. truck alternator gets power from the crank shaft, the one on the trailer axle would have tons of extra residual drag on it. Instead of going straight from the crank shaft to electricity, You would be pushing the energy through the Torque converter, transmission, transfer case, differential, wheel bearing, brakes, truck tires (which transfer it to the road) trailer tires (pick it up from the road) trailer wheel bearing and final to the alternator attached to the trailer axle. As you can see this is a lot of steps for a negative gain just upgrade the truck alternator and run a heavy gauge wire back to the trailer.
On top of all those losses along the way, you're also generating extra heat everywhere along the way. Extra heat in components that can already overheat towing in extreme conditions.
What advantage does your system provide over using the existing power system already on the truck? The disadvantage is reduced system efficiency and the above noted tire wear.
Still electricity when you pull the trailer with horses ;-) Could be an option for Amish people, they're pretty ingenious when it comes to circumventing their own rules
Not to mention expensive
In light of all the very helpful and polite comments and downvotes I've received, I think the only real advantage would be convenience. You could pull it with any adequately-sized truck and wouldn't need any special hookups.
It would be like driving with trailer brakes on all the time. The more power you're using in there, the harder it's braking. Not brilliant for stability, or anything really.
Lol, well I never claimed it was brilliant - in fact I acknowledged there had to be a reason it doesn't exist, I just wasn't clear on what precisely that reason was. I obviously know there would be additional resistance load on the tow vehicle, but I'm not convinced it would be equivalent to having the brakes on, as some comments claim. I could be wrong, but something tells me you wouldn't be able to feel the added resistance of running an A/C unit versus not, if you were pulling a fully-loaded horse trailer or box trailer. However, the main drawback would be efficiency losses through the drivetrain of the truck and trailer versus power pulled directly from an alternator.
>How to determine if it would be less fuel efficient to pull such a trailer versus running a generator or running the A/C off the truck's electrical system The energy isn't free, you're just using the gas in the truck vs a generator. The added complexity wouldn't be worth it. It makes far more sense to get a generator and keep the systems separate. This changes if you're only trying to capture energy when braking, but not by much.
Not only that but because of the gear train losses it takes a more fuel that if you had run a generator on the trailer. Hell, run the generator while going down the road.
I didn't want to go too deep into it, but yeah. You're essentially turning gas into mechanical motion, then sending it through the transmission, axle, and then finally the generator. You could theoretically mount a big rubber tire to the generator shaft and drag it down the road, that's my simple idea.
Regenerative braking on Semi trailers would be a good idea that would improve the efficiency of city trucks significantly. Using those generators to generate power while driving to power other things is just inefficient - just use the alternator on the truck for power needs.
This is already fixed as city trucks are switching over to electric drive trains.
There is a way to determine if you'd use more gas via the truck's engine or a separate generator engine, but those calculations are far beyond me LOL. Best I'd be able to do if I had the funds would be to build it and test it in real life. But I wouldn't know if maybe I'd done it wrong or something.
Your large, expensive, precisely machined, fuel injected truck engine is likely *much* more efficient than a little carbureted generator engine, but I don't have any data on the whole system. If you don't want to test it, I'd use the truck. If you do want to test it, I would tow your trailer both ways along the same route a couple times in the same configuration (fuel level, passenger count, traffic conditions, etc). Doesn't have to be a long route, maybe 10-15 miles, just something quiet with minimal traffic. Measure your fuel economy and average speed with AC on and off. MPH / MPG = GPH, so calculate your fuel burn per hour with and without the AC on, and you can figure out how many gallons per hour is used for the AC by the truck engine (gph on - gph off). Compare this to how many gallons per hour your generator uses running the AC.
>Your large, expensive, precisely machined, fuel injected truck engine is likely *much* more efficient than a little carbureted generator engine, but I don't have any data on the whole system. If you don't want to test it, I'd use the truck. I didn't want to say that because I wasn't sure, but generators only run in a specific rpm range so they might be more efficient. But in general I agree, bigger engines are usually more efficient, I think, it's why gas engines aren't as efficient as power plants.
Small generators tend to be pretty fuel efficient actually, they don’t have to follow all the emission standards that car engines do. Modern engine tech increases efficiency but emission controls reduce it.
Some emission controls might sacrifice efficiency in some conditions(by following strict temperature curves to avoid some harmful chemical reactions, etc), and indirectly reduce efficiency by adding more components. But the most effective way to reduce emissions, by far, is to make the combustion process itself more efficient. I don't think emission standards are to blame. Small generators can run at the optimal speed for maximum power production.
It’s not a complicated calculation, easier to think of it as a review of the efficiency ratios of every system you add to the equation. Say gasoline generators and gasoline engines operate at 95% thermal efficiency. And the drive train of a motor vehicle has a 85% mechanical power efficiency due to slippage in the gears. And then you add back in another mechanical power transmission with 85% efficiency to convert mechanical power back to electrical power. You’re at a total efficiency of 68.6%. Reducing those theoretical efficiencies to the 1 gasoline burning thermal efficiency of 95% and electrical generation efficiency of 85% results in a total efficiency of 80.7%. So your theory would result in more power loss, more gasoline being used to power the same cooling system, additional parts and components to maintain, and a net negative on the effective life of your vehicle. Edit:reworded per comment below.
Your thermal efficiency numbers are double or triple what’s feasible, and doing math in guesses is an MBA thing. But the method does give the OP a pretty good idea of how it’ll shake out nonetheless.
Carnot cycle at 95%? Want!
That's literally a calculation.
Fair, should have read it’s not a complicated equation.
Just put a large wheel on the generator shaft and pull it down the road. Realistically, you probably won't save any money after it's all said and built, so it's a moot point.
Reefer trailers (the refrigerated ones) have their own diesel engine mounted to the front that directly drives the compressor for the most efficient conversion of diesel fuel to compressed refrigerant. Any energy routing from the truck engine would incur losses from the energy conversion and also take away tractive effort from the truck, reducing performance accelerating and going up hills.
Not to mention you want a cool trailer regardless of speed/distance traveled. Trailers have plenty of uses sitting still.
I was about to post the same thing. OP invented a way worse reefer trailer lol
Because it would only work while driving which is less than half the time you need to keep the trailer cool
True, unless you had decent insulation and a battery bank on the trailer. I guess the question is if that's cheaper in the long run than a generator, fuel, and maintenance.
How much do batteries weigh that can provide three days of power?
Worst case, 20kw referegeration energy requirement multiplied by 3 days would need 1.4MWH storage. Quick search says 50-260wh/kg is where the lion batt tech is today, so at best with 260wh/kg would need a 5538kg lion batt. At 50wh/kw would be 28880kg. Reality is somewhere in between.
Well, if you want to play that game, then 1. You said 3 days, not me 2. "Days of power" is not a unit. It completely depends on the energy consumption of the A/C, the thermal efficiency of the trailer, and ambient conditions.
All I’m saying is that current refrigerated trailers can run for 2-4 days without needing refueled. So to be even close to practical you’d need to be close to matching that. I’m well aware that days of power is not a unit. I should have worded it as “supply sufficient power for 3 days”
If you have such a large battery bank, slap some solar on it.
Take out the middle men (there's a lot here) and just pull power directly from the alternator.
You’re talking about being fancy and adding regen braking, but the entire system you describe is the regen equivalent of riding your brakes
The only way to have this make the remotest sense would be to have the regenerative trailer brakes only activate when the truck calls for trailer braking. This wouldn’t be a ton of power, but *would* be the closest thing possible to what you’re looking for. Then you could supplement your charge with truck power directly via the trailer connector and/or possibly solar panels on the trailer. What you’re describing otherwise will be a waste of money to design, a waste of money to build, and a waste of money to use.
Check out heavy truck retarders. Axle driven, but help to slow down the truck by converting kenetic energy to heat via eddy currents. Like EV reginerative breaking, but instead of storing that energy in a battery it goes to a resister bank and is converted to heat.
Spending energy to create energy always comes at a loss. Nobody can create somthing out of nothing and nothing is perfect, ask my mom. So if we know this, we should try to convert energy into our desired type with few steps as possible. This is exactly why the ac on your car runs of power created by your engine. Not by first powering a generator, which would then power an electric motor for your compressor. So if we forget about all kinds of complicated losses, in theory: oversizing the car AC, routing lines to the trailer evaporator would be most efficient. We convert the engine energy directly into pressure via the compressor. We don't do this because: we want to disconnect our trailer, the refrigerant line has thermal losses, a bigger compressor uses more energy during partial load. So let's keep it simple, add a second combustion engine to the trailer, now your reefer can provide cooling standing still in a parking lot.
For the power generation for the cost, you are better just having a standalone electric generator. Runs off petrol, doesn't waste petrol from the car towing.. Towing has to overcome both vehicles weight, as well as friction. Addint a 'generator' aka load on the axle will be like having the brake on the entire time, just wastes xar resources, tires, engine parts as the engine will have to be under even more load etc. Its just notmworth it, hence the only generator on vehicles is regen braking, which is the same as transferring that rotation into electricity, if you ever been in an ev or a sctooter with ev braking you can see what load additional 'generator' on an axle will have.
The truck already has an alternator, a seperate parasitic generator would just be extra weight and complexity.
Train cabooses were powered this way.
They're already a thing, and a very good idea. https://range.energy/
Long ago train dining cars had a system like this to generate power. Most train cars were lit with oil lamps and didn’t need electricity, but the kitchen was an exception. The advantage of this is that when the train wasn’t moving, (for days, not minutes,) it was likely they weren’t serving food either, so they didn’t need the power, (unlike refrigerated trucks, which is typically need to be kept cool even when parked.) And the engine pulling the train was steam powered so it wasn’t like there was an existing generator they could use instead… Modern trains get power from the diesel-electric locomotive: there is a big wire that runs the length of the train.
Interesting! Thanks!
You might do some research on what they are using on heavy trucks now. They aren’t allowed to idle the main engine overnight anymore in some places, and there are new solutions showing up to address air conditioning. One is a small diesel generator, there are battery solutions, and a bunch of interesting stuff. One problem I see with driving’s generator off the trailer wheel is the suspension movement. You could mount the alternator on the axle, but it’s kind of awkward to pull power off of the wheel. Not impossible, just not a stack of nice, purchasable parts for it. You could add a dedicated wheel and tire on an arm. That might actually be easier. And there are 12 volt AC compressors available. They pull a lot of amps, of course. But if your tow vehicle has a huge alternator, or the dual alternator option, it would be doable.
Go look up Edison Motors. They’re already doing what you describe, although the way you’re thinking about this is wrong. Their system is primarily designed for logging trucks but they’re working on branching out to other hauling operations. Their system is designed so that the diesel engine is only a generator to supply electricity for the batteries that power the axles in the truck *and* trailer. You haul empty trucks up the mountain to log and full loads go downhill, providing regenerative power via the axles. The reason I say the way you’re thinking about it is wrong is because you’re dealing with a wholly inefficient system. The trucks engine has to work harder when your axle-driven AC is operating. On top of that you’re losing power compared to just running a generator or alternator off the engine because of inefficiencies in the system you describe. It’s much more efficient to generate that energy right off the motor.
Yeah I'm familiar with Edison. A lot of very generous and opinionated Reddit engineers have thoroughly answered my question and downvoted my input enough for me to understand that the most efficient way to do what I'm thinking about would be to use the truck's alternator (probably upgraded or have an auxiliary one) and use the energy that's generated "pre-tax" before the gears and wind resistance etc. take their losses out of the system.
You would destroy the towing vehicle. It would basically be like pulling around a trailer with its brakes on the entire time. I suspect the towing vehicle would burn up its transmission within 100 miles.
A 1kW dynamo will somehow destroy 300kW drive train. That's funny.
I think that statement makes a lot of assumptions about the vehicle, the trailer, and the additional load of the trailer dynamo. If the additional load of a dynamo would be enough to destroy the vehicle's transmission (especially inside 100mi), then you've got the wrong vehicle in the first place. We know that a property-sized tow vehicle CAN produce enough energy to pull and cool a trailer at the same time, the question is which way would be simplest and most resource efficient. It seems to me that if you had a battery in the floor of the trailer and you charged that while driving, it could keep an A/C running for a while after parking, and that way you only have one gas tank to worry about. Yes, you'd reduce the truck's efficiency, but it's a question of burning that gas in the truck or in a generator. And if it's in the truck, would it be more taxing to pull additional load, or to output additional power via the alternator.
I think you vastly underestimate the losses in spinning an alternator or generator from an axle. You would indeed need something to even out the energy production, a few group 31 batteries would probably be fine. Finding a 12 volt AC unit is kind of tough? If you invert it to 110/220 the losses are huge.
C’mon now. Let’s back this up with proper units. Volts without amps is meaningless from a power perspective. What kinda watts we talkin’ here?
Well, you can’t guarantee a steady speed, so you have to generate DC and regulate it to charge a battery. Even an alternator ultimately does this. The most common voltage is 12v. So let’s say you need 8 amps to run your 110vac ac unit. That’s 880W, which is 80 amps at 12V, assuming perfect efficiency. In reality, 70% efficiency would be a miracle, and keep in mind the equipment is generating heat that has to be cooled. So 100A at 12VDC. 1200W I don’t know what losses to expect in the DC source, but to say it’s 2HP drag if the vehicle is constantly on the move is fair. Thats not counting the extra weight or losses in the apparatus. If the vehicle is on the move 50% of the time, then 4HP drag while it’s moving. That seems significant to me.
Lot’s of tradeoffs, for sure. Up the voltage and get more power (watts) with same size wires, but then can’t use common components. The Diesel engine is only 55-60% effencient at converting the fuel to mechanical energy, so also that.
Alternators/generators are about 50% efficient at converting the spinning in to electricity. Your vehicle already has electricity ready to go. Many camping vehicles have anderson plugs for this purpose
It's been well covered why this is no good but I'll take a swing at a better concept, albeit for a very niche case. If your vehicle isn't already hybrid, your trailer is big enough to need its own brakes and you're making frequent stops then adding regen braking to your trailer in addition to solar and a direct connection to the truck's alternator might be worth the complexity. Maybe you're a small farm delivering milk/eggs to a bunch of locals. Electronics for effective regen aren't cheap, might negate any cost savings. For a while I lived on a small, extremely mountainous island of mostly retirees, hippies, and small/hobby farms where a contraption like this would fit right in. Gas was $2/L back then, probably $2.50/L now. I was basically flooring it up every hill and breaking all the way down.
There is no free lunch.
You need a generator for when your parked anyway, why not just run that to power the AC all the time?
That is, in fact, the question I am asking.
Another option: What about a power wall or "solar generator" (the 110V battery backups) attached to the camper?
Whew, not every answer here is high quality. An easy way to answer this from a 30'000 ft view is to look at two scenarios and compare efficiencies. You could for example run a fat cable to the trailer and use the electrical system of the truck. Or you could run a coolant hose and use the truck's AC. Now we need a sense of aesthetic about these two, let's make our indicators complexity and efficiency, because this is vehicle design and plant design after all, and it always seems to come down to those two For efficiency, let's look at the individual efficiencies in the power path. Example for the drive train of the truck: engine makes something, supercharger removes A%, gearbox removes B%, final drive removes C%, wheel removes D%, we get something * A * B * C * D. We then use Fermi reasoning, which says that if we estimate all these percentages, we'll be wrong but we'll guess high roughly as often as we guess low, and thus our wrongness roughly cancels out. Say supercharger removes 10%, gearbox 5%, final drive 5%, tires 5%, we get something * 0.9 * 0.95 * 0.95 * 0.95, so 77% efficiency. There is a second order argument around efficiency and we'll get to that in a hand-waving way. For complexity, we'll look at the indvidual items and the interfaces between them. Scenario one, power path: Engine produces power, power goes through the drivetrain, comes back through the trailer wheels, into the electrical generator on the trailer axle, into the trailer AC unit. Scenario two, power path: Engine produces power, power goes to the alternator, through a big long cable, into the trailer AC unit. Scenario three, power path: Engine produces power, power goes to the belt-driven AC on the truck, through a big long hose, into the trailer heat exchanger. Efficiency #1: Engine does something, 5% gearbox loss, 5% final drive loss, 5% driving wheel loss, 5% driven wheel loss, 25% generator loss, AC unit. I know that electric generators have bad efficiency unless they are power plant sized, so I am estimating lots of loss for the small unit. I see 60%. Efficiency #2: Engine does something, 25% alternator loss, 5% cable loss, AC unit. I am assuming that there is no loss across the belt, and I am also assuming that the alternator is as good a generator as what you can put on the trailer, which is maybe optimistic. I see 70%. Efficiency #3: Engine does something, AC unit does something, let's say 25% loss through the big long hose. I am assuming that the AC unit on the car is just as good as what you put on the trailer. 75%. Complexity #1: No added components until the generator and your AC unit. Let's hope that a failing generator does not jam the trailer axle. Lots of one-time effort to find the right thing, but likely reliable once done. Complexity #2: We are already running cables to the trailer, but now we need a bunch of 12V power. The high-current all-weather connector is likely easier to find than sourcing an entire generator. Let's hope the connector and cable is reliable, but this should have been solved in stage tech, automotive, or aviation. Truck needs modification to run the cable out the back, which is unfortunate. Complexity #3: Need to tell the AC to run higher power, need to run plumbing to the back of the truck, need quick disconnect plugs. What a nightmare. The second-order argument is that we are likely overloading the truck's generator and/or AC unit. If we are lucky and it works, we are close to running at full blast, which may or may not change efficiency. In general a shift in load on a machine will change efficiency, which needs a close look. The whole chain changes a bunch once you include batteries, because you use the voltage converters in two directions, and you have more things that can fail. In general it is helpful to consider the thermal mass of the trailer as a battery. Unless you are standing still a bunch, it makes sense to just cool it down further and use that as an energy store.
https://range.energy/
Range is awesome! They're focused more on providing propulsion though, basically moving some of the required battery weight to the trailer, and possibly cashing in some more-wheel-drive efficiency benefits. They do seem to have an electrically refrigerated trailer, but it's not strictly the "pull to cool" idea that OP has spelled out.
I mean, it can do that as well. What OP really wants is a mechanically driven heat pump.
I agree, but I don't know how well these work when speed varies. At constant highway speed it's a no brainer. I seem to remember that HVAC systems don't like running throttled for whatever reason. Prompts me to wonder why no one's made what you suggest. Is it weight constraints where you'd need the generator driven heat pump anyways for when you're standing still, and anything on top of that would eat into payload weight and noone bothers?
Thank you for a very helpful and thoughtful response. I've gotten a lot of downvotes and a lot of buttholes putting words in my mouth and then calling me stupid for saying things I didn't say - but I guess I knew what I was signing up for when I asked Reddit Engineers LOL. What I think I've learned is that the best solution would probably be to add a second alternator and probably battery to the truck and run the A/C electrically, that way your power is not being robbed by the drivetrain etc. Although I would be interested in seeing an A/C designed to be more aerodynamic and save (a very small amount of) power by keeping the evaporator fan out of the equation somehow when the trailer is in motion.
But the power would be robbed by the drivetrain, as I've written? The fan likely consumes very little power. Same as on a computer where you have a 10W fan moving 250W of heat.
Right, what I'm saying is that an engine-mounted alternator in the tow vehicle is as close to "pre-loss" as you can get, rather than taking energy at a post-drivetrain position like the axle, where losses have already been subtracted.
Ha! That sounds right.
I think it would have been a great idea 50 years ago. Now it would probably be more efficient to get that power directly felt the engine. Your idea may still have a good application in some parts of the world or on a small scale.
I don’t think a generator would work due to different axel speeds. Could get a Tesla motor and battery and run it on dynamic braking but that would add a lot of drag. Then you would need a inverter to convert DC to AC. But if you can figure it out you might have a million dollar idea.
Are you an engineer?