Is 12vdc compressor from solar possible

[hammet]

@almostDIR golf carts are typically 24v as an FYI. Reason being the amp draw is cut in half which makes all of the wiring much easier to manage. Electric golf carts are typically 3-5hp, so the right range for most smaller compressors, but to get that in 12v would be very difficult since you would first have to find a 12v motor that large. It is very uncommon to see a 12v motor larger than 1hp due to the amp draw.
Depending on the voltage of the solar system, it may be possible to wire the panels and battery banks to higher voltage which would allow DC drive in a reasonable range.

@NotWavingDrowning this project is possible if you are truly committed to do it. It is not recommended if you do not have significant experience with high current electrical systems and there is a very real possibility of a fire. Proceed with caution

tbone,
Lots of cruising sailboats use banks of deep cycle golf cart batteries. They are usually 6v batteries that are paired in series to make 12 volts and the pairs then wired in parallel to produce 'house' banks of anywhere from about 100 amp hours to 800 Ah. The reason these batteries are used is because they are built to take many cycles of deep discharges (to about 50%) and recharges.

Once you have a decent size bank and a proper way to charge it, an inverter can be used to get AC current. Marine grade inverters over 1000 watts are commonly available, though pricey and they must have short, heavy cables to the batteries to prevent voltage drop. Assuming OP had a large enough battery bank, he could run the compressor, however, he is going to need a way to charge his battery bank quicker than just solar. Mostly, that means running is engine for while until his batteries are at about 80% charged and then topping off with solar. Not a realistic way to fill multiple tanks for multiple days of diving.

There is no way you can add over 1000 watts of solar to a sailboat inexpensively. 50 to 200 amps is much more realistic. First of all, the real estate is just not there. Secondly, the panels must be pointed directly at the sun to reach close to ideal output. Then there is the cost of the panels, controller, and wiring. Even the slightest shading (less than 5%) such as clouds or the shadow of a mast will cause a severe drop in output. Most cruising sailors will use solar to keep their battery banks topped off but they are using much fewer amps over periods of days. Maybe 100 amp hours if they use refrigeration. So panels that lie flat on the bimini of a boat may average only about 4 hours of directlight per day in the tropics and that's when it's sunny out.

 

[tbone1004]

@hammet I'm well aware of how the banks are built, the problem is 12v is not enough voltage for this application. 1000w=~1hp=~1cfm *once you factor in efficiency losses from voltage boosting and change from dc to ac current. Anything over that is impractical to run from a 12vdc bus which is why home solar systems are 24v or 48v. Higher voltage means smaller wires, less voltage drop, and more efficient inversion to AC power.

A VFD does AC-DC-AC conversion and you can technically wire a DC power source directly into the VFD. This is how most electric cars work, but the battery voltage has to be sufficiently high since the VFD isn't doing true voltage augmentation. I.e. if it is fed 400v from the battery, it can feed the motor up to 400v, but it can't boost the voltage to 600v. The Tesla motors IIRC are rated at 320v, so the inverters are never applying full voltage *except in ludicrous mode*, so that way your power does not drop as a function of pack voltage. This is analogous to constant output lights like Underwater Light Dude vs say Big Blue that loses output as a function of pack voltage.
For this application, say a Bauer Junior II, you'd need to be able to run a 2hp motor, and would need at bare minimum a 2500w inverter to run that compressor. I would recommend having a separate inverter just for that compressor though. Even though 2hp is technically 1500w, you have a single phase motor which is only about 70% efficient, so that's 2200w just to run the motor, then the inverters are at best 80% efficient, which is about 2700w of actual DC consumption to run the thing.
That compressor is rated at 2.9cfm, and let's say that you're back on the boat with 500psi on an AL80, so need 65cf of fill which is around 22 minutes. 2700w*22min/60min*12v=83ah/tank of battery capacity and with discharge capacities and what not you need at bare minimum a 100ah battery bank per scuba tank to fill.
If you can find a 12vdc motor that's sufficiently large, by all means, but they won't be more than 90% efficient for a top quality one so you still will need about 1800w to run it and that's if you can find that motor.

 

[tbone1004]

would a small booster work for pumping normal air to the required pressures if you generate the low pressure working air using a small 12v compressor so small sized it can be powered from solar panels directly? would be very slow and a tad expensive and you would need to have custom filter assemblies but that would be easily scalable I think.

it is possible, and my booster actually has the ability to do that. Boosters are horrifically inefficient though so the power requirements would actually be worse for this application. My booster case has the ability to run like this, but the total power requirements would be significantly higher than using a smaller compressor.

 

[BRT]

alright, so a bit of weird info up there.
You can run any compressor you want on 12vdc. Don't care if it's a 50cfm monstrosity, or a Rix SA3. All you have to do is put a 12vdc motor with appropriate pulley on there and it will go. The problem is finding one big enough to run it at any reasonable speed.
On average, you need 1hp/cfm.
Important notes
1hp=746watts=torque*rpm/5252
Compressors are constant torque applications, so your motor needs sufficient torque capacity to actually turn it, but that's not all that much.
MOST compressors use splash lubrication which means they have a minimum RPM, generally somewhere around 70% of the top speed so you can't slow them down all that much.
Watts=volts*amps, so to get a 1hp/cfm compressor at 12v, you will be continuously drawing 65-70a depending on efficiency losses. You will have to run 2-4awg wire which is thick and expensive. That is why cars went from 6v to 12v systems, and why cars are moving towards higher and higher voltages. Smaller wires are cheaper and lighter. The motors can also be smaller which is ideal.

So. Should you? No, running it on 12vdc is stupid IMO. Use an inverter and run AC. Can you find a compressor that will do it? Not many new ones are small enough to handle it, and none currently in production that I'm aware of that are truly variable speed. You can find an older Rix and put a smaller motor on it which is what I would do if you need to keep it electric, but it sounds like you'll be far better off with either an electric compressor and a generator, or a gas powered compressor.

Why do you say that compressors are constant torque applications. I would call them constant speed applications with variable torque required.

 

[tbone1004]

Why do you say that compressors are constant torque applications. I would call them constant speed applications with variable torque required.

A true variable torque application is a fan. With a fan, the torque required is a function of speed. In this case, power is a cubic function of speed. I.e. if you double the speed, it requires 8x the power, and this is probably the most common type of load. Cars, fans, most Power is typically measured in watts, and 745.7watts=1hp

With a compressor, the torque required is a function of outlet pressure. Compressors have PMV's for filtration and balancing, and once you exceed the PMV setpoint, the torque requirement does increase, but it is not a function of speed. A compressor that needs to pump 4000psi will require the same torque at 1rpm as it does at 1000rpm. The power requirement directly proportional to the speed in this case. If you increase the compressor speed from 1rpm to 1000rpm, it will require 1000x the power.

Does that make sense?

 

[almostDIR]

The amps and volts and watts are getting mixed up a little in this thread the main point being still that it is difficult to fit enough solar panels in a sailboat to get enough watt hours per day to be able to compress the required amount of air for scuba purposes. If one would only use a tank or two a week it would be much easier. But if needing to use the engine to charge the batteries to get enough charge for the compressor it would be much more practical to just use gasoline powered scuba compressor instead, no point wasting energy on the intermediate step.

Charging batteries with 12v and discharging them at 24v is pretty easy to arrange actually. One can do this with for example switches or diodes... One can also just charge the batteries at 24v from panels if arranging them differently and using 24v controller. It is also pretty easy to modify a compressor to accept whichever motor you want, one just needs some machining tools and skills.

One could also do a compressor which has both electric and gasoline powering options easily interchangeable, just requires some extra parts and good machining tools. If I had couple of thousands of loose cash I could build a prototype for you in my garage

 

[BRT]

A true variable torque application is a fan. With a fan, the torque required is a function of speed. In this case, power is a cubic function of speed. I.e. if you double the speed, it requires 8x the power, and this is probably the most common type of load. Cars, fans, most Power is typically measured in watts, and 745.7watts=1hp

With a compressor, the torque required is a function of outlet pressure. Compressors have PMV's for filtration and balancing, and once you exceed the PMV setpoint, the torque requirement does increase, but it is not a function of speed. A compressor that needs to pump 4000psi will require the same torque at 1rpm as it does at 1000rpm. The power requirement directly proportional to the speed in this case. If you increase the compressor speed from 1rpm to 1000rpm, it will require 1000x the power.

Does that make sense?

Got you.

 

[Danseur]

Add a regenerative prop, and a genset.

Bank and inverter big enough.

Sure you can, but why would you, unless the Apocalypse has come...

 

[2airishuman]

I have a cruising catamaran and want tanks before hookah. I was thinking of using an 80cf tank and filling it to 2000psi. My math says I should be able to do it with around 0.6kWh of power which I can make with solar.

My problem is reality doesn't seem to agree (happens a lot), from what I have told I will need at least double that which I can't make.

Anyone know if it can be done.

I think it can be done though it may take somewhat more than 0.6 kwh depending on how full the cylinder is when you start and the efficiency of your compressor. We were having this discussion on cruisersforum.com the other day.

The trick is to use a DC motor. It is easy to find 24V motors up to about 4 HP that are in a standard NEMA 56 frame. 12V is harder but I believe they do exist. They are mainly used on battery-operated floor maintenance machines.

Here's a 12v DC motor in 1HP: CDP3445-V12 BALDOR 1HP MOTOR

They also show up on ebay but for 12v there's a lot of wading through the results because there are starter motors etc.

DC MOTOR 1.0HP 56C Frame 12V/1750RPM TEFC MAGNET Permanent Versatility Generally 882510651145 | eBay

Anyway you could run a smaller compressor with one of those with some adjustment to the pulley size.

Using a DC motor essentially removes problems with inverter size and starting load from the equation. DC motors, especially shunt wound ones, are tolerant of low voltage while starting while AC motors are not.

If you have a 24v electrical system on your cat then there are more options.

 

[2airishuman]

Anything is possible, but unless it's Waterworld or a BIG boat, not worth the trouble, IMHO

The OP has a cruising catamaran. They are big boats, designed for crossing oceans under sail, with considerable interior space. Many of them have scuba compressors aboard.

You can run any compressor you want on 12vdc. ..
So. Should you? No, running it on 12vdc is stupid IMO.

It may not be the best solution but it's certainly worth considering depending on the boat.

Cruising catamarans ordinarily have, in addition to sails, two auxiliary engines, one for the left prop and one for the right prop. Each of these is equipped with at least one large alternator, sometimes two. In an installation where electric power is important, these alternators will each deliver about 2000 watts continuous, either at 12 volts (most common) or 24 volts.

This is augmented by solar, and occasionally, other sources. On a cruising cat there is usually enough room for a considerable number of solar panels.

There is a widely held belief that adding additional internal combustion engines to such a craft is unnecessary. Weight is usually a limiting factor, not space, and engines are heavy.

Use an inverter and run AC. Can you find a compressor that will do it? Not many new ones are small enough to handle it, and none currently in production that I'm aware of that are truly variable speed.

Bauer is currently making compressors that ship with a 3hp electric motor (AC). I don't know whether they can be run at lower speeds than nameplate, but it doesn't matter -- inverters are widely available that will start a 3 HP induction motor.

https://www.wholesalesolar.com/cms/magnum-energy-msh3012m-inverter-specs-3462742499.2940345.pdf


Anyway, the most common solution is to run a compressor that has a gasoline engine. The second most common solution is to run a compressor that is powered by an onboard 120v or 240v generator set.

Other solutions are possible, such as using a DC Motor as described upthread, or powering the compressor with a belt drive or hydraulic drive from one of the main engines.