Originally Posted by SV THIRD DAY
An important thing to remember about the AC vs DC water maker question is that the only difference is how do you get sea water up to 800psi to drive a portion of it through the RO Membrane. I know that seems basic and almost obvious to say, but starting with that understanding can help with the decision. Sure different brands of water makers have varying levels of automation, control and features but at the end of the day that fact drives Cost, Fresh Water Production Rate, Power Usage, Maintenance
The focus of the DC approach is energy efficiency and giving the client the ability to run the system from their battery bank, so good DC systems use an energy recovery pump to get sea water up to 800PSI. But this focus on low energy usage also makes you will live with low production and need to run the unit for longer periods. The energy recovery pumps also add in cost, so expect to pay more for a lower output DC system.
The focus of the AC approach is High fresh water production, the AC approach says we have power available while running the generator, so lets make as much water as possible and use a energy hog off the shelf piston pump. This approach can give you a 30GPH water maker for almost 1/3 the cost of a 20GPH DC energy recovery system, so if the goal is having larger amounts of fresh water and you have a generator available then the AC system usually wins out. But if the goal is running off solar and your batteries, then you have to go with a lower output DC unit.
A common worry going the AC approach is what if my generator goes down...do I then lose my ability to make water? No, not if you have a 2000W inverter, you can make water through your inverter. Our approach is to use our generator to make water while at anchor
and then when underway with the alternator running, once the batteries become topped up, we turn on the water maker through the inverter being powered by the alternator. So that way you have a back-up approach for if something happens to your generator. Now unless you have LiFePO4 batteries, it is unrealistic to think that you will run a high output AC water maker off of your battery bank using just wind
and solar...but it can easily be done while running your alternator.
There is no "wrong answer" in the AC/DC decision, as long as you know the pluses and minuses to each approach and line that up with your cruising plans and expectations. Speaking in generalizations (which is always risky) the owner and crew of a 42ft Cat tends to have different expectations than a smaller mono hull
cruiser in terms of comfort, showers, deck and gear
rinse-downs, and general water use. It is much easier to meet those expectations with a higher output AC water maker than a lower output DC system.
Here is a link to download a copy of our SM30 (33GPH water maker for $5250)
Of course I am a predisposed to like the AC approach not just because I build and sell them, but because our family
of 4 would not be on year 9 of cruising and living aboard if I had to tell my wife and daughter to constantly be watching their water use. Part of meeting our expectations for comfortable cruising and living aboard was having unlimited showers and a washing machine, so the AC high output water maker was the best way to meet our expectations.
There is a chap in Cairns who built himself a small powercat running outboards and solar panels who is running both his watermaker and scuba compressor off inverter AC by using variable speed drives
(similar to inverter air conditioners - he is a air con/electrian). Indicates some interesting possibilities for a cat with over 1kw of solar panels
I dont think its a s simple as that Alan... the permeate flow rates given by DOW on their membrane is onlt a data point at a specific test condition ie water temp, pressure, total flow rate, recovery rate = 8% etc... its not an indication of what you always get.
Looking at the specs of the efficient marine
watermakers, without energy recovery devices, they seem to share one thing in common. They reduce the speed of the pump so the total high pressure flow rate is small, whilst keeping the membrane suface area the same (2540 size). This results in a higher recovery rate of somewhere near 20% - not the 8% data point quoted by dow in their specifications. The problem with doing this in a simple DIY setup is that its very difficult to direct couple the pump to a motor which only turns at circa 500rpm. all of the single
phase induction motors and most of the DC motors ive looked at spin at a minimum of 1400rpm and thus move alot more water than is required in order to get the 20% recovery rate. Echotec uses a gear reduction pulley system on theirs, but i dont want that extra complexity. I think where its heading for me, will be a 940rpm motor which is driven via a variable frequency drive so i can slow it down to circa 500rpm. The drive adds an extra $300 to the cost, but it also allows flexibility in terms of flow rates and power consumption
... An 240VAC induction motor plus variable speed drive ($600) still costs less than a 375w 12vdc motor on its own...
Once i get this all up and running, the efficiency should not be too far different to the commercial
units, the only difference being a slightly lower motor efficiency of im guessing around 10% compared to a brushed DC motor...
No plans mate - we are designing the system and building them ourselves...
All the parts are available in australia
- however we have chosen to source some of the more expensive bits from the USA as saves quite a bit of money compared to local prices...
The system consists of a saltwater lift
pump - im using my jabsco
deck wash pressure pump, - tee off a low pressure line to the desal system;
A couple of prefilters - the washable type - 20micron and a 5 micron in the plastic housings etc and a low pressure guage - to detect blocked filters etc;
the high pressure pump - Stainless steel
CAT pump 2sf15seel - driven by a 750w rated single
phase 4 pole 240vac induction motor - sourced and modified locally to fit the imperial pump mounting flange to a metric frame electric motor
high pressure housing and membrane 2540 size, high pressure guage, and Stainless needle valve to control brine pressure;
A few lines and Y valves etc for diverting water to tanks, buckets for testing, and freshwater flushing
Manual TDS meter to test product water before collecting.
Its all pretty simple, designed to be reliable, and low cost. the high pressure pump motor runs off the 240v inverter - which is probably the weakest link in the systems reliability
- im contemplating keeping a second inverter on board in case it fails - they are cheap enough at $200 for a 2kW pure sine these days...
projected power input is about ~45amps inc the 10% inverter loss, and the water output should be around 48L per hour... total cost is under $3500...
Good news - my variable speed drive arrived on friday and i connected it all to the dive compressor
... it ran fine on plugged into the mains but kept tripping the inverter when i tried to run it from the boat... good news is i added an inductor to the line as a line filter and everything is now ok - the variable speed drive generates quite a bit of noise
and harmonics on the incoming power supply which the inverter doesnt seem to like much - it wasnt a current
problem - the unit only draws 9amps at 240V and that is happening once the compressor is upto full pressure...
And one more thing about big motors...
Induction motors typically use between 5 and 8 times their full load current
on startup - its called inrush current, and its so large because the rotor is not turning and their is no inductive reactance to provide any other impedance to current flow. Only the resistance of the stator coil is there until the rotor gets moving, and typically a small induction motor like this will have a stator winding resistance of a few ohms - like 2-4ohms or similar. If you apply DC theory to that youll know that current I, = v/R. If you assume a winding resistance of 3 ohms (about typical) then you should see 240v/3ohms = 80amps @ 240VAC - woah! But yes its true if only for a split second... So why does the running current of the motor look more like 4 amps? As the rotor builds speed, it interacts with the magnetic field of the stator and you have a conductor moving in a magnetic field - so the rotor generates a back EMF which opposes the current flow - so even tho the winding resistance is still 3 ohms - the inductive reactance is also at play and adds to the resistance to give a total impedance. Impedance refers to circuits which are generally AC in nature and can be not just in the form of copper resistance like in DC theory, but also inductive reactance, and capacitive reactance which occur in AC circuits but have the same effect - more impedance mean less current flows...
The problem with motors runn from inverters is tripping the overload protection of the inverter. If you try to start an electric motor
and it pulls in excess of the burst current rating of the inverter - it will trip out and you loose all power until it resets and repeats. Net result is you cant get your motor moving and your SOL... So you need a big rating on the inverter so the inrush current wont trip it before it gets moving. Once the motor is turning, your all good provided the motors electrical
power input (remember you also need to account for power factor on an AC motor - typically 0.8) doesnt exceed the maximum continous current rating of the inverter. So my 3000w -6000w burst inverter will have an instant trip at 25amps (6000/240) and it will be able to sustain a current of 12.5amps - which is only 3000w at power factor = 1. Its less than 3000W as the power factor drops - which it will do with induction motors running off it - 2400W if you assume PF = 0.8 as the current is still 12.5A...
This gives you the idea as to why generators are rated in KVA and not KW - they dont know what the power factor will be as it depends on the loads the consumer connects to it!
So now i have a 2.2kW scuba
tank filling compressor running from my 3kW inverter - powered by solar (and engine
alts if need be)
The compressor isnt as nearly as noisy as the petrol engine powered versions - obviously there isnt a combustion engine running it. But its still noisy, just the pistons of the compressor do make a bit of noise
- at full speed its the same as any other small size electric
motor and belt driven air compressor you would run for air tools and spray painting etc
The good thing is now i have full control of the RPM
so i can slow it down a bit if i want to and the more i slow it down the more quietly it runs and the less power it uses - at 30hz it runs very quietly and uses nearly half the power... 35hz seems to be a good compromise on speed to fill a tank and noise levels are tolerable.
At full speed it fills a medium size tank in 20mins from empty, 15 mins if there is 50BAR already in it. Its output at full speed is 100L/min. So a 10L tank (85cu ft imperial) is 20 mins to input 200BAR.
Thanks for the tips on the watermaker - no doubt they will come in handy. I was going to add a rotometer for the product water flow but thought i wouldnt need it as i will be running it into a bucket for testing before diverting. I figured i would guage the flow from that as i collect a test batch before diverting it to tank... The maximum flow for these membranes is 115L /hour - youve seen it this high in real world use in estuaries?"
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