Generator and inverter in hybrid AC system

MDboater

It seems that the majority of cruisers these days offer generators as standard equipment or as an option for a source of AC power while away from the dock.  I have read of people adding inverters to batteries as an alternative to generators for a source of AC power.  What I haven't seen people discuss are hybrid systems that use both a generator and an inverter.  It seems to me that this type of system would offer advantages over either the generator or battery/invertor systems.  Generators can provide endless AC power (with enough fuel) but create extra noise, burn fuel, require maintenance, and often run inefficiently when powering a small load.  Inverters are quiet but suffer from AC power capacity that is limited by the battery bank. 

I envision a hybrid system where an inverter connected to house batteries would supply AC power until the batteries reach a certain level of discharge.  Then the generator would automatically turn on to supply AC power and recharge the batteries.  When the batteries are recharged or the AC load turns off, the generator would automatically turn off.  It seems like this type of system would offer numerous benefits:

1) The generator would run less often.

2) Since the generator would have the extra load of charging the batteries, in addition to whatever other AC systems demanded power, it would be operating more efficiently than when under low load conditions. 

3) The generator may not be needed while under way because the house batteries powering the inverter would be charged from the alternator.

4) It would offer a level of redundancy such that you would still have some amount of AC power if either the generator or inverter failed. 

I know that the hardware exists for such a hybrid system but I have not yet seen one offered in a boat.  Are they out there and I just haven't found them yet or are there reasons why a hybrid system as I have described should be avoided?  I'm interested to hear feedback from others more experienced than I.

Thanks,

MDboater

Michael T

@MDboater, BTW that's a fantastic overview and, if I may say so, a very intelligent assessment! Nicely done! There are many companies that offer exactly what you discuss either as complete systems or in stages to take you where you want to go for the price you want. You might wish to google Mastervolt who make an awesome array of the product with features you discuss, as do - to some extent - Xantrex and KISAE. I believe Charles also has product that you want as might Sinergex.

Stodge

A genny will produce a LOT more power than an inverter can.  I don't know that you could power an A/C unit with an inverter unless it was huge and had a lot of battery capacity to use.  TV, Microwave, couple things like that, sure.

MDboater

MT: Thank you for the kind words and for suggesting the vendors!  I looked at the web sites and yes, there are hardware solutions to assemble such a system.

MDboater

Stodge:  There are several manufacturers offering 3 kW continuous (6 kW surge) inverters.  A 16,000 BTU air conditioner draws about 1400 W so powering an A/C would not be a problem.  As to capacity, two group 31 batteries provide about 240 amp hours (AH) of capacity in total.  Discharging the batteries to low levels will shorten the life of the batteries.  So assuming the batteries were discharged to 50% capacity before recharging they would provide 120 AH.  This capacity is sufficient to run the 16,000 BTU A/C for about an hour.  When under way and taking power from the alternator, this time would increase.

I agree that when multiple A/Cs, TVs, and microwaves are in use simultaneously then a generator makes sense.  However, under low load conditions the generator is less efficient.  I looked at the specs for a 7.5 kW Kohler and it burns .72 gph at 25% load but only 1.12 gph at 100% load.  In transitioning these load conditions 56% more fuel provides 300% more power.

Taking the idea further to attempt to quantify the value.....   How many hours do you expect to get out of a generator before replacing? Is 1000 a reasonable assumption?  Assuming it would cost $10,000 to replace a generator then it costs about $13 per hour ($10 for the generator and $3 per hour fuel) to run your generator (minus maintenance).

Stodge

My genny has 60 hours on it.  The genny is 13 years old.  I basically use the genny for about 30 minutes to heat the water in the water tank, do some microwave and charge the batteries.  Never planned to sit with the A/C on while out on the hook.

A hybrid solution, I assume, includes a genny, just a smaller one.  So the maintenance issue still exists, just perhaps more spread out.

As for a pair of group 31 batteries, My house has 2 group 31 in parallel.  My fridges make pretty short work of them.  Adding an inverter to the mix would pull them down even faster.  In a hybrid solution that would mean the genny would be running often to keep the batteries up.

Now if you had a dedicated bank of group 31s for the power solution, separate from the normal house it would likely give you the advantage you're looking for.  Of course now you have 6 batteries on board versus 4.

The song "Money for Nothing" comes to mind.  

Dream_Inn

You also could add Solar into this equation.  One of the members on here that has our exact same boat (just a color hull) sets out solar panels that helps charge his batteries (directly plugs into cig lighter).  It also provides power to an inverter if desired.  I like the idea, but didn't like the idea of having to set them up each time.

As far as hours on your generator, I think you can far surpass 1000 hours.  I think up to 4 times that should not be any issue if maintained.  I boat with someone that runs his generator the entire weekend, every weekend when on the hook.  I make sure his output is always away from our boat.  He put over 200 hours just this year on it. 

I have a small inverter on my boat, but I really don't use it much, just for a blender here and there.   I don't run the genny much, maybe twice a day if we are the hook all weekend.  We use it mostly for cooking, keeping fridges going and charging batteries.  TVs are 12Vdc (not that we watch much, but once in a great while the kids at night).  Lights are all LED, but we still don't use those much either.  For me, I will definitely look into those batteries MT spoke about.

Michael T

If going solar and you want serious power may I recommend assessing the Rigid Monocrystalline type as they are, as far as I know, by far the greatest power producers square inch to square inch. I have seen amazing home designed mounts that can be manually articulated on two axis to track the sun. There are also automatic tracking systems available for the deep-pocketed! IMO, if generating any form of serious solar power amperage you should have a solar charge controller so you don't fry your batteries. At one time Arizona Light and Power or Power and Light? had an awesome web site dedicated to solar power and the latest battery technology - which for us is probably a regular series 31 AGm then progressing to the very latest - either thin plate pure lead Series 31 AGMs (see Northstar), Super 8D AGMs or for the connoisseurs among us, the Lithiums.

MarkB

MDboater said:

Stodge:  There are several manufacturers offering 3 kW continuous (6 kW surge) inverters.  A 16,000 BTU air conditioner draws about 1400 W so powering an A/C would not be a problem.  As to capacity, two group 31 batteries provide about 240 amp hours (AH) of capacity in total.  Discharging the batteries to low levels will shorten the life of the batteries.  So assuming the batteries were discharged to 50% capacity before recharging they would provide 120 AH.  This capacity is sufficient to run the 16,000 BTU A/C for about an hour.  When under way and taking power from the alternator, this time would increase.

I think one of the problems is the start up current when the A/C kicks on. I believe they can pull 10 amps or more at 110V.  That would require 100 amps on the 12V side. I'm not sure the 12V system will be capable of that kind of start up current. One solution may be to put in a capacitor.

However, between voltage regulation, switch-over controls, etc.  I can see it getting really expensive, and not really worth the investment.

MDboater

MarkB said:

MDboater said:

Stodge:  There are several manufacturers offering 3 kW continuous (6 kW surge) inverters.  A 16,000 BTU air conditioner draws about 1400 W so powering an A/C would not be a problem.  As to capacity, two group 31 batteries provide about 240 amp hours (AH) of capacity in total.  Discharging the batteries to low levels will shorten the life of the batteries.  So assuming the batteries were discharged to 50% capacity before recharging they would provide 120 AH.  This capacity is sufficient to run the 16,000 BTU A/C for about an hour.  When under way and taking power from the alternator, this time would increase.

I think one of the problems is the start up current when the A/C kicks on. I believe they can pull 10 amps or more at 110V.  That would require 100 amps on the 12V side. I'm not sure the 12V system will be capable of that kind of start up current. One solution may be to put in a capacitor.

However, between voltage regulation, switch-over controls, etc.  I can see it getting really expensive, and not really worth the investment.

Every inverter that I have seen has a continuous operation rating and a surge rating.  The surge capability would handle the additional start up current if it exceeded the inverter continuous operation rating.  The batteries can supply ~100 amp current because that is on the order of what is needed to start an engine.  However, you raise a point that high current use or rapid discharge may have negative consequences on the battery.  Or to reduce the per battery discharge rate you would need to add additional batteries (adding unwanted expense and weight).

It may not be a cost effective solution and that would be the bottom line.  I guess $15-2500 in hardware to add a 3 kW inverter and necessary control to a generator system.

MarkB

I agree a starter battery could unload that kind of current but can a deep cycle do that as well. Most boats have deep cycles on the house circuit. 

Stodge

I get the intent.  Use something more like a little Honda 2Kw with a heavy duty inverter to provide all your AC power needs.  But I think even going all AGM batteries the capacity and discharge may not be there yet.

Now we could all go diesel like the cruise ships do.  Have electric motors, lots of batteries and a diesel genny to keep things powered.  

I'm holding out for an electric/personal nuke plant hybrid solution before giving up my gas engines and genny.  

Dream_Inn

I know I talked with a large group of aft cabin style boaters that all had portable Honda generators running.  They all of course had the big gennys, but said they preferred the little Honda generators to keep their golf cart batteries charged using way less fuel and was much more reliable.  It almost made sense.

MarkB

For sure. And when the honda quits, you go to home depot and buy and new one for less than what it costs to repair your main genset.