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24vdc charging vs. 120vac charging
By K3CZ, Section Controls Posted on Sun Aug 28, 2005 at 12:58:44 AM MST
Should the backup genny run at 120vac or 24vdc?

Gents:
This may be a trivial question, but there must be some pool of knowledge out there that will help me decide on my system configuration.
I have a new grid-connected house (abt 1500 ftsq) under construction at the present time. Besides the normal 200a. AC utility svc, it will have a ground source ht. pump environmental system with a fireplace and woodstove, and a section of the electrical system segmented for emergency backup(water pump, limited lighting, refrigerator, etc). (App. 400w of Piggott-type windmill, and about the same in solar panels charging 8 or so L16's all operating at 24v.) Problems: Central MD has very poor wind resources and the location is wind poor in two quadrants; also, even with panel tracking I can probably expect less than 4hr/day of decent solar insolation due to site problems.
Consequently, in an emergency mode, the L16's can probably support the emergency systems for only a day or so, even with maximum RE input. I'm figuring on about 4/5 kw of AC with (2) 24v inverters daisy chained to give me 120/240volts for distribution.
How best to supplement the system during extended power outages? Via a normal 120v. generator via the inverter battery charging mode, or possibly a 24v DC genset directly charging the batteries? A good charge controller on the 24vdc genset should require it being run only to charge the batteries a few hours/day.
Does anybody have experience withs such an arrangement????? Van K3CZ

24vdc charging vs. 120vac charging | 5 comments (5 topical)

Re: 24vdc charging vs. 120vac charging (3.00 / 0) (#1)
by drdongle on Sat Aug 27, 2005 at 07:07:20 PM MST

A 24 v ( actually you need 28)is a more efficient choice as you eliminate the conversion of 120 VAC to 24VDC. However the benefit of a 120 V generator is that it can also power 120 Vac devices.
Chose wisely Grasshopper.
Carpe Vigor, Dr.D

Re: 24vdc charging vs. 120vac charging (3.00 / 0) (#2)
by bob g on Sat Aug 27, 2005 at 07:21:06 PM MST

the way i figure it is this:

example #1

genset running at 120 ac, hopefully not the 3600 rpm variety!
there are efficiency concerns with any generator, perhaps the generator is 90% efficient
then you have to run a battery charger, at perhaps another 90%
the cost of a good 1800 rpm genset, and a good 3 stage charger, combined with the losses
in my opinion likely is not the most cost effective method available to you

Example #2

a small single cyliner diesel, running a high output delco or leece/neville/prestolite alternator @ 24 vdc charging, coupled to a marine controller which would allow 3 or 4 stage charging with temp compensation would allow good and efficient charging. you are getting the highest charge rate and the best efficiency using one less conversion step

also example #1 would require decent rpm managment which adds cost that is not necessary with the 2nd example.

Example #2 can be set at whatever rpm that produces the needed output, highest efficiency and lowest noise.

also if anything goes haywire with example #1, you have to have expensive repairs done, and usually at a shop. in comparison Example #2, could be built and repaired easily at home as parts are widely available and various substitutions could be made, (ie. reverting back to the original alternator voltage regulator, replacement of belts or substitution of a gas engine if the diesel goes down.

for my money example #2 wins hands down, especially if you incorporate a watercooled diesel in the cogeneration mode, ie. harvest the heat to make domestic hot water or space heating

bob g

Re: 24vdc charging vs. 120vac charging (3.00 / 0) (#3)
by hobot on Sat Aug 27, 2005 at 07:36:14 PM MST

Go with the good Dr's advice, gas, propane or diesel genset in
120 V with 240 outlet to boot and use inverter battery chargers
you've already paid for on hand. This will allow the normal
top offs and equalization cycles to preserve bat bank and of cousre
if bank is dead, direct use and also can be a probable power plant
to boot. It is for emergency use you said so other wise the grid
and other sources tied to the stacked inverters can keep up battery.
Usaully its the worse cold or heat conditions durring the power outs
and engines not regularly cycled just seem to go stale, but d/t noise
and non emergency gaps ... well you know so be aware to be able to
start, which generally means also a starting battery to watch over.
I've hand cranked 20 hp diesel off in normal conditions but barely,
so no way when crank oil and block are frigid. If rare use and only
vitals to power smaller size for hand starts matters. Could get two.

I've similar set up with 10Kw diesel. Water bed heaater in insulated
battery box and oil heater on engine block as well as power draw on
24V bank, was tricky to balance it to start up as V's dropped but
not cycle an hour on for 30 min off. But in long lasting below zero
storms, diesel gets too cold to start so system would time out and
then really cool off if not caught and reset and nurised alive.
Best in real showdowns to just keep the beast snorting till crisis
over or set to start on intervals regardless of battery state.
Propane gens are about the most foolproof long wearing and
you can breath the exhuast like stoves inside homes but pricyer.

hobot

Re: 24vdc charging vs. 120vac charging (3.00 / 0) (#4)
by bob g on Sat Aug 27, 2005 at 10:25:05 PM MST

in defense of my example of small diesel driven alternator,

i was not in reference to a hand crank 20 hp diesel at sub freezing temps, for that yes it is a hand full and then some, rather

i was suggesting a smaller electric start diesel in the sub 10 hp class, and i would assume a moderately insulated enclosure, without auto start capability.

for colder temps i would incorporate a glow plug in the intake to aid in starting, or retrofit a starting aid (ether) injector.

the small chinese diesels are high compression units, that shouldnt be that bad to start even in very cold climates.

i have personally started 1693 cat's (probably the most difficult truck engine to start) at -20 degree's F , when everything including the fuel has either frozed or gelled. it takes a bit of work but can be done, these were truck engines setting outside in the wind.

also i might add, small gas engines are a bear to start in sub zero temps too. especially if the thing has not been started in a long time and has condensation in the fuel (ice).

as far as using the inverters to recharge the battery bank thru the use of a 120 volt genset, yes it can be done and very well controlled, but..

if the battery bank is large and below 50%DOD, you better have a large charging capability or you will be running the genset for a very long time.

with the marine alternator controllers, very large charging currents are directed to the battery bank to get from very low capacity to 80% SOC, (temp compensated of course), after which the charge rate is tapered back. Essentially doing the same job the inverter charger does, but also they are programmable.

perhaps your inverter charging capability can deliver 1.5 to 3 kwatt at 28 volts, if so maybe you would be better served using the 120 ac genset

also dont forget that you will also need to have somesort of disconnect if you plan on feeding your loads direct from the genset, this will add to the cost and complexity.

there are many variables envolved in this question, and clearly there is "one solution" to fit all circumstances.

bob g

[ Parent ]

Re: 24vdc charging vs. 120vac charging (3.00 / 0) (#5)
by FishbonzWV on Sun Aug 28, 2005 at 05:22:03 PM MST

I just went through a 24 hour power outage. A large oak fell and took out the local grid and a 24Kv feeder grid. Pulled the high voltage lines off of eight remote poles.
Fired up the 5.5K generator for daylight power and switched to the battery bank for night use. I'm not set up for RE, just some emergency power. There were three gens running in the neighborhood. One ran all night, one shut down about 10pm and the house was dark, shut mine down about 9pm and had lights and fridge running thru the night!

24vdc charging vs. 120vac charging | 5 comments (5 topical)

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