If you have a house battery bank and an inverter (as most larger boats do) why don't we use DC gensets instead of AC gensets?
Let's say you have 600 amps**12 volt capacity house batteries and two 120 volt airconditioning units which draw about 15 amps each. Let's also say you have a 3kW inverter and that you charge the house batteries with about 60 amps (10-13% of capacity).
You would need roughly a 5-7 kW genset to run both A/C units, the battery charger and some smaller loads. A 5-7 kW genset produces about 40-60 amps**120 volts which equals to 400-600 amps**12 volts.
Main problem I see is that you need to run the same genset as well just to charge the batteries or just to run one A/C. When running the genset just to charge the batteries you are utilizing only about 10-15% of the genset capacity (60 amp charge rate vs 400-600 amps available). You are waisting fuel and you are decreasing engine life because diesel engines do not like running at rated rpms with low loads for an extended period of time. And you can not throttle down the genset because it has to provide the voltage at a fixed frequency (50-60Hz).
Let's think the DC genset way:
Your one A/C unit draws 15 amps**120 volts which is about 1800 watts and equals to about 150 amps**12 volts. Your house bank battery charger needs only 60 amps.
Let's say you decide to purchase a 250 amp**12 volt DC generator. That is a 3kW genset. It can be throttled down to about 50% with less loads as the genny is producing DC only and that is not frequency (Hz) related. Best examples are the small Honda gensets on eco throttle which is a DC genset with a AC inverter or your alternators on your engines.
When you run the genset to just charge the batteries, it will provide about 125 amps**12 volts with 50% throttle and that will charge your batteries ** 60 amps and will still provide some amps for some other small loads. You are utilizing at least 50% of your gensets capacity with no other loads.
When you turn on one A/C it will go to 85% power and will provide 60 amps for the charger and 150 amps for your inverter which in turn converts that to 15 amps**120 volts to run your A/C.
In case of both A/C's running, there will be no charge current to the house batteries as the inverter is utilizing all of the available 250 amps of the genset plus it is drawing an additional 50 amps from the house batteries to provide both A/C's with 30 amps**120volts (300 amps**12 volts). You can run your two A/C's for 6 hours for a 50% battery discharge.
As soon as one of the units shuts down the charge to the batteries assumes.
A 3kW vs a 5-7kW genset. Lower rpms/noise for most conditions. No battery charger involvement for genset runtime=less conversion losses.
Although this is a total theoretical analysis what am I missing here?
Let's say you have 600 amps**12 volt capacity house batteries and two 120 volt airconditioning units which draw about 15 amps each. Let's also say you have a 3kW inverter and that you charge the house batteries with about 60 amps (10-13% of capacity).
You would need roughly a 5-7 kW genset to run both A/C units, the battery charger and some smaller loads. A 5-7 kW genset produces about 40-60 amps**120 volts which equals to 400-600 amps**12 volts.
Main problem I see is that you need to run the same genset as well just to charge the batteries or just to run one A/C. When running the genset just to charge the batteries you are utilizing only about 10-15% of the genset capacity (60 amp charge rate vs 400-600 amps available). You are waisting fuel and you are decreasing engine life because diesel engines do not like running at rated rpms with low loads for an extended period of time. And you can not throttle down the genset because it has to provide the voltage at a fixed frequency (50-60Hz).
Let's think the DC genset way:
Your one A/C unit draws 15 amps**120 volts which is about 1800 watts and equals to about 150 amps**12 volts. Your house bank battery charger needs only 60 amps.
Let's say you decide to purchase a 250 amp**12 volt DC generator. That is a 3kW genset. It can be throttled down to about 50% with less loads as the genny is producing DC only and that is not frequency (Hz) related. Best examples are the small Honda gensets on eco throttle which is a DC genset with a AC inverter or your alternators on your engines.
When you run the genset to just charge the batteries, it will provide about 125 amps**12 volts with 50% throttle and that will charge your batteries ** 60 amps and will still provide some amps for some other small loads. You are utilizing at least 50% of your gensets capacity with no other loads.
When you turn on one A/C it will go to 85% power and will provide 60 amps for the charger and 150 amps for your inverter which in turn converts that to 15 amps**120 volts to run your A/C.
In case of both A/C's running, there will be no charge current to the house batteries as the inverter is utilizing all of the available 250 amps of the genset plus it is drawing an additional 50 amps from the house batteries to provide both A/C's with 30 amps**120volts (300 amps**12 volts). You can run your two A/C's for 6 hours for a 50% battery discharge.
As soon as one of the units shuts down the charge to the batteries assumes.
A 3kW vs a 5-7kW genset. Lower rpms/noise for most conditions. No battery charger involvement for genset runtime=less conversion losses.
Although this is a total theoretical analysis what am I missing here?