Engine Drive vs DC Systems

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WHY A DC INSTEAD OF AN ENGINE DRIVEN SYSTEM?

Glacier Bay manufacturers all types of drive systems including engine and hydraulic as well as AC and DC electric. However, in most cases we do not recommend an engine driven system for serious offshore cruisers. The reason for this is simple - it will take much more engine running time to maintain your refrigeration system with an engine driven compressor than it will to replace the electricity used by our DC system. This is true even with our own engine driven system which, according to the Cruising World Magazine tests (June 95), requires 1/3 less run time than Sea Frost and Grunert and only about 1/2 that of Technautics.

While this comment may seem to fly in the face of conventional wisdom, the simple truth is that a properly set up battery bank and alternator system does a much better job of capturing the available power from an engine than does a directly driven refrigeration compressor. To illustrate how the DC approach can shorten your engine run time, consider that a typical engine drive system would usually take 45 minutes to 1 hour of engine run time twice per day to "charge" the plates. (In the Cruising World Magazine refrigeration systems test, the Glacier Bay engine-driven system took 1.1 hours per day, the Sea Frost and Grunert took 1.6 and the Technautics took 2 hours of engine run time per day to cool a 5 cu.ft. refrigerator and 3 cu.ft. freezer (with 4" of insulation) in the tropics.)

Remember these numbers while we investigate the efficiency of the DC alternative. The first step is to find out how much electrical power the DC system requires to cool these same boxes. Fortunately, the Cruising World test also gives us a good foundation for making this assessment. In the DC system portion of the test a high-efficiency Glacier Bay refrigeration system (cooling the same boxes) consumed 57 amp-hrs of electricity from the battery bank . Therefore, the question is, "Does it take less or more than 1.5 to 2 hours of engine running to replace 57 amp-hours of charge back into battery bank?". How long it will take to replace this energy depends primarily on three things.

The size of the battery bank.

The type of batteries used.

The power output of your alternator(s).

Lead-acid type batteries have an average "acceptance rate" (ie. recharge rate) of 25% of full bank capacity under normal 50% to 80% cycling. Gel-Cel type batteries have a 40% acceptance rate under the same 50% to 80% cycling conditions. The newest "AGM" (Absorbed Glass Mat) batteries have a full 100% acceptance rate. (NOTE: This is why the house battery bank should never be divided into two banks. To do so simply cuts the acceptance rate in half effectively doubling the recharge time that would otherwise be possible.)

Once your battery bank size and type are determined, your alternator(s) should be sized so that, when hot, it will provide an output equal to the acceptance rate of your battery bank. Remember that it is possible to charge a single battery bank with two separate alternators. Consider using that place on your engine where you might otherwise install an engine driven refrigeration compressor to be used as a home for a second alternator instead!

Now, considering the above information and the need to replace 57 amp-hours back into the bank you can use the charts below to accurately predict your required engine run time.

BANK SIZE (AMP-HRS)	RECOMMENDED ALTERNATOR(S)	ENGINE RUN TIME REQUIRED (Minutes per day)
400	100	43
600	150	26
800	200	21
1000	250	16
1200	300	13

BANK SIZE (AMP-HRS)	RECOMMENDED ALTERNATOR(S)	ENGINE RUN TIME REQUIRED (minutes per day)
400	160	25
600	240	16
800	320	12
1000	400	9

Now, if we look only at how long you must run your engine to replace the energy the refrigeration system is using, we see that, in all cases, it is reduced to only a small portion of that required by an engine driven system. In the most extreme examples given, the DC system requires only 6 minutes of engine running per day vs 2 hours for the engine-driven system. While this level of efficiency requires very heavy-duty alternators, even relatively modest alternator/battery bank combinations result in a Glacier Bay DC system which requires only about 1/3rd as much engine run time as the Sea Frost or Technautics engine-driven systems.

OPTIONAL ECM DIGITAL CONTROLS
Proponents of engine driven systems may rightfully point out that sometimes the engine is already being run for some reason other than just charging the refrigeration holding plates and the engine driven compressor offers the advantage of allowing you to "top off" the holding plates at that time. With Glacier Bay's optional ECM Digital Controls we incorporate this same advantage into our DC system as well. These microprocessor based controllers replace the standard thermostats (included in the above quotation) and offer an additional, energy saving feature "engine drive" feature. Under normal conditions (ie. at battery or shore power) the ECM controls monitor the cold plate and fully cycle the system in the same way as the standard thermostats. However, whenever an auxiliary engine or generator is started they automatically switch to "engine-drive" mode to maintain the hold-over plates in topped off, but not over frozen, condition for as long as the additional power is available. The end result is that, in many conditions, daily battery draw can be reduced by up to 50%.

OTHER DC SYSTEM ADVANTAGES
In addition to requiring less engine run time, your Glacier Bay DC system will -

free you from a mandatory engine running twice (or more) per day.
allow you to leave the boat unattended for days at a time without worrying about food spoilage.
allow you to further reduce (or eliminate entirely) engine running with the addition of alternative charging sources (solar panels, wind generator).
run directly from AC power sources using your battery charger.
give further reductions in energy consumption (engine run time) by allowing you to independently shut down box(es) when they are not needed.

See our Technical Library for a further discussion on setting up and using your battery bank.

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