Using DC vs AC

Imagine what you would say if you discovered an AC generator set in the trunk of your new Mercedes to run the lights, wipers and air-conditioning. As ridiculous as this sounds that’s what you could find in your new boat.

Your marine engine has a DC generator and battery system just like your Mercedes. As is or with minimal changes it is capable of supplying all your shipboard power needs.

Purpose-built marine equipment including your navigation and communications equipment is designed to operate on DC power. So why do boats have large AC generator sets? – mostly to power equipment that is really intended for onshore applications.

Why should you think twice before having AC on your boat?

  • Unlike DC equipment AC equipment operates at potentially lethal voltages – 115 or 240 Volts. In a marine environment this is doubly hazardous.
  • Galvanic corrosion can be a real problem due to the need to earth the AC wiring to the DC wiring for safety.
  • AC powered motors have a low starting torque requiring them and your generator set to be oversized – more weight and bulk and fuel wasting inefficient operation.
  • Single phase AC motors are inherently inefficient – not a problem onshore but onboard this means more power loss and increased weight.
  • AC power at the marina is generally limited in capacity by long cable runs. You cannot rely on this for anything but light power loads.

So what are the solutions?

  1. Install the high efficiency DC or hydraulic powered JEC Combi air conditioning and refrigeration unit. It can also provide high pressure water to a desalination unit.
  2. Install JEC high efficiency DC pumps and fans that are purpose built for marine conditions.
  3. Minimise or eliminate anything but the smallest AC powered appliances. DC to AC solid state inverters can readily power the small ones.
  4. Install a JEC lightweight universal voltage battery charger/power supply to keep your batteries topped up from marina shorepower anywhere in the world.
  5. For larger vessels install a small generator set to provide an alternative charging source for your shipboard batteries. This can be optimally sized for a continuous steady load.

The following table compares the difference between AC and DC generated power.

Alternating Current (AC) Efficiency Direct Current (DC) Efficiency
Diesel engine efficiency at 50% duty cycle 42% Surplus energy from propulsion engines stored in house batteries 95%
AC generator efficiency 80% DC has no generator N/A
AC motor single phase efficiency 75% DC motor efficiency 87%
Compressor efficiency 95% Compressor efficiency 95%
Losses due to distribution and connections 5% to 7% Losses due to distribution and connections 5% to 7%
Fuel losses in the process of AC conversion is 77.5 litres * Fuel losses in the process of DC conversion is 26.2 litres *

* Reference data: 100 litres of fuel was used and all losses were taken into consideration to produce cooling energy or fresh water. The final result is expressed in litres of fuel lost in the process of conversion.

Our Tests using the JEC Combi air conditioning/refrigeration plant, conducted over long running periods, prove that propulsion engine fuel consumption did not increase with the operation of the Combi. Operation of the Combi in a Marina is facilitated by a light weight DC power supply that can be connected to mains power anywhere in the world from 90-260V AC, 50 or 60Hz ranging from 2-5kW.

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