Discussion/Comments

Summary

Maximising the number of systems on DCDirect current while minimising those on ACAlternating current reduces the cost and complexity of the electrical system.

Design Goal

AAmpere (amp), SI unit of electrical current small-to-medium size passagemaker should be designed so that, when necessary, it can use shore power almost anywhere in the world. When cruising or anchored, it should provide its own power with the simplest possible redundant system, and a minimum of fuss for you and any neighbours. The major complication in this scenario is the different frequency (Hertz) of the world’sSecond different alternating current (AC) shore-power systems, and the effect that has on AC equipment and the battery charger.

To meet this goal, you could equip your boat with an AC system, just like in your house, whether you are in North America with a dual 120/240 Volt (VVolt) AC service or in Europe or elsewhere with a single 230-VACVolt alternating current service. This would be very simple in most respects. There are a lot of good AC appliances and fixtures, and wiring is inexpensive relative to direct current (DC).

[DC flows in one direction; AC switches (cycles) back and forth. Cycles are measured in Hertz (HzHertz, SI unit of frequency)].

Analysis

The problem is you can’t store AC energy in a battery. With a fully AC design, you would have to run an engine constantly to drive an alternator. If either failed, there would be no fallback source of electrical power. Also, AC is generally less efficient than DC.

What about DC then? You could have a DC battery system, typically 12 or 24 V, with an alternator to recharge it, just like in your car. With a big battery, you wouldn’t have to run the engine all the time.

This is a good idea but it turns out there are very few good DC appliances. All the better ones are AC.

Using AC appliances on a DC system requires a DC-AC converter, called an inverter. Unfortunately, large DC-AC inverters are expensive. Many produce quasi-sine wave (square wave) output. Some equipment, like computers, requires true sine wave and won’t run on square waves. Another problem is appliances like refrigerators have a large surge current when the AC motor kicks in, requiring a larger inverter. Some appliances are just plain energy hungry. For example, microwave ovens readily consume 1500 watts, ovens even more. Also, DC-AC conversion is less efficient overall due to power losses in the inverter.

An alternative to an inverter found on many boats is a separate AC generator (genset). But this would have to run anytime you wanted to use AC, bringing us back to square one.

The Solution

The only practical solution is a dual system consisting of a DC system with battery storage, and a DC-AC inverter for AC appliances. This system will have an engine-driven alternator to charge the batteries at sea and a battery charger for use with shore power. But because of the aforementioned cost and inefficiency of the inverter, we should try to keep AC requirements to a minimum.

With this in mind, the design objective of an electrical system is to run as much as possible on the DC system and use an inverter for AC while, hopefully, eliminating the need for a separate AC generator. This requires a careful balance in all the electrical systems, and maximum energy efficiency in appliances and fixtures.

The Design

Having decided to run as much as possible on DC, we have to figure out how to maximise the number of systems on DC while minimising those on AC. Hopefully, we can design a system that can be sustained by the batteries for most of the day, and not take more than an hour to recharge (least fuss to other people). As it turns out, today’s batteries and charging systems are advanced enough that we should be able to attain this goal. Because this solution resulted from an iterative process, it’s easier to outline it, and then explain it, rather than trying to take you through the iterations.

The oven will be a diesel-fired Dickinson to reduce AC electrical needs, but an electrical stovetop and microwave will provide flexibility. Diesel ovens throw off a lot of excess heat. In summer, it will be more comfortable cooking with the stovetop, microwave or a barbecue in the stern cockpit.

Having eliminated the oven as an AC energy hog, our point of departure is that the following will be DC:

• All electronics except the TVTelevision and computers
• Main lighting in the engine room
• All engine-room systems and motors
• The refrigerator

The following will be AC:

• All appliances except the refrigerator
• Television
• Computers
• Hot-water heater
• Electrical outlets
• Auxiliary lighting in the engine room

Electronics such as engine and navigational instruments, including two-way radios, satellite receivers, sonars and radars, are readily available in DC. Consumer electronics such as stereos and AM/FM/SW radios are also available, many developed for the automobile and recreational vehicle (RVRecreational vehicle) market. None of these will be considered further in this discussion. The exceptions are the TV and any computers, which will be AC.

Lighting is readily available in DC; we will add AC lighting in the engine room as a backup when using shore power. The refrigerator will be DC because we can design a custom DC cold-plate refrigeration system that is much more energy efficient than a store-bought appliance. Other appliances will be AC:

• Dishwasher
• Icemaker
• Kettle
• Microwave
• Stovetop
• Television
• Towel rails
• Trash compactor
• Vacuum cleaner
• Vapour cleaner
• Washer/Dryer

Finally; in a crossover between electrical and HVAC design, heated towel rails will be a combination of AC and hot water.

Safety

All electrical appliances must not have a neutral to ground wire. This is standard in residential installations, but very dangerous in a steel hull. Ground should go to the common grounding point and the AC circuit should be floating.

Related

HVAC Integration

LightingDesign