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Marine Energy Part 3 – Advantages Of Hybrid

In this article, I shall look at the configurations of hybrid propulsion systems – their advantages and disadvantages and why I believe hybrid is the way to go for boats.

What Advantages

A hybrid system is quite likely to be more expensive than a simple diesel system since you have an electric motor and batteries as well as a diesel engine As discussed in the previous article, most of your energy requirements are still likely to come from diesel fuel so why bother? What benefits might we get out of a hybrid set up? Well, personally, I would choose the following criteria.

  • Smooth and quiet cruising. We know the electric motor will give us this but what when the batteries start to run flat and we have to run on diesel. I’ll deal with this problem when considering hybrid configurations.

  • Fuel efficiency. It would be nice to save a bit of fuel. Canal boats especially, are fitted with oversized engines which are running well below their most efficient load / rev range and can spend a significant amount of time just ticking over in locks. I shall take a closer look at boat energy requirements and fuel efficiency in the next article.

  • Domestic Power. Today’s boaters expect increasing levels of domestic comfort including microwaves and washing machines. These currently require large alternators and battery packs specifically for domestic power. A hybrid set up already has a large battery pack which is easily capable of running such appliances.

  • Convenience. It would be nice to have a system that automatically looked after all you energy needs giving you propulsion just when you needed it and domestic power when you needed it. I develop electronic control systems for a living so I can see how easy it would be to do some quite sophisticated things with these automatic control systems.

  • Cost. Could the above features justify the extra cost. Indeed, can the cost be made comparable with a traditional diesel set up? I will consider the cost of hybrid set ups in the final article on the subject.

Hybrid Configurations

Let us consider the available hybrid configurations and see whether they can give us these benefits. There are two hybrid configurations used in marine applications known as ‘series’ and ‘parallel’. I shall consider them each in turn and consider their advantages and disadvantages.

Parallel

Parallel HybridThe basic parallel hybrid configuration is shown in the diagram. Here the propeller is driven by a conventional engine and gear box. The electric motor drives the output shaft of the gearbox in parallel to the main diesel engine. Electric drive is achieved by putting the gear box in neutral and stopping the diesel engine. The clutch on the propeller shaft allows the engine to be used to charge the battery while the boat is moored up.

Advantages

  • Better fuel economy than a diesel only set up as the engine can be run harder for shorter periods.

  • Full engine power plus electrical power available.

  • Can be retro-fitted to an existing diesel set up.

  • The electric motor acts as a generator when using diesel propulsion.

Disadvantages

  • Typically, a standard boat engine is used which is running, inefficiently, at around 1/5th of it’s rated power at cruising speed.

  • There is vibration and noise from rigidly mounted engine physically connected to the propeller.

  • A gearbox and clutch are required.

  • While moored up you can still end up running a large engine just to generate moderate amount of electricity.

  • Quite hard to automate because of the requirement to operate the gearbox and clutch.

Serial

Serial HybridThe basic serial hybrid configuration is shown in the diagram. Here, the engine drives a generator which charges batteries and drives propulsion motor. The system can be computer controlled so that the engine / generator system automatically provides power, only when required, whether this is for propulsion or domestic energy use.

Advantages

  • A much smaller (cheaper) engine can be fitted as this only needs to provide enough power for average cruising speed.

  • Vibratory internal combustion engine can be completely vibration isolated from the hull as it is only connected to the rest of the boat by wires and pipes.

  • Domestic power is automatically generated when required.

  • Energy management totally integrated and automatic to achieve the best balance between diesel and electric. Engine start / stop is automatic to optimise energy use.

  • The gearbox is no longer required.

Disadvantages

  • Fuel consumption may be poorer due to generator motor combination – though not too bad. See the maths in the next article.

  • Cannot be upgraded from a standard diesel set-up.

  • A separate generator is required.

Serial Versus Parallel Versus Conventional Diesel

Now we know what hybrids systems are capable of, how do the two hybrid systems, described above, compare with a conventional diesel set-up when measured against the criteria I outlined above.

OK I admit the following is subjective, simplistic and possibly a bit too “reality TV show”. Anyway I’ve assigned scores for each system for each criteria and totted them up at the end. If you disagree, feel free to contact me and make your point. The scores are all out of five.

Feature

Conventional Diesel

Parallel Hybrid

Serial Hybrid

Fuel consumption.

2

3

3

Smoothness

2

2

4

Domestic Power

1

3

5

Convenience

3

2

5

Installation (including upgrading)

4

4

2

Cost

3

1

4

Total

15

15

23

Conclusion

Serial comes out ahead on nearly all counts especially for smoothness, automation and integration. As I shall consider in a later article, the cost need not be prohibitive either.

In the next article, I shall digress into a little mathematics and attempt to put some figures on how much energy our boat really needs.

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