At first I was pretty leery of putting high voltages in my boat. After a little research, I found that different voltages and frequencies have different risks. Alternating current tends to cause fibrillation that can stop your heart while direct current tends to burn you. AC causes less pain and you can be electrocuted with high frequency AC without feeling it. DC is more painful and causes muscle contraction so you might not be able to let go of the wires. The most dangerous AC voltage range is 120 to 250 volts. Above 500 volts the danger lessens because the higher voltage causes defibrillation rather than stopping the heart.
When doing an electric vehicle, high voltage is best. Since you require the same amount of power to run the boat regardless of the voltage, the higher voltage means that you can have much smaller wiring and much safer terminations of the wiring. In motors, higher voltage means less current and that translates to less heat from the motors. The downside is that high voltage can arc and if it's DC the arcs can stick. Careful control of switching is mandatory.
Because it was a voltage of choice for some hybrid auto makers and a couple startup hybrid boat companies, I chose to build a battery bank of 326 volts, based on 24 batteries and a float voltage of 13.6 v per battery.
Higher voltage means that wires sizes can be much smaller to carry power from one part of the boat to another. For example, a 250A current requiring a 0000 gauge wire at 12V would be equivalent in power to less than one amp at 326 volts. It’s the difference between a wire that’s almost an inch thick and one that you can barely see. Because electricity Is transmitted primarily over the surface of the wire, lower currents need much smaller wires.
Yes, that was a poor, and certainly dated pun. But my point is that there has been a revolution in the past couple decades in the way we handle power conversion. When I was a young man there was AC with transformers to change the voltage and DC with resistors to get a voltage you wanted. Now we have high power semiconductor switches that can handle very high voltages and thousands of amps of current. Today you can take almost any AC or DC and convert it to whatever you want. Take a microprocessor and send some pulses to a big semiconductor relay and you can take DC and make an AC waveform. Pulse the DC differently and lower the DC voltage. Run it through a different kind of circuit and change the DC to whatever DC or AC you want. In the case of my setup, I decided to use this new technology to my advantage to have more a more robust and less expensive generator.
In the Barbara Ann, the 326 volt DC bus goes to only eight points: battery bank, AC inverter, 12 volt DC-DC converter, 24 volt DC-DC converter, hydronic space heating and domestic water heat, the generator, the propulsion motor, and separate 10kW motor for the hydraulic pump. Lights, most instruments, and watermaker, refrigeration, grey and black water pumps all run on 12 volts as they did before the refit. I use a single 200A DC-DC converter from a Finnish company, Powernet Oy.
The AC inverter is a 7kW model from Airpax Dimensions that operates from the 326 volt DC bus. AC powers TV, microwave, air conditioning, washer/dryer, and dishwasher.
Domestic hot water and our cabin heater consist of three stainless steel instant hot water heaters from Chronomite, each weighing three pounds and consuming 9kW when in operation.
I had numerous problems getting products from the various hybrid propulsion companies that I contacted. Finally I decided to piggyback on their work and what I had learned and use the experience to pick and choose the components for the system myself.
Adding a little weight can pay big dividends. One of the big advantages of the high tech DC generators is that they are light. However, they are very expensive and in many cases they are designed to stress the limits of the generator and the engine powering it. One generator that I almost bought runs at 2800 rpm in order to generate 320 volts. The fuel efficiency of this particular diesel engine is 40% less than it would have been at 1800 rpm. The manufacturer told me that they couldn’t guarantee more than 28kW output from this 32kW generator and they wouldn’t warranty the unit for continuous operation.
If your own hybrid project is a lightweight racing boat or catamaran, and you have very deep pockets, then go for a DC generator and lithium ion or lithium iron phosphate battery. The Li batteries will save about 80% of the weight of the battery bank (1600 lbs), and the DC generator will save you about 300 lbs. Unfortunately, these weight savings will at least double the cost of your power plant.
In my case, I decided to stay with conventional AGM batteries and a very conventional 3-phase AC generator. For the batteries I’m using Odyssey 2250 batteries that use thin plates of extremely pure lead to create a nearly indestructible battery that can be charged at five times its discharge rate.
The Westerbeke generator is based on a rugged Kubota diesel engine rated for continuous duty and running at 1800 rpm. This drives a Mecc Alta 3-phase generator that generates 277 volts leg-to-leg at 60Hz. For our application the frequency is irrelevant since we immediately convert the generator output to AC using a liquid cooled 12-diode full-wave bridge rectifier module that outputs DC at 326 volts. Without further filtering, ripple is less than 4% but we added a couple chokes for further filtering and the huge battery bank acts as a capacitor and smoothes the output so it’s quite clean. This whole solution is about 30% cheaper than an equivalent permanent magnet DC generator and uses components rated for decades of 24/7 operation. The only disadvantage is weight. Fortunately the weight is very concentrated and tends to make nice ballast.