Other Technologies
size and weight) combined with efficiencies double that of conventional alternators. For the same belt and pulley load you get twice the output. Over the next year or two we will see an increasing number of impressive offerings of this kind.
However, it’s not much use having 5, 10, or 15 kilowatts of alternator output if you can’t use the energy. We need batteries with very high charge-acceptance rates to high states of charge that can act as an energy storage “buffer.” Battery chemistry plays an important role here in as much as traditional wet-cell lead-acid batteries have a low charge-acceptance rate which rapidly becomes even lower as a battery approaches a full state of charge, whereas any form of lithium-ion has an extraordinarily high charge-acceptance rate to very high states of charge: a many-times-larger bank of wetcell batteries is required to match the performance of lithium-ion.
Although lithium-ion is currently the ultimate technology, it is extremely expensive and requires sophisticated management to keep it safe. In recent years there have been numerous advances in the lead-acid world, notably with variants of absorbed glass mat (AGM) batteries, which have steadily ramped up performance, especially in terms of charge-acceptance rates and deep-cycling capabilities. Noteworthy are thin-plate pure-lead (TPPL) batteries and, more recently, batteries with carbon-doped negative plates (e.g. Northstar ‘blue’ batteries) and carbon-foam negative plates (e.g. Firefly). The latter promises to be a breakthrough technology. mum rated power required in a generator, enabling the generator to be downsized, which, with no other changes to the system, has the effect of increasing the average load. The next step is to engage in some form of load management, either manual or automatic, such that high loads are not brought online simultaneously, and/or the major AC consumers are concentrated in designated generator runtime slots. In the former case, this too reduces the peak load on the AC system; in the latter case, it reduces the load on the inverter and batteries during nongenerator periods. A step up in technical complexity but also effectiveness comes with synchronizing inverters. These have the ability to track the output from a generator. The system is programmed with a load limit for the generator (ideally, around 70 percent of its rated output). If the load goes above this (maybe the inrush current from an old-fashioned air conditioner) the inverter draws power from the batteries and kicks in to boost the generator’s output. If the generator load goes below the preset limit, the inverter switches to battery-charging mode and adds whatever load is necessary to put the generator back at its set point, up until the batteries can no longer accept this rate of charge. This is powerful technology that is now well tested but considerably underutilized in the marine world.
On larger boats there is always a relatively high “background” AC load. In such a situation it makes no sense to transfer this to inverters and batteries. A better approach is to have a relatively small generator, running 24/7, which is optimized to handle this load, with an additional generator, inverters, and so on set up as described above to automatically kick in and handle the load whenever it goes above the base level. Both generators will now operate at, or close to, peak efficiency at all the times.
There are other iterations of this theme, including variable speed generators and DC generators. In the latter case, the entire AC system is powered by inverters with the DC generator firing up whenever the inverter demand plus the battery charge-acceptance rate loads the generator to its sweet spot. The generator shuts down whenever the load drops below this level.
Regardless of the approach, the goal is always the same: to only run an engine when it can be loaded at, or close to, its peak efficiency, and to find other mechanisms to power the boat’s electrical systems when this cannot be done. Properly engineered, the net result will typically be a considerable improvement in efficiency with an often substantial reduction in engine run-hours. What’s not to like about that?
Nigel Calder has literally written the book(s) on boat systems, including Boatowner’s Mechanical and Electrical Manual, Marine Diesel Engines, Refrigeration for Pleasureboats, The Cruising Guide to the Northwest Caribbean, and Cuba: A Cruising Guide.