Conventi onal Generators
The moment the shore-power cord is unplugged and a boat moves away from the dock, almost all energy on board is created by running one or more fossil-fueled engines. House energy comes primarily from alternators attached to the propulsion engine(s) and from AC generators. In both cases, energy production as it currently exists is extraordinarily inefficient, and frequently results in long hours of relatively unproductive engine run time, with the associated maintenance, noise, exhaust fumes, and inconvenience. Evolving technologies are making much of this engine runtime unnecessary. What happens if we supply our house power from an onboard AC generator? Let’s consider a generator powering a 16,000-Btu air conditioner (about what you’d need for a medium-sized saloon). The running load of the air conditioner will be around 1.5 kilowatts. In the past, electric-motor-driven devices, such as air conditioners, had high ‘start-up’ or ‘inrush’ loads, typically four to six times the running load—i.e. around 6 kilowatts in this case (this no longer applies to many newer devices as we will see below). It has been standard practice to specify at minimum a 6-kilowatt generator to power a 16,000-Btu air conditioner. But as soon as the air conditioner gets up to speed the running load on the generator is down to 25 percent of the generator’s rated output, and then when the cabin cools down and the air conditioner periodically cycles to ‘off ’ there is no load at all. At 25-percent load almost all generators are operating at far below peak efficiency while at no load the system is infinitely inefficient. We are oscillating between being marginally efficient and appallingly inefficient! The overall effect is fuel consumption that is three to 10 times higher than would be the case if the generator always operated at peak efficiency. One way or another, this has been, and continues to be, a common operating profile for AC generators.
The corollary to this inefficient mode of operation is the average load on the generator is a fraction of the generator’s rated load, which means the generator must be run long hours to deliver a given amount of energy. If the average load can be increased, not only does the fuel efficiency dramatically improve, but the number of hours the engine must be run to deliver a given amount of energy is also reduced directly in proportion to the increase in average load. This has obvious lifestyle benefits, substantially reduces maintenance requirements, and greatly reduces the cost of the energy being produced on board.
The key to improved efficiency and reduced engine run-hours is to find mechanisms to maintain the load on engines at around 70 percent of their rated power at whatever speed they are being operated, and to shut down the engines and supply house power from nonengine sources any time this load cannot be sustained. There are a number of available and emerging mechanisms that make this possible.