Aaron Porter | Professional BoatBuilder Magazine A tidy generator installation on the schooner Ernestina-Morrissey provides ample space to access the unit for service and maintenance.
Generator installations, variations, and optimizations
We continue our series on selecting and installing appropriate onboard generators from Generator Know-How, Part 1 in Professional BoatBuilder No. 213.
Installation
The mechanical installation of a generator is similar to that of a propulsion engine. You need a solid engine bed, and there are similar cooling-water, exhaust, and fuel-system requirements, though on a smaller scale. The generator will also require comparable levels of maintenance at the engine end. Note that engine oil change intervals may be shorter than those of a propulsion engine. And if the generator comprises a V-belt driving an alternator, the belt may need more frequent replacement because of the high ambient temperatures inside many insulated sound shields. Fischer-Panda, for example, recommends belt replacement every 100 hours on some models.
While the engine driving the generator is water-cooled, in most generators the electrical end is air-cooled, which leads to some unique vulnerabilities. Inside a generator are multiple insulated copper windings (see Part 1). Inevitably, the insulation has small defects and voids. In the humid marine atmosphere, moisture from cooling air can collect in these flaws. Fresh water is an insulator, while salt water is a conductor. But add a few impurities to fresh water, and it, too, becomes a conductor. In short, the combination of moisture and dirt degrades the insulation.
Compounding these technological vulnerabilities is the practical reality that generators are often installed in restricted compartments with high ambient temperatures. Heat lowers the ampacity of the insulation on the various windings and conductors and stresses control-circuit electronics. Generators are also often undersized for the inrush currents they encounter. Those peak currents further tax insulation already suffering from reduced capacity. When localized short circuits develop in the windings, this degrades the insulation even more. Meantime, the engine itself is also stressed more than it should be from a combination of the high ambient inlet air temperature and often restricted air supply for combustion.
This all points to the single most important installation consideration in terms of heading off unnecessary trouble: locating the generator where it will receive the coolest, driest, and cleanest cooling and combustion air possible. A close second, on the engine side, is preventing saltwater intrusion from the wet exhaust, which can be difficult, especially when a generator is installed off the centerline of a boat.
A critical safety issue of the installation is minimizing the potential for carbon monoxide poisoning, especially with gasoline-powered generators (see sidebar at end of article).
Nigel Calder All too often a generator is stuffed into too small a space with very poor access for maintenance. This generator also inhibits access to the propulsion engine located below it.
Electrical Integration
On the electrical side, if you have a North American-style 120/240V split-phase system, it is important to balance as much as possible the 120V loads between the two 120V buses. High unbalanced loads unnecessarily stress stator windings.
Note that some generators are supplied by the manufacturer without the neutral-to-ground connection required by the ABYC and ISO at the “source of power.” When installing a generator, you must ensure this connection is made at the generator, with the sole exception of installations with a polarization or isolation transformer on the shore-power inlet—those allow the connection at the AC grounding bus.
Many AC generators are installed to power the onboard AC system from either shore power or the generator. If no other AC sources are present, the generator must be able to handle the peak AC load, including peak inrush loads from electric motors, and it must run whenever AC power is needed, even if it’s just for a low-power device. Without load management, the generator must be rated close to the sum of all possible AC loads.
The electrical installation in this system is simple and uncomplicated: You need an appropriate two-pole selector switch—Shore/Ship/Off.
This conventional setup results in long hours of generator operation at light loads, which, aside from the noise and exhaust fumes, is extremely inefficient. It’s especially so if the generator is powered by a diesel engine with a traditional fuel injection system (i.e., not a high-pressure common rail system). With all generators, if you factor in amortization cost, there is an extraordinarily high cost for the energy generated (see “True Cost of a Kilowatt” at end of article). Light loads intensify maintenance and shorten the service life of diesel engines with traditional fuel injection and for gasoline engines with carburetors. Despite drawbacks, this combination remains the most common generator setup.
Aaron Porter | Professional BoatBuilder Magazine High-draw appliances like this air conditioner commonly exceed the capacity of alternators and require a generator.
Let’s look closely at a hypothetical example: A well-equipped, medium-size sailing yacht has a core daily energy requirement of around 4 kWh for navigation systems, refrigeration, lighting, water pumps, radios, etc. You can generate and supply this level of energy with a powerful alternator, a suitably sized battery bank, and possibly solar and wind energy. However, if you add AC consumers—air-conditioning, a watermaker, or a dive compressor—energy consumption rapidly increases to exceed the alternator capacity. With a generator, appliances such as a microwave or electric kettle become practical. As a result, it’s easy to conclude that a generator output of 8 or more kilowatts is necessary on such a boat. But that’s not necessarily true.
On this vessel, the only AC device running for long periods is the air conditioner, averaging 700 watts for roughly 12 hours (assuming it’s not used overnight). The microwave and kettle together run for maybe 40 minutes, and the watermaker for 2 hours. To power the air conditioner without other sources of AC power, the generator must be run for the entire 12 hours. If fully loaded, the generator could theoretically deliver 96 kWh during this time. Instead, we have the 4 kWh DC load that’s now supplied via a battery charger, the 8.4 kWh aggregate air-conditioning load, and maybe a combined 5 kWh load for the other devices. That’s a total load of 17.4 kWh (ignoring AC-to-DC conversion losses). During the 12 hours, the average load is 1.45 kW, which is less than 20% of the generator’s rated output.
In practice, the load swings between roughly 50% when the air conditioner cycles on to nearly zero when it cycles off and only small DC loads remain. This is an expensive and, for many engines, unhealthy duty cycle.
Improving Duty Cycles
There are several ways to improve this dramatically. The simplest is load management. If the boatowner can ensure no more than half the peak loads are running simultaneously, the required generator capacity drops to 4 kW. The generator will still run 12 hours a day, but it now has an average load above 50%, substantially improving efficiency and reducing energy cost.
A better approach is to view the AC and DC systems as a combined entity with battery chargers and inverters as the interface. If you properly design and manage such a system, all the light AC loads can be run from batteries via the inverter. (This includes modest air-conditioning needs in many applications.) Such a setup greatly reduces generator runtime. In an optimized system, whenever the generator is running, battery charging loads are added to the AC loads such that the former will be appropriately adjusted as the latter fluctuates, maintaining a near optimum load on the generator. The generator must still be powerful enough to handle at least the peak AC load when it is running, plus any additional battery charging load.
Another considerable step-up in sophistication and energy systems optimization is possible with a synchronizing inverter. These have the capability of paralleling their AC output with a shore-power connection or a generator’s output. When the generator is running and the peak AC load exceeds the generator’s rated output, the inverter kicks in to supply the necessary extra energy from the batteries. If the AC loads go down, the inverter switches into battery-charging mode to add load to the generator. Sophisticated inverter/chargers have software in which you can set the desired generator load to always run it at peak efficiency. Depending on the shifting AC loads, the inverter/charger will cycle back and forth between inverter and charger modes to maintain this load.
This technology shifts the calculation linking peak AC load and an onboard generator’s maximum rated output, which has traditionally determined generator size. The net result can be a dramatic improvement in generator efficiency, with an equally dramatic reduction in the cost of energy produced. You can either optimize for a substantial reduction in generator size, in which case the generator will still have to run relatively long hours, or for a substantial reduction in the generator runtime, in which case the generator will not be greatly downsized. In either case, the generator will be optimally loaded when running.
With such a system, a sizable battery bank is required to support inverter loads and absorb battery-charging energy when the generator is running. In modern systems, you can fully automate the functionality if the generator has an autostart function triggered by AC load or battery state of charge.
Years ago, I participated in extensive tests by Victron Energy demonstrating these advantages. The test results are still available on the Victron website.
Conclusions
To optimize efficiency and reduce the cost of power from a generator, it is best to load the unit to at least 50% of its rated output. On the other hand, generators in recreational boats are rarely built to run continuously at their full rated output. As a rule, generators should not be run for extended periods above 80% of their rated output. A load target of about 70% is ideal.
Until recently, generators were sized to handle the greatest anticipated short-term AC load on a boat, and most still are. However, since the average output is well below the rated output, most generators operate extensively at low loads, which drives up fuel, amortization, and maintenance costs, and—on carbureted gasoline engines and diesel engines with traditional injection—reduces service life.
With load management, the peak power demands and the corresponding generator rating can be substantially lowered. Adding a conventional inverter to the system can eliminate long hours of generator run time at low loads. A synchronizing inverter can break the link between peak demand and a generator’s rated output, with a dramatic reduction in generator sizing or runtimes. Reduction in generator run hours results in a disproportionate reduction in the cost of energy produced through a reduction in the kWh amortization cost.
The key installation considerations are: a dry location with a supply of clean, cool, dry air; an exhaust system for the engine that isn’t vulnerable to saltwater flooding at any conceivable heel angle or sea state; and good access for maintenance. Electrically, the installation must comply with American Boat and Yacht Council (ABYC) and International Organization for Standardization (ISO) standards, including the neutral-to-grounding bonding.
While I remain skeptical that generators are necessary for many marine applications in which they are commonly used, we now have mechanisms to ensure they run far more efficiently, with a greatly reduced cost for the electrical energy created, and with less maintenance and a longer service life, than was standard on the previous generations of recreational boats. As an industry, we do not take advantage of these mechanisms nearly as much as we should.
Energy Cost
True Cost of a Kilowatt
There is a common presumption that the principal cost of generating electricity on a boat is the fuel. In this case, the lower the efficiency of the engine driving the generator, the more expensive the electricity generated. Because fuel efficiency on all engines is reduced at light loads as compared to higher loads, light-load operation drives up the fuel cost per kWh delivered to the boat. This is especially true for carburetor-fed gasoline generators and diesel engines with traditional fuel injection systems, but not so much with fuel-injected gasoline engines and diesel engines with a high-pressure common rail injection system.
But fuel is only part of the equation. A major hidden cost lurks in amortization of the generator itself. Let’s say by way of example, the installed cost of a generator is $25,000 and its life expectancy is 5,000 hours. It costs $5 an hour to run it, whether or not it is doing any useful work. If it is generating 10 kW of electricity, the amortization cost is 50¢/kWh; if it is generating 2 kW, that jumps to $2.50/kWh. Then there are maintenance and other operating costs to be factored in, all of which bump up the kWh cost of electricity. Most times, these costs end up being substantially greater than the fuel cost per kWh of electricity generated, even at European prices for gasoline and diesel.
The total cost per kWh for the electricity generated is the sum of all these costs. Because of poor generator optimization, it is not unusual for it to be $3 to $10 per kWh for generators installed on recreational boats. It’s typically much lower in commercial applications.
Portable Gas Generators
Perils of Portable Gasoline-Powered Generators
Portable gasoline-powered generators are by far the most cost-effective way to add modest AC capability to a boat. They are widely available and relatively inexpensive, which is the reason that many cruising sailors keep one on board. However, these generators are not marinized, and they are almost never installed with proper grounding practices. The combination of corrosion over time and inadequate grounding can make them electrically unsafe.
Then consider the following American Boat and Yacht Council (ABYC) requirements, with which few portable gasoline generators comply:
- “All (gasoline powered) generator sets shall be ignition protected…”
- “If any electrical component is required to be ignition protected, the generator set, and the sound shield or enclosure, shall be visibly marked with a safety label…”
- “Generator sets installed with a sound shield shall have a firefighting port installed for discharging a suitably sized, gaseous fire extinguisher directly into the space immediately surrounding the generator set, without opening the sound shield.”
- “All exposed noncurrent carrying metal parts that could become energized due to a fault shall have metal-to-metal contact, or otherwise be electrically connected or bonded together, to provide a common ground connection.”
- “The generator set shall be provided with a designated bonding terminal. This terminal shall not be on a part of the machine disassembled during operation or routine maintenance. The bonding terminal shall be of adequate size for a flexible grounding conductor, and the terminal shall accommodate at least 8 AWG wire or its equivalent.”
- Carburetors must meet various requirements, including a flame arrestor.
Fuel lines must pass stringent Coast Guard permeation and fire resistance tests and cannot be fastened with common clamps that are dependent on spring tension.
Portable generators are frequently connected to a boat’s electrical system via extension cords with plug-in connections that are not suitable for water and corrosion resistance in a marine environment, and do not comply with ABYC and International Organization for Standardization (ISO) requirements, including the need to have a locking arrangement at both ends.
Finally, and critically, there’s carbon monoxide. All fossil-fuel engines generate some carbon monoxide. This is compounded with free-standing engines such as portable generators, in comparison to an engine that’s installed in compliance with ABYC and ISO standards. Many portable gasoline generators are fitted with carburetors, resulting in especially high levels of carbon monoxide. Almost none can pass EPA emissions certification standards for marine generators.
When a portable generator is run onboard, the exhaust has the potential to form pockets of carbon monoxide on deck; from there, it may find its way below decks. Let’s say, for example, a boat is at anchor facing into the wind. The generator is running on the aft deck with the idea the wind will blow away the exhaust. To ventilate the boat, there is an open aft-facing hatch in a forward cabin. The wind over the hatch in the forward cabin will create a vacuum. In the U.S., this phenomenon is known as the “station wagon effect,” will pull potentially contaminated air into the boat from the aft deck. There are numerous other scenarios in which carbon monoxide can migrate into spaces occupied by people.
Carbon monoxide is odorless, colorless, tasteless, and only a little lighter than air, so it tends to hang around. If there is any carbon monoxide in the air you breathe, your body will preferentially absorb it over oxygen. Even if there is an available supply of fresh air and oxygen, the carbon monoxide replaces critical oxygen molecules in your blood hemoglobin, which leads to poisoning.
Once attached to hemoglobin, carbon monoxide is relatively stable. It blocks the attachment point for oxygen molecules and slowly robs your body of its vital oxygen supply. Consequently, low levels of carbon monoxide over time can cause progressive poisoning. Initial symptoms are similar to seasickness, flu, or food poisoning, escalating to an inability to think coherently, headaches, drowsiness, nausea, dizziness, fatigue, vomiting, collapse, coma, and finally death. If carbon monoxide is present while you are sleeping, you may not wake up. This is not academic. Carbon monoxide poisoning claims multiple lives in the boating community every year.
The best antidote is an inboard generator that’s properly installed according to ABYC and ISO standards with its exhaust plumbed to the exterior of the hull. The generic ABYC standard for AC generators is A-27, Alternating Current Generator Sets. Aside from installation requirements, this standard contains numerous detailed criteria for the construction and manufacturer-testing of marine AC generators. Any generator installed in a boat should be labeled as complying with ABYC A-27.
The ABYC has strict requirements around ventilation and sealed bulkheads between any gasoline engine and accommodation spaces. These requirements are in the ABYC H-2 standard, Ventilation of Boats Using Gasoline. There is a separate standard with respect to the exhaust system for any onboard engine: ABYC P-1, Installation of Exhaust Systems for Propulsion and Auxiliary Engines.
If despite these warnings, a noncompliant portable gasoline generator is used, it should at the least be certified as complying with the second edition of the UL 2201 Standard for Carbon Monoxide Emission Rates of Portable Generators. This standard contains detailed carbon monoxide test procedures with a low limit for emissions. In the event these limits are exceeded in the immediate vicinity of the generator, the generator is required to shut itself down.
A carbon monoxide detector should be installed in all sleeping cabins.
About the Author: Nigel Calder is the author of Boatowner’s Mechanical and Electrical Manual and other marine titles. He is a member of the American Boat & Yacht Council’s Electrical Project Technical Committee. He recently teamed with OceanPlanet Energy to continue pursuing his passion for improving the efficiency of boat energy systems, and became one of the co-founders of BoatHowTo, a platform for accurate online marine technical education.
