24 FOGHORN Sizing a battery looks nothing like that. If you take peak power and multiply by mission duration, you will end up with a battery that is enormous and expen- sive. Battery size is driven by average power over the oper- ational profile, not peak. Peaks are absorbed easily; what drives the energy requirement is what the vessel is actually doing most of the time. This is where a different mindset is required. A battery is a tool that shapes power flow over time. To size it, you need to know the shape: when the vessel is heavily loaded, when it is lightly loaded, where the peaks are, how often they occur, what the average sits at, and what redundancy or spinning-reserve requirements look like. This is the key to success, the better this is done, the better the results will be. Every vessel has its own answer. A water taxi running short hops at high duty has a fundamentally different profile from a dinner cruise that sits at low load for hours, which is itself different from an excursion vessel that mixes long transits with extended dock time. Two sister ships on dif- ferent routes can have vastly different operational profiles. There is no off-the-shelf “passenger vessel”-sizing exercise, there is only the operational profile, and that is the input that decides almost everything. The same point holds across the industry. Offshore sup- ply vessels can use small batteries for spinning reserve in dynamic positioning—low usage, very high power. Some ferries face the harshest duty cycle for batteries on the plan- et: five-minute recharge, up to fifty cycles a day, with the charging infrastructure as much of a challenge as the bat- tery itself. Tugs and workboats use batteries to avoid low loading on diesels and to provide zero emissions in port. Passenger vessels, feeders, and coasters often look closest to the tug case as far as objectives—batteries used to keep die- sels in their efficient operating range, with zero-emissions capability at the dock as a meaningful secondary benefit— but obviously the operational profile is vastly different. These applications all use batteries, but the batteries are doing very different jobs. These extremities serve to paint the picture of why evaluating what is going on, what the vessel is doing, is so important in engineering the right hy- brid or battery electric solution. THE HYBRID LESSON: FOCUS ON THE GENSET For most passenger vessel operators, the realistic near-term path is hybrid rather than fully electric. And the hybrid les- son, learned the hard way across the industry, is counterintui- tive: the goal of adding a battery is not to try to push the diesel off center stage. The goal is to ensure that the diesel spends its time doing exactly what it does best and most efficiently. Take a representative operational profile: average load around 100 kW, with intermittent peaks reaching 400 kW. The natural specification, using diesel logic, is a 500-kW genset that can absorb the worst case. That genset will then spend most of its life running at 20 percent load, burning roughly 20 percent more fuel than is necessary. In reality, most diesels spend most of their lives running load percent- ages even lower than that. The intuitive battery response is peak shaving—let the bat- tery handle the peaks, leave the genset alone. This produces almost no benefit, because the genset is still oversized and still spending its life at 20 percent load. FOGHORN FOCUS GRAPHS: SPOC ENERGY
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