Is hybrid power generation the best choice for the next ten years of ownership? The answer to this question comes from reliable prediction of fuel consumption and cost, peak loads on battery systems and maintenance cost. Accurate measurements of power demand, presented to the Hybrid Grid Design tool developed at RH Marine, may provide a fair comparison of business models.
The maritime industry has started to adopt hybrid power solutions onboard ferries, mega yachts, cruise vessels, offshore support vessels and even large offshore construction vessels. The drivers to select these systems are numerous and differ between ship segments and owners. Among the drivers are: emission control areas (ECAs), local regulations, public opinion, increased comfort, redundancy, grid stability, reduction in operational expenditures and more. All ships have in common that the business case to invest in hybrid technologies should be viable and the outcome profitable.
We define a hybrid power generation system to consist of two or more different sources that can provide power. As such, it can be for example a combination of diesel generators and batteries, but also a combination of diesel engines and diesel generators as in the case of Power Take Off and Power Take In (PTO-PTI). The advantage of two or more power sources is “flexibility”, which in turn should be optimised towards the ship owner’s business goals.
Barriers to invest in sophisticated hybrid power systems are mainly driven by uncertainties in total cost of ownership (TCO). Important questions relate to the level of extra investments versus operational savings. Key performance indicators (KPIs) that influence the TCO equation are:
• Extra upfront investment costs of the hybrid power system;
• Expected fuel and emission savings;
• Expected maintenance savings;
• Additional availability of the hybrid power system;
• Future fuel prices, interest, CO2 emission and electricity prices;
• Lifetime of new technologies (for example batteries).
Figure 1. The OSD low-emission hybrid Azistern e-tug.
For the Netherlands based naval architecture company Offshore Ship Designers (OSD), the optimal power configuration for a low-emission hybrid Azistern e-tug was designed. To achieve this, engineers first started analysing detailed operational data. In the concept design of a ship’s power system, the operational profile is used as a starting point. The operational profile can be presented in several formats. The traditional presentation the average power demand and the time spent in each of the vessel’s operational modes. This simple representation of the ship’s power demand imposes two main disadvantages:
1. The sequences in which these modes occur are unknown
2. Power peaks are not visible, which makes it harder to make optimal usage of energy storage technologies.
In addition, a large deviation was experienced between the specified operational profile and actual usage, when the ship is in operation at several projects. This could lead to a hybrid power system that is not optimised to actual use. Measuring and logging the ship’s power system will solve this problem by providing very detailed insights into the actual power demand onboard a vessel.
Most ships measure the electrical power demand on the busbar. In a diesel direct configuration, the power on the shaft lines can be measured as well. This data gives much better insights into the actual power demand over time and is used as a basis to find the most optimal power system solution.
Figure 2. Traditional operational profile for the Azistern tug: power, modes and time.
Figure 3. Measured operational profile of the Azistern tug.
The next step in the process is to find the optimal power system for the ship owner. As input to the business case, three different power configurations are simulated and compared:
• diesel direct;
• diesel electric; and
• hybrid diesel electric and battery.
In this case of a measured operational profile with more than 5 million data points, a sophisticated simulation environment is needed. The engineering team used RH Marine’s in-house developed Hybrid Grid Design tool based on Matlab/Simulink software.
The three different configurations that were compared, are constructed with component models that are developed together with universities and research institutions. These models are validated by actual onboard measurements.
Figure 4. Hybrid grid design tool.
Determination and specification of an adequate battery system is a science in itself. The rates that the system charges and discharges, capacity, depth of discharge, state of charge, end of life, cycles and temperature: all of these parameters determine the performance and lifetime of the battery. The integration of the actual operational usage strategy in the Hybrid Grid Design tool also provides a solution to this issue. With a precisely described operational profile that is in accordance with the vessel’s actual operation, the engineering team is able to exactly simulate the usage of the battery such as illustrated in figure 5. This data is used to specify more precisely the optimal battery system in cooperation with the battery system manufacturer.
Another big advantage in this software tool, is the integration of the actual Power Management System (PMS) and Energy Management System (EMS) as they will be installed onboard. This enables simulation of not only the component behaviour and resulting performance, but also the actual onboard control strategies. These “usage” strategies play a substantial roll with respect to quantifying any operational gain.
The hybrid business case is based on the three major operational expenditures (OPEX) that may be presented as an outcome of the Hybrid Grid Design tool: fuel consumption, diesel engine running hours and average loading and battery usage. For the capital expenditures (CAPEX), the hardware and engineering cost are compared for the three configurations. In the end, based on a ten-year lifetime, the TCO is calculated. Two business cases are presented:
1. “today’s power system” with the current costs; and
2. “tomorrow’s power system” with future costs.
The results demonstrated in the table on the previous page, indicate that the TCO of the diesel electric and diesel electric hybrid configuration are more viable and profitable compared to the diesel direct configuration for both the current and tomorrow’s cost profiles for this specific vessel and its operational profile. The “tomorrow” scenario assumes high fuel costs, compulsory exhaust after treatment, low battery prices and high emission costs. With respect to these future predictions, one can certainly say that fuel costs are unpredictable, that exhaust after treatment is already compulsory due to strict emission regulations and that battery kWh prices are going down strongly (see Tesla power wall battery prices for instance).
Based upon a vast experience in supplying and servicing ship power generation, distribution and propulsion systems, RH Marine is dedicated to designing an optimal power configuration founded on a solid business case.
Contact us for more information about our Hybrid Energy products.