The technical lifespan of a ship is around thirty years. The economical lifespan, however, due to technological progress, is lower. Retrofitting battery based energy storage systems on existing ships can reduce operational costs. What are the available options?
Depending on priority, the focus can be on decreasing maintenance intervals, decreasing fuel consumption or increasing range. Without large changes to the equipment and their control philosophy, existing installations can still be improved. Measurements on the actual ship can be used to find the optimal solution for a specific ship.
The shipping industry is a competitive market. Therefore, it is important that the operational expenditures (OPEX) are as low as possible without a large impact on the capital expenditures (CAPEX). Maintenance and fuel costs impact the OPEX. This makes fuel savings and decreasing maintenance intervals important subjects.
For this article, a setup consisting of two or more auxiliary engines, a thruster and a load is considered. Without large implications, this system can be extended with a battery based energy storage system as shown in the dotted line in figure 1.
Figure 1. Typical setup with two auxiliary be engines expanded with batteries.
Extension of this system involves:
- extension of the switchboard for the converter;
- installation of the converter and the batteries; and
- adding the converter and batteries to the alarm, monitoring and control system.
With this battery based energy storage system, the load on the engines can be manipulated. The converter will infl uence the frequency on the main busbar. Based on the frequency droop curve of the generator, this will result in a constant load of the generator. Dependent on client demands, three types of strategies can be distinguished.
With this strategy, the system will prevent the load on the generator to exceed the threshold where a second generator will be started. As long as the battery is not fully discharged, it will keep the load on the generator on the maximum level. If the total load becomes lower, it will charge the battery and keep the load on the generator on the maximum level until the battery is charged. This principle is shown in figure 2.
This technique is especially useful for systems with long power peaks. Benefits are:
- no short starts and stops of diesel engines; and
- usage of smaller diesel engines.
Figure 2. Peak shaving.
With this strategy, the load of the generator will be averaged and the battery system will be discharged during short peaks and charged during short dips. Therefore, the load on the generator will be more constant, which improves fuel efficiency and reduces maintenance costs of the generator. Figure 3 explains the power smoothening principle.
This technique is useful for all systems. Benefits are:
- reduced fuel consumption;
- less engine wear and tear;
- usage of slightly smaller diesel engines;
- no short starts and stops of diesel engines; and
- it allows for more variation in the thruster load.
Figure 3. Power smoothening averages the load of the generator.
When opting for a fully electric strategy, the load delivered by the battery system is dependent on the state of charge (SoC) of the batteries. When the batteries are fully charged, maximum power will be delivered by the batteries and minimum power will be delivered by the generator. When the batteries are fully discharged, minimum power will be delivered by the batteries and maximum power will be delivered by the generator. With this strategy, initial power will be delivered by the batteries and the generator will assist when the batteries are discharged. This is especially useful in harbours.
The batteries can be charged with clean energy from a shore connection allowing a ship to sail out of the harbour without any emissions. This principle is shown in the figure below. This technique is useful for short sea shipping and yachts. The benefit is that the diesel engines are not used during the start of a trip.
Figure 4. A fully electric operation where batteries are charged from shore.
An additional benefit (independent of the chosen strategy) of hybrid technology is the increased level of reliability. When running with one generator, the battery system will act as an uninterruptible power supply (UPS) when the generator fails. This prevents black out situations when starting the second generator. Manoeuvring, for instance, can be performed with only one generator online.
Interaction of generator and converter per strategy
By changing the output frequency of the converter, the load of the generator will change. The converter can be considered as a grid running in parallel with the generator. The system prevents deep charge and discharge of the batteries and overloading of the batteries. How the generator and converter interact will now be discussed for each operating strategy.
Figure 5. Peak shaving droop behaviour.
Peak shaving: The interaction between the generator and the converter for the peak shaving strategy is presented in figure 5. The converter controls the load of the generator by determining the busbar frequency. As long as the batteries are not fully charged or discharged, the busbar frequency is set at a level corresponding to the maximum load of the generator. The various scenarios for this control philosophy are listed in the following table.
Table 1. Generator startup prevention scenarios.
Based on real life data, this results in the total load division and battery SoC as shown in figure 6.
Figure 6. Generator peak shaving.
Generator smoothening: Figure 7 shows the interaction between the generator and the converter when choosing for the generator smoothening option.
Figure 7. Generator smoothening droop behaviour.
The converter controls the load of the generator by determining the busbar frequency. As long as the batteries are not fully charged or discharged, the busbar frequency is set at a level corresponding to the average load of consumers. The various scenarios for this control philosophy are shown in table 2.
Table 2. Peak shaving scenarios.
Based on real life data, this results in the total load division and battery SoC as portrayed in figure 8.
Figure 8. Total load division and battery state of charge for generator smoothening.
Fully electric: Figure 9 shows the interaction between the generator and the converter by means of fully electric operation. The converter controls the load of the generator by determining the busbar frequency. Busbar frequency is set at a level corresponding to the state of charge of the batteries. During the start of a trip (for instance, when leaving the harbour), the batteries will be used and the system will run with minimal generator power.
Figure 9. Fully electric droop behaviour.
During the trip, the user can decide to charge the batteries by using spare generator power. At the end of a trip (for instance, when entering the harbour), the batteries will be used and the system will run with minimal generator power. The table below lists the various scenarios for this control philosophy.
Table 3. Fully electric scenarios.
Based on real life data, this results in the total load division and battery state of charge as shown in figure 10 on the previous page.
Figure 10. Load division and battery state of charge for fully electric operations.
Batteries will reduce operational costs
By using only limited hardware and limiting the implications for the remaining part of the installation, hybrid technology can result in significant benefits for a business. Depending on the type of business, hybrid technology for peak shaving, generator smoothening or fully electric operations with the correct dimensioning of the batteries can reduce a ship’s operational expenditures. Whether the focus is on extending a ship’s range, preventing unnecessary diesel engine starting, reducing fuel consumption or even faster ships, hybrid technology has the potential to contribute to all of these goals.
The most efficient solution depends on power demands, battery sizing, generator dimensioning and control philosophy. Based on data provided, simulations can be performed to determine the best possible solution to a specific situation.
This article was written for SWZ|Maritime by Stefan Bruins, Technical Specialist Marine & Offshore at Eekels Technology. It was published in the November 2019 issue.