2016_1 Hybrid technology for new emerging markets master

Hybrid technology for new emerging markets – inductive charging

The double ended car ferry ‘Folgefonn,’ as a part of the pre-commercial activities within hybrid systems introduction, has been converted from a diesel electric ferry to a complete plug-in hybrid and plug-in electrical ferry.

Text: INGVE SØRFONN Photo:

The double ended car ferry ‘Folgefonn,’ as a part of the pre-commercial activities within hybrid systems introduction, has been converted from a diesel electric ferry to a complete plug-in hybrid and plug-in electrical ferry. This technology demonstration has undergone successful testing during normal operation for about one year. This stage of the demonstration project has given the ‘Folgefonn’ energy storage capability in the form of a battery pack, hybrid control system, power transfer system and onshore energy storage system. Wärtsilä’s contribution to the ‘Folgefonn’ project is the concept development, including the inverter systems, the hybrid control, battery package and systems, power transfer and land-based energy storage system as well as the integration of the onboard systems. As a part of the project, renewable energy from shore is used to reduce the environmental impact and to make the ferry operation more efficient. The transfer of power is critical for a ferry dependent on “external fuel,” and a high-power, fast-charging concept is one of the critical issues for this type of operation. The search for reliable and safe systems that can be connected and disconnected quickly initiated the research and development of wireless high-power technology.

Hybrid Energy Storage Systems (ESS)

Hybrid power systems combine different power sources with energy storage devices. 

The introduction of the hybrid power system, and its integration with conventional diesel or dual- fuel engine generating sets, offers a significant improvement in efficiency by running the engines on optimal load and absorbing many of the load fluctuations through batteries. 

For plug-in hybrid systems, parts of the energy will be harvested from shore. As such, a higher degree of energy utilization is possible, due to the improved efficiency in the energy conversion.

A further step is a complete plug-in electrical system where the total energy demand is taken from external land-based energy sources, preferably renewable sources. The total efficiency from power supplied locally onshore to propeller will be more than 85%.

The key element in these types of power systems is how to store the energy safely and efficiently. The most available technology at present is batteries.

Currently, battery technology is improving in cost and performance, and as such this is a logical choice for energy storage. 

Another possible technology may be double-layer capacitors, especially for power-demanding applications.

Discussions about how to design the capacity of an energy storage system will depend on how the system is to be used. Knowledge about the actual vessel operation is therefore important. 

Batteries can deliver large power peaks, normally in the 3-6 C area (times nominal capacity), but they are limited by the investment in power electronics to deal with such high power peaks. 

For full operation, there may be a demand for both high power and energy. 

Many influencing factors have to be considered, such as charging strategies including renewable and onshore power limitations, the battery life, investment costs, and the configurations of other engine driven generating sets onboard

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Fig. 1 - Overview of existing capabilities within commercially available inductive charging technology. The yellow area has been the target for maritime applications.

Inductive charging

Inductive charging (wireless charging) uses an electromagnetic field to transfer energy between two coils. A sending induction coil is used to create an alternating electromagnetic field and a second induction coil takes the power from the electromagnetic field and converts it back into electric energy. To manage longer distances between sender and receiver coils the inductive charging system uses resonant inductive coupling.

Successful application of battery storage systems for plug-in hybrid vessels will require fast, safe and reliable power transfer from the onshore power system while the ship is docked. 

Coastal transportation systems, and especially ferries, are operating on a fixed schedule with short docking times. Therefore, maximum utilization of the time available for charging will be important for effective utilization of the energy storage capacity. In this context, existing solutions for high power electrical connections, with flexible cables and mechanical contacts in the docking area, impose time and availability limitations. The time required for connecting and disconnecting the power supply for on-board battery charging will limit the energy transfer during docking. Direct electrical connection also causes challenges related to safety and reliability in harsh environments.

A possible approach for overcoming the drawbacks of conventional electrical connections for high-power battery charging, would be to apply technology for wireless, inductive power transfer. Such technology has been undergoing rapid scientific and commercial development during the last years, especially for battery charging of electric vehicles, including private cars as well as public transportation systems like busses, trams and trains. However, the power level and transfer distance achievable with commercially available solutions for inductive power transfer is limited to a few hundreds of kWs across distances up to 20-30 cm, while large scale commercial ship applications require power transfer capability in the MW power range across distances of up to about 50 cm. 

Developing dedicated technology and practical solutions that enable high-power, wireless, inductive battery charging will allow integration of the technology into the ship and harbour-side structures. Thus, it will be possible to design systems that can allow for fully automated charging operations that will start immediately when the ship is docking. 

With this background, Wärtsilä and its partners conducted research in this field and created a lab prototype as the final proof of use of the technology. 

This can become an enabling technology for utilizing battery storage in various coastal transportation systems. Additionally, this solution should have the potential to serve as one of the enablers for complete automated charging systems and as a part of future autonomous operation.

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Fig. 2 - Typical inductive charging system.
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Fig. 3 - The new ferry design from Wärtsilä Ship Design.

Technology development

The feasibility of high-power, wireless inductive charging systems was first investigated by theoretical studies, based on analytical evaluation of various mechanical designs operating at different transfer distances and with different electrical frequencies. These analyses included the evaluation of electrical resonance circuits, required for achieving high power transfer capability in inductive charging systems, and of the power electronic converters required to control the system. From the analysis, the feasibility of power transfer capabilities exceeding 1 MW was confirmed for the range of transfer distances and within the space requirements expected for ship applications. Furthermore, it was demonstrated that this can be achieved by using our own standard inverter systems.

Results from the analyses were used to study the operation of the power electronic converters and to design control systems that will allow for seamless dynamic operation, including the controlled transfer of required power even with wide variations in the mechanical positioning of the transmitting and receiving coils. The system has been designed to transfer rated power of 1 MW within a range of 15-50 cm between the coils. This is a significantly larger variation in relative magnetic coupling conditions than encountered in battery charging systems for electric vehicles. 

The onboard batteries will normally be dimensioned for the full charging capacity of the transfer system, but utilization is depending on the type of chemistry, charging intervals and the lifetime prediction of the batteries. Charging rates of about 2-3 times nominal its battery capacity are usual with the present battery technologies, but with high power battery chemistry the charging rates can be in the area of 6 times the battery capacity.

The resulting system can be controlled so that the power converter operation during charging will automatically compensate for dynamic variations in the mechanical positions while required power flow is maintained. 

FEM-based simulation models developed during the project have been used to assess the practical construction of the full-scale prototype and to assess the need for the electromagnetic shielding required to comply with international standards for electromagnetic exposure. This analysis has revealed information about how the physical structure of the coil should be constructed to avoid additional losses. 

The main technical inventions developed within the project have been included in a patent application to secure relevant intellectual property rights (IPR). 

The specially constructed coils, the associated resonance circuits and the power electronic conversion systems have already been undergoing tests in Wärtsilä’s high-power laboratory facilities in Stord, Norway. The preliminary results are promising, and transfer of 1 MW at a distance up to 50 cm has already been demonstrated. Efficiency figures are for the moment >95%. These tests verified the developed concept, which is to be considered ready for further industrial product design and full scale testing in a vessel.

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Fig. 4 - Illustration of the combined vacuum mooring and induction charging system.
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Fig 5 - Typical DC distribution and plug-in hybrid solution with induction charging.

Market concepts

The inductive charging technology is very flexible to integrate and can be introduced in new market areas and applications.

Since the introduction of the technology, there has been interest from many market areas and industries such as road vehicles (trucks), military applications and demanding transfer applications where time restrictions and safety aspects are dominant.

The focus area at the time was coastal ferries, where new contract demands require strict targets for use of energy and release of climate gases. This is a new trend in this market segment and is driven by regulatory demands, political visions and incentives and a more sustainable way of running this business.

A close co-operation between Electrical & Automation and Ship Design has led to a complete new ferry design with an integrated Wärtsilä hybrid system and inductive charging.

The primary goal when designing plug-in power systems is to reduce the power demand to a minimum for the operation. The hull design and the utility system have been carefully evaluated and computational fluid dynamics (CFD) analyses have been performed to quantify the best hull for the actual number of cars and speed of the ferry.

The present charging systems available and used for this kind of application have been traditional plug connection or pantograph systems with necessary redundancy to secure availability of electrical energy. These have been used together with vacuum mooring. 

Mooring systems are an advantage as they keep the ferry in a steady position, and the main propeller can be stopped during the docking time to reduce the total energy use during the day.

As a spin-off of the ferry design, Wärtsilä has made an agreement with Cavotec to combine the use of their vacuum mooring unit and the inductive charging technology. 

Cavotec has a strong brand and a strong market position within vacuum mooring systems and in general power transfer systems.

For the vessel owner, the combined solution will give the following benefits:

  • Automatic mooring
  • Automatic charging
  • Maximum utilization of the charging time available while docking
  • High degree of availability of energy
  • High reliability
  • Minimum maintenance needs due to limited wear and tear
  • Optimisation of the onboard ESS investment due to high energy transfer rate
  • Galvanic isolation between shore and vessel
  • Increased safety during operation

Compared with a traditional plug-connection system, the losses during transfer will be somewhat higher with the inductive system. However, due to reduced maintenance costs, reduced operation costs and attractive cost for renewable energy, the total operation cost will be favourable. 

The investment cost for the inductive charging depends on how the integration is done both onshore and onboard. Compared with a redundant plug-based system with auto-mooring the combined inductive charging and mooring unit will have about the same cost.

Many influencing factors like the onshore infrastructure, the need for onshore energy storage and the operational conditions will decide the total cost.

Some of the ferry routes have more than 20,000 short dockings per year so maintenance and reliability issues are important.

Full-scale demonstration

To ensure a fast market introduction and a full-scale validation of the developed concept, it is important to verify the integrated design in a full-scale demonstration. The partners in the Folgefonn project succeeded in getting governmental funding to integrate a full-scale 1 MW unit in the ‘Folgefonn’ ferry to provide such verification and to demonstrate reliable, real-life operation of the technology. The estimated time for installation of the system is late 2016 and the first part of 2017, with a test program during regular operation completed within 2017. A commercial introduction of the technology will start this year, in parallel with the first full-scale pilot installation.

Electrical onboard DC distribution

DC bus is a natural common point of coupling in a modern hybrid system, as DC is the basic source for most of the modern power inverters. Energy sources and loads connected to the same DC source will reduce the number of conversion steps onboard. 

The bi-directional DC/DC converter controls the battery and steps up the voltage on the DC bus to the desired voltage level. 

A bi-directional active front-end inverter (AFE) is connected between the DC bus and the AC network. 

The inductive charging is connected to the DC grid through a DC/DC converter and is stabilizing the power from the induction receiving coil.

Electrical onshore system

The inductive sending coil is connected to a bi-directional active front-end inverter (AFE) that is connected to the on-shore grid supply. Alternatively the input can be from a DC supply from an onshore ESS if this is necessary for stabilizing the grid or to overcome tariff regimes that can be costly with large power peaks drawn from the grid.

The batteries and the power electronics can also be used as grid voltage support and as such be attractive for the power utility provider to improve the grid capacity. 

System control

The main objective of the control system is to keep a stable power transfer flow to the batteries with variable overlapping of the induction coils, a variable air gap and variable tilting. This is achievable by using our own inverter control and reconfiguring the hardware, firmware and software to stabilize and control the power flow. The advanced diagnostic system embedded in the inverter control secures remote on-line services for the operation.

 

Not only is the high power transfer wireless, but the control from the vessel to the shore system is controlled by a wireless Wi-Fi link.

Conclusion

The introduction of new Hybrid Power Systems with energy storage is a new and attractive way of reducing both fuel and exhaust emissions. 

With the new inductive charging technology, Wärtsilä can offer total electrical plug-in solutions as a part of their portfolio and complete integrated vessel design concepts.

The expected results from a full-scale operation will improve the availability and safety of these kinds of operations, and the concept will be the first fast-charging wireless technology in the ferry industry. The technology will act as an enabler for efficient use of electrical plug-in solutions in this industry. 

 

New Wärtsilä ferry concept

Wireless charging and mooring concept

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