The DDFlyTrain project has projected a fuel saving of around 10% based on the use of high speed flywheel brake recovery technology retrofitted to DMU rolling stock. This order of saving means that the technology has a potential ROI of inside five years. The project has also produced a proof of concept test rig used to demonstrate the technology to rail industry stakeholders and a concept for integration onto a Bombardier Turbostar DMU.
The project set out to demonstrate through rigorous simulation and rig testing, the practical feasibility, operational fuel and energy savings, and the economic investment case for the use of high speed flywheel energy storage technology on DMU trains. The hybrid solution is based on Ricardo’s TorqStor high-speed flywheel energy storage technology and the high efficiency Artemis Digital Displacement hydraulic pump-motor transmission.
The research project, which started in 2013 and was led by Artemis, was co-funded by Innovate UK and the Rail Safety and Standards Board. It included extensive simulation work based on field service data, which was used in the optimal sizing and design of a practical installation high speed flywheel brake energy system for rail-based applications, and the construction and commissioning of a test rig for demonstration purposes. The project has also completed an initial integration exercise to incorporate the hardware and software onto a Turbostar DMU. Significant effort has been focused on identification and management of the key operational concerns of such a solution.
Although it has previously been used exclusively on electric rolling stock, regenerative braking represents a particularly attractive fuel-saving proposition for DMUs. The launch phase efficiency of conventional diesel rail vehicle transmissions, which typically use a torque converter on starting from rest, can be as low as 30%. By capturing and storing energy otherwise dissipated during braking, a significant improvement in fuel consumption is possible if this energy is subsequently reused to augment acceleration from rest during the period of poor transmission efficiency. Other associated benefits can also be realised: improved longevity of braking system components as retardation is applied through absorbing energy into the flywheel, less noise and vibration from the engine during launch, and reduced exhaust emissions produced in the vicinity of stations.
In addition to its very promising application on DMU rolling stock, the high speed flywheel technology demonstrated in the DDFlyTrain project is also extremely attractive for electric multiple units operating on DC conductor rail networks such as the London DC electrified lines region. Network power capacity issues can place a significant restriction on the number of trains allowed to operate in some sectors, given the high current demand during launch. By applying high speed flywheel technology a significant element of this launch requirement can be eliminated, thus increasing effective capacity.
Ricardo’s TorqStor is connected to the DMU driveline via Artemis Digital Displacement hydraulic pump-motors in which computer-controlled solenoid valves coordinate the responses of individual pistons to the overall power and torque requirements. Compared with conventional hydraulics, Digital Displacement pump-motors have inherently high part-load efficiencies and controllability and this makes it economically feasible to use hydraulics in energy sensitive applications such as DMU regenerative braking.
Ricardo’s TorqStor differentiates itself from other flywheel designs with its unique permanent vacuum. This is made possible by its advanced magnetic gear system that enables the transmission of torque across a vacuum without the limitation of rotating seals or necessity for vacuum pumps. It provides an industrial design for deployment in real-world applications where durability and ease of serviceability are of paramount importance. It is also highly scalable depending on the requirements of each application. With the value attaching to this potentially disruptive technology, the key aspects of Ricardo’s TorqStor are understandably the subject of protection by a family of patents.
In the DDFlyTrain project, the optimal configuration for a DMU was found to be two 4.5MJ capacity TorqStor units with a maximum speed of 45,000rev/min. The proof of concept demonstration test rig was a scaled-down version of the DMU architecture, based on a single 220kJ TorqStor.
“Ricardo’s TorqStor high speed flywheel technology linked to the Artemis Digital Displacement high efficiency transmission provides a technically feasible and commercially attractive means of enabling regenerative braking on DMUs, so providing a pathway to reduce the carbon footprint of this form of rail travel,” commented David Rollafson, Vice President of Innovation, Ricardo. “The double-digit percentage fuel savings and short commercial payback demonstrated for this technology make it attractive both as a retrofit solution for existing fleets as well as for application on new-build rolling stock.”