True hybrid power for marine propulsion

Reliability and durability remain highly important considerations, particularly in the marine sector where ships can often be a long distance from specialist repair and maintenance facilities, and loss of trading revenue must be avoided at all costs.

Based on the UK’s south coast, but with a presence in 17 worldwide locations, Ricardo began in 1915 and now has a £179 million revenue earned by 1600 employees, over 1300 of whom are technically qualified.

As well as what are becoming standard methods of maximising fuel economy and controlling emissions, namely fully electronic control, exhaust gas after-treatment, water injection, exhaust gas recirculation (EGR) and waste heat recovery – all of which are familiar territory to Ricardo’s engineers from their experience in other engine applications, the company is exploring advanced technology in the whole power and propulsion system. Advanced computer modelling and simulation technology permits relevant and in-depth research and development into fuel cell and hybrid technology using various techniques, in some cases executed in real time.

The drawbacks of these technologies are very familiar to Ricardo; EGR for example does not lend itself to engines burning residual fuels, and current waste heat recovery systems tend to be costly and hard to implement, apart from some turbo-compound systems. The company sees considerable potential in radial turbines, bottoming- or organic- Rankine cycles, and split cycle regenerative engine technologies.

Other current activities include research into true hybrid technologies for ship propulsion. Often, in marine applications, ‘hybrid’ can be something of a misnomer, being applied to diesel-electric and/or gas electric and dual fuel propulsion systems. In fact true hybrid systems lend themselves well to applications such as ferries, supply vessels and tugs, where low power levels are needed most of the time, for transiting and station-keeping, with occasional comparatively short bursts of very high power. Battery-based systems are an obvious choice, similar to those used in hybrid road vehicles, but batteries have proved less than ideal in marine applications. They are heavy, bulky, costly in terms of initial investment and replacement (most have a limited lifetime). However, the company is able to apply its battery experience in automotive, commercial and military applications to NiMH and Li-Ion installations which include advanced control and management systems

Fuel cells represent a further technology that is presently in its marine infancy but which offers much future potential. Fuel reforming, i.e. creation of fuels from various media for use in fuel cells, has been an important part of Ricardo’s work. The company has worked with ExxonMobil, for example, on PSR (pressure swing reformer) technology to generate hydrogen from water and hydrocarbons.

Energy recovery and energy storage technologies also, according to Ricardo, have promising applications in the marine sector. One major benefit for road and rail vehicles – recovery of energy from braking systems – is missing from ship systems, but there are still benefits to be gained from being able to generate and store energy for use when bursts of higher power are required.

The company’s experience in developing hybrid powertrains for road vehicles, which includes many well-known examples, is applicable to energy storage for marine use. However, as with most technologies, existing hybrid propulsion technology is cost effective up to a point, but becomes a very expensive option as higher levels of CO₂ reduction are demanded. This is because the system is centred on battery technology. Several versions of batteries have been used, from conventional lead-acid, through more familiar NiMH and Li-Ion, to advanced systems such as Li-poly and ZEBRA. Most batteries score highly for technological maturity (though Li-poly batteries are largely untried) and, apart from lead-acid, do well for energy density, but less well on power density. Other parameters like cost, safety, temperature range and life expectancy vary considerably according to type.

So Ricardo has investigated other energy storage technologies, the most promising of which are hydraulics, ultra-capacitors, and flywheels. These all tend to score more highly than electro-chemical solutions (i.e. batteries) – particularly flywheels.

Existing flywheels suffer from comparatively low energy density. Using an example of flywheels capable of storing 1MJ of energy in a unit of 220mm diameter, an all-steel flywheel would have a mere 6.25 kJ/kg specific energy at the maximum speed of 42,500 rpm. The existing so-called state-of-the-art carbon flywheel is considerably better, with 200 kJ/kg and a 75,000 rpm maximum speed. Ricardo’s ultra-high-speed, lightweight ‘slingshot’ design, however, is claimed to be capable of 1000 kJ/kg specific energy – which compares with a figure of 350-600 for a Li-Ion battery pack – and thanks to advanced bearing technology can run at 145,000 rpm maximum speed.

The patented flywheel technology uses advanced composite materials, still of a confidential nature, and relies on passive magnetic bearings which need no lubrication and offer over 90% reduction in friction compared with ceramic bearings, with losses that reduce in linear fashion as speed increases. Magnetic couplings mean that no mechanical linkages are required for the energy storage and recovery system.

In a kinetic marine powertrain the flywheel can be rotated by a small CVT and clutch, or an electrical nmotor/generator, taking energy from the excess power generated by the on-board engines operating at optimal efficiency while load is reduced. When boost is required by the propulsion system, the energy from the flywheel is returned to the powertrain. As a purely mechanical system no transformation to electrical energy would be required.

The system is similar to some being developed for road vehicles, so will benefit from economies of scale. Ricardo estimates the cost of its ‘Kinergy’ flywheel-based kinetic system for a light vehicle could be as little as $1500-$1600, making it significantly more cost effective than battery-based solutions in terms of cost per unit of carbon reduction.

Ricardo recognises that advanced kinetic hybrid technology is still a long way off for mainstream marine applications, and is one of many solutions to lowering carbon emissions and improving fuel efficiency. To make full use of its current expertise as well as future technology and provide benefits to clients in the shorter and well as the longer term, the company has launched a scheme knowns as the ship energy storage assessment (SEsA) consortium. The purpose of the consortium is to enable Ricardo to work with ship operators and other industry partners to investigate energy management of the propulsion and auxiliary power systems, and consider the flexibility of various propulsion configurations under changing operating conditions. The study will use Ricardo’s experience and its advanced simulation and modelling tools. A variety of ship types will be considered, including a generic cruise vessel, ferry, tanker, bulk carrier, container ship, offshore ship and naval vessel, according to the interests of the various clients. It will be possible, within the study, to carry out simulations of specific ship systems, which will remain confidential to the client.

Phase 1 will assess energy storage combinations for the complete propulsion systems under different operating conditions including normal operation, slow steaming and other duty cycles as agreed among the consortium members. A more detailed analysis, and an investigation into efficiency of sub-systems, will follow in phase 2. The study will include possible contributions from renewable energy, and benefits of energy storage for in-port connections. Ricardo says that the aim of the study is to build a demonstrator of the selected energy storage solution, with an optimised control strategy. There will be eight phases in all, culminating with tests of the demonstrator.

Even in the first phase, advanced energy storage will be considered, including electrical, mechanical and thermal storage systems, while prime mover technology will consider gas engines, turbines, fuel cells and Stirling engines. This phase, which is scheduled to last six months, will also include exhaust gas after-treatment and waste heat recovery. Ricardo believes that its independent status will provide an objective view of the various technologies, their benefits and drawbacks.

The cost of being involved in the first phase is quoted by Ricardo as £50,000 to £70,000. At the end of the study Ricardo expects participants to be able to understand how to achieve 15% to 25% lower operating costs, equivalent to $600,000 to £1 million per year for a 10MW engine. There is a possibility of reducing capital investment through downsizing of installed engines, and participants will be equipped to comply with Tier III emissions legislation. The targeted return on investment is estimated at below two years.