Onboard Hybrid Propulsion and Sewage Treatment System Powered by a Stirling Engine Unit

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1 Onboard Hybrid Propulsion and Sewage Treatment System Powered by a Stirling Engine Unit Carlo Maria BARTOLINI a, Luca CIOCCOLANTI b,*, Gabriele COMODI c, Flavio CARESANA d a Università degli Studi e-campus, Via Isimbardi 10, Novedrate (CO), ITALY, carlomaria.bartolini@uniecampus.it b Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Via Brecce Bianche 1, Ancona, ITALY, l.cioccolanti@univpm.it c Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Via Brecce Bianche 1, Ancona, ITALY, g.comodi@univpm.it d Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Via Brecce Bianche 1, Ancona, ITALY, f.caresana@univpm.it Topic: Cogeneration Applications Keywords: diesel burner, off-grid Stirling engine, waste heat recovery, hybrid propulsion Abstract At present, boats have to comply with very strict environmental regulations on emission production and sewage disposal. In the near future, new environmental policies on reduction of water pollution will also affect sailing boats. To reach environmental targets, a few models of new boats on the market are equipped with hybrid propulsion systems in which an electric generator is combined with a diesel engine. The electric generator, driven by the diesel engine, charges a bank of batteries which in turn powers an electric motor. The electric motor can operate in parallel with the main propulsion engine to provide more power output or on its own, when a quieter and more fuel-efficient operating performance is required during idling or docking. In this paper a Stirling engine is considered for the onboard application. The main advantage is that a Stirling engine can run continuously to charge the bank of batteries in the hybrid propulsion system or to supply power on request for onboard appliances, all whilst producing a very little noise. In addition, the heat discharged by the cooling system of the engine can be utilized for onboard sewage treatment. New environmental regulations limit the sewage disposal at ports thus making it necessary to store and treat sewage during navigation. In order to reduce sewage disposal, the engine s thermal output is used to aid evaporation thereby reducing the quantity of waste disposed. In this work, the authors have studied the integration of hybrid propulsion and sewage treatment systems powered by a Stirling engine in order to meet these new environmental regulations and provide comfort during the navigation of sailing boats. It is anticipated that experimental tests will be carried out after once the design stage of the system has been completed. Nomenclature NO x = nitrogen oxides SO x = sulphur oxides VOCs= volatile organic compounds CO 2 = carbon dioxide PM= porous medium EM = electric motor ICE = internal combustion engine PV= photovoltaic CVT= continuously variable transmission SCF= success critical factor * Corresponding author. Tel.: ; fax: address: l.cioccolanti@univpm.it

2 1. Introduction Since the 1972 Stockholm Conference, the United Nations has realized the common need to inspire and guide people in the preservation and enhancement of the human environment [1]. In 1992, at the Rio Conference, a broad agenda was adopted regarding the environment and development, which, among other things, included a set of recommendations relating to shipping and the role of the International Maritime Organization (IMO) [2]. In 1997, a new annex was added to the International Convention for the Prevention of Pollution from Ships MARPOL (MARine POLlution). The Regulations for the Prevention of Air Pollution (Annex VI) intend to minimize airborne emissions from ships (SO x, NO x, ODS, VOC) and their contribution to global air pollution and environmental problems. It entered into force on 19 May 2005, and a revised Annex VI was adopted in October 2008 which entered into force on 1 July In 2007, international shipping was estimated to have contributed about 2.7% of the global emissions of carbon dioxide [3]. IMO has developed technical and operational energy efficiency measures to reduce the amount of CO 2 emissions from international shipping. Nevertheless, shipping activities are expected to further increase in the near future and meeting a target for emissions continues to represent a challenge. In addition, Regulations for the Prevention of Pollution by Sewage (Annex IV) came into force in 2003, and a revised Annex IV entered on 1 August This states that ships must be equipped with either an approved sewage treatment plant or an approved sewage holding tank. Discharge of sewage into the sea is prohibited, except when a ship has an approved sewage treatment plant in operation. Sewage which has not been comminuted or disinfected has to be discharged at a distance of more than 12 nautical miles from the nearest land. Furthermore, those annexes set special mandatory measures to prevent, reduce and control harmful atmospheric emissions of SO x, NO x and are of particular relevance in what are known as Emission Control Areas, such as the Baltic Sea Area. Sewage discharge from passenger ships will generally be prohibited under the new regulations, except when ships have a sewage treatment plant in operation, which should be of a type approved by the national Administration. At present, pleasure boats are not subjected to these rules but new environmental policies on reduction of water pollution will also affect sailing boats in the near future. 2. WEB: Wastewater treatment and Electricity on Board This study intends to fulfil the need to decrease the environmental impact of pleasure boats which is compounded by their large numbers. Propulsion, in particular, as it has by far the higher impact on the marine environment. At present, diesel-fuelled, internal-combustion engines are used to power boats. Compared to ICE propulsion, electric propulsion has many advantages, like silent running, less need for maintenance, autonomous electric sailing, lower CO 2 and VOCs emissions and access to waters which are otherwise prohibited to combustion engines. Electric propulsion by PV cells in boats is an upcoming technology [4]. A key feature of the electric propulsion is the amount of electrical energy needed to reach a certain speed and range. Efficiency in the electrical system, hull shape and lightweight design play an important role in achieving zero-emission boats. However, the necessary use of a power storage system, usually batteries, to power boats limits the diffusion of electrical-only propulsion. The main drawbacks of this system are the weight, limited operation time and the long period of time needed to recharge the batteries. A way of overcoming these shortcomings is to integrate an ICE with an EM to form a hybrid-electric propulsion system. Hybrid propulsion technology is of greatest interest by far in the automotive industry and, in recent years, also in shipping where several systems are already commercially available [5]. In 2008, Steyr Motor presented its first hybrid propulsion system for boats and later formed a working partnership for developing and distributing its hybrid propulsion systems. In 2009, Twin Disc SRL introduced its hybrid propulsion system in sailing boats. The system consists of a patent-pending 5kW electric generator/motor system (fitted directly on top of the saildrive) which turns in order to charge a bank of batteries while the boat is running on diesel power. No additional clutch or engagement devices are used, thus avoiding the typical failures or malfunctions of similar systems. At the same time, due to the widespread trend towards sailing boats which provide all modern comforts, electric energy demands aside from propulsion have become increasingly important. In these cases, an auxiliary power unit is usually installed on board to supply power to appliances. Compared to ICE, a Stirling engine fits this purpose thanks to its almost silent operation and impressive reliability. However, on a small scale (~1 kw) the high investment cost and inefficiencies in oil combustion represent significant drawbacks. Nevertheless, its characteristic quietness whilst in operation makes it possible to run the engine at any time of day and provide a significant amount of energy for appliances. For example, a 12h running operation of a 1kWe power Stirling engine would mean about 12 kwhe of energy per day, which is a significant amount of energy for sailing boats. According to Yanmar [6], approximately 2 kwe are required for a 35 ft vessel to achieve 4.5 knots. Travelling constantly for about 2 hours at 4.5 knots therefore requires about 4 kwhe. Apart from propulsion, the surplus electrical energy can be used for different purposes on board such as air-conditioning, lighting and cooking. In particular, through the use of an electric stove, use of LPG can be completely eliminated and onboard safety increased significantly. * Corresponding author. Tel.: ; fax: address: l.cioccolanti@univpm.it

3 Despite relatively low electrical efficiency, Stirling engines have a higher thermal power output which can be usefully recovered to achieve overall efficiencies higher than 80%. In a previous paper [7], some of the authors evaluated the thermal output recovery of a 1 kwe Stirling engine for fresh water production on board sailing boats. In addition, thermal power output can be used for many other purposes such as providing hot water and heating or treating grey water for disposal. Sailing boats are highly customizable and the options available on request often depend on geographical sailing areas. For example, heating is in great demand in Northern Europe yet rarely so in Mediterranean Areas. The authors are currently investigating the use of thermal energy for wastewater. The direct discharge of wastewater, especially of oily bilge water from ships, is prohibited by the IMO. Traditional shipboard wastewater treatment plants consist of two independent systems: one for the black/grey wastewater and one for the bilge water. A single, compact, onboard wastewater treatment system has been designed by Sun et al [8,9]. All these systems disinfect wastewater before discharge into the sea. However, separation of grey water from black water (toilet water) allows for the recycling of grey water and makes it possible to reduce black water. Furthermore, small quantities of black water can be thermally pretreated before their final disposal thus reducing the environmental impact of the sewage. With a 1 kwe Stirling engine, thermal power output can be partially recovered for all these purposes. 3. The Stirling Engine A range of different small-scale Stirling engine configurations are commercially available. The Linear Free Piston Stirling Engine (LFPSE) is the most suitable configuration due to the absence of a crank mechanism and the consequent lack of need for a lubrication system. The highly reliable and noiseless operating make it possible to run a LFPSE as a power supply unit at any moment such as when idling or in port. The 1 kwe Microgen Stirling engine has been considered as an onboard power unit, with the following features: Table 1: Microgen LFPSE specifications Electrical power output Overall thermal power output Thermal power input Fuel Estimated fuel consumption Weight Noise level * * fuelled by natural gas 1 kwe 5 kwt 7 kwt Oil 0.75 l/h 48 kg 45 1m Thermal power is obtained by recovering about 2 kwt from the cooling system and about 3 kwt from exhausts. The coolant is a water-glycol solution whose input temperature should be as low as possible for optimum Stirling engine performance, while its output temperature must be sufficiently high for heating purposes. Currently, Microgen Stirling engines work in a grid-connected configuration. However, a grid-independent module has already been realized and is presently under study for market entry [10]. Moreover, off-grid Stirling engines are of interest for many other applications [11]. In the grid-independent configuration, the engine starts thanks to a battery system which recharges while the Stirling engine runs. An inverter shifts the current in DC/AC mode during start up. Significant efforts must be made in order to improve the oil combustion process. At present, the 1 kwe Stirling engine is commercially available with a natural gas burner only, even though attempts and developments have been carried out on an oil burner. 4. Prototype definition Prototype definition must consider the future engine operation whilst taking into account the current configuration. This means that the following three main issues must be analysed: hybrid propulsion and the logic control unit; 1kWe Stirling engine oil burner; integration of the thermal output recovery in a wastewater treatment plant. * Corresponding author. Tel.: ; fax: address: l.cioccolanti@univpm.it

4 The hybrid propulsion Hybrid propulsion is by far the smartest solution to reduce CO 2 emissions from boats. Various hybrid powertrain configurations exist but the most common are series, parallel and power-split. In the series configuration, EM only provides power to the mechanical drive train, as reported in Figure 1b. This means that ICE works in the optimum torque and speed range, regardless of driving conditions, both as auxiliary power unit to drive the EM or the Generator to provide power to the energy storage system. It operates best with low-speed and high-torque applications. However, series configurations entail large energy conversion losses between the mechanical and the electrical systems. Therefore, the overall propulsion efficiency is reduced [5]. Moreover, although the ICE is smaller because it is designed to meet the average power demands, the EM and the energy storage systems required to satisfy the peak power demands cause a significant weight penalty on boats. On the contrary, in the parallel configuration, ICE and EM can run together or alone depending on the driving conditions, Figure 1c. In this configuration, ICE is directly coupled to the propeller shaft and could operate in its low efficiency regions. Therefore, a Continuously Variable Transmission (CVT) is usually integrated to mitigate ICE inefficiency. However, this implies higher complexities of the control strategy in order to set the torque from the power sources for maximum efficiency [12]. Finally, in the power-split configuration, a planetary gear is used to transfer the power generated by the ICE or the EM, and no direct connection exists between the various powerplants and the mechanical drive train as reported in Figure 1d. The most important advantage of this configuration is its comparative efficiency in reducing fuel usage and emissions as it efficiently combines the various power sources. Fig.1: Standard propulsion system Fig.1b: Hybrid propulsion system series configuration Fig.1c: Hybrid propulsion system parallel configuration Fig.1d: Hybrid propulsion system Power split

5 Fig.1e: Hybrid propulsion system Stirling engine configuration Using a 1kWe Stirling engine as an auxiliary power supply unit eliminates ICE recharge of the batteries, as shown in Figure 1e. Once in operation, the Stirling engine charges the battery pack and its energy is used for propulsion or onboard facilities (stove, lighting, appliances etc). The engine starts thanks to the battery system and an inverter shifts the current in DC/AC mode and vice versa. The control unit ensures the engine starts and stops during operation. The hybrid propulsion works by ensuring torque and speed combinations of both systems at which the ICE fuel consumption is minimal for steady-state conditions. In particular, the ICE produces a torque value which is often different to the torque demand. The difference between the torque values is provided by the EM when the batteries are charged. On the contrary, when the EM is off the ICE only meets the desired torque. According to the propulsion control unit, the system can work in hybrid normal mode, motor only mode, ICE only mode and hybrid maximum power mode. Hybrid is the default operating mode when the ICE works at its highest efficiency. In the second mode, the EM only powers the boat. Electric operation stops when battery charge is under 20%. In the ICE, the engine alone generates all the torque for boat speed. When maximum power is required both engines work at full throttle. Table 2 summarises the conditions for the switching of the operating modes. Table 2: switching conditions of operating modes Operating mode Hybrid normal Motor only ICE only Hybrid maximum power Condition -Default operating mode -Battery charge at least 20% -ICE off -Battery charge at least 20% -EM off or battery charge under 20% -Maximum speed rate -Battery charge at least 40% The oil burner Vaporizing oil burners were among the earliest machines used for domestic heating purposes [13]. Kerosene was burned rather than oil due to its lower evaporation temperature. In a Stirling engine, uniform head temperature must be assured for optimal engine performance and durability. At low-power size, fuel consumption is low and liquid fuel combustion becomes complex. At present, the majority of oil-fired systems rely on pressure, whereas a significant number of sleeve-type vaporizing burners exists. In a burner which relies on pressure, fuel is pumped under pressure and spray-injected into a combustion chamber, where the fuel is ignited. Atomization of fuel assists the combustion process. Alternatively, in a sleeve burner, fuel is fed under gravity to a fuel well, where it is drawn up a series of concentric wicks. At the top of the wicks the fuel vaporises and burns [14]. This type of burner has the advantage of low fuel consumption that is almost impossible to achieve with a high pressure burner. Use of a porous medium (PM) for combustion of liquid fuels is an effective method to increase the contact surface area and conduction of heat transfer for liquid fuel vaporization and flame stabilization [15]. In particular, a more efficient heat transfer occurs from burned gases to the unburned mixture than in conventional combustion devices. A PM burner consists of two different zones: the preheating zone, made of a low-porosity and less conductive material, and the combustion zone, made of highly radiating, conductive material [16]. This configuration offers a clear separation between vaporization and combustion zones which is highly desirable for liquid fuels. Selection of proper porous material causes combustion at the interface of the two zones which spreads over the entire volume of the combustion zone. Many phenomena are involved in a liquid-fuel, porous-medium burner, as reported in Figure 2 [17]. Due to an intense irradiative heat flux from the combustion zone to the preheating zone, the temperature of the latter increases to well above the saturation temperature of the liquid fuel. As the liquid fuel flow is driven by gravity and the capillarity in the preheating zone, evaporation occurs once it comes into in contact with the high-temperature surface of the matrix

6 (above the Leidenfrost temperature). Superheating of the fuel vapour within the zone may also be established, enabling self-ignition of the mixture followed by the homogeneous combustion within the combustion zone. Therefore, the use of a porous medium for liquid-fuel combustion is a very promising technology but has still not been adopted in practical and industrial applications. Moreover, the effects of material properties and structure of the matrix with temperature must be carefully examined [18]. Fig. 2: liquid fuel porous media burner The wastewater treatment According to Annex IV, ships have to be equipped with either an approved sewage treatment plant or an approved sewage holding tank. Sailing boats are not subjected to these stringent rules but wastewater treatment is still important. In fact, fresh water is usually scarce on board whereas grey water from shower and sinks can be usefully recycled after disinfection. Advanced wastewater treatments are already available in the market. On the contrary, black water from toilet tanks can be separated from the former and thermally treated. Therefore, the Stirling engine thermal output can be recovered for reducing the black water discharge and therby decrease environmental impact. Liquid phase evaporates and separates from solid whose content has been minimized. Alternatively, depending on the preferences of boat-users, the thermal output can be redirected for other purposes, as discussed above. Separation of wastewater treatment allows for the maximum recycling of water and limits fresh water needs to a few litres. 5. Market Analysis A 1kWe Stirling engine for hybrid propulsion and onboard electricity is suitable for small and medium size sailing boats and can be used on vessels of up to 40 feet in length. Noiseless operation, high efficiency and low environmental impact are certainly the success critical factors (SCFs) of the system. Moreover, new stringent environmental regulations on shipping have increased interest in low-emission propulsion systems. Compared to current technologies, the product represents a radical innovation and can significantly extend the product portfolio of sailing yachts, companies and distributors. When considering Italian production, 750 sailing boats were built in A market closed to innovation together with the current economic crisis increase difficulties in market entry. Nevertheless, a target of 10% of the whole production can be reasonably set up by Supposing a sail price of about 15,000, total revenues will be of around 1,120,000. In addition, foreign market are sensibly better. In fact, French and American sailing boat production represent more than 60% of the world production. This means that, entry into those markets is an important target to achieve. The opportunities for the Stirling engine market are even better when taking into account the retro-fitting of existing sailing boats where Stirling engines can be mounted as noiseless supply units for the charging of batteries.

7 6. Conclusions In this paper the authors investigated the integration of hybrid propulsion and sewage treatment systems powered by a 1 kwe Stirling engine in order to meet the new environmental regulations and provide comfort during the navigation of sailing boats. Thanks to their noiseless operation, Stirling engines can run continuously in order to charge the bank of batteries in a hybrid propulsion system or to supply power on request for onboard appliances. In addition, the heat discharged by the cooling system of the engine can be used for the treatment of black water. Liquid phase evaporates and separates from solid whose content is minimized allowing for a reduction in sewage discharge. The prototype definition must resolve several issues related to this new engine configuration. In particular, the logic control unit of the hybrid propulsion, the oil combustion process and the integration of the Stirling engine thermal output recovery in a wastewater treatment plant. Market analysis shows interesting industrial opportunities for this system, experimental tests of which will be run after the design stage has been completed. References [1] United Nations, Declaration of the United Nations Conference on the Human Environment, 1972, [2] International Maritime Organization, [3] International Maritime Organization, Air Pollution and Greenhouse Gas (GHG) Emissions from International Shipping, ssionsfrominternationalshippping/pages/default.aspx. [4] Gorter T, Reinders A.H.M.E, Comodi G, A comparison of 15 polymers for application in photovoltaic modules in PV-powered boats, Applied Energy 92: , [5] Kim C, NamGoong E, Lee S, Kim T, Kim H. Fuel economy optimization for parallel hybrid vehicles with CVT. SAE International, Technical Report; [6] Yanmar Marine, Yanmar SD20 Hybrid Saildrive, Brochure. [7] Bartolini C.M, Cioccolanti L, Comodi G, Vagni S, Onboard auxiliary power and desalination unit with a free piston Stirling engine, 14 th International Stirling Engine Conference, November [8] Sun C, Leiknes T, Weitzenböck J, Thorstensen B, Development of an integrated shipboard wastewater treatment system using biofilm-mbr, Separation and Purification Technology 75: 22-31, [9] Sun C, Leiknes T, Weitzenböck J, Thorstensen B, Development of a biofilm-mbr for shipboard wastewater treatment: The effect of process configuration, Desalination 250: , [10] Microgen Engine Corporation, Private Communication, [11] Bartolini C.M, Cioccolanti L, Comodi G, Distributed Stirling engines for pipelines corrosion protection, 15 th International Stirling Engine Conference, September [12] Chau KT, Wong YS. Overview of power management in hybrid electric vehicles. Energy Conversion and Management 2002;43(15): [13] Weinberger H, Residential Oil Burner 3 ed, Delmar Learning, [14] Barnes C.D, Garwood D.R, Price T.J, The use of biodiesel blends in domestic vaporizing oil burners, Energy 35 (2010); [15] Mujeebu MA, Abdullah MZ, Abu Bakar MZ, Mohamad AA, Abdullah, MK, Applications of porous media combustion technology A review, Applied Energy 86 (2009): [16] Mujeebu MA, Abdullah MZ, Abu Bakar MZ, Mohamad AA, Abdullah, MK, A review of investigations on liquid fuel combustion in porous inert media, Progress in Energy Combustion Science 35 (2009): [17] Jugjai S, Phothiya C, Liquid fuels-fired porous combustor-heater, Fuel86 (2007): [18] Kaplan M, Hall M.J, The combustion of liquid fuels within a porous media radiant burner, Experimental Thermal and Fluid Science (1995); 11: