New applications: Automotive
Chair: Prof. Dieter Brüggemann
EXHAUST HEAT DRIVEN RANKINE-CYCLE OPERATING IN WET VAPOR REGION FOR HD DIESEL ENGINES
Jan Wiedemann, Roland Span
Abstract: A basic ORC plant for waste heat recovery consists of four main components: a pump, a heat exchanger, an expansion device and a condenser. Most studies on this subject assume expansion devices, which require dry expansion to avoid liquid droplets at their later stages and thus working fluid in superheated state during the entire expansion process. Since the superheating process accounts for a major portion of exergy destruction in the heat exchanger, the use of dry organic fluids or expansion devices, which allow wet expansion, is suggested. Other studies even propose to heat up the working fluid just to its boiling point and to flash evaporate it in a two-phase expander, due to the fact, that the evaporation process in the heat exchanger also contributes a large portion of exergy destruction. Partial heating may reduce exergy destruction but also enlarge volume flows and the size of system components.
The expander considered in the present study is an oil-lubricated screw expander. The working fluid is a mixture of water and/or wet organic fluids and lubricating oil. To guarantee sufficient lubrication and to avoid oil deposition, a minimum liquid content of the working fluid in every system component is required to entrain the oil in the fluid circulation. To prevent superheating of the working fluid in the heat exchanger and to control its liquid content during the expansion process in transient system operation, some components have to be added to the system. Downstream of the heat exchanger the working fluid is separated into its saturated vapor component, which is injected into the expander and its oil-rich liquid component, which is stored in a tank. The oil-rich liquid from the tank is partly injected into the expander in the required quantity by an additional pump. Via a bypass it may also be fed back into the circulation upstream the heat exchanger.
The fuel savings, i.e. the ratio of recovered power to motor power, achieved by this process should at least amount to 5\%. Simulation results yield preliminary estimates regarding ideal process parameters and suitable working fluid compositions. They show that fuel savings of 5% can easily be achieved in marine applications. Due to more restricting recooling capacities the fuel savings are lower in rail application.
AUTOMOTIVE HEAT RECOVERY WITH PISTON EXPANDERS AND WET FLUIDS
Remi Daccord, Thiebaut Kientz, Julien Melis
To date, legislation of exhaust emission levels has focused on carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. Those still more stringent emissions regulations have caused engine manufacturers to limit combustion temperatures and pressures, thus lowering potential efficiency gains. However, future regulations will focus on CO2 emissions. It requires an increased efficiency of the Internal Combustion Engine (ICE) and a move to more costly Hybrid Vehicles (HEV) . As HEV technology has achieved considerable market share in recent years, Exhaust Heat Recovery (EHR) which has the potential to decrease fuel consumption without increasing emissions seems a promising technology. Coupled to fuel saving technologies for urban driving (stop-start – regenerative breaking), EHR which can spare fuel on extra urban driving is a key technology to go further in fuel cuts.
Among others candidates, Rankine technology seems favored for EHR by literature for its efficiency and storage capacity. A basic Rankine cycle (RC) is composed of a pump, an evaporator, an expander and a condenser as illustrated in figure 1.
SPECIFICATIONS OF A BOTTOMING RANKINE CYCLE
A simple enthalpy model was made in order to compare fluids in the particular operating conditions of transportation. The high cooling temperature set to 90°C makes it difficult to find a good efficiency with refrigerant fluids such as R245fa. Investigations made on alcohols revealed their potential as working fluids for automotive RC. However, the most interesting fluids appeared to be “wet fluids” that would condensate during expansion and they mostly required high expansion ratio to be fully exploited. On top of that, shortcomings of these fluids are numerous as state-of-the-art technology is not used to deal with them. It was chosen to consider water and ethanol for further experiment.
Expander technology preferred for running with these working fluids and with the transient behavior of the exhaust gas was the piston expander. Once this choice established, a list of the technical challenges was set up. Particular attention has been paid to lubrication. High steam temperature and hydrolytic environment are major difficulties that basic lubricants cannot easily deal with.
MONOCYLINDER TEST EXPANDER DEVELOPMENT
Since 2009, the company has work on piston expander for wet fluids at high temperature. Early designs were oriented toward an oil-free monocylinder for model calibration and wear assessment. A piston rod on ball bearings links a cantilevered crankshaft to a ceramic piston. No lubrication is provided on the piston skirt. Two poppet valves for intake and exhaust are actuated by two removable cams on the crankshaft. No oil is provided on the valve guide. Separate lubrication is used for the cams contact.
This expander was manufactured in two exemplaries: one for performance tests (figure 4), the other for endurance tests. The test benches work with pure water as it was safer than ethanol for this early phase of testing. Any control failure will not be grave regarding flammability.
The performance test bench is equipped with static temperature, pressure and flow sensors and three dynamic pressure sensors for in-cylinder measurements. Steam was provided by three types of boiler according to test requirements: electric (<8bars, <350°C, 30kg/h), biomass pellet (<80bars, <450°C, 25kWth) or propane (<32bars, <350°C, 80kg/h).
0D model (figure 5) of the expander was carried out. Two parameters have to be tuned for calibration . The filling factor qualifies the impact of pressure drops and leaks. The isentropic efficiency tells us how we exploit the enthalpy gradient. Tests data enabled us to plot these factors and perform a polynomial fit to obtain an accurate simple model of the expander.
The endurance test bench has moved the piston for more than 5000km demonstrating a low oil free wear rate of less than 10-7 mm3/Nm. New design and material are presently tested to lower ten times this rate to enable maintenance free running of the expander during its whole life in case of long haul truck application which is more demanding than cars in terms of lifespan.
Thanks to the valuable data collected on its test prototype and 0D model, Exoes has currently finished the design phase of a first expander prototype fully designed for vehicle integration. The first tests will occur before this summer on a new test bench including the hydraulic loop especially designed to be embedded in a vehicle.
During 2013, Exoes will also go on developing its modeling competences through the calibration of a dynamic complete RC model, essential for its efficiency optimization and the management of fast transient conditions including starting and warming up phases .
 Sprouse III, C. Depcik, C., Review of organic Rankine cycle for internal combustion engine exhaust waste heat recovery, Applied Thermal Engineering 51 (2013) 711-722.
 Quoilin, S. Sustainable energy conversion through the use of organic Rankine cycles for waste heat recovery and solar applications, PhD, University of Liège, 2011
 Horst, T. Rottengruber, H-S. Seifert, M. Ringler, J., Dynamic heat exchanger model for performance prediction and control system design of automotive waste heat recovery systems, Applied Energy 105 (2013) 293-303
SIZING MODELS AND PERFORMANCE ANALYSIS OF WASTE HEAT RECOVERY ORGANIC RANKINE CYCLES FOR HEAVY DUTY TRUCKS
Ludovic Guillaume, A. Legros, Steven Quoilin, Sebastien Declaye, Vincent Lemort
Abstract: The interest in organic Rankine cycles for waste heat recovery on internal combustion engines has grown significantly for the past few years. Indeed, in such engines, only about one third of the energy available is actually converted into effective power, what remains being dissipated into heat. Therefore, since it becomes really challenging to increase the engine efficiency itself, solutions that focus on the recovery of this waste heat are increasingly investigated to improve the energetic efficiency of vehicles. Among these solutions, Organic Rankine Cycle systems are particularly appropriate.
The adoption of such technology in the automotive domain requires a specific R&D activity to select and develop the components and identify the most appropriate system architecture. Particularly, the selection of the working fluid and of the expansion machine technology constitutes an important part of this research.
This paper attempts to address this problematic of selecting the architecture, the expander and the working fluid for a waste heat recovery organic (or non-organic) Rankine cycle on a truck engine. It focuses especially on three expander technologies: the scroll, the piston and the screw expanders, and three working fluids: R245fa, ethanol and water.