| 1. |
- Latz, Gunnar, 1984, et al.
(författare)
-
Comparison of Working Fluids in Both Subcritical and Supercritical Rankine Cycles for Waste-Heat Recovery Systems in Heavy-Duty Vehicles
- 2012
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627.
-
Tidskriftsartikel (refereegranskat)abstract
- In a modern internal combustion engine, most of the fuel energy is dissipated as heat, mainly in the form of hot exhaust gas. A high temperature is required to allow conversion of the engine-out emissions in the catalytic system, but the temperature is usually still high downstream of the exhaust gas aftertreatment system. One way to recover some of this residual heat is to implement a Rankine cycle, which is connected to the exhaust system via a heat exchanger. The relatively low weight increase due to the additional components does not cause a significant fuel penalty, particularly for heavy-duty vehicles. The efficiency of a waste-heat recovery system such as a Rankine cycle depends on the efficiencies of the individual components and the choice of a suitable working fluid for the given boundary conditions. Commonly used pure working fluids have the drawback of an isothermal evaporation and condensation, which increases irreversibility, and consequently decreases the efficiency during the heat transfer. Previous work has suggested that one way to overcome this problem is to use zeotropic mixed working fluids. These have already been applied in several stationary systems and refrigerant cycles but not yet in waste-heat recovery systems for portable applications. This theoretical study compares different pure working fluids and zeotropic mixtures in both subcritical and supercritical Rankine cycles. The main objective was to analyze the respective energy and exergy efficiencies by modeling the Rankine cycles. The results suggested that the final fluid and cycle choice is limited by the exhaust-gas temperature range of a heavy-duty diesel engine and realistic condensation conditions for the fluid. Further, environmental and safety concerns over working fluids in portable applications are important challenges, which need to be taken into account in selecting an appropriate fluid. Copyright © 2012 SAE International.
|
|
| 2. |
- Latz, Gunnar, 1984, et al.
(författare)
-
Performance Analysis of a Reciprocating Piston Expander and a Plate Type Exhaust Gas Recirculation Boiler in a Water-Based Rankine Cycle for Heat Recovery from a Heavy Duty Diesel Engine
- 2016
-
Ingår i: Energies. - : MDPI AG. - 1996-1073 .- 1996-1073. ; 9:7, s. 495-
-
Tidskriftsartikel (refereegranskat)abstract
- The exhaust gas in an internal combustion engine provides favorable conditions for a waste-heat recovery (WHR) system. The highest potential is achieved by the Rankine cycle as a heat recovery technology. There are only few experimental studies that investigate full-scale systems using water-based working fluids and their effects on the performance and operation of a Rankine cycle heat recovery system. This paper discusses experimental results and practical challenges with a WHR system when utilizing heat from the exhaust gas recirculation system of a truck engine. The results showed that the boiler’s pinch point necessitated trade-offs between maintaining adequate boiling pressure while achieving acceptable cooling of the EGR and superheating of the water. The expander used in the system had a geometric compression ratio of 21 together with a steam outlet timing that caused high re-compression. Inlet pressures of up to 30 bar were therefore required for a stable expander power output. Such high pressures increased the pump power, and reduced the EGR cooling in the boiler because of pinch-point effects. Simulations indicated that reducing the expander’s compression ratio from 21 to 13 would allow 30% lower steam supply pressures without adversely affecting the expander’s power output.
|
|
| 3. |
- Latz, Gunnar, 1984, et al.
(författare)
-
Selecting an expansion machine for vehicle waste-heat recovery systems based on the Rankine cycle
- 2013
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2
-
Konferensbidrag (refereegranskat)abstract
- An important objective in combustion engine research is to develop strategies for recovering waste heat and thereby increasing the efficiency of the propulsion system. Waste-heat recovery systems based on the Rankine cycle are the most efficient tools for recovering energy from the exhaust gas and the Exhaust Gas Recirculation (EGR) system. The properties of the working fluid and the expansion machine have significant effects on Rankine cycle efficiency. The expansion machine is particularly important because it is the interface at which recovered heat energy is ultimately converted into power. Parameters such as the pressure, temperature and mass-flow conditions in the cycle can be derived for a given waste-heat source and expressed as dimensionless numbers that can be used to determine whether displacement expanders or turbo expanders would be preferable under the circumstances considered. The goal of this theoretical study was to use this approach to analyze waste-heat recovery systems for a heavy-duty diesel engine and a light-duty gasoline engine. Given the different waste-heat rates of these two engines, the relationships between Rankine cycle performance and design aspects such as the expansion ratio and the locations of pinch points in the heat exchanger were evaluated. The calculated values of these parameters were used as inputs in a dimensionless analysis to identify an optimal expansion machine for each case. The impact of varying the working fluid used was investigated, since it had a large impact on the results obtained and provided insights into design dependencies in these systems.
|
|
| 4. |
- Latz, Gunnar, 1984
(författare)
-
Waste Heat Recovery from Combustion Engines based on the Rankine Cycle
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most of the energy in the fuel burned in modern automotive internal combustion engines is lost as wasteheat without contributing to the vehicle’s propulsion. In principle some of this lost energy could becaptured and used to increase the vehicle’s fuel efficiency by fitting a waste heat recovery system to theengine. This thesis presents investigations into the design and functioning of waste heat recoverysystems based on Rankine cycle technology for vehicular applications.To facilitate the design of such systems, the performance of different working fluids and expansiondevices was investigated using a zero-dimensional model of the Rankine cycle. Simulations using thismodel indicated that water-based fluids should perform well when recovering waste heat from a hightemperature source such as a combustion engine’s exhaust gas. In addition, evaluations based onsimilarity parameters indicated that displacement expanders are optimal in systems having low flowrates and high expansion pressure ratios, both of which are to be expected in vehicular systems usingwater as the working fluid. Organic working fluids allow higher flow rates in the cycle, making theefficient use of turbines possible.Data from the simulations using the zero-dimensional model were used to guide the design andconstruction of a demonstrator test bench featuring a Rankine cycle-based recovery system that recoverswaste heat from the exhaust gas recirculation system of a heavy duty Diesel engine. The test bench useswater as the working fluid and a piston expander as the expansion device. The Rankine cycle’s thermalefficiency was 10%, corresponding to 1-2% of the engine’s power output. To find ways of improvingthe system’s performance, one-dimensional models of the expander and the system as a whole werecreated and then validated by comparing their output to experimental data obtained with the test bench.The expander model suggested that reducing the compression ratio would make it possible to reduce thesteam inlet pressure by 30% without affecting the expander’s power output. This hypothesis was thenconfirmed experimentally.The expander model was used to rank the relative influence of selected steam boundary conditions andexpander geometry parameters on the performance of a piston expander. The inlet pressure, steam inletcut-off timing, expander speed and outlet pressure were found to be the most significant main effects onexpander performance. It was also shown that interaction effects between steam conditions and expandergeometry had considerable influence on both power output and efficiency.
|
|
| 5. |
- Latz, Gunnar, 1984
(författare)
-
Waste-Heat Recovery from Combustion Engines based on the Rankine Cycle
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The majority of the energy in the fuel burned by the combustion engines used in modern vehicles is lost in the form of waste heat and does not contribute to the propulsion of the vehicle. Three different technologies have been proposed for recovering some of this lost heat and thereby increasing the overall efficiency of combustion engines: the turbocompound, thermoelectric converters, and heat engines based on the Rankine cycle. This thesis is about systems based on the Rankine cycle and the challenges associated with their incorporation into vehicles that are not encountered in more conventional applications.One such challenge relates to the selection of a suitable working fluid. To address this issue, a range of candidate fluids were evaluated, including organic fluids, ammonia, and water. In simulations, the best results were achieved using water-alcohol mixtures. Mixtures with a water content of 80 % by mass were found to be particularly useful since they are non-flammable and do not suffer from the freezing problems encountered when using pure water. Pure organic fluids were found to present numerous problems, including their low thermal stability, safety issues and in case of most organic refrigerants the potential to increase global warming.Another key challenge in the development of Rankine cycle systems for vehicles relates to the design of suitable expansion devices. Two expander types are considered suitable for vehicular systems: turbo expanders and displacement expanders. In order to establish a method for determining which type will offer the greatest efficiency in any given case, an analysis based on dimensionless numbers was performed. Displacement expanders were found to have favorable performance characteristics in situations involving high expansion pressure ratios and low flow rates; such conditions tend to increase the thermal efficiency of the Rankine cycle. On the other hand, turbo expanders can be made more compact than displacement expanders and may therefore be more suitable in cases where space is at a premium. Moreover, by using a pure organic working fluid instead of the suggested water-alcohol mixture and decreasing the expansion pressure ratio, the cycle parameters can be adjusted to permit the efficient operation of turbo expanders.Based on the above analyses of the system’s components, a model heat recovery system was created using the GT-Suite 1-D flow simulation program. This model can be used in conjunction with a previously established model of a heavy-duty diesel engine created using the same software, which was in this work converted to mean-value model in order to permit faster computation.
|
|
| 6. |
- Latz, Gunnar, 1984, et al.
(författare)
-
WATER-BASED RANKINE-CYCLE WASTE HEAT RECOVERY SYSTEMS FOR ENGINES: CHALLENGES AND OPPORTUNITIES
- 2015
-
Ingår i: Proceedings of the 3rd International Seminar on ORC Power Systems.
-
Konferensbidrag (refereegranskat)abstract
- Much of the fuel energy in an internal combustion engine is lost as heat, mainly through hot exhaust gas. The high energy losses, and high temperatures of the exhaust gas, provide favorable conditions for applying a waste-heat recovery system. Among the available options, systems based on the Rankine cycle show the highest potential in terms of reducing fuel consumption. Water or water-based mixtures have several advantages over organic fluids as working fluids for such applications of the Rankine cycle, in terms of cost, thermal stability, safety and complexity of the system. They also have several disadvantages, including possible freezing for pure water, high boiling temperature and high heat of vaporization. Hence, higher temperatures and amounts of waste heat are needed for reliable operation of the system. However, few experimental investigations have addressed the practical challenges associated with water and their effects on the performance and operation of a system in a driving cycle. This paper presents results of experiments with a full-scale system for recovering waste heat from the exhaust gas recirculation (EGR) of a 12.8 L heavy-duty Diesel engine on a test bench. The working fluid used in the experiments was deionized water and a 2-cylinder piston expander served as an expansion device. The engine was kept in standard configuration, except for minor modifications required to implement the heat-recovery system. The prototype EGR boiler was designed to fit in the space initially designated for the production EGR cooler. The assembly was operated in the operating points of the European Stationary Cycle (ESC). The results show that the trade-off between boiling pressure, sufficient superheating of the water and cooling of the EGR caused by the pinch-point in the boiler poses a major challenge when using water as a fluid. Low flow rates at low load points were challenging for boiler stability. During operation, the blow-by of working fluid into the lubrication system of the expander and vice versa was also problematic. Special steam-engine oil with high viscosity and good water separation capability was used to weaken this effect. The Rankine cycle-based test system attained a thermal efficiency of 10% with EGR as the only heat source. Results, major constraints and possible means to improve the system when using water as a working fluid are presented here. Simulation models developed for the EGR boiler and the piston expander supported this effort.
|
|
