HD Diesel engine equipped with a bottoming Rankine cycle as a waste heat recovery system. Part 2: Evaluation of alternative solutions
Highlights
► This research broadens the scope of the investigations about waste heat recovery. ► New solutions for coupling WHR Rankine cycles and thermal engines are evaluated. ► The main objective is making the solution easier to implement in real engines. ► The energy recovered by an typical turbine and by a WHR Rankine cycle are compared. ► The maximum increment of the mechanical power delivered by the power plan is by 8%.
Introduction
The main advantage of direct injection Diesel engines is its high thermal efficiency. This competitive efficiency together with its high reliability makes the direct injection Diesel engine particularly suitable as a power plant for heavy-duty transport applications. Promoted by the increasingly strict regulations on pollutant emissions [1], the Diesel engine is being object of intense research to make it more environmentally friendly, especially regarding NOx and particulate matter. An attractive alternative for improving the overall thermal efficiency of Diesel engines consists of recovering the energy lost by means of a waste heat recovery (WHR) system.
Hountalas et al. performed a theoretical analysis comparing the most common WHR systems, including mechanical and electrical turbocompounding together with a bottoming Rankine cycle [2]. From the reported results, a reduction in fuel consumption up to 8–9% at full engine load is feasible.
In a previous stage of the present research and in other works available in the literature have been stated the benefits in terms of fuel consumption produced by the bottoming Rankine cycle strategy [3], [4]. Two different approaches were investigated, two Rankine cycles in cascade (binary cycle), and a single Rankine cycle neglecting the low temperature sources. The reduction in fuel consumption without considering internal irreversibilities ranged from 16% in the first configuration to 8.5% in the second configuration. Similar results have been reported in the literature regarding a potential decrease in fuel consumption of approximately 10% attainable by integrating a bottoming cycle [5].
However, most of the literature limited its scope to the energy flow analysis, omitting any reference about how to reintroduce the recovered energy into the Diesel plus Rankine engine power plant. The main alternatives consist of directly linking the turbine shaft with the crankshaft or converting the mechanical power into electrical power to make it suitable for its use [6]. The layout of the first alternative, although seemingly straightforward, is not easy to develop due to the extreme differences in rotation speed between the turbine and the Diesel engine, while the second solution requires an electrical generator and also a set of batteries to store the energy [7] aside from the complexity in development the required turbine or expander for ORC or steam bottoming cycles.
Considering the different Diesel engine subsystems, there is one turbomachine rotating at similar speed as that of the bottoming cycle turbine, the compressor. Therefore, the power produced by the Rankine cycle turbine could be directly used to drive the compressor required to supercharge the Diesel engine, thus replacing the conventional turbine placed at the exhaust line. This would reduce the engine back pressure and the mechanical pumping losses, which in turn could also increase the efficiency of the power plant. However, the authors have not found any reference in the literature discussing the feasibility of this alternative.
As a continuation of previous theoretical analysis reported in [4], [8], [9], and with the aim of improving the knowledge about the potential of a bottoming cycle as a WHR system for future HDD engines, this paper is focused on evaluating the energy balance between the energy recovered by the exhaust turbine in the conventional Diesel engine layout and the energy produced by the bottoming cycle, plus the energy recovered as pumping work for different engine adapted layouts. The possibility of driving the engine compressor only with the power produced by the turbine of the Rankine cycle has also been evaluated. This could make the solution easier to implement in real engines.
Section snippets
Materials and methods
The present theoretical analysis has been performed on basis of an open-software within the category of the 1D wave action model (OpenWAM™) developed at CMT-Motores Térmicos, which has been previously validated to reproduce the behavior of a state-of-the-art HD Diesel engine. The use of the model along this investigation is justified when considering the representative results reported in the literature, which were obtained with similar computational models [10], [11], [12], [13].
A detailed
Results and discussion
Initially, the power plant layout has been introduced and thereafter the potential of this layout for increasing the total power of the thermal engine is critically discussed. Subsequently the power required to drive the low pressure compressor and the possibility of supplying it exclusively with the bottoming Rankine cycle is analyzed. Finally, the main drawbacks detected in the power plant layout are discussed and after that, the power requirement of the low pressure compressor is evaluated.
Summary
Table 1 shows a summary with all the solutions studied in both the first and the second part of this paper. This table shows how the total mechanical power varies for each configuration and the total heat transferred at the heat exchangers also increases (the size of the heat exchangers increases). Finally, it also shows the gas temperature at the inlet of the Diesel oxidation catalyst (DOC). This temperature can be related with the DOC efficiency, higher temperatures can increase DOC
Conclusions
From a global point of view, the best solution is the "configuration with high temperature heat sources". That is, a simple water Rankine cycle aiming to use the residual heat of the IC engine with high temperature heat sources. This achieves about 15% increment in the global mechanical power, using a heat exchanger of larger surface because an increment in heat transferred is necessary. The "configuration with all the sources. binary cycle" has a power increment of 19% with respect to the
Acknowledgements
This work was partially funded by “Programa de Apoyo a la Investigación y Desarrollo de la Universidad Politécnica de Valencia”.
References (20)
Diesel engine waste-heat power cycle
Applied Energy
(1988)- et al.
Alternative ORC bottoming cycles for combined cycle power plants
Applied Energy
(2009) - et al.
Analysis of numerical methods to solve one-dimensional fluid-dynamics governing equations under impulsive flow in tapered ducts
International Journal Mechanical Sciences
(2004) - et al.
A model of turbocharger radial turbines appropriate to be used in zero- and one-dimensional gas dynamics codes for internal combustion engines modelling
Energy Conversion and Management
(2008) - et al.
Design and testing of the Organic Rankine Cycle
Energy
(2001) - Emission Standards for Model Year 2007 and Later Heavy-duty Highway Engines. U.S Environmental Protection Agency (EPA)...
- et al.
Study of available exhaust gas heat recovery technologies for HD Diesel engine applications
International Journal of Alternative Propulsion (IJAP)
(2007) Comparative Evaluation of Three Alternative Power Cycles for Waste Heat Recovery Form the Exhaust of Adiabatic Diesel Engines
(1985)Recovery from exhaust gas on a diesel engine
VDI Berichte
(1984)Diesel engine waste heat recovery utilizing electric turbocompound technology
Cited by (49)
Experimental investigation to study combustion and emission characteristics of diesel engine by application of EGR and heated intake air
2023, Materials Today: ProceedingsA critical review on waste heat recovery utilization with special focus on Organic Rankine Cycle applications
2021, Cleaner Engineering and TechnologyWaste heat utilization from internal combustion engines for power augmentation and refrigeration
2021, Renewable and Sustainable Energy ReviewsAir conditioning cycle simulations using a ultrahigh-speed centrifugal compressor for electric vehicle applications
2021, International Journal of RefrigerationOn the effects of increased coolant temperatures of light duty engines on waste heat recovery
2020, Applied Thermal EngineeringCitation Excerpt :The brake efficiency was improved in the study by 12.1% when reaching the maximum allowable working fluid pressure of 16 bar. Similarly, Dolz et al. [18,19] in their works also look at the waste heat recovery from a heavy duty diesel engine, evaluating 8 working fluids and multiple heat sources from the engine. They found the maximum power increment for the engine is obtained from a separated/binary cycle for the low temperature and high temperature heat sources.