Elsevier

Applied Thermal Engineering

Volume 36, April 2012, Pages 279-287
Applied Thermal Engineering

HD Diesel engine equipped with a bottoming Rankine cycle as a waste heat recovery system. Part 2: Evaluation of alternative solutions

https://doi.org/10.1016/j.applthermaleng.2011.10.024Get rights and content

Abstract

A theoretical investigation has been performed on the feasibility of introducing a waste heat recovery (WHR) system in a two-stage turbocharged HDD engine. The WHR is attained by introducing a Rankine cycle, which uses an organic substance or directly water as a working fluid depending on energetic performance considerations. A previous research was carried out to evaluate the maximum potential of this WHR concept, a conventional layout was used for coupling the Rankine cycle to the thermal engine. The objective of the present research is to broad the scope of the previous analysis by considering new alternative solutions for the problems related to the coupling between the WHR Rankine cycle and the thermal engine. These solutions are based on adapting one of the turbochargers by removing its turbine and trying to recover the energy by the Rankine cycle. Finally, the turbine of the Rankine cycle supplies the recovered energy directly to the compressor of this turbocharger. Thus, in these layouts the coupling is simpler as it involves only two turbomachines, which are supposed to share a similar rotating speed. From the results of the global energy balance, these alternative layouts produce slight benefits in fuel consumption but in all cases these benefits are lower compared to those attained with conventional layouts.

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”.

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