Elsevier

Energy Conversion and Management

Volume 123, 1 September 2016, Pages 643-645
Energy Conversion and Management

Correspondence
Comment on “Energy and entropy analysis of closed adiabatic expansion based trilateral cycles” by Ramon Ferreiro Garcia, Jose Carbia Carril, Javier Romero Gomez, Manuel Romero Gomez [Energy Convers. Manage. 119 (2016) 49–59]

https://doi.org/10.1016/j.enconman.2016.06.056Get rights and content

Abstract

The authors of the above mentioned paper claim that “recent developments related to the performance of thermal cycles composed of closed processes have led to the exceeding of the Carnot factor” (Garcia et al., 2016). However, this unusual result is based on an erroneous analysis of the proposed trilateral cycles. The authors forget to take into account the compression work of the working fluid. By considering both the expansion and the compression work of the cycle, the correct thermal efficiency is far lower than the Carnot efficiency.

Introduction

In Section 2 of their paper [1], the authors propose the analysis of a trilateral cycle composed of three ideal processes, namely a closed isochoric process, a closed adiabatic process and a closed isobaric process. Fig. 1 is taken from their original paper.

In their analysis [1], the authors consider that the only work to take into account is the expansion work W23. However, during the isobaric process (3–1) the piston moves from the bottom dead center to the top dead center so that the environment performs some work on the working fluid. In practice, this work can be provided by a flywheel connected to the crankshaft of the system. The net useful work produced by the working fluid is thus lower than the expansion work only [2], [3].

Section snippets

Correct analysis of the proposed cycle

In the presentation of the performance of the cycle in Section 2 of their paper, the authors tacitly consider that the working fluid is a perfect gas with constant heat capacity. Here we explicitly use the same assumption. All the processes are considered ideal. The heat and the work are assumed to be positive if received by the working fluid.

In the following, all the relations are expressed as functions of the temperatures T1 and T2. Table 1 gives the temperature, pressure and specific volume

Conclusion

The authors of [1] claim that the trilateral cycle they propose can exceed the Carnot efficiency in the case of low hot to cold sources temperature ratio. However, in their analysis, they wrongly consider that the compression work is zero, and they compute a thermal efficiency based on the expansion work only. When taking into account the net work over the cycle, the thermal efficiency of the trilateral cycle is lower than the Carnot efficiency, which is indeed an expected result.

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