Elsevier

Energy

Volume 100, 1 April 2016, Pages 115-128
Energy

Maximum-efficiency architectures for heat- and work-regenerative gas turbine engines

https://doi.org/10.1016/j.energy.2016.01.044Get rights and content

Highlights

  • Optimal architecture established for heat- and work-regenerative gas turbine engines.

  • Optimal air compression must be part intercooled and part adiabatic.

  • Air must be regeneratively heated to maximum allowed heat exchanger temperature.

  • Regenerative air heating must be done using product gas between expansion stages.

  • After heating, air must be further compressed prior to combustion.

Abstract

This study establishes maximum-efficiency architectures for heat- and work-regenerative gas turbine engines using a systematic irreversibility minimization approach. It considers engine architectures that employ two kinds of energy transfers: heat and work. It does not assume any cycle a priori (e.g., heat-recuperative reactive Brayton cycle). Instead, the maximum-efficiency architecture is directly deduced from first principles. Not surprisingly, the optimal architecture has some conventional features such as regenerative heat transfer from post-expansion combustion products to post-compression air, and external heat transfer out during compression (intercooling). But in addition it has three non-conventional features. First, unlike conventional heat recuperation heat is withdrawn between expansion turbine stages and transferred to post-compression air. Second, air is further compressed after heating. Third, compression is required to be part intercooled and part non-intercooled.

Introduction

Combustion engines have been developed based on a wide variety of simple-, regenerative-, and combined-cycle architectures that have been optimized for higher efficiency. All engine architectures can be understood as being sequences of three kinds of energy transfers—as work, as heat, and with matter—and the resulting mechanical, thermal, diffusive, and chemically-reactive equilibration processes. Each process in the sequence has a well-defined process length. For example, a reactive Brayton architecture is a sequence of work input, combustion (equilibration of fuel and air), and work output. The amount of work added in compression, amount of fuel burned, and the amount of work extracted in expansion are the respective lengths of these processes. Thus, optimization of engine architectures involves either varying the process sequence (creating a new engine cycle), or varying the process lengths (changing parameters of a cycle), or both.

Parametric optimization studies perform optimization of only the process lengths for any chosen engine cycle. For the Brayton cycle, a parametric study might involve optimization of the amount of compression work (pressure ratio), or the amount of fuel used (equivalence ratio) for maximum efficiency. However, the optimality of the underlying process sequence itself, i.e., compression, combustion, and expansion is not evaluated. For identifying the overall maximum-efficiency engine architecture, parametric analysis of pre-selected engine cycles is not sufficient. One must also identify the optimal process sequence allowed by physics and engineering constraints. Furthermore, in combustion-engine optimization the combustion process must be modeled accurately as a chemically-reactive process. It must not be treated as a heat-addition process, as is often done in heat-engine analyses. Combustion generates irreversibilities due to chemically-reactive, diffusive, thermal, and mechanical equilibration, whereas, heat addition has only thermal irreversibility [1], [2]. This work is aimed at establishing the overall maximum-efficiency engine architecture for heat- and work-regenerative engines and includes identification of the optimal process sequence (the underlying cycle) and the optimal process lengths.

Conventionally, regenerative heat transfer is employed in engine cycles in two ways: internal-regenerative heat transfer from post-expansion gases to post-compression air, and external heat transfer to the environment between air compression stages (intercooling) which have been widely studied in research literature [3], [4], [5], [6], [7] and in thermodynamics textbooks [8]. In addition, an alternative approach to regenerative heat transfer was studied by Dellenback [9], Cardu and Biaca [10], and Cai and Jiang [11]. In this approach, energy is transferred out as heat between two turbine stages, i.e., prior to complete work extraction, and supplied to post-compression air. Although intermediate removal of energy results in less work output from the downstream turbine, Dellenback shows that an overall higher efficiency can obtained. This is because such a heat transfer strategy raises the post-compression air to higher temperatures. However, in their studies Cardu and Biaca, and Cai and Jiang, show a corresponding decrease in air-specific work. The scope for efficiency increase is also shown to be limited due to practical limitations of heat exchanger effectiveness. In a re-assessed version of this approach Dellenback [12] shows that two stages of heat regeneration, between and after expansion stages, has a greater potential for increasing efficiency.

In this paper we consider all permissible ways to employ internal and external heat transfer using heat exchangers, work input/output using compressors and turbines, and combustion in burners to establish the architecture for heat- and work-regenerative gas turbine engines that has the highest efficiency over any existing or conceivable cycle. Our approach includes, but is not restricted to, traditional heat recuperation and intercooling. The paper employs attractor-trajectory optimization methodology previously developed by the authors [13] and is structured as follows. In Section 2 the thermodynamic model and methodology is presented. In Sections 3 Model problem I: feedback heat transfer from combustion to pre-combustion segments, 4 Model problem II: feed-forward heat transfer from combustion to post-combustion segments, 5 Model problem III: feedback heat transfer from post-combustion to pre-combustion segments, 6 Model problem V: external heat transfer the optimization is performed through four model problems. The optimal heat- and work-regenerative engine architecture obtained is summarized in Section 7 that concludes the paper.

Section snippets

Optimization objective

The objective is to identify the engine architecture that has maximum exergy efficiency. Such an architecture minimizes total entropy generation inside and outside the engine [14].Max exergy efficiencymin S˙genTotal=min [S˙genEngine+S˙genEnvironment]

Irreversibility inside the engine includes compressor and turbine irreversibilities, heat-exchanger irreversibility, and combustion irreversibility. External irreversibility includes the exergy in the exhaust and in heat transferred out from

Problem formulation

In the first model problem the optimal work-regenerative sequence is perturbed in a generic manner by adding a heat-transfer-in stage (Xin) in the compression segment. The corresponding heat-transfer-out stage (Xout) is placed in the combustion segment at the mth inter-burner location, also chosen arbitrarily. This evolution of optimal simple-cycle process sequence to the new heat-feedback base sequence is shown in notation below with the added heat transfer stage shown in bold.CBTBnTCXinCBTBm

Problem formulation

In the second model problem feed-forward heat transfer from the combustion to the post-combustion segment is considered. The optimal work-regenerative architecture is modified by replacement of one work-extraction stage (mth stage) in the isothermal combustion segment by a heat transfer stage that transfers energy to the post-combustion segment as shown in notation below and in Fig. 6.CBTBnTCBTBm1XoutBTBnmTXinT

Problem motivation

Similar to the previous problem, removing energy as heat for maintaining

Problem formulation

In the third model problem we consider heat transfer from the post-combustion to the pre-combustion segment. A single stage of heat transfer is added in a generic manner by splitting the air compression and products expansion processes as shown below and depicted in Fig. 9. The part-adiabatic and part-isothermal combustion segment remains unchanged from the optimal simple-cycle architecture.CBTBnTCXinCBTBnTXoutT

Problem motivation

It terms of irreversibilities, there is addition of heat transfer irreversibility.

Problem formulation

In this final model problem, external heat transfer (intercooling) is considered for optimization of engine efficiency. Intercooler stages can be added in a general manner in pre-combustion and post-combustion stages of the optimal architecture obtained in the previous problem. Intercooling is not permitted in the combustion segment because heat exchangers are not permitted at such high temperatures.C(Xin)mCB(TB)n(TXout)mCIC(XinI)mCB(TB)n(TXoutI)m

Problem motivation

External heat transfer introduces an additional

Conclusions

The optimal architecture for heat- and work-regenerative gas turbine engines has been established via a systematic and enumerative irreversibility-minimization approach. In Table 3, the different architectures considered in this study have been listed. Starting from the optimal work-regenerative architecture—the base sequence—each process studied is listed sequentially. The added process is highlighted in bold in the process sequence shown in the second column. Heat feedback from combustion

Acknowledgments

The authors would like to thank Global Climate & Energy Project (GCEP) at Stanford University for supporting this work.

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