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Publication Name: Transcritical or supercritical CO2 cycles using both low- and high-temperature heat sources
Original File Name Searched: 2012 - Y.M. Kim - Transcritical or supercritical CO2 cycles using both low- and high-temperature heat sources.pdf
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Energy 43 (2012) 402e415

Contents lists available at SciVerse ScienceDirect Energy

journal homepage: www.elsevier.com/locate/energy

Transcritical or supercritical CO2 cycles using both low- and high-temperature heat sources

Y.M. Kim a, *, C.G. Kim a, D. Favrat b

a ECO Machinery Division, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea

b Industrial Energy System Laboratory, Swiss Federal Institute of Technology of Lausanne (EPFL), Station 9, CH1015 Lausanne, Switzerland

articleinfo

Article history:

Received 17 November 2011 Received in revised form

26 February 2012

Accepted 31 March 2012 Available online 4 May 2012

Keywords:

Transcritical CO2

Supercritical CO2

Rankine cycle

Brayton cycle

Thermal energy storage (TES) Exergy

1. Introduction

Recently, interest in a supercritical CO2 power cycle has increased in conjunction with its application in nuclear reactors owing to its simplicity, compactness, sustainability, enhanced safety, and superior economy [1e5]. The supercritical CO2 cycle is expected to benefit fossil, renewable, and advanced nuclear power plants because CO2 is an extremely effective working fluid in its supercritical state.

In the case of CO2 cycles with high-temperature (HT) heat sources such as nuclear power, concentrated solar power, and combustion, the working fluid goes through both subcritical and supercritical states (transcritical cycle), or is used entirely above its critical pressure (supercritical cycle). Further, the CO2 cycles can be gas cycles (Brayton cycles) or condensation cycles (Rankine cycles). Feher [6] proposed a supercritical CO2 (S-CO2) power cycle that operates entirely above the critical pressure of CO2, is regenerative, and ensures the compression in the liquid phase to

* Corresponding author. Tel.: þ82 42 868 7377; fax: þ82 42 868 7305.

E-mail addresses: ymkim@kimm.re.kr (Y.M. Kim), cgkim@kimm.re.kr (C.G. Kim),

daniel.favrat@epfl.ch (D. Favrat).

0360-5442/$ e see front matter ! 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.03.076

abstract

In CO2 cycles with high-temperature heat sources that are used in applications such as nuclear power, concentrated solar power, and combustion, partial condensation transcritical CO2 (T-CO2) cycles or recompression supercritical CO2 (S-CO2) cycles are considered to be promising cycles; this is because these cycles cause a reduction in the large internal irreversibility in the recuperator owing to the higher specific heat of the high-pressure side than that of the low-pressure side. However, if heat is available in the low-temperature range, the T-CO2 Rankine cycles (or fully-cooled S-CO2 cycles) will be more effective than the T-CO2 Brayton cycles (or less-cooled S-CO2 cycles) and even than the partial condensation T-CO2 cycles (or recompression S-CO2 cycles). This is because the compression work is reduced while achieving the same temperature rise by heat recovery through the recuperator before the high-temperature heater.

The proposed T-CO2 Rankine cycles or fully-cooled S-CO2 cycles using both the low- and high- temperature heat sources can maximize the power output of the CO2 power cycle with the given high-temperature heat sources. Moreover, the proposed CO2 cycles combined with the low-temperature thermal energy storage offer the advantage of load leveling over other CO2 cycles, with the given high- temperature heat sources.

! 2012 Elsevier Ltd. All rights reserved.

minimize pump work [1]. Angelino [7] conducted one of the most detailed investigations on transcritical CO2 (T-CO2) cycles and primarily focused on condensation cycles [1]. However, it was found that the T-CO2 Rankine cycles exhibited a large internal irreversibility in the recuperator owing to heat transfer from the turbine exhaust stream with a low specific heat to the pump exit stream with a high specific heat [1]. Feher [6] first revealed the same problem associated with irreversibility in the recuperator used in the S-CO2 cycles. A recompression cycle was proposed to avoid the problem; the recuperator was divided into low- and high-temperature parts, each having different flow rates to cope with a large variation in the heat capacity of the fluid. Thus, only a fraction of the CO2 fluid flow is bypassed to the recompressing compressor before pre-cooling and is merged with the rest of the fluid flow, heated through the low-temperature (LT) recuperator, from the main pump (or compressor) before it enters the HT recuperator. The recompression cycle can be applied to both the S- CO2 and the T-CO2 cycles, and it has been studied as the most promising CO2 cycle for HT heat conversion [1e9]. Sarkar et al. [10] studied the effects of various operating conditions and perfor- mance of components on the optimization of the S-CO2 recom- pression cycle. Meanwhile, the T-CO2 Rankine cycles (fully condensation cycles) have been mostly studied for low-grade heat

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