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Publication Title | Energetic and exergetic analysis of CO2- and R32-based transcritical Rankine cycles for low-grade heat conversion

Organic Rankine Cycle

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Applied Energy 88 (2011) 2802–2808

Contents lists available at ScienceDirect Applied Energy

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

Energetic and exergetic analysis of CO2- and R32-based transcritical Rankine cycles for low-grade heat conversion

Huijuan Chen, D. Yogi Goswami ⇑, Muhammad M. Rahman, Elias K. Stefanakos Clean Energy Research Center, College of Engineering, ENB 118, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL 33620, USA

article info

Article history:

Received 8 October 2010

Received in revised form 12 January 2011 Accepted 13 January 2011

Available online 19 February 2011

Keywords:

Transcritical Rankine cycle Low-grade heat conversion Working fluid

1. Introduction

Research on the conversion of low-grade heat from sources such as geothermal, waste heat, and low temperature solar collec- tors, into electrical power has received a lot of attention in recent years. Organic Rankine cycle (ORC) is the most used cycle for this purpose. The slope of the saturation curve of a working fluid in a T–S diagram can be positive (e.g. isopentane), negative (e.g. R22) or vertical (e.g. R11), and the fluids are accordingly called ‘‘wet’’, ‘‘dry’’ or ‘‘isentropic’’, respectively [1,2]. Wet fluids, like water, usu- ally need to be superheated, while many organic fluids, which may be dry or isentropic, do not need superheating. Another advantage of organic working fluids is that ORCs typically require only single- stage expanders, resulting in a simpler, more economical system in terms of capital costs and maintenance [3].

However, the constant temperature boiling behavior of a pure fluid in a conventional ORC results in a pinch point and a mismatch between the temperature profiles of the working fluid and a sensi- ble heat source fluid, as seen in Fig. 1 and demonstrated by Saleh et al. [4]. The mismatch between the temperature profiles intro- duces significant irreversibilities. Transcritical Rankine cycles, on the other hand, can reduce the irreversibilities of the heating process.

A conceptual configuration and a P–h diagram of a transcritical Rankine cycle are shown in Fig. 2a and b. The working fluid is pumped above its critical pressure (a ? b), and then heated isobarically from liquid directly to supercritical vapor (b ? c); the

⇑ Corresponding author. Tel.: +1 813 974 0956; fax: +1 813 974 5250. E-mail address: goswami@usf.edu (D.Y. Goswami).

0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.01.029

abstract

Transcritical Rankine cycles using refrigerant R32 (CH2F2) and carbon dioxide (CO2) as the working fluids are studied for the conversion of low-grade heat into mechanical power. Compared to CO2, R32 has higher thermal conductivity and condenses easily. The energy and exergy analyses of the cycle with these two fluids shows that the R32-based transcritical Rankine cycle can achieve 12.6–18.7% higher thermal effi- ciency and works at much lower pressures. An analysis of the exergy destruction and losses as well as the exergy efficiency optimization of the transcritical Rankine cycle is conducted. Based on the analysis, an ‘‘ideal’’ working fluid for the transcritical Rankine cycle is conceived, and ideas are proposed to design working fluids that can approach the properties of an ‘‘ideal’’ working fluid.

Ó 2011 Elsevier Ltd. All rights reserved.

supercritical vapor is expanded in the turbine to extract mechani- cal work (c ? d); after expansion, the fluid is condensed in the con- denser by dissipating heat to a heat sink (d ? a); the condensed liquid is then pumped to the high pressure again, which completes the cycle.

The major difference between a subcritical and a transcritical Rankine cycle lies in the heating process of the working fluid as seen in Fig. 3. In a transcritical Rankine cycle, the working fluid is heated directly from the liquid state into the supercritical state, bypassing the two phase region (b ? c in Fig. 3). By bypassing the isothermal boiling process, the transcritical Rankine cycle allows the working fluid to have a better thermal match with the heat source, resulting in less exergy loss. Furthermore, by avoiding the boiling process, the configuration of the heating system is poten- tially simplified.

The choice of a working fluid is of key importance for a trans- critical Rankine cycle. Carbon dioxide (CO2), being abundant, non-flammable, non-toxic and inexpensive, has been extensively studied as a supercritical working fluid by a number of researchers. Zhang et al. [5–8] indicates the thermal efficiency of a CO2-based transcritical Rankine cycle to be 8.0–11.4% depending on the work- ing conditions. Chen et al. [9,10] found that under the same ther- modynamic mean heat rejection temperature, a CO2-based supercritical power cycle gives a slightly higher power output than a R123-based ORC. Wang et al. [11] and Cayer et al. [12] did para- metric studies on the thermodynamic performance and exergy destruction in a CO2-based transcritical Rankine cycle to find that parameters such as turbine inlet pressure and temperature have significant effects on the exergy efficiency of the supercritical CO2 power cycle. Beside CO2, hydrocarbons [13] and refrigerants [14–18] have also been studied as working fluids in transcritical

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