Thermal-economic analysis of a transcritical Rankine power cycle with reheat enhancement for a low-grade heat source Hanfei Tuo* ,† University of Illinois at Urbana Champaign, Urbana, IL, 61801 SUMMARY A thermal-economic analysis of a transcritical Rankine power cycle with reheat enhancement using a low-grade industrial waste heat is presented. Under the identical operating conditions, the reheat cycle is compared to the non-reheat baseline cycle with respect to the specific net power output, the thermal efficiency, the heat exchanger area, and the total capital costs of the systems. Detailed parametric effects are investigated in order to maximize the cycle performance and minimize the system unit cost per net work output. The main results show that the value of the optimum reheat pressure maximizing the specific net work output is approximately equal to the one that causes the same expansion ratio across each stage turbine. Relative performance improvement by reheat process over the baseline is augmented with an increase of the high pressure but a decrease of the turbine inlet temperature. Enhancement for the specific net work output is more significant than that for the thermal efficiency under each condition, because total heat input is increased in the reheat cycle for the reheat process. The economic analysis reveals that the respective optimal high pressures minimizing the unit heat exchanger area and system cost are much lower than that maximizing the energy performance. The comparative analysis identifies the range of operating conditions when the proposed reheat cycle is more cost effective than the baseline. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS transcritical power cycle; carbon dioxide; thermal-economic analysis; waste heat Correspondence *Hanfei Tuo, University of Illinois at Urbana Champaign, Urbana, IL, 61801. † E-mail: tuo1@illinois.edu Received 9 August 2011; Revised 30 November 2011; Accepted 15 December 2011 1. INTRODUCTION Due to the increasing electricity demand and environmen- tal issue, power generation using organic Rankine cycle (ORC) to recover low-grade industrial waste heat, biomass, solar, and geothermal energy has attracted more and more attention [1–5]. However, its performance is limited by its constant-temperature evaporation process. Supercritical power cycles show high potentials to recover such low grade heat, because working fluid temperature glide above the critical point provides a better temperature profile match in the vapor generator. In addition, carbon dioxide is a very promising natural fluid for the transcritical power cycle, because of its desirable qualities such as moderate critical point, little environment impact, and low cost. Many researches have been conducted regarding tran- scritical Rankine power cycles (TRC) utilizing low- or medium temperature heat sources. Gu et al. [6,7] investigated a TRC system using geothermal sources with a temperature higher than 190  C. Guo et al. [8] compared natural and conventional working fluids for use in TRCs using a low temperature (80–120  C) geothermal source. A comparative analysis of a TRC using zeotropic mixture as the working fluids and an ORC with R134a was carried out. It showed that TRC can achieve higher thermal efficiency and better heat exchange processes than ORC under the same thermal conditions [9]. In the transcritical CO 2 Rankine power generation field, Zhang et al. [10] first put forward this cycle powered by solar energy. They studied a similar cycle powered by solar energy for both power and heat generation [11,12]. Corresponding experimental study was conducted in order to validate the feasibility of the proposed cycle [13]. Chen et al. [14] analyzed the performance of a CO 2 TRC for recovering au- tomobile waste heat at 200  C. Chen et al. [15] found that when using the low grade heat source, the carbon dioxide INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2013; 37:857–867 Published online 14 February 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2886 Copyright © 2012 John Wiley & Sons, Ltd. 857