Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Thermodynamic analysis of simple and regenerative Brayton cycles for the concentrated solar power applications Alireza Javanshir ⁎ , Nenad Sarunac, Zahra Razzaghpanah Mechanical Engineering and Engineering Science, UNC Charlotte, Charlotte, NC 28223, USA ARTICLE INFO Keywords: Brayton cycle Working fluid selection Thermal efficiency Critical point ABSTRACT A systematic multi-step method was developed for selection of the best working fluid(s) for the simple and regenerative Brayton cycles. The first step of the method involves development of the thermodynamic model of a cycle and development of theoretical expressions for its thermodynamic performance (thermal efficiency and net specific work output). In the second step, parametric calculations were performed for nine working fluids over a range of cycle operating conditions corresponding to the concentrated solar power plants (CSPs) to determine the effect of cycle operating conditions and thermo-physical properties of the working fluids on cycle perfor- mance. The statistical regression analysis is used in the third step to develop correlations for the cycle efficiency and net specific work of the same form as theoretical expressions developed in Step 1. The last step in the process involves construction of performance maps, where performance parameters, such as thermal efficiency and net specific work output are presented as functions of the cycle operating conditions. The performance map for thermal efficiency shows that, depending on the operating conditions, either N 2 or CO 2 are the best choices of the working fluids. However, considering the net specific work output, He is the best working fluid over the entire analyzed range of the cycle operating conditions. 1. Introduction Solar energy as a source of clean and renewable energy has recently gained much attention as a potential alternative to fossil fuels. Different solar thermal systems are proposed for harvesting solar energy, such as solar power tower, parabolic trough, and dish systems. The con- centrated Solar Power (CSP) tower plants (i.e., solar power plants with a central receiver) operate with a high concentration ratio, which re- sults in a higher maximum operating temperature and efficiency of the power block (power cycle), compared to the parabolic through and dish systems [1]. Department of Energy (DOE) through its SunShot solar initiative has been supporting development of the more efficient solar- thermal power production technologies with lower cost [2]. The effi- ciency of a Brayton cycle operating at temperature higher than 600 °C is in a 50% range [3], thus, a Brayton cycle is a good choice for the high- temperature CSP applications. Previous studies [4,5] show that the choice of a working fluid has a significant effect on the power block performance. Carbon Dioxide (CO 2 ), Air, Nitrogen, Nitrous Oxide, and Helium have been considered as the working fluids for the Brayton cycle [4]. A heat transfer fluid (HTF) is used to transfer thermal energy from the solar receiver to the power block. Current HTF choices for the CSP plants include mineral and synthetic oils, solar salt, or water/steam. Thermo-physical properties of the HTF, such as the upper temperature limit of 400 °C for thermal stability of oils, limit plant thermal perfor- mance [6]. To avoid these limitations, a supercritical CO 2 (sCO 2 ) having higher operating temperature range has been proposed for the CSP applica- tions as both the HTF and the working fluid [7–9]. For example, a sCO 2 Brayton cycle operating at 800 °C and 30 MPa has a significantly higher cycle efficiency (52%), compared to a Helium-Brayton cycle (44%) [5]. Garg et al. [10] showed that to achieve a specified value of thermal efficiency with a lower maximum operating temperature, a power cycle needs to operate in the supercritical regime. Although this idea is not currently applied on a commercial scale [11], research is conducted on the small-scale test units and pilot-scale systems [12]. According to [13–15], a sCO 2 Brayton cycle has the following ad- vantages, compared to the other working fluids such as Air or N 2 : (a) higher cycle efficiency at the same turbine inlet temperature, (b) smaller turbomachinery and heat transfer equipment, (c) higher heat transport capacity (at constant pressure), and (d) lower and more constant cycle exhaust temperature compared to the conventional https://doi.org/10.1016/j.enconman.2018.02.079 Received 4 December 2017; Received in revised form 6 February 2018; Accepted 23 February 2018 ⁎ Corresponding author. E-mail addresses: ajavansh@uncc.edu (A. Javanshir), nsarunac@uncc.edu (N. Sarunac), zrazzagh@uncc.edu (Z. Razzaghpanah). Energy Conversion and Management 163 (2018) 428–443 0196-8904/ © 2018 Elsevier Ltd. All rights reserved. T