J. of Supercritical Fluids 76 (2013) 54–60 Contents lists available at SciVerse ScienceDirect The Journal of Supercritical Fluids jou rn al h om epa ge: www.elsevier.com/locate/supflu Supercritical carbon dioxide Brayton cycle for concentrated solar power Pardeep Garg, Pramod Kumar ∗ , Kandadai Srinivasan Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India a r t i c l e i n f o Article history: Received 12 December 2012 Received in revised form 4 January 2013 Accepted 13 January 2013 Keywords: Supercritical CO2 Brayton cycle Solar energy Irreversibility a b s t r a c t Supercritical carbon dioxide based Brayton cycle for possible concentrated solar power applications is investigated and compared with trans- and sub-critical operations of the same fluid. Thermal efficiency, specific work output and magnitude of irreversibility generation are used as some of the performance indicators. While the thermal efficiency increases almost linearly with low side pressure in the sub- and trans-critical cycles, it attains a maximum in the supercritical regime at ∼85 bar after which there are diminishing returns on increasing the low side pressure. It is also found that supercritical cycle is capable of producing power with a thermal efficiency of >30% even at a lower source temperature (820 K) and accounting for foreseeable non-idealities albeit with a higher turbine inlet pressure (∼300 bar) which is not matched by a conventional sub-critical cycle even with a high source temperature of 978 K. The reasons for lower efficiency than in an ideal cycle are extracted from an irreversibility analysis of compo- nents, namely, compressor, regenerator, turbine and gas cooler. Low sensitivity to the source temperature and extremely small volumetric flow rates in the supercritical cycle could offset the drawback of high pressures through a compact system. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Globally increasing electrical energy demands and environ- mental concerns preventing its matching with supply has posed challenges to the engineering community to look for alternative means of bridging the gap. A large fraction of that demand is met from water based power plants either from hydroelectric or thermal energy derived from fossil fuels or nuclear reactors. Environmental emotions are preventing addition of capacities to either of those categories of power plants. Distributed power generation, using renewable energy and non-water based working fluids together offer opportunities to meet the exigencies. In the past three decades there has been a spurt in research reporting interesting results on choice of working fluids for organic Rankine cycle (ORC) based power generation. Available options can be categorized as halocarbons [1], which have either or both of ozone depletion potential (ODP) and global warming potential (GWP) or hydrocarbons which are flammable [2] or mixtures [3]. Our earlier research addressed the issues of avoiding ODP, reducing GWP and suppressing flammability by adopting mixtures of hydrocarbons with either R-245fa [4] or carbon dioxide [5]. An approach adopted by the refrigeration community, which is strongly advocating use ∗ Corresponding author. Tel.: +91 80 2293 3361; fax: +91 80 2360 4536/3260 4356. E-mail addresses: pramod@mecheng.iisc.ernet.in, pramod k24@yahoo.com (P. Kumar). of carbon dioxide as a working fluid [6], prompted us to investigate this fluid also as a Brayton cycle working fluid for power genera- tion. Credence to this philosophy is lent by literature on this topic dating back to 1950 [7] and reasonably analyzed in 1960s [8–10]. The possibility of supercritical carbon dioxide (S-CO 2 ) cycle for nuclear applications [11,12], where heat source temperatures are in the range 600–700 ◦ C, steers to infer that for concentrated solar power generation also, perhaps, this cycle could be adoptable. This is corroborated by work of Chen et al. [13] who found transcritical CO 2 (T-CO 2 ) Rankine cycle to be more compact, eco-friendly and devoid of pinch point in the heat exchanger. T-CO 2 operating with a sink temperature of -50 ◦ C, attainable with LNG evaporation, has been studied [14,15]. Yamaguchi et al. [16] have tried trans- critical condensing cycle with reasonably good efficiencies even at a low source temperature of ∼440 K. Three options available with entirely gaseous phase operation of carbon dioxide based Brayton cycle are (a) conventional cycle operating entirely below critical pressure (SN-CO 2 ), (b) transcritical non-condensing cycle with the upper cycle pressure being in the supercritical region (TN-CO 2 ), and (c) supercritical cycle with low side pressure also being supercritical (S-CO 2 , this notation retained to be the same as that used in literature [11,12]). Objective of this paper is to explore all the three CO 2 Brayton cycles with specific application to concentrated solar power adoptable for distributed power generation for rural and remote settlements. Figs. 1 and 2 show the three possibilities on the pressure-enthalpy and temperature- entropy planes at 873 K source temperature. An assessment is made of the potential of these options through a set of performance 0896-8446/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2013.01.010