تحلیل اکسرژی کلکتورهای خورشیدی تشتک سهمی شکل با چرخه رانکین آلی و بخار ترکیبی به صورت یکپارچه

ABSTRACT Exergy analysis of parabolic trough solar collectors integrated with combined steam and organic Rankine cycles

Utilization of solar energy has become crucial and it is expected to increase significantly in the near future. Therefore, there is a need to improve the performance of thermal power plants integrated with solar thermal energy. Parabolic trough solar collector (PTSC) technology is considered the most established solar thermal technology for power production. It has been used in large power plants since the 1980s in California and has demonstrated a promising renewable energy technology for the future. Hence, this technology has been selected for this study.

A number of papers had examined ORCs integrated with PTSCs for electrical power production, e.g. [1–6]. He et al. [1] considered three organic working fluids, R113, R123, and pentane, for an ORC and found that pentane had the best performance. In another study, Quoilin et al. [2] carried out thermodynamic modeling of a proposed small scale PTSC integrated with an ORC for power production, considering different design options to be located in a rural location in Berea District of Lesotho, South Africa. In a different paper, Bamgbopa and Uzgoren [3] developed a transient model for a simple ORC in which the working fluid was R245fa and found that the heat exchanger was the critical part of the model. In a different study, the performance of a low temperature solar thermal electric system using an ORC and a compound parabolic trough was examined by Gang et al. [4]. It was shown that the overall electrical efficiency was about 8.6% when a solar irradiation of 750 W/m2 was assumed. They further examined their system for selected cities and considered an improved design of the oil and organic fluid heat exchanger [5]. Derscha et al. [6] carried out a study that compared the performance of integrated solar combined cycle systems (ISCCs) with a solar electric generating system (SEGS) and found that ISCCs provided a better option than SEGS.

A few studies considered integrating ORC with PTSC for cogeneration or trigeneration, e.g. [7–13].

Li et al. [7] assessed the performance of their system for both power and water production through reverse osmosis (RO) and their system had efficiency between 18% and 20%. In a different study, Nafey and Sharaf [8] conducted thermodynamic analysis for both power and water desalination using RO. They selected four refrigerants for the PTSC case: dodecane, nonane, octane, and toluene and found that toluene was the best option. In another study, Sharaf et al. [9] conducted thermo-economic analysis of PTSC integrated with an ORC and a multi-effect distillation. Two scenarios of generation were considered in their study: the first one was with only water production and the second one was with both power and water production. It was found that the first option is more attractive. Delgado-Torres and García-Rodríguez [10,11] conducted thermodynamic analysis of a thermal system consists of an ORC, a PTSC, and an RO. Initially, they analyzed the system assuming only water production through RO [10] and then they extended their study to include both electrical and water production [11]. The main objective of their study [11] was to examine the effect of different organic fluids on the aperture area of the PTSC. In another study, the energetic performance of PTSC integrated with an ORC in which the waste heat from the ORC is used for cogeneration was conducted by Al-Sulaiman et al. [12]. It was found that there was an energy efficiency improvement, when trigeneration was used, from 15% to 94% (utilization efficiency). On the other hand, using exergy analysis, Al-Sulaiman et al. [13] found that there was an exergetic efficiency improvement from 8% to 20% when trigeneration is used as compared to only power generation. Al-Sulaiman [14] conducted energy analysis of PTSC integrated with a steam Rankine cycle as a topping cycle and an ORC as a bottoming cycle. His study considered the energetic performance of his system and the effect of selected parameters on the size of the solar collector field.

It can be observed from the literature review that there is no exergy analysis that has been conducted on parabolic trough solar collectors integrated with combined steam and organic Rankine cycles. Furthermore, with fossil fuel depletion and significant increment of CO2 emissions, finding an improved thermal power system driven by a renewable energy, such as solar energy, is becoming more crucial. Therefore, the current study is original and of significant importance. The objective of the current study is to examine, in detail, the exergetic performance of a thermal power system driven by parabolic trough solar collectors that are integrated with combined steam and organic Rankine cycles. The steam Rankine cycle (SRC) is the topping cycle while the ORC is the bottoming cycle. Severn refrigerants for the ORC were considered: R134a, R152a, R290, R407c, R600, R600a, and ammonia. In this study, key exergetic parameters are examined: exergetic efficiency, exergy destruction rate, fuel depletion ratio, irreversibility ratio, and improvement potential.