Elsevier

Energy Conversion and Management

Volume 121, 1 August 2016, Pages 93-104
Energy Conversion and Management

Thermodynamic analysis and optimization of an integrated Rankine power cycle and nano-fluid based parabolic trough solar collector

https://doi.org/10.1016/j.enconman.2016.05.029Get rights and content

Highlights

  • The performance of an integrated nano-fluid based solar Rankine cycle is studied.

  • The effect of solar intensity, ambient temperature, and volume fraction is evaluated.

  • The concept of Finite Time Thermodynamics is applied.

  • It is shown that CuO/oil nano-fluid has the best performance from exergy perspective.

Abstract

In this paper, the performance of an integrated Rankine power cycle with parabolic trough solar system and a thermal storage system is simulated based on four different nano-fluids in the solar collector system, namely CuO, SiO2, TiO2 and Al2O3. The effects of solar intensity, dead state temperature, and volume fraction of different nano-particles on the performance of the integrated cycle are studied using second law of thermodynamics. Also, the genetic algorithm is applied to optimize the net output power of the solar Rankine cycle. The solar thermal energy is stored in a two-tank system to improve the overall performance of the system when sunlight is not available. The concept of Finite Time Thermodynamics is applied for analyzing the performance of the solar collector and thermal energy storage system. This study reveals that by increasing the volume fraction of nano-particles, the exergy efficiency of the system increases. At higher dead state temperatures, the overall exergy efficiency is increased, and higher solar irradiation leads to considerable increase of the output power of the system. It is shown that among the selected nano-fluids, CuO/oil has the best performance from exergy perspective.

Introduction

During the past decade, serious issues related to fossil fuels including limited energy resources in the world, environmental pollutions, and global warming have made the renewable energies to be utilized as alternative resources. Solar energy, as one of the most promising renewable resources of energy, can be converted to thermal energy using trough collector system and solar concentrators, and then to electrical energy using a steam turbine [1], [2]. Power generation systems based on parabolic solar collectors is well established and commercialized all over the world in the past two decades [3].

Parabolic trough collector is considered as a suitable device to derive power generation systems with operating temperatures up to 400 °C [4]. The main challenge in the operation of the parabolic collectors is how to minimize thermal losses during absorption and transfer of heat. Exergy analysis can be utilized as a useful method to evaluate the irreversibility associated with a solar system to determine the optimum operating conditions of the solar collector [5].

Extensive investigations have been done on solar thermal systems. Singh et al. carried out an exergy analysis on 35-megawatt parabolic trough power plant in India. The results of their study indicate that the most exergy loss of the system is related to high temperature heat exchanger and exergy losses in the low temperature heat exchanger is very low [6]. They proposed several methods to reduce exergy losses and to increase the efficiency of the system. Bakos et al. evaluated the efficiency of solar trough collector as a function of input heat flux, intensity of solar radiation and the aperture size of the parabolic collector. It is shown that the efficiency is improved by increasing the input heat flux and reducing the absorber pipe diameter [7].

Al-Sulaiman et al. applied exergy analysis to evaluate the performance of parabolic trough solar collectors integrated with a combined steam and organic Rankine cycle. The results show that the R-134a and R600a combined cycles have the highest and lowest exergy efficiencies, respectively, among the considered cases [8]. Reddy et al. reported an exergy analysis on the components of the solar thermal power plant system, including parabolic trough collector/receiver and Rankine heat engine. Also, they optimized the operating pressure of a Rankine heat engine in order to achieve the maximum efficiency. The results show that the energy and exergy efficiencies of thermal power plant increased by 1.49% and 1.51% with increasing pressure from 90 to 105 bar, respectively [9]. An exergy and energy analysis on a solar thermal power system is reported by Singh et al. [10]. The energy analysis indicates that the condenser of the heat engine has the maximum energy loss, however exergy analysis shows that the maximum exergy destructions take place in the collector and the receiver. Baghernejad and Yaghoubi performed exergy analysis to evaluate and optimize performance of integrated solar combined cycle. The result indicate that the major sources of exergy destruction are as follow: the combustor, collector, heat exchangers and pump/turbines [11].

A novel method to increase the efficiency of the solar collector system is to utilize nano-particles as a heat transfer medium since they have better thermal properties as compared to the base fluid. Yousefi et al. studied the effect of using Al2O3–H2O nano-fluid as an absorbing medium in a flat-plate solar water heater [12]. The results indicate that by using 0.2 wt% of Al2O3 nano-fluid the efficiency of solar collector enhances by 28.3% in comparison with water as working fluid. In another study, Liu et al. investigated the influence of water-based CuO nano-fluids in an evacuated tubular solar air collector integrated with simplified compound parabolic concentrator and open thermosyphon. The results indicate that if using nano-fluid as the working fluid of open thermosyphon, the air outlet temperature and collecting efficiency of the solar air collector will have significant improvement [13]. Faizal et al. demonstrate that nano-fluids have high energy and exergy efficiencies comparing with base fluids. The efficiency of solar collector increased up to 23.5% using SiO2 nano-fluid [14]. A research by Faizal et al. indicates that nanoparticles with higher density and lower specific heat have higher thermal efficiency, and CuO based nano-fluid leads to the highest efficiency in comparison with other nano-fluids including SiO2, TiO2 and Al2O3 [1].

The main objective of this study is to conduct an energy and exergy analysis on parabolic trough solar system integrated with a Rankine cycle, and to evaluate the overall system performance based on four different nano-fluids as working fluid in trough collector system, namely CuO, SiO2, TiO2 and Al2O3. The performance of a two-tank thermal energy storage system integrated with a solar concentrating collector is evaluated based on Finite Time Thermodynamics (FTT). The effects of solar intensity, dead state temperature, and volume fraction of different nano-particles on exergy and energy efficiency are studied. Also, the genetic algorithm is applied to optimize the net power output and exergy efficiency of the integrated cycle.

Section snippets

System description

A solar thermal power plant is considered that consists of solar collector system, thermal energy storage system (TES) and steam Rankine cycle. Fig. 1 shows a flow diagram of two-tank TES system that is integrated into a parabolic trough collector and Rankine cycle. The solar collector subsystem is a series of parabolic concentrators, in which the inner lining of reflectors concentrates solar energy on an absorber tube that is installed along the focal length. Heat is transferred to the heat

Parabolic trough collector

The following thermodynamic model is based on a mathematical method presented by Kalogirou [17]. Useful energy gain by a single solar collector can be written as follows:Q̇u=ṁrCpr(Tro-Tri)where mr, Cpr,Tro, and Tri are the mass flow rate in the receiver, the specific heat of fluid inside the receiver, the receiver working fluid outlet and inlet temperature, respectively. The heat capacity and density of nano-fluid can be calculated as [12], [14], [18]:Cp,nf=Cp,np+Cp,bf(1-)ρnf=ρnp+ρbf(1-)μnf

Optimization

Genetic Algorithm (GA) as one of the evolutionary algorithms is used to determine the optimal working conditions. GA generates a very large collection of possible solutions to solve a problem, then each of these solutions is evaluated using a “fitness function”. This algorithm is quite effective if the correct parameters are selected [28], [11].

A mathematical model for optimization consists of three main parts including objective functions, decision variables and the problem constraints. These

Validation of results

The solar collector thermodynamic model that is developed in this study is validated using the results presented by Singh et al. [10]. As shown in Table 4, the total energy absorbed by the solar collector (Qs) and the useful heat gain by the collector (Qu) are compared with those in the reference study [10].

The overall exergy efficiency of the integrated system at different dead state temperatures is compared with a study by Abid et al. [29] for Al2O3 nano-fluid. As shown in Table 4, at higher

Results and discussion

In this study, a solar Rankine cycle is analyzed in which parabolic trough solar collector system is used to provide the required energy for power generation. Four different nano-fluids based on CuO, SiO2, TiO2 and Al2O3 nano-powders are utilized as working fluid in trough collector system. A sensitivity analysis is performed to evaluate the performance of the solar Rankine cycle based on the variation of solar irradiation, environment temperature and nano-fluid void fraction.

The overall exergy

Conclusions

In this study, the performance of an integrated Rankine power cycle with parabolic trough solar collector and a thermal energy storage system is evaluated using energy and exergy analysis method. Four different nano-fluids, namely CuO, SiO2, TiO2 and Al2O3, as working fluid in the solar trough collector system are examined. A mineral oil is utilized to store required energy for producing steam during 4 h when sufficient sunlight is not available. Parametric Analysis of the integrated cycle is

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