Elsevier

Energy

Volume 40, Issue 1, April 2012, Pages 107-115
Energy

Fluid selection and parametric optimization of organic Rankine cycle using low temperature waste heat

https://doi.org/10.1016/j.energy.2012.02.022Get rights and content

Abstract

The paper presented a working fluid selection and parametric optimization using a multi-objective optimization model by simulated annealing algorithm. The screening criteria considered included heat exchanger area per unit power output (A/Wnet) and heat recovery efficiency (Ф). The independent parameters are the evaporation and condensation pressures, working fluid and cooling water velocities in tubes. A comparison of optimized results for 13 working fluids shows that boiling temperature of working fluids will greatly affect the optimal evaporating pressure. R123 is the best choice for the temperature range of 100–180°C and R141b is the optimal working fluid when the temperature higher than 180°C. When the exhaust temperature ranges from 100°C to 220°C, the optimal pinch point at evaporator is about 15°C. Economic characteristic of system decreases rapidly with heat source temperature decrease. When the heat source temperature is lower than 100°C, ORC technology is uneconomical.

Highlights

► The optimal working fluid is selected for different heat source temperature. ► The boiling temperature of working fluids can greatly affect the optimal evaporating pressure. ► The optimal pinch point at evaporator is about 15  C ► The ORC technology is unsuitable for the waste heat that the temperature is lower than 100 C.

Introduction

More than 50% total heat generated in industry is the low grade heat and it is emitted in the range of 100–220°C. Recovering energy from this waste flue gas and convert it to electricity can reduce fossil fuel consumption and alleviate environmental problems. The organic Rankine cycle (ORC) is one of the promising technologies of converting low grade heat into electricity [1], [2], [3]. Therefore, more and more attention has been paid to the technology in recent years and many researches about it mainly focused on working fluid selection and parametric optimization of organic Rankine cycle.

Saleh et al. [4] compared the thermodynamic performances of 31 pure working fluids for organic Rankine cycles on the basis of the BACKONE equation of state. Liu et al. [5] examined the effects of various working fluids on the thermal efficiency and total heat recovery efficiency. It showed that the wet fluid was regarded as inappropriate for ORC system. Hung et al. [6] did a comparative study between wet, dry, and isentropic fluids in ORC system and the isentropic fluids were considered to be the best. Tchanche et al. [7] analyzed the thermodynamic characteristics and performances of 20 fluids in a low-temperature solar organic Rankine cycle and R134a was recommended. In the high-temperature organic Rankine cycles, Fernendez et al. [8] proposed that siloxanes can show good efficiencies and ensure thermal stability in regenerative ORCs. Based on the thermal efficiency, cyclopentane was recommended as the best working fluid [9]. Hung et al. [10] investigated the thermal efficiency and irreversibility of an organic Rankine cycle to recover waste heat. Results showed that p-xylene had the highest efficiency and the lowest irreversibility in recovering a high temperature waste heat. Aljundi et al. [11] compared the thermal and exergetic efficiencies of different working fluids in ORC system. The results showed that n-hexane was the best working fluid while R227ea was the worst. Quoilin et al. [12] studied the thermodynamic performance of a solar organic Rankine cycle. It showed that the most efficient fluid was Solkatherm. Chen et al. [13] showed that the CO2 transcritical organic Rankine cycle gave a slightly higher power output than the organic Rankine cycle using R123. Moreover, a series of mixtures [14], [15], [16], [17], which could be used to reduce the system irreversibility, were also proposed in sub-critical ORC system.

Hettiarachchi et al. [18] presented an optimum design of an ORC system utilizing low temperature geothermal water. Based on the screening criterion of total heat transfer area to the net power out, the ammonia was recommended among four pure fluids. Roy et al. [19] carried out a parametric optimization and performance analysis of a waste heat recovery system using ORC technology. The considered performance parameters were work output and efficiencies of the system and R123 was recommended. Rashidi et al. [20] presented a parametric optimization of the regenerative ORCs. In their research, thermal efficiency, exergy efficiency and specific work were selected as the objective functions. Guo et al. [21] examined the optimum working fluid and parameters for a low-temperature geothermal ORC system. The results showed that optimum evaporation temperature and fluids vary with different screening criteria. When the net power output was selected as the objective function, Chao H. et al. [22] proposed the optimal evaporation temperature and working fluids for subcritical organic Rankine cycle. Wang et al. [23] presented the effects of thermodynamic parameters on the supercritical CO2 cycle performance and optimized the thermodynamic parameters by means of genetic algorithm. Choosing the exergy efficiency as an objective function, Dai et al. [24] compared the performance of 10 pure working fluids and R236EA was recommended. Cayer et al. [25] and Zhang et al. [26] conducted a parametric investigation for a trans-critical and sub-critical ORC system, respectively. They reported that the best working fluid and operation parameters varied with the objective function.

The brief review presented above clearly shows that the screening criteria were very important to working fluid selection and parametric optimization for the ORC system. Many researchers conducted the studies limited to the first law efficiency [4], [5], [6], [7], [8], [9], [10], [11], [13], [14], [15], [16], [17], [19], [20], [25], [26] and second law efficiency [6], [7], [8], [11], [15], [20], [24], [25], [26]. A few researches considered the optimal parameters of ORC system with different criteria [19], [20], [25], [26]. Although the optimal working fluid was recommended for a special indicator, these literature did not take two or more indicators in account at the same time. In fact, the ORC system should consider several indicators during the operation. Up to the present, none of the published studies focused on the working fluids selection and parameter optimization based on the multi-objective function. On the other hand, these works did not evaluate the effect of heat source temperature on the working fluids under the optimization condition.

In this study, a multi-objective optimization model was proposed and the screening criteria considered included heat exchanger area for per unit power output (A/Wnet) and heat recovery efficiency (Ф). The main objective of this study was focused on finding a suitable working fluid of the ORC for waste heat recovery. The cycle parameters were optimized using a simulated annealing algorithm. The performances of the ORC with different working fluids were compared under the optimization condition. The effects of waste heat temperature and pinch temperature on cycle performance were discussed. And the economic characteristic analysis of system has also been performed.

Section snippets

System description

The basic components of a subcritical ORC system consist of an evaporator, a turbine, a condenser and a working fluid pump. The corresponding T–s diagram of ORC is shown in Fig. 1. The working fluid pump lets the liquid refrigerant (state 1) available at the exit of the condenser into the evaporator. The high pressure liquid (state 2) is heated and vaporized by the waste heat resource. Then the hot pressurized vapor is delivered to turbine inlet (state 3) and drives the expander to generate

Optimization algorithm

In the optimal design, there are many conventional methods such as the steepest descent method, powell method. Usually these methods acquires local optimization solutions easily but meets difficulty in obtaining global optimization solution. And another weak point is that its performance depends on the choice of initial solution. Compared with the conventional algorithms, the simulated annealing (SA) method is recognized to have a better capability to find the global optimum solution. In

Working fluids

The working fluid is an important part of an ORC system. For ORC applications, there are some general criteria like moderate vapor pressure in the evaporator, stability, suitable critical temperature, ozone-safe and so on [2], [32]. As a result of these criteria, 13 fluids presented in Table 1 are selected as potential candidates.

Validation

According to the developed model, the ORC simulation was performed by using a simulation program written in Matlab. Numerical solution was validated with the results of Liu et al. [5] for the ideal Rankine cycle using R123 as the working fluid and for the same operating conditions. During the validation, waste heat temperature was 200°C and condenser temperature was 30°C.

As shown in Fig. 4, the comparison shows very good agreement between present solution and the results of Liu et al. This

Conclusions

In this study, the working fluid and parameters of ORC system has been optimized by simulated annealing algorithm. The effect of waste heat temperature and pinch point on the performance and economic characteristics of ORC system has also been compared under the optimal conditions. According to the optimization and comparison, the following results are concluded.

  • 1)

    The selection of working fluid can greatly affect the operation parameters and the evaporating pressure in the cycle increases with

Acknowledgment

The authors gratefully thank the financial support for this research from the Science and Technology Department of Hunan (2009Gk2009) and the Innovation Fund for Technology Based Firms of China (08C26224302178).

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