A comparative analysis of rankine and absorption power cycles from exergoeconomic viewpoint
Introduction
Energy plays a very important role in human beings’ daily life, economic development and activities. Over years various attempts have been done to develop more efficient energy conversion systems which have better environmental impacts. Therefore, developing and presenting some approaches to improve the design of energy conversion systems and to reduce environmental impacts is of high significance. From thermodynamic viewpoint, the Carnot cycle is the best possible cycle which can exists while there are no heat losses in this cycle. Then the Rankine cycle with steam as working fluid was introduced as a more practical cycle than a Carnot cycle and was widely utilized in the world for power generation, but in the case of heat sources with medium and low temperature, the heat cannot be converted efficiently to electrical power using typical cycles like Rankine ones. Therefore, Rankine cycle with organic working fluid and Kalina cycle were introduced to gain this aim in recent years. Organic Rankine Cycle (ORC) has been investigated by many researchers to recover heat from low and medium temperature heat sources such as geothermal and solar sources and also to recover heat from the exhaust gasses from diesel engines [1]. Thermodynamic performance and environmental properties of various organic fluids have been compared in Ref. [2]. Wang and Zhao [3] indicated that the thermal efficiency of the Rankine cycle with solar heat source will increase in comparison with simple Rankine cycle using zeotropic mixtures as working fluid and using superheater and internal heat exchanger. Selecting the working fluid is a basic parameter in power cycles. Several studies have been carried out to compare different working fluids in the Rankine cycle so far [4]. Binary mixtures have also been used as a working fluid in power cycles in recent years among which the most important one was the suggested cycle by Kalina which used NH3–H2O as working fluid for power generation. Theoretically, Kalina cycle converts almost 45% of heat of direct fire systems to power and this value can increase to 52% in the combined cycles while these efficiencies for Rankine cycle are 35% and 44% respectively [5]. Also, in the industrial waste heat application, the Kalina cycles compared to a Rankine cycle can concede to 32% more power [6].When both of the cycles of Kalina and Rankine are placed as the bottoming cycles with the same conditions of heat source, when the heat source temperature is below 537 °C, the Kalina cycle, based on the second law of efficiency, will be 10–20% more efficient than the simple Rankine cycle [7]. The major characteristic of Kalina cycle for being more efficient than other power cycles is due to the unique characteristics of NH3–H2O working fluid and the processes which occur in the heat-attainment in the evaporator and heat-rejection in the condenser. During the evaporation process in cycle with NH3–H2O as working fluid, the pressure is constant, but the temperature changes and unlike other working fluids, this will be used in the power cycles such as water because both pressure and temperature are constant in water during the evaporation process in the evaporator. Therefore in the cycles with ammonia-water working fluid, the matching temperature or pinch will better take place between the temperature profile of the streams of heat source and the temperature profile of NH3–H2O in evaporator and condenser. As a result, during the heat transfer processes the exergy destruction in evaporator and condenser will decrease and the heat transfer processes will take place more efficiently. Since ammonia concentration in ammonia water can be adjusted, we can have more flexibility to optimize heat recovery.
It is necessary for steam condensation to have a natural sink and a condenser temperature which is higher than condenser coolant in the Rankine cycle. Rankine-absorption configuration can be used to decrease the condensation temperature. Agnew et al. [8] investigated the combined cycle of Rankine and absorption cycle in which the exhaust gases from the boiler of the Rankine cycle used as the absorption cycle heat source. The absorption cycle was utilized to cool the Rankine cycle condenser. In recent years, many power and cooling cogeneration cycles have been suggested which have been made through combining a power generation cycle such as Rankine and Kalina and absorption cooling cycle and ammonia water working fluid has been used in these cycles. In this cogeneration cycles, instead of using the conventional condensation process, absorption-condensation process has been used [9], [10], [11], [12], [13], [14], [15].
Power generation systems are usually designed and optimized thermodynamically and economically. Thermo-economics is a powerful and relatively new method which uses combination of thermodynamic and economic concepts in order to let designers be able to have more useful information about the performance and costs of energy conversion systems that cannot be obtained in the separate thermodynamic and economic analyses and more importantly it can used in minimizing the final product costs of the energy systems [16]. The concept of exergy is used instead of energy in thermo-economic analysis [17], [18]. Different studies have been carried out on energy conversion systems from thermo-economic viewpoint. [19], [20], [21], [22], [23], [24]. Ahmadi et al. [25]presented the exergoeconomic analysis on trigeneration cycle including gas turbine cycle, ORC and single effect absorption cycle. Baghernejad and Yaghoubi [26] applied the exergoeconomic analysis on the integrated solar combined cycle and optimized it using genetic algorithm. They showed that the selected objective function in the optimal case is 11% lower than its base case. It was also observed that in the optimal conditions the capital investment cost increases to 13.3%. Kanoglu and Abusoglu [27] presented an overall review on exergoeconomic analysis and optimization of cogeneration systems based on different exergoeconomic approaches.
The present study has developed the investigation of absorption power cycle which is cited in Ref. [28] in which the low temperature heat source is used focusing on the exergoeconomic aspect of this cycle which, to our knowledge, has not been performed. This cycle uses LiBr–H2O as working fluid. Then this cycle has been compared from exergoeconomic viewpoint with Rankine cycle and the same cycle with NH3–H2O mixture as working fluid in the same conditions of heat sources. To do this, the energy and exergy analysis has been done on LiBr–H2O cycle and the result has been validated with Ref. [28]. After that a comprehensive exergoeconomic analysis has been done to all of the three cycles and then unit cost of power produced by turbine and other parameters of exergoeconomic analysis has been obtained and compared together. Then a parametric analysis has been done and the effect of different parameters of the cycles such as temperature and pressure of the generator, the outlet pressure of the absorber and concentration of solutions on the exergoeconomic performance of the cycles has been investigated. It is expected that the obtained results from the exergoeconomic analysis of these cycles provide a deep understanding for designers of power generation and energy conversion systems.
Section snippets
Description of the cycle
Scheme of the considered cycle has been shown in Fig. 1. Using a medium-temperature heat source (steam with temperature of 150 °C at saturated vapor state), refrigerant vapor is separated from solution in the steam generator. The separated steam (1) moves toward the turbine and is expanded in the turbine. Concentrated solution (6) at first passes through the internal heat exchanger and expansion valve respectively and then enters the absorber after its pressure reduction. In this cycle, the
Simulation and analysis of the system
In the system of Fig. 1, the steam with temperature of 150 °C which is entered to steam generator at saturated vapor state and exits from that at the saturated liquid state is used as heat sources. Condensation temperature in the Rankine cycle and the outlet temperature of the absorber in the LiBr–H2O cycle as well as the NH3–H2O cycle are considered to be 50 °C. Water at the ambient temperature and pressure is also used as absorber coolant. The input parameters of the cycles have been shown in
Results and discussion
Energy, exergy and exergoeconomic simulations of the considered cycles have been done by EES software. The thermodynamic models of LiBr–H2O absorption power cycle and Rankine cycle were validated using the published data by Garcia-Hernando et al. [28] and the comparison of simulation results with results in the Ref. [28] is shown in Table 4, Table 5. According to Table 4, Table 5, the values of performance parameters obtained in the present work are in a good agreement with those published by
Conclusions
In this paper LiBr–H2O cycle has been compared from exergoeconomic viewpoint with Rankine cycle and with the same cycle using NH3–H2O as working fluid in the same conditions of heat sources. Significant conclusions that can be drawn from the exergoeconomic analysis and parametric study of the cycles are as follows:
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The value of unit cost of power produced by turbine for LiBr–H2O cycle, Rankine cycle and NH3–H2O cycle equals 10.22 cent/kW-hr, 9.631 cent/kW-hr, and 8.592 cent/kW-hr respectively.
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