Elsevier

Energy Conversion and Management

Volume 108, 15 January 2016, Pages 190-201
Energy Conversion and Management

Advanced exergy analysis of the Kalina cycle applied for low temperature enhanced geothermal system

https://doi.org/10.1016/j.enconman.2015.11.017Get rights and content

Highlights

  • Conventional and advanced exergy analyses are applied for a Kalina cycle in an enhanced geothermal system.

  • The results of conventional and advanced exergy analyses are compared.

  • Kalina cycle improvement potential by advanced exergy analysis is defined.

  • The role of components interactions for performance enhancement is presented.

Abstract

In recent years, the possibility of using low temperature heat sources has been followed as a hot topic in different research and academic centers. In this regard, the Kalina cycle has been paid a lot of attention because of its promising features. Using the engineering equation solver (EES) software, conventional exergy analysis is carried out in this study for the Kalina cycle driven by a low temperature enhanced geothermal source. After validating the developed model for conventional exergy analysis, the advanced exergy analysis, i.e., splitting exergy destruction rate into endogenous, exogenous, avoidable and unavoidable parts, is performed to provide detailed information about improvement potential of the system components. The results of advanced exergy analysis show that the cycle has high potential for efficiency improvement. It is also revealed that the advanced exergy analysis gives the improvement priority first for the condenser, then for the turbine and the evaporator. From the conventional exergy analysis however, the exergy destruction calculated for the evaporator is higher than that for the turbine.

Introduction

Generating electricity from geothermal sources has a history of more than 100 years [1]. The worldwide installed capacity of electrical power generation from these sources in 2015 is around 12,635 MW and it is expected that this capacity will reach 21,443 MW in 2020 [2]. Nearly 11–12% of this power is generated from geothermal plants with binary working fluids [1]. As defined in literature, the binary cycle is the main technology for generating power from low to medium temperature (<180 °C) geothermal energy sources [3]. The organic Rankine cycle (ORC) and Kalina cycle (KC) are two major groups of these binary geothermal cycles [4]. Using the KC with a mixture of ammonia–water as working fluid brings about a performance enhancement of nearly 20% compared to some other power cycles [5].

The Kalina cycle was designed and developed by Alexander Kalina to be used as a bottoming cycle instead of the Rankine cycle in combined cycle power plants. Kalina showed that the efficiency of this new cycle is about 30–60% higher than that of the Rankine cycle [6]. A lot of research works have been carried out on the Kalina cycle all over the world. In this section, some of these works are reviewed. Hettiarachchi et al. [7] examined and compared the performances of an ORC and a KC (known as KCS 11) used for low temperature geothermal heat sources. They concluded that, under specified conditions and at moderate turbine inlet pressures, the KCS 11 performs better than the ORC. Using binary working fluids, the performances of a KC (KCS 34) and an ORC for producing electricity from geothermal sources in the Republic of Croatia was investigated by Guzović et al. [8]. In the proposed binary plants with ORC and Kalina cycles for this study, geothermal fluid has transferred heat to the working fluid by cooling. Their results showed that the ORC efficiency increases when the geothermal fluid is cooled from 175 °C to 69 °C. At the same time, the results emphasized that the cycle produces higher output power when the temperature of geothermal fluid increases. Guzović et al. concluded that for geothermal sources with lower temperatures, the KCS 34 demonstrates better performance. Using a variety of working fluids and working fluid compositions, Rodríguez et al. [9] carried out comparative exergoeconomic analyses for the KC and ORC employed for an advanced geothermal system in Brazil. They suggested R-290 and a mixture of 84% ammonia–16% water (in mass fraction) as working fluids for the ORC and KC, respectively and reported the superiority of the KC to the ORC from the viewpoints of thermodynamics and economics. Singh et al. [10] performed a parametric study on the combined KCS 11 – Rankine cycle and reported that the best cycle performance is achieved with an ammonia concentration of between 78% and 82% for the working fluid of KCS 11 and a moderate pressure of 4000 kPa for the ammonia turbine inlet. Li et al. [11] compared the performances of KC and CO2 transcritical power cycle (CTPC) utilizing low temperature geothermal sources in China. Results of this study showed that the KC has higher thermal efficiency and net output power and better economic performance than the CTPC. A detailed review about the results of the references [7], [8], [9], [10], [11] are shown in Table 1. Yari et al. [12] compared thermodynamic and economic performances of the KCS 11, ORC and trilateral power cycle (TLC) and showed that for both the KCS 11 and the ORC, the optimum operating conditions for maximum net output power is different from those for minimum product cost.

In most of the above mentioned research works, conventional exergy analysis plays an important role, especially in determining the exergy destruction in different components. The analysis however, doesn’t specify the internal or external sources of irreversibilities for system components (concepts which can be very useful for thermodynamic system designers). The concept of the advanced exergy analysis, which has been proposed in recent years, however, provides this information for the designers. In the advanced exergy analysis, the exergy destruction in each system component is split into avoidable and unavoidable parts and also into endogenous and exogenous components. The advanced exergy analysis offers a great opportunity for improving a system performance and identifies system components which play a major role in this improvement. The idea of advanced exergy analysis was proposed by Tsatsaronis et al. [13]. In recent years, Tsatsaronis and his research group (in technical university of Berlin) have presented several research works using advanced exergy, exergoeconomic and exergoenvironmental analyses for various thermodynamic systems. They applied advanced exergy method to analyze vapor-compression and absorption refrigeration machines and also to the gas turbine power systems [14], [15]. Petrakopoulou et al. [16] used advanced exergoeconomic method to analyze the performance of a gas turbine-low pressure steam turbine combined cycle power plant. This analysis showed that plant performance improvement can be obtained by modification of each component. Their results also indicated that component interactions play insignificant role in this improvement. In another works, advanced exergy analyses were carried out by Morosuk et al. [17], [18] on the liquefied natural gas-based cogeneration system to identify the interactions among components and the potential for performance improvement. Tesatrasonis et al. [19] estimated the combustion process exergy destruction resulting from different sources of irreversibility (heat transfer, chemical reaction, friction and mixing). Tsatsaronis and Morosuk [20] identified the value, location and causes of the irreversibilities as well as the costs and environmental impacts of gas turbine based cogeneration system using the advanced exergy, exergoeconomic and exergoenvironmental methods. They reported that the combustion chamber and heat recovery steam generator are important for improving the system efficiency. Using the advanced exergy method, Hepbasli et al. analyzed the Afyon geothermal district heating system [21], [22], and a gas engine heat pump for food drying processes [23], [24]. Açıkkalp et al. applied the advanced exergy method for a cogeneration and a trigeneration systems and identified the potential of economic improvement for each component and also the effects of components on one another in terms of exergy destruction and its associated cost [25], [26]. Soltani et al. [27] carried out an advanced exergy analysis on an externally fired combined cycle power plant using biofuel and reported that the exergy destructions in most of the components are endogenous and unavoidable. Tan et al. [28] evaluated a geothermal district heating system in terms of thermodynamic and economic aspects by using the advanced exergy analysis method and identified potentials of the system improvement and energy saving.

From the above mentioned review, it is clear that the Kalina cycle has been paid a lot of attention because of its promising features (especially in generating power from low temperature geothermal sources). Also, it can be seen that the advanced exergy analysis provides useful information not attainable from the conventional exergy analysis. To our knowledge, the Kalina cycle has not been analyzed using the advanced exergy method so far. This is important considering that the interaction between system components in the Kalina cycle can play a major role in identifying the weak points of the system from the viewpoints of second law of thermodynamics. The present work is an attempt to fulfill this lack of information and to reveal the real sources of irreversibilities and real potential of improvement in the Kalina cycle used for low temperature enhanced geothermal system. It is hoped that the results, which cannot be attained from the conventional exergy analysis, will be useful for engineers and thermal designers.

Section snippets

Description of the geothermal power plant

The process flow diagram of Kalina cycle (KCS 11) that is used in the enhanced geothermal power plant is shown in Fig. 1. As shown in the figure, the system consists of an evaporator, a separator, a turbine-generator, high and low temperature recuperators (HTR and LTR), a recirculation pump, a throttling valve, a condenser and a mixer. In the first stage, the ammonia–water mixture is heated in the HTR and also in the evaporator. Then the rich ammonia vapor is separated in the separator and sent

Conventional exergy analyses

By considering the mentioned assumptions, the mass, energy and exergy balances for the system components as control volumes can be written as:ṁi=ṁeQ̇-Ẇ=ṁehe-ṁihiĖQ-Ẇ=ṁeee-ṁiei+ĖD

In these equations, Q̇ is the rate of heat transfer to control volume, Ẇ is the rate of work leaving control volume, ṁehe is the rate of enthalpy leaving control volume, ṁihi is the rate of enthalpy entering control volume, ṁeee is the rate of exergy leaving control volume, ṁiei is the rate of

Results and discussion

The input data used in the present work are those considered by Rodríguez et al. [9] who analyzed the Kalina cycle proposed for an enhanced geothermal system in Brazil. These input data are considered for both the conventional and advanced exergy analyses in the present work. In the analysis, the ammonia percentage of the working fluid and the evaporation pressure are chosen as the optimal values reported by Rodríguez et al. [9]. A detailed model validation is performed using the data reported

Conclusion

In the present work, conventional and advanced exergy analyses are carried out for a Kalina cycle using a low temperature enhanced geothermal source. The main results can be listed as follows:

  • Conventional exergy analysis shows that the highest exergy destruction occurs in the condenser, followed by the evaporator, the turbine, the LTR and the HTR. However, the advanced exergy analysis suggests that the first, second and third priority of improvement should be given to the condenser, the turbine

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