Effect of reference state on the performance of energy and exergy evaluation of geothermal district heating systems: Balcova example

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Abstract

In this paper, we undertake a parametric study to investigate how varying reference temperature from 0 to 25 °C will affect the energy and exergy efficiencies of the Balcova geothermal district heating system (BGDHS) and develop two significant correlations (with a correlation coefficient of 0.99) that can be used for predicting the efficiencies. The exergy losses in the overall BGDHS are quantified and illustrated using exergy flow diagram particularly for a reference temperature of 11.4 °C for comparison purposes. This reference temperature is taken as an average value of the ambient temperatures measured during the past 5 years for the day of 2nd January to reflect the actual situation. The results show that the exergy losses within the system occur mainly due to the losses in pumps, heat exchangers, reinjection sections of the geothermal water back into reservoir and pipeline, and account for 1.75%, 8.84%, 14.20%, and 28.69%, respectively. In addition, we study energy and exergy efficiencies to determine the possibilities to improve the system, and energy and exergy efficiencies of the system are found to be 42.36% and 46.55%, respectively, for 2nd January 2004.

Introduction

Geothermal energy appears to be a potential solution to energy and environmental problems, where it is available, and a key tool for betterment of society and its sustainable future. The other advantage is that geothermal energy systems are simple, safe and adaptable (with modular up to about 50 MW). Due to their advantages, they are given priority in both developed and developing countries as an attractive option to replace fossil fuels [1].

Some energy and exergy values are dependent on the intensive properties of the reference state. Consequently, the results of energy and exergy analyses generally are sensitive to variations in these properties. Before energy and exergy analyses can be applied with confidence to energy systems, e.g., geothermal district heating systems, the significance of the sensitivities of energy- and exergy-analysis results in reasonable variations in dead-state properties must be assessed. Only very brief discussions of these sensitivities have been reported (e.g., [2], [3], [4]). The authors believe that one requirement for continued acceptance of exergy analysis is that the effects on the results of exergy analyses of variations in dead-state properties be fully examined and understood.

Based on the studies on the energetic and exergetic aspects of geothermal systems appeared in the open literature, we can classify them in three groups as follows: (i) exergy analysis of geothermal power plants [5], [6], [7], [8], [9] (ii) evaluation of geothermal fields using exergy analysis [10], [11], [12], and (iii) classification of geothermal resources by exergy [12]. The concept of exergy was first used to analyze a geothermal power plant by Badvarsson and Eggers [5]. Their illustrative example compared the performances of single and double flash cycles based on a reservoir water temperature of 250 °C and a sink condition of 40 °C with the exergy efficiencies of 38.7% and 49%, respectively [12]. In addition, numerous studies have been undertaken from fundamental studies to energy and exergy analyses of geothermal and thermal systems in general [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] and more recently, Ozgener et al. [14], [15], [16] and Ozgener [17] have conducted an exergy analysis of two local geothermal district heating systems in Turkey.

To the best of the authors’ knowledge, no examination of the effects of the reference state on the efficiencies of the geothermal district heating systems has appeared in the literature. This was the motivation behind the present work. The authors undertook a preliminary study [16] on energy and exergy analyses of the Balcova geothermal district heating system at a reference temperature of 13.1 °C only. Here, we now present an original and more practical version of the energy and exergy analysis of the system at a more realistic reference temperature of 11.4 °C as the actual average local temperature, rather than the standard environment temperature and investigate the energy and exergy losses in the system at this particular practical temperature. Furthermore, we study the effects of varying reference state temperature from 0 to 25 °C on the energy and exergy efficiencies of the system and develop some practical correlations that are capable of predicting efficiencies at various ambient temperatures.

Section snippets

Case study

Fig. 1 illustrates a schematic diagram of the Balcova geothermal district heating system in Izmir, Turkey, including hotels and official buildings as heated by the system. It is situated on a total area of about 3.5km2 with an average thickness of the aquifer horizon of 150 m. There is no feeding, and only 25% of the fluid contained in the reservoir is utilized. A maximum yield of the field is about 205 kg/s at a reservoir temperature of 118 °C. The BGDHS consists mainly of three circuits, e.g.,

Modeling

Here, Table 1 shows the general mass, energy and exergy balance equations to find the energy and exergy inputs and outputs, the rate of exergy decrease, the rate of irreversibility, and the energy and exergy efficiencies.

The balance equations are written for mass, energy and exergy flows in the system and its components as they are considered steady-state steady-flow control volume systems, and the appropriate energy and exergy equations are derived for this system and its components. The heat

Results and discussion

The temperature, pressure, and mass flow rate data for both thermal water and water are given in accordance with their state numbers as specified in Fig. 1. The exergy rates are calculated for each state as listed in Table 2. The data were obtained from the measured values in the system and they were converted into SI units; therefore, pressure values were determined precisely in Table 2. Note that here state 0 indicates the reference state for both thermal and water. Some investigators (e.g.,

Conclusions

We conduct an energy and exergy analysis of the geothermal district heating systems and apply to the BGDHS in Izmir, Turkey as a case study at a reference temperature of 11.4 °C. We also utilize actual thermal data taken from the system to perform a system performance assessment through energy and exergy efficiencies. Moreover, the exergy destructions (showing the losses) in the overall system are quantified and illustrated using an exergy flow diagram along with an energy flow diagram. Two

Acknowledgements

The authors gratefully acknowledge the support provided for this work by the Izmir-Balcova Geothermal Inc. (IBGI) and personal support by its general manager, Mr. Fasih Kutluay.

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