Analisis exergetico plantas binarias

ARTICLE IN PRESS
Renewable Energy xxx (2009) 1–10

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Exergetic analysis of various types of geothermal power plants
Mortaza Yari
Department of Mechanical Engineering, Faculty of Engineering, University of Mohaghegh Ardabili, Ardabil 179, Iran

a r t i c l e i n f o
Articlehistory: Received 14 September 2008 Accepted 29 July 2009 Available online xxx Keywords: Exergy Second-law efficiency First-law efficiency Geothermal power plant Organic Rankine cycle Single-flash

a b s t r a c t
Based on available surveys, it has been shown that Iran has substantial geothermal potential in the north and north-western provinces, where in some places the temperature reaches 240  C.In order to better exploit these renewable resources, it is necessary to study this area. Thus, the aim of this paper is a comparative study of the different geothermal power plant concepts, based on the exergy analysis for high-temperature geothermal resources. The considered cycles for this study are a binary geothermal power plant using a simple organic Rankine cycle (ORC), a binary geothermalpower plant using an ORC with an internal heat exchanger (IHE), a binary cycle with a regenerative ORC, a binary cycle with a regenerative ORC with an IHE, a single-flash geothermal power plant, a double-flash geothermal power plant and a combined flash-binary power plant. With respect to each cycle, a thermodynamic model had to be developed. Model validation was undertaken using available data fromthe literature. Based on the exergy analysis, a comparative study was done to clarify the best cycle configuration. The performance of each cycle has been discussed in terms of the second-law efficiency, exergy destruction rate, and first-law efficiency. Comparisons between the different geothermal power plant concepts as well as many approaches to define efficiencies have been presented. The maximumfirst-law efficiency was found to be related to the ORC with an IHE with R123 as the working fluid and was calculated to be 7.65%. In contrast, the first-law efficiency based on the energy input into the ORC revealed that the binary cycle with the regenerative ORC with an IHE and R123 as the working fluid has the highest efficiency (15.35%). Also, the maximum first-law efficiency was shown to be given by theflash-binary with R123 as the working fluid and was calculated to be 11.81%. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Geothermal energy is going to be an attractive energy source due to rising oil prices and environmental pollution concerns. Since the price of oil has reached its peak and efforts are necessary to find alternative energy resources, geothermal energy is more competitivewhen compared to conventional fossil fuel systems and direct use of geothermal energy has increased approximately twofold in the last five years [1]. Three major types of power plants are operating today: drysteam plants, flash-steam plants and binary-cycle plants where binary and combined flash/binary plants are relatively new designs. Geothermal energy is used to generate electricity and fordirect uses such as space heating and cooling, industrial processes, and greenhouse heating. High-temperature geothermal resources above 150  C are generally used for power generation. Moderatetemperature (between 90 and 150  C) and lower-temperature

E-mail addresses: myari@uma.ac.ir, myaari@yahoo.com 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2009.07.023

(below 90  C) geothermal resources are best suited for direct uses [2]. Exergy analysis is vital in designing, optimizing and modeling these kinds of cycles. Exergy analysis is now a mature methodology that accounts for the system’s inefficiency in terms of exergy destruction, i.e., the degradation of the system’s ability to perform work with respect to its surroundings [3]....