ARTICLE IN PRESS
Renewable Energy xxx (2009) 1–10
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Exergetic analysis of various types of geothermal power plants
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 . 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: firstname.lastname@example.org, email@example.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 . 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 ....
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