Alternative ORC bottoming cycles FOR combined cycle power plants
Introduction
Combined cycles comprise a topping cycle with high maximum temperature and a bottoming cycle with low or intermediate maximum temperature. For power production with gas turbine based combined cycles, virtually all bottoming cycles are Rankine cycles with steam due to very attractive features such as good thermal integration with the topping gas turbine cycle, high reliability and considerable past industry experience. In the same category, the Kalina cycle [1], [2] using a zeotropic mixture of ammonia and water has also been proposed though it has not reached commercial success yet. This cycle presents a very interesting evaporation process at variable temperature that allows for a more efficient heat recovery process from the topping cycle.
In the low temperature range, bottoming Organic Rankine Cycles (ORC) constitute another alternative, having shown good thermodynamic performance for low maximum temperature bottoming cycles [3], [4], [5]. This interest in organic working fluids for low temperature Rankine cycles is not new and it has been proposed for different applications: renewable energy and low temperature heat recovery [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Moreover, small scale ORC power plants are presently commercially available [11], [12], [13].
ORCs bottoming cycles in combined power plants have been proposed previously by Najjar [17], who analyzed a combination of ORC fluids and cycle layouts that resulted in a global combined cycle efficiency slightly below 45.2%, by Chacartegui et al. [18] for intermediate temperature thermosolar power plants with a carbon dioxide topping cycle, and by Invernizzi et al. [19], Caresana et al. [20] and Yari [21] for microturbine combined cycles. Despite these numerous works, a careful review in this subject shows that combined cycles comprising modern high efficiency gas turbines, like recuperative gas turbines, and ORCs in the medium and large scale power generation have not been analyzed carefully previously. Thus, the use of ORC bottoming cycles incorporated into the exhaust of recuperated gas turbines or very high pressure ratio gas turbines, which are characterized by their very high efficiency but low exhaust temperature (TET), are studied in this work. As shown later, this combination allows for high efficiencies to be achieved, which are similar to that of modern gas and steam combined cycles but apply to topping cycles with lower TITs.
The document has been organized in four main parts: first, an analysis of the main characteristics of ORC cycles; second, the study of the integration of some commercial gas turbines with different bottoming ORC cycles; third, a parametric optimization of the combined cycle to improve its global efficiency and, finally, a summary of economic considerations related to the use of ORCs in a combined cycle.
Section snippets
ORC cycles. Preliminary considerations
The aforementioned works include analyses that focus on fluid selection and optimization of cycle layout. For the former, a great number of organic fluids have been studied [6], [8], searching features like high boiling temperatures that increase cycle efficiency [7], [14]: propane [4], [14], n-pentane[4], n-butane [4], [5], [10], [14], n-pentane–n-butane mixtures [4], siloxanes mixtures [4], toluene [5], [7], cyclohexane [5], ammonia–water mixtures [5], [9], Benzene [7], p-Xylene [7] and
Low temperature bottoming cycles with commercial gas turbines
In this section, combined cycles that use commercially available gas turbines and ORC bottoming cycles are analyzed. The purpose of such analysis is to evaluate the interest of the proposed bottoming when integrated with ordinary commercial gas turbines. For the ORC cycle, the scheme shown in Fig. 1 is considered. For the topping cycle, the following gas turbine engines, whose main characteristics are shown in Table 4 [27], [28], [33], are evaluated:
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Four modern heavy duty gas turbines: GE
Preliminary considerations
This Section continues the previous analysis where the interest of combining low temperature bottoming cycles with low exhaust temperature gas turbines has been shown. A parametric optimization of the bottoming cycle depending on the turbine inlet temperature of the topping cycle is now presented. It must be noted that the gas turbines used in this Section do not necessarily correspond to any of the engines listed previously since their working cycle, i.e. operating parameters, are subjected to
Economic considerations
The previous thermodynamic analysis and optimization must now be completed with some considerations regarding the costs of ORC combined cycles. The objective of this economic analysis is to mark break-even costs for the development of this ORCRCC technology, i.e. costs that make this technology competitive, either at system (combined cycle) or subsystem (ORC and gas turbine) levels.
Estimating the costs of these bottoming cycles in comparison to conventional steam cycles, requires an evaluation
Conclusions
The main conclusions drawn from this work are the following:
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The analysis of combined cycles based on commercial gas turbine data and ORCs, Section 3, shows that ORCs are an interesting and competitive option when combined with high efficiency gas turbines with low exhaust temperatures. Among the fluids analyzed, toluene and cyclohexane ORCCCs present the highest global efficiencies.
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These high efficiency turbines with low exhaust temperatures to be used in combined cycles with ORC bottoming
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