Alternative ORC bottoming cycles FOR combined cycle power plants

Abstract

In this work, low temperature Organic Rankine Cycles are studied as bottoming cycle in medium and large scale combined cycle power plants. The analysis aims to show the interest of using these alternative cycles with high efficiency heavy duty gas turbines, for example recuperative gas turbines with lower gas turbine exhaust temperatures than in conventional combined cycle gas turbines. The following organic fluids have been considered: R113, R245, isobutene, toluene, cyclohexane and isopentane. Competitive results have been obtained for toluene and cyclohexane ORC combined cycles, with reasonably high global efficiencies.

The paper is structured in four main parts. A review of combined cycle and ORC cycle technologies is presented, followed by a thermodynamic analysis of combined cycles with commercial gas turbines and ORC low temperature bottoming cycles. Then, a parametric optimization of an ORC combined cycle plant is performed in order to achieve a better integration between these two technologies. Finally, some economic considerations related to the use of ORC in combined cycles are discussed.

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.