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Chapter 3 Abstract A critical component in designing efficient ORC plants is the expander, which is typically a turbine. The variety of possible working fluids, the complex gas dynamic phenomena encountered, and the lack of simplified design methods based on previous experience on similar machines make the design of efficient ORC turbines a complicated task. Relevant paths of research may thus be concerned with (i) the development of generalized design methodologies, and (ii) the assessment of non-conventional machine architectures: this chapter explores both. In particular, the first critical evaluation of the centrifugal or radial-outflow turbine (ROT) architecture as a candidate technology for ORC turbo-generators is presented. In the first part of the chapter, starting from basic turbomachinery theory, all the special fea- tures involved in ROTs design are enlightened. The main findings being that, in order to design efficient centrifugal turbines, particularly for low power output applications, it is needed that (i) the blade discharge geometric angles, the radial chords, the stage expansion ratios, and the re- action degrees are allowed to vary among each cascade, and (ii) the diameter and the speed of revolution are included among the optimization variables. It is discussed how simplifying assump- tions usually adopted in the axial turbines practice are typically not applicable. A novel design methodology is derived and presented for the preliminary sizing of ROTs in the power size range from several MWE down to few kWE, i.e., covering most of the applications foreseen today. An original in-house mean-line code coupled to an external optimizer is developed, which allows to determine the preliminary design of ORC turbines of various configurations and working with dif- ferent fluids. This tool, named zTurbo, is adopted to verify the novel method by presenting several exemplary design exercises. The second part of the chapter deals with the design of centrifugal machines with 1 MWE power output, handling expansion ratios of the order of 60, and rotating at 3000 rpm, thus representative of present industrial applications. Several simplifications derived from the axial-turbines practice are adopted in order to illustrate their consequences. The design of two different turbines is per- formed with zTurbo, a transonic six-stage and a supersonic three-stage ones. It is confirmed that the adopted simplifications lead to unwanted design features, such as converging meridional channels and large flaring angles on the last stages. The predicted fluid-dynamic efficiency for the transonic and the supersonic machine is around 86% and 81%, respectively. The third part of the chapter focuses on the assessment of the down-scaling potential of the ROT architecture, considering its implementation in the promising field of mini-ORC turbogenerators. The novel design methodology is applied to the sizing of two 10 kWE ROTs, handling an expansion ratio of 45: a 5 stages transonic, and a 3 stages slightly supersonic ones. The proposed design procedure proves valuable in sizing machines with a meridional channel which monotonically di- verges maintaining maximum flaring angles lower than 10◦. The resulting turbines are projected to exceed a fluid-dynamic efficiency of 79% and 77%, with speed of revolution around 124000 and 15400 rpm, respectively. The results show that the ROT architecture is a promising concept for future ORC power sys- tems, which allows for the realization of efficient, compact, and reliable expanders down to a power output of few kWE. However, the design of these machines presents several criticality which are unveiled here for the first time. 3.1 Introduction As anticipated in Ch. 2, ORC power systems have been demonstrated to be advantageous compared to steam Rankine cycles for a number of applications: this is mainly a consequence of the increase of specific cost of turbomachinery as the scale of the plant reduces. The use of organic fluids, char- 58PDF Image | New Concepts FOR Organic Rankine Cycle Power Systems
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