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Energies 2020, 13, 5846 2 of 23 1. Introduction The rising interest in small-scale organic Rankine cycle (ORC) plants for waste heat recovery (WHR) purposes, in applications characterized by time variable conditions associated with both the heat source and heat sink, raises the issue of off-design plant operation [1–3]. Whereas extensive literature deals with proving ORC thermodynamic feasibility in various WHR scenarios [4–7], the optimum plant design is relatively unexplored, based on the analysis of the plant hydraulics [8,9], i.e., the performance sensitivity to the (i) actual fluid charge in the plant [10]; (ii) revolution speed of the pump and expander [11,12]; (iii) lower and upper cycle pressures [13]; and (iv) heat exchangers behavior at part load conditions [14]. In general, ORC plant modeling considers user-imposed variables for the operating parameters, such as temperatures—superheating degree [15], subcooling degree [12–16], approach point temperatures [13]—and pressures, or a combination of temperatures and pressures [14], resulting in predictions that can deviate from real plant operation, which can vary significantly within a given envelope. Moreover, the possibility of actually implementing conditions close to such assumptions usually requires an increased plant complexity: a dedicated control strategy for pressure optimization [13], as well as the need to rely on rotary proper equipment if dynamic machines [14–16] or heat exchangers [17,18] highly customized for the specific application are used, rather than on-the-shelf technology. However, this means higher costs, reduced fail-safe, and not necessarily an extended range of the optimum operation of the plant. On the other hand, for small-scale power plants, the choice of rotary (scroll, sliding vane, screw) and linear (piston) volumetric expanders instead of dynamic machines ensures better facing the unsteady operating conditions, making the whole plant more reliable and robust [19,20]. Moreover, positive-displacement machines can operate at low operational speed, hence ensuring a less complex and expensive solution than that involving the scaling down of turbomachines [21]. As discussed extensively in Ref. [9] in the context of controlling ORC plant operation, the revolution speed of the rotary equipment is a same-rank variable as the working fluid properties or the characteristics of the heat source and sink. This is particularly critical in the case of both the pump and the expander when these are volumetric machines. With reference to the pump, the revolution speed directly controls the processed mass flow rate of the working fluid for each specific condition at the heat sink. At the expander, the revolution speed determines the expander permeability and the intake pressure. As a matter of fact, a significant breakthrough in ORC plant design and optimization should be sought in a less constrained off-design analysis, with a deeper investigation of the effects of the system’s inertia in transient operation [22], optimum selection of the rotational speed for the expander and pump [11], and an optimum working fluid mass flow rate for each operating condition, which, in turn, reflects on the limited operating pressures of the plant. Another fundamental aspect is the working fluid selection, and to this aim, molecular-based models were developed to predict the thermodynamic properties of organic working fluids [23]. Moreover, the use of working fluid mixtures ensures enhancement of the thermodynamic performance relative to pure fluids [24–26]. In order to meet the expectations of such a novel approach, which aims to develop a detailed representation of the ORC plant integrating both plant and cycle analysis, the research mainly focused on the in-depth modeling of the heat exchangers for various cycle layouts to allow the calculation of the working fluid mass at both the design point and off-design conditions [27–30]. On the other hand, very little has been investigated, to date, on the plant hydraulic behavior and in particular on the plant permeability, which is defined as the attitude of the circuit to allow the fluid to pass through. This property was found to be fundamental in the relation between the mass flow rate circulating inside the plant and the maximum pressure [9]. The plant permeability is basically determined by the volumetric expander, which according to its volumetric efficiency sets the intake pressure for a given mass flow rate sent by the pump and when it is rotating at a certain speed [9]. Consequently, the dependence between the volumetric efficiency, leakages, and rotational speed is worth investigating, particularly in small-scale ORC units for WHR from unsteady thermal sources.PDF Image | Design of ORC Plant for Low-Grade Waste Heat Recovery
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