Experimental and Numerical Characterization of the Sliding Rotary Vane Expander Intake Pressure in Order to Develop a Novel Control-Diagnostic Procedure

Abstract

Waste heat recovery via Organic Rankine Cycle (ORC)-based power units represents one of the most promising solutions to counteract the effects of CO2 emissions on climate change. Nevertheless, several aspects are still limiting its development on the on-the-road transportation sector. Among these aspects, the significant variations of the conditions of the hot source (exhaust gases) are a crucial point. Therefore, the components of the ORC-based unit operate far from the design point if the main operating parameters of the plant are not suitably controlled. The maximum pressure of the cycle is one of the most important variables to be controlled for the importance it has on the effectiveness of the recovery and on safety of operation. In this paper, a wide experimental and theoretical activity was performed in order to define the operating parameters that mostly affect the maximum pressure of the recovery unit. The results showed that the mass flow rate provided by the pump and the expander volumetric efficiency were the main drivers that affect the plant maximum pressure. Subsequently, through a validated model of the expander, a diagnostic map was outlined to evaluate if the expander and, consequently, the whole plant were properly working.

Design and Operational Control Strategy for Optimum Off-Design Performance of an ORC Plant for Low-Grade Waste Heat Recovery

“…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][2][3]. Whereas extensive literature deals with proving ORC thermodynamic feasibility in various WHR scenarios [4][5][6][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].…”

“…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.…”

“…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].…”

“…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.…”

“…It has been already observed [9,31,32] that real performance can be quite different from the idealized one evaluated by means of thermodynamic analysis without considering typical real plant features such as hydraulic permeability and the effect of expander leakages on the whole performance. From it, in fact, having fixed a specific cycle design for optimal operating conditions, the main relevant variables can be calculated in order to fulfill the desired goal: maximum efficiency, maximum thermal energy recovered, and maximum specific work, after which the optimum pressure or superheating degree can be easily determined.…”