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Energies 2020, 13, 5846 3 of 23 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. However, once the optimum pressure is calculated, it must be reached in a real plant, and this depends on a complex equilibrium between the permeability of the plant and the working fluid flow rate. The first parameter depends not only on many design variables (pipe diameter and length, circuit layout, presence of the reservoirs, dimension of the pump and expander, heat exchanger design, valve positions, etc.) but also on many operating conditions (revolution speed of the expander and of the pump, flow rate which calls for the enthalpy equilibrium at the heat recovery vapour generator (HRVG), valve operation, etc.) and some sub-design parameters (angular position of the inlet and exit ports of the expander, built-in volume ratio of the expander and on the pump, etc.). Indeed, all these parameters/variables determine the mass aspirated by the expander, setting the pressure of the working fluid at the expander intake and consequently the plant maximum pressure [9]. All these aspects should be managed in order to set up a desired maximum pressure on the plant equal to that determined by a thermodynamic analysis for a given mass flow rate circulating inside the plant. In general, volumetric machines can achieve a higher rate of operational flexibility [33] in terms of high expansion ratios if piston expanders are used [34–36], as well as good reliability and good off-design performance in the case of sliding-vane machines being employed [31,37]. According to extensive literature on the drivers that define plant operating pressure [31,38,39], it can be assured that this parameter is fixed by the expander: the flow leakages associated with normal operation play a significant role in determining the optimal cycle parameters. Particularly with sliding vane technology, a significant incidence of internal leakage on the machine energy and hydraulic performance must be expected, i.e., a loss of available work for a reduced mass flow rate, actually expanding and reduced volumetric efficiency [39]. As the flow leakage gets larger, the matching point between the hydraulic characteristics of the circuit and the pump shifts toward larger required mass flow rates to compensate for the pressure loss at the expander inlet. As a result, when the plant is conceived to operate with a constant upper pressure, an additional pump power consumption is expected with respect to design conditions and eventually a high backwork ratio (BWR) applies, which is the ratio between the work consumed by the pump and the work available at the expander shaft, and it could end up questioning the plant’s feasibility. In a complementary sliding pressure scenario, some variability on the expander inlet pressure is accepted: no additional pumping work is required, but the plant operates mostly under off-design conditions with obvious performance penalties. On the other hand, the discharge pressure is fixed by the condenser condition, thus defining the pressure ratio across the expander [40,41]. All these real plant features introduce complexities that should be taken into account in the off-design optimization and call for the setting up of a dedicated optimization strategy. In order to develop a predictive computational design tool for the optimum design of a small-scale ORC plant, a comprehensive model of the plant is developed in the present paper. The research work deals with the experimental validation of the comprehensive model based on data collected from an extensive experimental campaign performed on an ORC plant for WHR on the exhaust gases of a light-duty IVECO F1C 3-L supercharged diesel engine with R236fa as the working fluid. During the experimental and numerical analysis, the intrinsic variability of both plant operating parameters (e.g., working fluid mass flow rate) and driving conditions (e.g., internal combustion engine (ICE) speed and torque) were varied in order to induce a continuous off-design operation of the bottoming ORC plant. Once validated, the model allows us to analyze the ORC plant, reproducing its real layout, taking into account in particular the real pipe distribution, the volume of the components, and the mass of the working fluid charge. Therefore, it allows performing a plant analysis via the predictionPDF Image | Design of ORC Plant for Low-Grade Waste Heat Recovery
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