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UA Q (15) Tlm In the above equations Δhexp,is is the isentropic specific enthalpy drop in the turbine, Vexp,out and are the volume flow rates of the working fluid at the turbine inlet and outlet respectively, Q is the heat flux of the heat exchanger and ΔTlm is the logarithmic mean temperature difference Vexp,out (LMTD) between the heat exchange fluid streams. The first three parameters concern the operational applicability of the turbine and are related its cost. Extremely high rotor speeds may result in unrealistic or costly turbine design (involving expensive magnetic bearings for the turbine shaft), while increased VFR and SP values are associated with larger turbine size and technical complexity. More specifically, the size parameter, introduced by Macchi et al. [29] as a key efficiency optimization parameter for turbines operating with non conventional fluids, influences the turbine’s actual dimensions (such as blade height). Furthermore, the VFR is inversely correlated with the isentropic efficiency of the expansion. It has been reported that for achieving, for instance, isentropic efficiencies higher than 80%, the VFR value must be lower than 50 [30]. The UA value of a heat exchange process is indicative of the heat transfer equipment required. A high UA value means that more heat needs to be transferred between the heat exchange streams for the desired temperature increase/decrease to be accomplished, with direct implications on the total surface of the heat exchanger and its cost. 2.2. Assumptions 2.2.1. Heat source The thermodynamic simulations are carried out for three heat source inlet temperatures of 150, 225 and 300 oC, representative of low, moderate and high temperature levels of potential waste heat. Since the WHR source is modelled as dry atmospheric air, there are no practical limitations on the minimum temperature of the heat source stream at the heat exchanger outlet (due to condensation or other thermochemical phenomena). In principle this is not the case for waste heat recovery applications. For example, when the heat source stream consists of flue gases or geothermal water, it often cannot be cooled below a certain temperature because of issues such as corrosion of the heat exchange surfaces and other equipment. The characteristics of the heat source stream of the present work are summarized in Table 1. Table 1. Heat source characteristics for the application Mass flow Temperature Pressure Composition 2.2.2. ORC 5 kg/s 150, 225, 300 oC 2 bar 78 % N2, 21 % O2, 1 % Ar (mole fraction) One of the main parameters of ORC systems is the pinch point of the heat exchangers, which is defined as the minimum temperature difference between the hot and cold fluid streams during the heat exchange in the heater and the condenser. Smaller pinch point values lead to higher heat exchange efficiency, since the amount of heat transferred is higher. On the other hand, due to the limited LMTD between the heat exchange streams, a larger heat exchanger surface is may be required (15). Additional parameters of ORCs include the isentropic efficiency of the pump and the expander. The isentropic efficiencies indicate the deviation of the actual compression/expansion that takes place from the theoretical isentropic process. In the case of the WHR-ORC investigated, they are given by the equations: 8PDF Image | Thermodynamic investigation of waste heat recovery
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