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difference between the two fluids. In an ORC, each heat exchanger can be subdivided into 3 zones : liquid, twophase, and vapor. The temperature profiles in the heat exchangers (Figure 10) illustrate a point where the temperature difference is minimal. This point is a fundamental parameter for designing a practical ORC and is called the pinch point. The value of the pinch must always be positive, in order to make the heat exchange possible. A small pinch corresponds to a very “difficult” heat transfer and therefore requires more heat exchange area. A null pinch corresponds to an infinite exchange area. When sizing an installation, the choice of the pinch results of an economical optimization : ● A small pinch increases the performance of the heat exchangers, leading to a higher heat power in the evaporator and to a lower saturation temperature in the condenser ● A high pinch corresponds to smaller and thus less expensive heat exchangers. In refrigeration, a rule of good practice states that the value of the pinch should be around 5 to 10K to reach an economical optimum. In ORC applications, the value depends strongly on the configuration of the system and on the heat sink/source temperatures. The pinch point leads to an important limitation in ORC's by not allowing the heat source temperature to be lowered far below the evaporating temperature. For example, in Figure 10, one may believe that, since the refrigerant enters the evaporator at a temperature of about 25°C, the hot fluid can be cooled down to a temperature close to that value. The pinch point limitation shows that this is not possible : the hot fluid is cooled down to a temperature of about 90°C. In order to cool the hot fluid down to a lower value, the evaporating temperature of the cycle should be decreased, leading to a decreased cycle efficiency. The same limitation is stated in the condenser : the cold stream cannot be heated up to the temperature of the fluid leaving the expander. 6.5 Understanding the behavior of the ORC This section analyses the particular case of an ORC using volumetric (positive displacement) pump and expander (for example a piston pump and scroll expander). The aim is to understand how specific parameters of the cycle (overheating, pressures, etc.) can be adjusted by varying the working conditions. 1. The mass flow rate. Since the pump is a positive displacement machine, is imposes the volume flow rate. Since the fluid is in liquid state at the pump supply, the fluid is incompressible and the mass flow rate is also determined by the pump. It can be adjusted by modifying the swept volume of the pump or varying its rotational speed. 2. The evaporating pressure The expander being a positive displacement machine, the volume of fluid it absorbs at each revolution is fixed. This volume is called the swept volume. For a given rotating speed, the volume flow rate is also fixed and is given by : 17PDF Image | An introduction to thermodynamics applied to Organic Rankine Cycles
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