Fluid selection for the Organic Rankine Cycle (ORC) in biomass power and heat plants

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Fluid selection for the Organic Rankine Cycle (ORC) in biomass power and heat plants ( fluid-selection-organic-rankine-cycle-orc-biomass-power-and- )

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226 U. Drescher, D. Bru ̈ggemann / Applied Thermal Engineering 27 (2007) 223–228 Table 2 Fig. 4. Schematic T, S-diagram illustrating the decision pattern for the interaction between vapor pressure and process parameters. The filled rhomb marks the turbine inlet state without superheating and the triangle with superheating, each at 2 MPa. VP = vapor pressure, con = condensation. Input and constraints of the screening Parameter Melting temperature Maximum process temperature Condenser pressure Condenser temperature Dryness at turbine outlet Efficiency of internal heat exchanger Minimum DT of internal heat exchanger (T8–T9) Turbine efficiency Feed pump efficiency Maximum process pressure Value or constraint <275 K 0.9 0.95 10K 0.8 0.8 62 MPa or vapor pressure and isopentane. Aside from the critical point, relative dis- crepancies were found to be less than ±3% for calculated gas and liquid heat capacity and below ±1.5% for cycle efficiency. 4. Screening of fluids Many parameters have to be considered for fluid selec- tion and ORC thermal efficiency calculation (Table 2). The two main parameters are maximum and minimum process temperature. The upper limit of the maximum pro- cess temperature is the fluid stability and material compat- ibility. These are difficult to evaluate. Typical maximum temperature for existing biomass ORC is about 573 K. Thus, we analyzed this temperature in detail. In addition, maximum process temperature was varied from 523 to 623 K, aware that some fluids might not be adequate for this temperature range. The minimum temperature was set to 363 K. If necessary, the condenser temperature was raised to make the vapor pressure equal 5000 Pa, the lowest pressure accepted for the condenser (Fig. 4). The melting temperature should be below ambient tem- perature. Otherwise the fluid may solidify during shutdown time. For this study, the turbine efficiency was fixed at 80%, which is in the typical range from 75% [14] up to 85% [15]. The maximum process pressure was limited to 2 MPa to reduce safety measures and material expenses [16]. If the vapor pressure at maximum process temperature was lower than 2 MPa, the fluid was expanded directly from dew line. Otherwise, the fluid was superheated (Fig. 4). In the rare case of maximum process temperature and pressure being in the supercritical region of a fluid, the maximum process pressure was reduced by 0.1 MPa below critical pressure and the fluid was superheated. If the adjusted process pressure was below 0.3 MPa, the fluid was excluded, because the pressure ratio in the turbine was too low for efficient power generation. 5. Results About 700 substances of the DIPPR database passed the preselection described above and were included in the sub- sequent comparison. Fig. 5 shows the number of substances which reach given efficiency values with or without internal heat exchanger. The implementation of a recuperator increases efficiency considerably. The efficiency of the 100 best-suited fluids ranges from 25.4% to 24.3%. Thus, many fluids with similar thermal efficiencies are available for considering further criterions. As expected, there is a strong relation between efficiency and vaporization temperature (Fig. 6(a)). The highest effi- ciencies were obtained when superheating was avoided and the fluid was expanded directly from dew line. According to Fig. 6(b), the efficiency rises to approxi- mately 25% at 1 MPa with a slight decrease for higher pressures. In accordance with the law of corresponding states, flu- ids with low maximum process pressure at maximum Fig. 5. Number of substances exceeding the efficiency g in ORC processes with Tmax =573 K and Tmin =363 K.

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