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The main differences between ORCs and steam cycles are the following: Superheating. As previously stated, organic fluids usually remain superheated at the end of the expansion. Therefore, there is no need for superheating in ORC cycles, contrary to steam cycles. The absence of condensation also reduces the risk of corrosion on the turbine blades, and extends its lifetime to 30 years instead of 15–20 years for steam turbines [14]. Low temperature heat recovery. Due to the lower boiling point of a properly selected organic working fluid, heat can be recovered at much lower temperature (e.g. with geothermal sources). Components size. In a steam cycle, the fluid density is extremely low in the low-pressure part of the cycle. Since pressure drops increase with the square of the fluid velocity, the high volume flow rate necessitates an increase in the hydraulic diameter of the piping and the size of the heat exchangers. Similarly, the turbine size is roughly proportional to the volume flow rate. Boiler design. ORC cycles enable the use of once-trough boilers, which avoids steam drums and recirculation. This is due to the relatively smaller density difference between vapor and liquid for high molecular weight organic fluids. In contrast, the low vapor density in steam boilers can generate very different heat transfer and pressure drop characteristics between liquid water and steam. Complete steam evaporation in a single tube must therefore be avoided. Turbine inlet temperature. In steam Rankine cycles, due to the superheating constraint, a temperature higher than 450 1C is required at the turbine inlet to avoid droplets formation during the expansion. This leads to higher thermal stresses in the boiler and on the turbine blades and to a higher cost. Pump consumption. Pump consumption is proportional to the liquid volume flow rate and to the pressure difference between outlet and inlet. It can be expressed in terms of the Back Work Ratio (BWR), which is defined as the pump consumption divided by the turbine output power. In a steam Rankine cycle, the water flow rate is relatively low and the BWR is typically 0.4%. For a high temperature ORC using toluene, the typical value is 2–3%. For a low temperature ORC using HFC-134a, values higher than 10% are typical. Generally speaking, the lower the critical temperature, the higher the BWR. High pressure. In a steam cycle, pressures of about 60–70 bar and thermal stresses increase the complexity and the cost of the steam boiler. In an ORC, pressure generally does not exceed 30 bar. Moreover, the working fluid is not evaporated directly by the heat source (e.g. a biomass burner) but via an intermediary heat transfer loop. This makes the heat recovery easier since thermal oil can be at ambient pressure, and the requirement of an on-site steam boiler operator is avoided. Condensing pressure. To avoid air infiltration in the cycle, a condensing pressure higher than atmospheric pressure is advisable. Water, however, has a condensing pressure gener- ally lower than 100 mbar absolute. Low temperature organic fluids such as HFC-245fa, HCFC-123 or HFC-134a do meet this requirement. Organic fluids with a higher critical temperature on the other hand, such as hexane or toluene, are subatmo- spheric at ambient temperature. Fluid characteristics. Water as a working fluid is very convenient compared to organic fluids. Its main assets are low cost and high availability, non-toxicity, non-flammability, environmentally friendly (low Global Warming Potential and null Ozone Deplet- ing Potential), chemical stability (no working fluid deterioration in case of hot spot in the evaporator), and low viscosity (and thus Table 3 Advantages and drawbacks of each technology. S. Quoilin et al. / Renewable and Sustainable Energy Reviews 22 (2013) 168–186 175 Advantages of the ORC No superheating Lower turbine inlet temperature Compactness (higher fluid density) Lower evaporating pressure Higher condensing pressure No water-treatment system and deareator Turbine design Low temperature heat recovery, once-through boiler Advantages of the steam cycle Higher efficiency Low-cost working fluid Environmental-friendly working fluid Non-flammable, non-toxic working fluid Low pump consumption High chemical-stability working fluid lower friction losses and higher heat exchange coefficients). However, steam cycles are in general not fully tight: water is lost as a result of leaks, drainage or boiler blow-down. Therefore, a water-treatment system must be integrated with the power plant to feed the cycle with high-purity deionized water. A deaerator must also be included to avoid corrosion of metallic parts due the presence of oxygen in the cycle. Turbine design. In steam cycles, the pressure ratio and the enthalpy drop over the turbine are both very high. As a conse- quence, turbines with several expansion stages are commonly used. In ORC cycles, the enthalpy drop is much lower, and single or two-stage turbines are usually employed, entailing lower cost. Additional consequences of the lower enthalpy drop of organic fluids include lower rotating speeds and lower tip speed. A lower rotating speed allows direct drive of the electric generator without reduction gear (this is especially advantageous for low power-range plants), while the low tip speed decreases the stress on the turbine blades and simplifies their design. Efficiency. The efficiency of current high temperature Organic Rankine Cycles does not exceed 24%. Typical steam Rankine cycles show a thermal efficiency higher than 30%, but with a more complex cycle design (in terms of number of components or size). The advantages of each technology are listed in Table 3. In summary, the ORC cycle is more interesting in the low to medium power range (typically less than a few MWe), since small-scale power plants cannot afford an on-site operator, and because it requires simple and easy to manufacture components and design. It is consequently more adapted to decentralized power generation. For high power ranges, the steam cycle is generally preferred, except for low temperature heat sources [37]. 5. Working fluid selection The selection of working fluids has been treated in a large number of scientific publications. In most cases, these studies present a comparison between a set of candidate working fluids in terms of thermodynamic performance and based on a thermo- dynamic model of the cycle. When selecting the most appropriate working fluid, the following guidelines and indicators should be taken into account: (1) Thermodynamic performance: the efficiency and/or output power should be as high as possible for given heat source and heat sink temperatures. This performance depends on a number of interdependent thermodynamic properties of the working fluid: critical point, acentric factor, specific heat, density, etc. It is not straightforward to establish an optimumPDF Image | Techno-economic survey of Organic Rankine Cycle (ORC) systems
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