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200 150 100 50 0 -50 -100 R245fa T [C] 200 150 100 50 0 -50 4-086-12 JUMEL ET AL 4. UNDERTAKING HIGH IMPACT ACTIONS: TECHNOLOGY AND ... hso hsi 3 2,00E6 Pa 470000 Pa csi 4 cs5 cso 0,8 1,2 51000 Pa 0,4 0,6 1000 Pa 5 0,2 Dry Fluid Isentropic Fluid Wet Fluid 250 500 750 1000 1250 1500 1750 2000 2250 2500 s [J/kg-K] Figure 5. T-s diagram of R-245fa supercritical Rankine cycle. ronmental impact and economic viability (Chen, Goswami et al. 2010). Water is a good working fluid with numerous advan- tages (i.e. abundant, cheap, high thermal and chemical stability, low viscosity, non-toxicity, non-flammability, zero ODP, zero GWP) but it cannot economically be used for power generation from a low-grade heat source due to its relative high boiling point (99.974 °C, 101.325 kPa). For this purpose, the refriger- ants which are essentially organic compounds with low normal boiling temperature present their potentials. Many researchers have focused their study on working fluid selection for ORC system. Predominantly, the system’s thermal efficiency and second law efficiency are often used to evaluate working fluid potential. The organic working fluids are generally categorized into three groups thanks to slope of saturated vapour curve in T-s diagram (Figure 6). The dry fluid has a positive slope of satu- rated vapour curve, the wet one – negative, whereas the isen- tropic fluid features a vertical saturation curve. (Liu, Chien et al. 2004) have used x (= ds/dT) value (Equation 1) to predict working fluid classification, i.e. x > 0: dry fluid, x ~ 0: isentropic fluid and x < 0: wet fluid. (1) Where x(ds/dT) denotes the inverse of the slope of the satu- rated vapour curve on T-s diagram; TrH (TH/Tcrit) denotes the reduced evaporating temperature; DHH represents the enthalpy of fluid vaporization and the exponent n is suggested to be 0.375 or 0.380 (Poling, Prausnitz et al. 2000). The reliability of this equation was verified at the fluid’s normal boiling points by Liu, Chien et al. However, the calculations of (Chen, Goswami et al. 2010) based on the definition of the slope (ds/dT) showed that a large deviation can occur when using Equation (1) at off-normal boiling points. These authors recommended using the entropy and temperature data directly to calculate x if their values are available. Due to environmental concerns, CFC working fluids (R-11, R-12, R-113, R-114 and R-115) have been phased out at the 0 1000 2000 3000 4000 5000 6000 s [J/kg-K] n.T rH +1 Figure 6. Three types of working fluids: dry fluid, isentropic fluid and wet fluid. end of 1995 (in developed countries) and totally phased-out by 2010 in the developing countries. While HCFC fluids (such as R-21, R-22, R-123, R-124, R-141b and R142b) are being phased out in 2020 (for developed countries) and 2030 (for developing countries) (Government 2011). Many criteria of the refrigerant properties have been proposed for primary screening of work- ing fluids for Organic Rankine Cycles. Indeed, ideal fluid would have : • high enough critical point and low enough freezing point, • acceptable saturation pressure at the temperatures of the application; dry and isentropic fluid are desirable (to avoid excessive superheat in heat exchangers and condensation at the turbine exit) however if slope is too positive, the vapour leaves with substantial superheat which is waste in the con- denser; small heat vaporization and matching heat capacity, • high density, low viscosity, • high thermal conductivity, • thermal and chemical stability, • non-toxic, non-flammable, essentially non fouling, noncor- rosive, slight smell, benign to the global physical and bio- (1) logical environment etc., • easy leak detection, • low cost. The physical data and environmental data of the fluids studied in this work are encapsulated in Table 1. According to ASHRAE Standard 34 – Designation and Safety Classification of the Refrigerants, the toxicity and flammability classifications of the refrigerants are categorized in six separate safety group classifications (A1, A2, A3, B1, B2 and B3) and two subclasses (A2L and B2L) (Table 2). A rough procedure which is proposed by (Tettig, Lagler et al. 2011) with the following steps for working fluid selection: • Literature review on existing organic fluids • First selection taking into account the working temperature range (heat source and heat sink temperature) ξ=CP −1−TrH ΔH TT2H HH 562 ECEEE 2012 SUMMER STUDY on EnERgY EffiCiEnCY in inDUSTRY Hexane Ammonia T [°C] R141bPDF Image | Working Fluid selection and performance comparison of subcritical and supercritical organic Rankine cycle (ORC) for low temperature waste heat recovery
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