Comparative assessment of refrigerants and non refrigerants as working fluids for a low temperature Organic Rankine Cycle

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1 INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD , DECEMBER, Comparative assessment of refrigerants and non refrigerants as working fluids for a low temperature Organic Rankine Cycle A. Sunita V. Sanghani, B. Dr. Ragesh G. Kapadia Abstract -- The selection of working fluid in Organic Rankine Cycle (ORC) is of a great importance for better system operations, thermal efficiency and environment. A comparative assessment of refrigerants and non refrigerants as working fluid in a low temperature ORC is presented here. A program has been developed using Engineering Equation Solver (EES) [6] to compare 20 different fluids thermodynamically. Thermodynamic parameters like pressure ratio, mass flow rate, volume flow rate, thermal efficiency, second law efficiency, irreversibility, ODP and GWP were used for comparison. Of 20 different fluids are investigated, R134a appears as the most suitable for small scale applications because of R134a is of classa1 (non-flammable and non-toxic). R152a, R600a, R600 and R290 offer attractive performances but need safety precautions, owing to their flammability. T I. INTRODUCTION he conventional power plants based on Rankine cycle are economically not viable for power production with low temperature heat source. On the other hand, existing conventional power plants use fossil fuels or radioactive materials as inputs. Pollutants released in the atmosphere are responsible of the ozone depletion, global warming, acid rains and contamination of lands and seas. In this context, using renewable energies like solar energy, wind energy, biomass and geothermal heat as well as waste heat for electricity production becomes important. In such scenario ORC can be used for a small scale power production by utilizing low temperature heat source e.g. solar radiation, biomass combustion, ground heat source or waste heat from factories for commercial and residential utilities or in desalination plants. The main difference of conventional Rankine cycle and ORC lies with selection of working fluid. In conventional power cycle, steam is used as working fluid whereas in ORC low boiling substances such as refrigerants or hydrocarbons are used as working fluids. Therefore, vapor can be produced at low temperature which in turn runs the microturbine or expander to produce power. Proposed and studied different micro-orcs designed for electricity generation. [1] The economics of ORC cycle is linked with thermodynamic properties of working fluid [2]. A bad choice of working fluid increase plant cost decreases the thermal efficiency and produce greater environmental impact. Properties of a good fluid are: low specific volumes, high efficiency, moderate pressures in the heat exchangers, low cost, low toxicity, low ODP and low GWP among others. The main purpose of this study is to find working fluids that can be employed in low temperature ORC which can result in to a better thermodynamic performance and least environment impact. In this study 20 working fluids are selected from Refprop 8.0 database [9], for comparative assessment. The heat source temperature is assumed as 90 C this is a fair output from a flat plate collector utilizing solar energy. The capacity of the plant is assumed as 2 kw with micro power system [3] as heat engine. The EES [6] tool is used for thermodynamic analysis of ORC and to evaluate comparative performance of selected working fluids. II. PRELIMINARY SELECTION Organic working fluid can be characterized depending on the slope of the saturation curve on temperature - entropy diagram which may be infinity, positive, or negative. Based on the slope fluids can be classified as isentropic, dry, or wet respectively [4]. Dry or isentropic working fluids are more appropriate for ORC systems. This is because dry or isentropic fluids are superheated after isentropic expansion. Therefore there is no concern for existing liquid droplets at turbine outlet. Of 20 fluids selected from the REFPROP 8.0 database [9], four of them are isentropic, nine fluids are wet and seven fluids are dry. The physical properties of working fluids are presented in Table 1. Near critical pressure, small changes in temperature are equivalent to large changes in pressure that make the system unstable. Therefore a reasonable distance between the higher limit of the cycle and the critical point of the fluid should be considered. III. THERMODYNAMIC CYCLE The present ORC system consists of heat exchangers, a micro-turbine/expander, a condenser and a pump. The pump supplies the working fluid to the heat exchanger where it is heated and converted into a high pressure vapor and then it enters in to a micro turbine/expander. The low pressure vapor is exits at expander and is then enters into a condenser where it is condensed by cool air. The condensate is pumped back to heat exchanger. The said thermodynamic ORC is well depicted on T-s diagram in fig 1. Process 1-2 is an expansion process occurs in micro turbine/expander, 2-3 is heat rejection in a condenser, TABLE 1

2 2 INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 PHYSICAL AND ENVIRONMENTAL DATA OF THE WORKING FLUID The equations to calculate the different parameters to evaluate the performance of the saturated cycle are presented in this section. For simplicity, the internal irreversibility and, accordingly the pressure drops in the components other than the turbine, such as the pre-heater, evaporator, condenser and pipes, are ignored. The first and second laws of thermodynamics is applied to the individual components of the cycle to calculate heat transfer, work input and output and irreversibility of the cycle. Expander: t wf*(h 1 h 2s )*η st *η mt wf *(h 1 h 2s )*η mt (1) t T 0 * wf *(s 2 s 1 ) (2) Condenser: c wf *(h 2 h 1 ) (3) Fig.1 T s process diagram of ORC 3-4 is a pumping process, 4-5 is preheating and 5-1 is vaporization. The process of vaporization (5-1) in an ORC may end in a saturated vapor state or superheated vapor state. In this study, we consider a subcritical cycle where the vapor at the turbine inlet is saturated. The superheat was not found of interest as the incorporation of a superheated could bring additional cost. In addition increasing the maximum temperature of the heat source increases the heat loss of it. Because of these reasons the saturated Rankine cycle has been investigated in this study instead of a superheated cycle. Heat exchanger: uhx wf *(h 1 h 4 ) (5) Pump: p wf *v 3 *(P 4 P 3 )/ p (7) p T 0 * wf *(s 4 s 3 ) (8) First law efficiency: = ( t - p ) / uhx (9)

3 Second law efficiency: INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD , DECEMBER, System total irreversibility: Input enthalpy ratio: Fig. 3 Turbine outlet volume flow rate versus turbine inlet temperature for various working fluids at Tc = 35 C. B. Turbine outlet volume flow rate: Fig.2. Average ambient temperature of New Delhi, India IV. RESULT AND DISCUSION The input data for the evaluation of ORC are given in table 2. The isentropic efficiency of turbine and pump is assumed as 0.70 and 0.80 respectively. The heat source temperature is assumed as 90 C. The average ambient temperature of New Delhi is assumed as 28 C in year of 2009 as show in fig.2. A. Cycle pressure: According to Badr et al. [7] and Maizza and Maizza [8], good pressure values are in the range MPa and a pressure ratio (PR) of 3.5 is reasonable. From table 3, the following observation can be made concerning the pressure in condenser and heat source (heat exchanger). R113, cyclohexane, ethanol, R718 (water), methanol have low condenser pressures. R717, R32, R407 have higher evaporator pressure above 3Mpa. Ethanol, water, methanol, cyclohexane and R113 have higher pressure ratio. Ethanol and water have low evaporating pressure. Other fluids like RC318, R600a, R600, R114, isobutene, R500 and R152a have good condensing and evaporating pressures. Turbine outlet volume flow rate determines its size and the system cost. Results of calculations in Table 3 let see that R113, cyclohexane, water, ethanol, methanol, R123 and R141b have high volume flow rate. Fluids with low volume flow rate are preferred for the economic purpose. R32, ammonia, R407C, R290, R500, R134a and R152a, isobutene have low volume flow rate. As shown in fig.3, the volume flow rate decreases with an increase turbine inlet temperature. C. Cycle efficiency: The system thermal efficiency ranges from 2.61% for R32 to 4.89% for water. Fig. 4 shows the effects of the variation of the turbine inlet pressure. The temperature difference T ht maintained constant and vapor at inlet of expander saturated. Thermal efficiency of the system increases with increase of turbine inlet pressure. The range of second law efficiency is 15.29% of R32 to 28.71% of water from table 3. As shown in fig.5, the effect of turbine inlet pressure on system second law efficiency. TABLE 2 INPUT DATA FOR ANALYSIS OF THE ORC Fig.4. Thermal efficiency of system versus turbine inlet pressure for various working fluids Tc=35 C and T ht =15 C.

4 4 INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 these fluids required lower mass flow rate, and hence lower heat input required. Ammonia has higher evaporation pressure, but yields a low mass flow rate and high heat of vaporization. From Fig.9, it can be seen that the system mass flow rate decreases when the turbine inlet temperature increases. For economic reason, the fluids with low mass flow rates like methanol, ethanol and ammonia are interesting for large capacity system. Fig.5. Second law efficiency of system versus turbine inlet pressure for various working fluids at Tc=35 C and T ht =15 C. D. Irreversibility: From table 3, system total irreversibility varies in range of kw. Water and R407C have lower and higher irreversibility rates, respectively. Fig.6 shows the effect of heat source temperature on system irreversibility. The system total irreversibility rates decrease faster for low boiling points fluids as inlet pressure of turbine increase. The lowest irreversibility rate is obtained for RC318. The lower boiling point temperature of fluids total irreversibility rates increase of system with turbine inlet pressure increase. In second part of the analysis, the temperature of heat source kept constant. Fig. 7 and 8 shows the total irreversibility rate of system decreases as the turbine inlet pressure increase. As conclusion, small temperature difference between the fluid streams improves the performance of system. E. Mass Flow Rate: From table 3 methanol, ethanol, water have lowest maximum pressure and highest enthalpy of evaporation ( ). Therefore Fig.7. System total irreversibility rate versus turbine inlet pressure for various working fluids at Tc = 35 C and heat source temperature of 90 C. F. Analysis of the heat input: Heat input determines the size of the heat exchanger and it related with the system costs. From Table 3, the heat required for a 2 kw power output falls in the range kw. Fluids with high require low heat rates; these are: water, ethanol, methanol and ammonia. As shown in fig.10, high temperature saturated vapor reduce the amount of heat input. Fig.6. System total irreversibility rate versus turbine inlet pressure for various working fluids at Tc = 35 C. Fig.8. System total irreversibility rate versus turbine inlet pressure for various working fluids at Tc = 35 C and heat source temperature of 90 C.

5 INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD , DECEMBER, TABLE 3 COMPARISON OF PERFORMANCES OF DIFFERENT WORKING FLUIDS FOR A 2KW POWER OUTPUT: Fig.9. Turbine fluids outlet mass rate versus turbine inlet temperature for various working at Tc = 35 C G. Environmental consideration: Some substances, mainly refrigerants, deplete the ozone layer or/and contribute to the global warming. Because of their negative effects, there is a necessity to choose those with less harmful effects on the environment. R12, R113, R114 and R500 cannot be selected owing to their high ODP and high GWP. RC318 has a GWP of about and is excluded from the selection. Among these are: R141b, R123, R407, R134a, R407C and R32. Water, ammonia, and alkanes families are environmentally friendly substances. Fig.10. Heat input rate versus turbine inlet temperature for various working fluids at Tc = 35 C. H. Safety consideration: ASHRAE 34 provides a safety classification for fluids. Alkanes non-toxic but flammable are class A3. They require safety devices. R152a is classified A2 (lower flammability and non toxic). R123 isb1(non flammable but toxic). Ammonia classified B2 (toxic and has lower flammability limit). R134a is of class A1(non-flammable and non - toxic), i.e safer compared to other refrigerants and therefore is the preferred fluid.

6 6 I. Overall analysis: INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 From the analysis carried out in previous section (A to I), none of the fluids yields all the desirable properties. All the above mentioned parameters are important for ORC design. It is difficult to find an ideal working fluid which exhibits high efficiencies low turbine outlet volume flow rate, reasonable pressures, low ODP, low GWP and is non-flammable, non-toxic and non-corrosive. The following fluids are not selected: Cyclohexane has high volume flow rate and high pressure ratio. RC318 (high GWP), R407C (high evaporative pressure and low efficiency), R32 have high evaporative pressure, low efficiency and high moisture after expansion. R12, R113, R114 and R500 (high GWP, high ODP) R141b (high turbine outlet volume flow rate, high ODP). Ethanol, Water, Methanol have non convenient pressure value and high turbine outlet volume flow rates. R601 have high volume flow rates. Finally R134a followed by R152a, R600, R600a, and R290 are most suitable fluids for low temperature applications driven by heat source temperature below 90 C. advantages over the rest of fluids. From an efficiency point of view, fluids with high boiling point like ammonia, methanol, ethanol and water are very efficient but the presence of droplets during the expansion process is a drawback. Concluding, R134a followed by R152a, R600, R600a, R500, R12, RC318 and ammonia. High latent heat of vaporization presented by water, methanol, ethanol and ammonia have low mass flow rate and small heat input, which is a clear advantage over the rest of fluids. From the efficiency point of view, fluids with high boiling point like ammonia, methanol, ethanol and water are very efficient but the presence of droplets during the expansion process is a drawback. References: [1] S. Quoilin, M. Orosz, V. Lemort, Modeling and experimental investigation of an organic rankine cycle using scroll expander for small solar applications, in: Proc. Eurosun Conf., Lisbon, Portugal, 7 10 October, [2] W.C. Andersen, T.J. Bruno, Rapid screening of fluids for chemical stability in organic rankine cycle applications, Industrial and Engineering Chemistry 44 (2005) [3] Bertrand Fankam Tchanche, George Papadakis, Gregory Lambrinos, Antonios Frangoudakis, Fluid selection for a low-temperature solar organic Rankine cycle [4] Hung TC. Waste heat recovery of organic Rankine cycle using dry fluids. Energy Conversion and Management 1995; 42:539e53. [5] Rayegan R, Tao YX. A critical review on single component working fluids for Organic Rankine Cycles(ORCs). ASME Early Career Technical Journal 2009; 8:20.1e8. [6] S.A. Klein, Engineering Equation Solver (EES), Academic Professional Version, [7] O. Badr, S.D. Probert, P.W. O Callaghan, Selecting a working fluids for a Rankinecycle engine, Applied Energy 21 (1985) [8] V. Maizza, A. Maizza, Working fluids in non-steady flow for waste energy recovery systems, Applied Thermal Energy 16 (7) (1996) [9] Lemmon EW, Huber ML, McLinden MO. NIST standard reference database 23: reference fluid thermodynamic and transport properties- REFPROP, version 8.0. Gaithersburg: National Institute of Standards and Technology, Standard Reference Data Program; V. CONCLUSION Thermodynamic characteristics and performances of different fluids were analyzed for selection as working fluids in a low-temperature organic Rankine cycle. Several criteria were used for comparison: pressures, mass and volume flow rates, efficiencies, cycle heat input. Fluids favored by the pressure values are: isentropic fluids, butanes, n-pentane and refrigerants R152a, RC318 and R500. Low volume flow rates are observed for R32, R134a, R290, R500 and ammonia. High latent heat of vaporization presented by water, methanol, ethanol and ammonia has as have low mass flow rate and small heat input, which are