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The Use of Organic Zeotropic Mixture with High Temperature Glide As a Working Fluid in Medium-Temperature Vapor Power Plant

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The Use of Organic Zeotropic Mixture with High Temperature Glide As a Working Fluid in Medium-Temperature Vapor Power Plant Borsukiewicz, A.: The Use of Organic Zeotropic Mixture with … THERMAL SCIENCE, Year 2017, Vol. 21, No. 2, pp. 1153-1160 1153 THE USE OF ORGANIC ZEOTROPIC MIXTURE WITH HIGH TEMPERATURE GLIDE AS A WORKING FLUID IN MEDIUM-TEMPERATURE VAPOR POWER PLANT by Aleksandra BORSUKIEWICZ* ORC Power Plants Research and Development Centre, West Pomeranian University of Technology, Szczecin, Poland Original scientific paper https://doi.org/10.2298/TSCI141120088B The paper presents the idea of using organic substances as working fluids in va- por power plants, in order to convert the low and medium temperature thermal energy sources into electrical energy. The calculation results of the power plant efficiency for butane-ethane zeotropic mixtures of different mass compositions, for the power plant supplied with hot water having a temperature of 120 °C. Based on the results of thermal-flow calculations it was found that the use of zeo- tropic mixture does not allow to increase the efficiency and output of the power plant (these values appeared as slightly lower ones). However, it was found that, through the selection of a mixture of sufficiently large temperature glide, the heat exchange surface of the condenser can be reduced or a co-generation system can be implemented. Key words: organic Rankine cycle, zeotropic mixture, high temperature glide Introduction Energy, except at food production and medicine, is the most important aspect of hu- man development, having a direct impact on quality of life. The search for new sources of ener- gy, especially from renewables and waste, and more efficient ways of energy conversion into usable forms are all currently under intensive development. However, the characteristics of these sources are specific – there is usually a small heat flux/amount of energy from distributed sources, at rather not too high temperature. These aspects, as well as the global trend of increas- ing of fossil fuel prices have both contributed to the intensive development of organic Rankine cycle (ORC) technology i. e. to vapor power plant in which the working fluid is other than wa- ter, usually one with organic origin. The main reason for the use of organic substances is in the possibility to generate vapor at a relatively low temperature, without necessity of applying pres- sure that, in any of the system component, is lower than the ambient pressure. Until recently, natural and synthetic mono-component fluids have been used as the working fluids in the ORC power plants. The principle upon such systems work is similar to that of the classical Clausius- -Rankine vapor power plants. The ORC power plant, in the basic version, is composed of heat exchangers: heater, evaporator, and condenser, and of circulation pump and turbo-generator. However, the shape of the saturation curves for the majority of organic substances (particularly the curve x = 1) differs from that for water. This is schematically presented in fig. 1. –––––––––––––– Author’s e-mail: [email protected] Borsukiewicz, A.: The Use of Organic Zeotropic Mixture with … 1154 THERMAL SCIENCE, Year 2017, Vol. 21, No. 2, pp. 1153-1160 Figure 1. Saturation curves shape for various working fluids and the process of isentropic vapor expansion in the turbine (1-2 s) when using the so-called wet fluid e. g. water (a) and dry fluid (b) Figure 2. The cycle of thermodynamic changes of vapor power plant cycle implemented by using (a) wet fluid and (b) dry fluid For the so-called wet substances (e. g. water) the working medium should be super- heated before being directed to the turbine, process 6-1 in fig. 2(a), in order to avoid the ex- pansion of vapor in the wet vapor area. A characteristic feature of the so-called dry fluids ap- pears as no need to overheat them in order to implement the process of expansion in the su- perheated vapor area. Furthermore, in power plants with dry fluids, the internal regeneration may be used – the energy of the turbine outlet vapor, in the temperature range from T2s to T2r, can be used to heat the liquid medium in the temperature range from T4s to T4r, as indicated in fig. 2(b). The power plant schemes for different types of fluids are shown in fig. 3. Figure 3. Installation diagram of ORC power plant supplied with saturated dry vapor with (a) wet or dry fluid, without internal regeneration and (b) dry fluid, using internal regeneration The first experimental ORC type power plants with unary halogen-derivatives fac- tors (R12, R22) were already used in the last century, mainly for geothermal applications [1, 2]. Recently, the energy from biomass combustion [3], the energy obtained in focusing so- lar collectors [4], and the waste energy from technological processes [5] have all been used to supply the ORC power plant. The work towards searching for new, more environmentally Borsukiewicz, A.: The Use of Organic Zeotropic Mixture with … THERMAL SCIENCE, Year 2017, Vol. 21, No. 2, pp. 1153-1160 1155 friendly working fluids that meet thermodynamic and operational requirements is continuous- ly in progress. It should be emphasized that the use of any fluid (other than water) is associat- ed with the work on optimizing the structure of the turbine, pump and heat exchanger, and on appropriate choice of the material system. Due to the nature of the working substances used in ORC power plants the latter have to be provided with a hermetic design. Zeotropic mixtures as working fluids Zeotropic mixtures are the substances of two or more components characterized in that that during the process of phase transition at evaporation as well as at con- densation, both carried out at a constant pressure, the temperature of the mixture changes. The property of the temperature change in the isobaric process of the phase transition is called a temperature glide. Figure 4. Isobars course for zeotropic wet working That phenomenon has an influence on the fluid (based on 50/50 butane-ethane mixture) and shape of the comparative Clausius-Rankine the cycle of thermodynamic changes of the vapor cycle, as shown in fig. 4. The temperature power plant cycle implemented with the use of this difference in points 6 and 5 is a tempera- working fluid ture glide in the evaporator, while that in points 2s and 3 is a temperature glide in the condenser. Figure 5 shows the evaporation process at constant pressure, for zeotropic mixture of 50/50 butane-ethane composi- tion. As it can be seen from fig. 5, the change of the mixture composition affects the extend of the temperature glide. Table 1 shows the temperature values at the border lines for various compositions of the ethane-butane mixture at the pres- Figure 5. Isobaric process of boiling of a zeotropic sure of 5.2 MPa and the temperature glide working fluid value for the evaporation process ∆Tg,e [6]. It should be clarified that for ethane/butane Table 1. Saturation temperatures for various compositions of the ethane/butane mixture compositions 0.2/0.8, 0.1/0.9, and for pure (for p5 = p6 = 5.2 MPa) components the pressure of 5.2 MPa ex- ceeds the critical pressure, therefore for Ethane/butane 0.8/ 0.7/ 0.6/ 0.5/ 0.4/ 0.3/ these cases it is not possible to implement mass fraction /0.2 /0.3 /0.4 /0.5 /0.6 /0.7 the subcritical cycle for the evaporation T5 48.2 55.2 63.4 73.5 86.4 103.3 pressure of 5.2 MPa. T6 58.6 71.3 83.5 95.3 106.9 118.3 The possibility to apply the zeotropic ∆Tg,e 10.4 16.1 20.1 21.8 20.5 15 mixtures in the Rankine cycle is the subject of several publications. It is worth quoting the work of [7], which presents the results of calculations performed using the Stan Mix pro- gram for calculating thermodynamic properties of mixtures. This paper examines the efficiency of utilization of the butane (50%) + n-hexane (50%) mixture in the ORC power plant supplied with geothermal water having a temperature of 140 °C and cooled with air at 12 °C. Utilization Borsukiewicz, A.: The Use of Organic Zeotropic Mixture with … 1156 THERMAL SCIENCE, Year 2017, Vol. 21, No. 2, pp. 1153-1160 of the butane/hexane mixture allowed obtaining thermal efficiency at 10.2% as compared to the efficiency equal to 9.3% for the unary pentane. The paper also explores the possibility of using a multicomponent mixture: MM (3.18%) + MDM (34.14%), MD2M (45.51%) + MD3M (16.17%) + MD4M (1%) in the ORC power plant supplied with heat carrier at a temperature of 400 °C. This work includes the possibility of using an internal heat recovery, but mainly in the recovery of heat from the turbine outlet vapor, as the fluids chosen for calculation are character- ized by a high value of the ds/dT derivative. The work [8] presents the results of experimental studies of the ORC system that utilizes R245fa and a mixture of R245fa/R152a (0.9/0.1 and 0.7/0.3%), and is powered by the solar energy from a liquid manifold. It is the only work con- taining the results of experimental studies to which the author of the present paper managed to reach, but that experimental system did not include a turbo-generator (only the expansion valve) thus no values of the Rankine cycle efficiency could be obtained, which for pure R245fa amount to 4.16%, for the mixture with a composition of 0.9/0.1 amount to 4.29%, while for the mixture 0.7/0.3 – 5.59%.
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