Organic rankine system and turbine design

Atmospheric greenhouse gas is increasing day by day with the increase of world population and consumption. Energy consumption has the largest share in greenhouse gas emissions released to the atmosphere. According to the energy consumption data,the energy consumption in the industrial area is in the first place. In recent years, some efficiency enhancing components and processes have been incorporated into the operating process in order to reduce industrial energy consumption, but the waste heat released to the atmosphere can not be avoided. Inevitable waste heat losses can be utilised with energy conversion technologies. With the ORC (Organic Rankine Cycle)systems, one of the heat recycling technologies, energy efficiency can be increased by turning wasted heat energy into electric energy.Thanks to the organic fluids used in ORC systems, low volume and low cost systems enable energy recycling even at low temperatures. In addition, ORC systems are reliable, environmentally friendly and easy to maintain. These advantages make ORC systems one of the best solutions for energy recycling of industrial waste heat. In order to reduce energy consumption, it is necessary to increase the environmentally friendly energy recycling systems in the industrial area.The fact that there are very few systems that provide energy recycling from low temperature waste heat sources in Turkey and the fact that the installed systems are of foreign origin, this field shows the necessity of national research and development. For this reason, it is aimed to reach the output which can contribute to the research and development phases of the ORC systems as a result of this study.In this work, the theoretical knowledge about the steam power systems and Rankine cycle, which is the ideal cycle for these systems, is reached firstly. Equations used in Rankine cycle calculations were compiled and infrastructure for thermodynamic cycle design was established. Classification methods of ORC power systems have been examined and information about thermal energy sources has been obtained for understanding application areas. A brief, non-exhaustive survey of ORC technology and the components used in the system has been carried out. In this research, efficiency of cycle structure, working fluid selection criteria, heat exchangers and rotary equipments are examined. In the literature review of the general ORC systems, the data of the installed systems are finally obtained. The data of the systems established in Turkey are compiled and presented in tabular form. As a result of this analysis, the national need is exposed and the working goal is reinforced. In the last part of the literature review, expanders used in ORC systems, which is the main interest of the study, have been examined. It has been observed in this review that positive displacement expanders are often preferred for small ORC systems because of their simplicity and low cost, while turboexpanders are preferred over higher power ORC systems. Expanders used in the installed ORC systems have been researched and following results are observed: - Positive displacement expanders are preferred below 20 kW power capacity, - Both positive displacement (especially screw extender) and turbo expanders are being preferred between 20 kW and 110 kW , - While turbo expanders are only available as solutions above 110 kW. In order to have a better understanding of the expanders, research has been carried out on the construction of all varieties, working principles, power capacities, advantages and disadvantages. System design goals were established with the information obtained in the literature survey. Firstly, it is aimed that the system power capacity is 100 kW and the thermal efficiency is at least 15%. After determining the source temperature range to be 80-200 oC, the fluids that can work in this temperature range were investigated and the fluid selection criteria (critical and maximum temperaturepressure values, specific heat, molecular weights, safety levels, ozone depletion and global warming levels) , Resulting in the highest efficiency fluid being R245fa. After the fluid selection, the cycle structure was determined. By decreasing the high temperature in front of the condenser the power consumed in cooling is reduced. This increase the efficiency of the system and decrease the system volume. That is why the use of a recuperator is decided. Pump efficiency was set at 70%, assuming that the pump selection would be made appropriate for the system. As an expander, a radialoutflow turbine suitable for the targeted power capacity and capable of working efficiently at low speeds has been chosen. It is targeted that the efficiency of the selected turbine is at least 70%. Finally, it was decided to design the system by using water-cooled condenser with water temperature 28 oC.The thermal efficiency of R245fa, selected as the working fluid for the determination of the input parameters (evaporator pressure, condenser pressure, turbine inlet temperature) required for thermodynamic cycle design, was analyzed. Since the turbine inlet temperature directly affects the turbine efficiency, the maximum value 166,85 oC that the fluid can reach is selected. Analyzes made at different pressures at this value yielded the highest efficiency and the evaporator pressure value not exceeding the critical level was set at 31 bar. Care has been taken to ensure that the condenser pressure is as low as possible since the thermal capacity and therefore the volume is reduced as the condensation pressure drops as the condensation temperature also drops. When the pressure is determined, it is noted that the condenser inlet temperature does not fall below the inlet temperature of the coolant. As a result, the condenser pressure was set at 2 bar. The cycle design calculations were carried out using a computer code prepared within the scope of an ORC system design and manufacturing project carried out in university and industry collaboration. As a result of the thermodynamic cycle analysis, a system that produces 100 kW power with an efficiency of 16,118% was designed with 2,713 kg / s fluid flow. As a design output, the power capacities of the components of the pressure, temperature and enthalpy in the inlet and outlet stations (evaporator, condenser, recuperator and pump) are determined. Before the design of radial outflow tubine, the geometric structure of such turbines have been investigated. As a result of the research, it was decided to implement the radial outflow turbine design with zTurbo software especially developed fr ORC turbines. The structure of the zTurbo software is examined and the estimated value ranges for the input parameters are determined by literature search. In addition, studies in the literature have been examined to select the loss correlation to be used in design. In these studies, it was observed that Craig & Cox correlations can not predict the tip losses sufficiently, Traupel correlation is higher than the analytical values but still Treupel estimates yield more closely to the analysis than Craig & Cox correlation. It is also observed that Traupel provides values closer to the analysis values related to the total loss estimates.For this reason, it was decided to apply Traupel loss correlation in the design. Preliminary design experiments were carried out with zTurbo software. Several experiments have been made with different input data and loss correlations. In the preliminary design trial studies carried out for target design, the output effects of the inputs in different values are examined in detail. In four-stage turbine design experiments, it has been observed that the stages have increased performance due to their different expansion rates. The reaction degree of the stages are kept around 0.5. Turbine rotation speed has been determined as the end result of the design experiments carried out to provide the best efficiency. For blade clearences and trailing edge thickness, the minumum values were chosen taking into account mechanical vibration and manufacturability criteria. While the distance between the stator and rotor is small, the distance between the two stages is chosen to be about twice this value. Due to the decrease in fluid density, the flow area is expected to increase so that the speed does not rise to supersonic speeds, and therefore the height of the blades are expected to increase. For this reason, the first stage height and diameter were determined as a result of design experiments. Chord lengths were chosen equally in terms of ease of manufacture, with the exception of the first stage. Another important consideration in design experiments is the angles of the blade geometry. In order to keep the angles in the velocity triangles under control, it is ensured that the angles of the blade geometry consecutively increase. Three control points were determined by design results inputoutput diameters and angles, camber line length), and by using these points, lines of curvature were formed by NURBS curves. While desigining the camberline, flow path and flow dynamics have been taken into consideration, as a result, it has been decided to design the wing of the satisfactory geometry. The NACA A3K7 blade profile was used to construct the geometry. Thickness points were determined on the camber line and tangentially extruded to form the thickness taken into account the manufacturability and the distance ratios were determined considering the manufacturability factor and the distance between consecutive blades. When considered necessary, the required number of stages have been revised. As a result of the design studies, preliminary design of a radial output turbine which produced 122.77 kW mechanical energy with an efficiency of 79.5% was carried out.