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Figure 11: Proposed Design of ORC Turbine by Retrofitting a Turbocharger 6.1 Selection of Bearing Bearings are required to support a rotor shaft in the turbine assembly. The turbine assembly commonly requires radial bearings and thrust bearings to support both the radial load and axial thrust. Literature reviews on steel bearing and hybrid bearing show that hybrid bearing is the most suitable bearing for ORC application. In a study by Moleneaux, a hybrid bearing was found to be more suitable than steel bearings when used in an ORC turbine (Molyneaux & Zanelli, 1996). The application of hybrid bearings for refrigerant has also been suggested by Jacobson as the hybrid bearings have an optimal of raceway topography to ensure the maximum elastic deformation of the asperities in Hertzian contact when compared to a steel bearing (Jacobson & Espejel, 2006). The optimization of the raceway allows an effective separation of the mating surfaces, further allowing the bearing to withstand pure refrigerant lubrication for long periods (Jacobson & Espejel, 2006). The proposed design of ORC turbine would be equipped with a pair of hybrid bearings to provide a longer life under lubrication from pure refrigerants. 6.2 Selection of Lubricants In an ORC application, bearings are usually lubricated by the working fluid for small size turbo-generators, while bearings for medium size turbo-generator are lubricated by proper oil lubricants with a separate oil circuit and oil pump. Two journal bearings and a thrust bearing are lubricated with R134a in an 18 kW oil-free compressors investigated by Molyneaux (Molyneaux & Zanelli, 1996) and journal bearing and thrust bearing are lubricated with toluene in a 25 kW solar-powered ORC system investigated by Nesmith (Nesmith, 1985). Common issues in bearing failures by using refrigerants include the attack of bearing surfaces by some refrigerants such as ammonia (Jacobson & Espejel, 2006). Many refrigerants tend to dissolve into lubricants, such as R134a in ester oil. The mixture reduces the initial lubricant viscosity and pressure viscosity coefficient thus reducing the film thickness during operation. Furthermore, the boiling point of refrigerant-oil mixture is reduced at the local pressure (Yamamoto, Gondo, & Kim, 2001). The reduction in boiling point allows vaporization of lubricant on the bearing surface when the bearing surface temperature is higher than the boiling point of the mixture, thus reducing the lubricant film thickness (Jacobson & Espejel, 2006). Due to the high cost and complexity of the external lubrication system, the ORC turbine would use the working fluid as lubricant for the bearings. The rheology of three working fluids, R245fa, R134a and R123 was studied to choose an optimal lubricant for the bearing. Among the three working fluids, R245fa is found to be a better lubricant for rolling bearings followed by R134a and lastly R123 (Jacobson & Espejel, 2006). Though R134a is a relatively poor lubricant compared to R245fa, R134a is employed as the lubricant in oil-free compressors (Molyneaux & Zanelli, 1996) as the pressure viscosity coefficient of R134a is very similar to the pressure viscosity coefficient of poly alpha olefin oil (Jacobson & Espejel, 2006). The proposed design of turbo-expander in Figure 9 is designed to be lubricated by R134a and R245fa. 6.3 Casing Design and Seal Selection The turbine casing can be categorized into enclosed and open-drive designs. In an open drive design, the turbine shaft is coupled to an external generator for power generation. The working fluid will leak to atmosphere due to high differential pressure between the fluid inside the turbine and the atmosphere. The most common sealing system for an open drive design is a dry gas seal and a mechanical seal which are not suitable for small size turbo- generators. The need for an extra gas buffer system and lubrication system to the seal increases the cost of the turbo-generator. In a fully enclosed design, the turbine shaft is coupled to a high speed generator within the enclosed casing. Due to the scarcity of off-the-shelf high speed generators, the proposed design of ORC turbine is connected to magnetic coupling which facilitates the coupling to an external generator. The casing surrounding the magnetic coupling is made from aluminium to allow the operation of magnetic coupling. 7. CONCLUSION This paper presents the selection of turbine type, performance evaluation of the automotive turbocharger and modification of the turbocharger into an ORC turbine. A suitable type of turbo-expander was chosen from Balje’s turbine performance chart. Performance analysis was then conducted to study the performance behaviour of the chosen turbocharger. If two or more turbochargers are chosen during the selection stage, simulation of turbine off- design performance can assist the selection of the turbine wheel. Lastly, this paper demonstrates the conceptual design stage of ORC turbines, making consideration for the casing design, the selection of bearings, lubricants and seals. 8. FUTURE WORK The future work is categorized into a number of different sections, which are the investigation of the utilization factor of the turbo-expanders during prospecting phase and feasibility phase, construction of the ORC turbine and development of Expander Selection and Implementation (ESI) method). The first section aims to investigate the performance of the turbo-expanders. The utilization factors are then applied in the ORC plant prospecting phase and feasibility phase to facilitate the quantification of the economic merits of the ORC plants before proceeding into detailed design and construction phase. The second section aims to construct a prototype of the ORC turbine. Laboratory performance testing and rotor- dynamic balancing will then be included. Finally, an Expander Selection and Implementation (ESI) method will then be proposed and refined based on the experience in 35th New Zealand Geothermal Workshop: 2013 Proceedings 17 – 20 November 2013 Rotorua, New ZealandPDF Image | TURBOCHARGER AS TURBO-EXPANDER FOR ORGANIC RANKINE CYCLE
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