Distributed Renewable Energy Technologies:Design and Development of Scalable Transmission Systems
Abstract
Renewable forms of energy are being intensively pursued and investigated in systems connected to the grid as well as in standalone applications. The global energy generation is expected to grow 2.7 times by the year 2035. Today, renewable energy resources account for 14% of the total world energy demand. There are however, certain disadvantages associated with the use of alternative energy resources such as intermittency in energy resource, instability, and high initial investment. To meet the growing energy demand, solutions are being explored to overcome the drawbacks associated with the use of these forms of energy. The goal of this MS thesis is to develop a transmission system, with a focus on the gearbox, for low input speed applications. This system is designed for use in non- traditional renewable energy harnessing technologies such as wind and hydrokinetic energy generation. The goal is pursued through several objectives: • Conducting theoretical analysis to determine either the required torque or gearbox specifications based on the input torque or desired output torque at defined input speeds. • Conducting Finite Element Analysis (FEA) on the developed gearbox system to examine the structural stability, using the stress and displacement criteria. • Developing a prototype for experimental testing. • Validating the results based on material properties and literature. The Finite Element Analysis takes into consideration different mesh and geometry environments to predict the accuracy of the results. These predictions, however, are based on a number of assumptions such as perfectly elastic material behavior, disregarding losses due to friction and gear pair misalignments. The findings obtained from the stress distributions in each of the environments are compared with Hertzian contact stress analysis. It is observed that the contact stress (Von Mises) obtained through FEA approach the analytic stress (Hertz) values by determining the optimum mesh density. This is achieved by identifying and refining the mesh in the regions of localized stresses. In case of gear pairs, the maximum stress is concentrated at the contact region of the mating gear teeth. Thus, the gear faces in contact have a refined mesh with a face element size of 0.1 mm. Regions with comparatively lower stress values can be coarse (in this case the maximum element size is 10 mm). Using a "proximity and curvature" or "curvature" type of mesh ensures finer quad meshing in the stress concentrated areas. One case study for the proposed epicyclic gear design can be found in a distributed wind energy technology system, called Wind Tower Technology (WTT) to be used in Maryland, US. Epicyclic gears are known to have advantages over parallel shaft drives in terms of weight, number of components, and size. These make it a suitable choice for the WTT and similar concepts where a significant increase in the output shaft power is needed. In this case, the gear train configuration is designed based on the output torque requirement, taking into consideration, the materials and ease of machining and manufacturing. Manufacturing techniques such as laser cutting, CNC machining, 3D printing and silicone molding are used to fabricate a two-stage gearbox system and the set up. As a future scope, this setup will be connected to a Data Acquisition System – "LabVIEW" to test the feasibility of the gearbox design by determining the current-voltage characteristics of the generator connected to this system. The FEA results on the final design using Delrin as a material for gears showed a maximum gear tooth contact stress of 37 MPa (the allowable stress defined by ASTM D4181 is 98 MPa) and using steel for shafts showed a maximum stress of 206 MPa (the allowable stress defined by AISI 1020 is 350 MPa). The second part of this MS thesis is focused on conducting FEA on a speed converter to be used for renewable energy technologies. While current systems control the output power fluctuations electronically, the patented speed converter employ mechanical controls to obtain a smooth output power. Its application for the proposed case is studied and discussed when used in conjunction with the developed epicyclic gearbox system. The results show the potential of obtaining a smooth high-rated power using a combination of proposed epicyclic gearbox system and the speed converter. Further experimental research at different scales can be pursued as a follow up of this research.