The energy sector faces a pressing need for significant transformation to curb CO2 emissions. For instance, Czechia and Germany have taken steps to phase out fossil thermal power plants by 2038, ...opting instead for a greater reliance on variable renewable energy sources like wind and solar power. Nonetheless, thermal power plants will still have roles, too. While the conventional multistage axial turbine design has been predominant in large-scale power plants for the past century, it is unsuitable for small-scale decentralized projects due to complexity and cost. To address this, the study investigates less common turbine types, which were discarded as they demonstrated lower efficiency. One design is the Elektra turbine, characterized by its velocity compounded radial re-entry configuration. The Elektra turbine combines the advantages of volumetric expanders (the low rotational speed requirement) with the advantages of a turbine (no rubbing seals, no lubrication in the working fluid, wear is almost completely avoided). Thus, the research goal of the authors is the implementation of a 10 kW-class ORC turbine driving a cost-effective off-the-shelf 3000 rpm generator. The paper introduces the concept of the Elektra turbine in comparison to other turbines and proposes this approach for an ORC working fluid. In the second part, the 1D design and 3D–CFD optimization of the 7 kW Elektra turbine working with Hexamethyldisiloxane (MM) is performed. Finally, CFD efficiency characteristics of various versions of the Elektra are presented and critically discussed regarding the originally defined design approach. The unsteady CFD calculation of the final Elektra version showed 46% total-to-static isentropic efficiency.
The energy industry must change dramatically in order to reduce CO2-emissions and to slow down climate change. Germany, for example, decided to shut down all large nuclear (2022) and fossil thermal ...power plants by 2038. Power generation will then rely on fluctuating renewables such as wind power and solar. However, thermal power plants will still play a role with respect to waste incineration, biomass, exploitation of geothermal wells, concentrated solar power (CSP), power-to-heat-to-power plants (P2H2P), and of course waste heat recovery (WHR). While the multistage axial turbine has prevailed for the last hundred years in power plants of the several hundred MW class, this architecture is certainly not the appropriate solution for small-scale waste heat recovery below 1 MW or even below 100 kW. Simpler, cost-effective turbo generators are required. Therefore, the authors examine uncommon turbine architectures that are known per se but were abandoned when power plants grew due to their poor efficiency compared to the multistage axial machines. One of these concepts is the so-called Elektra turbine, a velocity compounded radial re-entry turbine. The paper describes the concept of the Elektra turbine in comparison to other turbine concepts, especially other velocity compounded turbines, such as the Curtis type. In the second part, the 1D design and 3D computational fluid dynamics (CFD) optimization of the 5 kW air turbine demonstrator is explained. Finally, experimentally determined efficiency characteristics of various early versions of the Elektra are presented, compared, and critically discussed regarding the originally defined design approach. The unsteady CFD calculation of the final Elektra version promised 49.4% total-to-static isentropic efficiency, whereas the experiments confirmed 44.5%.
In contrast to the current trend of converting waste heat into electricity in the small power range below 100 kWel by means of an ORC plant, the authors are pursuing the concept of a micro steam ...power plant equipped with a micro turbine. Water avoids many of the problems often associated with organic working fluids, such as flammability, toxicity, greenhouse gas effect and high fluid costs. However, water vapor makes turbine design more challenging. The physical reasons for this are repeated, and thereby it becomes clear why a velocity compounded two wheel Curtis turbine has been chosen. The used in-house 1D turbine design tool is briefly introduced. More focus is put on the shortcomings of the implemented 1D loss model and their negative impact on the current turbine design. Consequently, the authors continued actual turbine design by a parameterized approach in 3D CAD/CFD. This approach is explained, and finally, the CFD flow field and the performance maps of the designed turbine are discussed. The turbine is currently under construction and will be installed in 2022 in a waste heat recovery (WHR) plant in Nuremberg/Germany.
In order to meet the targets set out in the Paris climate agreement of 2015, the energy sector needs to be restructured. In Germany for example the goal is to move out of both nuclear and coal-fired ...power. In order to fulfil more demanding energy requests alternative sources of energy must be found and made available. One component is the usage of Waste Heat Recovery.
This paper deals with the evaluation simulation of an Elektra turbine, which concept is a velocity compounded radial re-entry single wheel turbine. Based on the first turbine design, multiple simulations were carried out in order to maximize the efficiency. With these simulations, optimization opportunities are described and identified. The paper concludes with a comparison of the results achieved by numerical simulation and on the experimental test bench. The main focus for this paper is to analyze the actual flow phenomena to identify possibilities for further improvements. All simulations were calculated with the commercial software F
INE
TM
/T
URBO
15.1.
Development of micro ORC systems with 1-15 kW power output for micro-cogeneration and waste heat recovery at the Czech Technical University in Prague, University Centre for Energy Efficient Buildings ...(CTU UCEEB) has over ten years of history with many successes. These include 6 different ORC units, all with in-house designed rotary vane expanders (RVE) of many versions throughout this development. Among main advantages of the RVE belong relatively simple and robust design at low cost even at very small series of single-unit production and all that with acceptable efficiency. The ORC units operate with hexamethyldisiloxane (MM) working fluid at high pressure ratios and expansion ratios and the isentropic efficiency of RVE has a limit at these conditions around 60%, often however only at values around 50%. While this might be enough on a cost side for commercialization of this technology, in pursuit of higher efficiency solutions, different expander technology needs to be selected. A turbo-expander is a logical choice with prospect of higher efficiency. At the same time, a literature review has found a lack of reported detailed experimental data for micro (5-50 kW) turbo-expanders, possibly hindering global development towards economically feasible solutions. A project named Dexpand, “Optimised expanders for small-scale distributed energy systems” aims at these issues by objectives in designing, optimizing, manufacturing and testing several ORC expanders with MM and isobutane and their subsequent performance mapping and comparison. One major task is a design of a turboexpander for a 120 kWth biomass fired microcogeneration ORC unit currently operated at the CTU UCEEB. An axial impulse single stage turboexpander was selected as a suitable choice, providing a prospect of a decent efficiency at technically manageable rotational speed and size. This paper provides a detail of currently performed design activities, starting from boundary conditions specification, over development and optimization of a 1D model, preliminary 2D CFD calculations and finishing in a state of a robust and detailed 3D CFD model with a real gas model. Note that the working fluid, high molar mass organic vapour, is highly non-ideal in its behaviour and the flow conditions with pressure design ratio around 13 is highly supersonic (nozzle outlet isentropic Mach number exceeds 2). The current results based on 3D CFD indicate a prospect of an isentropic efficiency 71% at mechanical power output of 11 kW. Lastly, ongoing and future work is outlined, which includes aerodynamic optimization based on the developed 3D CFD model and construction design of the entire turbine assembly.
Due to the various ORC applications, it is not expedient to design and build one standard turbogenerator. Therefore, authors developed a flexible “micro-turbine-generator-construction-kit ...(MTG-c-kit)” by means of which a customized turbogenerator can be made for any required power output between 1 and 200 kW, for a wide range of working fluids and boundary conditions – quickly, cost effectively and with high expander efficiency. The architecture and main features of this kit are briefly introduced.
However, more focus is put on the developed 1D turbine design tool. It allows to quickly design and optimize a single stage turbine for any fluid and boundary conditions. The implemented models are presented on two specific design examples - an axial impulse and a radial inflow cantilever turbine. Also, the experimental results of the two micro turbines designed and built according to this approach, are presented and discussed.
Focusing to further accelerate and cheapen the customized micro-turbogenerators for low temperature applications, we provide an outlook of possibilities of additive manufacturing methods like 3D printing of turbine nozzles and rotors. These new possibilities provide potential to improve the competiveness of small-scale ORC. A simple air turbine design with plastic and metal components has been manufactured and tested.
•Axial and radial impulse turbines were chosen as a basis of the 1–200 kWe MTG-c-kit.•The kit provides flexible, efficient and cost-effective design for ORC turbomachinery.•1D model for quick, yet accurate, design and performance prediction is presented.•3D printing brings potential for further customization and cost reduction.•Experimental verification is shown for two MM turbines and 3D printed air turbines.
Introduction: We aimed to examine whether student teachers' tendency to test anxiety relates to their later perspective-taking tendency and self-efficacy. The purpose of the presented study was to ...obtain first insights into the relationship between test anxiety, perspective-taking, and self-efficacy tendencies in student teachers. These tendencies may determine student orientation across different situations. We tested the hypothesis that the test anxiety components emotionality and worry relate to later low perspective-taking and self-efficacy. Methods: We conducted a longitudinal study with 275 student-teachers and analyzed the obtained data using structural equation modeling. Results: The data analysis revealed that only emotionality is related to lower perspective-taking and self-efficacy in later life. Discussion: We conclude that lowering student teachers' test anxiety emotionality (e.g. through interventions) would result to later increased perspective-taking tendency. Limitations: Participants attended the study voluntarily, so it was a self-selected sample. We employed a correlational design over two measurement times instead of experimental methods. Conclusions: The obtained results give an explanation of why student teachers focused rather on themselves than on school students in classes. The perspective-taking tendency is important for teachers' student orientation. The role of perspective-taking in different teaching situations might be investigated in further research.
The energy industry is undergoing a major upheaval. In Germany, for example, the large nuclear and coal-fired power plants in the gigawatt scale are planned to be shut down in the forthcoming years. ...Electricity is to be generated in many small units in a decentralized, renewable and environmentally friendly manner. The large 1000 MW multistage axial steam turbines used to this date are no longer suitable for these tasks. For this reason, the authors examine turbine architectures that are known per se but have fallen into oblivion due to their inferior efficiency and upcoming electric drives about 100 year ago. However, these uncommon turbine concepts could be suitable for small to micro scale distributed power plants using thermodynamic cycles, which use for example geothermal wells or waste heat from industry to generate electricity close to the consumers. Thus, the paper describes and discusses the concept of a velocity-compounded single wheel re-entry cantilever turbine in comparison with other turbine concepts, especially other velocity-compounded turbines like the Curtis-type. Furthermore, the authors describe the design considerations, which led to a specific design of a 5 kW air turbine demonstrator, which was later manufactured and investigated. Finally, first numerical as well as experimental results are presented, compared and critically discussed with regards to the originally defined design approach.
PDF
