Introduction The next-generation turbine designs for small-scale hydropower systems seek higher efficiency and lower design and manufacturing costs. This thesis studies the fluid dynamic mechanisms in crossflow turbines for improving their maximum efficiency, motivated by their inherent simplicity in design and manufacturing at low cost and the potential for performance improvement using recent advances in computational fluid dynamic simulations. Crossflow turbines are typically used for small-scale, low-head remote power systems. Current crossflow turbines are reported to achieve maximum efficiency upto 85%, but due to lack of proper design theory, most turbines in practice only achieve about 80% maximum efficiency. The computational study presented in this thesis shows that crossflow turbines can achieve more than 90% efficiency and maintain high efficiency at part-flow operations if the inlet flow is regulated using a circular slider type control device at the inlet periphery of the impeller. 1.1 Background Energy is at the heart of modern society, playing critical roles from human civilization to supporting modern technological developments, such as manufacturing, transportation, food, medicine and communication. Amongst different forms of energy, electricity is the most advanced. Electricity is possibly the most remarkable discovery in the history of human civilization, which has enabled tremendous transformations in the pace of technological developments and human civilization. Today almost every advanced technological product or device, which constitutes an essential part of modern society, requires electricity to 1 perform its intended function. Therefore, access to electricity has become an essential part of modern-life as well as an enabler to accelerate technological advancements and human civilization. At the present, however, not everyone in the world is equally fortunate to enjoy the benefits of electricity. The International Energy Agency (IEA) estimates that about 1.2 billion people, particularly those living in the developing nations, lack access to electricity, and about 80% of people live in rural areas (IEA, 2013). The main obstacle is that expanding electricity networks to those places is expensive as well as technically challenging, further exacerbated by limited technical and politico-economic capabilities of those nations. Lack of energy has hindered education, health, food, transportation and communication systems. Tackling this wicked problem requires partly politico-economic solutions, but technological advancements in clean and affordable energy generation and supply greatly accelerate the efforts. For this, innovative designs of renewable and climate-friendly energy technologies and decentralized rural electrification methods to deploy a large number of systems are thought to be the key elements in meeting the United Nations’ sustainable development goals 1. In developed countries, such as Europe and Americas, nearly all people have access to electricity. Power is generated mostly from fossil fuels. Amidst global competition in consuming natural resources 2 , global issues, such as climate change, environmental degradation, and declining fossil fuel resources, have become critical. Therefore clean energy systems are needed to meet these demands and tackle the global issues of environmental United Nations has set a number of sustainable development goals to address the issues of energy access in the developing world (UNDP, 2015). 2 In efforts to support increasing energy demand due to population growth and for improving the quality of modern life 1 2 pollution and climate change caused by fossil fuel consumption. As a result, the demand for renewable and cost-effective sustainable energy systems has grown rapidly worldwide. Amongst different sources of renewable and environmental-friendly technologies, hydropower is the oldest and the cheapest and reliable energy technology, and thus it is expected to continue as an important part of the future sustainable energy systems. Sustainable development 3 requires harnessing renewable energy resources cost effectively to meet the demand of clean energy in both developing and developed nations. Amongst different renewable energy technologies, small-scale hydropower systems, ranging from a few kWs to 200 kWs, offer economical and reliable means to generate electricity in both developing and developed nations, where small streams of water are locally available in many areas. The idea behind the small-scale hydropower is technically simple. These systems do not require a dam to impound the water, and thus have little impact on the ecology. Water is diverted from a small river to a canal. A surge tank or a small reservoir is built at the end of the canal. From the reservoir, the penstock pipe carries the water to the power house, where it turns a turbine that drives an electric generator, producing electricity. The typical smallhydro systems for remote power work under the head of a few meters to 100 m with flow rates upto 200 litres per second. Small-hydro systems that generate power in the range of 10 kW - 200 kW are most popular. Depending on the capacity of the systems, the electricity can be used for lighting, supporting water, health, education and communication systems, and running small industries. Small hydropower systems, which have been used for a long time, particularly in the rural villages of the developing nations, can provide the best solutions to the sustainable development goals by providing electricity A development framework that works well for many future generations without harming the environment, human health and economy (UNDP, 2015). 3 3 to support the local economy, education, health, food and communications systems. To ensure their cost competitiveness for deploying a large number of systems in the future, they require further technological advancements, primarily the cost reductions in design and manufacturing and improvements in efficiency of turbines. Within this broader context of sustainable development, this thesis seeks to contribute toward the design of high-efficiency crossflow turbines, which are most popularly used in small-scale hydropower systems. 1.2 Types of Turbines and Design Requirements The turbines used in hydropower can be broadly classified as impulse and reaction turbines based on the degree of reaction (Dixon and Hall, 2013). Degree of reaction is the relative amount of pressure drop in the rotor and the nozzle, and is defined as the ratio of static pressure drop in the rotor to the static pressure drop in the stator or nozzle plus the rotor. Impulse turbines, such as Pelton, work on the principle of impulse action of high jet velocity impinging on the impeller blades. All the available energy of the flow is converted into kinetic energy by the nozzle at atmospheric pressure before the flow passes through the turbine blades. Angular momentum is extracted by the blades mainly due to the dynamic pressure difference between the two surfaces of the blade, while the static pressure difference across the blade surface is atmospheric. Thus the impulse turbines have zero reaction. In contrast to this, the reaction turbines, such as Francis, Kaplan and Propeller, work on the principle of reaction forces developed across the blade surfaces. Angular momentum is extracted mainly due to the pressure drop across both the stationary guide vanes and the impeller blades. The pressure drop occurs both in the nozzle and the rotor (Shepherd, 1956). Thus the reaction of these turbines is non-zero. An alternative and widely used classification, from the viewpoint of selection of turbines and their design, can be made using the concept of specific speed Nsp given as (Shepherd, 1956): 4 (1.1) Equation (1.1) shows that the specific speed classifies turbines based on the operating variables: the flow rate Q, the head H, and the operating speed N. A high head turbine, such as Pelton, has a lower specific speed. The low-to-medium head turbine, such as Francis, has a medium specific speed. Similarly, a low head turbine, such as Propeller or axial flow turbine, has a very high specific speed and can handle very high flow rates. Crossflow turbine falls under low-to-medium head turbine with high specific speed. Figure 1.1 shows a typical Figure 1.1: Classification of various turbines based on head H and flow rate Q illustrating their application range (Benzon et al., 2016). classification of various turbines based on head H and flow rate Q. The typical efficiencies of those turbines are shown in Figure 1.2. In the selection of turbines for small-scale hydropower systems, the maximum efficiency 5 4, the simplicity in design and manufacturing, and the cost are tightly coupled. As shown in Figure 1.1, the crossflow turbine is the most suitable turbine for low-to-medium head small systems. The simplicity in design and manufacturing at low cost is the most prominent feature amongst its counterparts. However, as shown in Figure 1.2, its maximum efficiency tends to be in the range 70 - 86%, which is lower than that of more commonly used advanced turbines, such as Pelton, Francis, and Kaplan, which have typical maximum efficiencies above 90% (Dixon and Hall, 2013; Sinagra et al., 2014; Elbatran et al., 2015). It is also known that compared to other turbines, crossflow turbines have relatively flat efficiency curves over a wide range of operating conditions as shown in Figure 1.2. The most efficient impulse turbine, the Pelton wheel, normally operates above 50 m, whereas the reaction turbines like Figure 1.2: Comparison of typical efficiencies of different turbines (Sinagra et al., 2014). Note that the efficiency is the hydrodynamic efficiency, i.e. the efficiency of energy extraction by the blades, not the efficiency of conversion into electrical energy. efficiency is defined as the ratio of shaft power to total input power at the turbine inlet. Losses in the penstock pipe are not taken into account. 4 6 Francis and Kaplan can operate at low heads, but usually require high flow rates to operate at high efficiency. Use of these turbines in low-head systems requires the design of scaledversions, which tend to have lower efficiency than the full-scale versions. In addition, they are highly complex to design and manufacture locally and are also relatively expensive. Similarly, Turgo turbines, which require heads of about 50 m, is not as efficient as Pelton and Francis turbines, and has efficiency below 90% (Benzon et al., 2016). Moreover, their blade design and manufacturing is also complex (Benzon et al., 2016). Therefore, improving the maximum efficiency would make crossflow turbines an ideal choice for small-scale applications. This is the fundamental motivation underlying the work of this thesis. Chapter 2 Thesis Objectives 2.1 Motivation and Background The turbine is the most important component of a hydropower system. The turbine affects not only its own performance, but also the performance of the entire system. Crossflow turbines are widely adopted in small-scale, low-to-medium head hydropower systems. The well-known and highly desirable advantages of crossflow turbine over its counterparts, such as Pelton and Francis, are the simplicity in design and manufacturing, low cost, and relatively flat efficiency characteristics. Moreover, the turbine is mechanically rugged and reliable. Unlike complex design shapes, such as the blades of Pelton and Francis turbines, the most prominent design feature is the use of circular-section blades, which can be easily designed 7 and rolled from thin steel sheet using simple machines at low cost. This is crucial because it is the blade that presents many challenges to the design and manufacturing of highefficiency turbines at low cost. The circular section blades are arranged radially around the axis of rotation and the blades are fixed to two circular discs at the two ends. Despite these advantages (design simplicity, local manufacturability, affordability, reliability, etc), a wellknown major issue is that crossflow turbines suffer from a lower maximum efficiency (70 - 86%) compared to more advanced turbines, such as Pelton and Francis, which can easily achieve efficiencies more than 90% (Dixon and Hall, 2013; Sinagra et al., 2014; Elbatran et al., 2015). The flow mechanisms in those turbines have been extensively investigated and well-understood, and the achievable performance gains through advanced design studies can be assumed only marginal. In contrast to this, less research has been conducted on crossflow turbines to understand the fundamental flow mechanisms underlying the power extraction, and the literature review indicates that lower maximum efficiency is due to the lack of fundamental research and not an inherent limitation of the design. The fluid dynamics of the flow in a crossflow turbine is rather complex, and has not been well-studied in the past from the viewpoint of improving the design. This thesis aims to improve the efficiency of crossflow turbines by improving our understanding of the underlying flow physics, identifying the dominant performance limiting flow mechanisms, and developing a rigorous design strategy to improve efficiency. Crossflow turbines consist of two main components as illustrated in Figure 2.1: a nozzle to control the flow entering the impeller blades and an impeller to extract the power from the flow. The impeller is open to the atmosphere. The impeller blades extract the power from the flow by creating a change in angular momentum of the flow passing through the impeller. The energy exchange is based on the kinetic energy of the water that enters and leaves the impeller at atmospheric pressure. The high velocity water enters the impeller with a 8 significant circumferential velocity, traverses the central air-space almost diametrically, and then exits the impeller with reference to the axis of rotation. The flow first passes through the first stage blades, then crosses the central region of the impeller, and passes through the second stage rotating blades before exiting the impeller at atmospheric pressure. Due to this unique flow characteristic and its operation at atmospheric pressure, crossflow turbines are considered as an impulse radial turbine (Shepherd, 1956). Many crossflow turbines have a guide vane in the nozzle, which helps to maintain high velocity at a suitable flow angle during part-flow operations. The smaller models are usually designed without guide vanes. Figure 2.1: Schematic illustration of the basic design features of a crossflow turbine. 9 The key design features are illustrated in Figure 2.1 with the exception of the flow control device which is usually a guide vane. In general, there are two methods for the design of modern turbines, direct design and inverse design (Yang, 1991; Logan Jr, 2003). A standard turbine design process follows the direct design method, which involves two main steps (Lakshminarayana, 1996; Logan Jr, 2003). The first is the preliminary design, which consists of establishing the turbine configuration and the calculation of design parameters (e.g. radius of blades, inner to outer diameter ratio, outer and inner blade angles, number of blades etc.) based on experience. In conventional designs, the calculation of design parameters is mostly based on previous experimental studies, which provide an empirical understanding of factors that are important to turbine design. The author’s experience, however, with small-scale hydro turbine manufacture in Nepal is that no account is taken of the actual flow behaviour in the design. The second is the detailed design phase, which involves detailed investigations of the specific flow physics (e.g. flow separation on blades) or guiding the design improvement via high-fidelity computations of Navier-Stokes equations, such as Reynolds-Averaged NavierStokes (RANS) simulations or Large Eddy Simulations (LES). In this design phase, the entire flow field is computed and the flow field information is synthesized to examine the loss mechanisms as an aid for improving the performance. Numerical simulations provide deeper understanding of the flow mechanisms that govern performance. This two-step design process is then iteratively repeated until an acceptable design is obtained. The main obstacle here is the requirement of a large number of simulations and evaluations of the flow field data, which demand for high-performance computers, long computational time, and efforts on flow field analysis. In the inverse design method, the objective function (e.g. pressure distributions on the blades) is prescribed and an optimum blade geometry is then computed as part of the design solution (Yang, 1991). Some limited inverse design techniques using 10 simplified flow models have been developed for the blade design of radial inflow turbines (Yang, 1991), but the inverse design using high-fidelity simulations are currently computationally expensive to be useful for practical design purpose. In this thesis, the direct design approach is followed using RANS simulations to approximately compute the flow field in the turbine, which helps in understanding the underlying fluid dynamic design problem as an aid for improving the efficiency. 11
