Subido por Cristian Polo Vallejos


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.
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
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
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
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
, 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
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
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).
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.
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):
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
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.
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
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
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
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.
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
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