Novak P. Developments in hydraulic engineering

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LIST OF CONTRIBUTORS

G.D.ASHTON

US Army Cold Regions Research and Engineering Laboratory, Hanover, New

Hampshire 03755, USA

J.LUIJENDIJK

International Institute for Hydraulic and Environmental Engineering (IHE), Delft, The

Netherlands

D.M.MCDOWELL

Emeritus Professor of Civil Engineering, University of Manchester, UK. Present

address: 13 Powis Villas, Brighton BN1 3HD, UK

E.MOSONYI

Institut für Wasserbau und Kulturtechnik, Universität Karlsruhe, Kaiserstrasse 12, D-

7500 Karlsruhe 1, Federal Republic of Germany

E.SCHULTZ

IJsselmeerpolders Development Authority, Postbus 600, 8200 AP Lelystad, The

Netherlands

W.A.SEGEREN

International Institute for Hydraulic and Environmental Engineering (IHE), Delft, The

Netherlands

Chapter 1

WATER POWER DEVELOPMENT:

LOW-HEAD HYDROPOWER

UTILIZATION

E.MOSONYI

Institut für Wasserbau und Kulturtechnik, University of Karlsruhe,

Federal Republic of Germany

1 INTRODUCTION

The harnessing of water power resources had been abandoned or at least decelerated for a

few decades throughout the world after the prophecies of the 1950s: (a) that oil would be

available in sufficient quantities to cover all requirements and extremely cheap for a long

time, and (b) that nuclear power plants would very soon cover all the world’s further

energy demands without risk. It turned out, however, that both of these assumptions were

no more than naive expectations propagated unfortunately even by scientists and

professional engineers. Consequently, in some countries, the planning and

implementation of several promising hydroelectric projects has been delayed or

completely stopped for about two decades.

The author deems it unnecessary to discuss the above mentioned Utopian dreams;

facts have in the meantime justified the opinion of numerous hydraulic engineers

concerning the real value of water power.

Nevertheless, new obstacles to progress in hydropower utilization have recently

emerged because of its environmental impact. It cannot be denied that several defects,

even irreparable failures, occurred owing to ignorance or negligence of environmental

requirements. Overhasty planning and construction carried out under political pressure

has also been the reason for failure. On the other hand, unjustifiable attacks, even on

schemes planned with the utmost care for environmental protection, are becoming more

frequent in some countries. It is, however, the author’s firm conviction that recent

progress in research and up-to-date technology permits, in most cases, the selection of a

proper design and of measures compensating for or diminishing environmental impacts to

acceptable limits. Hence it can be presumed that the construction and operation of

hydropower projects entails, in general, disadvantageous environmental effects to a lesser

degree than that of any other type of power generating plants.

The competitiveness of hydroelectric plants has been significantly enhanced during

the last decades by progress in various fields of technology both in civil and mechanical

engineering, e.g. in new foundation and grouting methods, improved tunnelling

techniques, lightweight and thus less expensive steel structures, improved and new

designs of gates, turbines and generators, simplified superstructures, etc. A more efficient

technology permits a considerably shortened construction time in comparison with earlier

schedules, with substantial economic consequences.

It has to be borne in mind that preliminary studies, planning, design, construction and

installation, operation and maintenance of hydroelectric schemes are all of multi-

disciplinary character. Therefore stability, safety, operational reliability and economic

effectiveness of hydropower plants strongly depend on achievements in research in

several fields such as hydrology and meteorology, geology and soil mechanics,

hydromechanics of civil structures and machines, structural analysis and electrotechnics.

Especially research on vibration, cavitation, perfected instrumentation and measurement

methods in scale-model tests and new procedures of regional hydrological analyses have

contributed markedly to the improvement of plant designs. In addition, it must be

mentioned that a sound project evaluation may require, according to the specific features

of the case, a high competence in various other domains (biology, ecology, sociology,

economics, etc.)

Some remarks concerning the content of this chapter have to be made although typical

general layouts are displayed. Structures which may be a part of any other scheme except

hydropower plants are not further discussed in detail (e.g. dams, weirs, ship locks, fish

passes, intakes). A concise discussion of the turbines and electrical equipment is

essential, since the power plant planner is not able to elaborate even a preliminary sketch

of the plant without making an estimate of the number and type of generating units and of

approximate operational characteristics (specific speed, rated speed) and, consequently,

of the elevation and turbine runner diameter. It cannot be sufficiently emphasized that the

general arrangement of the entire powerhouse completely depends on the latter two

parameters.

Considering the fact that only about 10–20% of the economically feasible water power

resources of our world have been utilized up till now, it can undoubtedly be stated that

hydropower engineers may have still much confidence in their future. The reason for the

seeming uncertainty about the degree of exploitation indicated above can be explained by

the fact that the assessment of the quantity to which the harnessed power is related, i.e.

the economically feasible potential, continuously changes according to technological

progress and policy in economic evaluation.

The terms project, scheme and development are used as synonyms for plants,

comprising all structures erected and measures applied for utilizing a selected water

power resource. Water power, hydropower and hydroelectric plants are used as synonyms

too. The word station refers to the coherent complex of powerhouse and weir (spillway),

while a river barrage includes also ship lock(s). The cascade or chain of river barrages

rendering possible or improving navigation results in a canalization of the river. The

impounded headwater stretches are alternatively called reaches or ponds. Turbine and

wheel are applied synonymously. The terms unit, aggregate and set are synonymously

collective designations for the turbine and its generator, including, if any, the gear drive

too.

Developments in hydraulic engineering–5 2

Data in captions of figures showing projects and powerhouses respectively are given

in the following way: name, river, country, year of commissioning, plant discharge

capacity (plant design flow), rated net head (or in some instances the available head

range), rated (installed) capacity. This set of data, however, is not complete in every case.

A certain discrepancy in indication of the head could not be avoided because in some

publications only the gross head has been referred to.

2 SOURCES OF MECHANICAL ENERGY IN WATER

2.1 Physical Principles of Water Power Utilization

In order to obtain a clear picture of the practical possibilities for extracting power from

the natural potential of the watercourses, we must know what happens to the energy

inherent in natural rivers. Particles of flowing water possess potential energy depending

on the altitude and kinetic energy depending on the changes in the flow velocity.

Although kinetic energy of rivers arises at the expense of potential energy, it is

insignificant compared with the dissipated potential energy which is used to overcome

the friction of the turbulent flow, to supply energy to whirls, to scour the river bed and to

transport sediment. This mechanical work is finally completely converted into heat and is

lost once and for all.

Thus the fundamental principle of hydropower utilization is to reduce the dissipation

of energy. Consequently, a certain part of the potential power appearing in utilizable head

can be regained from nature. Friction losses can be diminished basically by reducing the

velocities by constructing a dam or by reducing the head required for the conveyance of

water by diverting the whole or a part of the flow into a by-pass canal, or finally by a

combination of the two methods.

Thus it depends mainly on topographical and fluvial-morphological conditions to what

degree the natural potential expressed in head can be harnessed from a technological

point of view. One has to bear in mind of course the various restricting conditions of the

region (anthropogenic development) and economic feasibility.

2.2 Units

In the field of hydroelectric development, power and capacity are usually given in

kilowatts (kW) or megawatts (MW), and the generated and producible energy in

kilowatt-hours (kWh) or megawatt-hours (MWh) or, in cases of high productivity,

gigawatt-hours (GWh). Formerly it was also usual to state turbine capacities or even plant

capacities in horsepower units. It has to be noted that there is a slight difference between

the metric horsepower (HP) and the British horsepower (hp):

(1)

Since mechanical energy (work) is usually expressed in newton-metres (Nm) or

kilonewton-metres (kNm) and the power (capacity) in newton-metres per second (Nm/s)

Water power development: low-head Hydropower utilization 3

or kilonewton-metres per second (kNm/s) it is necessary to introduce the relevant

conversion factors.

2,3

As (by SI definition)

(2)

and

(3)

it follows that

(4)

and

(5)

or, when comparing eqn (2) with eqn (5),

(6)

Since we are still in a transition period towards the universal usage of accepted

technological measures (kW and kWh) and SI units, and have to evaluate data from old

plants, it is desirable to recapitulate also older units for energy (work) (kg-force m or

kpm) and power (horsepower):

(7a)

(7b)

(8)

In connection with heat production and heat losses, the calorie (cal) and kilocalorie still

occur in publications:

(9)

(10)

and consequently

Developments in hydraulic engineering–5 4

(11)

To indicate the revolving speed of machines the ‘revolution per minute’ (rpm) is

commonly used, its dimension being 1/min. The frequency of vibration, cycles, waves or

impulses is mostly related to the second; thus, expressed in hertz units (SI equivalents):

(12)

and accordingly

(13)

2.3 Power in Flowing Water

When assuming uniform steady flow between two cross-sections of a watercourse, the

power represented by H metres difference in water surface elevation between two

sections and which dissipates along the stretch L can for a flow of Q m

/s be expressed as

(14)

where v

and v

are the mean velocities in the two sections. Neglecting the usually slight

difference in the kinetic energy and substituting for γ=ρ

=9810 N/m

, we obtain

(15)

or from eqn (4) (for Q in m

/s and H in m)

(16)

Equations (14)–(16) give the theoretical (physical or potential) power of the selected river

stretch at a specified discharge.

A watercourse consists of stretches between major tributaries and/or outflows, which

can be characterized, during a specified state of flow, by constant or averaged discharges.

Accordingly the theoretical power resource of any river or a river system, at the selected

state of flow is

(17)

where Q

and H

are the values pertaining to stretch i.

When evaluating integrated power resources for an entire watercourse or drainage

basin (river system) there arises the question of what values of H and Q should be used in

the calculation. If we disregard very short river stretches, the change of H with stage is

Water power development: low-head Hydropower utilization 5

negligible; on the other hand, a thorough examination of the discharge variation is of

great importance.

Potential power is evaluated to obtain an inventory of possibilities and for ranking the

feasible power sites. Therefore diverse magnitude of the potential hydropower resources

are usually determined on the basis of geodesical and hydrological measurements:

1. Minimum potential power is based on the smallest runoff, having a duration of 100%

p100%

). This value is usually of small interest.

2. Small potential power is calculated from the 95% duration discharge (P

p95%

3. Median or average potential power is gained from the 50% duration discharge (P

p50%

4. Mean potential power results by evaluating the annual mean runoff (P

The sum of these values for an entire stream is termed gross river power potential

(Economic Commission for Europe (ECE), Committee on Electric Power).

Since it is not economically feasible to harness floods, and therefore the utilization for

every site is usually limited to an individually selected plant discharge capacity, there is

no reason for including the entire magnitude of peak flows in the inventory either of the

potential power or of the potential annual energy. Therefore, the annual discharge

duration curve will be truncated at a certain t days duration of discharge, which can be

simply the median (~182 days or 50% duration, denoted by Q

182

or Q

50%

), or a higher Q

(t<182 days) can be selected by specialists who are familiar with the local conditions and

future plans for power supply. Accordingly the annual magnitude of potential

(theoretical) energy can be computed in kWh (Fig. 1):

(18)

where Q

denotes the daily mean flow during the period 365−t and A the hatched area cut

by Q

. (Dimensions kNh/(m

×day/s) pertain to the factor 235, while A is read in

×day/s.)

Neither all potential power nor potential energy is available for direct

FIG. 1. Annual discharge curve.

Developments in hydraulic engineering–5 6

utilization. Only a part of the geodetically evaluated difference can be regained by

providing a concentrated drop in the water surface, the so-called head available for the

turbines (see Section 2.1), and for many physical reasons and because of anthropogenic

influences not every river stretch can be earmarked for full or even partial utilization.

Besides, there are instances where, because of demands for water with higher priority

than power generation, the natural discharges are not fully available for the power station.

Accordingly, the duration curve has to be modified to obtain the reduced discharges

utilizable for the turbines. With diversion type projects, generally also a predetermined

permanent duty flow has to be provided for in the river stretch by-passed by the power

canal. Thus, after having selected a power site, the producible power, at any available

gross head and discharge, can be established as (see eqn (16))

(19)

where all head losses and the constant 9·81 are integrated in the overall power coefficient

of the plant c

which, according to the individual technical solution, shows a variation

roughly between 7 and 8·5.

Thus the technically available resources and the practically feasible power estimates

can be established. A further question is which of the envisaged schemes can also be

accepted as economically promising ones and, if so, at what discharge capacities. This

evaluation, however, changes in the course of time, since it greatly depends on regional

or even international technological power development and on the power policy.

The plant discharge capacity, also termed plant design flow, has undergone a

remarkable change during the last 100 years.

Figure 2 demonstrates this evolution,

reflecting progress in co-operation of power suppliers and in power technology. Thus the

degree of utilization at some modern plants attains magnitudes in the range of Q

25%

15%

(even up to ~12%).

The scope of this treatise does not permit either a detailed presentation of theoretical

potentials and installed capacities according to river basins and countries in the world, or

a survey of the changes of economic estimates and historical development;

6,7

only an

earlier summary by continents is given in Table 1.

2.4 Energy Capacity of Reservoirs

If a stream flows out of a reservoir created by a river barrage or high dam, the energy of

all water particles at the beginning of emptying is H

, i.e. height of water level in the

reservoir in relation to a selected datum level.

Water power development: low-head Hydropower utilization 7

FIG. 2. Examples revealing the

progress in degree of utilization.

(Compilation by the author.)

TABLE 1

ESTIMATE OF HYDROPOWER

EXPLOITATION IN THE WORLD

Capacity in GW (1000 MW) Continent

Available Exploited (installed)

Rate of exploitation (%)

Europe 290 190 65

North and Central

America 375 105 28

South America 690 15 2

Asia 1350 45 3

Africa 1100 13 1

Australia and Oceania 150 10 7

World 3955 378 10

Based on data compiled in the 1970s; the installed capacities may be, at present, about 5–20%

higher.

Developments in hydraulic engineering–5 8

Novak P. Developments in hydraulic engineering - Vol. 5

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