Novak P. Developments in hydraulic engineering - Vol. 5
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9 PRELIMINARY PLANNING OF THE POWERHOUSE
9.1 Powerhouse Equipped with Conventional Axial Turbines
Assuming that the power site, the general arrangement of the entire river barrage, the
highest possible HWL have been fixed and the necessary hydrological data are available,
the power curve for the station has to be established (see Fig. 36). The next step is an
estimate on the number of generating aggregates. This can be made on the basis of
experience or by optimization, i.e. by repeating the entire forthcoming procedure with
diverse numbers of units and by comparing the investment costs.
When the unit capacity, the rated head and, from eqn (54), the highest permissible
specific speed are known, the rated operational speed can be obtained from eqn (42). A
recalculation with the next synchronous speed ensues. Further, eqn (47) presents the
runner diameter. These are characteristics for the limit design. Based on experience and
studies
95,124
the author suggests an informative sketch on the general arrangement of the
complete system of passages with the limit values as shown in Fig. 55.
96
The limit setting, i.e. the highest permissible level of the runner exit in relation to
TWL, is established by using the Thoma equation (eqn (38)). If the head greatly varies
and, particularly, if the lowest head is much smaller than the rated one, the setting level
has to be checked for the limit heads too, since it may occur that the σ
max
value at
minimum head is so high that the σ
max
H
min
product significantly exceeds the one
pertaining to the rated head. Such a situation may require a lower setting, even when
taking into account that this smaller h
s
has to be related to a higher TWL (because, as
shown in Fig. 36, the lowest heads develop at highest tailwater elevations). Similarly, the
h
s
evaluated from a cavitation coefficient σ
min
and the highest head H
max
has to be related
to the lowest TWL.
The possibility of pondage (storage) has to be investigated as it may greatly augment
the economic value of the plant.
The height of the draft tube exit section depends partly on the selected overflow
velocity and partly on the criterion that the draft tube port must be submerged below the
lowest TWL. The length of the intake flume is determined by the requirement on space
for the installations within the passage. Following Fig. 11 from right to left the usual
arrangement can be observed.
The intake section has to be sufficiently submerged below minimum HWL to avoid air
intrusion. Sometimes an entrance sill is formed. In the case of large intake cross-sections
the racks (screens) are strongly supported and care must be taken to achieve a vibration-
free design.
125,126
Water power development: low-head Hydropower utilization 79
FIG. 55. Informative dimensions of the
water passages of a conventional
Kaplan setting (author’s suggestion).
The hydraulic loss at the trashrack can be calculated by eqn (25). Reliable manual or
mechanical cleaning of the rack is indispensable. Efficient raking machines are needed in
up-to-date plants, especially if much floating debris can be expected.
Stop-log closure must be possible. In most cases a head-gate (intake gate) is installed
between the stop-log slots and the spiral case. A single crane may serve both the stop-
logs and the head-gate. For repair purposes stop-logs are also foreseen in the draft tube or
at the end of it. In the case of very large dimensions the roofs of both passages (intake
flume and draft tube) can be supported by dividing walls.
127–129
The superstructure of the powerhouse protects the generator and other electrical
equipment from rigours of weather. Indoor arrangement means the establishment of a
machine hall with the major crane inside (Figs 11, 14–16, 24, 25). Generators of the
semi-outdoor solution are housed in a deeper located low-roof gallery or chamber, while
the crane moves outside, lifting machine parts through hatches in the roof (Fig. 21). In
Developments in hydraulic engineering–5 80
some plants no superstructure is erected at all and the generators are individually
protected by weatherproof metal enclosures (Fig. 13).
Main auxiliary installations of the plants are: various types of bridge and gantry
cranes, dewatering and drainage systems, water collecting sumps and pumps, lighting and
ventilation systems, fire alarm and protection equipment, oil supply systems, heating and
domestic water supply for the operating personnel.
Discussion of foundation methods and structural analysis of the various components of
the powerhouse
130
are beyond the scope of this treatise. Avoidance of vibration and
power swings is an important task of the designer.
131–134
Scale model studies for the
proper layout of the entire river barrage and for the powerhouse bays are in most cases
advisable.
53,135,136
9.2 Powerhouse Equipped with Tubular Turbines
9.2.1 Bulb-Generator Setting
The first part of the procedure is similar to that discussed above, but there are slight
differences as regards the estimate on permissible n
s
, runner diameter D and static draft
head h
s
, as explained in Section 7 and obtainable from recent studies.
118,137–140
Figure 56 illustrates a suggestion by the author for a rough estimate on main
dimensions of the passages as a function of the runner diameter. From the tubular
arrangement of the bulb unit it follows that the lowest TWL is always higher than the
upper tip of the runner blade (negative suction head). [Data from the literature have been
carefully checked since in many cases manufacturers and authors relate the TWL to the
axis elevation of the runner and define the suction head (static draft head) accordingly.]
Rack and rack-cleaning devices are necessary but the headgate is usually dispensed
with. Stop-log closures upstream and downstream of the machines are always provided
for (e.g. see Fig. 17). Hatches and shafts have to be provided to facilitate dismantling the
machines. Figure 57 shows three solutions: (a) in case of small bulb diameter a single
shaft for both generator and turbine is sufficient; (b) a large-diameter bulb needs a
FIG. 56. Informative dimensions of the
water passages of a tubular bulb setting
(author’s suggestion).
Water power development: low-head Hydropower utilization 81
separate hatch for bulb and generator dismantling; (c) the pit arrangement provides the
easiest way of handling the bulb. The interior of large bulbs is approachable by an access
shaft.
9.2.2 Rim-Generator Setting
At present the author has no satisfactory data to present general guidelines for the rim-
generator arrangement but, according to recent publications by experts from Escher
Wyss, the length of this arrangement can be designed shorter than that of a powerhouse
equipped with bulb machines, because the length of the inflow passage (i.e. the distance
between rack and runner) can be reduced.
80,141,142
This advantage, however, can only be
realized if the head is very low (see Fig. 46), while cases occur where the higher head
and/or the requisite deep-setting of the runner govern the length of the inlet flume (e.g. in
the powerhouse shown in Fig. 18).
FIG. 57. Arrangements for the
dismantling of bulb units.
118
10 SMALL SIZE HYDROPOWER PLANTS
In Section 3.2.1 reference has already been made to ‘small hydropower’ (SHP). Within
the SHP range limited by around 5 MW (or even 10 MW) some US publications
distinguish two further sub-groups: plants are termed micro up to 100 kW and mini
between 100 kW and 1 MW. The particular features and requirements respectively
concerning small-size developments can be summarized as follows:
— Simplified hydrological and other physical studies which should be based, if possible,
on regional analyses since the implementation of small plants can, from the economic
point of view, be completely jeopardized by long-lasting, sophisticated and
consequently too expensive investigations.
— Planning and construction needs highly qualified ‘all round’ engineers, capable of
carrying out many professional tasks, possibly with only some short-term assistance
from a few specialists and manufacturers.
— Use of local construction materials as far as possible.
— Use of standardized turbines. Individual turbine design is mostly too costly.
— The selection of simple (though not over-simplified) electrical equipment.
— The use of induction generators, if network conditions permit, may result in cost
reduction too.
Developments in hydraulic engineering–5 82
— In mini and micro plants ‘package’ machine units can also be installed
advantageously.
We are witnessing extremely intensive worldwide progress in SHP development; thus
SHP has become a very popular topic for national and international conferences. Since
innumerable papers have recently been published on scientific hydrological,
technological, ecological, economic and social aspects of SHP, the author instead of
quoting individual treatises would like to refer to the proceedings of the most important
meetings only.
143–146
Also a hand book
147
and specified chapters in text books on SHP
furnish theoretical guidelines, design criteria and instructive examples for the
planner.
148,149,149a
11 ENVIRONMENTAL AND AESTHETICAL ASPECTS
It is necessary to supplement the plan, even at the first phase of its elaboration, with an
assessment on environmental feasibility. It is obviously not an easy task to accomplish an
objective analysis and a quantitative evaluation about the impact of the project on the
environment. Although several effects can, with sufficient physical, biological and other
data, be more or less acceptably forecast (e.g. influence on groundwater table, change in
water quality, impact on wildlife and fish), others can be evaluated only qualitatively or
very subjectively (e.g. impact on micro-clima, recreation, landscape and architectural
design).
Evidently a general statement that ‘the structures of the project have to harmonize with
the landscape and must not disturb the scenic beauties of the surroundings’ would
definitely comply with the justifiable requirements; nevertheless, what is ‘harmonizing’
and what is ‘disturbing’ are fairly subjective judgements, though it can generally be
stated that the less that can be seen of the structures (i.e. the lower the superstructures
and, in the case of semi-outdoor or outdoor solutions, the more efficiently the huge gantry
cranes can be shifted into an inconspicuous position; see Section 9.1) the better.
The partly or completely quantifiable physical and ecological impacts have to be
analysed in the preliminary phase of planning so that ‘environmental surprises’ are not
encountered in the final stage of design, which may enforce a complete re-planning of the
project entailing an excessive increase in costs, or even jeopardize the implementation.
Recently efforts have been made in several countries to improve environmental
qualities of hydropower plants, and several measures have been devised to diminish or
even compensate for unfavourable environmental impacts.
150,151
As an example, the Lech
River development in the German Federal Republic may be quoted.
152
In order to replace
biotopes submerged in the headwater ponds, artificial islands and irregularly shaped, low
slope embankments without revetments were created. Drainage and canals stabilizing the
groundwater table were shaped to simulate natural brooks. Submersible plants without
any superstructure are not conspicuous in dense forests. Low-roof powerhouses equipped
with tubular aggregates were designed to harmonize with the landscape. The headwater
reaches, where no conflict with nature preservation can be presumed, are used for
recreation and sport both in summer and winter. Finally, an ancient diversion weir
(probably constructed as early as about 1200 AD) has been reconstructed according to its
Water power development: low-head Hydropower utilization 83
original picturesque design to re-establish the original fluvial landscape at the old city of
Landsberg.
Because of the very widespread and sophisticated nature of environmental
implications, checklists concerning all conceivable environmental aspects should be
established.
153
In most cases evaluations of social, land-use, health and legal requirements
are inseparable items of such an analysis.
154,155
The assessment process can be facilitated
by using a methodology for systematizing, weighing and quantifying both detrimental
and favourable environmental impacts.
156,157
Small hydropower plants do not usually cause serious environmental problems;
especially micro and mini developments may even offer ecological advantages for the
watercourse and its surroundings. SHP plants can substantially contribute to erosion
control in hilly and mountainous regions as worldwide destruction of forests could be
stopped or at least decelerated by replacing firewood with hydraulic energy. According to
the author’s opinion a 100–120 kW midget plant can replace a metric ton of firewood per
day.
158
On the basis of the above considerations it seems to be justifiable to evolve specific
methodology on environmental impact assessment for SHP developments.
159
12 ECONOMIC APPRAISAL
12.1 Methods for Economic Evaluation
The economic analysis of a water power project is based on the comparison of all
expenditures during its planning, design, construction and operation with the benefits
obtained from the energy production over its so-called financial lifetime (amortization
period). On the basis of experience over several decades, hydroelectric plants can usually
be credited with an effective lifetime (availability until their obsolescence) substantially
longer than their financial lifetime. Negligence of this fact not seldom leads to
undervaluation of hydropower plants.
The current methods of economic evaluation are:
(1) Present-value (present-worth) assessment
(2) Internal rate-of-return evaluation
(3) Annuity method
(4) Benefit-cost ratio analysis
(5) Estimate on unit production cost
Since, during implementation of the project (preliminary studies, investigations and
model tests, general planning, detailed designing, construction and installation), partial
costs of the investment arise at diverse times and, similarly, the cash flow after
commissioning manifests itself in annual OMR (operation, maintenance and repair)
expenditures and annual benefits, it is imperative to convert all these cost and revenue
items into equivalent monetary terms, i.e. to discount them to a selected common time by
using a predicted interest (discount) rate and amortization period. The salvage value
(buildings, lots, switching and sub-stations) should sometimes also be included as it may
improve the economic parameters. On the other hand, it happens too often that the
Developments in hydraulic engineering–5 84
indispensable supplementary measures for environmental control are not, or not reliably,
accounted for in the investment estimate, thus palliating the economic indicators.
Methods (1), (2), (3) and (5) may be termed as classical procedures of economic
analysis, while the benefit-cost ratio (B/C ratio) was first suggested a few decades ago
and at one time became so popular as to turn almost into a ‘fashion’ for evaluating
engineering and other projects. In the author’s opinion the B/C ratio is not a true
economic characteristic and, besides, it can be misleading because its mathematical
structure makes easier a broader manipulation than is rendered possible by application of
the other methods.
160
Still, the B/C ratio should not be excluded for it can efficiently be
used for preliminary screening of project variants, provided that common evaluation
procedures are used. The reader may find the detailed description of the calculation
processes of methods (1) and (4) in any textbook on economics
161–163
and also in
hydraulic engineering publications.
160,164–166
In some types of industrial enterprises, and also with hydropower plants, the fifth
economic indicator, i.e. the unit production cost, is preferred for decision making, i.e. the
overall production cost of one kilowatthour of energy is determined. The annual cost
consists of the fixed (first) cost and the running expenses (OMR). Assuming a uniform
annual repayment over the n-year amortization period, the so-called capital recovery
factor
(58)
including both capital repayment and interest charge, is calculated where i is the interest
rate. Sometimes (mainly in Europe) the q=1+i substitution is used, so that with the q
interest factor eqn (58) appears as
(59)
After uprating all investment costs to the beginning of the year in which the presumed n-
year power production commences, and thus obtaining an I $ integrated present-worth
investment, the annual fixed cost, according to the sinking-fund depreciation procedure,
equals
(60)
Denoting the annual running cost (OMR), including management and taxes, by C
2
and, at
least in the first approach, supposing its uniform distribution, the total annual cost to be
charged to the yearly power production equals C
1
+C
2
dollars. Accordingly, the unit
production is
(61)
where E indicates the producible kilowatt-hours in an average year.
Water power development: low-head Hydropower utilization 85
The experienced planner, in possession of some hydrological data, is often able to
make a reliable estimate on the utilization hours t
u
of run-of-river schemes (see Section
5.3 and Fig. 39). Thus, considering a rated plant capacity of P kW, the annual energy,
according to eqn (34), will roughly total to Pt
u
kWh, and eqn (61) can be expressed as
(62)
It has to be emphasized that the calculation of the capital recovery factor according to the
above extremely simplified procedure, i.e. by assuming a constant n value for the
complete investment, is only acceptable if the planner has sufficient experience for
assessing a fairly reliable fictitious amortization period being representative for the entire
project. Otherwise, as is common practice, diverse amortization periods have to be
assigned to the various elements of the plant. The heavy civil engineering parts (concrete
and reinforced concrete structures, canals with their revetments, stone- and earth-works)
have, assuming proper quality, a much longer life than the steel structures, machines and
electrical equipment. Since, however, it is not possible to appraise the different n periods
by an exact calculation procedure, the planner has to rely upon guidelines or directives
issued by competent authorities or major hydroelectric enterprises. A concise compilation
of some informative data collected from relevant publications is given in Table 5.
167,168
Since the result of the economic analysis is very sensitive to the choice of both interest
rate and amortization period, it is obvious that the fairly wide ranges in Table 5 involve a
high degree of uncertainty and can be accepted as rough information only. In every
individual case the specified guidelines depending on the features of the relevant capital
market and monetary policy have to be adopted. This principle is also valid when
choosing the discount rate to be used for the planning.
It is usual to estimate, on the basis of long experience, the annual OMR cost as a
function of the entire investment cost (capital):
(63)
where the running cost factor b[1/a] roughly varies between 0·005 and 0·025 (0·5–2·5%),
the lower values pertaining to high plant capacities and vice versa.
The capital demand, and in a very approximate way, the economic attractiveness of
power plants is usually characterized by the specific investment:
Developments in hydraulic engineering–5 86
TABLE 5
RANGES OF AMORTIZATION PERIODS FOR
ECONOMIC ANALYSIS OF HYDROELECTRIC
PLANTS
a
Nature of asset Period
(years)
Heavy earth, rock, stone, brick, concrete and reinforced concrete structures (canals,
tunnels, caverns, dams, weirs, intakes, etc.)
50–100
Timber structures with proper impregnation (sluices, gates, flumes, etc.) 20–25
Superstructures (machine halls, service buildings, dwelling houses, etc.) 35–80
Steel structures (gates, cranes, racks, etc.) 20–40
Machinery (turbines, generators, transformers, switchgear, auxiliary equipment,
cables, etc.)
20–45
Transmission lines 20–25
Preliminary investigations 50
a
Based on guidelines adopted by various government authorities and national power boards.
(64)
i.e. the capital required for one kilowatt installed capacity.
Substituting eqns (60), (63) and (64) into eqn (62) we obtain
(65)
The degree of utilization of a power site (head and/or plant design flow) and the number
of generating units are sometimes established by an optimization process. Nevertheless, a
reliable optimization needs fairly accurate physical, technological data and realistic
assumptions of various types of costs and, especially of the discount rate and
amortization period, since otherwise the result (optimum value) may be pointless and
misleading. To avert such a mistake a sensitivity analysis is essential. (In the words of
L.D.James and R.R.Lee: ‘Benefit-cost analysis can be and has been corrupted’.
169
)
Finally, a brief reference to multi-purpose hydroelectric schemes has to be made. A
detailed discussion of this topic, however, is beyond the scope of this presentation. From
a financial and economic point of view it is in most cases a very sophisticated task for the
planner to perform the allocation of the investment and OMR costs to the diverse project
objectives. Various cost allocation procedures, resulting in very different cost
distributions, have been suggested.
160,164,170,171
These methods have a common element:
the so-called separable costs must first be set aside, i.e. those costs have to be evaluated
which are clearly chargeable to every single project purpose. Thereafter, the so-called
Water power development: low-head Hydropower utilization 87
joint cost has to be distributed according to a—mostly subjectively—selected allocation
principle. From among the numerous proposals two are named here:
(a) The remaining-benefit method
(b) The alternatively justified expenditure analysis
In many cases, however, economical concepts are not decisive for the decision maker
since political and/or social aspects also govern the allocation of the investment.
12.2 Informative Data
It is not an easy task to collect data on investments and actual energy production costs.
Therefore, any comprehensive publication concerning actual financial and economic
features of projects deserves credit.
7,172
Because of the obviously enormous scattering of
the investments, and due to the significant escalation of prices in time, it does not seem
appropriate to present cost data of general validity. Besides, as Gordon pointed out,
172
the
costs at newer sites will be higher than those at previous sites, because the most economic
sites will be exploited first. Based on his former evaluations, Gordon derived empirical
formulae for average new sites. In general, it can be stated that the specific investment
cost (I
0
) diminishes with growing head and with increasing capacity. According to a
diagram by Gordon, the specific investment cost in the low-head region, provided that
optimum site conditions exist, can roughly be estimated as follows (at 1982 prices):
Let us evaluate the unit production cost for a high capacity run-of-river plant constructed
on a navigable river (e.g. on the Danube), providing good site conditions. The total
investment in the river barrage, also including expenditures for all necessary structures
along the backwater reach, can be estimated on a current price level as 550×10
6
US$. The
main design data of the project are: Q
p
=3150 m
3
/s, H=11 m, P=300 MW (nine bulb-
Kaplan units at 34·4 MW), E=1720×10
6
kWh/a.
The specific investment in the entire double-purpose project amounts to
(550×10
6
)/(300×10
3
)=1830 $/kW. In cases when, on rivers with heavy traffic, it is
assumed that the continuity and safety of navigation (with special regard to the low-water
periods) can be enhanced by construction of the barrage, it is usual for the government to
meet a significant portion of the expenditure by allocating it to navigation, i.e. investment
and OMR costs of the ship lock(s) and possibly a certain part of the weir and of the
measures in the backwater reach. In the case under discussion it would be 30%. Thus, the
specific investment cost to be charged to power generation is reduced to 1280$/kW. The
capital recovery factor has been assumed at 8%, the running cost factor 2%. Hence
a*+b=0·10. By substituting this value and the annual utilization hours
Developments in hydraulic engineering–5 88