Guide on how to refurbish low head small hydro sites

Guide on how to refurbish
low head small hydro sites
What’s peculiar? Definitions
When to refurbish
Basic technical-economic presuppositions
Fields of action
Site configurations
What’s peculiar? Technical and methodological issues
Preliminary studies
Actions on Q
By reducing waterways roughness (case study n. 9)
By increasing the area of the cross section of the waterways
[e.G. From earth-trapezoidal to concrete-rectangular] – (case study n. 9)
By increasing the flow rate by negotiating a new water right with a higher rated discharge (case study n. 7)
Actions on H
By increasing the weir elevation (case study n. 6 And case study n. 7)
By restoring the initial gross head
By replacing old screens equipped by rectangular bars with screens with hydrodynamic profile
Actions on ␩ and T - Turbine and generator replacement
What’s worth replacing?
Works below the tailrace water level (case study n. 3)
Fluid-dynamics design of the draft tube
Embedded parts
How many units?
Replacement of an old TG unit with a new one of the same type
Francis open flume turbines (FOF) - case study n. 4
Kaplan turbines (case study n. 5 and case study n. 6)
Replacement of an old TG unit with a new one of a different type (case study n. 3)
Replace an old FOF turbine with a vertical bulb turbine (VBT) in the same pit
Actions on T
By improving sediment management
By improving trash management
By improving floating debris management (case study n. 2)
By improving weir operation safety (case study n. 1)
By setting or improving plant automation
Environmental added value
Flood management added value
Guide on how to refurbish low head small hydro sites
EFFICIENT SOLUTIONS. SHAPES is a project partially funded by DG-TREN
(European Commission) within FP6 and it has been running for two and a half
years starting in December 2007.
The overall objectives of SHAPES is to facilitate and strengthen the co-operation
between EU Small Hydropower (SHP) Research and Market actors with the
overall objective of streamlining future research & development and promote
R&D results in order to enhance penetration of SHP and know-how within the
EU and on new markets in developing countries.
off low
h present guide
id is
i addressed
d to people
l interested
d in
i the
h refurbishment
f bi h
head small hydroelectric plants and already having some basic knowledge of
the main technical issues involved in the small hydro subject.
That’s why in the guide, in order to keep it of a reasonable size and as effective
as possible, the aforesaid basic knowledge is taken for granted.
Furthermore, the guide will be focused on its three cross-specificities:
refurbishment, low head and small hydroelectric plants. For general issues
related to small hydropower and hydropower in general the reader can refer to
the “Guide on How to Develop a Small Hydropower Plant” published by ESHA
and available at
According to this approach, addressed only to the specificities of the subject,
the guide doesn’t enter in any detail the usual activities connected with the
implementation of a small hydroelectric plant (collection and interpretation of
hydrological data, estimation of gross and net head, economic evaluations etc.),
but it refers to those activities only when they’re relevant to the subject and it’s
advisable to face them in a particular way.
Finally, at the end of this document, several case studies are presented, which
most of the following chapters will refer to.
What’s peculiar?
Restoration of the integrity of
the main parts of an existing
plant, with restoration or
improvement of the plant
in terms of energy output,
reliability of operation and
environmental soundness
Hydropower plants
with an installed capacity
up to 10 MW
Strong constraints
(spatial, functional,
environmental, operational)
Strong constraints
(spatial, functional,
environmental, operational)
Gross head = 1,5 ÷ 30 m
Pico: P ≤ 1 kW
Micro: 1 < P ≤ 100 kW
Mini: 100 < P ≤ 1.000 kW
Small: 1.000 < P ≤ 10.000 kW
Very Low Head: 1,5-3,0 m
Low Head: 3,0-30 m
Guide on how to refurbish low head small hydro sites
to refurbish
Refurbishment as a necessity
When some components of the plant need to be changed, because they
compromise the economics of the plant, there is no choice on which parts need
refurbishment, but only on how to operate. The analysis of the future risk of
failure can anyway be helpful in setting some priorities of actions.
Refurbishment as an opportunity
Even if there is no particular necessity, refurbishing can be an opportunity
to increase the overall efficiency and the energy production of the plant (also
taking advantage of possible incentives).
As you can choose what to refurbish, a priority scale on which parts of the plant
are to be upgraded can be defined on a technical and economic basis.
Basic technical-economic presuppositions
Refurbishing a low head plant needs money, and usually a lot of money, as small
hydro investments are capital intensive. Furthermore, low heads plants highly
suffer from scale effects, mainly because, if compared to a high head plant with
the same output, they have a greater incidence of civil works cost.
In any case, when you have to refurbish the plant you must rank different project
alternatives, included the zero alternative, that is no refurbishment at all. The
best method for doing that is through the NPV (Net Present Value) method:
I = lifetime period as year, quarter, month, etc
Ii = investment in period i [Currency unit]
Ri = revenues in period i [Currency unit]
Oi = operating costs in period i [Currency unit]
Mi = maintenance costs in period i [Currency unit]
Vr = residual value of the investment over its lifetime,
of action
where equipment lifetime exceeds the plant working life
[Currency unit]
Ev = environmental added value of the plant due to the
refurbishment [Currency unit]
r = periodic discount rate [%]
n = number of lifetime periods
For the scope of this guide it’s important to make more explicit in the NPV
formula the term related to revenues Ri
Pi = price of the energy produced in period i [Currency unit/kWh]
Ei = energy produced in period i [kWh/time unit]
␩i = overall efficiency of the plant in period i [%]
= unit weight of water [kN/m3 ]
Hi = net head in period i [m]
Qi = flow rate in period i [m3/s]
Ti = hours of operation in period i [h]
The said formula, coupled with the NPV formula, leads to:
Each factor of this formula represents a field of action in the context of a refurbishment, exception made, in general, for the
financial parameter r and the energy price Pi.
The environmental issue needs a short comment. Environmental added value can’t be reduced to a purely monetary
value. In this context there’s only the need of taking into account, in the following considerations, the fact that, during the
refurbishment, different alternatives can have different environmental consequences, both in terms of purely monetary
costs and even of environmental benefits.
Guide on how to refurbish low head small hydro sites
Site configurations
Diversion plants
Power house located
In addition to the action envisaged for at the toe of a weir.
toe-of-weir plants, many actions can be In this case the actions are mainly
addressed to hydraulic improvements addressed to units replacement and
of the waterways
to the improvement in trash and
sediment management
What’s peculiar?
Technical and methodological issues
Once defined the main relevant elements, it’s necessary to point out the
consequences of such definitions. To some extent, refurbishment needs a
general approach, independently from the head of the plant. This approach can
be a stepwise one:
Risk of failure analysis
Preliminary studies
Current performances
Setting economical
Current main plant
parameters (Q,H)
Technical constraints
Setting priorities
Legal constraints
Final technical
Low head plants have many technical peculiarities. The actions during refurbishment must be taken accordingly:
Guide on how to refurbish low head small hydro sites
Reliability of operation
(no higher efficiency at
any cost...)
Optimisation of
civil works AND
hydro units
Simpler, though
less efficient,
of sliding
Diversion plants
- Possibility of
Replace open
channel with
Reduction of
civil works
below water
Reduction of
with existing
Flap gates vs.
slide gates
Weir safety
Suitable span to
avoid clogging
Reshaping intake
Floating bodies
log and debris
Trash Rack
of slope
of physical
Energy backup system
(Diesel...) or better no
energy for gates operation
of leakages
Flow area
Open channel
Replace trapezoidal
with self-bearing
rectangular section
Desilting devices
Flow lines modification
Attraction flow to desilting
device improvement
Horizontal bars screens
Guide on how to refurbish low head small hydro sites
Just like for any other hydroelectric initiative, even in case of refurbishment
some basic data must be acquired or estimated in order to have the clearest
possible picture of the status of the plant.
Water rights
Before refurbishment
After refurbishment
Duration of concession [years]
Legal restrictions
Parameters influencing economics
Before refurbishment
After refurbishment
Annual production [MWh]
Tariff structure [€/MWh]
Annual income [€]
Age of electromechanical
equipment [years]
Age of civil works [years]
Age of gates, TRCM [years]
Age of control system [years]
Maintenance costs [€/year]
Annual Outages [hours]
Environmental requirements
Before refurbishment
After refurbishment
Before refurbishment
After refurbishment
Residual flow [l/s]
Fish by-pass
Technical parameters
Plant rated discharge
Plant rated gross head
Hydraulic losses: weir
Hydraulic losses: intake
Hydraulic losses: headrace
Hydraulic losses: trash rack
Hydraulic losses: forebay and
turbine intake and outlet
Hydraulic losses: tailrace channel
It’s vital to collect as many information as possible on the plant operation:
• Unit(s) trips: how many? how long? why?
• Plant outages: how many? how long? why?
• Repair: what, when, why
• Maintenance: actions taken, unexpected events encountered and solved
Actions on Q
10.1 Foreword
When the decision of refurbishing a small hydro low head plant is taken, an evaluation of the actual flow duration curve
(FDC) of the river and of the existing plant should be done in any case. It’s not unusual that the initial FDC of the river
has changed over the years and has to be updated. Furthermore, the effect of the obligation of releasing the reserved flow
(if never released) or a higher reserved flow due to stringent environmental requirements (if already released) should be
taken into account. At least the two aforesaid evaluations should be done, especially when the replacement of the unit is
envisaged in order to design properly the turbine for getting the highest possible weighted efficiency.
Also climate changes can affect the future FDCs: e.g. in Mediterranean countries more intense and short rainfalls are
expected with longer dry periods, so that a turbine designed to exploit higher flow rates at a higher efficiency could be an
option. But it’s not immediate that the plant can convey a higher flow without actions on weir, intake, waterways, etc. and
also has the legal right for diverting more water from the river (or that this right can be easily acquired).
That’s why, apart from legal aspects, it’s worth spending some words on the actions for increasing the flow rate conveyed
by the plant.
Guide on how to refurbish low head small hydro sites
10.2 By reducing waterways roughness (case study n. 9)
The easiest and cheapest way to increase flow rate in a low head plant is reducing the roughness of the waterways, namely
headrace and tailrace channels, the weir crest level being equal: smoother surfaces of walls and sills allow for smaller
friction losses and, water level being equal, for a higher flow rate conveyed.
The ways to get a smaller roughness depend on the structure of the waterways: in earthen channels (not very usual, especially
for headrace channels, but more frequent for tailraces) the possible actions are few and mainly linked with keeping the
slopes clean from weeds. In artificial channels the task is easier: on the market many different types of special products
(one or two components waterproof mortars incorporating admixtures, fibres and comprising liquid polymer and special
cements) are available for this purpose; it must be noted that those products must be applied very carefully taking into
account the conditions of the surface where the product is applied, the weather conditions and so on, because they are very
sensitive to all of these conditions, and in case of improper application the results can be a real disaster, even worsening
the situation before application. Anyway don’t trust too much those kinds of actions in the evaluation of the final plant
performances in terms of flow rate conveyed, because roughness is bound to increase in time due notably to the wearing of
the coatings, and to weeds and algae growing.
10.3 Byto concrete-rectangular]
increasing the area of the cross section of the waterways [e.g. from earth-trapezoidal
– (case study n. 9)
Increasing the cross section area of the headrace and/or tailrace channel, if it’s viable both economically and from the
spatial constraints point of view, is for sure a better solution than acting on roughness only. A typical solution is to change
from earth-trapezoidal or masonry-trapezoidal to concrete-rectangular cross section.
Many low head hydropower channels were built with a trapezoidal section. There’s a technical-economic reason for that:
this kind of section, in duly proportions, has the minimum wetted perimeter and volume of excavation, so that it is the
section with the best hydraulic performances and the minimum construction cost. Trapezoidal sections, both natural and
masonry-coated have many problems: natural sections are subject to sliding of slopes, weeds growing, stability problems;
masonry-coated are subject to cracks, water leakages, local coating heavy deterioration, all problems due to the interaction
with the substrate and to the fact that the coating isn’t of the self-bearing type.
A self-bearing rectangular section made by reinforced concrete can solve all the above mentioned problems and at the same
time can remarkably increase the flow rate conveyed by the channel being the levels equal.
The main disadvantage of this solution is the high cost of investment.
The following important aspects have to be taken into account:
• joints design and realisation;
• drainage of side walls and bottom sill, to be evaluated case by case;
• constraints coming from local boundary conditions;
• access and realisation problems.
10.4 Bystudyincreasing
the flow rate by negotiating a new water right with a higher rated discharge (case
n. 7)
For some cases, the rated discharge defined in the water right can be negotiated and increased.
It happens when the water course hydrology and environmental impact studies demonstrate that the discharge reduction
in the river bed is acceptable in front of the new positive impacts, resulting from the maintenance of the patrimony, the fish
migration improvement and the green electricity production increase. Indeed, most of the old sites were dimensioned for
the output necessary for the corresponding mills (saw mills, flour ones, ...)
Of course, this implies administrative authorisations that are not guaranteed and not for free, but they are worth achieving
in front of the added annual production.
Actions on H
11.1 Foreword
Actions on the head of an existing plant are not very frequent. The gross head is usually determined upstream by the weir
level and downstream by the water level at the outlet channel. Actions on net head could be something more, because net
head depends on the hydraulics of the waterways. Anyway a few actions can be actually envisaged for gross or net head. It
must be noted that low head plants are usually operated keeping a constant level at a forebay just upstream of the turbine.
That means that actions to reduce the roughness of the waterways upstream of the forebay have no influence on the head,
but those actions increase the flow rate conveyed, water levels being equal.
Guide on how to refurbish low head small hydro sites
11.2 By increasing the weir elevation (case study n. 6 and case study n. 7)
This action is mentioned even though it’s not very usual. Many different conditions must be met to increase the weir
elevation and, consequently, the gross head of the plant:
• global stability of the raised weir must be guaranteed with suitable safety factors. If it’s not the case, the cost for getting
the said safety factors must be duly compensated by higher energy production due to this higher gross head;
• higher water depth and greater length of the backwater area must be consistent to the environmental status of the river
and to the water rights of the water upstream users (the increase of this gross head could decrease the one of a possible
power plant located upstream);
• a higher water level determined by a higher weir implies a higher depth in the headrace channel, if any, which can lead
to problems of side stability and freeboard, which thus have to be closely examined in a way similar to the weir raising.
11.3 By restoring the initial gross head
In old small hydro low head plants, it’s not unusual that the gross head is decreased because of material sedimentation
downstream of the powerhouse or in the tailrace channel. Restoration of the gross head can be achieved by dredging
the tailrace channel or the river. The benefit of this action must be carefully evaluated because some cons are anyway
connected to it:
• dredging is an expensive activity;
• the material dredged from the river must be disposed off: if it has an economic value (gravel, sand) as a component
for concrete, the disposal is rather easy and it’s possible to have some revenue by reselling the material (if allowed by the
national legislation); if it hasn’t, the disposal itself is expensive: in medium or large rivers tailrace dredging can imply
handling thousands of cubic meters of material and finding a final destination for them is not an easy task;
• river dredging can have a negative environmental impact on the riverine ecosystem;
• sediment dynamics can be such that after a few years the problem must be faced again.
11.4 By replacing old screens equipped by rectangular bars with hydrodinamically profiled screens
Old small hydro low head plants are equipped with screens made by steel rectangular bars. The screens are usually located
at the forebay upstream of the turbine or at the intake or both. Screens are sometimes equipped with trash rack cleaning
machines (TRCMs). In very small plants and in some very old and never modernised ones the screens are manually cleaned.
When a plant is planned to be refurbished, an interesting option regarding screens can be the replacement of conventional
steel rectangular bars with hydro-dynamically profiled screens. These screens can greatly reduce head losses and thus
increase the net head of the plant. The steel solution is more expensive if compared with the rectangular one (approx.
+60 % including installation), but now GRP (glass reinforced plastics), are a less expensive solution with a comparable
reliability. Hydrodynamic profiles allow for a reduction of the head losses up to one third of the correspondent rectangular
bars and can be cleaned in an easier way. Sometimes it is necessary to undertake this action to compensate smaller distance
between bars due to demands of fishery
Actions on ␩ and T
Turbine and generator replacement
12.1 Foreword
Actions on ␩, global efficiency, are related to actions on turbine-generator (TG) unit.
As usually the main reason for refurbishment is the dramatic decrease of the TG efficiency and number and duration
increase of unit trips due to the unit age, an important effect connected to the replacement is the operation time increase.
It must be said that in many situations this side effect is even more important than the efficiency increase itself.
The most common and frequent way to refurbish a plant is to replace the turbine. In principle it’s a simple task for any
kind of small hydroelectric plant, but in case of low head plants the choice is not obvious at all. In case of high head plants
the choice is rather simple because the available unit types are few: Pelton turbines and, in some situations, cross flow
Guide on how to refurbish low head small hydro sites
In the great majority of existing high head plants, Pelton turbines are installed and if you have to replace the unit a new
Pelton turbine is the most common choice. Maybe you change the number of nozzles (from one to two, from two to four,
five or six) because the hydrological parameters of the plant are changed and you need multi-nozzles turbines to get higher
efficiency at partial loads (it’s a rather common situation in case of an obligation of higher reserved flow release with respect
to the past), but in general the unit replacement at a high head plant doesn’t present many choices or problems. In case of
low head plants the situation is completely different. But thanks to technological progress and R&D effort in the turbines
design, many more different options are available now with respect to the past to exploit the same head and flow rate. That’s
why the final choice of the unit must be the result of an optimisation process which must take into account the boundary
conditions and the local constraints into the decision process.
12.2 What’s worth replacing?
Even the simplest possible choice in a unit replacement, that is to replace the old unit(s) with a new one(s) perfectly the
same, is not so obvious. Yes, because, from one side, if you decide to replace even the parts in concrete you have higher
costs, probably higher efficiencies, less constraints and some risks; by the other side, if you keep the embedded parts as they
are (typically the draft tube and, if any, the spiral case), you have a strong constraint on the new unit construction. In fact
you don’t exactly know which is the geometry of the coupling between embedded and not embedded parts (you can only
partially trust in old construction drawings): you must wait for dismantling the old unit, check the real geometry, adjust
very precisely from the mechanical point of view the coupling surfaces; all this process can take a lot of time and prolong
the unit outage with the consequent loss of production.
That’s why in many cases even the embedded parts are worth replacing, independently from any consideration about the
efficiency improvement of the units.
But the replacement of the embedded parts makes further problems arise, mainly the replacement of the draft tube of both
Kaplan and Francis open flume units, such as:
1. problems due to the necessity of works below the tailrace water level, strongly constrained by the interaction with the
existing structures;
2. problems due to the fluid-dynamics design of the draft tube.
Certainly the most critical is the first one.
12.2.1 Works below the tailrace water level (Case study n. 3)
Only in a few cases of open flume Francis turbines replaced with the same type of unit you may not face this problem.
Actually, as open flume Francis turbines were usually built and installed before the cavitation effects were completely
known, many draft tubes of those units are installed well above the tailrace water level; but even though for reasons of
global optimisation the choice of replacing a Francis open flume with a similar unit is taken, you must necessarily lower the
level of the runner and consequently even the level of the draft tube with the consequent risk of works below the tailrace
This kind of works has peculiar features:
1. they are expensive;
2. they are subject to
a high degree of uncertainty and consequently to a high risk of unexpected increase of the
refurbishment costs.
In principle the actions to be taken when there’s the need for works below the tailwater level are well known, and many
different technologies are available.
The main need is to keep water away from the working area. Apparently it’s an easy task: it’s enough to install at a suitable
low level some suitable pumps.
Actually, the satisfaction of this simple need encounters many difficulties:
1. pumps may interfere with the progress of the works: typically, if you have to replace the old draft tube, once installed
the new draft tube, you must embed it into the concrete. Pumps, located at the lowest possible level, and anyway lower
than the lowest part of the draft tube, should be removed before concrete casting; but if you remove the pumps, the
working area is inundated by water and you should cast concrete under water with further difficulties and uncertain final
2. when you pump water coming from the water table, you can have a high risk of washing out even the finest particles
of the terrain around the powerhouse: this situation is very dangerous because of the risk that part of the powerhouse
foundations may collapse.
12.2.2 Fluid-dynamics design of the draft tube
It could seem strange that the fluid-dynamics design of the draft tube is a critical issue: with respect to the past excellent
CFD (Computational Fluid Dynamics) systems are available for the flow simulation in draft tubes. The use of those tools
Guide on how to refurbish low head small hydro sites
is widely diffused in the design of small hydro units, in order to cut laboratory
full model costs, usually not affordable for small hydro plants operators.
Nevertheless, wrong design of new draft tubes is frequent. It must be said that
the draft tube design has been quite improved over the years: recently designed
draft tubes have a shape very different from the past with undoubted higher
efficiency. Nevertheless many partially unknown factors can still affect the final
result of the draft tube replacement, especially at particular partial load of the
units. A large bibliography is available on this topic.
12.2.3 Embedded parts
New units generally have different shapes and dimensions from old ones, so
that when you replace an old unit you must evaluate also the possibility of
removing embedded parts of the unit, typically stay vanes, discharge rings, stay
rings, spiral cases. Any interaction with civil works is potentially a source of
risk, especially in case of old buildings whose actual conditions are very often
not completely known. Contingencies must be expected, affecting both time
and costs of refurbishment. The simplest way to overcome this problems is a
radical action on the civil works, that is actually realising a completely new civil structure avoiding to keep some parts of
them in the easy illusion of reducing costs. Moreover, keeping old embedded parts represents a strong constraint to an
optimal hydraulic design of the new unit and it usually results in lower efficiency.
12.3 How many units?
In the past, very old plants were equipped with more than one unit in order to have the best possible exploitation of the
available water resources and the best efficiencies of the units. Indeed, all Francis turbines, even open flume ones don’t
work well at partial loads, so that to exploit at its best the available water more than one unit was installed.
This is still a typical choice for large hydro plants, even though in that case also mechanical, transport and erection problems
are at the basis of the multiple-units choice. It must also be noted that old plants used to have many units even for another
important historical reason: many plants were built to supply mechanical force to drive mills, saw mills, textile workshops, so
it was important to be able to operate the plants at partial load to meet the instantaneous energy demand of the workshops.
Coming back to small hydro, nowadays, in case of refurbishment, one unit only (or less units than in the past) is preferred
for economic (in general one big unit is cheaper than two units with halved output) and operation (automation, control and
management of more units is more complex) reasons. In any case, as rather usual in refurbishment of low head plants, it’s
difficult to state a general rule. Once again the general optimisation of the project must be kept in mind: the installation
of one new unit instead of three old ones can be cheaper from the electro-mechanical side, but it can be unfeasible or very
expensive or subject to higher contingencies due to strong actions needed on civil works.
12.4 Replacement of an old TG unit with a new one of the same type
12.4.1 Francis open flume turbines (FOF) - Case study n. 4
The reason for replacing an old unit with a new one is usually the quick efficiency decrease or the more and more frequent
plant outages due to the unit unreliability . In many cases it deals not only with the runner itself but also with auxiliary
components, such as lubrication and cooling systems, bearings, and over-temperatures. Very old low head units are Francis
open flume turbines directly coupled to generators. After the invention of Kaplan turbines this kind of units has been
rapidly abandoned as possible technical solutions to exploit low heads, so that also the hydraulic research and development
on these units haven’t been carried out any more. Anyway, when old FOF must be replaced, the possibility of replacing it
(or, more frequently, them, because, as all Francis turbines, even open flume turbines don’t work well at partial loads, so
that to exploit at its best the available water more than one unit was installed) must be taken into due consideration. For
sure this choice isn’t in the direction of innovation, but, as we said many times, the purpose of the refurbishment is the
overall optimisation of the plant and not of one part only. The replacement of an old open flume Francis turbine with a new
one particularly makes sense in situations with strong transport and erection constraints. As the modern alternative to FOF
is a Kaplan turbine, the complete different size, configuration and weight of a Kaplan turbine with respect to a FOF must
be evaluated with reference to site management.
Pros and cons can be summarised as follow:
Interaction with civil
of units
Guide on how to refurbish low head small hydro sites
12.4.2 Kaplan turbines (Case study n. 5 and Case study n. 6)
Replacing an old Kaplan turbine with a new one is apparently the easiest possible task.
Actually, even though the plant parameters are the same (rated head and flow, FDC, etc.), it’s rather usual that the new unit
can be different from the old one in many aspects, such as:
• rated speed (higher speeds are now frequent)
• blades number (connected to speed, being Q and H the same)
• runner diameter
• draft tube shape (modern draft tube design is rather different from old ones)
• runner centreline elevation (actual knowledge of cavitation phenomena often entails lower elevations)
All those aspects require some adaptation of the new unit to the existing structures, both mechanical and civil.
During the preliminary design stage, the consequences of any mechanical and hydraulic optimisation must be carefully
examined by means of a deep cost/benefits/risks analysis. Sometimes what is simple or cheap in the workshop construction
phase can have negative impacts on the works on site because of higher contingencies or of longer refurbishing times.
Keeping inner and outer rings is a typical example. It’s obviously cheaper to keep these parts because their construction is
expensive and takes time. By the other side you must machine on site the existing rings and you can encounter two possible
risks: such a bad mechanical condition that the replacement is needed (more money and more plant outage) or prolonged
site machining due to the remarkable lack of planarity.
12.5 Replacement of an old TG unit with a new one of a different type (case study n. 3)
When the decision of replacing an old TG unit is taken, at a low head plant many options, different from the simple
replacement with a similar one, are now available. Basically even the new available options are of the reaction type. The
option of replacing an old turbine with an action or an Archimedean screw turbine can be considered only in limited cases.
But in the great family of the reaction turbines there are many different solutions that have some peculiarities from the
general configuration point of view. The final choice depends, as mentioned above, from the global optimisation of the
refurbishment and not only of the unit replacement
12.5.1 Replace an old FOF turbine with a Vertical Bulb Turbine (VBT) in the same pit
In order to reduce as much as possible the works on existing structures, a viable solution is the installation of a Vertical
Bulb Turbine in the same pit where the old FOF was. In this case the dimensions of the bulb are the main problem. A
conventional bulb usually has a large diameter and its fitting in the available space must be duly checked to be sure that the
unit hydraulic behaviour is satisfactory. A new opportunity to use this solution is given by the installation of a Permanent
Magnet Generator (PMG). If compared to a conventional one a PMG is more compact and has comparable efficiencies at
any load. That’s why in recent years, sometimes coupled to Variable Speed Operation (VSO) generators, PMG have known
a growing interest and application in the low heads field.
Actions on T
13.1 Foreword
Actions on T are addressed to the improvement of the operation reliability of the plant by reducing outages. By far those
are the actions to which a refurbishment is mostly aimed. One way or another, almost all the other actions have a beneficial
side effect of increasing time of operation (e.g. the replacement of an old turbine allows for increasing the plant efficiency
and decreasing the unit trips).
In the following paragraphs an overview of some actions particularly related to T will be described.
13.2 By improving sediment management
The operation of many low head plants is negatively affected by sediments transported by water and deposited in key parts
Guide on how to refurbish low head small hydro sites
of the plants. The main consequences are:
• Reduction of the capability of conveying water to the plant (sedimentation at intake)
• Reduction of cross section in the waterways (sedimentation in headrace and forebays)
• Reduction of head (sedimentation downstream of the powerhouse)
• Plant outages for sediment removal
In many cases the reasons for excessive deposition lies in original sins of the plants, namely:
• Incorrect position of the intake in the river (typically in the inner side of a bend instead of on the outer one)
• Insufficient dimensions of the flushing devices at intake or at forebays
• Underestimation of the quantity of sediments transported or incorrect evaluation of their prevailing size
• Insufficient or incorrect design of the desilting basins (if any)
• General layout of the intake and waterways favouring deposition.
Therefore, the actions taken during the plant refurbishment can positively affect the sediment management. The field of
improvement is directly connected to the cause:
•Modification of the shape of the river at the intake reducing the inner curvature (very difficult, in general, both from
technical and administrative point of view, with high environmental impact and probably only temporary results unless
combined with other actions)
• Improvement of the flushing devices at intakes and forebays, e.g. lowering the sill level of desilting gates, enlarging their
width or both
• Installation of bottom deflectors or of other devices capable of locally increasing flow velocity during flushing.
13.3 By improving trash management
It is never enough underlined that a good trash management is vital for the operation of a low head plant.
Some old and small low head plants have no trash rack cleaning machines (TRCM) and debris is removed manually from
This kind of operation has rather evidently high operation costs and is not efficient (hydro plants work day and night and
debris are conveyed 24 hours a day). If a TRCM is not present at the plant, in case of refurbishment, one essential action is
to set a new one. Even the replacement of an old or inefficient TRCM is an option.
Once stated the necessity of setting a new TRCM (and of the relevant screen), the choice of the typology is also essential.
There’s no general rule for that, but some criteria can be suggested:
• Debris load: for heavy loads (branches, logs) a hydraulic driven TRCM is usually preferred. If mainly light loads (leaves,
small wood) are expected, even a chain or rope rotating TRCM can be a viable (and usually cheaper) solution
• Accessibility of the site to dispose of the debris removed from the water
• Possibility of remove debris from screens avoiding the abstraction from water and the relevant disposal (with the cost
connected). There are TRCM solutions with screens equipped by horizontal bars, which remove debris from the screen
and move them on one side. By opening a gate (a flap gate or a sliding gate equipped with a flap gate on the top) the debris
are flushed downstream without being removed from water. These solutions have also the intrinsic environmental benefit
of keeping in the water the organic fraction of debris, which plays an important role for riverine ecosystems.
13.4 By improving floating debris management (case study n. 2)
One of the most frequent causes of plant outage is the inefficiency of systems for managing floating bodies.
Especially during high or medium water periods, (typically in spring or early summer after snowmelt time), rivers convey a
large amount of floating bodies, which tend to enter the plant waterways and to clog screens at intakes and forebays.
To avoid oversizing or overloading trash rack cleaning machines, a simple, rather cheap and efficient solution is to install
floating log and debris booms.
These are simple plastic buoyant bodies of given length (approx. 3-4 m each), properly shaped and duly connected, usually
by galvanized steel connectors, and anchored to the banks with a concrete foundation designed to withstand the thrust of
the floating material stopped by the booms themselves.
They should be located at the intake while considering the necessity of flushing downstream the floating material.
This is a key issue of the whole system. In case of fixed weirs the booms position and shape when floating debris are right
against them, must be carefully designed, in order to be sure that flow over the crest of the weir, in case of incoming
discharges higher than the plant rated discharge, is enough to flush downstream the floating bodies .
In case of moveable weirs, you have different options:
• to set a flap gate as near as possible to the booms. By opening, even partially, the gate you flush downstream the debris.
Guide on how to refurbish low head small hydro sites
In order to favour debris removal from booms, sometimes the flow rate diverted by the plant is reduced (and the approach
velocity to the booms, too) and at the same time the gate is lowered;
• to check whether existing gates of other types at the weir are able to flush floating debris. If sliding gates are installed,
you can anyway flush floating debris, but the gates must be completely raised and operating in spillway mode: in orifice
mode, in fact, floating debris can’t be efficiently flushed. An alternative to that is given by a modification of existing sliding
gates by the installation on top of them of a small flap gate.
A correct curvature of the booms under the thrusts of the floating bodies is essential for the efficient operation of the
13.5 By improving weir operation safety (case study n. 1)
When a weir of an old low head small plant is equipped with moveable gates, very often these gates have small width
with many intermediate piers. This kind of weir layout makes it vulnerable to clogging during floods because the large
material transported (trunks, branches, etc.) can obstruct the waterways. The reason for many small gates was essentially
an economic one: gates with wide span required special construction, sometimes not compatible with the economics of
a small plant. On the other hand, many small gates can cause more frequent outages and an operation costs increase
due to the necessity of removing material just against the gates, a complicated activity. That’s why, in the refurbishment
framework, it makes sense to take into consideration, if the preliminary assessment of the criticalities showed it as a weak
point, the modification of the weir layout by replacing many old sliding gates with few flap gates or sector gates or inflatable
rubber dams, possibly protected by steel panels.
Each one of the aforesaid options has pros and cons and their own more specific application, even though all of them are
in some way equivalent.
The following table summarizes the main features:
Suitability for
water level
Risk of damage
during floods
Flap gates
Sector gates
Rubber dams
Rubber dams
with steel
Civil works
It must be kept in mind that, in any case, a fundamental role is played by temporary works in the river for replacing the
gates. These works must be properly designed to withstand the project flood and to protect working areas from water. In
some situations, on the contrary, you must design temporary dykes in order to be overtopped by a predefined flood to avoid
damages at the structures located upstream (villages, various activities, etc.) but to protect from ordinary flow rates the
working site. It’s quite essential to plan carefully all the activities in the river, rather obviously choosing the river low flow
period, with the lowest risk of flood.
Another important issue in the choice among different alternatives is related to the interaction with civil works and primarily
with foundations.
Some types of gates (flap and sector ones, mainly) transfer hydraulic loads in concentrated points to foundations so that
special foundations (piles, sheet piles) can be necessary to withstand those loads. Special foundations are expensive,
require time and skill to be built, especially in the difficult conditions typical of works in rivers, and are subject to frequent
contingencies, so that a deep analysis of the opportunity of the alternative requiring those works must be carried out.
13.6 By setting or improving plant automation
In the frame of a refurbishment, it’s quite usual to implement the automation of the plant or to modernise and update the
existing one. About this topic there’s nothing really specific to low head plants.
In any case, even considering the rapidly decreasing costs of hardware and software, it’s advisable to acquire as many data
from the field as possible in order to have a clear picture of the situation of the plant and of the evolution of the main plant
parameters in time. Particular attention should be paid to:
• Units vibration (installing seismic transducers or accelerometers at each bearing)
• Generator and transformer temperatures
• Bearings temperature
• Upstream and downstream water levels
• HPU (hydraulic power unit) pressures and temperatures.
added value
Guide on how to refurbish low head small hydro sites
The refurbishment of a low head plant can be a good opportunity (sometimes
an obligation) to increase the environmental added value of the plant by
suitable actions aimed at improving the plant environmental sustainability.
It must be said that the environmental performance improvement of an
existing low head plant is a difficult task because of the many constraints
coming from acting on existing hydraulic works, so that, as for any other
action during refurbishment, an optimised solution rather than an optimum
solution can be achieved in the most favourable situations.
Possible actions related to this topic are:
• fish passages: many old low head small hydro plants aren’t equipped with
fish passages or are equipped with inefficient ones. Within a refurbishment the realisation of a fish passage can be a
good opportunity to improve the plant environmental performance. Unfortunately it’s not an easy task. The main risk
is the construction of an expensive passage with low efficiency because of the general layout of the weir and of the
intake. Typically it’s impossible to put the passage in a suitable position to properly attract fish to the passage itself in
both migration directions.
• architectural and visual impacts: refurbishment can also include actions on buildings, mainly the powerhouse,
connected with the hydroelectric units replacement, but also on the weir or on the intake. Some design choices can
be taken keeping in mind the reduction of the visual impact of the plant, e.g. installing new gates at the weir with
lower visual impact (flap gates, or rubber dams with steel protection panels which don’t require intermediate piers or
• Fish friendly turbines: for many years research on fish-friendly turbines has been ongoing all over the world, mainly
addressed to large hydro low head turbines. Small hydro application, exception made for very low head (with low
speed, rather large diameters and gaps) are undoubtedly more difficult, especially in case of refurbishment, where
the constraints coming from existing structures don’t generally allow for installation of units typology with better
environmental behaviour than conventional ones (e.g. Archimedean screws).
• Installation of screens with reduced space between bars: the reduction of space between bars implies a lower risk of
fish entering the plant and the turbines, but also higher head losses, so that in order to keep them at a reasonable level
this action can be connected preferably to the installation of hydrodynamically-profiled bars.
• Fish dissuasion or attraction systems: this action, in general rather expensive both from the investment and operation
point of view, can be envisaged whenever, because of the plant layout, it’s difficult to keep fish away from the intake or
to attract it to the passage.
added value
The rehabilitation of old sites often results in an improvement of the flood
management. It can be noticed especially for destroyed weirs.
Indeed for these sites, problems with the river banks can occur, that keep on
increasing if nothing is done. Then the rehabilitation of the site implies new
equipment and an opportunity to study the area as a whole, considering the
electricity production and the flood management.
Then, new valves can be integrated, that can be regulated with the water level
variations. Then by-passes and basins can be created to better manage the
high discharges. Moreover it can be here added that turbines are most of the
time regulated to maintain a steady upstream level.
This report on low-head small hydropower plant refurbishment shows how
complex it can be to reach an optimum, as each plant part is linked to the
whole site operation and interacts with the efficiency of each other part.
This infers that a close investigation of the plant and relevant site constraints
is highly recommended to increase the refurbishment production gain and
the plant reliability, and to decrease the further refurbishment numbers.
Guide on how to refurbish low head small hydro sites
Case study n. 1:
Improvement of weir operation safety and flood management
Case study n. 2:
Improvement of floating debris management
Case study n. 3:
Replacement of old open flume Francis turbines by turbines by vertical bulb
turbines in the same pit
Case study n. 4:
Units replacement. Restoring initial performances featuring strongly
constrained solution.
Case study n. 5:
Replacement of an old Francis turbine with 2 Kaplan type ones
Case study n. 6:
Site reconstruction and setting of a new Kaplan turbine
Case study n. 7:
Weir and intake rehabilitation, fish pass integration, new power plant
Case study n. 8:
replacement of an existing unit with a similar one
Case study n. 9:
increase of the cross section area (from earth-trapezoidal to concreterectangular
Case study n. 10:
plant automation
Case study n. 11:
improvement of sediment management
Case study n. 12:
improvement of environmental added value
Case study n. 1: Improvement of weir operation safety and flood management
Country: Italy
Hg = 8,9 m
BEFORE: 12 electrically operated slide gates
FIELD OF ACTION: improvement of weir operation safety
MAIN PROBLEM: risk of clogging during floods and normal
ACTION: replacement with three flap gates
FEATURES: hydraulically operated, back-up emergency
diesel generator
BENEFITS: reduced risk of flooding plains downstream,
reduced operation costs, increased operation time
Qmax = 22 m3/s
E = 10,4 GWh/year
Case study n. 2: Improvement of floating debris management
info: Studio Frosio
Country: Italy
Hg = 10,5 m
BEFORE: heavy debris load at turbine intake
FIELD OF ACTION: improvement of floating debris
MAIN PROBLEM: high costs for debris disposal and
ACTION: installation of floating log and debris booms
FEATURES: easy installation, low cost
BENEFITS: reduction of operation costs
WARNINGS: need for a system for flushing floating bodies
(flap gate at the weir)
info: Studio Frosio
Qmax = 60 m3/s
E = 25 GWh/year
Guide on how to refurbish low head small hydro sites
Case study n. 3: Replacement of old open flume Francis turbines by turbines by vertical bulb turbines in the
same pit
info: Studio Frosio
Country: Italy
Hg = 10,5m
Qmax = 4 x 15 m3/s
E = 25 GWh/year
BEFORE: 4 open flume Francis turbines, ␩ ~70 %
FIELD OF ACTION: improvement of units efficiency and
MAIN PROBLEM: small space available for units in the
turbine pits
1. replacement with Permanent Magnet Generators
double regulated Bulb Turbines
2. new draft tubes
FEATURES: compact units with high efficiency
BENEFITS: increased avg. efficiency ␩ ~87 %
SIDE EFFECTS: space rationalisation in the powerhouse
RISKS: nearly prototype units, works below tailwater levell
Case study n. 4: Units replacement. Restoring initial performances featuringg strongly
g y constrained solution. info: Studio Frosio
Country: Italy
Hg = 15,0 m
BEFORE: 2 open flume Francis turbines
FIELD OF ACTION: restoration of units efficiency and
MAIN PROBLEM: erection almost impossible for Kaplan
Qmax = 12 m /s
E = 9,20 GWh/year
ACTIONS: replacement with new open flum
ume Francis
BENEFITS: increased reliability
RISKS: open flume hydraulics no more deeply investigated
byy manufacturers
Case study n. 5: Replacement of an old Francis turbine with 2 Kaplan type ones
Country: Switzerland
Hg = 2,1 m
BEFORE: abandoned site, no production
FIELD OF ACTION: integration of new turbines in an existing
infrastructure (factory),
MAIN PROBLEM: obsolete machines, reparation possible
but not cost efficient
ACTION & FEATURES: integration of 2 siphon turbines,
setting of valves to manage river discharges, nominal
discharge, reserved flow, and floods.
BENEFITS: better discharge and flood management,
production increase (turbine flexibility and efficiency,
operation time)
info: MHyLab
Qmax = 5 m3/s
E = 0,350 GWh/year
Guide on how to refurbish low head small hydro sites
Case study n. 6: Site reconstruction and setting of a new Kaplan
p turbine
Country: France
Hg = 27 m
BEFORE: in 2002 a flood destroyed the whole site, apart
from the power plant with its Francis turbine
MAIN PROBLEM: no production
ACTION & FEATURES: new weir, head increase, setting of a
Kaplan turbine
info: MHyLab
Qmax = 1,5 m3/s
E = 1,8 GWh/year
BENEFITS: production increase (head, turbine
turbi efficiency
and flexibility, time operation), decrease of the operation
& maintenance cost
NOTE: the old Francis turbine is used during the Kaplan
rbine maintenance.
Case study n. 7: Weir and intake rehabilitation, fish pass integration, new power plant
Country: Switzerland
Hg = 5,2 m
BEFORE: weir and intake destroyed by a flood in 2001,
abandoned site
FIELD OF ACTION: weir, intake, inlet and outlet channels,
turbine, power plant
MAIN PROBLEM: risks of banks subsidence, limited amount
of water in the inlet channel, no production
ACTIONS: weir reconstruction, upstream level raising,
integration of a fish pass, equipment of the intake, new
channel crossing section, totally new powerhouse
FEATURES: rocks filling, boom equipped with a fish net, 2
new Kaplan turbines
BENEFITS: better flood management, upstream and
downstream fish migration, head and discharge increase for
info: MHyLab
Qmax = 6,0 m3/s
E = 0,820 GWh/year
the SHP, production increase, maintenance cost decrease.
NOTE: this site is being studied, and has not been
rehabilitated yet.
Case study n. 8: raplacement of an existing unit with a similar one
info: IWHW
Country: Austria
Hg = 1,56 m
Qmax = 5,5 m3/s
BEFORE: vertical axis Francis Turbine from1924
FIELD OF ACTION: efficiency and operation reliability
Replacement of the existing turbine
New spur gear unit replacing two old gear boxes
Installation of a new asynchronous generator
New Hydraulic Power Unit driving the turbine governor
New electronic turbine governor
TRCM connected to the centralised control unit
New vertical flushing gate hydraulically driven
Before renovation:
After renovation:
Pmax = 50 kW
Pmax = 65 kW
E= 0,3 4 GWh/year E = 0,4 GWh/year
Case study n. 9: increase of the cross section area (from earth-trapezoidal to concrete-rectangular
Country: Italy
Hg = 8,0 m
info: Studio Frosio
Qmax = 16 m3/s
E = 12,2 GWh/year
BEFORE: earth trapezoidal headrace channel
FIELD OF ACTION: increase of rated flow and reduction of
sliding problems of the channel sides
ACTIONS: turning to concrete rectangular self-bearing walls
BENEFITS: increased rated flow and safety of operation.
Solution of stability, leakages and sliding problems.
CONS: high cost of investment
Guide on how to refurbish low head small hydro sites
Case study n. 10: plant automation
info: Studio Frosio
Country: Italy
Hg = 6,35 m
BEFORE: not automated, fully
manual operated plant
FIELD OF ACTION: increase of the
reliability of plant operation
ACTIONS: full automation of the
BENEFITS: increased safety of
operation. Optimisation of plant
parameters and maximisation
of energy production
Qmax = 32 m3/s
E = 10,5 GWh/year
Case study n. 11: Weir and intake rehabilitation, fish pass integration, new power plant
Country: Italy
Hg = 12,87 m
info: Studio Frosio
Qmax = 75 m3/s
E = 30,1 GWh/year
BEFORE: heavy sedimentation problems causing frequent
FIELD OF ACTION: improvement of sediment management
ACTIONS: bank re-shaping, wider desilting span, lower
desilting gate bottom sill, rip-rap protection of bottom,
longitudinal wall to concentrate flow lines at the desilting
BENEFITS: reduction of outages for sediment management,
increase of rated flow of the headrace channel
CONS: works expensive and long outage to realise them
NOTE: works under authorisation procedure
Desilting gate
too small
Intake in the
inner side of
the river
Case study n. 12: improvement of environmental added value
info: LŽŪU
Country: Lithuania
Hg = 35,26 m
Qmax = 11,1 m3/s (3x3,7 m3/s)
BEFORE: significant reservoir water level fluctuations and
unstable tailwater flow regime
FIELD OF ACTION: replacement of a worn turbine,
improvement of ecological status in reservoir and
downstream power plant
MAIN PROBLEM: necessity of turbine refurbishment, clearly
artificial river flow regime
ACTION: replacement with turbine having more flexible
operating flow regime
FEATURES: flexibility of operation
BENEFITS: power generation remains as before, river
E = 7,025 GWh/year
P = 3,2 MW
ecological status depending on more natural fl
ow regime
has been improved.
Case study n. 13: Replacement of an old Francis turbine with
th a Kaplan type one
Country: Lithuania
Hg = 35,26 m
Qmax = 11,1 m3/s (3x3,7 m3/s)
BEFORE: limited production, high operation costs
FIELD OF ACTION: integration of a new turbine in an
existing infrastructure
MAIN PROBLEM: obsolete Francis turbine
ACTION & FEATURES: replacement of a Francis turbine
with a S-shape axial one, setting of a penstock, creation
of a powerhouse
BENEFITS: increase of the head, turbine flexibility and
efficiency, and therefore production increase and
operation costs decrease
info: LŽŪU
E = 7,025 GWh/year
P = 3,2 MW
Renewable Energy House
Rue d’Arlon 63-67
B-1040 Brussels, Belgium
+32 25461945/33
Studio Frosio
Chemin du Bois Jolens 6
1354 Montcherand Switzerland
Tél : + 41 24 442 86 24
Fax : + 41 24 441 36 54
[email protected]
Studio Frosio
Via P.F. Calvi, 9
25123 Brescia - Italy
Tél : + 39 030 3702371
Fax : + 39 030 396143
[email protected]
The Szewalski Institute of
Fluid-Flow Machinery of the
Polish Academy of Sciences
ul. Fiszera 15,
80-952 Gdansk - Poland
Tel: +48 58 6995139
Fax: +48 58 3416144
[email protected]
Water & Land Management
Faculty_Lithuanian University of
Universiteto 10, Akademija,
LT-53361 Kauno r., Lithuania
Tel: +370 37 752 337
Fax: +370 37 752392
E-mail: [email protected]
Institute of Water Management,
Hydrology and Hydraulic
Engineering – University of
Muthgasse 18, AT-1190
Vienna, Austria
Tel: +43-1-47654-5500
E-mail: [email protected]
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