Renewable Energy Resources
2008
HYDROELECTRIC
ENERGY
António F. O. Falcão
SOLAR ENERGY flux on the Earth surface:
18
1.5  10
kWh/year
About 25% consumed in evaporation of water
Almost all this energy is released
in water vapour condensation
(clouds, rain) & radiated back
into outer space
Only 0.06% remains as potential
energy stored in water that falls on
hills and mountains
HYDRO ENERGY RESOURCE
• Total resource:  40 000 TWh/year (about 15 times total world
hydroelectric production
• Technical potential: about: 14 000  15 000 TWh/year
• Total world electricity consumption: 16 400 TWh
Prefixes:
k  103
kilo
T  1012
tera
M  106
mega
P  1015
peta
G  109
giga
E  1018 exa
Regional hydro potential output
Techical potential
TWh/year
Annual output
TWh/year
Output as % of
technical potential
Asia
5093
572
11%
South America
2792
507
18%
Europe
2706
729
27%
Africa
1888
80
4,20%
North America
1668
665
40%
Oceania
232
40
17%
14379
2593
18%
Region
World
Based on average output 1999-2002
Source: G. Boyle, Renewable Energy, 2004.
15,8% of world electrical
energy consumption
Technical potential
Economic potential
Exploited potential
Africa
Asia
Australasia/
Oceania
Europe
North & Central South America
America
Exploited hydro potential by continent
Weir and intake (dique ou açude)
Forebay tank
(câmara de carga)
Canal
(canal)
Penstock
(conduta forçada)
Power house
(casa das
máquinas)
Small hydro site layout
Large hydro
10 MW
Small hydro
500 kW
Mini-hydro
100 kW
Micro-hydro
Note: there are other definitions.
Small hydroelectric plants (< 10 MW)
World totals
Installed capacity
(GW)
Annual production
TWh/year
740
 2700
Small (< 10 MW)
50 a 60
 150
Small/total
6 a 7%
 6%
Total (large + small)
Installed capacity (GW) in small hydroelectric plants:
 China
 26
 Japan
 3.5
 Austria, France, Italy, USA > 2 each
 Brazil, Norway, Spain
 Portugal
 TOTAL
> 1 cada
 0.3 (about 100 plants)
50 to 60 GW
Installed capacity and production of SHPs (<10MW) in 30 European countries
A
H b = gross head (altura de
queda bruta) in metres
L = losses in canal, pennstock, in metres
Canal
H b = gross head
Pennstock
H  H b  L = net head (altura de queda disponível)
(altura de queda
bruta)
Turbine
B
Q = flow rate or intake (caudal), in m3/s
Pb   g Q H b = gross power (potência bruta), in Watts
Pav   g Q H
= power available to turbine
Pt  t  g Q H
= turbine power output
Pe  et  g Q H = electrical power output
t  turbine efficiency
 e  electrical efficiency
Hydraulic turbine
t
H = (net) head
rated
Q = flow rate
N = rotational speed
Dimensional analysis
N, H = constant
Q
 N Q 

(Dimensionless) specific speed
3 4
 ( gH )  rated
H (m)
Q ( m 3 /s )
N ( rad/s)
g ( m/s 2 )
Ω is directly related to
geometry (type) of turbine
Francis
Pelton
Kaplan
Rotors of hydraulic turbines with different specific speeds Ω.
Correspondence between specific speed Ω
and type of hydraulic turbine (Pelton,
Francis, Kaplan)
 N Q 

3 4
 ( gH )  rated
Pelton turbines (low Ω)
 N Q 

3 4
 ( gH )  rated
Usually:
• High H
• Small Q
Twin jet Pelton turbine
wheel or runner
nozzle
pennstock
Large Pelton turbine
• Vertical axis
• 6 jets (6 nozzles)
Francis turbines (medium Ω)
 N Q 

3 4
 ( gH )  rated
Francis turbine
Spiral
casing
runner
Guide
vanes
draft
tube
Reversible Francis pump-turbine
In times of reduced energy demand, excess
electrical capacity in the grid (e.g. from wind
turbines) may be used to pump water,
previously used to generate power, back into
an upper reservoir.
This water will then be used to generate
electricity when needed. This can be done by
a reversible pump-turbine and an electrical
generator-motor.
Kaplan turbines (high Ω)
 N Q 

3 4
 ( gH )  rated
Usually:
• Low H
• Large Q
Kaplan turbine
Electrical
generator
Blade angle can be
controlled
spiral
casing
Guide
vanes
runner
Kaplan turbine
Double control
Guide-vane
control
Rotor-blade
control
Propeller turbine (small power plants)
Simple control: rotor blades are fixed
A variant of the Kaplan turbine: the horizontal axis
Bulb turbine
guide
vanes
Used for very low heads, and
in tidal power plants
Tidal plant of La Rance,
France
Cross-flow turbine (also known as Mitchel-Banki
and Ossberger turbine)
• Used in small hydropower
plants.
• The water crosses twice
(inwards and outwards) the
rotor blades.
• Cheap and versatile.
• Peak efficiency lower than for
conventional turbines.
• Favourable efficiency-flow
curve.
Cross-flow turbine
Head-flow ranges of small hydro turbines
H
(m)
Q (m3/s)
Ranges of application of Pelton, Francis and Kaplan turbines (adapted from
Bureau of Reclamation, USA, 1976). Recommended rotational speeds are
submultiples of 3000 rpm, for sinchronous generators.
How to estimate the type and size of a turbine, given (rated
values of):
• H = (net) head,
• Q = flow rate,
• N = rotational speed ?
 N Q 

3 4
 ( gH )  rated
Type (geometry)
Pelton turbine
gH

0
.
59
2 2
N D
Diameter D
D
Francis and
Kaplan
turbines
 N Q 

3 4
 ( gH )  rated
D
 D( gH )1 4 
Specific diameter


Q 
(dimensionless)

rated
1.0
Pelton
0.8
Efficiency
t
Crossflow
0.6
Kaplan
Francis
0.4
Propeller
0.2
0.0
0.0
0.2
0.4
0.6
0.8
Flow rate as proportion of design flow rate
Part-flow efficiency of small hydraulic turbines
1.0
HYDROLOGY
• Watershed (of hydropower scheme) (bacia
hidrográfica)
• Flow (rate) (caudal)
Basic hydrological data required to plan a (small) hydropower scheme:
• Mean daily flow series at scheme water intake for long period (~20 years).
• This information is rarely available.
• Indirect procedures are often necessary.
Streamgauging
station
Power
plant
Indirect procedure:
Usually consists of transposition of sufficiently long (≥20 years) flow-records
from other watershed (bacia hidrográfica) equipped with a stream-gauging
station (estação de medição de caudal).
• Watershed of hydropower scheme and water shed of stream-gauging
station should be located in same region, of similar area, with similar
hydrological behaviour (similar mean annual rain fall level) and similar
geological constitution.
• Rain gauges (medidores de precipitação) should be available inside (or
near) both watersheds, and be used for simultaneous rain-fall measurements.
Relation between annual
precipitation and runoff at
stream-gauging station (per
unit watershed area)
Water-year runoff (mm)
400
350
300
250
200
400
500
600
700
800
900
Mean basin water-year precipitation (mm)
By transposition → relationship between annual
precipitation and power-plant flow rate at hydro-power
scheme.
Mean annual flow duration curve
Q  mean annual
flow rate
10
8
6
Q
Q 4
2
0
0
0.2
0.4
0.6
0.8
1
Time fraction flow rate is equalled or exceeded
Dimensionless form of the mean annual flow duration curve
ENERGY EVALUATION – CASE 1
Water reservoir has small storage capacity.
• Run-of-the-river plant (central de fio de água).
• Case of many (most?) small hydropower plants.
• Storage capacity is neglected.
• Energy evaluation from the flow duration curve.
• No time-series (day-by-day prediction) of power output.
• At most, seasonal variations are to be predicted.
Run-of-river plant and flow duration curve.
Q
Q
Max. turbine
flow
Min. turbine
flow
Ecological flow
Time-fraction flow rate is equalled or exceeded
Run-of-river hydropower plant (fio de água)
Required data for energy evaluation:
• Flow duration curve for hydropower scheme.
• Maximum and minimum turbine flow rates (to be specified
from turbine characteristic curves).
• Ecological discharge (and others, required for the
consumption between the weir and the turbine outlet).
• Head loss L in diversion circuit as function of flow rate.
• Efficiency curves of turbine and electrical equipment.
Maximum and minimum turbine flow rates to be
decided based on turbine size and efficiency curve.
1.0
Pelton
0.8
Efficiency
t
Crossflow
0.6
Kaplan
Francis
0.4
Propeller
0.2
0.0
0.0
0.2
0.4
0.6
0.8
Flow rate as proportion of design flow rate
Part-flow efficiency of small hydraulic turbines
1.0
ENERGY EVALUATION - CASE 2
Second case: water reservoir (lagoon) has
significant or large capacity.
• Case of some small and most large hydropower
plants.
• Storage capacity must be taken into account.
• Energy evaluation is based on the simulation of a
scenario: daily (or hourly) flow-series and exploitation
rules.
• Basically the computation consists in the step-bystep numerical integration of a differential equation
(equation of continuity).
Hydropower plant with storage capacity
Required data for energy evaluation:
• Time-series of flow into the reservoir (simulated scenario).
• Maximum and minimum turbine flow rates (to be specified
from turbine characteristic curves).
• Ecological discharge (and others, required for the
consumption between the weir and the turbine outlet).
• Head loss L in diversion circuit as function of flow rate.
• Efficiency curves of turbine and electrical equipment.
• Reservor stage-capacity curve (surface elevation versus
stored water volume).
• Exploitation rules (e.g. concentrate energy production in
periods of higher tariff or higher demand).
Exercise
Consider a small run-of-river hydropower plant.
• Specify the turbine type and size.
• Evaluate the annual production of electrical energy.
Assume:
• Annual-average flow into reservoir.
• Flow duration curve.
• Gross head Hb .
• Loss L in hydraulic circuit.
• Efficiency curve of turbine, and rated & minimum turbine flow.
• Efficiency of electrical equipment.
• Ecological flow rate.
10
Exercise
8
q
Q
 ( )
Q
or
6
Q
Q 4
  F (q)
F(q) is fraction of time q is
exceeded.
q  ( ) or τ  F(q)
2
0
0
0.2
0.4
0.6
0.8
Time fraction flow rate is equalled or exceeded τ
d
F (q )  F (q )  F (q) is probability density function.
dq
 F (q) dq = probability of occurrence of flow between q and q + dq .


 F (q) dq  1
0
1

Q
 q F (q) dq  q   1
Q

0
Exercise
q Q Q
Choice of function F(q)
Weibull distribution (widely used in wind energy):
k
 q
F (q)  exp   
  c  
k = shape parameter
c = scale parameter
c
1
(1  k 1 )
  gamma function
k
c
0,5
0,50000
0,55
0,58740
0,6
0,66464
0,65
0,73192
0,7
0,79000
0,75
0,83988
0,8
0,88261
0,9
0,95040
1,0
1,00000
1,1
1,03636
1,2
1,06309
1,3
1,08275
1,4
1,09719
1,6
1,11536
1,8
1,12450
2,0
1,12838
Exercise
Choice of efficiency-flow curve for turbine (typical small Francis
turbine)
1
0,8
t
0,6
0,4
x
Qt
Qt , rated
0,2
0
0
0,2
0,4
0,6
x  Qt Qt , rated
0,8
1
t  25.293 x5  88.611x 4  119 .89 x3  79.306 x 2  26.659 x  3.0478
Set a minimum value for the turbine efficiency, e.g. 20% efficiency.
Set the minimum value of the turbine flow rate accordingly.
Exercise
Annual-averaged electrical power output (SI units):


Pe   Pe F (q)dq    g 
qrated
0
et ( x) Qt ( x) H b  L(Qt ) F ' (q) dq
qmin


  F ' (q) dq
  g et ,rated Qt ,rated H b  L(Qt ,rated ) 
(in W )
qrated

Qecol
Qt   q 
Q


 Q

Q  Qecol 
Qecol  Q

x

  q 
Qt , rated
Qt , rated
Q  Qt , rated

Qt
Qmin Qt , min  Qecol xmin Qt , rated  Qecol
qmin 


Q
Q
Q
qrated 
Qt , rated  Qecol
Q
Exercise
Total electrical energy produced in one year:
Ee,annual  Pe  3600  24  365 (in J/year )
Ee,annual  Pe  24  365 10 9
(in GWh/year)
Exercise
Procedure (suggestion)
• Fix annual-averaged flow rate into reservoir, e.g. Q  4 m 3/s
• Fix gross head, e.g. H b  50 m
• Fix head loss, proportional to Qt2 ,e.g. such that loss equal to a few percent of
gross head Q
• Fix flow duration curve, e.g. based on Weibull distribution
• Fix turbine type, turbine efficiency curve and Qt , rated
• Fix minimum (dimensionless) turbine flow rate xmin
• Fix ecological flow rate Qecol
• Assume Qt  Qt ,rated when Q  Qecol  Qt , rated
• Compute Pe and Ee
• Make comparisons as appropriate; look for “optimum” value of Qt , rated
H b  50 m
Q  4 m3 s
Francis
turbine
Some results from Exercise
Ecological flow rate = 0
Head losses = 0
k = 1.6 shape parameter of
Weibull distribution
rated
Cross-flow
turbine
rated
annualaveraged
Annualaveraged
Annual-averaged
Francis
Cross-flow
The two largest hydropower plants in the world
Three Gorges Dam, China
Itaipu, Brazil-Paraguay
THREE GORGES DAM – The largest hydropower plant in the world
Yangtze River, China.
• Construction: started in 1994; to
be completed in 2009.
• Dam - length: 2309m; height:
185m
•
• Reservoir – length: 600km
• About 1.5 million people had to
be relocated
Three Gorges Dam hydropower plant
• Installed power: 22500 MW
• 34×700 MW Francis turbines
Itaipu hydropower plant, Paraná River, BrazilParaguay
Construction: 1984-91
Reservoir area: 1350 km2
Installed power: 12870 MW
Total dam length: 7235 m
18 Francis turbines of 715 MW
Dam height: 196 m
Principais bloqueios ao desenvolvimento
de PCHs na EU
• Processo de licenciamento
• Exigências específicas locais
• Financiamento
• Ligação à rede eléctrica
• Venda de electricidade produzida
• Quadro regulador incerto
• Ausência de informações correctas
• Recrutamento e formação de técnicos
Principais bloqueios em Portugal
(FORUM Energias Renováveis em Portugal, 2002)
• Dificuldades na obtenção de licenciamentos, sujeitos a um processo
extremamente complexo, onde intervêm, sem aparente coordenação,
diversas instituições e ministérios.
• Dificuldade na ligação à rede eléctrica nacional por insuficiência da
mesma e, ainda, por outras dificuldades processuais e operacionais.
• Ausência de critérios objectivos na emissão de pareceres de
diversas entidades e na apreciação dos estudos de carácter
ambiental.
• Eventual opinião ou intervenção negativa de agentes locais.
• Dificuldades de maios humanos na Administração para tratamento
dos processos de licenciamento.
• "Em 2001, a situação podia resumir-e a um impasse quase completo
no licenciamento das PCHs" (situação pouco diferente da actual).
Aspectos económicos
• Maiores alturas de queda são factor favorável (menores
caudais para a mesma potência, menores custos de
equipamento).
• Frequentemente maiores alturas ocorrem em zonas menos
habitadas (consumo local, ligação à rede).
• Na Europa, a maior parte dos melhores locais (maiores
quedas) já estão aproveitados.
• Muito longo período de vida (frequentemente  50 anos) com
pequenos custos de operação e manutenção. Investimentos nas
grandes hídricas em geral do Estado.
• Mas a análise económica (investidores privados) baseia-se em
amortizações em 10 - 20 anos.
Costs of installation of small
hydropower plants
Comparison: cost of installation of a large onshore
wind turbine (> 1MW): about 1.0 - 1.1 M€/MW.
Note that lifespan of wind turbine (20-25 years?) is probably
shorter than lifespan of a hydro plant.
US$/kW
kW installed
Range of costs for small hydropower projects.
Small hydropower : specific costs of installed capacity
€/kW
Head (m)
ENVIRONMENTAL IMPACT - 1
The impact of the large hydropower plants is probably greater (afecting
larger areas) than any other power plants (not necessarily worse impact).
The impact from small plants (per unit power) is not necessarily smaller
than from large ones.
This impact is important during construction and during operation.
Do not forget that any renewable has environmental impact, namely
concerning construction/production phaes (energy and materials are
required).
The large hydro plants change the ecology over large areas.
Beneficial effects:
• Replaces fossile-fuel power plants (reduce greenhouse gases & acid rain).
• Flood control (especially plants with large reservoir).
• Irrigation.
• Valued amenity and visual improvement.
ENVIRONMENTAL IMPACT - 2
• The most obvious impact of large hydro-electric dams is the flooding of
vast areas of land, much of it previously forested or used for agriculture.
• Large plants required the relocation of many people (Aswan, Nile river:
80000; Kariba, Zambesi river: 60000; Three Gorges Dam, Yangtze river: 1.5
million).
• In large reservoirs behind hydro dams, decaying vegetation, submerged
by flooding, may give off large quantities of greenhouse gases (methane).
• Damming a river can alter the amount and quality of water in the river
downstream of the dam, as well as preventing fish from migrating
upstream. These impacts can be reduced by requiring minimum flows
downstream of a dam, and by creating fish ladders which allow fish to
move upstream past the dam.
• Silt (sediments), normally carried downstream to the lower reaches of a
river, is trapped by a dam and deposited on the bed of the reservoir.
This silt can slowly fill up a reservoir, decreasing the amount of water which
can be stored and used for electrical generation. The river downstream of
the dam is also deprived of silt which fertilizes the river's flood-plain during
high water periods.
Basic bibliography (in addition to pdf files available at site of
Renewable Energy Resources):
• Janet Ramage, “Hydroelectricity”, in: Renewable Energy (Godfrey
Boyle ed.), Oxford University Press, 2004, p. 147-194. ISBN 0-19926178-4.
• M. Manuela Portela, “Hydrology”, in: Guidelines for Design of Small
Hydroplants (Helena Ramos, ed.), 2000, p. 21-38. ISBN 972-96346-4-5
(available at CEHIDRO, IST).