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HYDROELECTRIC ENERGY
Renewable Energy Resources
2008
António F. O. Falcão
SOLAR ENERGY flux on the Earth surface:
kWh/year105.1 18
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: (about 15 times total world hydroelectric production
• Technical potential: about:
• Total world electricity consumption: 16 400 TWh
TWh/year00040
TWh/year0001500014
Prefixes:
giga10G
mega10M
kilo10k
9
6
3
exa10E
peta10P
tera10T
18
15
12
Regional hydro potential output
RegionTechical potential
TWh/yearAnnual output
TWh/yearOutput 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%
World 14379 2593 18%
Based on average output 1999-2002
Source: G. Boyle, Renewable Energy, 2004.
15,8% of world electrical energy consumption
Exploited hydro potential by continent
Africa Asia Australasia/Oceania
Europe North & CentralAmerica
South America
Technical potential
Economic potential
Exploited potential
Weir and intake (dique ou açude)
Penstock(conduta forçada)
Forebay tank(câmara de carga)
Small hydro site layout
Canal(canal)
Power house(casa das
máquinas)
10 MW
500 kW
100 kW
Large hydro
Small hydro
Mini-hydro
Micro-hydro
Note: there are other definitions.
Small hydroelectric plants (< 10 MW)World totals
Installed capacity (GW)
Annual production TWh/year
Total (large + small) 740 2700
Small (< 10 MW) 50 a 60 150
Small/total 6 a 7% 6%
Installed capacity (GW) in small hydroelectric plants:
China 26
Japan 3.5
Austria, France, Italy, USA > 2 each
Brazil, Norway, Spain > 1 cada
Portugal 0.3 (about 100 plants)
TOTAL 50 to 60 GW
Installed capacity and production of SHPs (<10MW) in 30 European countries
A
Canal
Pennstock
B
= gross head(altura de queda bruta)
bH
Turbine
bH = gross head (altura de queda bruta) in metres
L = losses in canal, pennstock, in metres
LHH b = net head (altura de queda disponível)
Q = flow rate or intake (caudal), in m3/s
bb HQgP = gross power (potência bruta), in Watts
HQgPav = power available to turbine
HQgP tt = turbine power output t turbine efficiency
HQgP tee = electrical power output e electrical efficiency
H = (net) head
Q = flow rate
N = rotational speed
Hydraulic turbine
t
Q
rated
N, H = constant
rated43)(
gH
QN
Dimensional analysis
(Dimensionless) specific speed
)m/s(
)rad/s(
)/sm(
)m(
2
3
g
N
Q
H
Ω is directly related to geometry (type) of turbine
Pelton
Francis
Kaplan
Rotors of hydraulic turbines with different specific speeds Ω.
Correspondence between specific speed Ωand type of hydraulic turbine (Pelton, Francis, Kaplan)
rated43)(
gH
QN
Pelton turbines (low Ω)
rated43)(
gH
QN
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)
rated43)(
gH
QN
Francis turbine
Spiral casing
Guide vanes
runner
draft tube
Reversible Francis pump-turbine
In times of reduced energy demand, excesselectrical 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)
rated43)(
gH
QNUsually:
• Low H
• Large Q
Kaplan turbine
spiral casing
Guide vanes
Blade angle can be controlled
runner
Electrical generator
Kaplan turbine
Guide-vane control
Rotor-blade control
Double control
Propeller turbine (small power plants)
Simple control: rotor blades are fixed
A variant of the Kaplan turbine: the horizontal axisBulb turbine
Used for very low heads, and in tidal power plants
Tidal plant of La Rance, France
guide vanes
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
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.
Q (m3/s)
H (m)
How to estimate the type and size of a turbine, given (rated values of):
• H = (net) head,
• Q = flow rate,
• N = rotational speed ?
rated43)(
gH
QN Type (geometry)
Pelton turbine
59.022
DN
gH
D
Diameter D
rated43)(
gH
QN
rated
41)(
Q
gHD
Francis and Kaplan turbines
DSpecific diameter
(dimensionless)
Part-flow efficiency of small hydraulic turbines
Cross-flow
Pelton
Kaplan
Francis Propeller
0.0 0.2 0.4 0.6 0.8 1.0
1.0
0.8
0.6
0.4
0.2
0.0
Efficiency
t
Flow rate as proportion of design flow rate
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.
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.
Stream-gauging station
Power plant
200
250
300
350
400
400 500 600 700 800 900
Mean basin water-year precipitation (mm)
Wat
er-y
ear
run
off
(m
m)
Relation between annual precipitation and runoff at stream-gauging station (per unit watershed area)
By transposition → relationship between annual precipitation and power-plant flow rate at hydro-power scheme.
Mean annual flow duration curve
0 0.2 0.4 0.6 0.8 10
2
4
6
8
10
Dimensionless form of the mean annual flow duration curve
Time fraction flow rate is equalled or exceeded
Q
Q
Q mean annual flow rate
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.
0 0.2 0.4 0.6 0.8 10
1
2
3
4
5
Q
Q
Time-fraction flow rate is equalled or exceeded
Max. turbine flow
Min. turbine flow
Ecological flow
Run-of-river plant and flow duration curve.
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.
Run-of-river hydropower plant (fio de água)
Part-flow efficiency of small hydraulic turbines
Cross-flow
Pelton
Kaplan
Francis Propeller
0.0 0.2 0.4 0.6 0.8 1.0
1.0
0.8
0.6
0.4
0.2
0.0
Efficiency
t
Flow rate as proportion of design flow rate
Maximum and minimum turbine flow rates to be decided based on turbine size and efficiency curve.
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-by-step numerical integration of a differential equation (equation of continuity).
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).
Hydropower plant with storage capacity
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.
0 0.2 0.4 0.6 0.8 10
2
4
6
8
10
Time fraction flow rate is equalled or exceeded τ
Q
Q
Exercise
or
)(qF
F(q) is fraction of time q is exceeded.
)()( qFqFdq
d is probability density function.
)(Q
F(q)τq or )(
= probability of occurrence of flow between q and q + dq .dqqF )(
)(qF
1)(
0
Q
QqdqqFq1)(
0
dqqF
Exercise
Choice of function F(q)
k
c
qqF exp)(
)1(
11
k
c
Weibull distribution (widely used in wind energy):
function gammac = scale parameterk = shape parameter
k c
0,50,5000
0
0,550,5874
0
0,60,6646
4
0,650,7319
2
0,70,7900
0
0,750,8398
8
0,80,8826
1
0,90,9504
0
1,01,0000
0
1,11,0363
6
1,21,0630
9
1,31,0827
5
1,41,0971
9
1,61,1153
6
1,81,1245
0
2,01,1283
8
QQq
Exercise
Choice of efficiency-flow curve for turbine (typical small Francis turbine)
0
0,2
0,4
0,6
0,8
1
0 0,2 0,4 0,6 0,8 1
t
rated,tt QQx
0478.3659.26306.7989.119611.88293.25 2345 xxxxxt
rated,t
t
Q
Qx
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):
rated,
ecol
rated,
ecol
rated, ttt
t
Q
Q
Q
Q
Q
Qx
Q
Q
QqQt
ecol
Q
QQx
Q
Q
Qq tt ecolrated,minecolmin,min
min
Q
QQq t ecolrated,
rated
)in W()(')(
)(')()()()(
rated
rated
min
rated,rated,rated,
0
q
tbtte
q
q
tbtteee
dqqFQLHQg
dqqFQLHxQxgdqqFPP
Total electrical energy produced in one year:
)J/yearin (365243600annual, ee PE
)GWh/yearin (1036524 9annual,
ee PE
Exercise
Exercise
Procedure (suggestion)
• Fix annual-averaged flow rate into reservoir, e.g.
• Fix gross head, e.g.
• Fix head loss, proportional to ,e.g. such that loss equal to a few percent of gross head
• Fix flow duration curve, e.g. based on Weibull distribution
• Fix turbine type, turbine efficiency curve and
• Fix minimum (dimensionless) turbine flow rate
• Fix ecological flow rate
• Assume
• Compute
• Make comparisons as appropriate; look for “optimum” value of
/sm 4 3Q
m 50bH
Q
rated,tQ
2tQ
minx
ecolQ
rated,ecolrated, when ttt QQQQQ
ee EP and
rated,tQ
0 2 4 6 8 10 12
turbine rated flowm3s0
250
500
750
1000
1250
1500
enibrutre
wopWk
0 1 2 3 4 5 6turbine rated flowm3s0
500
1000
1500
2000
2500
enibrutre
wopWk
0 2 4 6 8turbine rated flowm3s0
500
1000
1500
2000
2500
3000
enibrutre
wopWk
Some results from Exercisem 50bH
sm4 3QEcological flow rate = 0
Head losses = 0
Francis turbine
Cross-flow turbine
rated
annual-averaged
rated
Annual-averaged
Annual-averaged
FrancisCross-flow
k = 1.6 shape parameter of Weibull distribution
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
• Installed power: 22500 MW
• 34×700 MW Francis turbines
Three Gorges Dam hydropower plant
Construction: 1984-91
Reservoir area: 1350 km2
Total dam length: 7235 m
Dam height: 196 m
Itaipu hydropower plant, Paraná River, Brazil-Paraguay
Installed power: 12870 MW
18 Francis turbines of 715 MW
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.
Range of costs for small hydropower projects.
kW installed
US$/kW
Small hydropower : specific costs of installed capacity
Head (m)
€/kW
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 0-19-926178-4.
• 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).
