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HYDROELECTRIC ENERGY

HYDROELECTRIC ENERGY. Renewable Energy Resources 2008. António F. O. Falcão. HYDRO ENERGY RESOURCE Total resource: (about 15 times total world hydroelectric production Technical potential: about: Total world electricity consumption: 16 400 TWh.

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HYDROELECTRIC ENERGY

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  1. HYDROELECTRIC ENERGY Renewable Energy Resources 2008 António F. O. Falcão

  2. HYDRO ENERGY RESOURCE • Total resource: (about 15 times total world hydroelectric production • Technical potential: about: • Total world electricity consumption: 16 400 TWh SOLAR ENERGY flux on the Earth surface: 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

  3. Prefixes:

  4. 15,8% of world electrical energy consumption Based on average output 1999-2002 Source: G. Boyle, Renewable Energy, 2004.

  5. Technical potential Economic potential Exploited potential North & Central America Australasia/ Oceania Europe Asia South America Africa Exploited hydro potential by continent

  6. Weir and intake (dique ou açude) Canal (canal) Forebay tank (câmara de carga) Penstock (conduta forçada) Power house (casa das máquinas) Small hydro site layout

  7. Large hydro 10 MW Small hydro 500 kW Mini-hydro 100 kW Micro-hydro Note: there are other definitions.

  8. Small hydroelectric plants (< 10 MW) World totals • 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

  9. Installed capacity and production of SHPs (<10MW) in 30 European countries

  10. A = gross head (altura de queda bruta) in metres Canal = gross head (altura de queda bruta) L = losses in canal, pennstock, in metres Pennstock = net head (altura de queda disponível) Turbine B Q = flow rate or intake (caudal), in m3/s = gross power (potência bruta), in Watts = power available to turbine turbine efficiency = turbine power output = electrical power output electrical efficiency

  11. Hydraulic turbine rated H = (net) head Q = flow rate N = rotational speed N, H = constant Dimensional analysis Q (Dimensionless) specific speed Ω is directly related to geometry (type) of turbine

  12. Francis Pelton Kaplan Rotors of hydraulic turbines with different specific speeds Ω.

  13. Correspondence between specific speed Ω and type of hydraulic turbine (Pelton, Francis, Kaplan)

  14. Pelton turbines (low Ω) • Usually: • High H • Small Q

  15. Twin jet Pelton turbine wheel or runner nozzle pennstock

  16. Large Pelton turbine • Vertical axis • 6 jets (6 nozzles)

  17. Francis turbines (medium Ω)

  18. Francis turbine Spiral casing runner Guide vanes draft tube

  19. 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.

  20. Kaplan turbines (high Ω) • Usually: • Low H • Large Q

  21. Kaplan turbine Electrical generator Blade angle can be controlled spiral casing Guide vanes runner

  22. Propeller turbine (small power plants) Simple control: rotor blades are fixed Kaplan turbine Double control Guide-vane control Rotor-blade control

  23. A variant of the Kaplan turbine: the horizontal axis Bulb turbine Used for very low heads, and in tidal power plants guide vanes Tidal plant of La Rance, France

  24. 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.

  25. Cross-flow turbine

  26. Head-flow ranges of small hydro turbines

  27. 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.

  28. How to estimate the type and size of a turbine, given (rated values of): • H = (net) head, • Q = flow rate, • N = rotational speed ? Type (geometry)

  29. Pelton turbine D Diameter D

  30. Francis and Kaplan turbines D Specific diameter (dimensionless)

  31. 1.0 Pelton 0.8 Cross-flow Efficiency 0.6 Kaplan Francis Propeller 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Flow rate as proportion of design flow rate Part-flow efficiency of small hydraulic turbines

  32. 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.

  33. Power plant Stream-gauging station • 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.

  34. 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.

  35. Mean annual flow duration curve mean annual flow rate Time fraction flow rate is equalled or exceeded Dimensionless form of the mean annual flow duration curve

  36. 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.

  37. Run-of-river plant and flow duration curve. Max. turbine flow Min. turbine flow Ecological flow Time-fraction flow rate is equalled or exceeded

  38. 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.

  39. Maximum and minimum turbine flow rates to be decided based on turbine size and efficiency curve. 1.0 Pelton 0.8 Cross-flow Efficiency 0.6 Kaplan Francis Propeller 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Flow rate as proportion of design flow rate Part-flow efficiency of small hydraulic turbines

  40. 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).

  41. 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).

  42. 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.

  43. Exercise or F(q) is fraction of time q is exceeded. is probability density function. Time fraction flow rate is equalled or exceeded τ = probability of occurrence of flow between q and q + dq .

  44. k = shape parameter c = scale parameter Exercise Choice of function F(q) Weibull distribution(widely used in wind energy):

  45. Exercise Choice of efficiency-flow curve for turbine (typical small Francis turbine) Set a minimum value for the turbine efficiency, e.g. 20% efficiency. Set the minimum value of the turbine flow rate accordingly.

  46. Exercise Annual-averaged electrical power output (SI units):

  47. Exercise Total electrical energy produced in one year:

  48. 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

  49. Some results from Exercise Ecological flow rate = 0 Head losses = 0 k = 1.6 shape parameter of Weibull distribution Cross-flow turbine Francis turbine rated rated annual-averaged Annual-averaged Annual-averaged Francis Cross-flow

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