Water-for-hydroelectric-power.docx

Bulacan State University – Civil Engineering Department

WATER FOR HYDROELECTRIC POWER Electric power is developed from two types of plants: (a) hydro (b) thermal Hydroelectric generators are driven by water turbines while thermal (or steam) plants derive energy from combustion of fuel. Most thermal plants use steam turbines and fossil (coal, oil, or natural gas) or nuclear fuel. COMPARISON OF THERMAL- AND HYDROELECTRIC-POWER COSTS The initial cost of a hydroelectric plant is generally higher than that of a comparable thermal plant. The initial cost of the hydroelectric project includes 1) the cost of the dam 2) diversion works 3) conduits 4) land 5) water rights 6) railroad 7) highway and utilities relocation 8) value of improvements flooded by the reservoir and 9) the generating plant itself Often a hydroelectric plant is located at a relatively inaccessible location, which adds to the initial cost because of the expense of hauling materials and equipment and because of long transmission lines to carry the energy to market. In addition to the cost of transmission facilities, there is lost of energy during transmission. Thermal plants are usually located near their load centers, eliminating the need for long transmission lines. The site requirements for a thermal plant include the availability of an adequate supply of fuel and plenty of water for cooling the condensers. Hence, thermal plants are usually located at sea coast, or near a lake or river. Consideration of the ecological consequences of discharging large quantities of hot water into a water body is necessary in site selection and outfall design. Heated water has been shown to be useful for irrigation. Cooling towers may also be an alternate solution. Even though the initial capital investment is greater, taxes on a hydroelectric plant may not be as much as those for a thermal plant because of the lower tax rate in remote locations compared with the higher tax rate in areas where a thermal plant is likely to be located. The cost of insuring a hydroelectric plant may also be less than that for a thermal plant of comparable capacity because a smaller portion of the items comprising the initial cost of a hydroelectric plant are insurable. The cost of operating a thermal plant is much higher than that for a hydroelectric plant, mainly because of fuel costs. A thermal plant is also difficult to operate and maintain, and the cost of labor, maintenance, and repairs is much higher than for a hydroelectric plant. Expensive air pollution-control systems are required for most thermal plants, and cooling towers or cooling ponds are needed at many to avoid thermal pollution of streams or lakes. Fuel costs for a thermal plant vary with the unit price of fuel and the plant output. The cost of fossil fuels is dependent on the distance from the fuel source to the plant, and fuel cost in one locale may be as much as twice that in another area. Because of lack of bulk, the transport costs for nuclear power are quite low, and hence nuclear power plants are advantageous in locations where conventional fuels and hydroelectric power are unavailable. As thermal plants get older, their efficiency drops, and they are usually operated at reduced capacity, thus permitting newer, more efficient plants to carry heavier loads. WATER USE FOR ENERGY PRODUCTION Definition of terms WATER FOR HYDROELECTRIC POWER | Water Resources Engineering FTSamonte2017

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Bulacan State University – Civil Engineering Department

Load – refers to the demand for electricity expressed in terms of energy demand (or use power demand) or capacity demand (peak power demand). Daily load shape – is the power demand as a function of time of day. Load is divided into three segments: 1) Base load, which is continuous for 24 hours 2) Peak load, which is the highest load occurring for a few hours a day, and 3) Intermediate load, which is the portion between the load and the peak load.

TYPES OF HYDROELECTRIC POWER DEVELOPMENTS Water and energy are two resources that are very necessary for humankind and are intricately connected. Hydroelectric power is the most obvious use of water for production of energy. The energy in falling water is used directly to turn turbines that generate electricity.  The types of hydroelectric developments by facility type are: 1) Run-of-river developments 2) Pondage developments 3) Storage developments 4) Reregulating developments 5) Pumped storage 6) Tidal power 7) Underground power station Run-of-river developments have a dam with a short supply pipe (penstock) that directs the water to the turbines. The natural flow of the river is used with very little alteration to the terrain stream channel at the site and there is very little impoundment of the water. Typical run-of-river projects include: (1) navigation projects, (2) irrigation diversion dams, and (3) projects lacking storage as a result of topography. A pure run-of-river project has no usable storage. (Figure 4.1) Pondage developments are run-of-river projects with a small amount of storage (daily or weekly) that can be used to regulate discharges to follow daily and weekly load patterns. Pondage refers to the short time in ponding of water, as these projects have insufficient storage for seasonal flow regulation. Storage developments have an existing impoundment at the power plant or are at the reservoir upstream of the power plant. Storage is the long-time impounding of water to allow seasonal regulation capability. (Figure 4.2) Reregulating developments receive fluctuating discharges from large hydroelectric peaking plants and release the discharges to meet downstream flow criteria. Reregulating reservoirs are also called after-bay reservoirs. (Figure 4.3) Pumped-storage developments convert low-volume off-peak energy to high-value on-peak energy. Water is pumped from a lower reservoir to a higher reservoir using inexpensive power during periods of low energy demand. Water is then discharged through the turbine to produce to meet peak demands. Pumped storage projects can also be operated on a seasonal basis. (Figure 4.4) Tidal power uses the potential energy from the difference in height between high and low tides. A dam or barrage is normally built across a river estuary and is designed to trap a high tide of water, then allowing it to drive a set of turbines and generate power. Underground power station is generally used at large facilities and makes use of a large natural height difference between two waterways, such as a waterfall or mountain lake. An underground tunnel is constructed to take water WATER FOR HYDROELECTRIC POWER | Water Resources Engineering FTSamonte2017

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Bulacan State University – Civil Engineering Department

from the high reservoir to the generating hall built in an underground cavern near the lowest point of the water tunnel and a horizontal tailrace taking water away to the lower outlet waterway.  The types of hydroelectric developments by size are: 1) Large hydropower 2) Small hydropower (< 10 MW) 3) Mini hydropower (< 1 MW) 4) Micro hydropower (< 100 kW) 5) Pico hydropower (< 10 kW) 6) Family hydropower (< 1 kW) Hydropower installations can be classified by size of power output, although the power output is only an approximate diversion between different classes. There is no international consensus for setting the size threshold between small and large hydropower. For the United Nations Industrial Development Organization (UNIDO) and the European Small Hydropower Association (ESHA) and the International Association for Small Hydro (IASH) a capacity of up to 10 MW total is becoming the generally accepted norm for small hydropower plants (SHP). In China, it can refer to capacities of up to 25 MW, in India up to 15 MW and in Sweden small means up to 1.5 MW, in Canada 'small' can refer to upper limit capacities of between 20 and 25 MW, and in the United States 'small' can mean 30 MW. The German Federal Ministry for Environment, Nature Conservation and Nuclear Safety mentioned that a SHP is <1 MW, everything above is a large hydroelectric plant and usually comes along with a large dam. The International Commission on Large Dams (ICOLD) defines a large dam as a dam with a height of 15 m or more from the foundation. If dams are between 5-15 m high and have a reservoir volume of more than 3 million m3, they are also classified as large dams. Using this definition, there are over 45 000 large dams around the world.  The types of hydroelectric developments by site or location are: 1) Concentrated-fall hydroelectric development 2) Divided-fall hydroelectric development A concentrated-fall hydroelectric development is one in which the powerhouse is located near the dam, and is most common for low-head installations. The powerhouse may be located at one end of the dam, directly downstream from the dam, or even in a buttress-type dam. In a divided-fall development, water is carried to the powerhouse at a considerable distance from the dam through a canal, tunnel, or penstock. With favorable topography, it is possible to realize a high head even with a low dam. COMPONENTS OF HYDROELECTRIC PLANTS A hydroelectric development ordinarily includes 1) diversion structure 2) conduit (penstock) to carry water to the turbines 3) turbines and governing mechanism 4) generators 5) control and switching apparatus 6) housing for the equipment 7) transformers 8) transmission lines to the distribution centers 9) trash racks at entrance to the conduit, canal and penstock gates 10) forebay, or surge tank 11) tailrace

WATER FOR HYDROELECTRIC POWER | Water Resources Engineering FTSamonte2017

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Bulacan State University – Civil Engineering Department

The height of the dam establishes the generating head and the available storage for power plant operations. The reservoir is the water impoundment behind a dam. Storage capacity is the volume of reservoir available to store water. Draft tubes convey water from the discharge side of the turbine to the tailrace. A forebay serves as a regulating reservoir, temporarily storing water when the load on the plant is reduced and providing water for the initial increments of an increasing load while water in the canal is being accelerated. The forebay is provided with some type of intake structure to direct water to the penstock. Intakes should be provided with trash racks to prevent the entry of debris which might damage the wicket gates and turbine runners or choke the nozzle of impulse turbines. A forebay must be provided with a spillway, or wasteway, so that excess water can be disposed of safely if the need arises. Siphon spillways are often advantageous for this purpose. Penstocks convey water from the intake structure to the powerhouse. The structural design of a penstock is the same as for any other pipe. Because of the possibility of sudden load changes, design against water hammer is essential. Long penstocks are usually provided with a surge tank to absorb water hammer pressures and to provide water to meet sudden load increases. For short penstocks it is generally economical to forgo the protection of a surge tank and rely on a heavier pipe wall and slow-closing valves. Penstocks are most commonly made of steel, though reinforced-concrete and wood-stave pipes are also used. If the distance from the forebay to the powerhouse is short, separate penstocks are ordinarily used for each turbine. Long penstocks are usually branched at the lower end to serve several turbines. A powerhouse consists of a substructure to support the hydraulic and electrical equipment and a superstructure to house and protect this equipment. The substructure usually consists of a concrete block extending from the foundations to the generator floor with waterways formed within it. The superstructure of most powerhouses is a building housing all operating equipment. Basic types of turbines: (1) impulse turbines (commonly called Peltron turbines), in which a free jet of water impinges on a revolving element of the machine that is exposed to the atmosphere, and (2) reaction turbines, in which the flow takes place under pressure in a closed chamber. Reaction turbines extract power from both the kinetic energy of the water and the difference in pressure between the front end and back of each runner blade. The tailrace is the channel into which the water is discharged after passing through the turbines. If the powerhouse is close to the stream, the outflow may be discharged directly into the stream, while in other situations there may be a channel of considerable length between the powerhouse and the stream. Figure 4.1 Run-of-river development

Figure 4.3 Reregulating development FTSamonte2017

Figure 4.2 Storage development

WATER FOR HYDROELECTRIC POWER | Water Resources Engineering

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Bulacan State University – Civil Engineering Department

Figure 4.4 Pumped-storage development

Figure 4.5 Concentrated fall development

Figure 4.4 Divided-fall development

DETERMINING ENERGY POTENTIAL Hydrologic Data The most common data for a hydropower feasibility study is streamflow data, which is used to develop estimates of water available for power generation. The most commonly used stream flow data is mean daily flows, mean weekly flows, and mean monthly flows. This data is used to develop flow-duration curves , which show the percentage of time that flow equals or exceeds various vales during the period of record. Flow duration curves summarize the stream flow characteristics and can be conducted from daily, weekly, or monthly stream flow data.

WATER FOR HYDROELECTRIC POWER | Water Resources Engineering FTSamonte2017

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Bulacan State University – Civil Engineering Department

Other types of hydrologic data include: (1) tailwater rating curves, (2) reservoir elevation-area-capacity relationships, (3) sediment data, (4) water quality data, (5) downstream flow information, (6) water surface fluctuation data, and (7) evaporation seepage loss analysis. Power Rule Curve In general rule curves are guidelines for reservoir operation. They are generally based on detailed sequential analysis of combinations of critical hydrologic conditions and critical demand conditions. Rule curves are developed for flood control operations and for conservation storage for irrigation, water supply, hydropower, and other purposes. A power rule curve is defined as a curve, or family or curves, indicating how a reservoir is to be operated under specific conditions to obtain best or predetermined results. The general shape of a power rule curve is governed by the hydrologic and power demands. The curve defines the minimum reservoir elevation (corresponding minimum storage) that is required to generate firm power anytime of the year. Firm energy is the generation that exactly draws the reservoir level to the bottom of the power pond during the most severe drought of record. Multipurpose Storage Operation Most storage projects that have power storage also have space for flood control regulation and conservation storage space for other water needs. A joint-use storage zone is one that can be used for flood regulation during part of a year and for conservation storage the remainder of the year. This is allowed in many river basins because major floods are concentrated in one season of the year. Joint-use storage allows less total reservoir storage than having separate zones for flood control and conservation. Planning the hydroelectric power development 1) Assemble hydrologic data on streams, and determine the amount of water available and its distribution throughout the year and from year to year. Extend the data by simulation and/or stochastic methods, if necessary. 2) Make preliminary designs for all installations which seem competitive in cost, and determine the most economic design at each site by comparison of costs and estimated power revenues. 3) Determine the requirements to be satisfied (maximum instantaneous load in kilowatts, total energy in kilowatthours per year, and variation in kilowatts with time). 4) Select feasible projects as close to the load center as possible. 5) Compare the best designs from the several sites, and select the site or combination of sites which proves best for production of the required power. Often this selection is guided by estimated future requirements and the possibilities of future expansion to meet them. 6) Compare the cost of the hydroelectric-power plant with that of an equivalent thermal plant. 7) If hydroelectric power is competitive with steam, proceed with the detailed design of the hydroelectric installation.

WATER FOR HYDROELECTRIC POWER | Water Resources Engineering FTSamonte2017

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