12.1 Hydropower

Earlier when commercial electric power generation using hydropower was not available, it was used for irrigation and operation of various machines, such as watermills, textile machines and sawmills. Greeks were able to harness the power in the moving water thousands of years ago when they used water wheels, which picked up water in buckets around a wheel. The water's weight caused the wheel to turn, converting kinetic energy into mechanical energy for grinding grain and pumping water. In the 1800s, the water wheel was often used to power machines such as timber-cutting saws in European and American factories. It was then realized that the force of water falling from a height can turn a turbine which when connected to a generator can produce electricity. Thus, the world's first hydroelectric station (of 12.5 kW capacity) was commissioned on 30 September 1882 on Fox River at the Vulcan Street Plant, Appleton, Wisconsin, USA, used for lighting two paper mills. Hydropower is considered a renewable source of energy as it uses water but does not consume it. It, therefore, offers significant potential for carbon emission reductions. The installed capacity of hydropower by the end of 2013 was 1000 GW which contributed 16.4% of worldwide electricity supply [1]. Hydropower remains the largest source of renewable energy in the electricity generation. Hydropower systems use the energy in flowing water to produce electricity. Although there are several ways to harness the moving water to produce energy, run-of-the-river systems, which do not require large storage reservoirs, are often used for microhydro, and sometimes for small-scale hydro projects.

Hydropower plants (HPPs) today span a very large range of scales, from a few watts to several gigawatts. The largest projects, Itaipu in Brazil with 14,000 MW and Three Gorges in China with 22,400 MW, both produce between 80 and 100 TWh/yr. The great variety in the size of hydropower plants gives the technology the ability to meet both large centralized urban energy needs and decentralized rural needs. The primary role of large hydropower system is in providing electricity generation as part of centralized energy networks. The small hydropower plants can operate in isolation and supply independent systems, often in rural and remote areas of the world. In this chapter, small hydropower for generation of electricity is described.

12.2 Classification of Hydropower Plants

Classification of HPPs can be based on different factors:

Depending on Head:

• Low (less than 50 m)
• Medium (between 50 and 250 m)
• High (greater than 250 m)

Based on exploration and storage:

• With flow regulation (daily or seasonal) reservoir type
• Without flow regulation, run-of-river type

Depending on the size of HPPs, these are classified as follows:

• Large hydro – having capacity of more than 100 MW and feeding to large grid
• Medium hydro – 15–100 MW usually connected to grid
• Small hydro – 1–15 MW usually connected to grid
• Mini hydro – 100 kW–1 MW and connected to grid
• Micro hydro – less than 100 kW

Small hydro are generally “run-of-river” type as shown in Fig. 12.1 which has very small dam or barrage usually just a weir and little or no water stored. Therefore, run-of-river installations do not have the same kinds of adverse effect on ecological balance as in large-scale hydro. Small hydro plants have many advantages such as the following:

• High efficiency (70–90%)
• High capacity factor (more than 50%)
• High level of predictability

12.2.1 Basics of Hydropower Generation

Hydraulic power can be captured wherever a flow of water is from a higher level to a lower level. The vertical fall of the water, known as the “head”, is essential for power generation. When the water flows from higher level to lower level, the potential energy of the water is converted to equivalent amount of kinetic energy. Thus, the height of the water is utilized to calculate its potential energy, and this energy is converted to speed up the water at the intake of the turbine and is calculated by balancing these potential and kinetic energies of water.

where m = mass of water in kg, g = acceleration due to gravity

H = effective water head at the turbine, and v = jet velocity of water at the turbine blades

Hydro turbines convert water force into mechanical shaft power, which can be used to drive an electricity generator or other machinery. The power available is proportional to the product of head and flow rate. The general formula for any hydro system power output is

12.1

where Q is the volume of water flow passing through the turbine in m³/s, is the density of water and is efficiency of turbine. The best turbines can have hydraulic efficiencies in the range of 80% to over 90%, although this will reduce with size. Micro-hydro systems (<100 kW) tend to be 60–80% efficient. Corresponding energy over a period of time will be

12.2

12.3 Resource Assessment

Adequate head and flow are necessary requirements for hydro generation. It is therefore important to know both the flow of water and the gradient at a particular location to assess the potential of hydroelectric power availability. In many countries, stream flow records are maintained by hydrological institutes. These records are useful in determining the flow at a particular site. However, if the records are not available, the discharge should preferably be directly measured for at least a year. A single measurement of instantaneous flow in a watercourse cannot be used for this purpose. There are numerous methods of stream flow measurement also known as stream gauging. These include direct methods, such as volumetric gauging, and dilution methods, as well as indirect methods involving stage-discharge relations or rating curves [2, 3]. Since the velocity of a stream varies with depth and width across a stream, it is important to know the main quantity to measure when choosing a stream gauging method. If the interest is to measure the stream surface velocity, the float method is well suited. This method involves throwing some buoyant, highly visible object into the stream and measuring the time it takes to float a known distance.

12.3.1 Velocity Area Method

The process of measuring stream flow (volume rate of flow), or discharge, is called stream gauging. For obtaining a more accurate stream discharge measurement, the velocity–area method is used. Discharge is the volume of water flowing down a stream or river per unit of time, commonly expressed in cubic metre per second. This is a conventional method for medium-to-large rivers, involving the measurement of the cross-sectional area of the river and the mean velocity of the water through it. Discharge, or the volume of water flowing in a stream over a set interval of time, can be determined with the equation:

It is a useful approach for determining the stream flow with a minimum effort. Stream water velocity is normally measured using a current meter. A current meter consists of a propeller or a horizontal wheel with small, cone-shaped cups attached to it. The cups get filled with water and turn the wheel when placed in flowing water. The number of rotations of the propeller or wheel cup depends on the velocity of the water flowing in the stream. If purely laminar flow is assumed, the stream velocity is expected to vary vertically following a parabolic function because of the zero velocity (no-slip condition) at the bottom of the stream bed. In case of a turbulent flow, it will be a logarithmic function. For this reason, velocity should ideally be measured at several depths for each interval along the river cross section. Alternatively, a single measurement is taken to measure flow at 0.6 times the total depth, which typically represents the average flow velocity in the stream. Velocity also varies within the cross section of a stream, where stream banks are associated with greater friction and hence slower moving water. Thus, it is necessary to take velocity measurements along a cross section of a stream as shown in Fig. 12.2. Since stream channels are rarely straight, it is helpful to measure velocity across an “average” reach of the stream (e.g. average width and depth) with a single channel, a relatively flat stream bed with little vegetation and rocks and few back-eddies that hinder current meter movement.

The stream is divided into sections based on where velocity and stage height measurements were taken in the cross section of the stream. By multiplying the cross-sectional area (width of section × stage height) by the velocity, the discharge for that section of stream can be calculated. The discharge from each section is then added to determine the total discharge of water from the stream.

12.3.2 Float Method

The basic idea of float method is to measure the time that it takes the object to float a specified distance downstream as shown in Fig 12.3. Float method can be used to measure the surface velocity of river flow. Because surface velocities are typically higher than mean or average velocities, the mean velocity is obtained by using a correction factor.

where k is a coefficient that generally ranges from 0.8 for rough beds to 0.9 for smooth beds (0.85 is a commonly used value).

This method is simple and inexpensive but not as accurate as the velocity–area method. Surface velocity measurements should be carried only on windless days to avoid deflection of the floats due to wind. Even under these conditions, surface floats may be diverted from a direct course between measuring stations because of surface disturbances and crosscurrents. To determine the velocity of the discharge, about 8–30 m long section of the channel that includes the part where the cross section was measured is marked. The length will be dependent upon the speed of the water. In many channels, 8 m length would be too short a distance because the float would travel too fast to get an accurate time estimate. In such streams, a longer distance may be used. The float is gently released into the channel slightly upstream from the beginning of the section. The time taken by the “float” to travel the marked section is measured, and this process is repeated at least three times to calculate the average time. The surface velocity is then computed by dividing the length of the section by the time it took the float to move through the section.

12.4 System Components

Small run-of-the-river hydropower systems consist of these basic components [4, 5]:

• Diversion weir, channel or pipeline
• Turbines
• Generators

12.4.5 Spillways

Spillways are designed to permit controlled overflow at certain points along the channel. Figure 12.4 depicts a flood spillway in detail, including flow control and channel emptying gates. Flood flows through the intake can be twice the normal channel flow, so the spillway must be large enough for diverting this excess flow.

The spillway is a flow regulator for the channel. In addition, it can be combined with control gates to provide a means of emptying the channel.

The spill flow must be fed back to the river in a controlled way so that it does not damage the foundations of the channel.

12.5 Turbines

Water wheels and turbines can be used to convert the energy of running water into mechanical energy. Turbines are more commonly used nowadays to power small hydropower systems as turbines are more compact in relation to their energy output than water wheels. They also have fewer gears and require less material for construction. The moving water strikes the turbine blades, to rotate a shaft which is connected to a generator. Conventional pumps can also be used as substitutes for hydraulic turbines. When the action of a pump is reversed, it operates as a turbine. Since pumps are mass produced, these are more readily available and less expensive than turbines. However, for adequate pump performance, the micro-hydro site must have fairly constant head and flow. Pumps are also less efficient and more prone to damage.

There are two general classes of turbines: impulse and reaction. The type of turbine selected for a particular project is based on the height of standing water called head and volume of water flow [5–7].

12.6 Impulse Turbines

Impulse turbines use the velocity of water which moves the runner blades of the turbines. The water stream hits each bucket on the runner. There is no suction on the down side of the turbine, and water remains at atmospheric pressure after and before hitting the runner. An impulse turbine is generally used for high-head and low-flow applications. There are three types of impulse turbines used for small hydro systems; these are (i) Pelton turbine, (ii) cross-flow and (iii) Turgo turbine.

12.6.1 Pelton Turbine

A Pelton turbine essentially consists of one or more injectors for generating the high-speed jet and a wheel with a series of split buckets for receiving the jet energy as shown in Fig. 12.5. An injector nozzle converts the pressure energy of the water into the kinetic energy of the high-speed jet. It also regulates the flow rate via a built-in needle which is driven by a servomotor. The jet hits each bucket, and the impact of water on the buckets causes the runner to rotate and produce mechanical energy. Nearly all the energy of water is expended in rotation of the runner, and deflected water is discharged in tail race. The buckets are so shaped that the water enters tangentially in the middle and is split in half. Each half is turned backwards and flows again tangentially in both the directions to avoid thrust on the wheel. Often, two buckets are mounted side by side, thus splitting the water jet in half. This balances the side-load forces on the wheel and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel.

The number of jets is not more than two for horizontal shaft turbines and is limited to six for vertical shaft turbines. The flow partly fills the buckets, and the fluid remains in contact with the atmosphere. Therefore, once the jet is produced by the nozzle, the static pressure of the fluid remains atmospheric throughout the machine. Because of the symmetry of the buckets, the side thrusts produced by the fluid in each half balances each other. For maximum power and efficiency, the turbine system is designed such that the water-jet velocity is twice the velocity of the bucket. A very small percentage of the water's original kinetic energy will still remain in the water; however, this allows the bucket to be emptied at the same rate it is filled, thus allowing the water flow to continue uninterrupted.

For a constant water flow rate from the nozzles, the speed of the turbine changes with changing loads on it. For quality hydroelectricity generation, the turbine should rotate at a constant speed. To keep the speed constant despite the changing loads on the turbine, the water flow rate through the nozzles is changed. To control the gradual changes in load, servo-controlled spear valves are used in the jets to change the flow rate. And for sudden reduction in load, the jets are deflected using deflector plates so that some of the water from the jets does not strike the blades. This prevents over-speeding of the turbine.

Depending on water flow and design, Pelton wheels operate best with heads from 15 to 1800 m, although there is no theoretical limit.

12.6.2 Cross-Flow Turbine

A cross-flow turbine as shown in Fig. 12.6 has a drum-shaped runner consisting of two parallel discs connected together near their rims by a series of curved blades. A cross-flow turbine always has horizontal runner shaft l (unlike Pelton and Turgo turbines which can have either horizontal or vertical shaft orientation). It uses an elongated rectangular section nozzle directed the against the full length of the runner.

A cross-flow turbine is drum-shaped and uses an elongated, rectangular section nozzle directed against curved vanes on a cylindrically shaped runner. It resembles a “squirrel-cage” blower. The cross-flow turbine allows the water to flow through the blades twice. The first pass is when the water flows from the outside of the blades to the inside; the second pass is from the inside back out. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross-flow was developed to accommodate larger water flows and lower heads than the Pelton.

The Banki–Michell turbine is a simple and economic turbine appropriate for micro-hydropower plants. The peak efficiency of this turbine is somewhat less than that of a Kaplan, Francis or Pelton turbine, but its relative efficiency is close to one within a large range, especially above the optimum discharge value. The Banki–Michell turbine has a drum-shaped runner consisting of two parallel discs connected together near their rims by a series of curved blades as shown in Fig. 12.7. The turbine has a horizontal rotational shaft, unlike Pelton and Turgo turbines, which can have either horizontal or vertical shaft orientation. The water flow enters through the cylinder defined by the two disc circumferences (also called impeller inlet), and it crosses twice the channels confined by each blade couple. After entering the impeller through a channel, the particle leaves it through another one. Going through the impeller twice provides additional efficiency. When the water leaves the runner, it also helps to clean the runner of small debris and pollution. So the cross-flow turbines get cleaned as the water leaves the runner (small sand particles, grass, leaves, etc., get washed away), preventing losses. Other turbine types get clogged easily and consequently face power losses despite higher nominal efficiencies.

12.7 Reaction Turbine

The reaction turbines considered here are the Francis turbine and the propeller turbine. A special case of the propeller turbine is the Kaplan shown in Fig. 12.8. In all these cases, specific speed is high, that is, reaction turbines rotate faster than impulse turbines given the same head and flow conditions. This has a very important consequence that a reaction turbine can often be compiled directly to an alternator without requiring a gear drive system. Combined turbine–generator sets are also available in the market. Significant cost savings are achieved in eliminating the drive, and the maintenance of the hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while the propeller is more suitable for low heads. Micro-hydropower project uses vertical-type Francis turbine system. The turbine mainly consists of water diversion chamber, turbine runner or wheel, water guide vane and draft tubes. With adjustable guide vanes, the water flow is regulated as it enters the runner, and the vanes are usually linked to a governing system which matches the flow to turbine loading. When the flow is reduced, the efficiency of the turbine is reduced. The runner blades are profiled in a complex manner and direct the water so that it exits axially from centre of the runner. In doing so, the water imparts most of its pressure energy to the runner before leaving the turbine via a draft tube. The spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner, and the guide vanes feed the water into the runner at the correct angle.

On the whole, reaction turbines require more sophisticated fabrication than impulse turbines because they involve the use of larger and more intricately profiled blades together with carefully profiled casings. The extra expenses involved are offset by high efficiency and the advantages of high running speeds at low heads from relatively compact machines.

Fabrication constraints make these turbines less attractive for use in micro-hydro in developing countries. Nevertheless, because of the importance of low head micro-hydro, work is being undertaken to develop propeller machines which are simpler to construct. Most reaction turbines tend to have poor part-flow efficiency characteristics.

12.7.2 Reverse Pump Turbines

Centrifugal pumps as shown in Fig. 12.9 can be used as turbines by passing water through them in reverse. Reversible machine sets consist of a motor generator and a centrifugal pump turbine that works either as a pump or as a turbine depending on the direction of rotation. The advantages of using these pumps as turbines are low cost due to their mass production and availability of spare parts. Furthermore, a well-designed, compact powerhouse saves equipment and civil costs. With a wide range of specific speeds, pump turbines can be installed at sites with heads from less than 50 m to more than 800 m and with varying unit capacities.

The main problem in using these pumps is that manufacturers do not normally provide characteristic curves of their pumps working as turbines. This makes it difficult to select an appropriate pump to run as a turbine for a specific operating condition.

12.8 Generators for Small Hydro Plants

Induction generators or synchronous generators can be used for small hydropower generation [8]. Induction generators are commonly used for small hydro schemes due to advantages such as availability, low cost and robustness. The cost per kilowatt of a single-phase generator is generally higher than a three-phase generator. Hence, a three-phase generator, which produces a single-phase output, is normally used. A three-phase generator can be converted into a single-phase generator, which produces approximately 80% of the machine rating, by connecting two capacitors as shown in Fig. 12.10. In order to further minimize the capital cost, very simple method of controlling voltage and frequency control is used. A capacitor-excited induction generator used with a hydraulic turbine in a stand-alone generating system can provide high-quality voltage and frequency control not matched by other small generating units. This is achieved without a turbine governor by using a controllable additional impedance on the load side. The control is achieved using a static power converter. In these schemes, the generator operates under manual control of the sluice gate, and if the consumer load changes, then the generated voltage and the frequency also vary. If the load is light, the generator speed can increase, leading to runaway condition. The control technique used to maintain the generated voltage and the frequency at its rated value is to maintain the total load connected to near-constant value by connecting a resistive ballast, which maintains the sum of the consumer load and the ballast load at a constant value.

By using self-excited induction generators rather than synchronous generators, cost savings and reliability improvements can be achieved, due to the simple construction and inherent robustness of cage induction machines. However, until recently, the extra cost and complexity of the voltage and frequency control equipment have more than offset the advantages of using stand-alone induction generators.

12.9 Design Considerations of Micro-Hydropower Plants

In the design of a micro-hydropower system, the following parameters are considered. The selection of turbine type, size and speed is based on the net head and maximum power flow rate. Most small hydropower sites are categorized as low- or high-head. The higher head is better because less water is needed to produce a given amount of power and smaller and less expensive equipment can be used. Low head is classified when there is change in elevation of less than 3 m. A vertical drop of less than 0.6 m will not be suitable to make a small-scale hydroelectric system. However, for extremely small power generation amounts, a flowing stream with as little as 35 cm of water can support a submersible turbine, as the type used originally to power scientific instruments towed behind oil exploration ships. Small-scale hydropower systems, as well as mini-hydro systems or micro-hydro systems, can be designed using either water wheels or impulse-type hydro turbines. Once the turbine power, specific speed and net head are known, the turbine type, the turbine fundamental dimensions and the height or elevation above the tail race water surface that the turbine should be installed to avoid cavitation phenomenon can be calculated. In case of Kaplan or Francis turbine type, the head loss due to cavitation, the net head and the turbine power must be recalculated. In general, the Pelton turbines cover the high-pressure domain down to (50 m) for micro-hydro. The Francis type of turbines cover the largest range of head below the Pelton turbine domain with some overlapping and down to (10 m) head for micro-hydro. The lowest domain of head below (10 m) is covered by Kaplan type of turbine with fixed or movable blades. For low heads and up to (50 m), the cross-flow impulse turbine can also be used. Once the turbine type is known, the fundamental dimensions of the turbine can be easily estimated.

Once the turbine is selected, the mean flow duration curve for the river is obtained to determine power potential of stream. In case of low discharge rivers (less than 4 m³/s), it may be possible to build a weir. It is a low wall or dam across the stream to be gauged with a notch through which all the water may be channelled. A simple linear measurement of the difference in level between the upstream water surface and the bottom of the notch is sufficient to quantify the flow rate (discharge). In order to prevent the trash entering into the entrance flume, bars at certain spacing (called trash rack) are placed in a slanting position (at an angle of 60°–80° with horizontal). The maximum possible spacing between the bars depends on the type of turbine used. Typical values are 20–30 mm for Pelton turbines 40–50 mm for Francis turbines and 80–100 mm for Kaplan turbines. The water carried by the power channel is distributed to various penstocks leading to the turbines through the forebay. Water is temporarily stored in the forebay in the event of a rejection of load by the turbine, and it can be withdrawn when the load is increased. In addition, the forebay acts as a sort of regulating reservoir.

Pipes are used for conveying water from the intake to the power house. They can be installed over or under the ground, depending on factors such as the nature of the ground, the penstock materials, the ambient temperature and the environmental requirements.

When the turbine and generator operate at the same speed and can be placed so that their shafts are in line, direct coupling is the right solution. In this case there, will be no power loss incurred and maintenance is minimal. Turbine manufacturers will recommend either rigid or flexible type of coupling although a flexible coupling that can tolerate certain misalignment is usually preferred. In the lowest power range, turbines can run at less than (400) rpm and speed is increased through gears to (1500) rpm of standard alternator. In the range of powers produced in small and micro-hydro schemes, this solution is always more economical than the use of a direct-coupled alternator. The rotational speed of a turbine is a function of its power and net head. In the small and micro-hydro schemes, turbine selection is made keeping in mind whether it will be coupled directly or through a gearbox to reach the synchronous speed of generator.

Standard generators should be installed when possible, so in each, the runner profile is characterized by a maximum runaway speed.

The runaway speed of a hydraulic turbine is the speed at which the turbine coupled to the generator runs at the maximum possible speed due to loss of load. The runaway speed of a hydraulic turbine is the speed at which the turbine coupled to the generator runs at the maximum possible speed due to loss of load. The runaway speed of the turbine is determined by the turbine designer and is influenced by the maximum discharge of water from the penstock, the combined inertia of the turbine runner and the generator and the flywheel. Depending on the type of turbine, it can attain 2 → 3 times the nominal speed. The cost of generator and gearbox may be increased when the runway speed is higher, since they must be designed to withstand it.

A governor is a combination of devices and mechanisms, which detects the speed deviation and converts it into a change in servomotor position. Several types of governors are available. The purely mechanical governor is used with fairly small turbines. In modern electric–hydraulic governor, a sensor is located on the generator shaft to sense the turbine speed. The turbine speed is compared with reference speed. The error signal is amplified and sent to the servomotor to act in the required sense. To ensure the control of the turbine speed by regulating the water flow, certain inertia of rotating components is required. Additional inertia can be provided by a flywheel, on the turbine, or generator shaft. The flywheel effect of the rotating components is stabilizing, whereas the water column effect is destabilizing. For MHP schemes at remote locations (not connected to the grid), the parameter that needs to be controlled is the turbine speed, which controls the frequency. In an off-grid system, if the generator becomes overloaded, the turbine slows down. Therefore, an increase in the flow of water is needed to ensure that the turbine does not stall. If there is not enough water to do this, then either some of the load must be removed or the turbine will have to be shut down. On the other hand, if the load decreases, then the flow to the turbine is decreased or it can be kept constant, and the extra energy can be diverted into a ballast (dummy) load connected to the generator terminals.

In a run-of-river hydro scheme, the flow of the water is not altered, so its minimum flow rate needs to be the same or higher than that of the proposed turbine output power, ensuring maximum efficiency. The result is that the costs involved for a run-of-river scheme are much lower and have less environmental impact than other small-scale hydro plants. The disadvantage is that the water flow rate is variable throughout the year, and the system is unable to store the water's energy.

The development of a small-scale hydropower electrical scheme which uses a small dam or weir, water storage reservoir (impoundment) or requires a diversion of the rivers, water flow through tunnels or canals requires far more water usage in total as well as more complex civil and ground engineering works to match the site elevation, not to mention the environmental impact that is proportional to the size of the scheme.

12.9.1 Example

A small stream drops 20 m down the side of a mountain producing a water flow rate of 500 l/min past a fixed point. How much power could a small-scale HPP generate in kilowatts, if the type of water turbine used has a maximum efficiency (η) of 85%.

The data given: Head = 20 m, flow rate = 500 l/min, efficiency = 0.85 and gravity = 9.81 m/s². But first we must convert the water flow rate of 500 l/min into m³/s.

1000 l is equal to 1 m³, so 500 l is equal to 0.5 m³. One minute is equal to 60 s, then a flow rate of 0.5 m³/min is equal to 0.00833 m³/s.

References

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