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CASE STUDY AND ANALYSIS OF MICRO
HYDRO POWER PLANT
(DAPUR MAIDAN, DIR LOWER KPK)
Muqeem Ud Din
Umar Farooq
Supervised by:
Engr. Naveed Ullah
Final year project 2011-2012, submitted as a partial fulfillment for
the Degree of B.Sc in Mechanical Engineering
DEPARTMENT OF MECHANICAL ENGINEERING
UNIVERSITY OF ENGINEERING & TECHNOLOGY,
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CASE STUDY AND ANALYSIS OF MICRO
HYDRO POWER PLANT
(DAPUR MAIDAN, DIR LOWER KPK)
Muqeem Ud Din
Umar Farooq
_______________
Engr. Naveed Ullah
Project Supervisor
___________________
Prof. Saeed Javed Tajik
Chairman
FINAL YEAR PROJECT 2011-2012
DEPARTMENT OF MECHANICAL ENGINEERING
UNIVERSITY OF ENGINEERING & TECHNOLOGY,
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ABSTRACT
The total available hydro power potential in Khyber Pukhtunkhwa is about 30000MW in which 18% has been harnessed. Local communities living in distant far and flung mountainous areas are still in dark and lack resources and development. MHPPs could bring revolutionary change in their lives and help in the development of society. NGOs and local communities have installed MHPPs but most often with locally built accessories and average expertise. The aim of this project was to analyze the already installed MHPPs in KPK by considering a case study and to recommend for improvement. In order to accomplish this objective, extensive literature review has been carried out. For site selection and acquisition of data regarding MHPPs in KPK, various GOVT. and Non-Govt. organizations have been consulted. Different sites were visited and finally a 10 KW MHPP was selected in “Dapor Maidan” Dir (l). For evaluation of the site, theoretical power output was calculated after finding site parameters. Efficiency was calculated which was 21.76%. The much lower efficiency than optimum range (60-75%) depicted shortcomings in civil structures and electro mechanical components. Finally causes of limitations were identified and recommendations were made for the improvement of efficiency.
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ACKNOWLEDGMENT
Special gratitude to Engr. Navidullah, Semester Coordinator Mechanical Department UET Peshawar, for his matchless support and guidance. We would like to thank Prof Iftikhar, Director Undergraduate and Prof Saeed javid Tajik, Chairman Mechanical Department for their support. We acknowledge the helping hand extend to us from SHYDO( Sarhad Hydal Development Organization), PCRET( Pakistan council of renewable energy and technology) and SRSP( Sarhad Rural Support Program). Thanks to Asmatullah, our host in Maidan dir (l) who cordially helped us in arranging visits to site and accompany us.
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CHAPTER 1
INTRODUCTION
The objective of a hydro power scheme is to convert the potential energy of a mass of water, flowing in a stream with a certain fall (termed the head.), into electric energy at the lower end of the scheme, where the powerhouse is located.
The power of the scheme is proportional to the flow and to the head. Micro-hydro schemes produce power from streams and small rivers. The power can be used to generate electricity, or to drive machinery. Micro-hydro can bring electricity to remote communities for the first time, replacing kerosene for lighting, providing TV and communications to homes and community buildings, and enabling small businesses to start.
Micro-hydro schemes are already benefiting many remote communities and hilly areas of Pakistan. In the developed world, micro-hydro schemes supply power to existing mains electric grids.
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1.1. HISTORY OF SMALL HYDRO POWER TECHNALOGY
Hydropower is a renewable, non-polluting and environmentally benign source of energy. Hydropower is based on simple concepts. Moving water turns a turbine, the turbine spins a generator, and electricity is produced. Many other components may be in a system, but it all begins with the energy in the moving water. The use of water falling through a height has been utilized as a source of energy since a long time. It is perhaps the oldest renewable energy technique known to the mankind for mechanical energy conversion as well as electricity generation. In the ancient times waterwheels were used extensively, but it was only at the beginning of the 19th Century with the invention of the hydro turbines that the use of hydropower got popularized.
Small-scale hydropower was the most common way of electricity generating in the early 20th century. The first commercial use of hydroelectric power to produce electricity was a waterwheel on the Fox River in Wisconsin in 1882 that supplied power for lighting to two paper mills and a house. Within a matter of weeks of this installation, a power plant was also put into commercial service at Minneapolis. India has a century old history of hydropower and the beginning was from small hydro. The first hydro power plant was of 130 kW set up in Darjeeling during 1897, marked the development of hydropower in the country. Similarly, by 1924 Switzerland had nearly 7000 small scale hydropower stations in use. Even today, Small hydro is the largest contributor of electricity from renewable energy sources, both at European and world level. With the advancement of technology, and increasing requirement of electricity, the thrust of electricity generation was shifted to large size hydro and thermal power stations. However, it is only during the last two decades that there is a renewed interest in the development of small hydro power (SHP) projects mainly due to its benefits particularly concerning environment and ability to produce power in remote areas. Small hydro projects are economically viable and have relatively short gestation period. The major constraints associated with large hydro projects are usually not encountered in small hydro projects. Renewed interest in the technology of small scale hydropower actually started in China which has more than 85,000 small-scale electricity Hydropower stations which will continue to play important role throughout the 21st Century, in world electricity supply. Hydropower development does have some challenges besides the technical, economic introducing, hydropower
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plants environmental advantages it shares above other power generation (fossil fuel based) technologies.
At the beginning of the new Millennium hydropower provided almost 20% (2600 TWh/year) of the electricity world consumption (12900 TWh/year). It plays a major role in several countries. According to a study of hydropower resources in 175 countries, more than 150 have hydropower resources. For 65 of them, hydro produces more than 50% of electricity; for 24, more than 90% and 10 countries have almost all their electricity requirements met through hydropower.
1.2 SMALL HYDRO POWER PROJECT
CLASSIFICATION
Hydro power projects are generally categorized in two segments i.e. small and large hydro. Different countries are following different norms keeping the upper limit of small hydro ranging from 5 to 50 MW. The world over, however, there is no consensus on the definition of small hydropower. Some countries like Portugal, Spain, Ireland, Greece and Belgium, accept 10 MW as the upper limit for installed capacity. In Italy the limit is fixed at 3 MW (plants with larger installed power should sell their electricity at lower prices) and in Sweden 1.5 MW. In France the limit has been recently established at 12 MW, not as an explicit limit of MHPP, but as the maximum value of installed power for which the grid has the obligation to buy electricity from renewable energy sources. In the UK 20MW is generally accepted as the threshold for small hydro. Though different countries have different criteria to classify hydro power plants, a general classification of hydro power plants is as follows:
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Table 1.1 Hydro power plants classification
Apart from the above classification, some of the other terms in vogue nowadays when describing very small hydro power plants are „Pico Hydro‟ (less than 5 kW) and „Tiny Hydro‟ (less than 1kW).Small hydro plants are also classified according to the “Head” or the vertical distance through which the water is made to impact the turbines. The usual classifications are given below:
Table 1.2 MHPP classifications on basis of Head
Type Head range High head 100m and above Medium head 30-100m
Low head 2- 30m
These ranges are not rigid but are merely means of categorizing sites. Schemes can also be defined as:-
Run-of-river schemes
Schemes with the powerhouse located at the base of a dam Schemes integrated on a canal or in a water supply pipe Type Capacity
Large- hydro More than 100 MW and usually feeding into large electricity grid Medium-hydro 15-100 MW usually feeding a grid
Small-hydro 1-15 MW usually feeding into a grid
Mini- hydro Above 100KW but below 1MW;either stand alone schemes or more often feeding into the grid
Micro-hydro From 5KW up to 100KW;usually provided power for small community or rural industry in remote areas away from grid Pico-hydro From a few hundred watts up to 5KW
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Most of the small hydro power plants are “run-of-river” schemes, implying that they do not have any water storage capability. The power is generated only when enough water is available from the river/stream. When the stream/river flow reduces below the design flow value, the generation ceases as the water does not flow through the intake structure into the turbines. Small hydro plants may be stand alone systems in isolated areas/sites, but could also be grid connected (either local grids or regional/national grids). The connection to the grid has the advantage of easier control of the electrical system frequency of the electricity, but has the disadvantage of being tripped off the system due to problems outside of the plant operator‟s control.
1.3 GENERAL PRINCIPLE OF MHPP
Power generation from water depends upon a combination of head and flow. Both must be available to produce electricity. Water is diverted from a stream into a pipeline, where it is directed downhill and through the turbine (flow). The vertical drop (head) creates pressure at the bottom end of the pipeline. The pressurized water emerging from the end of the pipe creates the force that drives the turbine. The turbine in turn drives the generator where electrical power is produced. More flow or more head produces more electricity. Electrical power output will always be slightly less than water power input due to turbine and system inefficiencies.
Water pressure or Head is created by the difference in elevation between the water intake and the turbine. Head can be expressed as vertical distance (feet or meters), or as pressure, such as pounds per square inch (psi). Net head is the pressure available at the turbine when water is flowing, which will always be less than the pressure when the water flow is turned off (static head), due to the friction between the water and the pipe. Pipeline diameter also has an effect on net head.
Flow is quantity of water available, and is expressed as „volume per unit of time‟, such as gallons per minute (gpm), cubic meters per second (m3
/s), or liters per minute (lpm). Design flow is the maximum flow for which the hydro system is designed. It will likely be less than the maximum flow of the stream (especially during the rainy season), more than the minimum flow, and a compromise between potential electrical output and system cost.
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1.4 POWER FROM MHPP
To know the power potential of water in a stream it is necessary to know the flow quantity of water available from the stream (for power generation) and the available head. The quantity of water available for power generation is the amount of water (in m3 or liters) which can be diverted through an intake into the pipeline (penstock) in a certain amount of time. This is normally expressed in cubic meters per second (m3/s) or in liters per second (l/s). Head is the vertical difference in level (in meters) through which the water falls down.
The theoretical power (P) available from a given head of water is in exact proportion to the head and the quantity of water available.
P= Q × H × e × 9.81 (kW) 1.1 Where,
P = Power at the generator terminal, in kilowatts (kW)
H = the gross head from the pipeline intake to the tail water in meters (m) Q = Flow in pipeline, in cubic meters per second (m3/s)
e = the efficiency of the plant, considering head loss in the pipeline and the efficiency of the
Turbine and generator, expressed by a decimal (e.g. 85% efficiency= 0.85)
9.81 is a constant and is the product of the density of water and the acceleration due to gravity
This available power will be converted by the hydro turbine in mechanical power.
1.5 COMPONENTS OF A MICRO HYDRO SYSTEM
Basic components of a typical micro-hydro system are as follows:
• Civil works components (headwork, intake, gravel trap with spillway, headrace canal, forebay and distilling basin, penstock pipe, powerhouse and tailrace)
• Powerhouse components (turbines, generators, drive systems and controllers) • Transmission/distribution network
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1.6 SYSTEM LAYOUT
The three types of waterway routes shown below are examples of possible layouts of micro-hydropower system. The „short penstock‟ option, in most cases, is considered the most economic scheme, but this is not necessarily the case.
Figure 1.2 Channel and penstock options
1.6.1 SHORT PENSTOCK
In this case, the penstock is short but the channel is long. The long channel is exposed to the greater risk of blockage, or of collapse or deterioration as a result of poor maintenance. Installing the channel across a steep slope may be difficult and expensive. The risk that the steep slope may erode makes the short penstock layout an unacceptable option, because the projected operation and maintenance cost of the scheme could be very expensive, and it may outweigh the benefit of initial purchase cost.
1.6.2 LONG PENSTOCK
In this case, the penstock follows the river. If this layout is necessary, because the terrain would not allow the construction of a channel, certain precautions must be taken. The most important consideration is to ensure that seasonal flooding of the river will not damage or deteriorate the penstock. It is also important to calculate the most economic diameter of penstock; in the case of a long penstock, the cost will be particularly high.
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1.9.3 MID-LENGTH PENSTOCK
The mid-length penstock may cost more than the short penstock, but the cost of constructing channel that can safely cross the steep slope may also be avoided. Even if the initial purchase and construction costs are greater in this case, this option may be preferable in case there are signs of instability in the steep slope.
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CHAPTER 2
HYDRAULIC STRUCTURES
2.1 WEIR & INTAKE
The large majority of small hydro schemes are of the run-of-river type, where electricity is generated from discharges larger than the minimum required to operate the turbine. In these schemes a low diversion structure is built on the streambed to divert the required flow whilst the rest of the water continues to overflow it. When the scheme is large enough this diversion structure becomes a small dam, commonly known as a weir, whose role is not to store the water but to increase the level of the water surface so the flow can enter into the intake. Sometimes, in remote hilly regions, where annual flooding is common it may be prudent to build temporary weir using local resources and manpower. The temporary weir is a simple structure at low cost using local labor, skills and materials. It is expected to be destroyed by annual or bi-annual flooding. However, advanced planning has to be done for rebuilding of the weir. The intake of a MHPP is designed to divert only a portion of the stream flow or the complete flow– depending upon the flow conditions and the requirement. MHPP schemes use different types of intakes distinguished by the method used to divert the water into the intake. For MHPP schemes, intake systems are smaller and simpler. The following three types of intakes have been described here: side intake with and without a weir and the bottom intake.
Side intake with weir: The weir used in this arrangement can be partly or completely submerged into the water.
Bottom intake: At a bottom intake the whole weir is submerged into the water. Excess water will pass the intake by flowing over the weir.
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2.1.1 SIDE INTAKE HEIGHT CALCULATION
In the case of side intake, following Case (a) or Case (b), whichever is higher, is adopted.
a
. Weir height (D1) determined in relation to the bed elevation of the scour gate of the Intake weirD1 = d1 + hi 2.1
b
. Weir height (D2) determined by the bed gradient of the settling basin D2 = d2 + hi+ L (ic – ir) 2.2Where,
d1: height from the bed of the scour gate to the bed of the inlet (usually 0.5 – 1.0 m) d2: difference between the bed of the scour gate of the settling basin and the river bed at the same location (usually around 0.5 m) hi : water depth of the inlet (usually determined to make the inflow velocity approximately 0.5 – 1.0 m/s)
L: length of the settling basin ic: inclination of the settling basin bed (usually around 1/20 – 1/30) ir: present inclination of the river
Figure 2.1 Sectional view of side intake and weir
2.1.2 TYROLEAN INTAKE HIGHT CACULATION
A Tyrolean intake where water is taken from the bottom assumes that the front of the weir is filled with sediment and, therefore, the weir height is determined by Case D2 for side intake.
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Figure 2.2Sectional view of Tyrolean intake and weir
Figure 2.3 Tyrolean Intake
2.2 POWER CHANNEL
The power channel or simply a channel conducts the water from the intake to the FBT. The length of a channel depends upon the topography of the region and the distance of powerhouses from the intake. Also the designing of the MHPP systems states the length of the channel – sometimes a long channel combined with a short penstock can be cheaper or required, while in other cases a combination of short channel with long penstock would be more suitable. Generally power channels are excavated and to reduce friction and prevent leakages these are often lined with cement, clay or polythene sheet. Size and shape of a channel and material used for lining are often a dictated by cost and head considerations. During the process of flowing past the walls and bed material, the water loses energy. The rougher the
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material, the greater the friction loss and higher is the elevation difference needed between channel entry and exit. In hilly regions it is common that the power channel would have to cross small streams. In such situations it is often prudent to build a complete crossing over the channel, as during rainy season, flash floods and/or rocks/mud may block the channel or worse still, wash away sections of the channel. Sometimes just the provision of a drain running under the channel (in case of very small streams along stable slope) is usually adequate.
2.2.1 TYPES AND BASIC STRUCTURE OF HEADRACE CANAL
Headraces, that have a function to covey water from intake to forebay/head tank, are classified into pressure waterways and non-pressure waterways. In term of hydraulics, non-pressure waterways are open channel and the pressure waterway is a conduit. The headrace structures are open canal, covered canal, culvert, tunnel, aqueduct, inverter siphon, etc. Because of the generally small amount of water conveyance, the headrace for a small-scale hydropower plant basically adopts an exposed structure, such as an open channel or a covered channel, etc. In general, the construction cost of open channel is the most economical.
2.2.2 HEAD RACE DESIGN
The size of cross section and slope should be determined in such a matter that the required turbine discharge can be economically guided to the head tank. Generally, the size of cross section is closely related to the slope. The slope of headrace should be made gentler for reducing head loss (difference between water level at intake and at head tank) but this cause a lower velocity and thus a lager cross section. On the contrary, a steeper slope will create a higher velocity and smaller section but also a lager head loss.
Generally, in the case of small-hydro scheme, the slope of headrace will be determined as 1/500 – 1/1,500. However in the case of micro-hydro scheme, the slope will be determined as 1/50 –1/500, due to low skill on the survey of leveling and construction by local contractor.
The cross section of headrace is determined by following method: Method of calculation:
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Where;
Qd: design discharge for headrace (m3/s)
A: area of cross section (m2) R: R=A／P (m)
P: length of wet sides (m) refers to next figure.
SL: longitudinal slope of headrace (e.g. SL= 1/100=0.01) n: coefficient of roughness
2.2.3 SETTLING BASIN
The water diverted from the stream and carried by the channel usually carries a suspension of small particles such as sand that are hard and abrasive and can cause expensive damage and rapid wear to turbine runners. To get rid of such particles and sediments, the water flow is allowed to slow down in „settling basins‟ so that the sand and silt particles settle on the basin floor. The deposits are then periodically flushed.
The design of settling basin depends upon the flow quantity, speed of flow and the tolerance level of the turbine (smallest particle that can be allowed). The
maximum speed of the water in the settling basin can thus be calculated as slower the flow, lower is the carrying capacity of the water. The flow speed in the settling basin can be lowered by increasing the cross section area. The settling basin must have a structure which is capable of settling and removing sediment with a minimum size which could have an adverse effect on the turbine and also a spillway to prevent excessive water inflow into the headrace. The basic configuration of a settling basin is illustrated below.
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Each of these sections has the following function.
Conduit section: Conduit section connects the intake with the settling basin. It is necessary that the conduit section should be curtailing its length.
Widening section: This regulates water flow from the conduit channel to prevent the occurrence of whirl pools and turbulent flow and reduces the flow velocity inside the settling basin to a predetermined.
Settling section: This section functions to settle sediment above a certain size and its required length (l) is calculated by the following formula based on the relation between the settling speed, flow velocity in the settling basin and water depth.
The length of the settling basin (Ls) is usually determined so as to incorporate a margin to double the calculated length by the said formula
2.5
Where
l: minimum length of settling basin (m) hs: water depth of settling basin (m)
U: marginal settling speed for sediment to be settled (m/s) usually around 0.1 m/s for a target grain size of 0.5 – 1 mm.
V: mean flow velocity in settling basin (m/s) usually around 0.3 m/s but up to 0.6 m/s is tolerated in the case where the width of the settling basin is restricted.
V = Qd/(B×hs) 2.6
Qd: design discharge (m3/s)
B: width of settling basin (m)
Sediment pit:This is the area in which sediment is deposited
2.2.4 SPILLWAYS
Spillways along the power channel are designed to permit overflow at certain points along the channel. The spillway acts as a flow regulator for the channel. During floods the water flow through the intake can be twice the normal channel flow, so the spillway must be large enough to divert this excess flow. The spillway can also be designed with control gates to empty the channel. The spillway should be
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designed in such a manner that the excess flow is fed back to the river without damaging the foundations of the channel.
Spillway drains the submerged inflow which flows from the intake. The sizes of spillway will
be decided by following equation.
Qf= C×Bsp×hsp1.5 →hsp={Qf /(C×Bsp)}1/1.5 2.7
Where
Qf: inflow volume of submerged orifice (m3/s, see Figure 2.4)
C: coefficient =1.80
hsp: water depth at the spillway (m, see Figure 2.4)
Bsp: width of the spillway (m, see Figure 2.4)
2.3 FOREBAY/HEAD TANK
The FBT serves the purpose of providing steady and continuous flow into the turbine through the penstocks. Forebay also acts as the last settling basin and allows the last particles to settle down before the water enters the penstock. Forebay can also be a reservoir to store water –depending on its size (large dams or reservoirs in large hydropower schemes are technically forebay).
A sluice will make it possible to close the entrance to the penstock. In front of the penstock a trash rack need to be installed to prevent large particles to enter the penstock. A spillway completes the FBT.
2.3.1 HEAD TANK CAPACITY
The head tank capacity is defined the water depth from hc to h0 in the FBT length L as shown in Figure
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Head tank capacity
Vsc = As×dsc＝B×L×dsc 2.8 where
As: area of head tank B : width of head tank L: length of head tank
dsc: water depth from uniform flow depth of a headrace when using maximum discharge (h0) to critical depth from top of a dike for sand trap in a head tank (hc) In oblong section, uniform flow depth:
ho=H×0.1／(SLE)0.5 2.9 SLe: slope of tail end of the headrace critical depth:
hc= {(α×Qd2) ／(g×B2)}1/3 2.10
α: 1.1 g : 9.8
2.3.2 DETERMINATION OF HEAD TANK CAPACITY
The head tank capacity should be determined in consideration of load control method and discharge method as mentioned below.
In case only the load is controlled: The case only control load (demand) fluctuation is considered, a dummy load governor is adopted. A dummy load governor is composed of water-cooled heater or air-cooled heater, difference of electric power between generated in powerhouse and actual load is made to absorb heater. The discharge control is not performed. The FBT capacity should be secured only to absorb the pulsation from headrace that is about 10 times to 20 times of the design discharge (Qd). A view showing a frame format
of load controlled by a dummy load governor is shown in figure 2.5
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In case both load and discharge is controlled: In the case of controlled both load and discharge, it used for load control a mechanical governor or electrical governor. These governors have function of control vane operation to optimal discharge when electrical load has changed. Generally a mechanical governor is not sensitive response to load change; FBT capacity in this case should be secured 120 times to 180 times of Qd. On the other hand, an electrical
governor will response of load change, therefore FBT capacity is usually designed about 30 times to 60 times of Qd.
2.4 PENSTOCK
The penstock is the pipe which conveys water under pressure from the FBT to the turbine. Penstock is a significant component of the MHPP scheme and needs to be designed and selected carefully as it represents a major expense in the total budget (for some high head installations this alone could cost as much as 30% of the total costs). Here the main aspects to consider are head loss and capital cost. Head loss due to friction in the pipe decreases dramatically with increasing pipe diameter. Conversely, pipe costs increase steeply with diameter. Therefore a compromise between cost and performance is considered for design and selection of pipe diameter and material.
While designing penstocks, the first principle is to identify available pipe options and then to decided upon acceptable head loss (5% of the gross head is generally considered). The details of the pipes of various materials and diameters with losses close to this target are then tabulated and compared for cost effectiveness. A smaller penstock may be lighter on pocket, but the extra head loss may account for lost revenue from generated electricity each year.
2.4.1 PENSTOCK MATERIALS
The factors to be considered while deciding upon the material to be used for a particular penstock are:
terrain, soil type
weather conditions Weight and ease of installation,
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accessibility of the site
likelihood of structural damage availability surface roughness,
design life and maintenance method of jointing design pressure Relative cost.
The following materials can be considered for use as penstock pipes in micro hydro schemes:
wooden planks or tree bark (for very small installations) Spun ductile iron
GI Pipes mild steel,
unclassified polyvinyl chloride (UPVC), high density polyethylene (HDPE), asbestos cement,
Pre stressed concrete,
Glass reinforced plastic (GRP).
Mild steel, UPVC and HDPE are the most common used materials.
2.4.2 CALCULATION OF STEEL PIPE THIKNESS
The minimum thickness of steel pipe of penstock is determined by following formula
2.11
Where
T0: minimum thickness of pipe
P: design water pressure i.e. hydrostatic pressure + water hammer (kgf/cm2), in Micro-hydro scheme P=1.1×hydrostatic pressure.
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Turbine is 25m, P=2.5×1.1=2.75 kgf/cm2. d: inside diameter (cm)
θa: admissible stress (kgf/cm2
) SS400: 1300kgf/cm2 η: welding efficiency (0.85～0.9)
δt: margin (0.15cm in general)
2.4.3 PENSTOCK DIAMETER
The diameter is selected as the result of a trade-off between penstock cost and power losses. A simple criterion for diameter selection is to limit the head loss to a certain percentage. Loss in power of 4% is usually acceptable. A more rigorous approach is to select several possible diameters, computing power and annual energy. The present value of this energy loss over the life of the plant is calculated and plotted for each diameter. In the other side the cost of the pipe for each diameter is also calculated and plotted. Both curves are added graphically and the optimum diameter would be that closest to the theoretical optimum. Actually the main head loss in a pressure pipe are friction losses; the head losses due to turbulence passing through the trashrack, in the entrance to the pipe, in bends, expansions, contractions and valves are minor losses. Consequently a first approach will suffice to compute the friction losses, using for example the Manning equation
2.12
2.13
2.14
2.4.4 PENSTOCK JOINING
Pipes are generally available in standard lengths (it is easier for transportation also) and have to be joined together on site. There are several methods of joining
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penstock pipes and the factors to be considered when choosing the best joint system for a particular scheme are:
pipe material,
whether any degree of joint flexibility is required, ease of installation
skill level of personnel, Costs.
Generally, the pipes are joined by one of the following four methods: flanged,
spigot and socket, mechanical, Welded.
2.4.5 BURYING OR SUPPORTING THE PENSTOCK
Penstock pipelines can either be laid upon the surface or buried underground. This generally depends upon the material of the pipe, the nature of the terrain and environmental and cost considerations.
While burying a penstock, it is very important to ensure proper installation because any subsequent problems such as leaks are much harder to detect and resolve. In case vehicles are likely to cross over the buried pipelines, they must be buried at least 750 -1000 mm below ground level. Burying the pipeline carefully and correctly enhances the life of the MHPP scheme and greatly reduces the chances of disruption in power generation especially in hilly terrain with heavy landslides.
If the natural terrain does not permit burying the penstock then the penstock is run over ground. In such conditions piers, anchors and thrust blocks are needed to stabilize the pipeline (especially if these happen to be very long) to withstand the weight of the pipes plus water and expansion and contraction of the pipe (due to changing temperature).
Support piers are used basically to bear the weight of the pipes plus water being carried. Anchors are large structures fixed along the length of a penstock, restraining all movements (horizontal or vertical) by anchoring the penstock to the ground. For a bend or contraction in the pipeline, a thrust block is used to oppose the specific force generated by the bend or contraction. All of these structures are usually
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built of rubble masonry or cement concrete. Sometimes, the anchor blocks may need steel reinforcement (for long pipelines).
2.5 TAIL RACE
After passing through the turbine the water returns to the river trough a short canal called a tailrace. Impulse turbines can have relatively high exit velocities, so the tailrace should be designed to ensure that the powerhouse would not be undermined. Protection with rock riprap or concrete aprons should be provided between the powerhouse and the stream. The design should also ensure that during relatively high flows the water in the tailrace does not rise so far that it interferes with the turbine runner. With a reaction turbine the level of the water in the tailrace influences the operation of the turbine and more specifically the onset of cavitations. This level also determines the available net head and in low head systems may have a decisive influence on the economic results.
2.6 FOUNDATION OF POWER HOUSE
Powerhouse can be classified into „the above ground type‟, the semi-underground type‟ and „the semi-underground type‟. Most of small-scale hydropower plants are of „the above ground type, The dimensions for the floor of powerhouse as well as the layout of main and auxiliary equipment should be determined by taking into account convenience during operation, maintenance and installation work, and the floor area should be effectively utilized. Various types of foundation for powerhouse can be considered depending on the type of turbine. However the types of foundation for powerhouse can be classified into „for Impulse turbine‟ (such as Pelton turbine, Turgo turbine and Cross flow turbine) and „for Reaction turbine‟ (Francis turbine, Propeller turbine)
2.6.1 FOUNDATION FOR IMPULSE TURBINE
In case of impulse turbine, the water which passed by the runner is directly discharged into air at tailrace. The water surface under the turbine will be turbulent. Therefore the clearance between the slab of powerhouse and water surface at the afterbay should be kept at least 30-50cm.
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2.13
Where,
hc: water depth at after bay (m)
Qd: design discharge (m3/s)
b: width of tailrace channel (m)
The water level at the after bay should be higher than estimated flood water level. Then in case of impulse turbine, the head between the center of turbine and water level at the outlet became head loss.
Figure 2.7 Foundation of Powerhouse for Impulse turbine
2.6.2 FOUNDATION FOR REACTION TURBINE
In case of reaction turbine, the head between center of turbine and water-level can be use for power generation. Then it is possible that turbine is installed under flood water level on condition to furnish the following equipment:
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Figure 2.8 Foundation of Powerhouse for Reaction turbine
2.7 HYDROULIC STRUCTURES OVER THE SITE
We have selected a Run of river site in the hilly area of district Dir (L).the MHPP is located on the stream which is fed by snow melt and small sprigs. The stream flows from a narrow Canyon. The MHPP is made at the entrance of the Canyon. The following Hydraulic structures present at the site.
Weir: a temporary weir is made. All the water of the stream is diverted to the Headrace canal by simply putting some stones at the front of the stream. Headrace canal: the headrace canal is an open earth channel which conveys
water from intake to the FBT .The length of the channel is 200 meter. A lot of flow is lost due to leakages from the channel.
Spill way: a spillway is made near the weir i-e at the beginning of the headrace canal.
FBT: a FBT is made at the end of headrace canal. The FBT is situated at wrong position so a lot of head is lost.
Penstock: A locally made penstock of 30 inches diameter is installed at wrong position to FBT. The penstock is not jointed well so a lot of water is lost on the way. A trash rack is fixed at the opening of penstock .The trash rack is not well designed.
Foundation for Impulse CFT turbine: an above ground type concrete foundation for CFT is made.
Tailrace: the tailrace is an open channel which conveys water back to the stream.
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CHAPTER 3
ELECTRO-MECHANICAL EQUIPMENT
3.1 HYDRAULIC TURBINES
Turbine is the main piece of equipment in the MHPP scheme that converts energy of the falling water into the rotating shaft power. The selection of the most suitable turbine for any particular hydro site depends mainly on two of the site characteristics – head and flow available. All turbines have a power-speed characteristic. This means they will operate most efficiently at a particular speed, head and flow combination. Thus the desired running speed of the generator or the devices being connected/ loading on to the turbine also influence selection. Other important consideration is whether the turbine is expected to generate power at part-flow conditions. The design speed of a turbine is largely determined by the head under which it operates. Turbines can be classified as high head, medium head or low head machines. They are also classified by the operating principle and can be either impulse or reaction turbines. The basic turbine classification is given in the table below:
Table 3.1 Turbine types
Type High Head Medium Head Low Head
Impulse turbines Pelton Turgo
Cross-flow Turgo Multi-jet Pelton
Cross-flow
Reaction turbines - Francis Propeller Kaplan
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The potential energy in the water is converted into mechanical energy in the turbine, by one of two fundamental and basically different mechanisms:
The water pressure can apply a force on the face of the runner blades, which decreases as it proceeds through the turbine. Turbines that operate in this way are called reaction turbines. The turbine casing, with the runner fully immersed in water, must be strong enough to withstand the operating pressure. The water pressure is converted into kinetic energy before entering the runner. The kinetic energy is in the form of a high-speed jet that strikes the buckets, mounted on the periphery of the runner. Turbines that operate in this way are called impulse turbines. As the water after striking the buckets falls into the tail water with little remaining energy, the casing can be light and serves the purpose of preventing splashing.
3.2 IMPULSE TURBINES
Impulse turbines are more widely used for micro-hydro applications as compared to reaction turbines because they have several advantages such as simple design (no pressure seals around the shaft and better access to working parts - easier to fabricate and maintain), greater tolerance towards sand and other particles in the water, and better part-flow efficiencies. The impulse turbines are not suitable for low head sites as they have lower specific speeds and to couple it to a standard alternator, the speed would have to be increases to a great extent. The multi-jet Pelton, cross flow and Turgo turbines are suitable for medium heads.
3.2.1 PELTON TURBINES
Pelton turbines are impulse turbines where one or more jets impinge on a wheel carrying on its periphery a large number of buckets. Each jet issues through a nozzle with a needle (or spear) valve to control the flow (figure 3.1). They are only used for relatively high heads. The axes of the nozzles are in the plane of the runner to stop the turbine. e.g. When the turbine approaches the runaway speed due to load rejection- the jet may be deflected by a plate so that it does not impinge on the buckets. In this way the needle valve can be closed very slowly, so that overpressure surge in the pipeline is kept to an acceptable minimum. Any kinetic energy leaving
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the runner is lost and so the buckets are designed to keep exit velocities to a minimum. The turbine casing only needs to protect the surroundings against water splashing and therefore can be very light.
Figure 3.1 Pelton runner
3.2.2 TURGO IMPULSE TURBINES
The Turgo turbine is an impulse turbine designed for medium head applications. These turbines achieve operational efficiencies of up to 87%. Developed in 1919 by Gilkes as a modification of the Pelton wheel, the Turgo has certain advantages over Francis and Pelton designs for some applications. Firstly, the runner is less expensive to make than a Pelton wheel while it does not need an airtight housing like the Francis turbines. Finally the Turgo has higher specific speeds and at the same time can handle greater quantum of flows than a Pelton wheel of the similar diameter, leading to reduced generator and installation cost. Turgo turbines operate in a head range where the Francis and Pelton overlap. Turgo installations are usually preferred for small hydro schemes where low cost is very important.
Turgo turbine is an impulse turbine where water does not change pressure but changes direction as it moves through the turbine blades. The water's potential energy is converted to kinetic energy with a penstock and nozzle. The high speed water jet is then directed on the turbine blades which deflect and reverse the flow and the water exits with very little energy. Like all turbines with nozzles, blockage by debris must
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be prevented for effective operation. A Turgo runner looks like a Pelton runner split in half. For the same power, the Turgo runner is one half the diameter of the Pelton runner, and so twice the specific speed. The Turgo can handle a greater water flow than the Pelton because exiting water doesn't interfere with adjacent buckets.
Figure3.2 Turgo impulse turbine
3.2.3 CROSS FLOW TURBINE
Also called a Michell-Banki turbine a cross flow turbine 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 its runner shaft horizontal (unlike Pelton and Turgo turbines which can have either horizontal or vertical shaft orientation). Unlike most water turbines, which have axial or radial flows, in a CFT the water passes through the turbine transversely, or across the turbine blades. As with a waterwheel, water enters at the turbine's edge. After passing the runner, it leaves on the opposite side. Going through the runner twice provides additional efficiency. When the water leaves the runner, it also helps clean the runner of small debris and pollution. The cross-flow turbines generally operate at low speed.
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Figure 3.3 CFT
CFTs are also often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit (the guide vane system in the turbine's upstream section) provides flexible operation, with ⅓, ⅔ or 100% output, depending on the flow. Low operating costs are obtained with the turbine's relatively simple construction. The water flows through the blade channels in two directions: outside to inside, and inside to outside. Most turbines are run with two jets, arranged so that the two water jets in the runner will not affect each other. It is, however, essential that the turbine, head and turbine speed are harmonized. The turbine consists of a cylindrical water wheel or runner with a horizontal shaft, composed of numerous blades (up to 37), arranged radially and tangentially. The edge of the blades is sharpened to reduce resistance to the flow of water. A blade is made in a part-circular cross-section (pipe cut over its whole length). The ends of the blades are welded to disks to form a cage like a hamster cage and are sometimes called "squirrel cage turbines"; instead of the bars, the turbine has trough-shaped steel blades.
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Figure 3.4 Horizontal/vertical inflows in CFT
The water flows first from the outside of the turbine to its inside. The regulating unit, shaped like a vane or tongue, varies the cross-section of the flow. These divide and direct the flow so that the water enters the runner smoothly for any width of opening. The guide vanes should seal to the edges of the turbine casing so that when the water is low, they can shut off the water supply. The guide vanes therefore act as the valves between the penstock and turbine. The water jet is directed towards the cylindrical runner by a fixed nozzle. The water enters the runner at an angle of about 45 degrees, transmitting some of the water's kinetic energy to the active cylindrical blades. The turbine geometry (nozzle-runner-shaft) assures that the water jet is effective. The water acts on the runner twice, but most of the power is transferred on the first pass, when the water enters the runner. Only ⅓ of the power is transferred to the runner when the water is leaving the turbine.
The cross-flow turbine is of the impulse type, so the pressure remains constant at the runner. The peak efficiency of a CFT is somewhat less than a Kaplan, Francis or Pelton turbine. However, the CFT has a flat efficiency curve under varying load. With a split runner and turbine chamber, the turbine maintains its efficiency while the flow and load vary from 1/6th to the maximum.
The CFTs are mostly used in mini and micro hydropower units less than 2 MW and with heads less than 200 m, since it has a low price and good regulation. Particularly with small run-of-the-river schemes, the flat efficiency curve yields better performance than other turbine systems, as flow in small streams varies seasonally. The efficiency of a turbine is determined whether electricity is produced during the periods when rivers have low heads. Due to its better performance even at partial loads, the CFT is well-suited to stand-alone electricity generation. It is simple in construction and that makes it easier to repair and maintain than other turbine types.
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Another advantage is that the CFTs gets cleaned as the water leaves the runner (small sand particles, grass, leaves, etc. get washed away), preventing losses. So although the turbine's efficiency is somewhat lower, it is more reliable than other types. Other turbine types get clogged easily, and consequently face power losses despite higher nominal efficiencies.
3.3 REACTION TURBINES
The more popular reaction turbines are the Francis turbine and the propeller turbine. Kaplan turbine is a unique design of the propeller turbine. Given the same head and flow conditions, reaction turbines rotate faster than impulse turbines. This high specific speed makes it possible for a reaction turbine to be coupled directly to an alternator without requiring a speed-increasing drive system. This specific feature enables simplicity (less maintenance) and cost savings in the hydro scheme. The Francis turbine is suitable for medium heads, while the propeller is more suitable for low heads.
The 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 higher costs are often offset by high efficiency and the advantages of high running speeds at low heads from relatively compact machines. Expertise and precision required during fabrication make these turbines less attractive for use in micro-hydro in developing countries. Most reaction turbines tend to have poor part-flow efficiency characteristics.
3.3.1 FRANCIS TURBINES
The Francis turbine is a reaction turbine where water changes pressure as it moves through the turbine, transferring its energy. A water tight casement is needed to contain the water flow. Generally such turbines are suitable for sites such as dams where they are located between the high pressure water source and the low pressure water exit.
The inlet of a Francis turbine is spiral shaped. Guide vanes direct the water tangentially to the turbine runner. This radial flow acts on the runner's vanes, causing the runner to spin. The guide vanes (or wicket gate) are adjustable to allow efficient turbine operation for a wide range of flow conditions. As the water moves through the
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runner, it‟s spinning radius decreases, further delivering pressure acting on the runner. This, in addition to the pressure within the water, is the basic principle on which the Francis turbine operates. While exiting the turbine, water acts on cup shaped runner buckets leaving without any turbulence or swirl and hence almost all of the kinetic or potential energy is transferred. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
Francis turbines can be designed for a wide range of heads and flows and along with their high efficiency makes them one of the most widely used turbines in the world. Large Francis turbines are usually designed specifically for each site so as to gain highest levels of efficiencies (these are typically in the range of over 90%). Francis turbines cover a wide range of head – from 20 meters to 700 meters, and can be designed for outputs power ranging from just a few kilowatts to one Gig watt.
Figure 3.5 Francis Turbine
3.3.2 KAPLAN TURBINE
The Kaplan turbine has adjustable blades and was developed on the basic platform (design principles) of the Francis turbine by the Viktor Kaplan in 1913. The main advantage of Kaplan turbines is its ability to work in low head sites which was
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not possible with Francis turbines. Kaplan turbines are widely used in high-flow, low-head power production.
The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features. The inlet is a scroll-shaped tube that wraps around the turbine‟s wicket gate. Water is directed tangentially through the wicket gate and spirals on to a propeller shaped runner, causing it to spin. The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy.
The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube that may lead to cavitations due to the pressure drop. Typically the efficiencies achieved for Kaplan turbine are over 90%, mainly due to the variable geometry of wicket gate and turbine blades. This efficiency however may be lower for very low head applications. Since the propeller blades are rotated by high-pressure hydraulic oil, a critical design element of Kaplan turbine is to maintain a positive seal to prevent leakage of oil into the waterway.
Kaplan turbines are widely used throughout the world for electrical power production. They are especially suited for the low head hydro and high flow conditions – mostly in canal based MHPP sites. Inexpensive micro turbines can be manufactured for specific site conditions (e.g. for head as low one meter). Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades.
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3.4 PUMPS WORKING AS TURBINES
Centrifugal pumps can be used as turbines by passing water through them in reverse. The potential advantages are the lower costs due to mass production (also local production), the availability of spare parts and the wider dealer/support networks. The disadvantages are that their performance characteristics have not been studied extensively and these poor part-flow efficiencies. Pumps as turbines have been used at several locations but the technology still remains unproven.
3.5 TURBINE SELECTION
Selection of an appropriate turbine to a large extent is dependent upon the available water head and to a lesser extent on the available flow rate. In general, impulse turbines are used for high head sites, and reaction turbines are used for low head sites. Kaplan turbines with adjustable blade pitch are suitable for wide ranges of flow or head conditions, since their peak efficiency can be achieved over a wide range of flow conditions.
Small turbines (less than 10 MW) may have horizontal shafts and even fairly large bulb-type turbines up to 100 MW or so may be horizontal. Very large Francis and Kaplan machines usually have vertical shafts because this makes best use of the available head, and makes installation of a generator more economical. Pelton turbines may be installed either vertically or horizontally. Some impulse turbines use multiple water jets per runner to increase specific speed and balance shaft thrust. Turbine type, dimensions and design are basically governed by the following criteria:
Net head
Range of discharges through the turbine Rotational speed
Cavitations problems Cost
3.5.1 NET AVAILABLE HEAD
The gross head is the vertical distance, between the water surface level at the intake and at the tailrace for reaction turbines and the nozzle level for impulse
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turbines. Once the gross head is known, the net head can be computed by simply subtracting the losses along its path. The first criterion to take into account in the turbines selection is the net head. Table 3.2 specifies for each turbine type its range of operating heads. The table shows some overlapping, so that for a certain head several types of turbines can be used.
Table 3.2 Range of Heads
Turbine type Head range in meters Kaplan and propeller 2 < H < 40 Francis 10 < H < 350 Pelton 50 < H < 1300 Michell-Benki 3 < H < 250 Turgo 50 < H < 250
The selection is particularly critical in low-head schemes, where to be profitable large discharges must be handled. When contemplating schemes with a head between 2 and 5 m, and a discharge between 10 and 100m3/sec, runners with 1.6-3.2 meters diameter are required, coupled through a speed increaser to an asynchronous generator. The hydraulic conduits in general and water intakes in particular are very large and require very large civil works, with a cost that generally exceeds the cost of the electromechanical equipment.
In order to reduce the overall cost (civil works plus equipment) and more specifically the cost of the civil works, several configurations, nowadays considered as classic, have been devised. All of them include the only turbine type available for this job .the Kaplan- in a double or a single regulated version.
The selection criteria for such turbines are well known: Range of discharges
Net head
Geomorphology of the terrain
Environmental requirements (both visual and sonic) Labor cost
The configurations differ by how the flow goes through the turbine (axial, radial, or mixed) the turbine closing system (gate or siphon), the speed increaser type (parallel gears, right angle drive, epicycloidal gears).
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As a turbine can only accept discharges between the nominal and the practical minimum, it may be advantageous to install several smaller turbines instead of a one large. The turbines would be sequentially started, so all of the turbines in operation except one will operate at their nominal discharges and therefore will exhibit a higher efficiency. Using two or three smaller turbines will mean a lower unit weight and volume and will facilitate transport and assembly on the site. The rotational speed of a turbine is inversely proportional to its diameter, so its torque will be lower and the speed increaser smaller and more reliable. The use of several turbines instead of one large one with the same total power will result in a lower ratio kilogram of turbine/cubic meter of operating flow, although the ratio equipment cost / cubic meter of operating flow will be larger. Increasing the number of turbines decreases the diameter of their runners, and consequently the support components in the powerhouse will be smaller and lighter. As the water conduits are identical the formwork, usually rather sophisticated, can be reused several times decreasing its influence in the concrete cost. Notwithstanding this, generally more turbines means more generators, more controls, higher costs.
3.5.2 DISCHARGE
The rated flow and net head determine the set of turbine types applicable to the site and the flow environment. Suitable turbines are those for which the given rated flow and net head plot within the operational envelopes. A point defined as above by the flow and the head will usually plot within several of these envelopes. All of those turbines are appropriate for the job, and it will be necessary to compute installed power and electricity output against costs before taking a decision. It should be remembered that the envelopes vary from manufacturer to manufacturer and they should be considered only as a guide.
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Table 3.3 Turbine Application Chart
3.5.3 SPECIFIC SPEED
Turbine type needs to be selected in consideration of the specific speed and turbine characteristics. The specific speed, which is the important factor to select turbine type, is defined as following formula.
3.1 Where,
Ns: Specific speed [m-kW] N: Rotation speed [min-1] P: Turbine output [kW] H: Effective head [m]
The proper range of the specific speed has already been known as shown in Table 3.4.The rotation speed of the turbine is limited. Therefore, it should be checked whether the specific speed is within the proper range. The larger the rotation speed is, the smaller the equipment is. The small equipment shall reduce the equipment cost. In addition, the rotation speed affects draft head.
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Table 3.4 Range of Ns
Turbine type Range of Ns(m-kW) Pelton 8-25
Francis 50-350 Diagonal flow 100-350 Propeller 200-900 Tubular More than 500
3.5.4 CAVITATION
When the hydrodynamic pressure in a liquid flow falls below the vapor pressure of the liquid, there is a formation of the vapour phase. This phenomenon induces the formation of small individual bubbles that are carried out of the low-pressure region by the flow and collapse in regions of higher low-pressure. The formation of these bubbles and their subsequent collapse gives rise to what is called cavitations. Experience shows that these collapsing bubbles create very high impulse pressures accompanied by substantial noise (in fact a turbine undergoing cavitations sounds as though gravel is passing through it). The repetitive action of such pressure waves close to the liquid-solid boundary results in pitting of the material. With time this pitting degenerates into cracks formed between the pits and the metal is spilled from the surface. In a relatively short time the turbine is severely damaged and will require being shut-off and repaired If possible.
Experience shows that there is a coefficient, called Thomas sigma, which defines precisely enough under which parameters cavitations takes place.
This coefficient is given by the equation
3.2
Where
Hsv is the net positive suction head and H the net head of the scheme
3.3 Where,
Hsv is the net positive suction head
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Hatm is the water vapor pressure
Z is the elevation above the tail water surface of the critical location Ve is the average velocity in the tailrace
Hl is the head loss in the draft tube
Neglecting the draft-tube losses and the exit velocity head loss, Thomas sigma will be given by
3.4
To avoid cavitations the turbine should be installed at least at a height over the tailrace water level Zp given by the equation
3.5
The Thomas sigma is usually obtained by a model test, and it is a value furnished by the turbine manufacturer. Notwithstanding the above mentioned statistic studies also relates Thomas sigma with the specific speed. There under are specified the equation giving óT as a function of ns for the Francis and Kaplan turbines
It must be remarked that Hvap decreases with the altitude, from roughly 10.3 m at the
sea level to 6.6 m at 3000 m above sea level. So then a Francis turbine with a specific speed of 150, working under a 100 m head (with a corresponding óT = 0.088), that in
a plant at sea level, will require a setting: z = 10.3 - 0.09 - 0.088 x 100 = 1.41 m
Installed in a plant at 2000 m above the sea level will require z = 8.1-0.09 - 0.088 x 100 = -0.79 m
A setting requiring heavy excavation
3.5.5 ROTATIONAL SPEED
The rotational speed of a turbine is a function of its specific speed, and of the scheme power and net head. In the small hydro schemes standard generators should be installed when possible, so in the turbine selection it must be borne in mind that the turbine, either coupled directly or through a speed increaser, should reach the synchronous speed, as given in table 3.5
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Table 3.5 Generator synchronization speed
3.5.6 RUNAWAY SPEED
Each runner profile is characterized by a maximum runaway speed. This is the speed, which the unit can theoretically attain when the hydraulic power is at its maximum and the electrical load has become disconnected. Depending on the type of turbine, it can attain 2 or 3 times the nominal speed. Table 3.6 shows this ratio for conventional and unconventional turbines.
It must be remembered that the cost of both generator and gearbox may be increased when the runaway speed is higher, since they must be designed to withstand it.
Table 3.6 Turbine runaway speed
3.6 DRIVE SYSTEM
In order to generate electrical power at a stable voltage and frequency, the drive system needs to transmit power from the turbine to the generator shaft in the required direction and at the required speed. Typical drive systems in micro-hydropower systems are as follows:
• Direct drive: A direct drive system is one in which the turbine shaft is connected directly to the generator shaft. Direct drive systems are used only for cases where the
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shaft speed of the generator shaft and the speed of the turbine are compatible. The advantages of this type of system are low maintenance, high efficiency and low cost. • “V” or wedge belts and pulleys: This is the most common choice for micro-hydropower systems. Belts for this type of system are widely available because they are used extensively in all kinds of small industrial machinery.
• Timing belt and sprocket pulley: These drives are common on vehicle camshaft drives and use toothed belts and pulleys. They are efficient and clean-running and are especially worth considering for use in very small system drives (less than 3 kW) where efficiency is critical.
• Gearbox: Gearboxes are suitable for use with larger machines when belt drives would be too cumbersome and inefficient. Gearboxes have problems regarding specification, alignment, maintenance and cost, and this rules them out for micro-hydropower systems except where they are specified as part of a turbine-generator set.
3.7 GENRATOR
Generators transform mechanical energy into electrical energy. Although most early hydroelectric systems were of the direct current variety to match early commercial electrical systems, nowadays only three-phase alternating current generators are used in normal practice. Two types of current are produced by electrical generators, either alternating current (AC) or direct current (DC). In the case of AC the voltage cycles sinusoidally with time, from positive peak value to negative. Because the voltage changes its sign the resulting current also continually reverses direction in a cyclic pattern. DC current flows in a single direction as the result of a steady voltage. DC is not usually used in modern power installations except for very low-powered systems of a few hundred watts or less.
Alternating voltage can be produced in a stationery coil or armature by a rotating magnetic field, but more usually a coil is rotated in a stationary magnetic field. The magnetic field can be produced either by a permanent magnet or by another coil (i.e., an electro-magnet) know as a field coil which is fed by direct current known as the excitation current. A generator supplying alternative current is described as an alternator to distinguish it from a machine designed to supply DC current which is known as a DC generator or dynamo. Current flows when a voltage difference is place across a conducting body. In AC circuits the magnitude and timing of the
