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Low Head Pico Hydropower
World Renewable Energy Congress VI (WREC2000)
© 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh
1475
L O W HEAD PICO H Y D R O P O W E R : A REVIEW OF A V A I L A B L E TURBINE T E C H N O L O G I E S
A. A. WILLIAMS and D. R. UPADHYAY
Department of Mechanical and Manufacturing Engineering, The Nottingham Trent University, Burton Street, Nottingham NG 1 4BU, UK G. M. DEMETRIADES
Marine Surveyor, Department of Merchant Shipping, Limassol, Cyprus N.P.A. SMITH
Department of Electrical & Electronic Engineering, The Nottingham Trent University.
ABSTRACT Very small hydroelectric schemes, with power outputs of less than 5kW have been found to be cost effective where standardised equipment is available. Whilst a number of designs of small Pelton turbine are available which enable new manufacturers to produce these machines, this is not the case with low head turbines. This paper presents a review of low head turbine technology for pico hydro drawn from University research and information from manufacturers. The designs have been assessed particularly in relation to their suitability for use in developing countries. Many have problems in terms of cost or reliability. The paper concludes by describing the research being carried out by the Nottingham Trent University and collaborating organisations to achieve a robust, low-cost design of Pico propeller turbine suitable for local manufacture in developing countries.
KEYWORDS Pico hydro; micro hydro; low-head turbines; rural electrification; renewable energy; propeller turbine
INTRODUCTION Lack of electricity is a serious constraint to development in most of the rural communities in developing countries. The extension of the national grid to remote communities in developing countries is prohibitively expensive because of factors such as lack of transport facilities, unfriendly topographical conditions and lack of capital. In such cases, small hydroelectric schemes are found to be a suitable means for providing electrical energy to the rural communities. Such schemes with power output up to 100 kW are referred to as micro hydro schemes. Very small schemes, with power outputs of less than 5 kW have been found to be cost effective where standardised equipment is available. Such technology has come to be known as Pico Hydro(Maher et al, 1998). In Nepal, for example, many high head schemes have been installed in recent years using batch-produced Pelton turbine-induction generator units. However, a standard turbine design suitable for low head schemes (i.e. below 5 m) is not widely available.
1476 This paper reviews known designs of miniature low head turbine, and evaluates them against a set of criteria that can be used to determine their applicability to developing countries. These turbines are required for installation in small communities in remote rural areas to provide stand-alone electricity supplies. The main criteria for 'appropriateness' are: • • • •
capital cost per kW output(which depends partly on efficiency) ease of installation (usually better with a directly-coupled standard generator) turbine life local manufacture and local repair/maintenance capability
Experience has shown that low capital cost and long turbine life can best be achieved by using standardized turbines that are locally manufactured by batch-production. Any imported equipment is likely to become inoperable from the first breakdown because spare parts and expertise in equipment repair are not available. Local manufacture, on the other hand, requires use of locally available materials and production using basic workshop facilities and available skills. Turbine sites may often be inaccessible by road and therefore ease of installation may require, among other features, portability.
NOVEL DESIGNS Hydro Pneumatic Devices. Novel water to air power conversion systems have been developed specifically for low head hydroelectric power generation. The working principle of such devices is based on water power conversion to air power. The use of air avoids the need for gearboxes and allows the use of Wells air turbines to drive high speed generators. One prototype scheme was developed by the Department of Civil Engineering at Queen's University of Belfast (Gould, 1989) and another type was developed by Coventry University (Bellamy, 1995). A new development, known as 'Bubblegen' has recently been proposed (Anon., 1999) but there is little evidence to suggest that high efficiencies are achievable. Moreover, the Wells turbine is not suitable for production in developing countries because the design is fairly sophisticated and requires advanced manufacturing skills and methods. For most of these designs, there is no evidence of use other than the prototypes described in literature, which are all of 50 kW or above and unlikely to be suitable for pico hydro.
Positive Displacement Low head Hydropower Machine. Two different designs have been put forward - the AUR water engine (Wilson et al, 1984) and the Salford Transverse Oscillating machine(STO). These are both positive displacement machines for the production of energy at sites with 0.5-3 m of head. Positive displacement machines are novel and unique from an engineering point of view. However, they require considerable civil works and the range of application is likely to be for schemes of over 100 kW.
Axial Flow Pumps as Turbines. The use of axial flow pumps for micro hydro schemes has been investigated by a number of researchers (Cooper et al, 1981; Yang, 1983). Despite their high specific speeds and direct coupling to generators, axial flow pumps are considered to be unsuitable for low head pico hydro schemes due to unavailability of designs for less than 20 kW.
The Helical turbine. This patented design was developed at the Hydro Pneumatic Power Laboratory of the Northeastern University of Boston in the USA. The helical turbine is a reaction turbine capable of providing a high speed unidirectional rotation under a reversible ultra low head (less than 3m) and/or high flow velocities (Gorlov, 1995a). It is suggested that the helical turbine will be ideal for schemes of up to 1 kW (Gorlov, 1995b). However, test results on this novel turbine show poor efficiency. Aerofoil profiled blades requiring high dimensional accuracy are unlikely to be suitable for manufacture in developing countries.
1477 PROPELLER TURBINE DESIGNS
Chinese Micro Hydro Generators. The leading market and manufacturer of small, low head hydropower units is China (Waltham et al, 1996 and Xuein, 1989). Micro hydropower systems, in the 650W to 12kW range, are produced in about 50 factories in seven provinces in Southern China. The installation of very small units, with power output up to 2 kW, is said to have spread to most of the provinces of China (Xuein, 1989). These units come in a package form, with axial guide vanes and a fixed blade runner directly coupled to a single phase permanent magnet generator. Their operating head range is between two and four metres. They weigh no more than 30 to 50kg and cost around US$300/kW installed. Small turbine-generator units are exported to many countries of the Far East for the electrification of rural households.
Family Hydro in Vietnam. In the early days of micro hydro development, Vietnam was one of the major consumers of Chinese technology. Family hydro installations, which comprise of a 50 W to 1 kW turbine -generator sets, are being widely used in Vietnam, where approximately 3000 sets have been installed. They have the advantages of low cost and easy installation. They can be purchased in the market place for as little as US$28 for a turbine and generator set (US$300/kW installed) which will supply in the region of 80 W (Green, 1993).
Generator •
1.0- 2:5 metres
• tube
Fig. 1 Schematic of a Family Hydro unit in Vietnam Similar equipment is now being manufactured in Hanoi by the Renewable Energy Research Center (RERC). RERC is presently manufacturing 200 W and 1 kW family hydro sets which are sold locally for US$75 and US$350 respectively (Green, 1993). The 200 W units operate at 1.5 to 2.5 m of head and require a flow rate of about 0.02 m3/s whereas the 1 kW units have a head range between 2 and 4 m and a flow rate of around 0.08 m3/s. Though more expensive than those imported from China, the Vietnamese units are more efficient and provide greater power outputs (Nguyen, 1991). The development of these units is a significant achievement towards the efforts for electrification of the rural areas of developing countries. The units described have a number of advantages such as low cost, compact design, high speed for direct coupling to the generator and ease of installation and operation. However, the major drawbacks of these units have been identified as low efficiency and reliability problems with bearing and seals(Green, 1993). Additionally, there are problems associated with local manufacture such as use of non-standard sizes and casting to manufacture the runner hub and aerofoil section blades as one piece. Such designs can only be produced at low cost in large production runs.
1478 Axial Flow Turbine in Indonesia. The Energy Research Laboratory of the Republic of Indonesia, has developed a propeller turbine for low head, small capacity hydropower plant for village electrification. The turbine runner of four blades was designed to run at 1000 rpm, an operating head of 2 m, 0. 15 m3/s of flow rate and producing 2.66 kW at 86% hydraulic theoretical efficiency(Susanto, 1981). The major drawbacks of this design are use of cast iron and complexity of runner blade profiles that demands use of high level of skills and relatively well-equipped workshops with milling machines.
Propeller Turbine with a Helical Bladed Runner- India. A turbine with eight runner blades with a constant thickness of 3mm was developed by the Department of Civil Engineering at the Indian Institute of Science in Bangalore, for a 5kW low head pico hydro scheme. The turbine was designed to operate at 850rpm and 5m of head. Tests of the turbine have shown good cavitation resistance with the overall efficiency of the turbine reaching 67% (Rao, 1986; Rao et al, 1988a & b). The major drawbacks of this particular design can be summarised as low operating speed, helical blades and use of non-standard dimensions.
Innovative Micro Hydro Plant - Papua New Guinea. A prototype propeller turbine rotating at 200 rpm and directly coupled to a generator-gearbox assembly with 12 V dc supply has been developed by the Mechanical Engineering Department of the Papua New Guinea (PNG) University of Technology. The eight runner blades have a constant thickness of 3 mm and were made on a Computer Numerical Control (CNC) milling machine. The turbine was designed to operate at a head of 1.5 m and a flow rate of 0.06 m3/s with power output of 200 W. The disadvantages of this design can be summarised as poor performance, use of gearbox, high number of runner blades in a hub of 0.08 m giving rise to difficulty in assembling, use of long turbine shaft and reliability problem with the seal (Ranatunga et al, 1991).
Propeller Turbine for Low Head Schemes - New Zealand. A vertical shaft propeller turbine was developed, built, installed and operated by the Department of Mechanical Engineering of the University of Canterbury in Christchurch, New Zealand(Parker et al, 1993). It had an operating head of 2.8 m and a flow rate of 0.4 m3/s. The design speed is 612 rpm and the electrical output of the installed turbine was 3.7 kW, (overall efficiency of generator and turbine around 34%). This turbine has achieved a very simple approach with regard to blade design and fabrication and yet is relatively reliable and efficient to operate. Mild steel was used to fabricate the turbine. However according to the literature available, further development work is required in order to increase the turbine power output without any alterations to the simplicity of the design. Despite the low production cost resulting from using blades made of constant thickness metal sheets, there are a number of potential disadvantages related to the design of this turbine such as low operating speed, use of non-standard dimensions and reliability problems with water lubricated bearings. Being an open flume design, the head range will be limited by the drive shaft length.
Low Head Axial flow Turbines from the Nottingham Trent University
Open Flume Axial Flow Turbine. A 1 kW prototype propeller turbine with five guide vanes and four blades made out of constant thickness steel plates was designed and constructed by a student of the Mechanical Engineering Department of Nottingham Trent University (Heitz, 1993). The turbine was designed to operate at 2100 rpm, 2.9 m of head and a flow rate of 0.06 m3/s, though its best efficiency speed was found to be closer to 1000 rpm. A similar turbine was designed and installed at a low head site in London using the same design approach. The turbine was designed to operate at a speed of 650 rpm. A belt drive system was used to drive a 4-pole induction generator using a belt drive. Tests carded out by (Williams et al, 1995) show a turbine efficiency of 24%. An improved design of runner, with 6 instead of 4 blades, shows some improvement in performance.
1479 Propeltric Unit by Demetriades. There was another turbine developed at the Nottingham Trent University based on the work by Demetriades. The propeller turbine design developed was suitable for direct coupling to an induction generator and was given the name propeltric unit. It has tried to incorporate most of the features that make it suitable for use in developing countries. The layout of the prototype unit is shown in the figure. Power is extracted upstream the overhung runner. The major advantage of this arrangement is the direct coupling of the propeller runner onto the generator shat~, which requires a small extension as successfully employed in Nepal for Pelton turbines(Williams et al, 1998).
~ F
RADIALGUIDEVANES / - - SHORTPENSTOCK .
_
/
,,<------
SPIRALCASING RUNNERBLADES tl ~-- (made°~a(~ ,n~t~ .cknes' ,
~--
,//--- DRAFTTUBE
Fig.2 Propeltric unit layout A series of tests on the new design was carried out by Demetriades, and the peak efficiency was xx%. Analysis presented by him has shown that the performance of the turbine has been affected by the use of a simplified velocity distribution in the spiral casing, and the losses associated with it. As a result, the effectiveness of the guide vanes was reduced. Based on the tests, it was concluded that: • • • •
the range of best efficiency is limited by the fixed geometry of the runner blades and guide vanes the main losses in the turbine are within the runner the turbine performance is dependent on the accuracy of manufacture of the runner blades more detailed analysis is required to improve the design of the spiral casing
CONCLUSIONS Propeller turbines are well suited for low head micro hydroelectric schemes with their main advantages being compact design due to high specific speeds and direct coupling to generator sets. A directly coupled propeller turbine-generator unit offers several advantages over belt drive systems and gearboxes (Smith, 1992a) such as improved system efficiency, increased turbine and beating lifetime, reduced costs, simplicity of installation and low maintenance requirements. The need for low head power generation equipment has led over the last few years to the development of a number of novel devices and water turbines - particularly fixed geometry propeller turbines. Of all the designs reviewed, only the Chinese turbine has been widely implemented, but it has a poor record of reliability. None of the current designs is available for local manufacture in other countries, so research and development work is continuing with this as a final goal. The current research at the Nottingham Trent
1480 University aims to produce a more efficient design of the scroll casing and the runner and will try to complement the work carried out by various other organisations to develop an acceptable low head turbine design. Extensive CFD modelling techniques will be used and prototypes will be tested using Laser Doppler Velocimetry to verify mathematical models.
REFERENCES Anonymous(1999). Solutions to Last Months Design Break. Eureka December pp. 65 Bellamy, N.W. (1995) Low head hydroelectric power using pneumatic conversion. Power Engineering Journal. 3(3), pp. 109-113. Cooper, P. and R. Worthen (1981 ). Feasibility of using large vertical pumps as turbines for small scale hydropower. Final Technical report DOE/ID/12160-T1. Idaho, USA: US Department of Energy Idaho. Gorlov, A. M. (1995a). Hellical turbine for low head hydro. In: Hidroenergia 95, 4th International Conference & Exhibition. Italy pp. 177-185. Gorlov, A.M. (1995b). The helical turbine: a new idea for low head hydro. Hydro Review. 14(5), pp. 44-55. Gould, M.H. (1989). A novel hydro/pneumatic device for power generation. In: Proceedings of the 24 th University Power Engineers Conference, pp. 313-315. Queen's University, Belfast, UK Green, J.P. (1993). Family Hydro in Vietnam. Hydronet. 2/93, pp. 2-3. Heitz, M. 1993. Design and construction of a small axial flow water turbine, Final year project, Mechanical Engineering Department, The Nottingham Trent University, UK. Maher, P., N.P.A. Smith and A.A. Williams (1998). Pico hydro power for rural electrification in developing countries. International Journal of Ambient Energy., Vol. 19, No. 3, pp 143-148. Nguyen, D.L. (1991). New and renewable source of energy application in Vietnam. In: Proceedings of Asia Energy 1991 Cot!ference, Thailand. Parker, G.J., S.A. Faulkner and E.P. Giddens (1993). A low head turbine for micro hydropower. RERIC Internation Engery Journal. 15(2), December, pp. 137-146. Ranatunga, S. and N. Indrus (1991). Innovative micro plant for electricity generation in remote areas. Intemational Mechanical Engineering Congress, Australia. pp. 117-121. Rao, G.J. (1986). Investigation on a hydraulic turbine with helical bladed runner for micro power development. M.Sc. Thesis, Indian Institute of Science, Bangalore, India. Rao, G.J., R. Prasad and B.S. Kulkami (1988a). Simplified turbine runner shows good cavitation resistance. Water Power and Dam Construction. (August). pp. 20-22. Rao, G.J., R. Prasad and J.K.S. Rao (1988b). Investigations of an axial flow runner for micro hydro power development. In: Proceedings o.]'8th Fuid Machinery Conference, Budapest, Hungary. pp. 616-23. Smith, N.P.A. et al. (1992a). Directly coupled Turbine - Induction Generator systems for micro-hydro power. In: Proceedings oJ the z World Renewable Energy Congress, Pergamon Press. pp. 2509-2516. Susanto, D. (1981). Micro hydro axial flow turbine for battery charging system in remote aras. In: Proceedings of Small Hydropower for Asian Rural Development, Bangkok, Thailand. pp. 24-244. Waltham, M et al (1996). Low head micro hydro potential: Final report to ODA under TDR contract R6482. Rugby UK: Intermediate Technology Development Group. Williams, A.A. and P.G. Holmes (1995). A novel low head hydro shceme using an induciton generator. In: Proceedings of the 30 th Universities Power Engineering Conference, UPEC 95, pp. 545-549. Williams, A.A. and N.P.A. Smith (1998). Appropriate drives and transmissions for small scale hydroelectric power plants in developing countries, International Conference on Mechanics in Design, Nottingham Trent University, pp 139-148. Wilson, E.M., D.E. Bassett and I.D. Jones(1984). Two new machines for hydraulic power from low heads. Hydraulic Machinery in the Energy Related Industries, International Association for Hydro Reserves(IAHR), pp. 530-548. Xuein, C. (1989). Hydropower: China brings power down to an individual level. World Water. 12(7), pp. 3839. Yang, C.S. (1983). Performance of the vertical turbine pumps as hydraulic turbines. ASME conference FED6, pp. 97-102.
© 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh
1475
L O W HEAD PICO H Y D R O P O W E R : A REVIEW OF A V A I L A B L E TURBINE T E C H N O L O G I E S
A. A. WILLIAMS and D. R. UPADHYAY
Department of Mechanical and Manufacturing Engineering, The Nottingham Trent University, Burton Street, Nottingham NG 1 4BU, UK G. M. DEMETRIADES
Marine Surveyor, Department of Merchant Shipping, Limassol, Cyprus N.P.A. SMITH
Department of Electrical & Electronic Engineering, The Nottingham Trent University.
ABSTRACT Very small hydroelectric schemes, with power outputs of less than 5kW have been found to be cost effective where standardised equipment is available. Whilst a number of designs of small Pelton turbine are available which enable new manufacturers to produce these machines, this is not the case with low head turbines. This paper presents a review of low head turbine technology for pico hydro drawn from University research and information from manufacturers. The designs have been assessed particularly in relation to their suitability for use in developing countries. Many have problems in terms of cost or reliability. The paper concludes by describing the research being carried out by the Nottingham Trent University and collaborating organisations to achieve a robust, low-cost design of Pico propeller turbine suitable for local manufacture in developing countries.
KEYWORDS Pico hydro; micro hydro; low-head turbines; rural electrification; renewable energy; propeller turbine
INTRODUCTION Lack of electricity is a serious constraint to development in most of the rural communities in developing countries. The extension of the national grid to remote communities in developing countries is prohibitively expensive because of factors such as lack of transport facilities, unfriendly topographical conditions and lack of capital. In such cases, small hydroelectric schemes are found to be a suitable means for providing electrical energy to the rural communities. Such schemes with power output up to 100 kW are referred to as micro hydro schemes. Very small schemes, with power outputs of less than 5 kW have been found to be cost effective where standardised equipment is available. Such technology has come to be known as Pico Hydro(Maher et al, 1998). In Nepal, for example, many high head schemes have been installed in recent years using batch-produced Pelton turbine-induction generator units. However, a standard turbine design suitable for low head schemes (i.e. below 5 m) is not widely available.
1476 This paper reviews known designs of miniature low head turbine, and evaluates them against a set of criteria that can be used to determine their applicability to developing countries. These turbines are required for installation in small communities in remote rural areas to provide stand-alone electricity supplies. The main criteria for 'appropriateness' are: • • • •
capital cost per kW output(which depends partly on efficiency) ease of installation (usually better with a directly-coupled standard generator) turbine life local manufacture and local repair/maintenance capability
Experience has shown that low capital cost and long turbine life can best be achieved by using standardized turbines that are locally manufactured by batch-production. Any imported equipment is likely to become inoperable from the first breakdown because spare parts and expertise in equipment repair are not available. Local manufacture, on the other hand, requires use of locally available materials and production using basic workshop facilities and available skills. Turbine sites may often be inaccessible by road and therefore ease of installation may require, among other features, portability.
NOVEL DESIGNS Hydro Pneumatic Devices. Novel water to air power conversion systems have been developed specifically for low head hydroelectric power generation. The working principle of such devices is based on water power conversion to air power. The use of air avoids the need for gearboxes and allows the use of Wells air turbines to drive high speed generators. One prototype scheme was developed by the Department of Civil Engineering at Queen's University of Belfast (Gould, 1989) and another type was developed by Coventry University (Bellamy, 1995). A new development, known as 'Bubblegen' has recently been proposed (Anon., 1999) but there is little evidence to suggest that high efficiencies are achievable. Moreover, the Wells turbine is not suitable for production in developing countries because the design is fairly sophisticated and requires advanced manufacturing skills and methods. For most of these designs, there is no evidence of use other than the prototypes described in literature, which are all of 50 kW or above and unlikely to be suitable for pico hydro.
Positive Displacement Low head Hydropower Machine. Two different designs have been put forward - the AUR water engine (Wilson et al, 1984) and the Salford Transverse Oscillating machine(STO). These are both positive displacement machines for the production of energy at sites with 0.5-3 m of head. Positive displacement machines are novel and unique from an engineering point of view. However, they require considerable civil works and the range of application is likely to be for schemes of over 100 kW.
Axial Flow Pumps as Turbines. The use of axial flow pumps for micro hydro schemes has been investigated by a number of researchers (Cooper et al, 1981; Yang, 1983). Despite their high specific speeds and direct coupling to generators, axial flow pumps are considered to be unsuitable for low head pico hydro schemes due to unavailability of designs for less than 20 kW.
The Helical turbine. This patented design was developed at the Hydro Pneumatic Power Laboratory of the Northeastern University of Boston in the USA. The helical turbine is a reaction turbine capable of providing a high speed unidirectional rotation under a reversible ultra low head (less than 3m) and/or high flow velocities (Gorlov, 1995a). It is suggested that the helical turbine will be ideal for schemes of up to 1 kW (Gorlov, 1995b). However, test results on this novel turbine show poor efficiency. Aerofoil profiled blades requiring high dimensional accuracy are unlikely to be suitable for manufacture in developing countries.
1477 PROPELLER TURBINE DESIGNS
Chinese Micro Hydro Generators. The leading market and manufacturer of small, low head hydropower units is China (Waltham et al, 1996 and Xuein, 1989). Micro hydropower systems, in the 650W to 12kW range, are produced in about 50 factories in seven provinces in Southern China. The installation of very small units, with power output up to 2 kW, is said to have spread to most of the provinces of China (Xuein, 1989). These units come in a package form, with axial guide vanes and a fixed blade runner directly coupled to a single phase permanent magnet generator. Their operating head range is between two and four metres. They weigh no more than 30 to 50kg and cost around US$300/kW installed. Small turbine-generator units are exported to many countries of the Far East for the electrification of rural households.
Family Hydro in Vietnam. In the early days of micro hydro development, Vietnam was one of the major consumers of Chinese technology. Family hydro installations, which comprise of a 50 W to 1 kW turbine -generator sets, are being widely used in Vietnam, where approximately 3000 sets have been installed. They have the advantages of low cost and easy installation. They can be purchased in the market place for as little as US$28 for a turbine and generator set (US$300/kW installed) which will supply in the region of 80 W (Green, 1993).
Generator •
1.0- 2:5 metres
• tube
Fig. 1 Schematic of a Family Hydro unit in Vietnam Similar equipment is now being manufactured in Hanoi by the Renewable Energy Research Center (RERC). RERC is presently manufacturing 200 W and 1 kW family hydro sets which are sold locally for US$75 and US$350 respectively (Green, 1993). The 200 W units operate at 1.5 to 2.5 m of head and require a flow rate of about 0.02 m3/s whereas the 1 kW units have a head range between 2 and 4 m and a flow rate of around 0.08 m3/s. Though more expensive than those imported from China, the Vietnamese units are more efficient and provide greater power outputs (Nguyen, 1991). The development of these units is a significant achievement towards the efforts for electrification of the rural areas of developing countries. The units described have a number of advantages such as low cost, compact design, high speed for direct coupling to the generator and ease of installation and operation. However, the major drawbacks of these units have been identified as low efficiency and reliability problems with bearing and seals(Green, 1993). Additionally, there are problems associated with local manufacture such as use of non-standard sizes and casting to manufacture the runner hub and aerofoil section blades as one piece. Such designs can only be produced at low cost in large production runs.
1478 Axial Flow Turbine in Indonesia. The Energy Research Laboratory of the Republic of Indonesia, has developed a propeller turbine for low head, small capacity hydropower plant for village electrification. The turbine runner of four blades was designed to run at 1000 rpm, an operating head of 2 m, 0. 15 m3/s of flow rate and producing 2.66 kW at 86% hydraulic theoretical efficiency(Susanto, 1981). The major drawbacks of this design are use of cast iron and complexity of runner blade profiles that demands use of high level of skills and relatively well-equipped workshops with milling machines.
Propeller Turbine with a Helical Bladed Runner- India. A turbine with eight runner blades with a constant thickness of 3mm was developed by the Department of Civil Engineering at the Indian Institute of Science in Bangalore, for a 5kW low head pico hydro scheme. The turbine was designed to operate at 850rpm and 5m of head. Tests of the turbine have shown good cavitation resistance with the overall efficiency of the turbine reaching 67% (Rao, 1986; Rao et al, 1988a & b). The major drawbacks of this particular design can be summarised as low operating speed, helical blades and use of non-standard dimensions.
Innovative Micro Hydro Plant - Papua New Guinea. A prototype propeller turbine rotating at 200 rpm and directly coupled to a generator-gearbox assembly with 12 V dc supply has been developed by the Mechanical Engineering Department of the Papua New Guinea (PNG) University of Technology. The eight runner blades have a constant thickness of 3 mm and were made on a Computer Numerical Control (CNC) milling machine. The turbine was designed to operate at a head of 1.5 m and a flow rate of 0.06 m3/s with power output of 200 W. The disadvantages of this design can be summarised as poor performance, use of gearbox, high number of runner blades in a hub of 0.08 m giving rise to difficulty in assembling, use of long turbine shaft and reliability problem with the seal (Ranatunga et al, 1991).
Propeller Turbine for Low Head Schemes - New Zealand. A vertical shaft propeller turbine was developed, built, installed and operated by the Department of Mechanical Engineering of the University of Canterbury in Christchurch, New Zealand(Parker et al, 1993). It had an operating head of 2.8 m and a flow rate of 0.4 m3/s. The design speed is 612 rpm and the electrical output of the installed turbine was 3.7 kW, (overall efficiency of generator and turbine around 34%). This turbine has achieved a very simple approach with regard to blade design and fabrication and yet is relatively reliable and efficient to operate. Mild steel was used to fabricate the turbine. However according to the literature available, further development work is required in order to increase the turbine power output without any alterations to the simplicity of the design. Despite the low production cost resulting from using blades made of constant thickness metal sheets, there are a number of potential disadvantages related to the design of this turbine such as low operating speed, use of non-standard dimensions and reliability problems with water lubricated bearings. Being an open flume design, the head range will be limited by the drive shaft length.
Low Head Axial flow Turbines from the Nottingham Trent University
Open Flume Axial Flow Turbine. A 1 kW prototype propeller turbine with five guide vanes and four blades made out of constant thickness steel plates was designed and constructed by a student of the Mechanical Engineering Department of Nottingham Trent University (Heitz, 1993). The turbine was designed to operate at 2100 rpm, 2.9 m of head and a flow rate of 0.06 m3/s, though its best efficiency speed was found to be closer to 1000 rpm. A similar turbine was designed and installed at a low head site in London using the same design approach. The turbine was designed to operate at a speed of 650 rpm. A belt drive system was used to drive a 4-pole induction generator using a belt drive. Tests carded out by (Williams et al, 1995) show a turbine efficiency of 24%. An improved design of runner, with 6 instead of 4 blades, shows some improvement in performance.
1479 Propeltric Unit by Demetriades. There was another turbine developed at the Nottingham Trent University based on the work by Demetriades. The propeller turbine design developed was suitable for direct coupling to an induction generator and was given the name propeltric unit. It has tried to incorporate most of the features that make it suitable for use in developing countries. The layout of the prototype unit is shown in the figure. Power is extracted upstream the overhung runner. The major advantage of this arrangement is the direct coupling of the propeller runner onto the generator shat~, which requires a small extension as successfully employed in Nepal for Pelton turbines(Williams et al, 1998).
~ F
RADIALGUIDEVANES / - - SHORTPENSTOCK .
_
/
,,<------
SPIRALCASING RUNNERBLADES tl ~-- (made°~a(~ ,n~t~ .cknes' ,
~--
,//--- DRAFTTUBE
Fig.2 Propeltric unit layout A series of tests on the new design was carried out by Demetriades, and the peak efficiency was xx%. Analysis presented by him has shown that the performance of the turbine has been affected by the use of a simplified velocity distribution in the spiral casing, and the losses associated with it. As a result, the effectiveness of the guide vanes was reduced. Based on the tests, it was concluded that: • • • •
the range of best efficiency is limited by the fixed geometry of the runner blades and guide vanes the main losses in the turbine are within the runner the turbine performance is dependent on the accuracy of manufacture of the runner blades more detailed analysis is required to improve the design of the spiral casing
CONCLUSIONS Propeller turbines are well suited for low head micro hydroelectric schemes with their main advantages being compact design due to high specific speeds and direct coupling to generator sets. A directly coupled propeller turbine-generator unit offers several advantages over belt drive systems and gearboxes (Smith, 1992a) such as improved system efficiency, increased turbine and beating lifetime, reduced costs, simplicity of installation and low maintenance requirements. The need for low head power generation equipment has led over the last few years to the development of a number of novel devices and water turbines - particularly fixed geometry propeller turbines. Of all the designs reviewed, only the Chinese turbine has been widely implemented, but it has a poor record of reliability. None of the current designs is available for local manufacture in other countries, so research and development work is continuing with this as a final goal. The current research at the Nottingham Trent
1480 University aims to produce a more efficient design of the scroll casing and the runner and will try to complement the work carried out by various other organisations to develop an acceptable low head turbine design. Extensive CFD modelling techniques will be used and prototypes will be tested using Laser Doppler Velocimetry to verify mathematical models.
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