Performance Investigation of a Screw Turbine Operating Under Low Head and Less Flow Rate Requirement

Transcription

1 Performance Investigation of a Screw Turbine Operating Under Low Head and Less Flow Rate Requirement M. Amjad 1, *, F. Fazal 1, F. Noor 1, A. Qamar 1, M. Farooq 1 1 Department of Mechanical, Mechatronics and Manufacturing Engineering, University of Engineering & Technology Lahore, KSK Campus. * Corresponding Author: [email protected] Abstract- Hydro power is the renewable source of energy and world is trying to use this energy at the maximum rate, but the hydro power plant requires certain head and flow of the water to work efficiently. This requirement results in high initial capital cost of the hydro power plant. Countries like Pakistan where a natural river draft is available have a great potential for micro hydro power generation. Screw turbine is one of the turbines which works on low head and requires less flow rates that can be used for generating electricity at micro level. The screw turbine has low initial cost and running cost is also low as no fine water screening is required, no need to build dams and it is also very friendly to marine life as fishes can easily pass through the turbine blades without any harm. In this study, the screw turbine is designed to be installed on the run of the river fashion. The purpose of current study is to investigate the torque developed on the turbine shaft at different heads and flow rate. It is concluded that maximum speed (RPM) of the turbine shaft is achieved when it is inclined at an angle of 36 o. Any increase in the flow rate for a given inclination and load will result in increased RPM of the screw shaft which makes it suitable for producing electricity at micro level to electrify the areas of Pakistan located along the natural water draft. Key Words- Screw Turbine, Mechanical Power, Low Head, Less Flow Rate, Speed, Torque, Run of the River. I. INTRODUCTION Small and large sources of hydropower have been the most demanding and important renewable source in world s power production scenario and contribute about 19% of the earth s electricity. Electricity plays a major role in the development of a country. In these days, electricity can be produced by number of ways like hydro, coal, steam, wind and nuclear. The energy which is produced by hydro is cheapest among all these [i]. Run of river small scale hydro power units which require neither dam nor water storage must be the prime consideration for electrification of rural and less developed areas [ii-iv]. Large dams are used to generate power in megawatt. To build these dams, large capital cost and large land reforms are required. So, in these days micro hydro power plants are common for simple run of river. For low head the most suitable turbine is hydraulic Archimedes screw turbine [v-viii]. Archimedes Screw is one of the oldest hydraulic machines, but this was used for long time for pumping water [ix-x]. Firstly, it was used for pumping water from lower level to upper level but it can be used in reverse as a hydraulic screw turbine for energy conversion. The generation of electricity through hydro-power plants has increased in last decade [xi]. Different run of river dams are created in different regions. The countries which are major hydropower producers are Canada, USA, Brazil, China, Russia, Norway, Japan, India, Sweden, and France [xii-xiii]. Pakistan is facing energy crisis due to lack of adequate investment in mobilizing energy resources and due to increase in demand. Government of Pakistan is focused to increase energy supplies for economic development of Pakistan. Pakistan is highly dependent on oil and gas for power production at the moment. To meet 30% of energy requirements, crude oil and LPG are imported, spending 14.5 billion. All the energy resources and their contribution in overall energy consumption are given in Table 1. Pakistan has a great potential for micro hydropower generation and the utilization of this potential is the need of the hour. Depending upon the availability of water head and flow rate, numerous turbines like Pelton wheel turbine [xiv-xv], Francis turbine [xvi-xvii], Kaplan 166

2 turbine [xviii-xix], etc. as shown in Figure 1, have been developed. Screw turbine is basically a low head turbine and its working range is less than about 5m head [xx]. The basic working principal of screw turbine is very simple and it can be used in run of river fashion. Water moving downward through the blades of the screw under gravity turns the turbine shaft which can be coupled to a generator to generate electricity at micro level. The use of hydraulic screw as a turbine which is the reverse of Archimedes screw pump [xxi] needs attention to develop micro hydro power plants in the present scenario of energy crisis in Pakistan which is the basic motive of this study. Table 1: The energy mix of Pakistan in 2012 Resources Contribution Oil 30.8% Gas 49.5% LPG 0.5% Coal 6.6% Hydro 10.5% Nuclear 1.9% Source: Pakistan Energy Year Book 2012 II. EXPERIMENTAL DESIGN The design parameters are carefully selected and are shown in Table 2. The material used for fabrication of this model screw turbine is mild steel. Mild steel sheet of 0.08 inch thickness was first bended into desired profile of the screw and then welded with 2 inch hollow shaft. The outer diameter of the screw is 5.6 inch and maximum length is 36 inch for this study. The variables in this study include inclination angle of the screw turbine, flow rate of water and applied torque to calculate the power produced by the screw turbine under loaded conditions. The inclination angle is varied between 21 degree to 36 degree, water flow rate from 0.4 liter/second to 1.0 liter/second and applied torque from 0 to 2.8 Nm using a spring balance of range from 0 to 10N with 0.1N resolution. The experimental range of these input parameters are also shown in Table 3Table 3 and a schematic of this experimental setup is given in Error! Reference source not found.. Figure 2: Schematic of experimental setup (where L is length, H is height and β is the inclination angle) Figure 1: Scope of screw turbine amongst other turbines operating under different flow rate and head. Hydro screw turbine has certain advantages over any other hydro turbine. The main positive aspects of a screw turbine include low capital cost, less installation time, no need for fine screening, high efficiency as compare to Kaplan turbine, high efficiency over variable flow rates as the inclination angle can be adjusted to compensate variable flow of water and low maintenance cost. It is also fish friendly and safe for other marine life because leaves, fishes, etc can easily pass through the helix of the screw. Keeping in mind all the above mentioned benefits, an experimental investigation is presented in this study to analyze the performance of a screw turbine under different operating conditions and the results suggest its suitability and applicability in run of river fashion to generate electricity. The ends of the model the screw turbine are supported by UC-206 bearings. Experiments were performed on the turbine by varying different parameters to see the effect on RPM (revolutions per minute) developed under no load and different loading conditions. The RPM of the turbine were measured by varying the flow of water and keeping the inclination angle at a certain value in the experimental domain and then by changing the inclination angle to another value. A ball valve was used to maintain the flow to a certain value within the range of this study. A tachometer (Digital Tachometer, ST-623B) was used to measure the RPM of the screw turbine shaft. Torque was varies by changing the force applied through a pulley, a belt and spring balance. Then torque was calculated using formula T = F x r, Where T torque of the turbine (Nm), F is the applied force (N) and r is the radius of the turbine shaft. 167

3 Table 2: Design parameters of screw used. Design Parameters Units Values Length of screw Inch 36.0 Outer diameter Inch 5.6 Inner diameter Inch 3.0 Blade thickness Inch 0.08 Pitch Inch 6 Material Mild steel Table 3: Experimental ranges of different process parameters. Parameter Units Range Inclination angle (β) degree 21 to 36 Flow rate Liter/s 0.4 to 1.0 Applied torque Nm 0 to 2.8 III. ANALYSIS AND DISCUSSION During experimentation, the rpm of the screw turbine were measured by varying the flow rate and inclination angle using a digital tachometer as described above. Flow was varied from 0.4L/s to 1.0L/s and inclination angle from 21 o to 36 o. The mechanical power of the turbine is calculated using the relation P = 2πNT/60 where T is torque applied on the turbine shaft (Nm) and N is rpm of the turbine. Figure 3 through Figure 7 shows various plots showing the variation in process responses as a result of variation in the input parameters. The inclination angle of the screw turbine was fixed at a certain value within the experimental range, the flow was varied and the corresponding change in speed of turbine shaft was noted. Figure 3 shows the change in speed of the turbine shaft at different water flow rates. It can be seen that as the flow rate is increased the speed of the turbine shaft is increased. Also with the increase in inclination, the speed also increases. This is because when at some particular inclination the flow rate is increased, water running through the blades of the screw turns the turbine shaft rapidly and the kinetic energy of water is converting into fast turning of turbine screw. Hence speed of the turbine shaft increases linearly with the inclination and water flow rate within the experimental range. Figure 3: Variation of RPM developed at different inclination under changing flow rate under no load conditions. Variation of turbine shaft speed at different inclinations under different loads at flow rate of 1L/s and 0.7L/s is shown in Figure 4 and Figure 5. There was no motion of turbine shaft at a flow rate of 0.4L/s so its effect is not shown here. It can be seen from Figure 4 and Figure 5 that there is no motion when a torque of 2.8Nm is applied on the turbine shaft even when the inclination and flow rate are increased to maximum in the experimental range. This is due the overloading condition of the turbine with in the experimental range. This phenomenon can be described as self-safety of this setup. When the load on the turbine shaft is greater than its designed capacity, it will simply stop turning without damaging the appliances operating with this turbine shaft. At a particular load value, the speed of the turbine shaft increases with the increase in inclination of the shaft and at a given load and inclination angle, speed varies directly with the water flow rate as cam be seen from Figure 4 and Figure 5. Figure 4: Effect of applied torque on RPM at different inclinations when flow of water is 1L/s. 168

4 Figure 5: RPM variation at different torque values vs inclination angle when flow of water is 0.7L/s. As described earlier that mechanical power developed on the turbine shaft is calculated using the relation P = 2πNT/60, power is zero when any of N or T is zero. When the turbine shaft is running at some inclination and at a particular flow rate, applied torque is varied and the effect is noted in the form of increase or decrease in the speed of turbine shaft. The rpm of the turbine shaft decreases when torque is increased and vice versa. When the applied torque is increased to 2.8Nm, turbine shaft is stopped irrespective of the inclination and flow rate which determines the maximum operating capacity of the turbine within experimental range. This effect is shown in Figure 6 and Figure 7 at flow rate of 1L/s and 0.7L/s respectively which explain that best operating conditions can be achieved when the load is between 1.0 to 1.5Nm in the selected range of this study. Figure 6: Power developed against applied torque at different inclinations when flow is 1L/s. Figure 7: Power developed against applied torque at different inclinations when flow is 0.7L/s. It is evident from the above discussion that screw turbine have a potential to be used in run of the river fashion to produce electricity at micro level to electrify the areas along the natural draft of the rivers in Pakistan. It can be seen that this requires a very little set up cost with no significant labor and capital investment which is normally required for any other turbine to be used for hydro power generation. IV.CONCLUSION The conventional fossil fuel resources are lessening and also becoming very expensive with the every passing day. Ever increasing energy demands can be controlled by installing small renewable energy power plants and micro hydro is the best choice for countries like Pakistan lacking in meeting energy demands but having a great natural water draft. Keeping in mind the current energy crisis in Pakistan, micro hydro power plants are one of the best solutions. Run of the river potential can be utilized by installing screw turbines which operate under low head and flow requirements. The present study investigates the operating conditions of a screw turbine which can be used to meet the energy demand of people who are residing in areas where it can be installed easily with a very small investment. Screw turbine in this study is operated at different inclination angles by changing the water flow rate against varying applied load. It can be concluded that speed of the turbine shaft increases with increase in both water flow rate and inclination angle for a given applied load within the experimental range. A maximum output is achieved at an inclination of 36 degree and flow rate of 1L/s which suggests its suitability for producing electricity on micro level by coupling its shaft with a generator. REFERENCES [i] Y. R. Pasalli and A. B. Rehiara, Design Planning of Microhydro Power Plant in Hink River, Procedia Environ. Sci., vol. 20, pp ,

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