J. Energy Power Sources Vol. 1, No., 014, pp. 7-78 Received: July 1, 014, Published: August 30, 014 Journal of Energy and Power Sources www.ethanpublishing.com Towards an Innovative Radial Flow Impulse Turbine and a New Bernhard Mayrhofer 1, Alkistis Stergiopoulou, Bernhard Pelikan 1 and Efrossini Kalkani 1. Department of Water, Atmosphere and Environment, University of Natural Resources and Life Sciences, Institute for Water Management, Hydrology and Hydraulic Engineering, Vienna, 1190, Austria. Department of Water Resources and Environmental Engineering, National Technical University of Athens, School of Civil Engineering, Athens, 15780, Greece Corresponding author: Alkistis Stergiopoulou (alkisti_ster@yahoo.gr) Abstract: The present paper explores the performance characteristics of two innovating hydropower systems, a radial flow impulse turbine and a new horizontal Archimedean screw system. Concerning the radial flow impulse turbine a functional description with a theoretical analysis of the turbine principle is given and the results of the first hydraulic measurements of a 7.5 kw prototype are shown. The analysis shows that the turbine efficiency is with 36.6 % low, in comparison to other turbines. In terms of the horizontal Archimedean screw the principle idea and a preliminary theoretical calculation of the screw are given. The preliminary theoretical calculation indicates an efficiency of %. To verify the calculated efficiency a practical experiment arrangement is planned and described in this paper. Key words: Small hydropower, radial impulse turbines, Archimedean screw turbines, kinetic energy. 1. Introduction All turbines convert hydraulic energy into rotational mechanical energy, which is subsequently converted in electric energy. There are three types of turbines, the reaction, the impulse and the Archimedean screw turbines, the difference being mainly the manner of water head conversion. In the reaction turbines, the fluid fills the blade passages, and the head change or pressure drop occurs within the runner. An impulse turbine first converts the water head through a nozzle into a high-velocity jet, which then strikes the buckets at one position as they pass by. The runner passages are not fully filled, and the jet flow past the buckets is essentially at constant pressure. Impulse turbines are ideally suited for high head and relatively low power [1].The third type of hydropower turbine concerns the low and zero head Archimedean screw systems with inclined and horizontal axis efficient and technically feasible machines. In the last centuries many turbines, which work on the basis of the three types, have been invented. This doesn t mean that there can t be new inventions in this field. The paper presents experimental research of an IRFIT (Innovative Radial Flow Impulse Turbine) and preliminary research efforts for a HAAHT (Horizontal Axis Archimedean Hydropower Turbines) harnessing the kinetic energy of rivers, currents and open channels works.. Functional Description of the IRFIT Fig. 1 shows a schematical picture of the researched IRFIT and of its 8-bladed rotor []. The supplied water, which is under pressure, comes from the bottom and flows through a reduction to the jet nozzle. The jet nozzle converts the energy of the water stream into kinetic energy and deflects the water into the radial direction. The water jet passes centrifugally the rotor,
Towards an Innovative Radial Flow Impulse Turbine and a New 73 where rotor blades deflect the water. The deflection leads to a momentum, which moves the rotor and causes a torque. The energy from the rotor is led by a shaft to the generator. The rotor and the shaft are fixed with a bearing. This bearing absorbs all the forces and contains the sealing, which prevents flow out of the housing. After having passed the rotor the housing collects the water and brings it back to the river channel. It also contains the mounting of the turbine. The housing is not completely filled with water, so that the rotor is surrounded by air, which results in lower friction losses and ensures atmospheric pressure conditions. This also allows air to enter the rotor through an air gap which is the most important characteristic of the turbine, as explained later. Due to the air there is no force locked connection between upstream and downstream water, which results in head losses. Fig. shows the cylinders used for the flow rate regulation of the radial impulse turbine. With these cylinders it is possible to lift and lower the rotor. This leads to a change of the jet nozzle gap, which is among Fig. 1 Turbine description. Fig. IRFIT regulation. other factors responsible for the flow rate. After the air gap the jet nozzle is necessary, because all the available potential energy should be converted into kinetic energy. The air also ensures that there is almost no retroactivity from the rotor to the jet nozzle. In the rotor channel the air should reduce the friction losses, because of the resulting higher hydraulic diameter and is necessary for the flow rate regulation. For this turbine a high relative output velocity is important. The air allows a maximum speed of the water jet because it reduces the water filled cross-section area, so that theoretically by a lower flow rate more air is in the channel, the water filled cross-section area decrease and according to the equation of continuity the velocity of the water jet remains constant. The ambient air pressure around and inside the rotor, the radial jet inlet flow and the loading principle differentiate the researched turbine from other types, such as Francis as well as Pelton turbines. 3. A First Theoretical Analysis of IRFIT With the theoretical analyses the maximum efficiency of the researched turbine should be determined. This research is based on an ideal turbine without losses. This ideal turbine has an endless number of rotor blades with a thickness of zero. The water jet enters the rotor inlet normal to the circumferential direction and leaves the rotor outlet tangential against the circumferential direction. No air in the rotor is considered. By using this assumption and the law of conservation of energy, the ideal turbine efficiency η ideal can be determined, as Eq. (1) shows: η ideal = 1 - P P 1 = 1 - m c m c 1 = 1 - c c 1 (1) P 1 and P represent the input and output power of the water jet, the mass flow and c 1 and c the input and output velocity of the water jet [3-4]. The input velocity is a function of the head H and the gravitational acceleration g, as written in Eq. (): c 1 = g H () The output velocity can be calculated with the relative
74 Towards an Innovative Radial Flow Impulse Turbine and a New Fig. 3 Theoretical maximumm turbine efficiency. water velocity at the rotor outlet w and the outlet circumferential velocity u, see Eq. (3): c = w + u (3) Both velocities w and u can be calculated with Eqs. (4)-(5), where r representss the rotor radius and n the rotation speed: w = g H + u (4) u = r π n (5) Fig. 3 illustrates the ideal turbine efficiency depending on the outlet circumferential velocity. The diagram shows, that a high outlet circumferential velocity and a low head increases the maximal efficiency of the idealized turbine. Simple calculations of a turbine with friction losses indicate that with the increase of the outlet circumferentia al velocity also the friction losses rise, so that an optimal rotation speed exists. This calculation also shows that low heads are not necessarily positive for a high efficiency, because with low heads the friction losses can be relatively higher than with high heads. So practically the turbine has an optimal head. The theoretical analysiss also shows that the turbine principle is similar to the Segner Wheel, which has the same behavior according to the theoretical maximumm turbine efficiency under the mentioned assumption [5-6]. However, the researched turbine features some significant construction details. For example the jet nozzle of the new turbine is located before the rotor so that a flow regulation can be realized. 4. IRFIT Measurements To determine the Radial Flow Impulse Turbine efficiency a prototype has been built and tested on a test Fig. 4 Radial Flow Impulse Turbine Prototype with main dimensions. Fig. 5 rig. The prototype has been designed for a head of 30 m and a discharge of 5.5 l/s, so that the hydraulic input power is around 7.5 kw. For the measurements higher heads operational rage of the turbine. In Fig. 4 the prototype with the main dimensions is shown. The diagrams illustrate the influence of the rotation efficiency. Fig. 5 presents the connection from turbine efficiency and rotation speed by a pressure from around 3.8 bar. It shows that the highest efficiency from 36.6 % occurs by a rotation speed of approximately 18 s -1 and is lower than the planned rotation speed of 5 s -1. A short deviation from this optimal rotation speed has a small Efficiency depending on the rotation speed. and discharge were used to identify the speed, the pressure and the flow rate on the turbine influence on the efficiency. For example a rotation speed reduction of 0 % reduces the efficiency by around 1.3 %. The influence of the flow rate, which is represented by the jet nozzle gap, on the optimal rotational speed is small. At the point with the highest measured efficiency the turbine has a specific rotation
Towards an Innovative Radial Flow Impulse Turbine and a New 75 Fig. 6 Efficiency depending on the pressure. Fig. 7 Efficiency depending on the flow rate. speed from approximately 13 min -1. The efficiency depends also on the pressure, as the Fig. 6 shows. For each pressuree and jet nozzle gap the operation point with best measured rotation speed is shown. The course of the characteristic curves indicates that at higher pressures higher turbine efficiency could be reached. In the third diagram (seee Fig. 7), the efficiency as a function of the flow rate is presented. This diagram shows thatt an increase of the flow rate raises the turbine efficiency. It furtherr indicates that with a higher flow rate the turbine efficiency could rise too. In general the measurement shows that the maximum measured turbine efficiency is too low for a commercial usage and it is not likely that optimizations can change this fact, if the theoretical principle is considered in detail. Despite the turbine is easy to build and may be used in developing countries with a high quantity of water. 5. Towards Horizontal Axis Archimedea an Hydropower Turbines (HAAHT) The possibility of exploiting kinetic hydraulic energy of watercourses, hydraulic networks, marine and Fig. 8 Design of the horizontal screw rotor. tidal currents, for power generation, have both been given little attention, although such currents represent a large renewable energy resource which could be exploited by modern technologies [7-10].The geometrical characteristics of the new Horizontal Archimedean Screw Rotor recently installed and experimented in the hydraulic channel of the Laboratory of the Institute for Water Management, Hydrology and Hydraulic Engineering, in Vienna, with the dimensions L chan nnel = 4.17 m, b channel = 1.4 m, h channel (depth) = 1 m, are given in Fig. 8. The length L, the diameters (output and input), the pitch S and the number of blades of the screw rotor are: L = 1 m, D o = 00 mm, D i = 100 mm, S/D o = 1, S = 00 mm, n = 3 (number of blades). The horizontal screw rotor could rotated horizontally and change direction (Δθ =100 ), forming an upstream angle of 50 with its initial position and a downstream angle of 50 with its initial position (see Fig. 9). Archimedean screw Some informationn concerning two possibilities for the rotation of the horizontal screw is given in Fig. 10 below. According to the first possibility the screw rotor stays horizontal and is based on a guide at his right edge. The rotor could be moved along its guide towards above and below (inside the hydraulic channel of a
76 Towards an Innovative Radial Flow Impulse Turbine and a New Fig. 9 The horizontal screw rotor could change horizontally direction (Δθ = 100 ). Fig. 1 Views of the new 3-bladed horizontal axis Archimedean screw turbine. depth 1 m). According to the second possibility the horizontal screw rotor could be rotated horizontally (θ1, θ, θ3 ) with the help of the right guide, which could also be rotated. An artistic photorealistic view of a series of horizontal screw rotors in the natural channel of Euripus Strait, Greece, is given below in Fig. 11. Some views of the new 3-bladed horizontal axis Archimedean screw turbine, before installing in the experimental channel of the Laboratory are given in Fig. 1. Fig. 10 Rotation possibilities of the horizontal screw. Fig. 11 Artistic photorealistic view of a series of horizontal screw rotors in the natural channel of Euripus Strait. 6. Preliminary HAAHT Theoretical Efforts To illustrate a quite simple basic theory of horizontal screw machine based on the drag principle, the classical undershot waterwheel with flat blades is considered as a good example to simulate a horizontal screw waterwheel. Consider the screw waterwheel, having an effective radius R, a blade area A b and an angular velocity ω rotating in a stream flow of velocity V c (see Fig. 13). The force exerted on the flat blade, simulating one screw blade, is given by Eq. (6): F d = 0.5 ρ A b C d V r (6) where ρ represent the fluid density, C d is the blade coefficient of drag and V r is the difference (V c - V b )
Towards an Innovative Radial Flow Impulse Turbine and a New 77 Fig.13 Screw waterwheel rotating in stream flow of velocity V c = V. between stream flow velocity V c and blade velocity V b = ω * R. The relative velocity V r decreases as ω/r approaches V c and the force F d decreases toward zero. The power produced by the blade can be calculated with Eq. (7) P T = F d V b (7) and combining the previous relations gives the power: P T = 0.5 ρ A b C d ω R V c -V b = 0.5 ρ A V 3 b C d ω R 1-λ c (8) where λ is the tip-speed ratio. By using Eq. (9) C p = C d λ 1-λ (9) the power equation reduces to 3 P T = 0.5 ρ A b C p V c (10) The derived function for C p gives a value of zero for λ = 0 and λ = 1 and a maximum value of 0.148 for λ = 1/3. However, based on the total projected area of the screw waterwheel, the maximum power coefficient is about 0.06. The power for constant-speed operation is proportional to (C p /λ 3 ) but as the stream velocity increases the power continues to increase. Τhe flow and the geometrical data are V c = 1.76 m/s, Α b = blade wet area = L b * (R o - R i ) = 1 m * 0.05 m = 0.05 m, L b = length of the blade, R o and R i are the outlet and the inlet radius of the blade. The theoretical power of the input current in the open channel could be: P th = 0.5 ρ A b C d V 3 c = 136.9 W (11) The power produced by the horizontal screw wheel is P T = ω T = π n T (1) where ω is the angular velocity of the runner, T is the torque acting on the turbine shaft, and N is the rotational speed of the runner. The hydraulic efficiency of the screw turbine is defined as the ratio between the mechanical power developed by the turbine to the available theoretical water power, as Eq. (13) shows: η hyd = P t =0. (13) P th For the above preliminary HAAHTcalculations, the following characteristic values are used: F d = 51.6 N, ω = 7.88 r/s, T = F d * R = 3.8175 Nm, V b = V c /3 = 0.5866 m/s. 7. Conclusions and Further Researches This paper has explored the performance characteristics of two very promising hydropower systems, an innovative radial flow impulse turbine and a new horizontal Archimedean hydropower screw system. The functional description of the radial flow impulse turbine shows the innovative concept of the researched turbine. Theoretically the turbine is similar to the Segner Wheel, although the design and operating principle show considerable differences. The measurement results have shown that the efficiency of the investigated prototype reaches only 36.6 %. The measured data indicates that at higher pressures and flow rates an increased efficiency can be obtained. Without the maximum efficiency it is hard to analyses the turbine performance, so that further measurements with more operation points are necessary. Due to optimizations, for example of the jet nozzle, the rotor, the air gap and so on, the efficiency could be increased. Therefore more measurements of different prototypes, mathematic calculations or simulation are needed. The preliminary theoretical calculation of the horizontal Archimedean screw indicates an efficiency of %. To verify this efficiency practical measurements on the described experiment arrangement are necessary. With these measurements also the optimal operation condition for a high efficiency can be determined and general advices for the use of horizontal Archimedean screws can be given. References [1] A Layman s Guidebook on How to Develop a Small Hydro Site, European Small Hydropower Association (ESHA), 1998.
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