Abstract
The Swansea Bay tidal lagoon will use Kaplan turbines to produce electricity. These turbines have low working heads, which allow a large flow of water, and hence the production of more power. The electricity is produced from the swirl in the blades which derives from the kinetic and potential energy of the water flow.
Introduction
A water turbine is a machine that converts the potential and kinetic energy of water into mechanical energy. Currently, turbines are being used to generate electricity. They are mostly located in dams to generate power from water kinetic energy (Amonkar et al., 2016). In the turbine, the flowing water is directed to the turbine runner blades which are always spinning. The spinning force acts from a distance (work). The water flow energy is transferred to the turbine. Water turbines are divided into impulse and reaction turbines (Bolland & Stadaas, 2013). The turbine selected for the Swansea Bay tidal lagoon is bi-directional, low head and Kaplan bulb which will be manufactured by Andritz hydro. This tidal lagoon project was proposed by Tidal Electric Limited.
Theory
Turbine and water flow
The turbine diameter should be matched with water jet velocity to ensure maximum generation of power and also to achieve proper rotational speed (Huwang et al., 2009). Based on the jet velocity, the equation to find the rpm (revolutions per minute) of the turbine is:
Where:
v=velocity
D= diameter of pipes
Ft/s- feet per second
rpm= revolutions per minute
Assuming the diameter of the turbine is 6 inches, the turbine turns at 1230 rpm with 100 feet of static head, 4” pipe, and 355 gpm (gallons per minute). The equation is also illustrated figure 1 below.
The energy is provided by the difference in elevation between the point of inlet and outlet of the whole system (Walter, 2016). The inlet is the surface elevation of the source of water while the outlet is the nozzle. The general equation for liquid flow between two points is
Where z1 and z2 are the elevation points in 1 and 2 respectively. H=height
Relation of flow to power
The Kaplan turbine has low working head to allow large flow rates of water. The guide vanes provide passage for water; they are aligned to a certain degree for the swirl determined by the turbine's rotor (Bozik & Benisek, 2016). The flow passes through the curved passage which forces the radial flow to an axial direction with the first swirl imparted by the guide on the inlet. The axial flow combined with a swirl from the blades loses momentum in linear and regular to produce power in the shaft (Walter, 2016).
Discussion
Decreasing the blades translates to a lower angular mass. Hence, the rotor speed increases. This leads to more mechanical cavitation and stress. On the contrary, an ncrease in the blades translates to an increase in the angular mass. Thus the rotor speed reduces (Huwang et al., 2009). The most important point is for the given water flux and speed to find the most suitable number of turbines. Furthermore, P = ωΤ; reducing the number of turbines means lower angular mass. If the speed is constant, then the power output reduces. If the speed remains constant, the turbines produce less power since it is weak and faces more resistance that is cavitation. Additionally, if the power remains constant, then the speed for fewer turbines increases (Waters & Aggidis, 2016).
The load on each of the blades is reduced when they are increased, hence there is low pressure between the pressure and suction points in the turbines. An increase in the blade number accompanied by the blade the blade angle, reduces the load, thus increasing the pressure. Thus cavitation (momentum= (mass) (velocity)) reduces.
Conclusion
Water turbine technology converts water flow into electric energy through a well-designed process of blades inlets and outlets in the whole set. The relation of water flow to the production of electric energy takes place at the axial and swirl of blades that changes momentum to produce power. Reducing or increasing the number of turbines reduces or increases power output respectively.
References
Amonkar, P. P., Naik, A. U., Devashetty, S., Kolar, V., & AS, K. 2016. Structural Analysis on Micro-Hydro Kaplan Turbine Blade. International Journal of Innovative Research in Science & Technology (IJIRST), 2(11), 2349-6010.
Bolland, O., & Stadaas, J. F. 2013. Comparative evaluation of combined cycles and gas turbine systems with water injection, steam injection, and recuperation. In ASME 1993 International Gas Turbine and Aeroengine Congress and Exposition (pp. V002T09A002-V002T09A002). American Society of Mechanical Engineers.
Božić, I., & Benišek, M. 2016. An improved formula for determination of secondary energy losses in the runner of Kaplan turbine. Renewable Energy, 94, 537-546.
Constant, E. W. 2010. The origins of the turbojet revolution (No. 5). Maryland: Johns Hopkins Univ Pr.
Hwang, I. S., Lee, Y. H., & Kim, S. J. 2009. Optimization of the cycloidal water turbine and the performance improvement by individual blade control. Applied Energy, 86(9), 1532-1540.
Waters, S., & Aggidis, G. (2016). Tidal range technologies and state of the art in the review. Renewable and Sustainable Energy Reviews, 59, 514-529.
Walter, S. (2016). A world first: Swansea Bay tidal lagoon in the review. Renewable and Sustainable Energy Reviews, 56, 916-921.
