Performance of Undershot Water Wheel with Bowl-shaped Blades Model

Water power is a type of power obtained from the force created by flowing water. Energy created from flowing water can be harnessed as a form of mechanical energy that can be utilized to generate electricity. Undershot water wheels have been extensively used to take advantage of the water flowing from rivers or waterfalls. This research was conducted by using water turbines with bowl-shaped blades made of iron and acrylic. The diameter of the turbines was 30 cm, and the diameter of the blades was 9 cm. Four, six, and eight blades were used in the three water turbines for this research. The blades were discharged and loaded to turn the turbine to generate force. The results of the study showed that the highest efficiency (? = 74.22%) was found in the six-blade turbine with a discharge of 0.01228 m³/s. It can be concluded that water turbines with bowl-shaped blades can be used as an alternative energy in small-scale electric generators. 

Blades; Efficiency; Energy; Undershot; Water wheels

The need for energy is increasing, especially in developing nations or areas. Appropriate generation of energy must be achieved to fulfill this increasing need. In Indonesia, the supply of energy still mostly relies on power plants that are run on fossil fuels, such as coal, oil, and natural gases (Altan and Atigan, 2008; Borg et al., 2014). These fuels are available only in limited amounts and will run out one day, while the demand for electricity continues to grow. Therefore, present energy consumption is shifting toward the use of renewable energy resources available in nature, including hydroelectric, wind, and solar energy, among others. This is because renewable energy sources are easily available and can be recycled, unlike fossil fuels, such as petroleum and coal (Saha et al., 2008; Singh and Ahmed, 2013).

A hydropower plant converts the energy of harnessed flowing water into mechanical and electrical energy. Hydroelectric energy is mostly generated by water mills (water wheels) or turbines in a waterfall or a river or stream (Denny, 2004; Khan et al., 2009; Sule et al., 2013; Sule et al., 2014). In a hydropower design evolution, the kind of turbine to be selected is decided by several specifications that depend on the conditions in the wished location (Budihardjo et al., 2015; Warjito et al., 2017).

Water the wheels are built with different shapes of plates. Bowl-shaped (i.e., half-sphere) plates have one of the highest drag coefficient values (1.42), especially in comparison to spherical plates (drag coefficient of 0.47).  The greater  the value of the drag coefficient, the greater the ability to harness the power of rushing water (Pujol and Montoro, 2010; Tjiu et al., 2015). The bowl shape can create a flowing stream of fluid when placed inside a generator (Deendarlianto et al., 2015).

        Water wheels are built with different shapes of plates. Bowl-shaped (i.e., half-sphere) plates have one of the highest drag coefficient values (1.42), especially in comparison to spherical plates (drag coefficient of 0.47). The greater the value of the drag coefficient, the greater the ability to harness the power of rushing water (Pujol and Montoro, 2010; Tjiu et al., 2015). The bowl shape can create a flowing stream of fluid when placed inside a generator (Deendarlianto et al., 2015).

Although there are many works on different types of low-flow wheels with various blade models, there are no studies on bowl-shaped blades, which can achieve 50% efficiency (Denny, 2004). The bowl-shaped blades model is more effective than other shapes because the momentum on the surface of the blade exposed to water produces high water pressure. However, it needs to be studied in more depth the resulting performance related to the number of blades using the same diameter and bowl size.

1.1. Type of Water Wheel

Water wheels can be broadly classified into three groups depending on the way in which water moves the water wheel. The three groups are as follows: (a) based purely on the gravity of the water; (b) based partially on the gravity of the water and partially on the flow of the water; and (c) based purely on the impulse of the water. In addition, there are three types of water wheels based on how the water is utilized: overshot wheel, breastshot wheel, and undershot wheel (Muller and Kauppert, 2003).

First, in the overshot wheel, water is inserted into the blade (bucket) at the top of the wheel. This type of water wheel uses only the gravity of water to operate. Basically, there is a small amount of force from the flow of the water into the bucket. Water from the top surface, begins to move through the sluice gate, which can be opened in a predetermined way (Warjito et al., 2017). Gravity pushes the blade down and makes the wheel rotate. When the blade approaches the bottom of the wheel, the water gradually begins to decrease. The advantages of using an overshot wheel are that it does not require heavy flow because the gravity of the water falling into the blade causes the water wheel to spin, it has a simple construction, and it is easy to maintain.

Second, in the breastshot wheel, water enters the blade at the center of the wheel (i.e., breast). The wheel is driven by a combination of gravity and the force of the water (Pujol and Montoro, 2010). Water flows from the top of the wheel (head race) into the blade through a number of channels, which are opened and closed through a rack and pinion mechanism and are designed to avoid changes in the flow. The bucket moves downward due to the gravity of the water and turns the wheel.

Third, in the undershot wheel, waterwheel activate when flowing water hits the bottom of the blade, which will rotate the wheel on its axis. This type of wheel is suitable for installation in a shallow, flat area where the water flows opposite to the direction of the blade in order to turn the wheel (Muller and Kauppert, 2003).

        The research results demonstrate that of the turbines with various numbers of blades, the six-blade model has the highest efficiency (?): 74.22% for discharge of 0.01228 m³/s. The maximum efficiency, ?, is directly proportional to the maximum turbine power (power of wheel) and that maximum power occurs when maximum efficiency is reached. The deviation of the test results from the ideal line on the graph is due to the opposing pressure of the current flow on the bowl-shaped blades (i.e., flow turbulence).

     This research was conducted in the Fluid Machines Laboratory of Mechanical Engineering, Hasanuddin University, Indonesia

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