Performance parameters of Savonius type hydrokinetic turbine – A Review
Introduction
Energy is a significant factor which impacts on the economic and social growth of a nation. Due to increased pace of urbanization and industrialization in the developed and developing countries, energy consumption rate per capita is increasing day by day. The sources of energy are broadly categorized as conventional (fossil fuels) and non-conventional [1], [2]. Fossil fuels are not only limited and inadequately distributed on the earth's crust but are also associated with serious environmental problems [3], [4], [5], [6], [7]. The growth of energy demand and depletion of oil reserve and environmental problems related to fossil fuels as well as rising oil fuel prices are the major issues encouraging the use of renewable energy technologies. Among all the renewable energy sources available, hydropower generation (large and small scale) holds leading position as regards of input to the world's electricity generation [8], [9], [10], [11]. Due to their negative environmental impact, large-scale hydropower stations equipped with large dams and huge water storage reservoirs received considerable criticism [11], [12] while small hydropower (SHP) is regarded as an environmentally friendly technology for rural electrification.
In most of the cases, conventional micro hydro systems work on run-of-river type scheme that lacks the reservoir capacity to store water which causes necessity of backup electricity supply due to seasonal variations resulting in severe output power drop [8], [13]. While conventional hydroelectric technology does well in harnessing the potential energy of water, the kinetic energy of moving water is disregarded. The technique to extract kinetic energy from different water currents such as tidal, ocean, river and irrigation canals is known as hydrokinetic energy technology which is a new category of hydropower energy. It does not require large dams or diverted flow to produce electricity [14], [15], [16], [17]. In order to capture as much potential in water masses as possible with low velocity, large flow openings are required in free flow turbines [18].
Many remote villages might be located in close proximity to rivers with little or no elevation. For such cases, hydrokinetic technology is considered to be suitable to install in comparison to the traditional hydropower generation [19]. High energy density, good predictability and minimal environmental impacts are the key subjects encouraging the use of hydropower technology [20], [21], [22].
The hydrokinetic energy technology may provide inevitable and secured power extracted from free streams. That's why; interest in the development of this technology has been grown-up extensively since the last couple of decades [23]. So far, a number of technological innovation, scientific research and development and the elaboration of public policies have been carried out to insert this Hydrokinetic technology within the rural electrification programs [24]. Due to these efforts, the pilot phase of hydrokinetic projects having an array of full-scale devices was installed in various part of the world. These projects exhibit advantages and disadvantages associated with Hydrokinetic technology. However, Hydrokinetic technology (HKT) has a number of advantages over other renewable sources. One of the most notable advantages is its ability to use existing infrastructure and avoid long-term construction for projects [25]. It leads to the reduction of the installation cost of the small-scale hydrokinetic project and power production starts much sooner than any other renewable sources such as hydro, wind or solar. As huge land is required for other clean energy sources while necessary land is rarely hindrance for the development of hydrokinetic power. The unpredictable or inconsistent output is the biggest hurdle for all other clean energy sources. Conversely, energy from the hydrokinetic source is much more predictable with only seasonal variations (winter) or variations during unusual events such as floods [26]. Canals and controlled rivers are even more predictable and consistent as stream flow are available 24 hours a day with gradual changes as the seasons change. HKT offers unique characteristic of base load power source for a remote community which can provide reliable power for critical applications such as refrigeration, medical, or schooling [26]. This makes hydrokinetic power generation as a strong alternative to meet energy demands of a remote location which improve regional economy [27]. HKT is effective at low water speed even at 1 m/s.
There is no greenhouse gas (GHG) emission and any audible noise at the surface during the operation phase. As a result, there is no anticipated impact quality of life. But there is GHG and atmospheric contaminant emissions during infrastructure manufacturing and installation phase which is one of the disadvantages of HKT [28]. Because of being in the earlier stage of testing, unknown issues i.e. durability of technology and true overall cost arise as a major obstacle to the development of HKT. These unknown issues make investing a risky decision for some business owners. HKT also adversely disturbs environmental issues such as impacts on habitats, flora, and fauna, salinity, dissolve oxygen, faecal bacteria level, sediment transport, seabed scour, structural instability, and disturbance of marine life [23]. As a result of the installation of Hydrokinetic turbine, uses of water body in various activities such as fishing, water sports, coastal tourism, commercial and military navigation, archeological work, communications (submarine cables) and aquaculture are affected and conflict arises between relevant agencies [28]. Due to this conflict, clearance is required from relevant agencies which almost require additional research, time [29].
As per Electric Power Research Institute (EPRI), recoverable hydrokinetic resource of rivers in the USA is estimated as 119.9 TWh per year [30]. EPRI expects that hydrokinetic technology may add 13,000 MW of new generation capacity to the United State by 2025 [31]. The problem associated in this area is its poor conversion efficiency. Therefore, it is a challenge to improve the performance of hydrokinetic turbines. A number of studies have been carried out and are available in the literature. Under the present study, an attempt has been made to review the performance related parameters of Savonius type hydrokinetic turbine considered most suitable under low-velocity water stream.
Section snippets
Concept of hydrokinetic turbines
Hydrokinetic is the kinetic energy of a water mass due to its movement. Amount of hydrokinetic energy contained in water masses depends on the water velocity. There are two sorts of hydrokinetic energy supported by water movements i.e. wave based and current based which can be found in river streams, artificial waterways, irrigation canals, dam head/tailrace etc. Theoretically, hydrokinetic power can be expressed as [32];
The turbines that convert the kinetic energy of flowing
Classification of hydrokinetic turbines
As discussed earlier, hydrokinetic technology captures the kinetic energy of water flowing in rivers, canals, and irrigation systems, without the use of large civil structure. Hydrokinetic turbines are different from the conventional turbines like Pelton, Francis, and Kaplan. In past few years, concepts of several hydrokinetic conversions such as piezo polymer conversion, vortex induced vibration, oscillating hydrofoil etc. have been developed to produce the electricity from free-flowing water
Savonius hydrokinetic turbine
The basic shape of Savonius rotor is an ‘S’ type, having a small overlap between two semi-circular blades [46]. Savonius rotor is simple in construction with low cost, low noise. It has an ability to accept fluid from any direction with good starting characteristics. It has low aerodynamic efficiency in comparison of Darrieus type turbine [47].
The working principle of Savonius rotor is based on the difference of drag force between concave and convex parts of the rotor blades when they rotate
Design parameters of Savonius hydrokinetic turbine
Savonius hydrokinetic turbine is geometrically analogous to Savonius wind rotor; hence design or geometrical parameters i.e. end plates, aspect ratio, gap ratio, overlap ratio, the number of blades, the number of rotor stages, blade profiles, etc. are the main parameters that affect the performance of a Savonius hydrokinetic turbine. In order to enhance the performance of rotor, researchers optimized these design parameters numerically or experimentally. Other than these geometrical parameters,
Challenges and future scope
Different studies suggested that Savonius turbine can be used in water channel as a hydrokinetic turbine. Limited studies are reported on Savonius turbine in water channel (irrigation canal or river). Based on extensive literature review carried out, following future scopes are observed for Savonius hydrokinetic turbine:
- (1)
Most of the studies are performed on semi-circular blade shape of Savonius rotor in the water channel. This has left a potential scope to investigate optimum twist angle of
Conclusions
In order to improve the performance of Savonius turbine, numerous studies were carried out. In earlier studies it was observed that unambiguity priority was granted by various investigators to the design parameters and flow parameters to enhance the efficiency of Savonius rotor. Several studies were aimed to optimize the designing parameters i.e. aspect ratio, addition of end plate, overlap ratio, gap ratio, number of blades, rotor angle, rotor stages and shape of the rotor and flow parameter
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