Full-Scale Experimental Assessment of a Horizontal-Axis Hydrokinetic Turbine for River Applications: A Challenge for Developing Countries

Abstract

This paper presents an experimental investigation into the performance of a horizontal-axis turbine deployed in the Sinú River. The primary objective is to assess the turbine’s functionality under actual water flow conditions. The prototype is equipped with a rotor measuring 1.58 m in diameter, featuring three blades, which are specifically engineered to produce 1 kW of power at a flow velocity of 1.5 m/s. To evaluate the turbine’s efficiency, measurements of both the electrical power output and the kinetic energy present in the river flow were conducted. The findings reveal a power coefficient ($C_P$) of 0.4087 for the turbine. Before the integration of hydrokinetic technology can proceed, essential preliminary assessments must be undertaken, including a hydrokinetic energy resource evaluation. This process entails multiple stages of reconnaissance and prefeasibility studies to identify optimal locations along river courses for the technology’s deployment. Additionally, this work highlights the significant challenges associated with adapting hydrokinetic technology in developing nations, particularly in the context of Colombia.

1. Introduction

As fossil fuels are finite and their consumption has significant environmental repercussions, there has been a growing global awareness of the need for sustainable energy solutions over the past few decades [1,2,3]. Renewable energy sources, including wind, solar, biomass, hydropower, and geothermal energy, have emerged as effective alternatives to traditional fossil fuels [4]. These renewable energy sources are crucial in mitigating greenhouse gas (GHG) emissions, which are key contributors to global warming and shifts in climate patterns. By leveraging multiple renewable sources through hybrid systems, energy efficiency can be enhanced and environmental benefit can be maximized. Consequently, hybrid renewable energy systems are considered a promising and eco-friendly solution for sustainable power generation [4,5,6,7].

Rivers represent one of the most important sources of renewable energy, holding considerable kinetic energy that can be transformed into mechanical energy and, subsequently, into electrical energy using hydrokinetic turbines integrated with electric generators [8,9,10]. This method offers a sustainable and continuous approach to energy generation, utilizing natural water flows without requiring extensive infrastructure. Thus, hydrokinetic technology stands out as an attractive option for clean power production [11,12,13,14].

Hydrokinetic turbines operate by harnessing the kinetic energy present in moving water currents, directly converting it into electrical energy. These turbines can be mounted on floating platforms or anchored structures along riverbanks [8,9,10]. The primary application of hydrokinetic turbine technology is to provide electricity for rural and remote communities that lack access to traditional power grids. By delivering a reliable source of renewable energy, this technology not only enhances the quality of life for residents but also stimulates local economic growth and fosters social development [8,12,15].

One of the key advantages of hydrokinetic technology is its potential to generate substantial electricity with minimal environmental impact. Unlike conventional hydropower systems, hydrokinetic turbines do not require water-retaining structures, such as dams, nor do they necessitate extensive civil engineering projects [8,9,10]. Additionally, the visual impact on the environment is minimal, as most turbine components are submerged. However, to maximize energy production throughout the year, it is crucial to optimize the system’s output while considering local water level variations [13,14]. It is noteworthy that under ideal free-flow conditions (without lateral or surface obstructions), the theoretical maximum energy conversion efficiency for a hydrokinetic turbine is 59.3% (Betz limit) [12,13,14,15]. However, in a confined channel (such as a river, artificial canal, or conduit), some of the flow that would normally bypass the turbine is forced through the rotor. This increases the flow velocity in the turbine zone, allowing more energy to be extracted from the current. As a result, the Betz limit can be exceeded, achieving efficiency values greater than 59.3%. A higher blockage ratio means a greater portion of the flow is directed through the turbine, enhancing efficiency. Nevertheless, excessive blockage can lead to undesirable effects such as cavitation or turbulence losses, ultimately reducing the expected efficiency gains [16,17].

Despite the promise of hydrokinetic turbines, the understanding of their sustainable development is still evolving, as many systems remain in the experimental phase, even though some of them have been deployed at full scale [12,13,14,15]. The primary environmental concerns associated with hydrokinetic turbines include alterations to river currents, changes in sediment transport, impacts on aquatic vegetation, disruption of the movement and migration of certain aquatic species, and potential injury or mortality of aquatic fauna due to contact with the moving rotor [12,13,14,15].

However, it is important to highlight that these systems produce zero emissions of GHG and atmospheric pollutants during operation [11,12,13,14,15]. In general, these systems demonstrate a low environmental footprint across their entire lifecycle. Studies suggest that hydrokinetic turbine systems represent a more sustainable solution for electricity generation in areas rich in water resources when compared to alternative energy generation processes like wind turbines, diesel generators, and standalone photovoltaic systems. This benefit is especially pronounced in terms of initial capital expenditure, energy production expenses, total present costs, and the capacity constraints of the systems [18].

Hydrokinetic turbines are available in various designs to capture energy from tidal, ocean, and river currents [8,9,10,11,14]. These turbines can be divided into two primary categories, as illustrated in Figure 1: horizontal-axis and vertical-axis turbines, depending on the alignment of their rotational axis with respect to the water flow. Typically, horizontal-axis turbines demonstrate higher efficiency than vertical-axis designs. Among the vertical-axis turbines, Darrieus, Savonius, and Gorlov helical models are the most common [13,19,20]. Both horizontal- and vertical-axis turbines function similarly to wind turbines, extracting energy through hydrodynamic processes [13,19,20].

A comparative analysis shows that horizontal-axis hydrokinetic turbines generally exhibit higher efficiency due to lower incidence losses, which refer to the energy lost when the flow direction changes as it interacts with the rotor. Vertical-axis turbines experience a radial inward flow on one half of the rotor, with water emerging radially outward on the other half [12,13,14,15]. This flow pattern results in increased velocity near the center of the vortex, ultimately leading to lower efficiency. The design of horizontal-axis turbines includes blades with adequate taper and twist, ensuring a uniform distribution of lift forces along their length [13,14,15]. Moreover, horizontal-axis turbines are inherently self-starting, while vertical-axis turbines often struggle with low or negative torque at low tip-speed ratios, making it difficult for them to reach operational speeds in areas with fluctuating water currents.

Additionally, horizontal-axis turbines are less prone to vibrations since they do not experience the continually changing angle of attack encountered by vertical-axis turbines. The latter endure cyclic tangential forces on their blades, resulting in significant torque ripple at the output [12,13,14,15]. When the frequency of these vibrations aligns with the resonant frequency of the supporting structure, it can lead to substantial operational challenges. Overall, horizontal-axis hydrokinetic turbines demonstrate to have superior efficiency due to their lower incidence losses, reduced vibrations, and more consistent lift forces, although vertical-axis turbines may offer advantages in terms of manufacturing and maintenance [21,22,23]. In the design and operation of hydrokinetic turbines, understanding the relationship between resonant frequencies and dynamic forces is crucial for ensuring long-term reliability and efficiency. When the frequency of these vibrations matches the natural frequency of the supporting structure, significant operational issues can arise. In general, horizontal-axis hydrokinetic turbines exhibit higher efficiency because of their lower incidence losses, minimized vibrations, and more stable lift forces. On the other hand, vertical-axis turbines may provide benefits in terms of ease of manufacturing and maintenance [21,22,23]. For the design and operation of hydrokinetic turbines, it is essential to analyze the interaction between resonant frequencies and dynamic forces to ensure both long-term reliability and optimal performance. The resonant frequency of a turbine refers to the natural frequency at which its components, such as blades and support structures, vibrate most readily. This frequency is determined by factors like the turbine’s material properties, mass, and structural design. On the other hand, excursion frequencies arise from dynamic forces induced by the river’s flow, such as turbulence, vortex shedding, and blade passing frequency. These forces can vary depending on water velocity and environmental conditions, and if they coincide with the turbine’s resonant frequency, they may lead to amplified vibrations, risking damage to the system. Therefore, careful consideration of both resonant and excursion frequencies is essential for designing hydrokinetic turbines that can withstand variable and unpredictable flow conditions while minimizing the risk of structural failure.

This study presents the experimental analysis of a full-scale horizontal-axis hydrokinetic turbine installed in the Sinú River, located in Colombia. It will also discuss the significant challenges related to adapting hydrokinetic technology in developing countries. The motivation for implementing hydrokinetic turbines in Colombia arises from the high costs of grid connections in remote areas, along with the unreliability of the national electricity supply. Furthermore, the presence of sites with favorable kinetic energy from rivers provides substantial opportunities for generating distributed energy without relying on traditional grid infrastructure. This aspect is increasingly relevant as both developing and developed nations seek to transition toward sustainable energy solutions. Finally, this work identifies key barriers to the successful deployment of hydrokinetic turbines in developing regions.

2. Challenges in Adapting Hydrokinetic Technology in Developing Countries

To date, full-scale commercial applications of hydrokinetic turbines have not been implemented in developing countries yet. In particular, Colombia faces several key challenges in adapting this technology.

2.1. Energy Resource Assessment

In Colombia, small hydropower studies have focused on hydraulic head, while hydrokinetic systems depend on flow velocity, cross-sectional area, and discharge. The outdated hydrokinetic resource database requires updated assessments at macro and micro scales. Key factors for power extraction include velocity, gradient, width, depth, and obstacles. High-velocity, low-turbulence sites are ideal, with proper turbine alignment to minimize energy losses. Turbines should be strategically placed to avoid navigation and ecological disruption, utilizing locations near riverbanks or downstream of hydropower plants for residual energy capture [24,25,26,27,28,29,30].

2.2. Increasing Energy Density

Increasing energy density in hydrokinetic turbines is essential for a sustainable energy transition. Optimizing turbine design and arrangement maximizes generation in rivers and oceans. In rivers, small turbines require careful spacing analysis to reduce interference and enhance energy capture [10,31,32]. In turn, in marine environments, larger turbines harness steady currents but must withstand harsh conditions. In both cases, an efficient layout balances space usage and water flow, promoting sustainability and energy efficiency.

2.3. Inadequate Research and Development

A key challenge in adopting hydrokinetic technology is the limited knowledge base. While various nations and organizations advance hydrokinetic energy through modeling, prototypes, and early commercialization [33,34,35,36,37,38,39,40], optimizing these technologies for local conditions, especially in Colombia, remains a gap. Multidisciplinary research is needed to develop suitable turbine designs considering Colombia’s hydrology. Additionally, protective measures must safeguard turbine blades from sediment, debris, and aquatic life interactions, ensuring durability and efficiency in complex environments.

2.4. Hybrid Energy System Integration

Hybrid energy systems integrating wind, solar, and hydrokinetic technologies optimize renewable resource use by balancing their complementary generation patterns. Solar energy peaks during the day, wind energy varies with weather conditions, and hydrokinetic energy provides a stable supply, improving overall reliability [41,42].

Key benefits include greater grid stability, reduced reliance on a single source, and minimized energy storage needs. However, integration requires advanced management systems, optimized site selection, and high initial investment costs. Effective coordination through smart grids, control algorithms, and real-time monitoring enhances efficiency and sustainability, supporting a more resilient energy infrastructure [41,42].

2.5. Reliability of Hydrokinetic Power Supply Due to River Flow Variability

Seasonal river discharge variability challenges hydrokinetic power reliability in developing countries. While more stable than wind energy, it remains less consistent than traditional sources. Hybrid systems and energy storage can help stabilize supply, while optimized turbine design enhances efficiency. Excess energy from high-flow periods could be stored as hydrogen for later use. Long-term river flow studies are crucial for system reliability, ensuring a sustainable energy solution.

2.6. Environmental Impacts

Hydrokinetic technology has a lower environmental impact than other energy systems but still poses challenges. Energy extraction alters water flow, affecting sediment transport and aquatic habitats. Tidal installations may disrupt marine life and human activities like fishing and navigation. Additionally, turbine blades pose risks to aquatic species through potential collisions or habitat disturbances.

Downstream effects, such as reduced flow velocity, oxygen levels, and nutrient transport, could impact ecosystems. Mitigation strategies include eco-friendly turbine designs and adaptive flow control. Further research is needed to optimize energy generation while minimizing ecological disruption [43].

2.7. Policy and Regulatory Framework

The shift toward renewable energy sources, such as hydrokinetic power, is crucial for decreasing reliance on fossil fuels and addressing the challenges of climate change. However, high initial costs necessitate supportive policies, incentives, and subsidies to enhance competitiveness [44].

In Colombia, the Ministry of Mines and Energy (MME) regulates the electricity sector, promoting diversification to improve energy security. Key legal frameworks include Law 142 and Law 143 (1994), which structure the electricity market, and Law 1715 (2014), which incentivizes renewable energy adoption through tax benefits and funding mechanisms.

Colombia’s commitments under COP21 and COP22 reinforce its renewable energy strategy [45]. To advance in hydrokinetic energy, clear regulations, environmental guidelines, and investment incentives are needed to foster sector growth [46,47].

3. Materials and Methods

3.1. Hydrokinetic Turbine Modelling

The design and modeling of a horizontal-axis turbine is a highly intricate process, presenting engineers with numerous decisions that directly influence both turbine performance and structural integrity [48,49,50]. As shown in Figure 2, the turbine consists of multiple key components, with the rotor, which includes the blades and hub, being the most critical element in determining overall power output. The efficiency of the rotor blades is influenced by several design parameters, including tip speed ratio ($\lambda$), blade number (B), blade radius (R), blade profile, angle of attack ($\alpha$), twist angle ($\beta$), and chord length (C). These factors collectively govern the rotational speed of the blades and significantly impact the turbine’s power generation and its overall efficiency [48,49,50]. Consequently, optimizing these parameters is crucial for maximizing the turbine performance and ensuring a balance between energy capture and structural robustness.

The electrical power output ($P_e$) of horizontal-axis hydrokinetic turbines, similar to that of wind turbines, can be calculated using Equation (1).

$$P_e={\displaystyle \frac{C_P\rho \pi R^2{V}^3}{2}}$$

(1)

$P_e$ is influenced by several factors, including fluid density ($\rho$), the area swept by the rotor ($A=\pi R^2$, where R represents the radius of the blades), water velocity (V), and power coefficient ($C_P$), which is a dimensionless parameter that represents the efficiency of a turbine in converting the kinetic energy of the fluid into mechanical power. It accounts for energy losses due to aerodynamic performance, mechanical transmission, and electrical generator inefficiencies. This relationship is expressed in Equation (1) [48,49,50]. $P_e$ can be measured using voltage and current sensors, which capture real-time data and allow for the calculation of power as the product of voltage and current.

In the design of blades for hydrokinetic turbines, both symmetric and asymmetric airfoils are frequently utilized. These airfoils were initially created and optimized for aviation purposes under varying physical conditions [51,52]. However, they may not perform optimally in hydrokinetic turbines that operate at low Reynolds numbers [52,53]. Nonetheless, airfoils specifically designed for low-altitude gliders could serve as effective alternatives for hydrokinetic blades. Traditional airfoil designs, such as those from the NACA series developed by the National Advisory Committee for Aeronautics, were primarily intended for operation at high Reynolds numbers and for full-scale aircraft applications [51,52,53].

At elevated Reynolds numbers, the boundary layer transitions prior to laminar separation, thereby circumventing the issues commonly associated with low-Reynolds-number aerodynamics. Conversely, in low-Reynolds-number scenarios, laminar separation predominates. Conventional airfoils, frequently employed in the design of small horizontal-axis wind turbines (HAWTs), typically exhibit a diminished performance at low Reynolds numbers due to the effects of laminar separation [51]. Consequently, the optimal aerodynamic performance for small HAWTs is attained by utilizing airfoils specifically engineered for low-Reynolds-number conditions [51,52].

There are relatively few airfoils designed for small HAWTs operating at low Reynolds numbers, with the NREL airfoil family (S822, S823) being one of the most prominent. The S822 profile, in particular, has been employed for hydrokinetic turbine blade design [53,54,55]. Hence, the blade profile used in this work is based on the S822 hydrofoil.

To evaluate the performance of the S822 hydrofoil, several investigations were conducted utilizing JavaFoil software V.2.20. This tool employs a potential flow panel method combined with an integral boundary layer model to analyze the fluid dynamics around airfoils or hydrofoils [56]. The analyses can be performed for defined angles of attack ($\alpha$) and varying Reynolds numbers. In this study, the geometry of the hydrofoil was designed using a CAD program, which produced x-y coordinates saved in ASCII file format. These coordinates, along with separator lines, were subsequently imported into the “Geometry card” of the JavaFoil software V.2.20.

After importing the airfoil geometry, adjustments could be made using the “Modify card”. In JavaFoil, all coefficients are computed based on a standard chord length of 1.0. Following the creation of the airfoil, the lift ($C_L$) and drag ($C_D$) coefficients could be determined using the “Polar card”, where the specific Reynolds number and the desired range of angle of attack ($\alpha$) are specified. For each combination of Reynolds number and $\alpha$, JavaFoil first calculates the velocity distribution and subsequently conducts a boundary layer analysis. In this investigation, the hydrofoil was examined at a Reynolds number typical for hydrokinetic turbines (approximately 750,000). The lift ($C_L$) and drag ($C_D$) coefficients were assessed for every $1^{°}$ across a broad range of $\alpha$ values. The resulting data are presented in Figure 3.

The analysis shows that the S822 hydrofoil can deliver optimal turbine efficiency at an angle of attack ($\alpha$) close to $5^{\circ }$, due to its superior lift-to-drag coefficient ratio ($C_L/C_D$), which reaches a value of 47.77, surpassing other tested angles. Therefore, a $\alpha$ of $5^{\circ }$ was chosen for the turbine design.

The design of an optimized turbine blade can be accomplished by integrating actuator disk theory with the blade element method (BEM) [22,50]. The chord length (C) is determined by equating the rotor torque obtained from the momentum theory with the torque calculated through the blade element theory, incorporating the effects of drag. The calculation of C is performed using Equation (2), while the twist angle ($\beta$) at any given point along the blade can be computed using Equation (3). Both equations rely on the flow angle ($\varphi$), which denotes the angle between the relative water flow and the plane of rotation. This angle can be derived using Equation (4).

$$C={\displaystyle \frac{8a^{\prime }r{\lambda }_r\pi Si{n}^2\varphi }{(C_{L}Sin\varphi -C_{D}Cos\varphi )B(1-a)}}$$

(2)

$$\beta =\varphi -\alpha$$

(3)

$$Tan\varphi ={\displaystyle \frac{(1-a)}{{\lambda }_r(1+a^{\prime })}}$$

(4)

The parameter a, known as the axial induction factor, represents the proportional reduction in water velocity as it passes from the free-stream flow to the rotor plane. This value is constrained by the momentum theory, which holds true only when $a\le 0.5$. Conversely, the term $a^{\prime }$ refers to the angular induction factor, which quantifies the relative increase in angular velocity due to the blades’ rotational acceleration, as dictated by the conservation of momentum. The relationship between a, $a^{\prime }$, and the angle of relative water flow is given by Equation (3). Additionally, the local tip speed ratio, ${\lambda }_r$, can be determined using either Equation (5) or Equation (6) [22,50].

$${\lambda }_r={\displaystyle \frac{\omega r}{V}}=\lambda {\displaystyle \frac{r}{R}}$$

(5)

$${\lambda }_r^2={\displaystyle \frac{a(1-a)}{a^{\prime }(1+a^{\prime })}}$$

(6)

The rotational velocity of the hydrokinetic turbine, represented by $\omega$ (in rad/s), is critical for determining the geometry of the blades. For this design, the turbine was configured to achieve a power output of 1 kW, with the water velocity set at 1.5 m/s. The choice of a 1.5 m/s velocity was based on the fact that rivers in developing countries typically have flow speeds that rarely exceed 2 m/s and are often not deep enough to support large-diameter axial turbines [57,58].

$C_P$ depends on the tip speed ratio, which is expressed as $\lambda$ or TSR (tip speed ratio), and the turbine pitch angle $\theta$, similar to wind turbines. Maximizing the $C_P$ is crucial for optimizing energy extraction from water currents. The parameter $\lambda$ represents the ratio between the blade tip speed and the water flow velocity, which strongly influences the turbine’s efficiency. This relationship is defined by Equation (7).

$$\lambda ={\displaystyle \frac{R\omega }{V}}$$

(7)

The theoretical limit of power coefficient ($C_{P_T}$), as derived by Betz, is 0.593. However, this value has been reported in the literature for HAWTs as a nonlinear function of $\lambda$ and $\theta$, as represented in Equation (8) [59].

$$C_{P_T}(\lambda ,\theta )=0.22\left({\displaystyle \frac{116}{{\lambda }_i}}-0.4\theta -5\right)e^{{\displaystyle \frac{-12.5}{{\lambda }_i}}}$$

(8)

$${\displaystyle \frac{1}{{\lambda }_i}}={\displaystyle \frac{1}{\lambda +0.08\theta }}-{\displaystyle \frac{0.035}{{\theta }^3+1}}$$

(9)

Using the function defined in Equation (8), the $C_P$ curve can be plotted for different values of $\lambda$ and $\theta$. For each $\theta$, there is a specific $\lambda$ that maximizes the $C_P$. The highest $C_{P_T}$ value is 0.4382 when $\theta =0$ and $\lambda =6.325$. This has two important implications. First, since the maximum $C_{P_T}$ occurs at $\theta =0$, any deviation from this angle results in a lower power capture; second, the optimal efficiency is achieved at $\lambda =6.325$.

Table 1. Comparison of the power coefficient ($C_P$) with results from earlier rotor tests documented in the existing literature.

Reference	Test Facility (m)	Rotor Diameter (m/s)	Flow Velocity	TSR	${\mathit{C}}_{\mathit{P}}$
[66]	Water channel	0.402	2.76	2.60	0.390
[64]	Field test	1.500	1.00	3.00	0.340
[63]	Water channel	0.800	1.73	6.00	0.460
[67]	Towing tank	0.400	1.436	3.50	0.278
[68]	Towing tank	0.762	1.00	3.53	0.285
[61,62]	Towing tank	0.700	1.20	4.80	0.440
[60]	Towing tank	1.200	2.00	5.51	0.470
[65]	Field test	1.500	1.10	3.10	0.330
[34]	Towing tank	1.200	0.50	5.37	0.330
[69]	Numerical result	1.580	1.50	6.739	0.419
[69]	Water channel	0.120	0.56	6.48	0.394

presents a comparison of $C_P$ from various rotor tests reported in the literature. The results show a wide range of $C_P$ values, with the highest reaching 0.470 and the lowest at 0.278. The maximum theoretical $C_P$ obtained in this study (0.4382) is comparable to the values observed in previous experiments, particularly those conducted in towing tanks [60,61,62,63]. Notably, some field tests [64,65] reported lower $C_P$ values, which may be attributed to real-world environmental factors, such as turbulence and flow variability. Overall, the results indicate that the designed rotor achieves a competitive $C_P$ within the range observed in experimental studies.

The axial flow induction factor (a) can be determined using Equation (10) once $C_{P_T}$ has been defined.

$$C_{P_T}={\displaystyle \frac{4a{(1-a)}^2}{\eta }}$$

(10)

where $\eta$ represents the overall efficiency of the system, accounting for both mechanical and electrical losses that affect power conversion performance [48,49,50]. Previous studies have documented transmission efficiency values ranging from 95% to 98% [69,70,71]. However, for this blade design, a conservative and realistic transmission efficiency of 70% was used to ensure robust performance.

Using the specified parameters, the geometry of the turbine blades was calculated. The blade length, chord length distribution, and twist distribution were determined through Equations (1)–(3). According to Equation (1), the calculated blade length is approximately 0.79 m. When applying Equation (2), a corrective factor (Fc) of 3.8 was implemented to enhance the structural integrity of the blades, thereby increasing their strength. As a result, the chord length values obtained from Equation (2) were adjusted by this factor. Figure 4 depicts the calculated distributions of both chord length and twist angle along the blade.

The calculated maximum blade width measures 0.381 m, with the blade root designed as a rectangular section to facilitate straightforward attachment to the hub. The blade incorporates 10 distinct stations [48,49,50]. The hydrofoil profile utilized in this design, developed by the National Renewable Energy Laboratory in collaboration with Airfoils, Inc., is specifically optimized for small horizontal-axis turbines [72]. This particular hydrofoil presents several significant advantages compared to conventional profiles designed for aircraft. Notably, it exhibits reduced sensitivity to surface roughness, which enhances energy capture even in scenarios where the blades may accumulate debris, such as insects. Additionally, the hydrofoil’s thicker root and tip sections (S822, 16%) contribute to a lighter blade, reduced manufacturing costs, increased stiffness, and a better resistance to fatigue stress [72].

This design approach ensures both high performance and durability for the turbine, particularly in small-scale applications where weight, cost, and efficiency are critical.

Table 2. Geometric parameters of a hydrokinetic turbine.

Parameter	Value
Diameter (D)	1.58 m
Number of blades	3
Hydrofoil	S822
Angle of attack ($\alpha$)	5°
Twist angle ($\beta$)	Depend of radial location
Chord length (C)	Depend on radial location

presents the geometric parameters used for the design of the turbine rotor. These parameters define the rotor’s dimensions and shape, ensuring optimal performance and efficiency under the given operating conditions.

The optimal blade design required balancing hydrodynamic efficiency with structural integrity. Initially, different hydrofoil profiles were distributed along the blade to maximize power extraction, taking into account the chord length, thickness, and twist angle distribution. Once the hydrodynamic design was established, the blade’s structural strength was evaluated to ensure it could withstand operational stresses. To achieve this, the finite element method (FEM) was employed to identify areas of critical stress on the turbine blade and assess its potential for a structural failure [49,50]. The FEM analysis accounted for various factors, including the blade’s material properties, geometry, and applied loads.

Given the blade’s relatively large size, the pressure exerted by water flowing at high speed across its surface generated significant bending moments, particularly near the blade root. To assess the structural response under these conditions, three distinct blade configurations were analyzed: (A) a solid blade, (B) a blade with facesheets and a core, and (C) a blade with facesheets, shear webs, and stations, as shown in Figure 5.

Configuration B included two surfaces, suction and pressure sides, designed using hydrofoil profiles optimized for hydrodynamic performance. These surfaces enabled the necessary pressure distribution to drive blade rotation, while the core, made of a lightweight material, improved structural stability without significantly increasing the blade’s weight. In contrast, configuration C consisted of two external layers, structural shear webs, and internal supports. The shear webs and supports, which were positioned within the shell formed by the two layers, played a vital role in managing and resisting various loads. Their placement, quantity, and shape had a notable influence on the blade’s structural behavior.

Each blade geometry was modeled in CAD software, with all configurations analyzed as cantilever beams. The FEM simulations showed that none of the designs exceeded the material’s yield or the tensile strength under the combined effects of centrifugal, gravitational, and hydrodynamic forces. For the solid blade made of Prolon MS (Castnylon plus Molybdenum), the maximum stress was found at the root, reaching 2.3228 MPa, which is only 1.71% of the material’s yield strength [49,50].

The hydrokinetic turbine, which is depicted in Figure 2, is suitable for deployment in multi-unit arrays designed to harness kinetic energy from rivers in Colombia. Similar to wind farms, these arrays can efficiently generate power, but their design must take into account site-specific factors such as bathymetry, available space, and potential conflicts with other water uses, such as shipping or recreational activities. These considerations help determine the optimal layout and size of the turbine array.

3.2. Hydrokinetic Turbine Manufacturing

The quality of the blade prototype is significantly influenced by the precision of its geometric specifications and surface finish. To achieve these standards, the blades were fabricated using CNC machining on blocks of Prolon MS (Castnylon + Molybdenum), a material known for its dimensional stability and machinability [73]. This material not only possesses commendable strength and stiffness, ensuring that the blade maintains its shape under operational loads, but also offers resistance to the corrosive conditions of river water while preserving its mechanical properties.

When manufacturing large blades via CNC machining, two critical challenges must be addressed: (a) the dimensions of the workpiece that can be accommodated by commercially available CNC machines in developing countries, and (b) the selection of materials based on cost, dimensional stability, and machining efficiency. The blade fabrication was carried out on a Milltronics VM20 CNC machining center, which features a 4-axis indexed configuration (see Figure 6).

The machining operation utilized a flat end mill with a diameter of 25.4 mm that is accompanied by two ball end mills with diameters of 12 mm and 8 mm, respectively. To optimize the machining time and quality, the process was divided into two roughing stages followed by a finishing operation. The first roughing stage utilized a deeper cut to expedite material removal, while the second stage employed finer cutting parameters to prepare the surface for finishing. This final operation ensured the blade achieved the high-quality characteristics necessary for its reliable performance.

During the roughing and finishing processes, three potential tool paths were considered: cyclic, spiral, and straight linear trajectories. Analysis indicated that spiral movements yielded superior results in terms of efficiency and quality for both stages of machining. Ultimately, a total of three blades were successfully produced using this method [73]. The hub of the hydrokinetic turbine was manufactured from Prolon MS, (Castnylon combined with Molybdenum) using a CNC machining process. After the blades and hub were completed, the entire turbine was mounted onto an adjustable frame, which was attached to a custom-designed floating platform (see Figure 2). This platform consisted of a raft built from multiple modular units, enabling flexible arrangements to suit different surface shapes and sizes. The individual floating cubes were securely linked using specialized connecting pins, ensuring the structure’s stability. Constructed from high-density polyethylene resin, these lightweight components are easy to handle and highly resistant to impacts, environmental fluctuations, and water-related degradation. The floating assembly was engineered to support a maximum load of 680 kg, while the turbine and its support structure weighed approximately 250 kg. This floating raft offered a stable base for both operational and maintenance activities. The adjustable height frame was constructed using conventional methods, incorporating commercially available components such as structural angles measuring 2 × 2 × 2 × 3/16 inches and 2-inch schedule 40 steel pipe. The assembly involved cutting and welding these elements to form a rigid skeleton that securely holds all critical components of the hydrokinetic turbine together. In addition to the mechanical components, the hydrokinetic turbine requires an electric generator, which was submerged alongside the rotor. For this purpose, a stainless-steel housing was fabricated (see Figure 7). This housing encloses the shaft, gearbox, and electric generator. The type of electrical energy generated—whether direct current (DC) or alternating current (AC)—is determined by the specifications of the generator employed. The selected generator was a 3-phase permanent magnet AC synchronous model, the FT-1000P (Wuxi Fengteng New Energy Technology Development Co., Ltd., Jiangsu, China), rated for 1000 W with a rotational speed of 500 rpm. It utilizes NdFeB (Neodymium iron boron) magnets, has a rated voltage of 48 V, and carries a protection grade of IP54. Permanent magnet generators offer several advantages, including a simpler design and a reduced volume and mass compared to electrically excited generators. They feature low startup torque and RPM, making them easy to install, maintain, and repair, with an efficiency exceeding 90%. While a permanent magnet synchronous generator can be directly integrated with the hydrokinetic turbine, this design incorporates a planetary gearbox (model 86XG-05, Sumtor (wuxi) Electric Co., Wuxi, China) featuring a gear ratio of 1:5. While gearboxes can introduce mechanical noise, energy losses, and increased maintenance costs, they also enhance the overall performance of the system when chosen correctly. For this project, a gearbox with an impressive efficiency rating of 96% was selected. The main turbine shaft transmits rotational energy from the rotor to the gearbox, and the gearbox’s output shaft is connected to the generator shaft using a flexible coupling. The generator housing was engineered to be watertight and also serves as a support structure for the turbine shaft.

Prior to conducting full-scale turbine experiments, a series of preliminary tests were performed, including a flotation test and a stability evaluation of the platform supporting the turbine. These tests were aimed at assessing the stability of the floating surface under various turbine positions relative to the water level. The testing took place at the University of Antioquia’s pool facility. Figure 8 shows a step-by-step sequence of the turbine assembly process during these pool tests.

The flotation and stability tests demonstrated excellent performance of the floating platform. However, to further enhance the reliability of the system, it was decided to add extra flotation modules to the sides of the original surface.

3.3. Experimental Setup

The prototype of the hydrokinetic turbine underwent testing in the Sinú River, situated in the Department of Córdoba in Colombia. In Figure 9, the testing site location is shown. Several site criteria were established for the turbine testing, including a minimum depth and width of 1.6 m to accommodate the 1.58 m rotor diameter, consistent water current velocity, and access to mounting facilities in a location that was both secure and easy to access. A suitable site was found near the University of Córdoba, meeting most of these requirements. One major advantage of the selected site was its road accessibility, which facilitated the installation and maintenance of the turbine.

At the test site, the river measured approximately 10 m in width and 5 m in depth. The average flow velocity (0.625 m/s) was calculated from measurements taken at three points along the turbine’s diameter: at the center of the turbine axis (1.2 m below the surface) and at two points 0.5 times the turbine radius on either side of the axis, all at 1 m upstream from the turbine. Velocity measurements were taken four times over a 2 h period, spaced 30 min apart, using the Flow Watch FW450 flow meter (±0.01 m/s resolution) from JDC Electronics SA, Yverdon-les-Bains, Switzerland. This device provided an average velocity based on multiple readings, ensuring accuracy. Additionally, surface velocity was verified by timing a floating object over a 1.5 m distance, with at least five measurements averaged. The floating object method yielded a surface velocity of 0.752 m/s, while the Flow Watch FW450 recorded 0.745 m/s.

It is crucial to note that a hydrokinetic turbine achieves optimal performance and maximum energy output when the water flow is smooth and linear, particularly at elevated velocities. However, river currents inherently display variability, which is influenced by seasonal and daily changes. Moreover, water velocity differs across potential installation sites, depending on the river’s cross-sectional characteristics. Therefore, selecting an appropriate location for the turbine installation is vital to enhance energy capture [12,13,14]. Additionally, the position of the turbine within the river’s cross-section is critical for two primary reasons. First, energy flux tends to be higher at the surface of the river compared to the lower regions, with current velocities changing to the distance from the banks [7,11,12,13]. Second, there are often competing users of the river, such as boats, fishing vessels, and bridges, which can limit the effective area that is available for the turbine deployment. Furthermore, factors, such as riverbed characteristics and suspended materials like fish and rocks, can affect turbine placement. Therefore, proper site selection requires a comprehensive understanding of the factors influencing water velocity at any point in the river. The performance of the turbine was evaluated by determining the $C_P$. It quantifies how effectively the turbine converts the kinetic energy contained in the water into electrical energy. Specifically, it is expressed as the ratio of the electrical power output ($P_e$) to the power available from the flowing water, as detailed in Equation (11).

$$C_P={\displaystyle \frac{P_e}{0.5\rho \pi R^2{V}^3}}$$

(11)

In this equation, the water density, which is denoted by $\rho$, is $997{\mathrm{kg}/\mathrm{m}}^3$ (density of water at 25 °C), while R represents the rotor radius of $0.79\mathrm{m}$, and V indicates the velocity of the water flow. Equation (11) assesses the turbine’s efficiency by accounting for the hydrodynamic forces acting on the blades, specifically the lift and drag components. Furthermore, mechanical losses arise from the gearbox, drive train and generator components [12,13,14,15]. To accurately determine the $C_P$, the turbine was installed and tested in the Sinú River. An encoder was placed before connecting to the gearbox, ensuring precise measurement of the turbine’s angular velocity. This measurement is crucial for calculating the turbine’s performance metrics. By applying various loads to the FT-1000P generator, a three-phase permanent magnet synchronous model, and measuring both voltage and current, $P_e$ was determined. The generator was connected to a resistive load directly coupled to its output terminals. Electrical power was measured by recording the line-to-line voltage ($V_{L-L}$) and phase current ($I_{phase}$) in each of the three phases. The total power supplied to the resistive load was then calculated using the standard equation for three-phase systems, as shown in Equation (12). Thirteen variations of the resistive load were tested, and for each load, current and voltage measurements were taken three times to determine an average $P_e$ value for each resistive load.

$$P_e=\sqrt{3}{V}_{L-L}{I}_{phase}PF$$

(12)

PF represents the power factor, which is approximately 1 for a purely resistive load. This method provides an accurate estimate of the electrical power generated and allows for an evaluation of the generator’s performance under real operating conditions. This arrangement enables a thorough evaluation of the turbine’s operational efficiency and performance under real-world conditions. After being assembled on the riverbank, it was transported to the installation site and anchored to the riverbed using galvanized steel bars, each 8 m long (Figure 9). The installation process was completed in 2 h, demonstrating the practicality of hydrokinetic systems, especially for smaller, portable turbines, which offer even simpler setups. Once the turbine was properly installed, it was submerged in the Sinú River. A key factor in selecting the installation site was ensuring that the area had an adequate width and a depth of approximately 2 m. Fortunately, the chosen site met these requirements, featuring a depth of about 5 m. This particular area was characterized by a remarkably stable river flow. Upon becoming operational, the hydrokinetic turbine generated power that was transmitted through cables to the power control center. This center featured a resistor bank functioning as a load. To assess the system’s performance effectively, measurements of the generated power were taken using a clamp current meter and a voltmeter. This setup is essential for ensuring accurate data collection and efficient energy management, thereby contributing to the overall effectiveness of the hydrokinetic turbine system in generating renewable energy. The selected testing site experiences distinct seasonal variations, with a dry season from December to March and a rainy season from May to October. Rainfall is minimal during the dry months, occurring on approximately 0 to 4 days per month, whereas the rainy season sees significantly higher precipitation, with an average of 15 to 18 rainy days per month. April and November serve as transitional months, each averaging around 8 rainy days. To ensure optimal river conditions for turbine submersion, testing was strategically scheduled for a period of average rainfall. As a result, evaluations were conducted during the first week of June, when the Sinú River maintained an appropriate water level. Nevertheless, to gain a comprehensive understanding of the turbine’s performance, future studies should extend testing across multiple seasons. Seasonal variations in river flow and velocity can significantly affect efficiency and energy output, providing valuable insights into the turbine’s long-term performance. Conducting tests throughout the year would allow for a more accurate estimation of annual energy production, including variations during high- and low-flow periods. While the present study was limited to a single test period due to budget constraints, future research should aim to address these gaps by assessing the turbine’s operation under different hydrological conditions.

4. Results and Discussion

The hydraulic turbine was initially designed to generate 1 kW at a flow velocity of 1.5 m/s. However, it is capable of operating efficiently across a range of flow velocities. The test was conducted at a lower velocity of 0.625 m/s, as finding a location in the river with the design velocity proved challenging. Despite this, the turbine’s performance was reliably assessed using the $C_P$, a key metric for evaluating its efficiency in converting the kinetic energy of flowing water into mechanical energy. Upon deployment of the hydrokinetic turbine in the flowing water, the kinetic energy was converted into lift force, which subsequently generated the torque required to drive the generator shaft. Nonetheless, power generation was limited due to the low flow speed of 0.625 m/s at the turbine inlet, with the maximum available power calculated to be only 238.621 W. Throughout the experiments, the electrical output from the generator was measured under various load conditions. During the tests, the electrical output from the generator was measured under various load conditions. Figure 10 presents the curve of $C_P$, plotted against the tip speed ratio ($\lambda$). The results show that the maximum efficiency of 40.87% was achieved at a $\lambda$ of 6.64, indicating optimal performance at this specific operating point. Table 1 presents various $C_P$ values reported in the literature. The results reveal a broad range, with the highest value reaching 0.470 and the lowest at 0.278. The maximum experimental $C_P$ obtained in this study (0.4087) is comparable to previously reported values, particularly those from experiments conducted in a water channel [66,69]. Overall, these findings suggest that the designed rotor achieves a competitive power coefficient within the range observed in experimental studies.

The experimental results were compared with the study validated by Aguilar et al. [69]. As shown in Figure 11, Aguilar et al. [69] conducted both numerical and experimental studies on a hydrokinetic turbine, using an Eppler 420 airfoil profile in both cases. The numerical analysis was performed on a full-scale turbine with a diameter of 1.58 m, while the experimental tests were conducted on a 0.12 m diameter laboratory-scale model. Their numerical study reported a maximum $C_P$ of 0.419 at a $\lambda$ of 6.739, whereas their experimental results showed a $C_P$ of 0.394 at a $\lambda$ of 6.48. In comparison, the current study achieved a $C_P$ of 0.4087 at a $\lambda$ of 6.64, demonstrating a similar trend and further validating the performance of the designed turbine.

During the experimental tests, the turbine demonstrated self-starting characteristics at the testing velocity of 0.625 m/s. This ability to initiate autonomous rotation highlights the turbine’s effective design and functionality under the tested conditions. After successfully completing the testing phase, the turbine was removed from the Sinú River. Despite the kinetic energy available in the Sinú River falling short of generating 1 kW of electrical power, primarily due to the flow velocity of 0.625 m/s being lower than the designed velocity of 1.5 m/s, the tests conducted effectively validated the turbine’s design under real operating conditions. Importantly, the testing process did not negatively impact the quality of the river water or disrupt the local flora and fauna. It is crucial for the turbine model to be accurately aligned with the flow direction to minimize friction, unwanted loading, and vibrations that may arise from misalignment. During the testing phase, several technical challenges were encountered regarding the development of hydrokinetic turbines in developing countries. One key issue was the absence of historical data on river flow, velocity, and depth at the test site, which are essential for designing and optimizing hydrokinetic systems. This lack of data meant that direct measurements during the testing phase must be conducted, significantly increasing the complexity of the analysis. Additionally, no information was available regarding the seasonal variations in the river’s energy potential, which are crucial for understanding how the system would perform over an entire year. Given the variability in flow rates and water levels, it became clear that a hybrid energy system would be required to reliably meet the target of 1 kW of power, as initially designed for the turbine. This would ensure a stable energy output, especially during low-flow periods when the hydrokinetic system alone may not be sufficient. Furthermore, the current regulations in Colombia support the integration of renewable energy sources into the national grid and allow for self-generation at the point of consumption. This regulatory framework provides an opportunity to implement hybrid systems that can combine hydrokinetic energy with other sources, such as solar or storage systems, to address the intermittent nature of the river flow. Therefore, from a technical perspective, integrating such systems is essential for ensuring a reliable and continuous power supply, while maximizing the potential of hydrokinetic energy in regions with a limited infrastructure.

5. Conclusions

A prototype hydrokinetic turbine was successfully designed, built, and tested in the Sinú River, located in the Department of Córdoba in Colombia. The turbine’s blades and hub were fabricated from Prolon MS, which is a composite of cast nylon and molybdenum, using a four-axis CNC machining process. The remaining components were constructed from 304 stainless steel to ensure durability. The blades have a solid cross-section to provide structural integrity, and the turbine is anchored to a modular floating platform made from high-density polyethylene resin. This platform offers ease of installation, adaptability, and long-term resistance to environmental conditions. The supporting frame, made from stainless steel, enhances the overall robustness of the turbine assembly. The turbine demonstrated self-starting capabilities at water velocities as low as 0.625 m/s, achieving an estimated overall efficiency of approximately 40.87%. The design phase focused on optimizing the rotor configuration and simplifying blade manufacturing to reduce costs, making the technology more accessible for local production in rural areas. The turbine can be manufactured using different methods, including manual crafting and CNC machining, which increases its feasibility for decentralized energy generation in remote regions. Although hydrokinetic turbines are a promising solution for river applications, their deployment is constrained by low free-stream velocities and shallow water depths. These factors limit both the system’s size and power output. The prototype was designed to generate up to 1 kW of electrical power at current speeds of approximately 1.5 m/s, providing a sustainable and renewable energy alternative for isolated communities that lack access to national power grids. The modular and scalable design, combined with its low environmental impact, makes it a viable solution for river-based power generation. Unlike conventional hydroelectric systems, which require significant differences in water levels and often involve more invasive installations, this turbine operates without major alterations to the river’s natural flow. For future work, it is important to conduct long-term durability tests to evaluate the structural integrity and performance of the turbine under extended operation. Additionally, environmental impact assessments should be performed to analyze potential effects on aquatic ecosystems. Further research should explore alternative rotor geometries to improve efficiency and adaptability to different river conditions. These advancements will help optimize hydrokinetic turbine technology, increasing its feasibility for widespread implementation in developing regions. Colombian rivers and canals hold considerable untapped potential for renewable energy, particularly in rural and remote regions. Depending on river flow speeds, it may be necessary to either adapt the turbine design or deploy multiple smaller turbines to meet the energy demands of a particular location. The turbines can be installed following various configurations, such as on pilings driven into the riverbed or on existing structures like bridge piers. They can also be arranged in arrays, with spacing between units determined by site-specific conditions, including water flow and depth, to optimize power generation.