Effect of cone on efficiency improvement of a self-powered IoT-based hydro turbine

Abstract

This study aims to improve the power generation performance of the self-powered IoT turbine flowmeter developed in our previous study. To achieve this, a cone was installed at the front-center of the rotor to accelerate the water flow and streamline the resulting high-speed flow toward the blade near the periphery of the rotor. The experiments and flow simulations served to explore the effects of the cone on the characteristics of the flowmeter. The results demonstrate that both the power generated by the cone-equipped rotor and the pressure loss increases with increasing cone diameter $d$ , Consequently, the power efficiency is maximum at $d/$ D = 0.375, where D is the outer diameter of the blade. The pressure difference between the front and rear of the blade for the cone-equipped rotor with $d/$ D = 0.375 is significantly larger than that of the rotor without the cone. This pressure difference increases the torque acting on the blade, which increases the rotational speed and output of the rotor. In addition to the improvement in performance, the introduction of the cone facilitates flowrate measurement because it enables the relationship between the flowrate and the rotational speed of the rotor to be expressed as a linear function.

Introduction

Several studies have attempted to develop micro-hydraulic turbines that convert hydraulic energy in small rivers and drainage canals into electric power.^1–8 Micro-hydraulic turbines with propeller-type rotors have been developed to utilize the water flow of the low head of less than 10 m.^9–13 The transient stability of the water flow of each of the low-head pump stations is also investigated.^14,15 Singh and Nestmann ¹⁰ and Tran and Kim ¹¹ developed micro-hydraulic turbines that could be installed at drainage canals and the water circulation pipes in fish farms, respectively. We ¹² developed a propeller-type micro-hydraulic turbine with excellent performance in terms of the passage of foreign matter included in the water flow. Du et al. ¹³ proposed a pump-turbine that realized pressure reduction and electric power generation simultaneously.

Electric power generation techniques that convert miniscule amounts of energy in the environment such as vibration and heat into electric power have attracted considerable attention. ¹⁶ Such energy harvesting techniques have been successfully used to supply electric power to sensors measuring temperature, ¹⁷ pressure, ¹⁸ and pH of water. ¹⁹ With recent advances in the energy harvesting, micro-hydraulic turbines can be effectively applied as power sources in sensors designed to detect the state quantity of the flow inside pipes. In order to obtain electric power necessary for the monitoring system of water leakage and water quality of various pipes such as water supply and drainage pipes, micro-hydraulic turbines for lift-force,^20,21 drag-force,^20,21 radial-flux, ²² and propeller type ²³ have been developed. We have previously developed a turbine flowmeter with a power generation function. ²⁴ The turbine flowmeter is connected to the pipes through which water flows, and the rotation of the rotor with four blades generates electric power. The generated power is used to operate a microcomputer and a low-power wide area (LPWA) communication module. The microcomputer detects the rotational speed of the rotor, and the LPWA communication module uploads the detected value to a server on the Internet. As the relationship between the flowrate and the rotational speed of the rotor is expressed by a linear function, the flowrate can be calculated based on the value of the rotational speed downloaded from the server. Thus, the flowmeter is a self-powered Internet of Things (IoT) sensor. A power of 0.245 W was obtained at a flowrate of 0.0012  ${\mathrm{m}}^3$ /s; this power was then used to operate the microcomputer and LPWA communication module. The full-scale accuracy corresponding to 3 $\sigma$ of the measured flowrate was 1.2%.

In this study, we focus on improving the power generation performance of the self-powered IoT turbine flowmeter reported in the previous study. ²⁴ A considerable amount of power is required to extend the IoT flowmeter to a multifunction sensor for monitoring various parameters that describe the state of water flow, such as the temperature and pressure. Therefore, in this study, a cone was introduced at the front-center of the rotor to improve the power generation performance. The cone accelerates the water flow at the front of the rotor and streamlines the resulting high-speed flow toward the blades near the periphery of the rotor. Such flow control using the cone can potentially increase the torque acting on the blades, thereby efficiently converting hydraulic energy into the rotational energy of the rotor. The experimental results showed that the introduction of the cone leads to an increase in the power output and power generation efficiency, while ensuring the linear relationship between the rotational speed of the rotor and the flowrate. The cone diameter corresponding to the maximum power generation efficiency was also determined. The effect of the cone on the power generation performance was also analyzed through a numerical simulation of the flow inside the rotor.

Turbine flowmeter and experimental setup

Self-powered turbine flowmeter and cone-equipped rotor

The self-powered IoT turbine flowmeter reported in the previous study ²⁴ is based on the design of a previously developed micro-hydraulic power generator. ¹² In this study, we aim at increasing the power generation performance of the turbine flowmeter by modifying the geometry of the rotor. Figure 1 shows the cross-sectional and 3D sectional views of the turbine flowmeter. The flowmeter is connected in series to the pipes through which water flows. A rotor with four blades is installed on the central axis of the flowmeter and is supported by two bearings. The axial length is 34 mm and the outer diameter of the blade is D= $ø30$ mm. Fourteen magnets are embedded in the periphery of the rotor. The rotor is surrounded by a stator core comprised of 28 0.35 mm thick electric steel plates. Copper wire is wound around the stator core, which has an axial length of 9.8 mm. The gap between the stator core and the magnets is 1 mm. Electric power is generated owing to the rotational motion of the rotor around the central axis, which is caused by the water flow in the axial direction.

Figure 1.

Cross-section of turbine flowmeter with power generation function.

The configuration of the blade of the rotor is shown in Figure 2. Figure 2(a) presents the geometry of the blade used in the previous study. ²⁴ The four blades are joined on the axis of rotation. The blade geometry used in this study is shown in Figure 2(b). A cylindrical hub with a diameter $d$ is attached along the axis of rotation. Thus, the blades near the rotation axis are replaced with the hub, and a cone is attached to the upstream end of the hub. The height and bottom diameter of the cone are $h$ and $d$ , respectively. The hub combined with the cone decreases the blade area around the rotational axis of the rotor, thereby adversely affecting the performance of blade. However, the cone is considered to efficiently convert hydraulic energy into the rotational energy of the rotor. This can be inferred from the observation that the torque generated on the blades near the rotor periphery is larger than that generated around the rotational axis of the rotor.

Figure 2.

Configuration of rotor blade: (a) blade without cone and (b) blade with cone.

In this study, the cone height $h$ was set to $h/$ D = 0.33, and the cone diameter $d$ corresponding to the highest power generation performance was determined. The apex angle of the cone is denoted by $\alpha$ . Figure 3 shows the rotors examined in this study. The apex angles at $d/$ D = 0.125, 0.25, 0.375, 0.5, and 0.625 are $\alpha =21.2°,41.1°,58.7°,73.7°$ , and 86.3°, respectively.

Figure 3.

Blade with a cone having various diameter.

Figure 4 shows the 2D development of the rotor blade at the periphery ( $r/$ D = 0.5). The width $B$ is 11 mm, and the blade is in a flat-plate-cascade shape with thickness $t$  = 1.6 mm. The inlet and outlet angles ${\beta }_1$ and ${\beta }_2$ , respectively, are equal at all the radial positions.

Figure 4.

2D development of rotor blade ( $r/$ D = 0.5).

As shown in Figure 5, the angles ${\beta }_1$ and ${\beta }_2$ change in the radial ( $r$ ) direction. The angles are independent of the cone diameter $d$ . At the rotor periphery ( $r/$ D = 0.5), ${\beta }_1$ and ${\beta }_2$ are both 65°.

Figure 5.

Variation of angles ${\beta }_1$ and ${\beta }_2$ of rotor blade.

Power generation efficiency

The self-powered IoT turbine flowmeter is connected in series to water pipes. To measure the power generation efficiency, two pressure sensors, positioned 30 mm upstream and 75 mm downstream of the flowmeter, are attached to the pipes.

The power generation efficiency $\eta$ can be defined as

$$\eta =\frac{P}{Q(p_1-p_2)}$$

(1)

Here $P$ is the output from the generator, $Q$ is the water flowrate, and $p_1$ and $p_2$ are the upstream and downstream pressures of the flowmeter, respectively.

Experimental setup

Figure 6 shows the experimental setup to investigate the power generation performance and the water flowrate measurement accuracy. Water is stored in a rectangular tank with a width, depth, and height of 790, 790, and 600 mm, respectively. The water pumped by a submerged pump passes through a commercially available electromagnetic flowmeter (FD-MH500A, KEYENCE CORPORATION). Then, the water flows into the turbine flowmeter to be tested and is finally released back into the tank. The water flowrate is controlled by the inverter drive of the pump.

Figure 6.

Closed-loop test rig for laboratory experiment.

Experimental results and discussion

The power output of the turbine flowmeter $P$ changes as a function of the water flowrate $Q$ measured using the electromagnetic flowmeter, as shown in Figure 7. The fluctuation of the flowrate measured by the electromagnetic flowmeter is 2% or less at any flowrate. Irrespective of the cone diameter $d$ , the power output is obtained at $Q$  ≥ 0.6 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s, and $P$ increases with increasing $Q$ . The rotor with larger $d$ generates a higher output $P$ . It should be noted that the cogging torque, which acts between the magnets embedded in the rotor and the stator, prevents the rotor from rotating.^25,26 Moreover, the friction torque of the bearings acts on the rotor. In addition, at $Q$  < 0.6 ×  ${10}^{-3}$ ${\mathrm{m}}^3$ /s, the torque induced on the rotor by the water flow is smaller than the sum of the cogging torque and the friction torque; therefore, the rotor does not rotate.

Figure 7.

Effect of the cone diameter on relationship between water flowrate $Q$ and generator output $P$ .

Figure 8 shows the relationship between the water flowrate $Q$ measured using the electromagnetic flowmeter and the rotational speed of the rotor $N$ . The rotor with larger $d$ rotates at a higher rotational speed. Irrespective of the $d$ value, $N$ increases linearly with an increase in $Q$ . Previously, ²⁴ the $Q$ value was successfully calculated by substituting the detected $N$ value in the linear expression for the relationship between $Q$ and $N$ , by employing a rotor without a cone ( $d/$ D = 0). It was found that when the rotor has a cone or in the case of $d/$ D > 0, the $Q$ value can be obtained from the measured $N$ value in the same manner.

Figure 8.

Effect of cone diameter on the relationship between water flowrate $Q$ and rotor rotational speed $N$ .

The upstream and downstream pressures of the flowmeter, $p_1$ and $p_2$ , respectively, were measured. The pressure difference $\Delta p$  =  $p_1$  −  $p_2$ is shown in Figure 9. $\Delta p$ increases with increasing flowrate $Q$ . The rate of the increment becomes larger with increasing $d$ , because the pressure loss increases with increasing $d$ , owing to the decrease in the effective flow passage area between the blades.

Figure 9.

Effect of cone diameter on relationship between water flowrate $Q$ and pressure difference $\Delta p$ .

The power generation efficiency $\eta$ calculated using equation (1) varies with the water flowrate $Q$ measured with the electromagnetic flowmeter, as shown in Figure 10. In the case without the cone or $d/$ D = 0, $\eta$ has a maximum value, 0.066, at $Q$  = 1.17 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s. In the case of $d/$ D = 0.125, $\eta$ is nearly equal to that in the case of $d/$ D = 0. In the cases of $d/$ D = 0.25 and 0.375, $\eta$ is higher than that in the case of $d/$ D = 0 at $Q$  ≤ 1.6 ×  ${10}^{-3}$ ${\mathrm{m}}^3$ /s. This is attributable to the fact that the output $P$ becomes larger with increasing $d/$ D, as shown in Figure 7. In the case of $d/$ D = 0.375, $\eta$ markedly increases in a wide range of the flowrate, and reaches the maximum value 0.074 at $Q$  = 0.67 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s. However, in the cases of $d/$ D = 0.5 and 0.625, $\eta$ is lower than that in the case without the cone ( $d/$ D = 0) at $Q$  ≥ 0.8 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s. This is because the cone increases the pressure loss $\Delta p$ , as shown in Figure 9. These experimental results demonstrate that the cone with a diameter of $d/$ D = 0.375 successfully increases the power generation efficiency.

Figure 10.

Effect of cone diameter on power generation efficiency $\eta$ .

Flow simulation

Simulation conditions

The power generation performance is closely related to the flow within the rotor. Thus, flow simulations were performed to explore the effects of the cone introduced at the front-center of the rotor on power generation performance. The simulation was performed using SCRYU/Tetra, a general-purpose thermofluid simulation software that uses a hybrid mesh to represent the surface shape of Software Cradle Co. Ltd. The fluid simulation was modeled on a Reynolds-Averaged Navier-Stokes (RANS) equation, whereas the turbulence is modeled with the $k-ϵ$ model. The finite volume meshes of tetrahedral elements were generated automatically with the appropriate manual mesh-size control of the lattice grid, which is called “octant” in SCRYU/Tetra. The flow for a cone diameter of $d/$ D = 0.375, which yielded the highest power generation efficiency in the experiment, was simulated. The flow simulation in the case without the cone ( $d/$ D = 0) was also performed. The flowrate $Q$ was set at 0.5 ×  ${10}^{-3}$ ${\mathrm{m}}^3$ /s, at which the cone effectively improved the power generation efficiency in the experiment.

Figure 11 shows the simulation domain and the computational mesh. The finest mesh size was approximately 0.4 mm, which was used near the center of blades and the cone, and the coarsest mesh size was approximately 0.6 mm, which was used near the wall of the pipe. The domain consists of the flowmeter (generator) and the pipes 1.67D upstream and 1.67D downstream of the flowmeter, respectively. The domain is discretized with tetrahedron meshes. The number of meshes for the flowmeter is 910,000, and those for the upstream and downstream pipes are 11,000 and 56,000, respectively. The flowrate is prescribed at the upstream boundary, and the pressure is given at the downstream boundary. The no-slip condition is applied to all solid walls. The mesh on the blade surface is shown in Figure 12.

Figure 11.

Computational domain and grid cells.

Figure 12.

Grid cells on the blade: (a) blade without cone and (b) blade with cone.

In this simulation, the load torque of the rotor was adjusted so that the rotational speed of the rotor matched that used in the experiment. Currently, the error of power generation efficiency $\eta$ was less than 3% in the simulation and experiment, which is in good agreement, quantitatively.

Results and discussion

The water velocity distribution between the blades is shown in Figure 13. The distribution on a cross-section at a radial position of $r/$ D = 0.33 is indicated as a representative position since the same distribution tendency of the water velocity was observed in the range of $0.2\le r/$ D $\le 0.47$ . The velocity between the blades in the case of $d/$ D = 0.375, shown in Figure 13(b), is significantly higher than that in the case without the cone, as presented in Figure 13(a). This is because the flow passage area decreases due to the cone. This demonstrates that the cone effectively accelerates the water flow and guides the resulting high-speed flow toward the blades.

Figure 13.

Water velocity distribution between blades at $Q$  = 0.5 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s: (a) without cone (d/D = 0) and (b) with cone(d/D = 0.375).

Figure 14 shows the pressure distribution on the same section as that shown in Figure 13. The pressure difference between the front and rear of the blade in the case of $d/$ D = 0.375, as shown in Figure 14(b), is larger than that in the case without the cone shown in Figure 14(a). It is found that the cone increases the fluid force or torque acting on the blade.

Figure 14.

Pressure distribution between blades at $Q$  = 0.5 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s: (a) without cone (d/D = 0) and (b) with cone (d/D = 0.375).

The pressure distribution on the blade is shown in Figure 15. The pressures on the front and rear of the blade are plotted against the nondimensional distance $s/l$ from the leading edge of the blade, where $l$ is the distance between the leading and trailing edges. The top panel of Figure 15 shows the distribution for a section at a radial position of $r/$ D = 0.2. The pressures on the front and rear are denoted as $p_f$ and $p_b$ , respectively. When the pressure difference $\Delta p_{\mathrm{fb}}$ = $p_f$ - $p_b$ is positive, the torque produced by $\Delta p_{\mathrm{fb}}$ acts in the rotational direction of the rotor. When $d/$ D = 0.375, the region of $s/l$ at which $\Delta p_{\mathrm{fb}}$ $d/$ 0 is approximately the same as in the case without a cone ( $d/$ D = 0). It should be noted that $\Delta p_{\mathrm{fb}}$ in the case of $d/$ D = 0.375 is much larger than that in the case without the cone ( $d/$ D = 0), which indicates that the cone produces a larger positive torque. In this region, as seen in Figures 13 and 14, positive pressure was locally observed at the front side of the blade and negative pressure was observed at the rear of the blade. The middle and bottom panels of Figure 15 show the results at $r/$ D = 0.33 and 0.47, respectively. The $\Delta p_{\mathrm{fb}}$ value in the case of $d/$ D = 0.375 is markedly larger than that in the case without the cone, similar to the result at $r/$ D = 0.2. At $r/$ D = 0.47, the cone successfully increases the pressure on the front side in the broader area. The increase in the torque near the periphery of the rotor can be confirmed. It is observed that the cone contributes to an increase in the torque acting on the blades. The area of the blade decreases due to the cone and hub, but the effect of the increase in $\Delta p_{\mathrm{fb}}$ can dominate the effect of the decrease in the area. Accordingly, the cone increases the rotor rotational speed and output as demonstrated in the experiments.

Figure 15.

Pressure distribution on blade at $Q$  = 0.5 ×  ${10}^{-3}$   ${\mathrm{m}}^3$ /s.

Conclusion

To increase the electric power generation performance of the self-powered IoT turbine flowmeter developed in a previous study, a cone was introduced at the front-center of the rotor. The effect of the cone diameter $d$ on the characteristics of the flowmeter, such as the power generation performance and the relationship between the flowrate and the rotational speed, was explored experimentally by varying $d$ in the range of $d/$ D ≤ 0.625, where D is the rotor diameter and the cone height $h$ is set at 0.33D. A simulation of the flow within the rotor was also performed under the condition, at which the power generation efficiency was found to be maximum in the experiment. The results are summarized as follows:

• (1) The power generated by the cone-equipped rotor increases with increasing $d$ , and the pressure loss also increases with $d$ . Therefore, the power efficiency becomes maximum at $d/$ D = 0.375, and the cone can increase the efficiency for a wide range of flowrates.

• (2) The velocity between the blades of the rotor with the cone for $d/$ D = 0.375, is significantly higher than that of the rotor without the cone because the cone decreases the flow passage area decreases. This demonstrates that the cone effectively accelerates the water flow and guides the resulting high velocity flow toward the blades.

• (3) The difference between the pressures on the front and rear of the blade for the rotor with the cone of $d/$ D = 0.375 is larger than that for the rotor without the cone. This pressure difference increases the torque acting on the blade; accordingly, the rotational speed and output of the rotor increase.

• (4) The cone-equipped rotor facilitates the measurement of the flowrate because the relationship between the flowrate and the rotational speed of the rotor is expressed by a linear function.