Development and performance testing of a new in-pipe hydropower prototype towards Technology Readiness Level (TRL) 6

Abstract

A new in-pipe hydropower prototype having a novel internal flow separator, denominated as P20, was developed and tested to achieve Technology Readiness Level (TRL) 6. The P20 separates water into main channel where the water flow is uninterrupted and two power generation (PG) purpose channels with nozzle and turbine system, respectively, at each side. The P20 also has valves for turbines maintenance and service work purposes at the PG channels’ inlets and outlets. From the performance test, 28.8% of pressure loss is created at the maximum flow condition (0.076 m³s⁻¹ flow rate that produced 1.25 bar of stagnation pressure). At this condition, each turbine produced mechanical power of 55 W with 35.9% of turbine efficiency. The highest turbine efficiency recorded in this study was about 42% obtained at a flow rate 59% of the maximum condition, where less water filled the turbine casing as compared to the maximum flow condition. PG channel size also affects the turbine efficiency. Overall, the P20 has been functioning according to the design that allows the majority of water to flow uninterruptedly while concurrently generating in-pipe hydropower.

Keywords:

• Hydropower
• in-pipe hydropower
• technology readiness level (TRL) 6
• turbine efficiency
• performance test

Review Editor:

• D T Pham, University of Birmingham, United Kingdom

Subjects:

• Mechanical Engineering
• Mechanical Engineering Design
• Power & Energy

Introduction

Nowadays, hydropower has become the third biggest source in electricity generation with around 62,500 plants operating around the world where 1397 GW of global energy capacity was recorded in 2022 (Slater-Thompson & Johnson, 2016; Zarf et al., 2019; IHA, 2023). Specifically, the Three Gorges Dam in China is the largest hydropower plant (LHP) that produced 22.6 GW of electricity (Kumar, 2021; Zhou et al., 2021). Although these LHPs are still very important in powering the world, there are also efforts to generate hydropower from a more environmental-friendly and economical way through excess pressure in water pipelines used to transfer drinking water from one point to another, called in-pipe or in-line hydropower (also known as in-conduit or in-situ hydropower) (Ma et al., 2018; Abdullah et al. 2021; Titus & Ayalur, 2019). A report released by the United State (US) Department of Energy’s Oak Ridge National Laboratory looked at water conduits across the nation that could be sources of hydropower, including agricultural canals and ditches, municipal and industrial water-supply pipelines, and wastewater-discharge systems all places where water flows. It concluded that an extra 1.41 GW of hydropower could be captured enough to power more than a million homes adding to the 530 MW of hydropower capacity in conduit projects currently in the US (Kornelis, 2023). In the European Union, an assessment has been performed on in-pipe hydropower, finding 3 TWh/y of potential energy generation (Quaranta et al., 2022).

The concept of the in-pipe Hydropower System (IPHS) itself is simple, where the excess pressure in the pipeline drives the turbine that is connected to an electric generator and produce energy (Hussein and Farouk, 2019; Titus & Ayalur, 2019; Jiyun et al., 2017). In 1985, Boulder in Colorado installed eight small scale hydroelectric plants in its municipal water system, which provided roughly 11% of the city’s electricity (Vella, 2013). A total of 40.8 GWh of electricity was generated from those plants in 2016, which is equivalent to the power generated from 20,399 tons of coal (Vella, 2013; Segerstrom, 2018; Gesner, n.d).

LucidPipes, Soar Energy and Rentricity are among the IPHS developers in the current market and operate widely around the United States and Canada. LucidPipes has been lightning 150 houses by 1.1 GWh produced annually in Portland (Soar, n.d; Schlabach, 2013). Soar Energy has provided electricity for 30 houses in Oregon with 32 kW of energy capacity and generated annual revenue of $20,000 (Power Technology, 2018; Soar, n.d). Rentricity also installed a system in the drinking water distribution pipeline (Hiyate, 2020; Zammataro et al., 2017). Recently, a new technology called InPipe Energy system has been installed in Hillsboro, Oregon. The system generates electricity of 185 − 200 MWh per year by a bypass from the existing pressure relief valve (Directors Hydro Review Content 2020; Water World, 2020; Semler, n.d). In Europe, Easy Hydro is one of companies involves in the implementation of IPHS (Kougias et al., 2019; Team, 2023). In Japan, Daikin Kogyo has also actively implemented and promoted in-pipe technology using its own system (Daikin, 2017).

Besides the implementation in the drinking water pipelines, the IPHS also can be implemented in other sectors, such as agriculture sector like fish farm at water discharge in-out (reservoir-fish farm) for water circulation and water waste utility sector at the treated water discharge to river (Rakibuzzaman et al., 2021; Bousquet et al., 2017; Titus & Ayalur, 2019).

However, the aforementioned systems seem to have certain technical and operational weaknesses. For example, LucidPipes constructed its specially designed turbine inside the main pipe to tap the kinetic energy. While energy could be generated, the water flow must be stopped to allow for maintenance and service work to be done. This water flow interruption surely will affect the consumers and is something that the water operators want to avoid. Rentricity, InPipe Energy and Daikin Kogyo IPHSs tapped the excess pressure from a bypass constructed parallel to the main pipe line (Soar, n.d; Daikin, 2017; Semler, n.d). This way, water flow will not be interrupted during the maintenance and service work. However, a significant amount of pressure loss will occur at the bypass line, reducing the power generation (PG) amount. Additionally, extra space and cost are needed to construct the bypass line.

Recently, Abdullah et al. (2021), proposed a new design of IPHS – a vertical axis parallel turbines system. His team proposed a unique system of water separation in the pipe. The water flow is divided into the main water channel and two PG channels with a turbine mounted at each side of the pipe. Water flow in the PG channels is controlled by valves at the inlet and outlet of the PG channels. Thus, water can still flow uninterrupted through the main water channel while the PG channel is closed for maintenance and service work. The turbine diameter also can be made bigger than the pipe diameter to produce higher torque and eventually power. To prove his concept, a prototype has been developed based on a 5-inch pipe, with a design that would allow 10% of the water in the pipe to flow to each PG channel (Abdullah et al. 2021). While the design concept was proven to a certain extent, certain key parameters such as the flow property and pressure condition at the PG channel were not able to be measured. As such, the turbine efficiency was not able to be determined. The workability of the prototype also was not able to be confirmed because the valves at the PG’s inlet and outlet were not there. All of these were due to the difficulties in prototype fabrication method then.

The prototype of a vertical axis parallel turbines system by Abdullah et al. (2021) was developed mainly to prove the conceptual design. It did not exactly take into consideration the practicality and economical aspect of the fabrication method. The prototype was built mainly from aluminium pipe and plate and joined together through welding. The welding was not able to perfectly join the valves to the body. It lacked fabrication preciseness especially when involving thin aluminium plates. Thus, the workability of the valves that control the water flow in the PG channels were not able to be tested.

In this continuation work, a complete prototype of the IPHS proposed earlier by Abdullah et al. (2021) was developed with the objectives to measure the prototype’s PG channels flow property and pressure condition that would eventually enable the prototype’s turbine efficiency to be determined. The complete prototype also would enable for the workability of the prototype to be evaluated and confirmed. The aim of this work is to conduct testing and evaluation towards achieving the Technology Readiness Level (TRL) 6. TRL 6 is where the prototype or model is demonstrated in the relevant environment (Tzinis, 2012; Engel et al., 2012).

The IPHS here was developed using a steel casting method where it offers reliability, seamless design and is conventionally more practical (Design Engineering Product n.d; Kinematics General 2020). Subsequently, a more complete performance testing of the system was able to be conducted. The successfulness of the study will lead to the implementation of the IPHS in the actual environment or higher TRLs.

Design concept of the in-pipe Hydropower System – P20

As explained earlier, the prototype was developed based from the design by Abdullah et al. (2021), where each of the PG channel at both sides of the prototype is estimated to receive up to 10% of the water flow – in total 20% of the whole water flow. The prototype here is denominated as P20 and shown schematically in Figure 1(a). The design follows the American National Standards Institute (ANSI) Schedule 40 requirement (ToolBox Engineering 2003; Savovic, 2007). The whole P20 body is divided into three main portions – inlet body, main body and outlet body – joined by flanges. The key parts are labelled and listed in Figure 1(a). For pressure measurement at locations that are considered critical, a series of pressure sensor points are prepared as shown in Figure 1(b). The designs of the turbine and nozzle are shown in Figure 2. Here, the turbine disc and 12 blades are casted as a unit. The nozzle is made of 3D-printed polylactic acid (PLA) and subsequently slotted into the PG channel. While there were no physical gate valves in Abdullah’s earlier prototype, there are four here, two at each PG channels at the inlet and outlet points.

Figure 1. P20 Prototype image; (a) 3D view and (b) top view with pressure point (Pi, P1 – P6, Po); T1: Turbine 1, T2: Turbine 2; scale 1:8.

Figure 2. Schematic diagram (dimension in mm); (a) nozzle and (b) turbine.

Methodology

Material

Cast-iron or flake graphite grey cast iron (FC 200/ASTM class 30) was selected for fabrication of the main body and turbine casing-blades for its high tensile strength, high hardness, low production cost and good machinability. It also has excellent high resistance to deformation and oxidation. Low carbon steel (EN 9/AISI 1015) is used for the shaft due to its excellent surface hardness and wear resistance. Meanwhile, for all valves part’s component, the casing is made of mild steel while the knife is made of brass. As explained earlier, the nozzle is built of 3D-printed PLA. In the actual environment, a steel-based material will be used. The used materials and properties are summarised as in Table 1. The total of the P20 is about 70 kg.

Table 1. Material selection for P20.

Part	Material	Specific weight (kg/m^3)	Tensile strength (MPa)	Hardness (Hv)	Overall weight (kg/unit)
Main, Inlet and Outlet body	Cast-iron	7120	214	220	63.0
Turbine casing-blade	Cast-iron	7120	214	220	0.90
Shaft	Low carbon steel	7760	420	131	0.16
Valve Casing	Mild steel	7760	500	149	0.45
Valve Knife	Brass	8480	21–1030	80 − 140	0.22
Nozzle	PLA	1240	19 − 54	20	0.06

Testing and evaluations

Testing was conducted based on the American Society of Mechanical Engineers (ASME) ‘Performance Test Code 18 (PTC 18): Hydraulic Turbines and Pump Turbines’ concept (ASME, 2021). A reticulation test rig was constructed to simulate the actual flow pattern of the actual environment. Figure 3 shows the schematic diagram and the actual image of the test rig. Water that is stored in an 8 m³ concrete reservoir is pumped out using a 7.5 kW rating motor pump through an 8-inch pipe (pump inlet) and 5-inch pipe (pump outlet) before entering the P20 prototype that is mounted to the test rig pipe using flanges. After the water flows out from the P20, it will flow back into the reservoir. In order to measure the stagnation pressure P_st (pressure when the flow of the water is totally restricted), pipe was fully closed at a point 2 (X-sign in Figure 3(a)) before the prototype. There are two pressure sensors fixed at the test rig before and after the P20. The flow rate is controlled by a variable speed drive (VSD). Flow is measured at 1.4 m before P20. The overall length of the piping system in the test rig is about 15.5 m.

Figure 3. Diagram of test rig; (a) schematic drawing and (b) actual image.

Tests under controlled environment were conducted first at a condition where the P20 was not mounted on the test rig so the water was able to flow without restriction (fully open condition). The flow rates were varied by controlling the VSD – Siemens Sinamics V20 with efficiency factor 98%. The respective flow rates at point Q_in were measured by the ultrasonic flow meter – GUF100 with accuracy ± 1%. In the second condition, the main pipe was fully closed (at a point before the prototype – refer to Figure 3) where the water was fully restricted to flow. Pressure at different flow rate was measured at P_i and was the pressure was designated as stagnation pressure P_st (bar).

The hydraulic power ($W_{\mathit{\text{Hy}}}$) generated from the water flow can be calculated using Equation (1)(1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) : (1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) where $\rho$ is water density (1000 kgm⁻³), $g$ is gravity acceleration (9.81 ms⁻²), $Q$ is flow rate (m³s⁻¹) and $H$ is meter head (m). The hydraulic power at respective flow rate was obtained by replacing the $H$ in Equation (1)(1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) with P_st. The pressure unit here was converted from the unit bar where 1 bar is equal to 10 m.

For the performance test, it was carried out firstly with P20 mounted to the test rig with all the valves at the PG channels opened. Similar to the earlier controlled environment condition, pressure under different flow rate condition was measured at point P_i. The pressure here is designated as P₂₀. Concurrently, pressures at the inlet and outlet of both PG channels, turbine rotational per minute/speed (RPM), and torques of the turbines were measured (Nm). Since it is not feasible to directly measure the flow rate in the PG channel using a flow meter, it is determined theoretically and will be explained in the later section. The mechanical power ($W_{\mathit{\text{Mech}}}$) generated by the turbine, was calculated based on the RPM values of the turbines and the torque using the following equation: (2) $${\mathrm{ }W}_{\mathit{\text{Mech}}}=\frac{2\pi \tau N}{60}$$(2) where is $\tau$ torque (Nm) and $N$ is turbine RPM. The turbines RPM was measured by Dynatool SM62336E Digital Tachometer with precision of ± (0.05% + 1 digit), while digital torque wrench MXITA (0.3–30 Nm) with accuracy: ± 1% (clockwise), ± 2% (counter clockwise) is used to measure the torques of the turbines. The turbine mechanical power is compared with the hydraulic power (theoretically determined) to obtain the turbine efficiency. The turbine torque and RPM values were measured based on the average reading of the digital torque wrench and tachometer for 30 s continuously.

Since the flow rate at the PG channels cannot be directly measured using a flow meter, the flow rate is measured theoretically can be calculated using the following Bernoulli’s Equation (3)(3) $${\mathrm{ }P}_{1\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_1^2+\rho g{H}_1=P_{2\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_2^{2\mathrm{ }}+\rho g{H}_2$$(3) and Continuity Equation (4)(4) $${\mathrm{ }Q}_1=Q_2;A_1{v}_{1\mathrm{ }}=A_{2\mathrm{ }}{v}_2$$(4) : (3) $${\mathrm{ }P}_{1\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_1^2+\rho g{H}_1=P_{2\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_2^{2\mathrm{ }}+\rho g{H}_2$$(3) (4) $${\mathrm{ }Q}_1=Q_2;A_1{v}_{1\mathrm{ }}=A_{2\mathrm{ }}{v}_2$$(4) where $P_{1\mathrm{ }}$ and $P_{2\mathrm{ }}$ are the pressure in the PG channel (Pa, at P₁ point) and after nozzle (Pa), $\rho$ is water density (1000 kgm⁻³), $g$ is gravitational acceleration (9.81 ms⁻²), $Q_1$ and $Q_2$ are the flow rate in the PG channel and nozzle (m³s⁻¹), $A_1$ (0.001134 m²) and $A_2$ (0.000284 m²) are the surface area of the PG channel and nozzle, $v_{1\mathrm{ }}$ and $v_{2\mathrm{ }}$ are the water’s velocity in PG and after nozzle (ms⁻¹).

Water velocity at nozzle and turbine linear velocity are used to determine tip speed ratio as follows: (5) $$\text{Water velocity a nozzle},v_W=\frac{\text{Flow rate in PG channel}}{\text{Nozzle surface area}}$$(5) (6) $$\text{Turbine linear velocity},v_T=\frac{2\pi \mathit{\text{rN}}}{60}$$(6) (7) $$\text{Tip speed ratio},\lambda =\frac{v_T}{v_W}$$(7) where $r$ is radius of turbine (m) and $N$ is turbine rotational speed (RPM).

Hence, turbine efficiency can be determined by dividing Equation (2)(2) $${\mathrm{ }W}_{\mathit{\text{Mech}}}=\frac{2\pi \tau N}{60}$$(2) with Equation (1)(1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) as follows: (8) $$\text{Turbine Efficiency}=\frac{W_{\mathit{\text{Mech}}}}{W_{\mathit{\text{Hy}}}}\times 100\mathrm{\%}$$(8)

Accordingly, tests were conducted with all the valves closed. These were carried out to appraise the valves functionality and workability. The flow of development and performance test of the P20 can be summarised in Figure 4.

Figure 4. Flow chart of development of P20.

Results and discussions

P20 prototype outcome

The P20 prototype actual image mounted on the test rig is shown in Figure 5. Based on the flowing direction, the inlet, main and outlet body of the P20 are clearly seen together with the pressure points. Figure 6 shows the comparison of the main body cross section area between the original drawing and the actual prototype. From both figures, the P20 has been successfully fabricated almost according to the original design. There are some areas at the valve region where small modifications had to be made during fabrication process, resulting in slightly reduced cross-section area of the PG channel (around 10% from the original drawing).

Figure 5. P20 mounted on the test rig with pressure points (P_i, P₁–P₆ and P_o).

Figure 6. Cross section of P20 main body; (a) schematic drawing and (b) actual image.

P20 performance testing

Figure 7 shows the relationship between the stagnation pressure (P_st) and flow rate (Q_in) when the pipe was fully closed. The P_st gradually increases with flow rate until reaches 1.25 bar at the maximum flow rate of 0.076 m³s⁻¹. Figure 8 shows the hydraulic power ($W_{\mathit{\text{Hy}}}$) at different flow rates. The hydraulic power also increases with flow rate until it reaches a power of 9.32 kW at the maximum flow rate. From Figure 8, the hydraulic power increases proportionally (not linear) with the flow rate.

Figure 7. Relationship between stagnation pressure and flow rate.

Figure 8. Relationship between hydraulic power and flow rate (from Equation (1)(1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) ).

Figure 9 shows the relationship between the pressure at point P_i (P₂₀) and flow rate when the P20 was mounted on the test rig and water was pumped into the system. The pressure gradually increases with the flow rate and reaches 0.36 bar at the maximum flow rate. The P₂₀ values from the figure were compared with the P_st values obtained earlier in Figure 7 and presented in Figure 10. From the comparison, it is clear that by the installation of P20 a pressure difference was observed. Basically, the water was able to flow into the P20, but it flowed with some restrictions or pressure loss. Figure 11 shows this pressure loss, which is the ratio in (%) between the P₂₀ and P_st, relationship with the flow rate. From Figure 11, at the lowest flow rate, there was almost no difference in both pressures. As the flow rate further increases, a significant drop in pressure was observed and eventually settled at pressure loss of around 28.8% at the maximum flow rate. In another word, at the maximum flow rate condition the P20 absorbs around 28.8% of the water pressure.

Figure 9. Relationship between pressure (P₂₀) at P_i and flow rate.

Figure 10. Relationship between pressure (P_st and P₂₀) and flow rate.

Figure 11. Relationship between pressure loss and flow rate.

Figure 12 shows the relationship between the pressures recorded at PG channels’ inlets (P₁ and P₃) and at inlet body main channel (P₂) with the flow rate. The pressure at these three points increases with the flow rate although at different trends. Pressure of 0.52 ± 0.02 bar, 0.18 ± 0.01 bar and 0.54 ± 0.02 bar are recorded, respectively, for pressure at P₁, P₂ and P₃ at the maximum flow rate. Pressure at P₁ and P3 are almost identical, which indicates that the condition in both PG channels is almost the same. Pressure at P₂ is much lower, indicating that the flow was less restricted unrestricted. The relatively high pressure at P₁ and P₃ is basically because the water flows in PG channels are restricted by the nozzles ahead. Pressure at the other points, P₄, P₅, P₆ and P_o, are almost equal to zero at all ranges of flow rate.

Figure 12. Relationship between pressure and flow rate at prototype when valves are opened.

Mechanical power conversion

Figure 13 shows the relationship between the turbines RPM at each PG channel and the flow rate. For both turbines, the RPM increases with flow rate, and show almost identical speed. The RPM values are in the range of 145–660. While the pressures at both PG channels’ inlets are almost identical (Figure 12), the RPM results here suggest that both turbines were accurately fabricated according to the design.

Figure 13. Relationship between turbine rotational speed and flow rate.

The relationship between turbines torque with turbines RPM is shown in Figure 14. A torque of 0.2 Nm is exerted by the turbines at 356.7 ± 3.5 RPM. Then, the turbine torque increases up to the highest value of 0.8 Nm at 660.0 ± 5.0 RPM.

Figure 14. Relationship between torque and turbine rotational speed.

Based from the turbines RPM and torque values, the turbines mechanical power ($W_{\mathit{\text{Mech}}}$) can be determined using Equation (2)(2) $${\mathrm{ }W}_{\mathit{\text{Mech}}}=\frac{2\pi \tau N}{60}$$(2) . Figure 15 shows the relationship between the mechanical power ($W_{\mathit{\text{Mech}}}$) of both T₁ and T₂ turbines with the flow rate. The $W_{\mathit{\text{Mech}}}$ increases obviously after the flow rate of > 0.04 m³s⁻¹. At the maximum flow rate, each turbine produces around 55 W of power. Collectively, both turbines produce a total $W_{\mathit{\text{Mech}}}$ of 110 W at full flow.

Figure 15. Relationship between mechanical power and flow rate for each turbine.

Turbine efficiency

In order to determine the turbine efficiency, firstly the flow properties (flow rate in the PG channel and velocity at the nozzle area) have to be obtained. Note that the pressures at the PG channels, P₁ and P₃ are almost identical at the maximum flow rate – 0.52 and 0.54 bar (Figure 12) – while the diameter of the PG channel is 38 mm and nozzle diameter is 19 mm (Figure 2(a)). From this, the surface area of the PG channel and nozzle can be calculated – 1134 mm² and 284 mm², respectively, for the PG channel and nozzle. Since the pressure after the nozzle is expected to be very low – assumed to equal zero (highest velocity) – the flow rate in the PG channel theoretically can be calculated using Equations (3)(3) $${\mathrm{ }P}_{1\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_1^2+\rho g{H}_1=P_{2\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_2^{2\mathrm{ }}+\rho g{H}_2$$(3) and (4)(4) $${\mathrm{ }Q}_1=Q_2;A_1{v}_{1\mathrm{ }}=A_{2\mathrm{ }}{v}_2$$(4) . Since $H_{1\mathrm{ }}$ and $H_{2\mathrm{ }}$ are similar, the equation can be simplified as follows: (9) $${\mathrm{ }P}_{1\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_1^2=P_{2\mathrm{ }}+\frac{1}{2}\mathrm{ }\rho v_2^{2\mathrm{ }}$$(9) where $P_{1\mathrm{ }}$ and $P_{2\mathrm{ }}$ are the pressure in the PG channel (52,000 Pa, at P₁ point) and after nozzle (0 Pa) and $\rho$ (1000 kgm⁻³). Therefore, the flow rate (Q) in each PG channel is calculated to be about 0.003 m³s⁻¹ – around 3.9% of the 0.076 m³s⁻¹ flow rate. Using the pressure of 0.52 bar at P₁ point and flow rate of 0.003 m³s⁻¹, the hydraulic power in the PG channel can then be derived using Equation (1)(1) $${\mathrm{ }W}_{\mathit{\text{Hy}}}=\rho \mathit{\text{gQH}}\mathrm{ }\left(\mathrm{W}\right)$$(1) , resulting a power value of 153 W. Since the mechanical power generated from each turbine was 55 W, the more accurate turbine efficiency could be obtained by using the same approach earlier (Equation (8)(8) $$\text{Turbine Efficiency}=\frac{W_{\mathit{\text{Mech}}}}{W_{\mathit{\text{Hy}}}}\times 100\mathrm{\%}$$(8) ) as follows: (10) $$\text{Turbine efficiency}=\frac{55}{153}\times 100\mathrm{\%}=35.9\mathrm{\%}$$(10)

The hydraulic power of 153 W obtained here was determined theoretically without considering the friction between the water and PG channel wall. The turbine efficiency would be slightly higher if the friction effect is considered. While only the turbine efficiency at the maximum flow rate condition was specifically derived here, the overall flow and turbine properties are displayed in Table 2 and Figure 16. In Table 2, the water velocity at nozzle ($v_W$), turbine linear speed ($v_T$), and tip speed ratio ($\lambda$), are also included. The PG channel inlet pressure values are also included. From Figure 16, although the turbine efficiency values after the flow rate of 0.041 m³s⁻¹ seem to show a constant trend, the highest turbine efficiency of 42% was obtained at the flow rate of 0.045 m³s⁻¹. Figure 17 shows the relationship between turbine efficiency and RPM. The curve pattern is similar to the one in Figure 16. Figure 18 shows the relationship between turbine efficiency and dimensionless flow rate (the flow rate is normalised with respect to the flow rate at maximum efficiency). The relationships between mechanical power and turbine efficiency with tip speed ratio are shown, respectively, in Figures 19 and 20. Although the absolute power increases with the flow rate, from these figures, the highest efficiency of the P20 turbines is obtained at about 59% of the maximum flow rate (0.045 m³s⁻¹/0.076 m³s⁻¹). The flow rate is directly proportional with the water-velocity at the nozzle. The higher the flow rate the higher the velocity at the nozzle. When the high-velocity water hits the turbines, the water would splash onto the turbine casing wall, filling the turbine casing with water and affecting the turbine’s performance. This can be shown in Figure 21 (picture taken from the earlier prototype of Abdullah et al. (2021)). This is the reason why the turbine efficiency at the maximum flow rate was lower. The efficiency of turbines may vary, depending on parameters, such as flow rate, pressure, material and pipe diameter (Patel & Pakale, 2015). The PG channel and nozzle diameter here are relatively small. While the turbine efficiency obtained here is comparable to a case study that used Savonius turbine (Payambarpour et al., 2019), the turbine efficiency is lower as compared to LucidPipe and Pump as turbine (PAT) systems elsewhere (Schlabach, 2013; Rakibuzzaman et al., 2019). However, since the current P20 PG channels are relatively much smaller as compared with the said LucidPipe and PAT systems, with better design especially a prototype with bigger PG channel and nozzle diameter, a much better efficiency can be achieved. Studies on Pelton turbine have shown that at turbine efficiency of more than 90%, the tip speed ratio varies between 0.46 and 0.48 (Mulders et al., 2019; Shah et al., 2021). Since there is no such condition where the water will fill the Pelton turbine’s casing as experienced in the P20 turbine, to directly compare its performance with the P20 turbine is difficult. Further study with variations in turbine blade numbers and nozzle diameter is necessary to better understand the P20 turbine efficiency relationship with the tip speed ratio.

Figure 16. Relationship between turbine efficiency and flow rate.

Figure 17. Relationship between turbine efficiency and turbine rotational speed.

Figure 18. Relationship between turbine efficiency and normalised flow rate.

Figure 19. Relationship between mechanical power and tip speed ratio.

Figure 20. Relationship between turbine efficiency and tip speed ratio.

Figure 21. Water splash phenomena in turbine casing.

Table 2. Flow and turbine properties.

Flow rate, Qin (m^3s^−1)	Water velocity after nozzle, $v_W$ (ms^−1)	Turbine rotational speed (RPM)	Turbine speed, $v_T$ (ms^−1)
0.021	0.894	149.8	0.510
0.029	1.033	251.0	0.854
0.037	1.366	324.6	1.105
0.041	1.461	356.7	1.214
0.045	1.549	388.7	1.323
0.053	2.000	454.2	1.546
0.061	2.129	524.8	1.786
0.069	2.309	590.5	2.010
0.076	2.633	660.0	2.246
Hydraulic power, $W_{\mathit{\text{Hy}}}\mathrm{ }$ (W)	Mechanical power, $W_{\mathit{\text{Mech}}}$ (W)	Turbine efficiency (%)	Tip speed ratio, $\lambda$ ($v_T$/$v_W$)
4.88	0.00	0.00	0.628
8.99	0.00	0.00	0.544
20.07	0.00	0.00	0.411
25.42	7.47	29.39	0.385
38.36	16.28	42.44	0.363
61.61	23.78	38.60	0.281
80.37	32.97	41.03	0.264
106.95	43.29	40.47	0.243
153.14	55.29	36.11	0.213

Functionality testing

Figure 22 shows the relationship between pressure and flow rate when all valves at the PG channels are closed. In general, all of the pressures increase with the flow rate. P₁ and P₃ show almost identical values. As compared with the earlier valves open condition, the pressure here is slightly higher. Pressure for P₂ is almost similar to the all valves opened condition, suggesting smooth water flow. Both turbines are not rotating as expected. Pressures at P₄, P₅, P₆ and P_o are almost equal to zero, similar to the all valves opened condition. All these had been confirming the functionality of the valves and the workability of the P20 prototype.

Figure 22. Relationship between pressure and flow rates.

From the above, the performance of the P20 prototype was evaluated and at the same time its workability was confirmed. At its current condition the P20 has achieved the TRL 6 and is ready to be tested at the actual environment. For the future plan, a different set of PG channels and turbines size will be developed with the anticipation of higher PG and turbine efficiency. This is very important to make the development of the IPHS not only technically but also financially viable.

Conclusions

In this study, a new in-pipe hydropower prototype denominated P20 was developed and evaluated based on its performance and functionality. From the study, the following conclusions were obtained:

1. At the maximum flow rate of 0.076 m³s⁻¹, each of the PG channels at both sides of the P20 allowed 7.8% of the water to flow through them and at the same time the P20 absorbed around 28.8% of the 1.25 bar stagnation pressure of the flow.

2. At the maximum flow rate, both turbines showed almost identical performance, generating a total mechanical power of 0.11 kW.

3. The highest turbine efficiency of about 42% was obtained at the flow rate of 0.045 m³s⁻¹ at tip speed ratio of 0.36.

4. All the valves at the inlets and outlets of the PG channels functioned perfectly not to let water flowing through them to allow for maintenance work of the turbines’ system when necessary.

5. While not compromising the primary objective of the water pipeline, which is to deliver water at the required flow rate and pressure, it is recommended to conduct a rigorous study aimed at increasing the efficiency of the prototype turbine. This study would involve improvising its design, particularly its nozzle and turbine, with the additional aim of gaining a better understanding of the relationship between turbine efficiency and tip speed ratio.

6. With significant design improvements resulting in much higher efficiency, it is proposed that the prototype be implemented in a real-world environment, meeting the requirements of TRL 6 or higher.

Patent

This prototype has been patented with Intellectual Property Corporation of Malaysia (MyIPO), pattern number: PI2019007588.

Acknowledgements

We would like to express our gratitude to Malaysia Ministry of Energy and Natural Resources for the financial support under grant Akaun Amanah Industri Bekalan Elektrik (AAIBE) and Department Mechanical Engineering, Universiti Malaya for facilities and support during experiment and fabrication processes.

Disclosure statement

We, the authors declare that we have no conflict of interest for this manuscript.

Additional information

Funding

This work was supported by Malaysia Ministry of Energy and Natural Resources under grant Akaun Amanah Industri Bekalan Elektrik (AAIBE) [grant number: GA012-2019]