Journal of Mechanical Science and Technology 31 (11) (2017) 5089~5097 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online)

DOI 10.1007/s12206-017-1002-7

Hydraulic design and performance analysis on a small pump-turbine system for ocean renewable energy storage system† Patrick M. Singh1, Zhenmu Chen1 and Young-Do Choi2,* 1

Graduate School, Department of Mechanical Engineering, Mokpo National University, Muan-gun 58554, Korea Department of Mechanical Engineering, Institute of New and Renewable Energy Technology Research, Mokpo National University, Muan-gun 58554, Korea

2

(Manuscript Received April 6, 2017; Revised July 19, 2017; Accepted August 21, 2017) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Korea has a myriad of islands in the south western coast that extensively rely on diesel generators for power production, which increase cost and environment pollution. The small hydro pump-turbine system for ocean renewable energy storage system is a kind of hybrid system that can reduce the usage of diesel generators and help to contribute to the environment in a positive manner by helping to reduce carbon emissions. The study focuses on initial hydraulic design and numerical analysis of a 30 kW-class pump-turbine system for energy independent islands in South Korea. The purpose of the study is to propose an ocean renewable energy storage system using a small pump-turbine system working with seawater. A 30 kW-class pump-turbine does not require a large head; approximately 30 m is sufficient for the design and application. Several other renewable energy systems like wind turbines, tidal turbines, wave energy converters and solar energy could be used to make a hybrid system with pump-turbine. The initial design achieved more than 85 % efficiency in both pump and turbine modes. However, further optimizations of the impeller blade shape and number of guide vane and stay vanes could improve the overall efficiency of the system. Keywords: Small hydro pump-turbine; Ocean renewable energy storage; Hydraulic design; Performance ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Renewable energy and ESS (Energy storage systems) can be combined to effectively replace or reduce diesel power generation in small outer islands that are far from the main land [1]. It can be a very promising new business that can be advanced into both domestic and overseas markets, especially the island nations in Asia and the South Pacific Ocean can benefit from this type of power generation systems. There are around 50 thousand islands in the world with a total area of over one sixth of global land area [2, 3], with the islands inhabiting more than 740 million people according to Geographic information system (GIS) analysis [4]. Korea’s Jeonnam province (Shinan-gun) alone consists of more than 1000 islands where 270 are inhabited islands, among which 107 islands are not connected to the electricity grid. These islands heavily rely on diesel power generation. Korea Electric Power Corporation (KEPCO) supplied diesel to 30 inhabited islands in the region in the year 2014 that accounts to ap*

Corresponding author. Tel.: +82 61 450 2419, Fax.: +82 61 452 6376 E-mail address: [email protected] This paper was presented at the ISFMFE 2016, LOTTE City Hotel, Jeju, Korea, October 18-22, 2016. Recommended by Guest Editor Hyung Hee Cho. © KSME & Springer 2017

proximately 13223 kL (136.3 billion KRW) and the numbers keep rising every year. The local government administration operating in this region also consumes diesel that accounts approximately 14000 kL (165 billion KRW) [5]. Although the islands face severe energy insecurity, renewable energy sources are abundantly available in the form of wind, solar, wave, biomass, and hydropower enabling clean renewable power conversion opportunities [6, 7]. Usually, each island is blessed with at least one renewable energy source. With the increasing demand for low-emission generation and rapid development of sustainable energy technologies, the utilization of renewable energy presents promising prospects for island power hybrids. It is quite feasible to eliminate fossil fuels with renewable energies in islands from economical and technical aspects [8, 9]. There are some energy independent islands in Korea, such as in Gasado and Sammado, with wind and solar hybrid systems that store excess electricity using batteries which were developed in October 2014. Other prospective islands include Geochado, Jodo, Sapsido and Geomundo [10]. Shin et al. conducted a study on the capacity design and operation planning of a hybrid PV-windbattery-diesel power generation system in the case of Deokjeok Island in Korea. The study was conducted for optimal design of Hybrid power generation system (HPS) in which

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diesel generator and renewable energy generators are incorporated to supply electricity to islands isolated from the national grid [11, 12]. Pumped storage hydro power is being developed rapidly over the years in the world. Pumped hydro storage (PHS), with large storage capacity, is the most common energy storage method at present. In 2013, global pumped storage capacity increased by 2 GW and the cumulative installed capacity reached about 140 GW [13]. PHS is also a solution to maintain a steady electricity supply for islands and improve the penetration level of renewable energy integrated into power grids. The power output of PHS stations can be regulated rapidly, flexibly and reliably to accommodate the volatile and stochastic power from other renewable energy. Thus, PHS can be employed for peak shaving and frequency regulation in islands to improve the utilization of renewable energy [2]. Fujihara et al. investigated on the development of pump turbine for seawater pumped storage power plant [14]. It is the first seawater pumped storage plant that was designed and constructed in Kunigami Village in Okinawa Prefecture, Japan, to execute verification tests for five years, especially in regards to the corrosive environment of the system. It is a 30 MW capacity system with heads of more than 150 m. However, this system is quite large in contrast to what the current study proposes for smaller islands in Korea. The current study focuses on an initial hydraulic design and numerical analysis of a 30 kW-class pump-turbine system for ocean renewable energy storage systems for islands in South Korea. A pump-turbine system can provide stable electricity to the grid in times when other renewables such as wind, solar or ocean energy converters are not operational. The pumped storage system has high efficiencies of more than 85 % and is more reliable in contrast to other renewable energies. The study proposes an ocean renewable energy storage system by using a small pump-turbine system that works with seawater for which performance prediction by numerical analysis is conducted. This system is a hybrid system that can contribute to the environment positively by helping to reduce the usage of diesel generators and carbon emissions. Moreover, a 30 kW-class pump-turbine can be developed with low heads of approximately 30 m. In the past five to ten years there has been several studies conducted on tidal current turbines and it is a feasible option to install these between islands where the current speed is economical. Several other renewable energy systems like wind turbines, tidal turbines, wave energy converters and solar energy could be used to make a hybrid system with pump-turbine.

2. Pump-turbine design and numerical methods 2.1 Ocean renewable energy storage system design An excellent example of micro grid system has been presented by Ishida et al. by constructing micro grid systems on remote islands that incorporate photovoltaic power and wind turbine renewable power together with storage batteries that

Table 1. Proposed micro grid system capacity. Energy resource

Capacity (kW)

Diesel generator (existing)

100~250

Batteries

10~30

Wind turbines

20

Tidal turbines

20

Wave energy converter

10

Photovoltaic (solar)

10~50

Pump-turbine (proposed)

30

Pump-turbine (potential) for larger islands

100~200

Fig. 1. Proposed pump-turbine hybrid system for ocean renewable energy storage system.

can be remotely monitored [15]. However, the current study proposes to include more renewable energy systems depending on whichever resource is available at a particular island. In Korea many islands are grouped close by and a standalone network grid could be constructed with several renewable energy converters incorporated together, such as wind, tidal current, wave and solar with pumped hydro storage system to provide a more stable grid as illustrated in Fig. 1. Table 1 summarizes the capacity of the proposed micro grid system for ocean renewable energy storage system. The upper dam or tank capacity can be calculated to approximately 5000 m3 by applying simple mathematics and the hydraulic power Eq. (1). This size is achieved assuming that the turbine mode will operate daily for eight hours and pumping mode will operate for twelve hours on average. The hydraulic power capacity can be calculated by Eq. (1): P = r gHQ × h

(1)

where P is the power, H is the total head and Q is the flow rate and η is the estimated efficiency of the pump-turbine system. By reverse calculation of flow rate from Eq. (1), the volume of the tank can be calculated by assigning a fixed operating time for the two modes of operation. 2.2 Impeller design and pump-turbine model The basic hydraulic design method discussed has been

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adopted from Ref. [16]. The specific speeds of the pump nsp and turbine nst can be calculated by Eqs. (2) and (3), respectively: n Q nsp = 0.75 HP

nst =

(2)

n P . H T1.25

(3)

The gradient of discharge variation to head variation gQ/H is the most important factor that affects the overall size of the pump-turbine. The peripheral speed u2 at the impeller exit at best efficiency point can be expressed by the following equation: æ 1 + 1 gQ/ H ö u2 =ç ÷ H P è 0.0765 ø

Table 2. Design specifications of pump-turbine. Parameters

Value

Total head H

PM: 34, TM: 30 m

Flow rate Q

PM: 0.11, TM: 0.125 m3/s

Rotational speed n

1800 min-1

Impeller inlet D1

0.20 m

Impeller exit D2

0.29 m

Impeller exit width Bg

0.035 m

Impeller blade number Z1

7

Guide/stay vane number Z2

20

0.5

.

(4)

From Fig. 2, the average value of gQ/H can be selected at nsp = 42 m-m3/s from the three curves shown. After obtaining the peripheral speed the diameter at the impeller exit, D2 can then be found by Eq. (5): D2 =

60 u2 × . p n

(5)

For the purpose of a rough estimation, the relation of cm1P/HP0.5 versus nsp is shown in Fig. 2. From this, the width Bg can be found by the following equation: Bg =

Q

p D2cm1P

(6)

.

From Eq. (6) we get the width Bg according to the diameter calculated earlier. Finally, a rough estimation of the impeller entrance diameter D1 can be determined by: æ 4Q D1 = çç è p cm1P

ö ÷÷ ø

0.5

.

(7)

As presented in Table 2, for this model there are 7 impeller blades, 20 stay vanes and 20 guide vanes. The rated power of the pump-turbine is 30 kW with flow rate of 0.125 m3/s and 30 m head in turbine mode. The flow rate is 0.11 m3/s and 34 m head in pump mode and rotational speed for both modes is 1800 min-1. A scaled up casing, stay vanes and guide vanes have been used from a 3 kW-class pump-turbine experiment facility available at Mokpo National University [17]. The pump-turbines three dimensional fluid domain is illustrated in Fig. 3(a). There are three designs of the impeller blade as shown in Figs. 3(b) and (c) that were studied in more detail to investigate the effect of different blade angles (loading) from Leading edge (LE) to the Trailing edge (TE) at the hub and

Fig. 2. Gradient of discharge variation to head variation of pumping performance versus specific speed; meridional velocity at the impeller exit of a Francis pump-turbine [16].

shroud sections. The meridional shape was fixed to a single shape and the dimensions of D1, D2 and Bg are also kept constant. The main difference that can be observed from the figure is that the length of the blade and curvature increases from design 1 to 3. Chen et al. demonstrated a similar method by investigating the blade loading on the Francis turbine model [18]. Fig. 4 illustrates the three different blade angle distributions for the three designs. The blade angles at the leading edges range from 12.5~15° for the shroud and 22.5~25° for the hub, whereas the blade angles at the trailing edges range from 17.5~25° for the shroud and 25~30° for the hub. The blade angles for design 1 are very similar to a centrifugal pump design. However, as the design has been modified to balance the performance of both modes, the blade angles have been reduced, especially near the trailing edge region. The current study has only investigated a linear distribution of the blade angles from the leading edge to the trailing edge. The blade angle distribution is contrary to a Francis turbine in the sense that it has a positive slope from the leading edge to the trailing edge, whereas the study conducted by Chen et al. showed a negative slope. 2.3 Numerical methods A similar numerical method from the previous study has been used for this study [17-21]. ANSYS CFX a commercial

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Table 3. Numerical methods and boundary conditions. Calculation type

Steady state

Turbulence model

SST

Mesh type

Hexahedral

Total element number

5.6×106

Rotor/stator interface

Frozen rotor/stator

Wall

No slip

Inlet (turbine/pump)

Total pressure/static pressure

Outlet (turbine/pump)

Static pressure/mass flow rate

(a) Blade angle distribution at hub

(b) Blade angle distribution at shroud Fig. 4. Blade angle distribution at hub and shroud.

(a) Full fluid domain

(a) Hexahedral full domain mesh (b) Impeller design 1 to 3

(b) Meshing of vanes with O-grid (Close up view) Fig. 5. Numerical mesh of fluid domain.

(c) Impeller blade design comparison Fig. 3. Pump-turbine fluid domain and impeller designs 1 to 3.

code is utilized for the numerical simulation [22]. Table 3 presents the numerical methods and boundary conditions for the hydrodynamic performance analysis and Fig. 5 shows the full fluid domain of the pump-turbine model for numerical analysis. The simulation does not consider leakage and fric-

tion losses. The hexahedral meshing with 5.5×106 nodes and elements were used. The Shear stress transport (SST) turbulence model is selected for all calculations in this study because it is generally used for rotating applications like pumps and turbines and shows relatively better convergence in contrast to the other models. The boundary condition in Turbine mode (TM) is set to total pressure at inlet and static pressure at outlet to maintain the design head, whereas the flow rate is controlled by the guide vane opening and this value is ob-

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(a) Pump mode

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(b) Turbine mode

Fig. 6. Comparison of performance curves of pump and turbine modes for all three impeller designs.

tained from the calculated result. For the Pump mode (PM) the static pressure boundary condition was applied at the inlet and mass flow rate was set for the outlet. Only a single guide vane opening was utilized for the pump mode calculations.

3. Results and discussion 3.1 Performance curves Fig. 6 presents the performance results of the pump-turbine comparing the head, power and efficiencies in the pump and turbine modes separately. The expected head of 30 m is achieved by all three designs in the pump mode. The head was set constant in the turbine mode while three guide vane open-

ings were investigated for low, medium (Design point: Blade angle 30°) and high flow rates. A single guide vane opening (Design point) was used for the pump mode analysis while the flow rate was changed to achieve the performance curve. The expected power of 30 kW is achieved by all three designs in the turbine mode. However, in pump mode designs 1 and 2 requires a higher power input, up to 40 kW, to achieve the required head and flow rate. Considering both modes, design 3 shows a balanced result. It requires lower input power in pump mode over a wide range of flow rate, and produces sufficient output power at a lower flow rate (flow similar to pump). Highest efficiency is achieved by design 3 with 85 % and 86 % in pump and turbine modes, respectively. The pump

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(a) Pump mode

(b) Turbine mode

Fig. 7. Pump-turbine head loss as percentage of efficiency at best efficiency point for both modes.

achieves best efficiency near the design point. The turbine also achieves best efficiency near expected flow rate. The hydraulic design matches well with the CFD results. 3.2 Head loss analysis The loss analysis is a good method for checking how each component affects the overall performance of the system in a significant way, more specifically in different operating modes the loss analysis shows quite different results. For the loss analysis, the equations are defined as following: H Loss =

Dptotal rg

H Loss:impeller ,TM

H Loss:impeller ,PM

æ tw ö Dptotal - ç ÷ èQ ø = rg æ tw ö ç ÷ - Dptotal Q ø =è rg

turbine mode. Additionally, in the pump mode a significant drop in head loss is observed in the stay vanes and guide vanes in the third design. However, a more detailed investigation on the blade loading could reduce the head loss of the impeller [18]. In the turbine mode the overall losses are low in contrast to the pump mode, which explains the difference of efficiencies for both the modes. Even though the losses on the draft tube are very low in pump mode, the large losses in the guide vanes, stay vanes and casing contribute to a larger loss in comparison to turbine mode. This is why there is a trade up and down relationship between each mode for a reversible pump-turbine system.

(8) 3.3 Internal flow analysis (9)

(10)

where Eq. (8) gives the total pressure loss for the casing, stay vanes, guide vanes and draft tube, Eqs. (9) and (10) gives the total pressure loss for the impeller in turbine and pump modes, respectively. Fig. 7 shows the loss analysis results for the pump and turbine modes at the best efficiency point for all the three impeller designs. The head losses are illustrated using the percentage loss in the efficiency of the pump-turbine. From this analysis it can be inferred that in both modes the impeller causes the largest head losses in contrast to the casing, stay vanes and guide vanes and draft tube. This is mainly because the impeller is a rotating component whereas the other components are stationary. However, in the pump mode the draft tube shows very small head losses in contrast to the casing, stay vanes and guide vanes. Comparing to the turbine mode the losses in draft tube is increases significantly making it very difficult to effectively reduce the loss by improving a specific component. Significant drop in head loss is observed in the third design for the

The separate components in the pump-turbine system were investigated qualitatively by studying the streamlines in the middle section of each component. The streamline distributions that show similarities between each design in the pump and turbine modes have been presented in Fig. 8. Design 1 results are shown on the left, while design 2 is in the middle and design 3 is shown on the right. From Figs. 8(a) and (b) the streamlines on the impeller suction and pressure sides are presented for the pump and turbine modes, respectively. It can be observed that there is very small variation on the flow pattern at the suction side but the flow in the pressure side improves from design 1 to design 3. Fig. 8(c) also shows improvement in recirculation flow from design 1 to design 3 in pump mode, especially near the tongue region. The most significant improvement is observed in the turbine mode at the draft tube. Large recirculation flows are observed in designs 1 and 2, however in design 3 the flow from runner exit is close to a 90° angle. This also gives a reason for the significant reduction in head loss in the draft tube for design 3. Furthermore, the tangential velocity and meridional velocity distributions were also investigated in the draft tube for the turbine modes of the three designs. The measuring location of the velocity distribution is illustrated in Fig. 8(d). This location is selected because it showed large recirculation flows. The velocity distributions in the draft tube are presented in Figs. 9 and 10, showing the comparison among

P. M. Singh et al. / Journal of Mechanical Science and Technology 31 (11) (2017) 5089~5097

Design 1

Design 2

Design 3

(a) Impeller blade in pump mode (Top: Pressure side, bottom: Suction side) Design 1

Design 2

Design 3

(b) Impeller blade in turbine mode (Top: Pressure side, bottom: Suction side) Design 1

Design 2

Design 3

(c) Casing, stay vanes and guide vanes in pump mode Design 1

Design 2

(d) Draft tube in turbine mode Fig. 8. Streamline distributions that show significant comparisons in each mode for all three designs.

Design 3

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both pump and turbine modes. Modifications to the impeller blade angle distribution also improved the flow distribution in the runner/impeller and draft tube passage, and helped reduce head loss and contributed to improved performance. Additional modifications and or optimizations are needed to improve the overall efficiency of the pump-turbine system [23, 24]. It can be proposed from this study that this kind of system would be beneficial to the energy independent island project in the near future. As the renewable energy sector tends to keep on growing, soon many renewable energy systems will be getting commercialized into hybrids.

Acknowledgment Fig. 9. Tangential velocity distribution in draft tube.

This work was supported by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy (No. 20163010060350). Moreover, the authors are very grateful to Professor Kazuyoshi Miyagawa of WASEDA University, Japan, for his valuable advices.

Nomenclature------------------------------------------------------------------------

Fig. 10. Meridional velocity distribution in draft tube.

the three designs for the tangential and meridional velocities. The tangential velocity remaining in the draft tube contributes to the kinetic energy loss, which gives a reason for the head loss discussed earlier. A value closer to zero is preferred for minimum losses in the draft tube but design 1 shows larger values of tangential velocity in contrast to designs 2 and 3. Moreover, a positive meridional velocity that indicates that the flow direction has changed opposite the actual flow and it gives a reason for large recirculation flows, especially in design 1. Larger negative values indicate smooth flow exiting the draft tube. It is observed that as the design was improved by modifying the blade angle, the meridional and tangential velocities showed significant improvements.

4. Conclusions The study proposes a hybrid system with a pump-turbine to increase the stability of the overall electrical grid system in small remote islands. The performance analysis of a 30 kWclass pump-turbine has shown satisfactory results for an initial hydraulic design with efficiency of 85 % in both modes. The initial design requirements match well with the numerical simulation results. Modifications to the impeller blade angle distribution contributed to improvement in performance for

cm1P g gQ/H HLoss HLossimpeller,TM HLossimpeller,PM HP HT nsp nst P u2 ω τ ∆ptotal

: Meridional velocity : Gravitational acceleration : Gradient of discharge variation to head variation : Pressure head loss in component : Pressure head loss in impeller in turbine mode : Pressure head loss in impeller in pump mode : Pump head : Turbine head : Specific speed of pump : Specific speed of turbine : Turbine rated power : Velocity at impeller exit : Angular speed : Torque : Total pressure difference

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Patrick Mark Singh received his B.E.Tech. degree from The University of the South Pacific and his M.S. degree from Mokpo National University, Korea. He is currently a doctorate candidate in the Graduate School, Department of Mechanical Engineering, Mokpo National University. His research interest includes fluid machinery and new & renewable energy. Zhenmu Chen received his B.E. degree from Wenzhou University, China, and his M.S. degree from Mokpo National University, Korea. He is currently a doctorate candidate in the Graduate School, Department of Mechanical Engineering, Mokpo National University. His research interest includes fluid machinery and new & renewable energy. Young-Do Choi received his B.S. and M.S. degrees from Korea Maritime University, and his Dr. Eng. from Yokohama National University, Japan. Since 2009, he has been a Professor at Department of Mechanical Engineering of Mokpo National University, Korea. His research interests include fluid machinery and new & renewable energies, such as ocean energy, wind power, hydro power.