Comparison of cylindrical and conical basins with optimum position of runner: Gravitational water vortex power plant
$Sagar Dhakal
a,d,n, Ashesh B. Timilsina
a, Rabin Dhakal
a, Dinesh Fuyal
a, Tri R. Bajracharya
a,d, Hari P. Pandit
b, Nagendra Amatya
c, Amrit M. Nakarmi
a,daDepartment of Mechanical Engineering, Central Campus, Institute of Engineering Tribhuvan University, Pulchowk, Lalitpur, Nepal
bDepartment of Civil Engineering, Central Campus, Institute of Engineering Tribhuvan University, Pulchowk, Lalitpur, Nepal
cScience and Humanities Department, Central Campus, Institute of Engineering Tribhuvan University, Pulchowk, Lalitpur, Nepal
dCenter for Energy Studies, Central Campus, Institute of Engineering Tribhuvan University, Pulchowk, Lalitpur, Nepal
a r t i c l e i n f o
Article history:
Received 9 July 2014 Received in revised form 9 March 2015
Accepted 3 April 2015
Keywords:
Basin
Gravitational vortex Runner position Velocity Output power
a b s t r a c t
Demand of energy is ever increasing, especially in developing countries. Renewable energy such as hydropower has become one of the most demanded sources of energy for its clean generation. Low head hydropower plant is demanded in area which cannot see grid extension due to difficult geographical terrain and other reasons. Gravitational water vortex power plant is one of such low head turbine in which the mechanical energy of free surfaceflowing water is converted to kinetic energy by tangentially passing the water to a basin, which forms a water vortex. This study is the analysis of different basin structures which has ability to form a gravitational vortex stream from low head, lowflow water streams with the optimum runner position in the basin to maximize the output power. The analysis wasfirst carried out by development of the model using CAD software, SolidWorks and it was simulated in commercial CFD code ANSYS Fluent for the measurement of velocity. Secondly, the result so obtained was experimentally verified by measuring the output power.
&2015 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . 662
2. Nepal and its perspective . . . 663
3. Study of past researches . . . 663
4. Model development and solution procedure . . . 664
4.1. CFD model and mesh for two basins . . . 665
4.2. Simulation result . . . 667
4.3. Analysis of simulation result . . . 667
5. Experimental setup and data collection: . . . 667
6. Cost differential of basins and operational challenges. . . 668
7. Conclusion . . . 668
Acknowledgment . . . 669
References . . . 669
1. Introduction
Water energy being a clean, cheap and environment friendly source of power generation is of great importance for sustainable future; being aware of this fact, still major of the hydro energy is under-utilized [8].There are mainly two approaches to harness Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2015.04.030 1364-0321/&2015 Elsevier Ltd. All rights reserved.
☆You tube video of project:https://www.youtube.com/watch?v=iMSTpjDIF5M.
nCorresponding author.
E-mail address:[email protected](S. Dhakal).
energy from water, namely, hydrostatic and hydrokinetic methods.
Hydrostatic approach is the conventional way of producing elec- tricity by storing water in reservoirs to create a pressure head and extracting the potential energy of water through suitable turbo- machinery [11]. In hydrokinetic approach, the kinetic energy inside theflowing water is directly converted into electricity by relatively small scale turbines without impoundment and with almost no head[10].
Gravitational water vortex turbine is an ultra-low head turbine which can operate in as low head range of 0.7–2 m with similar yield as conventional hydroelectric turbines used for production of renew- able energy characterized with positive environmental yield [24].
Austrian Engineer Franz Zotloterer invented this power plant while he was looking for an efficient way to aerate water. The gravitational vortex is a milestone in hydrodynamic development because in the past we needed energy to aerate water, but now this technique uses a water aeration process to produce electrical energy[21].
The water passes through a large, straight inlet through the channel and then passes tangentially into a round basin, which forms a powerful vortex; an exit hole is made at the bottom of the basin through which the vortexfinds its outlet[14]. The turbine does not work on pressure differential but on the dynamic force of the vortex; not only does this power plant produce a useful output of electricity, it also aerates the water in a gentle way[19]. Said aim is achieved by as hydroelectric power plant which supports the formation of a stable gravitational vortex which tends to be formed also in the upper reaches directly in front of the turbine inlet of conventional river stations as a lost vortex and is therefore prevented as much as possible there. The inventive hydroelectric plant, however, ensures that the necessary current-related condi- tions are fulfilled for reinforcing the rotational movement of the water, which is created when the waterflows off, in an unimpeded manner into a stable gravitational vortex without using pressure lines and directing devices. A turbine that rotates in a coaxial manner within the gravitational vortex and is impinged upon along the entire circumference thereof withdraws rotational energy from the gravitational vortex, which is converted into electric power in a generator[24].
In addition, gravitational vortex power plant is found to be advantageous due to the following properties of water vortex:
a. Increases the water surface area.
b. Maximizes the velocity offlow on the water surface area.
c. Disseminates homogenously contaminants in the water.
d. Increases the contact surface of the disseminate contaminants for microorganisms and water plants.
e. Aerates the water naturally, because of the high velocity of the flow on the water surface area.
f. Increase the heat of evaporation so water can reduce the temperature itself at rising temperatures in summer.
g. Concentrates dense water (water at 40C) in the ring shaped center to ensure the survival of microorganisms as long as possible[24].
h. The BOD removal efficiency of aerobic biological treatment processes depends on a number of factors including (but not limited to): influent BOD loading, F:M ratio, temperature, nutrient levels, and dissolved oxygen (DO) concentrations [17]. Through the creation of vortex dissolved oxygen concen- tration can be improved.
2. Nepal and its perspective
Nepal boasts snowy mountains (Himalayan range) in the North which acts as a perennial source for many free flowing rivers
establishing the country as second richest in water resources in the world after Brazil[2]. About 6000 rivers with total length of around 45,000 km and an annual discharge of 174 billion cubic meters are available in the nation[18].Nepal has about 83,000 MW of economically exploitable resources, but only 650 MW have been developed so far; about 63% of Nepalese households lack access to electricity and depend on oil-based or renewable energy alter- natives; the disparity in access is stark, with almost 90% of the urban population connected, but less than 30% of the rural population[3].
The majority of Nepal's rural populations have been meeting their energy needs (mainly for cooking and heating) by burning various forms of biomass (forest wood, crop residues and dried animal dung) in open hearths or in traditional stoves. In Nepal, the campaign of rural electrification started more than 40 years ago;
however, the provision of electricity to remote, rural communities is unrealistic and challenging [12]. The marginal cost of grid extension is greatly increased in rural areas by physical isolation, lower electricity loads, high upfront equipment costs, higher costs of supply and maintenance and low population density with scattered low-income consumers which results linking of rural areas to national electricity grid difficult and implausible[1,4,15].
Development and implications of low head turbines thus may be a good alternative to light up such areas.
3. Study of past researches
Mulligan and Casserly did their research project on“Design and optimization of a water vortex hydropower plant”carried out at the Institute of Technology, Sligo in Civil Engineering [13]. This research concludes that optimum vortex strength occurs within the range of orifice diameter to tank diameter ratios (d/D) of 14– 18% for low and high head sites, respectively. Thus, for cylindrical basin, to maximize the power output, the range of orifice diameter to basin diameter ratios lies within 14–18%.
Bajracharya and Chaulagai focused on developing innovative low head water turbine for free flowing streams suitable for micro-hydropower in Terai region of Nepal[5]. In this study, water vortex was created byflowing water through an open channel to a cylindrical structure having a bottom whole outlet. The research concluded that for afixed discharge condition, the height of basin, diameter and bottom exit hole arefixed, i.e., the basin geometry depends on the discharge supplied. This study suggests that, in sufficientflow condition, vortex minimum diameter is at bottom level and is always smaller the exit hole.
Wanchat and Suntivarakorn studied the effect of basin struc- ture in formation of water vortex stream [21,22]. Their study indicates the important parameters which can determine the water free vortex kinetic energy and vortex configuration and they include the height of water, the orifice diameter, conditions at the inlet and the basin configuration. It was found that a cylindrical tank with an orifice at the bottom center with the incomingflow guided by a plate is the most suitable configuration to create the kinetic energy water vortex.
The power production varies along with head andflow. There- fore, for a given head and flow the different geometrical para- meters that can be varied of conical basin for gravitational water vortex power plant are: (i) basin opening, (ii) basin diameter (iii) notch length (iv) Canal Height and (v) Cone Angle and among these parameters for a given basin diameter, all other parameters has significant contribution for the change in velocity except notch angle [9]. Although the objective of study with Panditet al. is different with similar principle, their study also suggests that the geometry of hydrocyclones is very sensitive to its hydraulic and particle removal capability[16].
Wanchat et al. studied the analysis and design of basin structure which has ability to form a gravitational vortex stream.
Their study investigated the suitable outlet diameter at the bottom center of the vortex basin. In the case of 1 m diameter cylindrical vortex basin, computationalfluid dynamics (CFD) and experiment using the model indicate that the suitable outlet diameter was in range of 0.2–0.3 m. The operating head of the free vortex was in the range of 0.3–0.4 m. The maximum power output was 60 W at 0.2 m outlet diameter and the head of the free vortex was at 0.4 m.
The total efficiency of the model system was 30%[22].
Till now all the researches have focused on the different geometrical conditions of the cylindrical basin and conical basins but there is no any study to compare these two different basin structure[6,14,22]. Our previous study was also focused on conical basin[9]. However, there is no any researches carried out with the basin structure along with numerical simulation and practical verification. Although the power output has been studied in different basins, but the suitable position of the runner in the basin has not been studied and this study aims to study the efficient runner position in the basins of the plant. Moreover, this study focuses for the comparison of two different basin structure with the help of ANSYS simulation followed by practical verifica- tion. And for the simulation, RNG k-
ε
model is more suitable than standard k-ε
model to the rapidly strained and great curving streamlineflows[7].4. Model development and solution procedure
Computational Fluid Dynamics can be defined as thefield that uses computer resources to simulate flow related problems. To simulate aflow problem you have to use mathematical physical and programming tools to solve the problem then data is gener- ated and analyzed. In order to provide easy access to their solving power all commercial CFD packages include sophisticated user interfaces to input problem parameters and to examine the results.
Hence all codes contain three main elements: (i) a pre-processor, (ii) a solver and (iii) a post processor[20].
In nearly all the previous researches, the air–core vortex was considered based on the assumption of steady, axisymmetric and incompressible flow. The continuity equation and the Navier–
Stokes equations in cylindrical coordinates are described as follows:
∂Vr
∂rþ∂Vz
∂zþVr
r ¼0 ð1Þ
Vr
∂Vθ
∂r þVz
∂Vθ
∂z VrVθ
r ¼v ∂2Vθ
∂r2 þ∂Vθ r∂r
Vθ r2þ∂2Vθ
∂z2
ð2Þ
Vr
∂Vr
∂r þVz
∂Vr
∂z V2θ
r þ∂
ρ
ρ
∂r¼v ∂2Vr∂r2 þ∂Vr
r∂r Vr
r2þ∂2Vr
∂z2
ð3Þ
Vr
∂Vz
∂r þVz
∂Vz
∂zþ∂
ρ
ρ
∂z¼gþv ∂2Vz∂r2 þ∂Vz
r∂rþ∂2Vr
∂z2
ð4Þ where, Vθ, Vr and Vz are tangential, radial and axial velocity components, respectively,
ρ
is fluid density, g is gravitational acceleration andν
is kinematic viscosity. Due to the complexity of the equations, it is extremely difficult to get an analytical solution directly[23].Computationalfluid dynamics (CFD) have become a cost effec- tive tool for predicting the performance of Fluid machines and also thefluidflow through a region of interest. In the present study, the
simulation has been carried out forflow visualization in the basins and velocity distribution in the plane where the turbine has extracted more power withfinite volume method. For the purpose of simulation thefluidflow domain was modeled as shown infigure using CAD software, Solid Works. The model was then imported in a commercial CFD code ANSYS Fluent and was simulated.
The modeling and meshing of the proposed model is done using software ICEM CFD forfluid analysis. First, denser mesh was taken near wall and at air core region but later the grid was refined and uniformly dense mesh was generated. This was done because the vortex velocity was found to be maximum in region between the air core and wall. As this velocity was subject of concern for basin optimization the CFD model was re-meshed. The canal and
Fig. 1. Boundary condition for cylindrical basin.
Fig. 2. Boundary conditions for conical basins.
Fig. 3. Design parameters for cylindrical basin.
outlet region was modeled with Cut cell mesh and Tetrahedron was used for basin modeling. The boundary condition and design parameters for both basins are shown inFigs. 1–4.
4.1. CFD model and mesh for two basins
The computational domain used is shown inFigs. 5 and 6and the Mesh independency is shown inFigs. 7 and 8.
The simulation has been done for a steadyflow to investigate the performance of different basin geometry on vortex velocity distribution. The main assumptions include a steadyflow, no slip conditions. The working fluid, water is assumed as an
incompressiblefluid with density of 998.2 kg/m3and viscosity of 0.001003 kg/m-s. The RNG k-
ε
turbulent model was used to investigate theflow pattern of the system.The Computational Fluid Dynamics simulation was run with no-slip conditions at the wall and pressure outlet condition at the outlet. The inlet was velocity inlet with initial inlet velocity offluid (water)flow is set to be 0.1 m/s. The upper surface was subjected to atmospheric pressure. The no. of elements in thefinal computa- tional domain used for the simulation was 308,851 and minimum and maximum element sizes were 310 5m, 610 2m, respectively.
The initial inlet velocity offluid (water)flow is set to be 0.1 m/s and the outlet was pressure outlet with wall of the fluid flow
Fig. 4.Design parameters for conical basin.
Fig. 5.Computational domain of cylindrical basin.
Fig. 6. Computational domain of conical basin.
0 0.05 0.1 0.15 0.2 0.25 0.3
5633 7582 15796 32783 68943 84165
No. of Nodes
Velocity
Poly.
(Velocity)
Velocity
Fig. 7.Grid convergence for cylindrical basin.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
2192 3211 7502 11252 18877 122772
Velocity(m/s)
No. of Nodes
Velocity
Log.
(Velocity)
Fig. 8. Grid convergence for conical basin.
Fig. 9.Flow simulation of cylindrical basin.
domain stationary. As it does not have any drastic change on vortex structure whether the canal was open or closed; so, we have simulated the cylindrical basin by considering it as closed channelflow. We have considered thefluid wasflowing through the area having hydraulic diameter of 0.2667 m.
The governing equations are discretized by thefinite volume method (FVM) using the commercial CFD package ANSYS FLUENT 14.5. FVM is used to discretize the governing equations with suitable discretization schemes for each governing equation. To solve the discretization equation, steady pressure based segre- gated solver with double precision and implicit scheme is used.
The second order method is used for the steady terms.
A SIMPLE method was used to solve the discretized equations.
The second order up-winding method is used for the discretization of the momentum equation and other equations. This method provides a proper view of the physics offlow. The convergence criterion for all the equations is 10 4.
Fig. 8 shows the velocity distribution in confined vortex chamber. The velocity contours obtained seems to agree with these plots.
Fig. 10. Flow simulation of conical basin.
Fig. 11.Contour of velocity for cylindrical basin.
Fig. 12.Contour of velocity for conical basin.
Table 1
Velocity result for cylindrical and conical basin.
Position in the basin (from top surface) Maximum velocity (m/s) Cylindrical Conical
0.425 0.2125 0.3
0.625 0.225 0.325
0.775 0.25 0.325
0.875 0.525 0.6
R² = 0.9362
0.38 0.4 0.42 0.44 0.46 0.48
0 5 10 15 20 25
Velocity(m/s)
Cone Angle ( degree ) Fig. 13.Cone angle vs. velocity.
Fig. 14.Design modification in the outlet tube.
4.2. Simulation result
The basins were modeled andflow simulation was done in each. It was seen that for same inlet conditions, the maximum velocity measured was 0.525 m/s for cylindrical basin and 0.6 m/s for conical basin with all the similar inlet conditions (Table 1). From theflow simulation we can see that the average velocity in conical basin is more than cylindrical basin for nearly similar conditions of head and discharge (Figs. 9and10). This can be explained as with decrease in flow area in conical basin, velocity increases thereby maintaining a constant flow rate. Also the gradient of velocity is less in case of cylindrical basin. From these results it one can expect that same turbine should extract much power from conical basin than in cylindrical basin. Thus computational study suggests us that conical basin is much better than cylindrical basin for the given head and discharge (Figs. 11 and12;Table 1).
4.3. Analysis of simulation result
From this simulation result, we came to know that the conical basin is better than cylindrical basin. As we change the basin design from cylindrical to conical, velocity gets increased accordingly with increase in cone angle which will result in more power production (Fig. 13). For maximum cone angle, the side walls should be directly
attached to the outlet tube so that there is no disturbance in the smoothflow of vortex through the outlet (Fig. 14).
5. Experimental setup and data collection:
The preliminary design of basin was carried out using general laws of Fluid mechanics like Continuity equation and Bernoulli equation[5,6]. Series of test runs were done by varying various parameters that effect vortex formation in the basin with the help of CFD. The best found basin design along with the robust runner was fabricated and tested. The purpose of the research was to extract maximum energy form thus formed vortex.
The inlet velocity was measured by using a simple methodol- ogy, i.e., a float method as no other devices for measuring inlet velocity was available. In this method an object was dropped in the canal. The time taken by thefloat to cover 1 m distance was measured for several times and average was calculated. Then the average velocity of top surface of water was approximated as reciprocal of the average time taken. Later the correction factor of 0.85 was chosen as wall of flow channel are smooth. Thus, the mean velocity was approximated to be 0.1 m/s.
We have done experiment in the laboratory setup of cylindrical and conical basin having diameter of 600 mm and height of 850 mm with all similar inlet conditions. Runner having 6 numbers of blades with diameter of 420 mm was used and tested in those basins for the measurement of output power through torque setup (Figs. 15–17).
The output power in case of cylindrical and conical basins was found to be maximum at the runner position of 65–75% of total height of basin from top position. Greater flexibility in runner position of conical basin could not be achieved due to the geometrical constraints.
In case of conical basin the power output increased with the increase in runner position but in case of cylindrical basin it started to decrease from a certain height. The output power at the bottom position of cylindrical basin was decreased due to the weak vortex formation in opposite direction to that of the dominant vortex formation. However, the same effect was seen less in case of conical basin. As the diameter of the vortex gets decreased along the converging cone with increase in vortex strength and decrease in turbulence created by interaction with runner, there is increase in power production (Tables 2 and 3).
Fig. 18shows various power outputs of runner in two different basins. It can be seen that, the power harnessed by the runner in conical basin increases along with the increase in runner position, which is measured from top of the basin. But in case of cylindrical basin, the power harnessed by the runner decreases after some height from the upper bearing.
Fig. 15. Design of lab setup.
Fig. 16. Fabricated lab setup.
Fig. 17.Runner used for power extraction.
6. Cost differential of basins and operational challenges According to the Ruby Iron, Patan Industrial Estate, Patan, Nepal, the cost estimation for the installation of 7 KW of conical basin at our proposed site of installation at Bagmati river, Gokarna-1, Kathmandu is NRs. 922232 for only metal parts including that of runner, basin, canal and all metal support, excluding that of transmission lines. If we replace this conical basin with cylindrical basin then the cost estimation would be NRs. 940246. Therefore there was no substantial cost difference in the installation of two different basins. However, the conical basin is significantly cheaper for mass production in industries.
There are many challenges related to successfully implement- ing a hydroelectric facility in different irrigation canals and small rivers and rivulets. Although the supportive structure of Dam for irrigation purposes can help to reduce the cost for installation at different irrigation canal, it is not available in all the canals. We have to construct a separate pathway for rivers which may increase the cost of installation. Moreover, the seasonality of irrigation diversions, seniority of water rights, locations remote from power service, and the variable nature of the flow and reservoir releases are also the major problems associated with it.
There are also opportunities afforded by the existing, engineered infrastructure of irrigation systems. Similar to our proposed site of installation at Bagmati river, Gokarna-1, Kathmandu, Nepal there is a lot of irrigation canals and pipelines and drop structures already in place and many are not in operation; in these places this plant can be successfully installed with low cost.
Sediments in water source,fluctuating water level andflooding are some of the major environmental challenges associated with
this plant. Since the GWVPP does not constitute of any mechanical parts to create the water vortex, the sediments have virtually no effect on the turbine performance unlike conventional hydro powers. In addition, afiltering mechanism is incorporated in the design to prevent undesirable particles to reach the fields. To counteract thefluctuating water levels in a river or a canal, GWVPP incorporates a water overflow mechanism where the turbine is fixed. Flooding could provide a threat to the turbine pump when it is put in a river. The turbine is designed with selected materials and it is easily portable so that communities can easily take the turbine out of and into the river. As a result, the turbine can be moved easily in case of a flood prediction to save it from damaging. Electrical interconnection also presents both challenges and opportunities. Ideal hydropower sites – like ideal wind or photovoltaic sites – are those with ready access to electrical service. Pulling new electrical service over any distance is often cost‐prohibitive for small renewable systems.
Most irrigation system sites are quite small. Permitting costs do not scale well to small sites, creating disproportionally high up‐ front costs that often kill economic viability or dissuade potential investors from seriously considering small hydropower projects.
However, this plant can be successfully installed in the low head water resources areas where the power grid expansion cost is high, geographical disturbances counts a lot, local demand is low.
Hence the successful implementation would decrease the small distributive local demand and help to mitigate the national energy crisis.
7. Conclusion
Low head turbine can be the most suitable option for rural electrification. Gravitational Water Vortex Power Plant ( GWVPP) is a new and emerging technique in context of low head hydro power. The numerical and experimental study on this plant asserted that output power and efficiency is maximum in conical basin compared to that of cylindrical basin for all similar inlet and outlet condition with maximum power extraction at runner position of 65–75% of total height of basin from top position.
Although there are some operational challenges, GWVPP is an excellent choice for power generation in the remote villages where grid line supply is tough and developing countries like Nepal can address the scattered low income consumers fulfilling their low demand, which results in the improvement of the rural electricity.
Table 2
Experimental reading for runner in conical basin.
Torque measurement Power output Power input Efficiency (n%) Runner position
W1(kg) W2(kg) Torque W(rpm) Pout(Watt) Head (h) Flow rate (Q) Pin(Watt)
3.0 1.5 0.882 130 12 0.85 0.01 101.73 11.97 0.325
4.38 1.16 1.89 143 28.32 0.85 0.01 101.73 27.83 0.4
5 1.5 2.058 174 37.48 0.85 0.01 101.73 36.84 0.605
Table 3
Experimental reading for runner in cylindrical basin.
Torque measurement Power output Power input Efficiency(n%) Runner position(m)
W1(kg) W2(kg) Torque (N-m) W(rpm) Pout(Watt) Head (h) Flow rate (Q) Pin(Watt)
2.3 1.6 0.41 136 5.85 0.85 0.01 101.73 5.72 0.37
5.2 2.5 1.587 170 28.24 0.85 0.01 101.73 27.75 0.58
3.5 1.2 1.3524 162 22.93 0.85 0.01 101.73 22.54 0.68
3.8 1.8 1.176 170 20.92 0.85 0.01 101.73 20.56 0.78
3.4 1.5 1.1172 165 21.31 0.85 0.01 101.73 20.94 0.825
R² = 1
R² = 0.8982
0 5 10 15 20 25 30 35 40 45
0 0.2 0.4 0.6 0.8 1
Power Output(watt)
Runner Position (m)
Conical Basin
Cylindrical Basin
Poly. (Conical Basin )
Poly. (Cylindrical Basin)
Fig. 18. Runner position vs. power output for cylindrical and conical basin.
It is realized that study on other basin structure and optimizing the runner shape shall be new areas to explore on this system.
Acknowledgment
This study acknowledges thefinancial support from Alternative Energy Promotion Center, Ministry of Science, Technology &
Environment, Government of Nepal (Budget line: 42050, Activity no.: 2.12.6.2, Contract number: 2070.CESC.006) and Himalaya College of Engineering, Chyasal, Lalitpur, Nepal. We are obliged to Center of Energy Studies, Central Campus, Pulchowk for technical and financial support. We are thankful to Ruby Iron and Magi Engineering for fabrication of our experimental setup.
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