HYDROKINETIC ENERGY EXTRACTION: PROGRESS, MODELING, AND ENVIRONMENTAL CONCERNS Veronica Miller and Laura Schaefer

Abstract To alleviate our pending energy crisis new, sustainable technologies are emerging. For sustainable validation, they are in need of environmental impact qualification and quantification. Hydropower has proven to be a viable source of energy to exploit, with smaller environmental impact than those of common use: coal, natural gas, etc. It must be preceded with caution, however, as typical hydropower set-ups (dams and small hydropower) have caused irreversible ecological and environmental damage. Both of these hydropower types cause changes in flow speed, patterns, and temperature, which degrade and even destroy local aquatic organisms due to the changes in their habitat. Hydrokinetic energy extraction has come forward as it harnesses energy in river flows, making a reservoir unnecessary, and proving to be less harmful to the environment. Basic principles of hydrokinetic energy extraction are presented along with a computational fluid dynamics model of the system. Through examining these principles it is clear that more research is required in hydrokinetic energy extraction with emphasis towards lower environmental and ecological impact. In addition to model enhancements, further development in this form of renewable energy includes a Life Cycle Assessment for full system evaluation of its environmental impact.

Figure 1: MULTIDISCIPLINARY APPROACH TO HKD DEVELOPMENT. Due to these effects, hydrokinetic technologies are being investigated and implemented. A hydrokinetic device (HKD) is one that extracts energy in a river from its free flow component. Rather than needing to restrict the flow for potential energy development, the energy may be extracted from its natural movement. The overall goal of research in hydrokinetic energy extraction is to achieve energy extraction that is more sustainable and healthier for the environment. The objectives to accomplish this are: • to look at velocity profile mapping with the environment and • optimize shapes and device orientation to maximize energy extraction while minimizing environmental impact. In achieving this goal a multidisciplinary approach is necessary shown in the following Figure 1. As this type of system is investigated the mechanics of it are inherent such as electrical set-up, mechanical design, and river geomorphology. Beyond these, it becomes crucial to take into account policy formulation and execution, and environmental impacts. As the diagram shows, these research prospectives are all interlinked to give a holistic view of hydrokinetic energy extraction.

INTRODUCTION The U.S. Department of Energy has identified as much as 3400 MW of unexploited hydroenergy by small and potentially free-flowing systems [1]. Additionally, small hydropower has been noted as a high potential source of energy internationally. Places such as the United Kingdom, Greece, Turkey, and India have already identified the source and have performed initial supply analysis [2–4], whereas other regions such as Ghana and Brazil are identified as having high potential for this technology [5; 6]. Utilizing these potential renewable resources gives a promising scenario for addressing the problem of the pending energy crisis we face as a global community [7]. Hydropower development must be preceded with vigilance though, as past forms have been proved harmful to the environment. Common forms of hydropower maybe shown in Table 1.

HYDROKINETIC EXTRACTION TECHNOLOGIES The current types of hydrokinetic extraction technologies are given in Figure 2. They range from axial flow to use of piezoelectric materials. Dynamic augmentation makes it onto the list as it has been researched for increased efficiency. The focus of this 1

Table 1: TRADITIONAL AND OTHER DEVELOPING HYDROPOWER SYSTEMS AND THEIR ENVIRONMENTAL EFFECTS [8]. Hydropower Systems Environmental Effects Reservoir type Changes of habitat and social impacts due to reservoir Modification of river flows Pumped-storage

Impacts related to elevated storage reservoir

In-stream diversion

Reduction in flow downstream of diversion

Run-of-river

Limited flooding River flows unchanged

study is also on environmental safety, leaving dynamic augmentation outside of the scope. Cross-flow turbines have shown the highest efficiencies within this research and will be given the most focus [9]. Many of the base HKD aside from being developed from tidal extraction device, have been inspired from wind energy given the similarity in energy extraction mechanics. They are shown in Figure 3. Turbines that have typically shown low performance in wind have demonstrated otherwise for river energy extraction [10; 11].

PRINCIPLES OF HYDROKINETIC ENERGY EXTRACTION A simple underwater wheel is chosen for a base hydrokinetic energy extraction model, which is designed so that other types of devices maybe also be modeled within it. It attempts to provide flow mapping for extraction devices that can assist with extraction efficiencies, shape and position optimization, and environmental impact associated with them.

RIVER MODEL River flow is turbulent, so the appropriate model must be chosen for evaluating the mean-flow field. All turbulent flows are characterized by continuity (Equation 1) and the Reynolds equations for turbulent motion [12] (Equations 2- 3). ρ 5 •V¯ = 0 DV¯ = ρg − 5¯ p + 5 • τij Dt   ∂u ¯i ∂u ¯j τij = µ + − ρu¯i 0u¯j 0 ∂xj ∂xi ρ

Figure 3: HYDROKINETIC ENERGY EXTRACTION DEVICE CLASSIFICATIONS.

(1) (2) (3)

2

Figure 2: HYDROKINETIC TECHNOLOGIES. k −  is one of the more commonly used turbulent any rotating system, neglecting friction effects. models to account for fluctuating velocities within Reynolds equations. It is a two equation model in˙ s = P = ωTo W troducing two additional variables: k represents turbulent kinetic energy and  is the dissipation rate (Equation 4). − To = ρQr (Vo − Vi )

(5) (6)

1 h| u~i 0 |i , 2  ν

:= | gradu~i 0 + gradu~i 0T |2 2

In these equations, ρ is the fluid density, r is the arm length of the device, Q is the river’s volumetric (4) flow rate, and Vi and Vo are the inlet and outlet ve˙ is the shaft work from the locities to the device. W This method using uniform turbulence is basic and device rotating in the flow, P is the power extracted offers a reliable initial estimate of the system [13; 14]. from the turbine, ω is the angular velocity, and To is Other, more developed models, such as Reynolds the torque occurring in the device. Given the similar stress transport model (RSTM) can be considered for nature of hydro extraction to wind, often the ideal power extraction equation, Equation 7, is used for future model development. hydropower extraction in these scenarios. k:=

Power Extraction Pideal = 0.5ρAVi3 Cp

The final hydrokinetic energy extraction design is dependent upon energy extraction efficiencies and its overall environmental impact. Research has shown that fish swimming patterns are affected by eddies in the stream [15], so emphasis will also be given to the computational fluid dynamics (CFD) analysis which reveals the best possible scenario for fish and local flora and fauna. A CFD model will be used to give an initial performance of the hydrokinetic energy extraction device. The CFD model demonstrates whether the assumptions were appropriate in the mathematical modeling component. Using this information assists in framing the parameters for the hydrokinetic energy extraction device (HEED) in that its overall function will detour fish from the device. In modeling the HEED, the energy equation is applied, which can be reduced to Equations 5 and 6 for

(7)

In the ideal power equation, A is the area from one HEED arm, and Cp is a turbine power coefficient and is defined by Equation 8. Equation 7 is derived from continuity. It is an approximation of the amount of energy that is extracted through a wind turbine, but a detailed analysis with blade shape and surface, fluid interaction would give more accurate results [16].  Cp =

1+

Vo Vi

 1− 2

Vo 2 Vi

 (8)

If friction is taken into account, Equation 9 will be used in the energy extraction. CD1 and CD2 are drag coefficients based on the geometry of the device. 3

Figure 5: VELOCITY MAGNITUDE IN M/S.

outlet. Other parameters include atmospheric pressure and water density at atmospheric pressure and 20 C. A result from running the relation in FLUENT is shown in Figure 5. The ordinate is velocity in m/s and the abscissa is the distance along a river bed in the downstream direction. Figure 4: POWER COMPARISON.

RESULTS AND DISCUSSION  −Tdrag

2 rAρ

 = CD1 (Vi cos θ − ωr)

2

−CD2 (Vi cos θ − ωr)

2

+CD1 (Vi sin θ − ωr)

2

−CD2 (Vi sin θ − ωr)

2

Figure 5 shows that initial estimates of velocity decreases or power extraction from the device are reasonable. It also shows a valid river profile which can be verified through numerical analysis or a groundup CFD model. Some circulatory flows are seen as a result of the device rotation, but further analysis is required to see the potential impact this might have towards fish and other marine organisms. This model provides a basis for a two-dimensional CFD analysis of hydrokinetic turbines, which can be applied to other designs than what is shown in the figure. The simulation presented attempts to model a hydrokinetic energy extraction device in a real case condition where the full Navier-Stokes equations are solved and surface-fluid interaction is accounted for. Other models of similar set-ups use ideal power models, which as shown in Figure 4, over-predict the amount of power that may be extracted from them. This model advances the field and contributes to understanding of hydrokinetic technology. Enhancements to this model would include extension to the third dimension and verification through experimentation. Environmental assessments are necessary for final implementation of HEEDs and include life cycle analysis (LCA) to assist in environmental impact quantification and flow profiles generated through particle image velocimetry in cooperation with experimentation. These profiles will further demonstrate the device’s interaction with its lo-

(9)

The comparison of these approaches is shown in Figure 4. It is interesting to see how the power predictions compare. It is expected that less power would be extracted when accounting for drag, and the similarity between the ideal and drag models shows that using the true geometry in calculations can change the amount of energy extracted versus the ideal model.

FLOW SIMULATION The river model and power extraction have been integrated in F LU EN T T M . A staggered grid of the flow field with hydrokinetic device is created in GambitT M and is 78 x 302 nodes. The angular rotation for the device is set at 0.4536 rad/s, based on a preliminary analysis. In this model, the bottom edge is defined as a wall and the top edge is defined as a pressure inlet, while the edge to the left is the flow inlet, which is set to 0.313 m/s (given for the Allegheny River, a typical U.S. river [17]), and the edge to the right is an 4

cal environment and will verify what is exhibited in the CFD models, assisting in quantifying the overall impact of the device.

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CONCLUSIONS

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