Session R11: Energy: Water Power I

Optimizing energy harvesting performance of a tandem hydrofoil turbine array with vortex-foil interactions

Tidal flow energy harvesting with oscillating hydrofoil turbines has become an important area of research as the need for more sustainable energy sources has increased in recent decades. As these turbines will be deployed in array configurations, it is essential to characterize optimal array kinematics and to understand how wake-foil interactions affect performance. Due to their structured wakes, it might be possible to improve the performance of hydrofoil arrays using constructive wake-foil interactions. In this study experiments were performed with prescribed-motion oscillating hydrofoils in a water flume where force and time-resolved PIV measurements were obtained in order to study array performance and wake-foil interactions. Three distinct types of interaction were observed: one where the trailing foil avoids the wake structures, another where interaction with wake vortices leads to worsened hydrofoil performance, and a case where vortex interactions lead to improved hydrofoil performance. From these observations it is apparent that in order to optimize the array performance the hydrofoils in the array do not share the same kinematics nor do they operate with the optimal kinematics of a single hydrofoil.   

Experimental study on a flapping-foil hydrokinetic turbine with a fully passive activating mechanism and crank-rocker power transmission

A rotary turbine is commonly used to extract energy from water flow. However, a new type of hydrokinetic turbine called a flapping-foil hydrokinetic turbine (FHT) has been developed, taking inspiration from underwater creatures. In this study, a passive pitch control method was implemented by synchronizing the movements of the front and rear foils. By linking the flapping movement of one hydrofoil with the pitch movement of the other hydrofoil, a portion of the flapping energy induced by the flow was utilized for pitch control. Through experiments, it was observed that the pitch angle and flapping angle were effectively interlocked by adjusting the gear ratio of the connecting sprocket, allowing the turbine to be self-driven.

The power transmission method used a crank-rocker mechanism. A transmission equation was derived using the rocker angle as the input angle and the crank angle as the output angle. The angular velocity and torque of the crank were obtained from measurements of the rocker's angular velocity and torque during experiments. The transmission power efficiency to the crank was predicted to be over 90% for both foils, and this was validated against the data obtained from the experimentally measured crank information.

The passive pitch control and crank-rocker power transmission proposed in this study considerably improved the performance of the FHT. With further research, it is expected that the FHT could serve as a promising alternative to rotary turbines.   

Scaling the Power Extraction Efficiency of Three-Dimensional Oscillating Hydrofoils

We present experiments of power extraction performance of combined heaving and pitching NACA 0015 foils of varying aspect ratio (AR). These surface-piercing foils are pitching about their one-third chord at a Reynolds number of Re = 10,000. As expected, the efficiency increases with increasing AR, and the optimal Strouhal number and pitch angle that maximize the efficiency shift to slightly lower and higher values, respectively. We demonstrate that classic Prandtl scaling laws that account for the upwash/downwash effects of finite-span foils can collapse power extraction efficiency performance maps around their peak performance of AR = [3, 4, 6, 8] foils. This simple scaling works well when the effect of the water surface acting as an image plane is accounted for. We further investigate the scaling of the forces and moments, as well as the heave and pitch power. Using the scaling laws, 2D simulations can be corrected for finite-span effect and are compared with the experiments. This study provides an improved understanding of AR effects on the power extraction efficiency of oscillating hydrofoils.    

Dynamic strain measurement of a horizontal axis tidal turbine in elevated levels of freestream homogeneous turbulence

Tidal turbines operate under turbulent inflow, which causes dynamic loading on turbine blades. The elevated levels of freestream velocity fluctuations in tidal flows become a critical source of induced fatigue loading on the blades.  A detailed understanding of blade loading at elevated levels of inflow turbulence, specifically the influence of turbulence intensity (T_i) and integral length scale (L_U), is crucial for prolonging the design life cycles of the blades. In this context, we investigate the real-time strain measurement along the span of a laboratory model of a horizontal axis tidal turbine subjected to anisotropic turbulent inflow conditions generated in a water tunnel using an active grid turbulence generator. The strain measurements are carried out using the Fiber Bragg Grating sensors attached to the in-house clamp-shell designed blades. In particular, the turbine is subjected to three inflow conditions: Quasi-laminar (T_i~2%), elevated-T_i (T_i~12-14%, L_U~0.35D), and elevated-T_i-L_D having similar T_i as elevated-T_i case but higher integral length scale (~D). The detailed measurements will be presented and compared for each case and estimates of blade root bending moments will be discussed.   

Effects of hydro-abrasive erosion on efficiency reduction of a Pelton turbine in Indian Himalayas

The problem of hydro-abrasive erosion due to sediment is becoming more severe for hydropower plants (HPP) owing to extreme floods caused as a result of climate change, especially in geologically young mountains, like the Andes and the Himalayas. In high-head HPPs, even small sediment particles cause critical hydro-abrasion resulting in loss of efficiency, interruptions in power production and downtime for repair/maintenance. Currently, only limited information is available as there is a lack of practically proven measurement techniques as well as reliable measurement data on suspended sediment properties and turbine erosion with efficiency losses. Hence, simultaneous measurement of suspended sediment properties, hydro-abrasive erosion and reduction in efficiency was carried out to find the optimum strategy for an HPP located in Indian Himalayas. The curved portion of Pelton buckets towards the bucket root was found to be the most severely affected region by hydro-abrasive erosion apart from splitter and cut-out. The operational strategy to operate the units near rated power after the erosion was recommended to yield more efficiency compared to operations at part load conditions. This study is useful for the optimum operation of an HPP with respect to hydro-abrasive erosion.   

On the abrasion in riverine streams relevant to marine energy converters

Marine energy converters and structures in riverine streams are subjected to varying sediment loads, consisting of organic and inorganic materials of a wide range of sizes and densities, which can impact these structures at different relative angles and velocities. Abrasion can lead to substantial damage and reduced performance in the case of marine turbines. The quantitative characterization and understanding of material response to abrasion play a crucial role in developing new resilient materials and coatings for durable structures, thus extending the lifespan of marine energy converters. Here, we will briefly explore the relevant parameter space for assessing such material response and present the features of a basic facility that enables long and customized testing.   

Efficient power density amelioration strategy for capacitive salinity-gradient and pH-gradient energy harvesting systems

Osmotic energy released from the mixing of  dilute and concentrated solutions is a new source of renewable energy. Our team developed previously an efficient capacitive concentration cell by using one single membrane and two inexpensive carbon-based capacitive electrodes. Such system converts efficiently the osmotic energy into electricity. It is safe, sustainable and environmental-friendly, but remains economically unviable today.

For power density amelioration,  we developed a promising strategy by using an additional booster system in the circuit. The booster system generates an alternative electric voltage in phase with the period of flux reversal. The net power density output is calculated as the difference between gross power density dissipated and the power density generated by the booster system. We  measured a maximal power density of 5.26 W.m^-2 (salinity difference of 0.17 mol.L^-1 and 5.13 mol.L^-1), corresponding to a 59.8% increase compared with the value 3.29 W.m^-2 without booster.

We extended this approach to processes involving solutions with pH gradients. These solutions are obtained by injecting CO₂ from plant stacks into electrolytes such as Na₂CO₃ and NaHCO₃. Under boosting strategy, we generated a higher net power density of 3.2 W.m^-2 (pH gradient of 2.4) compared with the result of 1.7 W.m^-2 reported in literature. A maximum net power density of 4 W.m^-2 is obtained for a higher pH difference. In both situations, the process approaches economical viability.