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To meet the demands of a larger share of the electrical energy produced by intermittent renewable energy sources, an increasing amount of plannable energy sources is needed. One solution to handle this is to increase the amount of energy storage in the electrical grids. The most widespread energy storage technology today is by far pumped hydro storage (PHS). In an attempt to enable PHS at low-head sites, the ALPHEUS (augmenting grid stability through low head pumped hydro energy utilization and storage) EU Horizon 2020 research project was formed. In ALPHEUS, new axial flow, low-head, contra-rotating pump-turbine (CRPT) designs are investigated. A CRPT has two individual runners rotating in opposite directions. CRPTs developed within the ALPHEUS project have already been thoroughly analysed at stationary and transient operating conditions by the authors. However, the effects on the CPRT's performance due to potential cavitation on the runner blade surfaces have previously not been investigated. For that reason, the current study focuses on running cavitation simulations on a model scale CRPT using the OpenFOAM computational fluid dynamics (CFD) software. In the CFD simulations, cavitation is modelled as a two-phase liquid-vapour mixture using the interPhaseChangeDyMFoam solver. The two runner domains have a prescribed solid body rotation. Condensation and evaporation processes are handled with the Schnerr-Sauer model. Turbulence is managed with the k-omega shear stress transport-scale adaptive simulation (kOmegaSSTSAS) model. Flow-driving pressure differences over the computational domain are achieved with the headLossPressure boundary condition to emulate a larger experimental test facility of which the CRPT is part. Figure 1 shows a snapshot in time of an iso-surface (light blue) of cavitating cloud with alpha_liquid=0.9 in turbine mode. At this operating point, a small amount of cavitating flow is found by the suction side of the leading edges of the left runner, which is facing a lower reservoir. In Figure 2, the same type of iso-surface is shown, however now in pump mode. It is seen that the pump mode operating condition is much worse than the turbine mode. The cavitating cloud covers most of the suction side of the left runner, additionally, the tip-clearance region is also exposed to cavitation. Furthermore, traces of cavitation are found on the leading edges of the right runner as well as on the left small-support struts. It is thus important to, at least, analyse the pump mode to determine if and how much cavitation affects the CRPS's operating performance.

Many industrial and biological interfacial processes, such as welding and breathing depend directly on wettability and surface tension phenomena. The most common methods to control the wettability are based on modifying the properties of the fluid or the substrate. The present work focuses on the use of high-frequency acoustic waves (ultrasound) for the same purpose. It is well known that ultrasound can effectively clean a surface by acoustic cavitation, hence ultrasonic cleaning technology. Besides the cleaning process itself, many authors have observed an important wettability enhancement when liquids are exposed to low and high (ultrasonic) frequency vibration. Ultrasound goes one step further as it can instantly adjust the contact angle by tuning the vibration amplitude, but there is still a lack of comprehension about the physical principles that explain this phenomenon. To shed light on it, a thermodynamic model describing how ultrasound decreases the contact angle in a three-phase wetting system has been developed. Moreover, an analytical and experimental research has been carried out in order to demonstrate that ultrasound is an important competitor to surfactants in terms of energy efficiency and environmental friendliness.

To tackle the growing demand for grid-scale energy storage, the ALPHEUS project proposes a novel low-head pumped hydro storage system aimed for coastal application in countries where the topography does not allow for traditional high-head storage. This system consists of a reversible pump-turbine technology with two contra-rotating runners coupled to their respective axial-flux motor-generators as well as a dedicated control, optimising for energy balancing and the provision of ancillary services. To better understand the integration and dynamic interaction of the individual components of the plant and to allow for the simulation of a wide variety of operating conditions and scenarios, this research aims at developing a system model coupling the hydraulic, mechanical and electrical components. Numerical results are compared and verified based on CFD simulations. While some inaccuracies have to be expected, the comparison shows an overall good match with only minor deviations in dynamic behaviour and steady state results.

Penetration of intermittent renewable energy sources into the power grid requires large-scale energy storage to ensure grid stability. Pumped Hydro Energy Storage (PHES) is among the most mature, environmentally friendly, and economical energy storage technologies, but has traditionally only been feasible at sites with large natural topographic gradients. ALPHEUS addresses this by developing reversible pump-turbines efficient at low heads, that operate between an enclosed inner basin (that functions as the upper or lower reservoir) and a shallow sea or lake.

With the increased amount of energy produced from variable renewable energy sources, such as wind and solar power, the need to store energy increases. The reason is that it is necessary to cope with the variation in energy being produced by the renewables to stabilise the electrical grids. The most widely used technology for energy storage on a large scale is today pumped hydro storage (PHS). For PHS to be economically feasible, a high head is typically required, which puts topographical constraints on where it can be built. However, the EU project ALPHEUS aims to develop PHS for low-head applications, hence allowing PHS at yet unexplored sites. In the project, new reversible counter-rotating pump-turbine (CRPT) concepts are explored as an alternative runner design for low-head situations. The CRPT consists of two runners rotating in opposite direction from one another and it is suggested that it can reach higher efficiencies and be more compact compared to a single runner arrangement. In the present work a model counter-rotating pump-turbine for the ALPHEUS project is numerically analysed with computational fluid dynamics (CFD) simulations. The simulations are carried out using unsteady CFD in OpenFOAM-v2012. In the simulations, the two runners rotate individually via prescribing a solid body rotation to the runner domains. The individually rotating runners causes a intricate rotor-rotor interaction which is resolved by the numerical model. An example of this is shown in Figure, where a complex vortical structure is developing by the runners and support-structures. Furthermore, the CRPT is in reality part of large hydraulic system which effects the performance of the machine. The system includes bends, valves, long pipes, and two large water reservoirs. To restrict the size of the computational domain, the novel \verb|headLossPressure| boundary condition, developed by Fahlbeck et al., is used to include the main effects of the hydraulic system. To summarise, this study will show the potentials with a CPRT in a PHS application through CFD simulations, explain the used numerical framework, and demonstrate a use case for the new headLossPressure boundary condition.

AbstractFreely suspended nanofibers, such as spider silk, harnessing their small diameter (sub‐micrometer) and spanning fiber morphology, behave as a nonresonating acoustic sensor. The associated sensing characteristics, departing from conventional resonant acoustic sensors, could be of tremendous interest for the development of high sensitivity, broadband audible sensors for applications in environmental monitoring, biomedical diagnostics, and internet‐of‐things. Herein, a low packing density, freely suspended nanofiber mesh with a piezoelectric active polymer is fabricated, demonstrating a self‐powered acoustic sensing platform with broad sensitivity bandwidth covering 200–5000 Hz at hearing‐safe sound pressure levels. Dynamic near‐field electrospinning is developed to fabricate in situ poled poly(vinylidene fluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) nanofiber mesh (average fiber diameter ≈307 nm), exhibiting visible light transparency greater than 97%. With the ability to span the nanomesh across a suspension distance of 3 mm with minimized fiber stacking (≈18% fiber packing density), individual nanofibers can freely imitate the acoustic‐driven fluctuation of airflow in a collective manner, where piezoelectricity is harvested at two‐terminal electrodes for direct signal collection. Applications of the nanofiber mesh in music recording with good signal fidelity are demonstrated.

Abstract Large-scale energy storage solutions are crucial to ensure grid stability and reliability in the ongoing energy transition towards a low-carbon, renewable energy based electricity supply. This article presents the evaluation of a novel low-head pumped hydro storage system designed for coastal environments and shallow seas. The proposed system addresses some of the challenges of low-head pumped hydro storage including the need for larger flow rates and reservoirs as well as the requirement of machinery with high efficiencies across a wide operating range to accommodate larger changes in gross head during storage cycles. It includes several units of contra-rotating reversible pump-turbines connected to axial-flux motor generators within a ring dike, as well as dedicated machine- and grid-side control. The technology allows for independent control of each runner, making it possible to adapt to the specific operating conditions of low-head systems. In this work, a numerical approach is used to simulate the system's performance and dynamic behaviour under various operational conditions, including energy generation, storage, and grid support of a 1 GW system with 4 GWh of storage capacity. The potential system performance for energy balancing cycles is evaluated, and a sensitivity analysis is conducted to assess the influence of scaling the motor-generators on performance and footprint of the plant. Additionally, the capability and limitations of the system to respond to grid demand fluctuations and provide frequency regulation services are assessed. The results demonstrate that the low-head pumped hydro storage system is a viable large-scale energy storage solution, capable of round-trip efficiencies above 70% across a wide operating range. By increasing the maximum power of the electric machines, the maximum head range of the whole system is increased which correlates with a threefold increase in energy density per unit area. The dynamic simulations further show that the system can rapidly change its power output allowing it to provide frequency regulation services. Allocating 20% of its nominal power as a reserve, the new power setpoints can be reached within a maximum of 5 s independent of its initial state of charge.

The Rayleigh index has been used for decades by a large number of researchers as an indicator to determine if a flame is driving or damping thermoacoustic interaction mechanisms. The use of the Rayleigh criterion has found applications in rocket combustors, gas turbine combustion technology and basic combustion research. The global Rayleigh index or integral is obtained by integrating the product of heat release rate and pressure fluctuations over space and time. Depending on the phase between pressure oscillations and heat release rate response, the oscillations can be enhanced or damped. It is commonly assumed in literature that the sign of the Rayleigh index from steady state data can be used to determine if the thermoacoustic feedback loop is stabilizing or destabilizing. However, we show in this paper that under fairly general conditions, a correctly measured Rayleigh index is always positive if evaluated from statistically stationary data. This proves to be true even if the heat release rate response to pressure fluctuations is in phase opposition to those pressure fluctuations. This is shown in a straightforward manner by substituting the wave equation with a heat release rate source term into the Rayleigh index. This was verified experimentally on a fully premixed combustion system by measuring the flame chemiluminescence using a photo multiplier and pressure fluctuations using a microphone placed sufficiently close to the flame to ensure acoustic compactness for the frequency range of interest. A large range of operating conditions have been tested, spanning linearly stable and unstable stationary thermoacoustic states, respectively corresponding to resonance or a limit cycle driven by the inherent stochastic forcing from the turbulent combustion noise. The experimental results corroborated the analytic finding: the Rayleigh index is found to be positive for all frequencies and all operating conditions. Proceedings of the Combustion Institute, 39 (4) ISSN:1540-7489 ISSN:1873-2704