• Energy Research

A comparison between centrifugal impeller pumps with and without splitter blades in terms of suction performance is presented by experimental tests and numerical analyses. The design of both pumps was carried out by preserving the impeller meridional shape, number of blades, and volute casing. Blade shapes were obtained by adopting a 1D inverse design method. The same blade loading distribution was assumed for the full blades of both impellers, while the loading distribution of the splitter blades was modified until a close matching between the two head-capacity curves was achieved. The fulfilment of this performance requirement and the use of the same number of blades were needed to describe accurately the role played by the splitter blades in cavitation inception and development. Differently from other experimental comparisons, where previous requirements were not met, a noticeable improvement in suction performance was found at large flow rates but not at partial ones, where a small deterioration in suction performance was observed. The crucial role played by the blade thickness blockage on the incidence flow angle at the leading edge of the full blades was also investigated.

To achieve the carbon free electricity generation target for 2050, the penetration of renewable energy sources should further increase. To address the impacts of their unpredictable and intermittent characteristics on the future electricity grid, Pumped Hydro Energy Storage (PHES) plants should enhance their regulation capability by extending their continuous operating range far beyond the optimal normal working range. However, for the time being, the regulation capability of the new generation of PHES, equipped with reversible pump-turbines due to their cost-effectiveness, is limited at part load by instability problems. The aim of this paper is to analyse, during a pumping power reduction scenario, the onset and development of unsteady phenomena leading to unstable behaviour. A 3D transient numerical simulation was carried out on the first stage of a variable-speed two-stage pump-turbine from full load to the unstable operating zone by progressively reducing the speed from 100% to 88% rpm corresponding to a power reduction from full load to about 60% with a ramp rate of 1.5% per s. Two three-dimensional unsteady flow structures affecting the return channel and the wicket gates at the end of the first stage were identified and their evolution in the power regulation scenario was fluid-dynamically and spectrally characterized to determine the fluid-dynamical conditions causing the head drop in the hump zone.

A novel paradigm of the original particle swarm concept is presented.Two types of agents are identifiable in the swarm: explorers and settlers.The particles dynamically exchange their role in the search process.The particle task differentiation has demonstrated effectiveness and accuracy. The paper presents a novel paradigm of the original particle swarm concept, based on the idea of having two types of agents in the swarm; the "explorers" and the "settlers", that could dynamically exchange their role in the search process. The explorers' task is to continuously explore the search domain, while the settlers set out to refine the search in a promising region currently found by the swarm. To obtain this particle task differentiation, the numerical coefficients of the cognitive and social component of the stochastic acceleration as well as the inertia weight were related to the distance of each particle from the best position found so far by the swarm, each of them with a proper distribution over the swarm. This particle task differentiation enhances the local search ability of the particles closer to gbest and improves the exploration ability of the particles as the distance from gbest increases.The originality of this approach is based on the particle task differentiation and on the dynamical adjustment of the particle velocities at each time step on the basis of the current distance of each particle from the best position discovered so far by the swarm.To ascertain the effectiveness of the proposed variant of the PSO algorithm, several benchmark test functions, both unimodal and multi-modal, have been considered and, thanks to its task differentiation concept and adaptive behavior feature, the algorithm has demonstrated a surprising effectiveness and accuracy in identifying the optimal solution.

Abstract In last decades, more and more complex models have been developed to investigate hybrid energy systems in order to define optimal size and management strategies. However, the results accuracy of these analyses is strictly related to the accuracy not only of the models but also of the input data, among which one of the most important is a proper assessment of investment costs. Even though small deviations from real investment costs are expected due to the peculiarities of each energy system, the meaningfulness of optimization analyses is affected by the availability of cost correlations characterized by sufficient accuracy. Although some technologies are characterized by standardized costs, other ones, as small hydro power plants, are characterized by investment costs that cannot be properly estimated with too simplified models, based on power and net head. This paper proposes a new approach for the estimation of the electro-mechanical equipment cost, according to which the final cost is divided into three terms with a distinct dependence not only on power and net head, but also on design flow rate. The resulting cost estimation correlations for Pelton, Francis and Kaplan turbines were characterized by mean errors smaller than those of the best performing literature correlations.

Abstract Hydropower is not only a renewable and sustainable energy source, but its flexibility and storage capacity also makes it possible to improve grid stability and to support the deployment of other intermittent renewable energy sources such as wind and solar power. As a result, a renewed interest in pumped-hydro energy storage plants (PHES) and a huge demand for the rehabilitation of old small hydropower plants are emerging globally. As regards PHES, advances in turbine design are required to increase plant performance and flexibility and new strategies for optimizing storage capacity and for maximizing plant profitability in the deregulated energy market have to be developed. During the upgrading of old small hydropower plants, the main challenges to be faced are the design of new runners, that had to match the existing stationary parts, and the development of optimal sizing and management strategies to increase their economic appeal. This paper traces an overview of the prospects of pumped-hydro energy storage plants and small hydro power plants in the light of sustainable development. Advances and future challenges in both turbine design and plant planning and management are proposed. PHES and hybrid wind/solar-PHES are illustrated and discussed, as well as the limits and peculiarities of the new design strategies, based on computational fluid dynamics, for both PHES and small hydropower plants.

Abstract Intermittent renewable energy sources are characterized by a highly fluctuating, unpredictable and delocalized energy production, which significantly limits their penetration in the grid due to the great problems caused in the balance between demand and supply. Pumped Hydro Energy Storage plants represent an ideal solution because of their ability to provide large storage capacity with excellent grid connection properties, high cycle efficiency range and competitive costs. However, to provide primary and secondary regulation services, PHES have to increase their operation at part loads and to be able to switch fast and frequently between pump and turbine modes. At these operating conditions, pump-turbines suffer from behaviour instabilities, thereby constituting a limit when considering their exploitation in a wider continuous working range. So, the definition of a new concept of pump-turbines able to provide the full benefit of regulation in pumping mode and a wide range of power in generation mode is an urgent need to increase the exploitation of renewable energy sources. This paper clarifies the effects of the stable and unstable behaviour of pump-turbines on the power regulation capacity of pumped hydro energy storage plants, by presenting a description of the possible operating modes of PHES and by focusing on the impact of the hydraulic characteristics of pump-turbines on the capability of plant to start-up, shut-down or change its operating modes. A detailed review of the studies published in literature on the topic revealed the main characteristics of the hydraulic instabilities and the influence of one or more geometrical parameters on their onset. Even though some geometry modifications aimed at improving the RPT's stability in one operating mode were proposed in literature, the definition of a comprehensive design strategy, globally optimizing the pump-turbine design by considering simultaneously the complexity of the phenomena in both the operating modes, still represents a challenge.

Scroll compressors are widely adopted machines in both refrigeration systems and heat pumps. However, their efficiency is basically poor and constitutes the main bottleneck for improving the overall system performance. In fact, due to the complex machine fluid dynamics, scroll design is mainly based on theoretical and/or semi-empirical approaches. Designs strategies that do not guarantee an in-depth analysis of the machine behavior can be supplemented with a Computation Fluid Dynamics (CFD) approach. To this purpose, in the present work, the scroll compressor inner fluid dynamics is numerically analyzed in detail using two CFD software and two different modelling strategies for the axial gap. The analysis of the fluid evolution within the scroll wraps reveals unsteady phenomena developing during the suction and discharge phases, amplified by the axial clearance with negative impact on the main fluid flow (e.g., −13% of average mass flow rate for an axial gap of 30 μ) and on the scroll performance (e.g., +26% of average absorbed power for an axial gap of 30 μ). In terms of accuracy, the k-ε offers good performance on the estimation of average quantities but proves to be inadequate for capturing the complexity of the unsteady phenomena caused by the axial gap (e.g., −19% of the absorbed power in case of perfect tip seal). The need for considering specific geometric details in design procedures is highlighted, and guidelines on the choice of the most suitable numerical model are provided depending on the analysis needs.

In this article a systematic procedure aimed at achieving the best compromise between flow deflection, static pressure recovery, and total pressure loss is proposed for aerodynamic diffusers with incompressible flow. Such a result was accomplished by using a neural network to generalize the radial diffusers performance data obtained by numerical analyses, a multi-objective approach based on the employment of fuzzy sets, and a swarm particle algorithm to find a good compromise between flow deflection, static pressure recovery, and total pressure loss. Useful design tools, obtained by collecting the results in proper design charts, are finally proposed to simplify the design of radial diffusers without resorting to expensive and time-consuming procedures of optimization. The influence of the Reynolds number on the overall performance was also taken into account.

This paper presents an experimental analysis of the unsteady phenomena developing in a vaneless diffuser of a radial flow pump. Partial flow operating conditions were investigated using 2D/3C high repetition rate PIV, coupled with unsteady pressure transducers. Pressure measurements were acquired on the shroud wall of the vaneless diffuser and on the suction pipe of the pump, whereas PIV flow fields were determined on three different heights in the hub to shroud direction, inside the diffuser. The classical Fourier analysis was applied to both pressure signals to identify the spectral characteristics of the developing instabilities, and the high-order spectral analysis was exploited to investigate possible non-linear interaction mechanisms between different unsteady structures. A dedicated PIV averaging procedure was developed and applied to the PIV flow fields so as to capture and visualize the topology of the spectrally identified phenomena. The influence of these phenomena on the diffuser efficiency was also investigated.