Martino Marini

Performance of vertical axis wind turbines with different shapes

Abstract Different types of vertical axis wind turbines (VAWT) are presented and compared as regards their aerodynamic performances. A blade with a given airfoil section and length can be fastened to spokes in several ways giving rise to many configurations. Two models are used in the analysis of Darrieus turbines: the single streamtube momentum model and the free wake vortex model. The former is more suitable to carry out an extensive study because of its short CPU time, the latter, which guarantees the best accuracy, can be used to verify the most significant results. The diagrams of the average coefficient of power and of the power for a given speed of the attacking wind are shown. The work-exchange between the wind flow and the machine is investigated by defining an internal efficiency of the wind turbine.

Analysis of Transient Performance of a Compressed Air Energy Storage Plant

Electrical energy storage might become a strategic topic if distributed generation will be matched with stochastic sources as wind or sun. Compressed Air Energy Storage (CAES) is one of the most promising options today: energy is stored as pressurized air in a cavern. Transient phenomena, occurring during the charging process, are analyzed in this paper. Two kinds of systems are considered with or without pressure compensation; in fact a water column can be used to link the cavern to a pond in order to compensate the pressure oscillations. A lumped parameter model has been adjusted by the authors to simulate the initial charging and the subsequent surge. The obtained results supply some insight about the safe working conditions and also the surge operation.Copyright

Axial Flow Compressor Design Optimization: Part II — Through-Flow Analysis

A new technique is presented for the design optimization of an axial-flow compressor stage. The procedure allows for optimization of the complete radial distribution of the geometry since the variables, chosen to represent the three dimensional geometry of the stage, are coefficients of suitable polynomials. Evaluation of the objective function is obtained with a through-flow type calculation, which has acceptable speed and stability qualities. Some examples are given of the possibility to use the procedure both for redesign and, together with what was presented in Part I, for the complete design of axial-flow compressor stages.Copyright

Semi-Empirical Methods for the Analysis of Vertical Axis Wind Turbines With Helical Blades

The Vertical Axis Wind Turbine (VAWT) was not successful for large size installations but it is suitable for small size applications. It is likely to integrate better in an urban environment due to its architectural features and its intrinsic capability to exploit winds from any direction without an orientation system. The performances of these machines can be evaluated with enough accuracy by means of the semi-empirical models that allow to explore various configurations in a straightforward way. They need the support of experimental investigations for aerodynamic coefficients and they have to refer to a limited class of airfoils. However they can be matched with CFD tools usefully to enlarge the field of investigation keeping the above advantages. In the paper the performance of the H type (giromill) configuration as well as of the helical rotors are calculated and compared. The latter geometry has received recently a special attention on account of some aspects that are discussed.Copyright

Performance Analysis of a Wind Powered Gas Storage System

All over the world huge masses of gas are compressed in a number of storage stations to compensate seasonal fluctuations of the users’ demand versus the methane extraction from geological deposits. In the great majority of such plants, turbo-compressors are used, namely centrifugal machines. Since in this kind of machines compression is essentially adiabatic, gas temperature rises up even to dangerous values. Natural gas cannot be injected into the reservoirs too hot without risk of geological damage, so often an after-cooler has to be provided. Natural gas compressors are driven by gas turbines (GT), fuelled by part of the gas flowing through the station; otherwise electric motors connected to the general grid are used. In the paper the exploitation of the renewable energy of the wind to drive the compressors of the system is proposed. The matching with the driving wind turbine is different from the matching with a gas turbine or an electric motor. However whereas the stochastic character of the wind source affects power generation seriously, in the proposed use it is not a real problem: the only constraint consists of having enough wind energy to complete a charge all over a season. An in-house code, based on the lumped parameter approach and a quasi-steady dynamics, has been developed in order to simulate the system performance during a complete charge for a known wind distribution. The turbo-compressor is modeled through its characteristic maps. Similarly the wind turbines, that drive the storage station, and the fans, that counterbalance the friction losses of the after-cooler, are replaced with their characteristic curves. The after-cooler, which is a gas-air compact heat exchanger, is modeled by means of the overall heat transfer coefficient and the total pressure losses. Finally the reservoir is supposed omothermal and isothermal. In order to investigate the plant performance, different kinds of wind distributions have been considered and the corresponding operation paths as well as power and pressure evolutions are shown and discussed.© 2010 ASME

Performance of a Water Compensated Compressed Air Energy Storage System

In a growing energy scenario, electric utility companies have to take into account new managing strategies. The increasing seasonal gap in energy demand, the penetration of stochastic sources (wind and sun) and of combined heat and power plants are making more and more difficult to schedule power production. Energy storage can balance supply and demand over different time scales, with technical and economical benefits. The two options for large size plants are pumped storage hydro and Compressed Air Energy Storage (CAES). In the present paper, a CAES plant both with and without water compensation, is considered. The time window is an entire year as there is a remarkable difference between the seasons. Indeed in winter and summer the price fluctuation amplitude can be profitably exploited while between seasons are less suitable in a storage perspective because of the relative flatness of the daily price pattern. The adopted strategy is based on two price thresholds: below the former, a single charging step is carried out at night, above the latter, one or more steps of electricity production are carried out at peak hours. Finally, amid the thresholds, the plant works as a mere gas turbine or is shut off. Of course the mere GT working is available only if turbo compressor and expander are consistent and this affects the performance of each machine during charge or discharge phases. The shape of the daily price pattern strongly impacts on the cash flow. The proposed model is applied to the present Italian scenario as the energy market, taxes and services are concerned. The water compensated plant attains a storage density nearly twice higher than without compensation.Copyright

An Integrated Software Procedure to Support Teaching of Radial Inflow Turbine Design

The authors decided to organize their design/analysis computational tools in an integrated software suite in order to help teaching radial turbine, taking advantage of their research background and a set of codes previously developed. The software is proposed for use during class works and the student can either use a single design/analysis tool or face a complete design loop consisting of iterations between design and analysis tools. The intended users are final year students in mechanical engineering. The codes output are discussed with two practical examples in order to highlight the turbomachinery performance at design and off-design conditions. The above suite gives the student the opportunity of getting used to different concepts (choking, blade loading, performance maps, [[ellipsis]]) that are encountered in turbomachinery design and of understanding the effects of the main design parameters.Copyright

Parametric Analysis of Thermal Energy Storage for Gas Turbine Inlet Air Cooling

Gas turbine efficiency and power output are strongly dependent on the inlet air condition. Thus, several authors proposed the use of different inlet air cooling systems. Such systems include, as examples, spraying water in the inflow air stream or air cooling through a chiller during GT operation. In the latter case, it is possible to operate the chiller at night time, taking advantage of the remarkable price gap between peak and off-peak hours. A parametric analysis of such a system is presented, focusing on the effect of price gap, chiller and storage design parameters and climatic conditions on the optimal sizing of the plant. Both the gas turbine performance changes, due to the different inlet conditions, and thermal losses related to the storage system are taken into account. The economic return of the system is evaluated through the year-round integral of gas turbine fuel consumption and chiller electricity requirements, for given scenarios of electricity price tag, ambient temperature and humidity profile. For different boundary conditions (market constraints and climate) the optimal configurations are identified and discussed.Copyright

The design of a radial turbine for microgasturbine applications

An automatic software procedure, previously developed by the authors, is used for the design of a radial inflow turbine for microgasturbine applications. The procedure is formed by algorithms of different complexity levels ranging from the meanline one-dimensional design tool to the fully three-dimensional Navier-Stokes based analysis. Each code gives complementary information to the designer. The codes have been written and developed by the authors at DIMSET. The present application demonstrate the use of the above procedure and allows for a critical analysis of the tools involved.Copyright

Theoretical aerodynamic methods for VAWT analysis

The peculiarities of vertical-axis wind turbines (VAWTs), such as simple blade building and the absence of an orientation system, are studied. A comparison is made of three theoretical methods quantified by means of an overall parameter as the average coefficient of power. The importance of advanced-flow analysis media, such as vortex models or finite-difference models, is shown in their ability to capture flow details, as illustrated by some examples. Such an accurate knowledge of wind evolution near the rotor is necessary to project high-efficiency turbines and to design their structural elements.<<ETX>>