Marco Torresi

Part Load Performance and Operating Strategies of a Natural Gas–Biomass Dual Fuelled Microturbine for CHP Generation

The focus of this paper is on the part load performance of a small scale (100 kWe) combined heat and power (CHP) plant fired by natural gas and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative micro gas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with natural gas takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for pre-heating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant.Different biomass/natural gas energy input ratios are also modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions.Copyright

Design of a ducted wind turbine for offshore floating platforms

Within the Marine Energy Laboratory project, funded by the Italian Ministry for Education, University and Research, one of the first offshore wind plants on floating hulls, hosting ducted wind turbines, has been considered. The confinement of horizontal-axis wind turbines inside divergent ducts is reconsidered, in light of material innovation and direct drive coupling. Ducted wind turbines can take advantage of the flow rate increase due to the effect of the divergent shrouds. The conventional blade element momentum theory has been reformulated in order to deal with ducted turbines. Furthermore, computational fluid dynamics simulations have been carried out based on the solution of the steady two-dimensional Reynolds-averaged Navier–Stokes equations for axisymmetric swirling flows. In order to avoid any expensive mesh refinement near the actual rotor blades, the turbine effect on the flow field is taken into account by means of source terms for the momentum equations solved inside the domain swept by the rotor. This technique allowed us to optimize the geometry of the ducted wind turbine in an extremely effective way.

Externally Fired Micro Gas Turbine and ORC Bottoming Cycle: Optimal Biomass/Natural Gas CHP Configuration for Residential Energy Demand

Small scale Combined Heat and Power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small scale micro turbine for heat and power cogeneration, a bottoming ORC system can be coupled to the cycle, however this option reduces the temperature and quantity of cogenerated heat available to the load. In this perspective, the paper presents the results of a thermo-economic analysis of small scale CHP plants composed by a micro gas turbine (MGT) and a bottoming Organic Rankine Cycle (ORC), serving a typical residential energy demand. For the topping cycle three different configurations are examined: 1) a simple recuperative micro gas turbine fuelled by natural gas (NG), 2) a dual fuel EFGT cycle, fuelled by biomass and natural gas (50% energy input) (DF) and 3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming cycle is a simple saturated Rankine cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild and hot climate conditions) and CHP plant operational strategies (baseload vs heat driven vs electricity driven operation mode) on the global energy efficiency and profitability of the following three configurations: A) MGT with cogeneration; B) MGT+ ORC without cogeneration; C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following tradeoffs: (i) lower energy conversion efficiency and higher investment cost of high biomass input rate with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration in the bottoming ORC; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.Copyright

CFD Analysis of the Flow Through Tube Banks of HRSG

The prediction of the performance of HRSG (Heat Recovery Steam Generator) by means of CFD codes is of great interest, since HRSGs are crucial elements in gas turbine combined cycle power plants, and in CHP (combined heat and power) cycles. The determination of the thermo-fluid dynamic pattern in HRSGs is fundamental in order to improve the energy usage and limit the ineffectiveness due to non-homogeneous flow patterns. In order to reduce the complexity of the simulation of the fluid flow within the HRSG, it is useful modeling heat exchangers as porous media zones with properties estimated using pressure drop correlations for tube banks. Usually, air-side thermo-fluid dynamic characteristics of finned tube heat exchangers are determined from experimental data. The aim of this work is to develop a new procedure, capable to define the main porous-medium non-dimensional parameters (e.g., viscous and inertial loss coefficients; porosity; volumetric heat generation rate; etc...) starting from data obtained by means of accurate three-dimensional simulations of the flow through tube banks. Both finned and bare tube banks will be considered and results presented. The analysis is based on a commercial CFD code, Fluent v.6.2.16. In order to validate the proposed procedure, the simulation of an entire fired HRSG of the horizontal type developed by Ansaldo Caldaie for the ERG plant at Priolo (Italy) has been performed and results have been compared with their data.Copyright

An innovative polyimide microchannels cooling system for the pixel sensor of the upgraded ALICE inner tracker

The ALICE Inner Tracking System (ITS) is the key detector for the study of heavy flavour production at LHC. In order to enhance the ALICE physics capabilities and the tracking performance, an upgrade has been planned for replacing the existing ITS in its entirety with seven new layers of monolithic silicon pixel detectors. A novel light cooling system for the ITSs innermost layer has been developed and fabricated. It consists of polyimide microchannels able to remove the power dissipated by the active electronics through forced convection of water coolant. The development, the construction, the mechanical-thermal characterization and the integration of this cooling system will be presented.

A Gas-Steam Combined Cycle Powered by Syngas Derived from Biomass☆

Abstract In this paper, an innovative power plant, constituted by a gas turbine in combined-cycle fuelled by a synthesis gas (or syngas), produced in a local biomass gasifier, is analyzed. The plant is integrated with an external combustion sys- tem, fed by cellulosic biomass, connected to a heat exchanger able to increase the air temperature, as in a regenerative cycle. The combustion products pass through a primary heat exchanger placed in the external combustion system, heating the compressed air, which flows into the principal combustion chamber, where a defined quantity of syngas, coming from the gasifier, reacts with the compressed air in a combustion process. The expanded gas, at the tur- bine exit, before going back into the external combustor, passes through a Heat Recovery Steam Generator (HRSG1) transferring heat to the bottoming Rankine cycle. The superheated steam undergoes an expansion in a steam turbine providing electrical energy. The syngas used in the combustion chamber is produced by a gasification process, based on a Fast Internally Circulating Fluidized-Bed (FICFB). Heat is transferred from the hot syngas (coming from the gasifier) to water, through a second Heat Recovery Steam Generator (HRSG2), producing steam, which is introduced in the gasifier, reacting with the pomace biomass in order to produce the syngas; since the produced quantity of steam is not sufficient for the gasification process, a further quantity of steam is produced in an auxiliary boiler fed by diesel oil, or in different ways, as described in the paper. This kind of plant is especially interesting for regions, like Italian Apulia, where there is a wide culture diffusion for the use of biomass, particularly from olive products, where there are available technologies for use of pruning, virgin and exhausted pomace, and where there are the market conditions for the commercialization of these resources and the incentives available for their energy development. Finally, the overall plant performance is calculated, shown and discussed.

A 3D Unsteady Analysis of a Wells Turbine in a Sea-Wave Energy Conversion Device

A Wells turbine designed for power generation in an innovative OWC (Oscillating Water Column) device for sea-wave energy conversion is investigated. This work aims to predict the turbine performance under a continuously variable reciprocating flow due to the action of the sea waves. When the amplitude of the oscillating flow reaches high values, stall occurs around the blades with a drop in the turbine performance. CFD simulation has been carried out aiming to provide an insight of flow behavior over the blades approaching these large amplitude flow conditions. Three test cases, preliminarily examined in order to assess the capability of the numerical methods, are presented and discussed: the first test-case concerns the 2D unsteady turbulent flow past a symmetrical airfoil undergoing oscillating pitching motion; the second test case concerns the 3D analysis of a high solidity Wells turbine in presence of different constant axial fluxes; the third test case concerns the 3D analysis of a high solidity Wells turbine in presence of an oscillating axial flux. Finally the flow past the low solidity Wells turbine rotor is simulated. The analysis has been performed by considering the flow to be unsteady, incompressible and viscous, while turbulence was modeled using the one-equation Spalart Allmaras model or the two-equations k-ω model.Copyright

An Efficient 3D CFD Model for the Analysis of the Flow Field Around Darrieus Rotors

An efficient numerical technique has been developed in order to investigate flow characteristics and global performance of Darrieus rotors. The interest for this kind of vertical axis wind turbines arises from their great capacity for integration within urban areas and for distributed generation.The proposed methodology is based on the solution of the steady three-dimensional governing equations, by means of a robust commercial CFD code. Since the effect of the turbine blades on the flow field is simulated through the introduction of momentum sources in the porous shell representing the volume swept by the turbine blades, any expensive refinement of the grid, near the rotor, is avoided. This approach dramatically reduces the computational costs, with respect to conventional unsteady flow simulations. The model efficiency enables the simulation of the flow field around Darrieus rotors considering complex and realistic computational domains, for instance when these turbines are clustered within a wind farm or placed inside urban areas.The methodology is validated by reproducing the performance of the Sandia 17-meter Darrieus rotor with approximate troposkien shape. Comparisons with other codes are also presented in order to highlight the advantages of the proposed method.Copyright

Numerical investigation of a Darrieus rotor for low-head hydropower generation

Abstract The aim of this paper is to numerically investigate the performance of a cross-flow water turbine of the Darrieus type for very low head hydropower applications. The interest for this kind of vertical axis turbine relies on its versa- tility. For instance, in the field of renewable energy, this kind of turbine may be considered for different applications, such as: tidal power, run-of-the-river hydroelectricity, wave energy conversion. Until now, low head hydropower, with heads less than 2 meters, has remained scarcely developed due to the relatively low energy density, which makes the cost of generation higher than traditional hydropower applications. However, in the spirit of distributed generation, the use of low head hydropower can be reconsidered, having the advantage of lower electricity transmission losses due to the localization near the consuming area. Nonetheless, it is fundamental to improve the turbine performance and to decrease the equipment costs for achievement of “environmental friendly” solutions and maximization of the “cost-advantage”. In the present work, the commercial CFD code Fluent is used to perform 2D simulations, solving the incompressible Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations discretized by means of a fi- nite volume approach. The implicit segregated version of the solver is employed. The pressure-velocity coupling is achieved by means of the SIMPLE algorithm. The convective terms are discretized using a second order accurate up- wind scheme, and pressure and viscous terms are discretized by a second-order-accurate centered scheme. A second order implicit time formulation is also used. Turbulence closure is provided by the realizable k− [turbulence model. The model has been validated, comparing numerical results with available experimental data.

Design of an Axial Impulse Turbine for Enthalpy Drop Recovery

In industrial process plants, often there is the need to reduce the pressure of the operating flow. Generally this is performed by means of valves which expand the flow without any work done. The same operation could be performed by replacing these valves with turbines, with the advantage of energy recovery, hence improving the overall efficiency of the system.In this work, a simple and rapid method is shown in order to design a single stage, straight bladed, axial impulse turbine for enthalpy recovery. Assigned the desired flow rate and the minimum power output, the turbine design is performed according to a one-dimensional study into which loss effects are considered by means of appropriate coefficients. From the one-dimensional analysis the heights, the pitch angle, the inlet and outlet angles of both rotor and stator blades are obtained. Actually, the rotor and stator blade profiles are defined by means of several analytical functions. The blade design is then validated by means of CFD simulations. The definition of loss coefficients and blade geometrical parameters is clearly an iterative process, which needs to be repeated until convergence is reached. Furthermore, by means of fully 3D simulations, the effect of the rotor-stator distance is investigated in order to maximize the turbine performance.Copyright