The paper shows novel technologies for heat and power generation for residential and industrial end users. The paper is focused on micro gas turbines of about 100 kWe and shows their profitability in the Italian scenario, making use of thermoeconomic methodologies. The paper explores different options that make use of biomass and natural gas in dual fuel arrangements. The possibility to set up combined cycle schemes by coupling the micro gas turbine to a bottoming organic Rankine cycle is also examined. The work is the basis of the research project “CLEVER” submitted to the European Union within the program Horizon 2020.

A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model and solved in the frequency domain. The paper shows that a complex eigenvalue problem is obtained that can be solved numerically by implementing the governing equations in a finite element code. This procedure allows one to identify the frequencies at which thermo-acoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, when the hypothesis of linear behavior of the acoustic waves can be applied. The method can be applied virtually to any three-dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by computational fluid dynamics methods proposed for analyzing the combustion chamber under instability condition. Furthermore, in comparison with the "lumped" approach that characterizes popular acoustics networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber. The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet, as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model. To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation in the parameters of the heat release law can influence the transition to instability.

A renewed interest on Vertical Axis Wind Turbines (VAWTs) arose from their ability to be effectively integrated within urban contests in the spirit of distributed generation. In order to improve their performance, a deeper comprehension of their fluid dynamic behavior is necessary. In the last years, at Politecnico di Bari a great effort has been addressed toward the numerical and experimental investigation of both lift- and drag-driven VAWTs. In particular, constant temperature hot wire anemometry (CTA) is used for the evaluation of the unsteady flow field downstream the VAWT prototypes tested in the subsonic closed-loop wind tunnel of the Politecnico di Bari, whilst, torque measurements are obtained directly from the servo amplifier monitor. Furthermore, by means of CFD analysis, a deep insight into the complex fluid-dynamics of the VAWTs has been obtained. All the acquired experience ended in the development of an innovative lift-driven VAWT prototype currently under investigation

The introduction of piezo-electric sensor on mass produced engines for in-cylinder pressure measurement represents a near future step in order to improve combustion control. Piezo-electric sensors provide pressure measurements affected by offset error and drift due to thermal sensitivity: a pressure reference is needed for each cycle in order to obtain the actual pressure value (pegging). Three methods for the evaluation of the offset, based on the hypothesis of polytropic compression, have been analyzed in this work: (1) a three-point referencing method, (2) a linear least-square method and (3) a non-linear least-square method. From such a comparison the three-point referencing method appeared the best suited for on-board calculation since it has the lowest computational cost even if it suffers from noise sensitivity. Hence, the accuracy and the efficiency of this method have been improved by means of an original methodology. The proposed method has been applied both to simulated and to experimental signals gathered from a 4-stroke spark-ignited engine. The results are compared to those obtained by using the other two methods showing much better accuracy in all the examined tests. Finally, a computational cost analysis proves the feasibility of the proposed method for efficient on-board pegging calculation with modern engine control units.

Aim of this paper is to provide a deep insight into the dynamic behavior of the flow through a Savonius rotor by means of computational fluid dynamics (CFD). The analysis is carried out solving the incompressible Unsteady Reynolds Averaged Navier-Stokes equations, providing fundamental information concerning the complex unsteady flow field in and around the rotor. The motivation for employing a numerical approach relies on the consideration that detailed analysis of wind turbines, aiming to improve their design, cannot be easily performed by means of experimental full-scale field-testing due to the lack of control on the test conditions. At the same time, few are the wind tunnels where large turbine prototype testing is possible, so that experimental tests are usually carried out inside wind tunnels having dimensions comparable with those of the prototype. Moreover, if the available wind tunnel has a confined test section, the turbine performance could be quite different from those expected in open field. Therefore, in this paper, the turbine is firstly supposed to operate in open field and then in a bounded test section, in order to analyze the effect of flow confinement and to correlate the turbine performance in open field with experimental results obtained from prototypes tested in small wind tunnels of assigned blockage.

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 system, 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 turbine 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.

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.

The focus of this paper is on the part load performance of a small scale (100kWe) 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 base load (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.

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 versatility. 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 finite 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 upwind 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 − epsilon turbulence model. The model has been validated, comparing numerical results with available experimental data.

In the last years, solar photovoltaic (PV) systems have had great impetus with research and demonstration projects, both in Italy and other European countries. The main problems with solar PV are the cost of solar electricity, which is still higher compared with other renewables (such as wind or biomass), due to the cost of semi-conductors, and the low conversion efficiency. However, PV panel prices are rapidly decreasing benefiting from favorable economies of scale. For instance, according to the Energy Information Administration (EIA) the US average levelized costs for plants entering service in the 2018 should be 144.3$/MW h for solar PV, whereas 111.0$/MW h for biomass and 86.6$/MW h for wind (Levelized Cost of New Generation Resources in the Annual Energy Outlook, 2013). In order to increase the electric yield of PV modules (which can be even doubled with respect to constant tilt configurations), without significantly increasing the system costs, it was decided to consider the addition of inclined mirrors at both sides of the PV modules, so as to deflect more solar rays towards them, as in Mirror-Augmented Photovoltaic (MAPV) systems. The system preserves its constructive simplicity with commercial flat PV modules even though dual axis tracker must be implemented, since MAPV systems harness mainly the direct radiation. The performance analysis of MAPV systems starts from the calculation of the global irradiation on the surface of the PV module which is a sum of the direct sunlight on it and the irradiation reflected by the mirrors. A mathematical model of a MAPV system is presented, which takes into account not only the increase of direct (or beam) radiation, due to the mirrors, but also the reduction of both the diffuse and reflected radiations due to the shadowing effect of the flat mirrors. In particular, under an isotropic sky assumption, a simplified analytical expression, applicable in the case of MAPV systems, for the sky-view factor has been developed. The deterioration in the performance of the PV system as a result of the increasing cell temperature with radiation augmentation due to mirrors has been also evaluated. Moreover, in order to provide a more realistic view of the process, the energy analysis is accompanied by the exergy analysis. Finally, in order to analyse the economics of MAPV systems, Net Present Value, Discounted Payback Period, Internal Rate of Return and Life-Cycle Costs, have been considered and compared with both a constant tilt building-integrated photovoltaic (BIPV) system and a system with a dual axis tracker.