Adriano Milazzo

A Thermodynamic Analysis of Multistage Adiabatic CAES

Adiabatic compressed air energy storage (A-CAES) represents a valuable and environmentally friendly option for massive energy storage. Existing examples of CAES refer to underground storage at medium pressure level. But for widespread utilization, independent from the availability of underground storage volumes, artificial reservoirs would be required. This requires rather high air pressure within the storage, which in turn will demand a carefully optimized recovery of the thermal energy released in the compression phase. Starting from a detailed thermodynamic analysis of the relevant design parameters and their influence on the system efficiency, we propose a comprehensive set of criteria for the design of the system, with particular attention to heat transfer devices.

Membrane systems for CO2 capture and their integration with gas turbine plants

Abstract The capture and sequestration of the CO2 emitted from fossil-fuelled power plants is gaining widespread interest for controlling anthropogenic emissions of greenhouse gases. Among technology options for CO2 capture, membrane-based gas separation systems are noteworthy owing to their low energy requirements, promising technology evolution and effective integration with power plants. This paper presents a mathematical model for membrane-based separation systems that is able to cover the most significant membrane types and configurations. This model has been integrated in a general simulation method for analysing and optimizing advanced energy conversion systems. Performance of these simulation tools is demonstrated by evaluating the influence of different operating conditions on the behaviour of pre-and post-combustion separation units, based on metallic or polymeric membranes. Finally, the feasibility of integrating a metallic membrane system into a chemically recuperated gas turbine (CRGT) power plant is explored, obtaining encouraging results for CO2 capture.

Carbon dioxide removal via a membrane system in a natural gas combined-cycle plant

Abstract Gas separation using polymeric membranes is generally believed to be poorly suited for CO2 capture from gas turbine based power plants, because the high air-to-fuel ratios produce large amounts of exhaust gas containing strongly diluted CO2. The driving force for selective permeation is obtained by compressing the entire exhaust stream, resulting in a significant energy penalty. Additional energy is required for permeate compression, which is impaired by the other gases emitted with CO2. In spite of these difficulties, the present study is concerned with membrane systems, which the authors believe warrant further investigation. The attention was focused on natural gas combined cycles (NGCC), so as to have a starting point with high energy efficiency and low baseline emissions. To reduce the exhaust gases and to concentrate the CO2, the NGCC is fitted with a flue gas recirculation system, which has proven to leave plant performance substantially unaffected. The energy requirements for exhaust gas compression are reduced by introducing a heat exchanger between the compressed and residual exhaust gas and recovering heat from the latter. The membrane separator has been modeled using conservative data reported in the literature. Various configurations, such as multiple compression-expansion and multistage membrane units, are considered. Permeate compression is also modeled, so as to highlight the influence of permeate purity on this process. The results show a rather moderate performance penalty. A specific emission of 100 g CO2/kWh corresponds to 48 percent efficiency. Admittedly a CO2 separator for a power plant would be larger than any other membrane system designed for other purposes. However, the system appears technically feasible and would offer valuable benefits in terms of emissions.

The surface roughness effect on the performance of supersonic ejectors

The paper presents the numerical simulation results of the surface roughness influence on gas-dynamic processes inside flow parts of a supersonic ejector. These simulations are performed using two commercial CFD solvers (Star- CCM+ and Fluent). The results are compared to each other and verified by a full-scale experiment in terms of global flow parameters (the entrainment ratio: the ratio between secondary to primary mass flow rate - ER hereafter) and local flow parameters distribution (the static pressure distribution along the mixing chamber and diffuser walls). A detailed comparative study of the employed methods and approaches in both CFD packages is carried out in order to estimate the roughness effect on the logarithmic law velocity distribution inside the boundary layer. Influence of the surface roughness is compared with the influence of the backpressure (static pressure at the ejector outlet). It has been found out that increasing either the ejector backpressure or the surface roughness height, the shock position displaces upstream. Moreover, the numerical simulation results of an ejector with rough walls in the both CFD solvers are well quantitatively agreed with each other in terms of the mean ER and well qualitatively agree in terms of the local flow parameters distribution. It is found out that in the case of exceeding the “critical roughness height” for the given boundary conditions and ejector’s geometry, the ejector switches to the “off-design” mode and its performance decreases considerably.

Ejector CFD Modeling

The prediction of the supersonic ejector dynamics implies the accurate description of all the complex flow features discussed in Chap. 2. Unfortunately, the theoretical modeling approach necessitates a number of simplifying assumptions and empirical constants that introduce significant uncertainty and reduce the capability of capturing a number of relevant flow features. In this respect, computational fluid dynamics may represent a tool to overcome these difficulties and analyze the flow details of arbitrary ejector geometries.

Physics of the Ejectors

The global behavior of the ejector results from a combination of complex flow features including shock trains, turbulent mixing layers bounded by wall regions, shock-induced separations, boundary layers subject to adverse pressure gradients, non-equilibrium phase change, etc. It is because of this complexity that ejector design and performance have thus far been difficult to characterize and optimize.

Ejector Chillers for Solar Cooling

Ejector chillers are being studied at Department of Industrial Engineering of Florence (DIEF) since 2000, both theoretically and experimentally. This chapter discusses the application of solar-powered chillers in air conditioning and details the fundamental parameters. This technology is not far from being a technically feasible alternative to commercially available single-stage absorption chillers, but obviously a huge effort is still needed to improve its performance and gain access to the market.

External combustion engine with Stirling open cycle

An open-cycle engine approximating the Stirling cycle but retaining external combustion is being developed. The Stirling engine is being built on a motorcycle V-engine; however, the heat exchangers have been designed from scratch. Some thermodynamic considerations justifying the use of the open cycle are presented. A numerical simulation of the proposed open cycle is described. An optimization of the heat exchanging system, making the use of an entropic parameter, is attempted, and the results are presented. The design of the experimental motor has been completed and is waiting for experimental testing to verify the cycle effectiveness and the optimization criteria.<<ETX>>