Giuseppe Grazzini

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.

Irreversible refrigerators with isothermal heat exchanges

Abstract A maximum for the coefficient of performance (COPf) is verified for a temperature difference between reservoirs and the hot isotherm of a closed irreversible cyclical refrigerator working at steady-state conditions. A maximum also exists when the COPf is considered as a function only of a parameter depending on the thermal characteristics of the heat exchangers. The influence of the parameter and entropy generation on the COPf maxima is described.

Numerical optimisation of a two-stage ejector refrigeration plant

Abstract Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.

Entropy parameters for heat exchanger design

A general expression for entropy generation in counter-current heat exchangers is developed. It is applicable to incompressible liquids and perfect gases. Two new entropy generation numbers are defined, NM and NQ. The analysis is applied to an air-air counter-current heat exchanger. The three entropy generation numbers, NS, NM and NQ, have a different variation with NTU at the various values of the capacity flow rate ratio employed in the calculations.

Thermodynamic parameters for energy sustainability of urban areas

This paper presents some thermodynamic indicators useful for energy planning of urban areas, and for defining the scenarios of integrated low environmental impact energy strategies and actions in an urban area. It is based on the results of the energy balance of a reference volume obtained by the Geographical Information System (GIS) techniques applied to the built-up area studied. The defined parameters allow evaluation of the sustainability of energy use in an urban area and its areal distribution in different building meshes superimposed on the area considered, using entropy.

Global analysis of dissipations due to irreversibility

A lot of studies performed in order to analyse irreversibilities has yielded both local and global stability evolution criteria. A global physical interpretation of the local dissipation potential and, from this, its expression, independent from the choice of f-space, are obtained.

Work from irreversible heat engines

A maximum for available work is verified for a temperature difference between reservoirs and a closed cyclical system working at steady-state conditions. There is also a maximum for efficiency with internal irreversibility. The dependences of the maxima on entropy generation and variations of the maxima with temperatures in the heating and cooling fluids are described. The maximum work per cycle is estimated from an exergy balance for an open system consisting of two input and output fluids that served as the heat sources.

A simple program to design a multi-stagejet-pump for refrigeration cycles

A PC quickbasic program was developed in the aim to design a jet-pump device for a water refrigeration cycle. At given conditions of generator, evaporator and condenser, with a defined cooling power, the design is automatic, yielding to a configuration which allows the maximum pressure gain the system can achieve. The physical model used is the classical one-dimensional with constant-area mixing. A second stage is used when the single one cannot achieve the necessary compression ratio, due to boundary condition. An annular primary flux is used for the second stage so a compact structure is obtained leading to an efficient utilization of kinetic energy of the secondary stream. The failure of perfect gas model for steam is discussed so a third stage was excluded. The comparison with some experimental literature data shows a good agreement for single stage model. The multistage annular jet-pump appears as a very promising device in the field of refrigeration.

Constructal thermodynamics combined with infrared experiments to evaluate temperature differences in cells.

The aim of this work was to evaluate differences in energy flows between normal and immortalized cells when these distinct biological systems are exposed to environmental stimulation. These differences were considered using a constructal thermodynamic approach, and were subsequently verified experimentally. The application of constructal law to cell analysis led to the conclusion that temperature differences between cells with distinct behaviour can be amplified by interaction between cells and external fields. Experimental validation of the principle was carried out on two cellular models exposed to electromagnetic fields. By infrared thermography we were able to assess small changes in heat dissipation measured as a variation in cell internal energy. The experimental data thus obtained are in agreement with the theoretical calculation, because they show a different thermal dispersion pattern when normal and immortalized cells are exposed to electromagnetic fields. By using two methods that support and validate each other, we have demonstrated that the cell/environment interaction can be exploited to enhance cell behavior differences, in particular heat dissipation. We propose infrared thermography as a technique effective in discriminating distinct patterns of thermal dispersion and therefore able to distinguish a normal phenotype from a transformed one.

Thermodynamic optimal design of heat exchangers for an irreversible refrigerator

Thermodynamic optimisation of energy systems is essential in reducing the environmental impact of energy utilisation. Yet, the refrigerators commonly used for this purpose have improvable efficiency levels. Their performance, as shown by the literature, is highly influenced by the size of the heat exchangers and by internal irreversibilities. In this paper the maximum coefficient of performance (COP) is obtained for an irreversible inverse Rankine cycle refrigerator working with the environmentally friendly fluid R134a. This is a steady-state refrigerator working as an open system which consumes external work, subtracts heat from a cold fluid stream at an inlet fixed temperature and assigns it to a higher fixed inlet temperature stream. Heat transfer irreversibilities in the shell-and-tube heat exchangers and external friction losses in the water streams are considered, ignoring only the internal pressure drop of vapor. A simulation program was developed to search the maximum COP at given external fluid temperatures, as a function of mass flows, dimensions and temperature differences in the heat exchangers. Owing to the large number of control variables involved, a numerical optimisation method was used to determine the maximum COP. The proposed method is fast, producing the maximum with acceptable approximation. It provides the refrigerating fluid evaporating and condensing pressures, the heat exchanger dimensions, and the water flow rates for a given cooling power with predefined inlet temperatures of cold and hot water streams. The heat exchanger area closely conditions the COP, so each maximum represents the optimum thermodynamic working conditions for a given area of the heat exchangers.