Marco Lucentini

Evaluation of the overall efficiency of a low pressure proton exchange membrane fuel cell power unit

Polymer electrolyte membrane fuel cells (PEMFC) at present can be considered the best choices to set up the auxiliary power unit of an hybrid electric vehicle. This paper describes how it is possible and convenient to manage a PEMFC at low pressure, using a blower instead of a compressor/expander. The polarization curve obtained from experimental tests on the stack confirms a typical behaviour at different values of pressure; although high pressure brings best performance, in the balance of the plant the air compressor generally absorbs more then 40% of the power generated from the stack. As a matter of fact, the overall efficiency of the whole FC apparatus can be improved by managing it at low pressure. In an FC/batteries hybrid electric vehicle, the FC is asked to operate not at partial load, but only at full load by using an ON-OFF control. Consequently it is possible to arrange a very compact and reliable apparatus in which the dynamic mass flow and pressure controls are not required and the mechanical and electric hardware is very simple.

The potential market analysis of a small cogeneration system based on Stirling cycle

In the past decade there have been rapid improvements in small-scale combined heat and power technologies. With the deregulation of electric power, observers see little future for large, expensive central power plants that require miles of transmission wires and take decades to pay off. Future demand will be met by smaller power generation systems located closer to where the electricity is used. In the coming years, the worlds electricity systems will change beyond recognition. This change is being driven by the need to address growing environmental challenge. The global policy response to climate change will probably be most important influence on the power sector in the future. Localized forms of the power production-based on cogeneration, renewable energies, fuel cells and other forms of distributed power system-have less environmental impact than conventional centralized electricity systems. In this paper, the authors emphasise Stirling technology. The Stirling engine (SE) is a heat machine which operates on a closed thermodynamic regenerative cycle, is capable of very high efficiency and can be used as a power system, refrigerator or heat pump. SEs may be used as power systems, converting heat to work, as refrigerators for cooling, and as heat pumps for elevating the temperature of heat drawn from an ambient temperature source.

Conceptual Study of a Thermal Storage Module for Solar Power Plants with Parabolic Trough Concentrators

Technologies and methods for thermal energy storage have been well tested in CSP Con‐ centrated Solar Power – plants [1, 2]. Solar tower plants (e.g. Solar Two, USA) and advanced parabolic trough plants (e.g. Archimede by ENEA, Italy) use molten salts both as heat trans‐ fer and thermal storage fluid. Differently, traditional trough plants (e.g. Andasol, Spain) dis‐ tinguish the fluid through the solar field (synthetic oil) from the one used in the storage system (molten salt). Hence, storage applications have only been proven in liquid state and in large scale plants.

A Model for the Evaluation of Energy Indicators in End Users Audit for Hospital Sector

Hospitals are complex building systems and they uses a large amount of energy in many different ways. Generally energy efficiency is not one of the main priorities in such structures, where the management mainly focuses on the continuity and the reliability of the energy supply. As a consequence this sector shows a high potential for energy savings. Furthermore the energy efficiency measures enforce the energy systems reliability. The first step in implementation of energy efficiency objectives and measures is the audit phase where the initial energy performance is assessed. Generally a complete audit needs the collection of many parameters and data (indicators) and that makes this procedure very long and expensive. Sometimes it is impossible to completely estimate the energy performance because of a lack of information. This paper proposes an algorithm able to operate a substantial reduction of all the indicators to be collected for the evaluation of the performance of energy end users. The model makes possible to estimate all the energy performance indicators (dependent) as a function of a small number of indicators (independent) whose estimation is simpler. It has been applied to the hospitals sector, but can be applied also to domestic energy sector and to a large number of the tertiary and commercial end users. The model operates the implementation of the mutual relationships among all the indicators involved in the energy performance evaluation. The indicator values are defined by applying mainly a statistical analysis of an adequate number of energy efficiency case studies. Furthermore design standards are applied in order to improve the indicators definitions. The model has been tested and validated in some case studies discussed.© 2011 ASME

Energy transfer processes of working gas-in pulse tube

The pulse tube cooler is a device for cooling to low temperatures. It runs on a modified Stirling cycle but has no cold moving parts and the gas flow in the pulse tube can carry heat energy away from a low temperature point under certain conditions. An orifice or other device controlling the flow at the end of the cooler provides the condition for cooling to occur. Pulse tube coolers have great commercial potential; offering low cost, low vibration and high reliability and they can be designed in variety of configurations including linear, U-tube (folded 180 in the middle) and concentric. They can be designed to cool to 200 K, to as low as 2 K, or to any temperature in between. The cooling power ranges from milliwatts to kilowatts depending on cooler size and operating temperature. In the development of pulse tube coolers, it is important to study the energy transfer process of the working fluid in the pulse tube. The enthalpy flow in the pulse tube is the gross cooling power of the cooler. Since the pulse tube section is nearly adiabatic, there are large temperature oscillations yielded by pressure oscillation. To evaluate the temperature oscillation, a mathematical model was developed. With this model, the analytical result is that the temperature ratio of the working fluid is the function of pressure ratio between hot and cold heat exchanges.

Photovoltaics in Italy, Mechanisms of Promotion: A Cost-Benefit Analysis of the Italian "Conto Energia" and Evaluation of Externalities

Italy is currently one of the fastest growing photovoltaic markets worldwide. The introduction of the feed-in-tariff mechanism called “Conto Energia” has raised and developed the market, contributing to the reduction of GHGs emissions and to increase the diffusion of the smart grid model.

Energy Efficiency Management Protocol in the Large Scale Retail

The paper presents the results of a two years research on the field of energy management systems. These systems range from the largest ones found in industrial plants down to the smallest utilized by the residential consumers. The goal is to define an energy management system specific to supermarkets sector including hypermarkets operating in large scale retail. The need to ensure continuity and quality of energy services, the high energy consumptions, the complexity of operation and maintenance facilities and, in general, the lack of strategies in energy end uses are just some of the reasons that have oriented the research in this sector. On the other hand, the groups operating in the large-scale retail, in order to contain costs, have always walked the path of maximum standardization of plants characteristics or, in most cases, the implementation of economy of scale in purchasing and maintenance; strategies not sufficient to ensure real savings considering the rising costs of energy. The starting points of the research have been some energy efficiency actions applicable to this type of consumers: buildings and facilities design, optimization of energy purchasing, management of maintenance, monitoring and collecting energy data, promotion of best practices in end uses, energy benchmarking, etc. For the purpose an energy audit was developed in the biggest supermarket and hypermarket chain in Italy during the last two years; the results led to the implementation of an operative protocol that makes possible to achieve energy savings in excess of 20%. The large number of supermarkets analyzed, the rigorous approach to the measurement and the monitoring of energy data, the possibility to verify the results in a ongoing way and the use of simulation models and software, permit a feasible extension to similar contexts.Copyright

Development and design of cold heat exchanger of pulse tube cooler

During the past fifteen years, heat transfer in oscillating flows has become a subject of increasing interest in the engineering community. Applications of oscillating flows include, for example, cooling electric equipment or alternative, environmentally safe refrigeration technologies, such as pulse tubes or Stirling refrigerators. Important components of these refrigerators are their heat exchangers. In such devices, the working fluid is subject to oscillatory forcing which is a key part of the process, as opposed to situations where oscillations are generated with the aim to enhance heat transfer. Heat transfer in oscillating, and often compressible flow, has not yet been completely understood, and the lack of design methodologies for heat exchangers in such flow is one reason that efficiencies of these devices are limited. A practical design method of a cold heat exchanger was developed in this paper. The influence of the working gas oscillation on heat transfer was considered in this method. The design parameters related to gas oscillation can be calculated using known methods.