• Energy Research

Natural gas is becoming increasingly important to meet the growing demand for energy, guaranteeing a reduction in polluting emissions. Transportation in form of Compressed Natural Gas (CNG) could be an alternative to the traditional transportation by pipeline or, as liquefied gas, by ships, but the ratio between the mass of transported gas and the container weight is currently too low. One of the many projects focusing on the development of innovative lightweight pressure cylinders is GASVESSEL, which proposes composite cylinders with a diameter of more than 3 m: loaded on a ship, they could allow transporting quantities of CNG as big as 10,000 tons. The related loading and unloading processes affect both the overall time required for transport and the quantity of transported gas; therefore, they have an impact on the economic feasibility of the whole project. In this paper, a newly developed process simulation model is presented that allows assessing the duration of the loading and unloading processes, the mass of transported CNG, and the amount of power and energy required by the process. The model is useful to support the design of the system considering different plant components and operating strategies. It is applied to the analysis of the loading and unloading of a ship that meets the GASVESSEL project specifications. The results show that the duration of the process is of the order of magnitude of 100 h, depending on ambient temperature, and that the energy consumption can vary in the range of 150–180 kJ for a unit mass of CNG. Finally, the model is used to simulate the same process with hydrogen, an energy carrier that allows meeting, together with the use of fuel cells, the requirements of zero local emissions. The results show increments of both the final loading temperature and compressor power with respect to the CNG case.

Reducing emissions from internal combustion engines is becoming one of the most important tasks for engine manufactures and transport regulatory organizations. In particular, the marine transportation sector is one of the most polluting, due to the intense maritime activity and the use of low-quality fuels, burned in Heavy Duty Diesel Engines, for ship propulsion and auxiliary power generation. In order to reduce the global shipping environmental impact, the IMO (International Maritime Organization) is restricting NOx and SOx ships’ emissions through the introduction of the IMO Tier III legislation, which requires to consider a wide spectrum of emissions reduction technologies and strategies, which are going to have an impact on the engine performance and fuel consumption. In this work, the main solutions being currently developed or adopted for low and medium speed Diesel engines have been reviewed from a qualitative, and sometimes quantitative, point of view, but, in comparison to previous literature, focusing more on their potential with respect to possible waste heat recovery systems utilization, such as, in particular, steam Rankine cycles and Organic Rankine Cycles (ORC). Indeed, even though many of the considered emissions mitigation technologies lead to a certain amount of penalty in fuel economy, the use of waste heat recovery systems to recover wasted engines energy could become interesting in order to develop more efficient but, at the same time, cleaner engines.

Micro Combined Heat and Power (CHP) systems based on Polymer Electrolyte Membrane (PEM) fuel cells are gaining increasing interest since they allow the production of electricity and heat in a decentralized, quiet, efficient and environmentally friendly way. However, the use of PEM fuel cell still involves different economical and technical issues to be overcome before widespread commercialization begins. In the last years, significantly research efforts have been made to the development of High Temperature PEM (HTPEM) fuel cells which could reduce system complexity and cost. In this paper, with the aim of reducing fuel cell stack size to enhance cost reduction, the possibility of integrating a batt ery storage system in a HTPEM based micro CHP system is evaluated. The proposed system encompasses a HTPEM fuel cell stack, a steam reforming fuel processor and a battery pack. The fuel processor allows the HTPEM fuel cell to be fuelled with conventional hydrocarbon based fuels while the battery pack allows peak shaving thus reducing fuel cell stack size.. Furthermore the choice of implementing a battery storage system contributes to the development of the smart grid technology by enabling off-grid and on-grid storage of energy. The overall system should provide total electrical and part of the thermal power requirements for a typical single family house. In order to evaluate the influence on system cost, two fuel cell system sizes have been considered and three different household electric load profiles have been analyzed. System performance, in terms of conversion efficiency, have been calculated and compared

The study presented in this paper aims to evaluate the performance degradation of Polybenzimidazole (PBI) based High Temperature PEM (HTPEM) fuel cells subjected to different ageing tests, according to a methodology already used by the authors. Three HTPEM Membrane Electrode Assemblies (MEAs) were characterized before and after different aging tests and performance compared. The three MEAs have been named MEA C, MEA D and MEA E. MEA C was subjected to 100,000 triangular sweep cycles between Open Circuit Voltage (OCV) and 0.5 A/cm2 with 2 s of permanence at OCV at each cycle. MEA D and MEA E were subjected to 440 h of operation at constant load of 0.22 A/cm2. In order to assess the cell performance, polarization curves, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) were recorded during the ageing tests. Degradation rates have been obtained for MEA C (44 mV/h), for MEA D (30 mV/h) and for MEA E (29 mV/h). ECSA (Electrochemical Surface Area) has been calculated for the three MEAs showing a reduction of approximately 50% for MEA C and of approximately 30% for MEA D and MEA E. Polarization curves during aging tests confirm that load cycling is more detrimental. A comparison with data obtained by the authors

ORCs are widely recognized as one of the most suitable solution for energy recovery, if the temperature of the heat source is of about 200°C, or lower. In case of heat sources of about 100 kW or smaller, the more common solutions prescribe a simple cycle and a single stage expander, in order to reduce complexity and costs. Scroll expanders, derived from scroll compressors, are expected to be available at very low unit costs. The drawbacks of this kind of solutions, originally designed for automotive, or HVAC applications, are mainly two: the low fixed volumetric expansion ratio and the small volumetric flow rate, that are not always well-suited for the requirements of power production. In this paper, different ORCs with multiple expansions are evaluated with the aim of achieving a better exploitation of small scale-low temperature waste heat sources. The comparison takes in consideration different possible solutions for the multiple expansions, with internally recuperated and not-recuperated cycles, whilst the data describing the actual behaviour of compressors derived scroll expanders have been previously obtained by a test rig, set up at the University of Trieste, using R245fa as working fluid.

The catalyst layer is one of the most critical components of a PEM fuel cell. The catalyst, in the form of platinum nanoparticles, tends to aggregate and sinter during operation, with the consequence to reduce its active area thus the efficiency of the entire fuel cell. This paper aims to highlight the efficiency of scanning electron microscopy (SEM) and synchrotron small-angle X-ray scattering (SAXS) to characterize the catalyst layer degradation of a commercial MEA for high temperature PEM fuel cells. SEM is an effective tool for the surface analysis of the samples, since it is able to obtain a magnification from 10x up to 105x. SAXS is a suitable technique to investigate the structural features of colloidal scale (between 10 and 103 Å). The paper presents in detail the analysis of three MEAs: one is a virgin sample, one was operated discontinuously in a single fuel cell, one operated in a 25 cells stack. The experiments show the degradation of the catalyst layer after operation. In particular, the agglomeration of the Pt particles leads to a growth in size from 5 nm up to 8 nm.

HTPEM fuel cells based on PBI-H3PO4 MEA have introduced some improvements respect to Nafion based PEM fuel cells: they withstand higher temperatures (up to 180°C) and therefore CO tolerance is increased. This simplifies fuel cell operation with hydrocarbon based fuels because some purification steps in the fuel processor are avoided. In this work an experimental system composed of a steam reformer with a capacity of 1 Nm3/h of hydrogen fuelled with propane and an HTPEM fuel cell has been described. Reformer start-up transient and steady state operation have been investigated. HTPEM polarization curves have been measured at steady state condition, operating the reformer at 3 different S/C ratios.

Recently, several efforts have been devoted to the improvement of the thermal efficiency of small gas turbines, in order to approach the typical values of the internal combustion engines in the same range of power. One possibility is represented by a combined cycle, obtained coupling the gas turbine to a bottoming organic Rankine cycle (ORC). This paper deals with the definition of the main features of an ORC system aimed to recover heat from a 100 kWe commercial gas turbine with internal recuperator. After the optimization of the thermodynamic cycles, involving a comparison between six working fluids, different expanders are analyzed, with the aim of detecting, if possible, the best suited machine. First, single stage turbines, in both radial and axial flow configuration, are designed specifically for each considered fluid, in particular investigating the opportunity of mounting the ORC expander directly on the high-speed shaft of the gas turbine. Then, the performances of these dynamic machines are compared with those of positive displacement expanders, such as scroll devices, obtainable from commercial HVAC compressor with minor revisions, and reciprocating ones, here newly designed.