[en] This paper deals with the main features of two emerging technologies in the field of small-scale power generation, micro turbines and Solid Oxide Fuel Cells, discussing the extremely high potential of their combination into hybrid cycles and their possible role for distributed cogeneration

[en] The article, following and completing the issues dealt with in part 1 (CH4 Energia Metano, 1/99, p. 7), describe the operating characteristic and construction features of molten carbonate and solid oxide fuel cells (MCFC and SOFC). For the latter type, construction cost are evaluated by various authors and research institutes. The article ends by presenting some tables showing the classification and the main characteristics of various fuel cells, and well as the effect of some gases on the behaviour of some of them

[en] Direct conversion of chemical energy into electricity (without intermediate heat generation) is a long-established method to improve the efficiency of power generation, as well as to reduce polluting emissions from thermal plants. The origins of fuel cells, as well as their operating principles, are dealt with. Then, various types of cells are taken into consideration, on the basis of both their characteristics and the operating principles of electrolytes. Finally, structure and operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC), Alkaline Fuel Cells (AFC) and Phosphoric Acid Fuel Cells (PAFC) are described

[en] This paper deals with the main features at present state-of-the-art fuel cell and hybrid cycle technologies, discussing their actual performance, possible applications, market entry perspectives and potential development

[en] This work discusses the design and the development of a Laboratory of Micro-Cogeneration (LMC) at Politecnico di Milano. The LMC laboratory is a unique structure devoted to small-scale power generation, with the main goals of testing and improving the performance of systems that produce or utilize electric and thermal (hot and/or cold) power in a very general sense, spanning from combined heat and power (CHP) units to heaters, from absorption chillers to heat pumps, but also able to perform tests on fuel processors and electrolyzers. The laboratory features a supply of natural gas as well as H₂ and O₂ from a high pressure electrolyzer and of CO, CO₂ and N₂ from bottles, permitting to carry out experiments with simulated synthesis fuels. The maximum allowable electrical power produced, exported to the grid or to an electronic loadbank, or consumed by the system under test is 100 kW; maximum allowable thermal power is roughly 200 kW with variable temperature water circuits (from chilled water up to a 150 °C at 8 bar superheated water loop). This work outlines also the instruments used for on-line recording of thermodynamic properties, emissions and power, aiming at monitoring and reconstructing mass and energy balances. One of the first experimental campaign has been carried out on a CHP system based on polymer electrolyte membrane fuel cells (PEM), a promising candidate for distributed CHP thanks to low pollutant emissions and good efficiency, rapid startup and flexibility, although affected by a rather complex fuel processing section to provide the appropriate fuel to the PEM. This work presents the experimental analysis of a 20 kW prototype PEM CHP system complete of natural gas processor. The prototype is operated at LMC to characterize the processing section and the thermodynamic performances of the overall system. Despite its non-optimized layout, the unit has shown encouraging total efficiency (76%) and primary energy saving index (6%). - Highlights: • It is presented a new Laboratory of Micro-cogeneration systems. • Laboratory layout, infrastructure and instrumentation is discussed. • Test conceptual operation and circuit arrangement are presented. • Test results on a 30 kW CHP unit based on PEM are presented

[en] Highlights: • A Stirling rated at 1 kWe and 8 kWth is analyzed experimentally and numerically. • The developed model is an extension of the work by Urieli and Berchowitz. • The initial pressure of the working fluid (nitrogen) is varied from 9 to 24 bar_g. • The initial pressure influences strongly the fuel input and the electrical power. • Major losses: wall heat conduction and non-unitary regenerator effectiveness. - Abstract: Micro-cogeneration Stirling units are promising for residential applications because of high total efficiencies, favorable ratios of thermal to electrical powers and low CO as well as NO_x emissions. This work focuses on the experimental and the numerical analysis of a commercial unit generating 8 kW of hot water (up to 15 kW with an auxiliary burner) and 1 kW of electricity burning natural gas. In the experimental campaign, the initial pressure of the working fluid is changed in a range from 9 to 24 bar_g – 20 bar_g being the nominal value – while the inlet temperature of the water loop and its mass flow rate are kept at the nominal conditions of, respectively, 50 °C and 0.194 kg/s. The experimental results indicate clearly that the initial pressure of the working fluid – Nitrogen – affects strongly the net electrical power output and efficiency. The best performance for the output and efficiency of 943 W and 9.6% (based on the higher heating value of the burnt natural gas) are achieved at 22 bar_g. On the other hand, the thermal power trend indicates a maximum value of 8420 W at the working pressure of 24 bar_g, which corresponds to a thermal efficiency of 84.7% (again based on higher heating value). Measurements are coupled to a detailed model based on a modification of the work by Urieli and Berchowitz. Thanks to the tuning with the experimental results, the numerical model allows investigating the profiles of the main thermodynamic parameters and heat losses during the cycle, as well as estimating those physical properties that are not directly measurable. The major losses turn to be the wall parasitic heat conduction from heater to cooler and the non-unitary effectiveness of the regenerator.

[en] Highlights: • Comparison of three 10 kW_el micro-CHP systems based on PEM fuel cell was assessed. • Micro-CHP performance at rated power and partial load was investigated. • Annual micro-CHP energy balance in the distributed generation scenario. • Definition of the specific bearable cost of the three systems. -- Abstract: This work, developed within the Italian project MICROGEN 30, aims at investigating the benefits of 10 kW_el Proton Exchange Membrane fuel cell based system with an innovative membrane reformer when applied to a residential application. Results are compared to solutions where a steam reformer is coupled with low temperature or high temperature Proton Exchange Membrane fuel cell stacks. The three cogenerator systems are integrated in a distributed generation scenario, working as suppliers of electricity and heat to two or more residential users. Micro-cogenerators energy and economic balance is evaluated using an in-house software, based on an heuristic algorithm that explores and defines the optimal system operating strategy versus defined load and tariff profiles. The innovative configuration achieves the highest micro-cogeneration economic saving on an yearly basis. The economic analysis also sets the maximum investment cost of the innovative cogenerator system being economically competitive with respect to centralized power generation and conventional boilers.

[en] Highlights: • SOS-CO₂: hybrid cycle integrating a SOFC with oxy-fired semi-closed Brayton cycle. • Natural gas fired power plant with high efficiency, zero emissions and CO₂ Capture. • Cycle modelled in Aspen Plus and operative conditions optimized for max efficiency. • Optimal SOS-CO₂ with net electric efficiency up to 76%_LHV and zero CO₂ emissions. • Efficiencies close to 70% even with uncooled expander, Uf = 0.75 and p_MAX = 2.75 MPa. This paper presents a new hybrid cycle based on the integration between a pressurized solid oxide fuel cell (SOFC) and a semi-closed regenerative intercooled Brayton cycle using a CO₂-rich stream as the working fluid. Nearly pure oxygen is used as oxidant for both the Brayton cycle combustor and the fuel cell. The cycle is conceived to produce electricity while capturing 100% of the produced CO₂ using natural gas or other fuels suitable for SOFC fuel cells. If the maximum cycle pressure is above the CO₂ critical pressure, the semi-closed Brayton cycle becomes a supercritical CO₂ cycle with the related efficiency advantages. In this work, the cycle is modelled with Aspen Plus and its design variables are optimized to find the maximum electric efficiency using an ad-hoc optimization approach. In the case study assessed (natural gas thermal input of 500 MW), the optimized cycle, working at 40 MPa with a cooled expander, achieves an outstandingly high efficiency of 75.7% (LHV basis) with CO₂ capture. The sensitivity analysis shows that similar efficiency values can be achieved even with less challenging operating conditions for both the Brayton cycle and fuel cell (maximum cycle pressure of 27.5 bar, uncooled turbine and fuel utilization factor of the fuel cell equal to 0.75).