Residential Energy Supply: Fuel Cells ppsx

Residential Energy Supply: Fuel Cells LJo ¨ rissen, Zentrum fu ¨ r Sonnenenergie- und Wasserstoff-Forschung Baden-Wu ¨ rttemberg, Ulm, Germany & 2009 Elsevier B.V. All rights reserved. Introduction For most of the time, electric energy supply was based on large centralized pow er stations, the big ge st of them located close to coal mines. Electricity distribution traditionally is done by means of high voltag e pow er lines feeding the electricity distribution grid. With time, electric utilities became large enterprises integrating electricity generation, distribution, and customer relations. Electricity generation mainly is powered by heat engines, their e fficienc y being determined by the temperature difference of the work fluid. Due to their location, most central po w er stations could not mak e use of the lo w-temperature w aste heat resulting as a by-product of electricity generation. Due to cheap primary energy supply, no charges on emissions, and the position of the utilities, there was no incentiv e to optimize the overall utilization of energy on the e xpense of electrical efficienc y. Meanwhile, the general situation has changed. De- regulation of the energy markets starting already in 1978 in the United States now also takes place in the European energy markets. Furthermore, global warming and en- vironmental concerns are putting pressure on the utilities to act. Efficient use of energy as well as the reduction of air pollution and carbon dioxide emissions are the key challenges in the twenty-first century. It is expected that a future energy portfolio will be consisting of a balanced mix of centralized power plants, the use of renewable energies, distributed generation (DG), and combined heat and power generation (CHP). Fuel cells are electrochemical po wer sources co n v erting the chemical energy contained in the fuel d irectl y into electrical energy. Since the efficienc y of fuel cells is not restricted by the Carnot c ycle as is the case in heat engines, one can expect higher efficiency of e lectricity generation. Furthermore, since most of the fuel is conve rted electro- chemically, one can expect low er emissions resulting from combustion processes. In the c onte xt of small p ower DG and CHP, fuel cells are considered to be a promising so- lution to pro vide electricity and heat with high efficiency and low emissions. In principle, small fuel cell systems could be installed in indi vidual homes or s mall buildings. Such systems are called residential fuel cell systems. Residential fuel cell applications may be divided in the following categories: • Backup and emergency power generation: Here the fuel cell is used to provide electri c power only for example during power outages or as a part of power management system. • Combined heat and power generation: Electricity and heat provided by the fuel cell are used simultaneously on site. Residential fuel cells are developed to power single- or multi-family homes. For single-family homes, the electric system power level typically is in the range of 1 kW. Fuel cell systems for multi-family homes or small businesses are to provide an electric power of approximately 5 kW. Full benefits of e mission reduction and efficiency g ain can only be obtained w hen the fuel cell systems are used in CHP mode. Therefore, further discussion will be limited to fuel cell use in combined heat and pow er applications. The following benefits are expected when using fuel cells as compared to separated generation of elec tricity and heat: • low primary energy demand, • very low emissions, • low noise operation, and • modular design allowing cost reduction by mass manufacturing. The implementation of small power CHP on a broad basis will impose challenges to the traditional business model of the utilities since in current electric grid str uctures the energy flow is oriented top-down from the central power station s to the final customer. In the ex- pected situation involving increasing amount of small- scale DG, for example, from residential CHP or feed in from renewable energies such as photovoltaics or wind power, smarter grids will be required in order to handle the energy flow as well as the accompanying data flow. The major development efforts for residential fuel cells have been devoted to proton-exchange membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs). No serious efforts are known to date on the development of molten carbonate fuel cells (MCFCs), alkaline fuel cells (AFCs), or phosphoric acid fuel cells (PAFCs) for resi- dential fuel cell systems. However, some of the so-called high-temperature PEMFC systems are in fact using phosphoric acid imbibed in a polymer membrane as the electrolyte. Table 1 shows a compilation of characteristic properties of PEMFC- and SOFC-based CHP systems. Typical Fuel Cell System Design Inputs to a fuel cell CHP system are fuel, ambient air, auxiliary e nergy (mainl y in the form of electricity), and necessary-makeup w ater for the fuel processor . Outputs are conditioned electric pow er, recov ered heat, and exhaust 108 . generation (CHP). Fuel cells are electrochemical po wer sources co n v erting the chemical energy contained in the fuel d irectl y into electrical energy. Since the efficienc y of fuel cells is not restricted. handle the energy flow as well as the accompanying data flow. The major development efforts for residential fuel cells have been devoted to proton-exchange membrane fuel cells (PEMFCs) and solid oxide fuel. solid oxide fuel cells (SOFCs). No serious efforts are known to date on the development of molten carbonate fuel cells (MCFCs), alkaline fuel cells (AFCs), or phosphoric acid fuel cells (PAFCs)