Energy Generation and Storage DOI: 10.1002/anie.200903603 Electrochemical Versus Heat-Engine Energy Technology: A Tribute to Wilhelm Ostwalds Visionary Statements Julia Kunze and Ulrich Stimming* Dedicated to Wilhelm Ostwald Carnot process · electrochemistry · energy technology · fuel cells · heat engines 1. Introduction Our present situation concerning major fossil energy carriers and energy consumption is characterized by limited reserves and resources, along with emission problems consid- ering crude oil, natural gas, coal and uranium as primary energy sources. At the same time, the worldwide demand for electrical energy has increased from 8.3 million GWh in 1980 to 18.9 million GWh in 2006, and is estimated to further increase to 30.7 million GWh in 2030. Energy management is a rather complex problem. In- creasing contributions of renewable energies, such as wind, solar, and wave power, tend to complicate the grid manage- ment. Therefore, generation of electrical energy is only part of the challenge. The management and storage of electrical energy will become essential to maintain the present grid quality. Energy conversion processes today are under special consideration because of two factors associated with them, namely the limited availability of primary energy carriers and the emission of pollutants, with inherent local and global negative effects on the environment. Energy conversion processes that aim at generating electrical power reveal efficiencies not much higher than 30 %, which indicates that losses in form of heat or chemical substances amount to more than two thirds of the primary energy. Conventional process- es, for example those in a heat-engine-based power plant, are volume processes, such as combustion, which results in mechanical and then in electric energy. By contrast, other technologies, such as photovoltaics or electrochemistry (bat- teries, fuel cells, or super caps), are based on interfacial transfer of energy and/or charge. In contrast to the perfor- mance of heat engines, which is limited by the Carnot efficiency, interfacial reactions are usually of much higher thermodynamic efficiency. The purpose of this essay is to critically compare Carnot- based and electrochemical methods that are used for the generation and storage of energy, and to reflect advantages and disadvantages of both alternatives. One of the first scientists who was aware of the impact and the importance of energy conversion and storage devices based on electro- chemical interfacial reactions was the Nobel Laureate Wil- helm Ostwald (1853–1932), who was at the time professor at the first chair of physical chemistry at the University of Leipzig. The view of Ostwald and what is todays under- standing of energy conversion will both be apparent in this essay, in order to reflect the development of the opinion about this topic from the 18th century up to now and to rise important questions in this context. 2. Classical Heat-Engine Cycles Thermodynamic considerations gave rise to the first heat engines, which were used to generate mechanical or electrical energy. Since the 18th century, conventional reciprocating steam engines have served as mechanical power sources, with notable improvements being made by James Watt. The first commercial central electrical generating stations in New York and London, in 1882, used steam engines. [1] In 1894, Wilhelm Ostwald predicted a technical revolution caused by the fuel cell, which would eclipse the invention of the steam engine. Despite Ostwalds predictions however, the first-generation heat engines are still serving as power plants today. Further developments have lead to the introduction of combined-cycle power plants. Usually a combination of several cycles, operating at different temperatures, yield a considerably higher system efficiency. Heat engines are only able to use a portion of the energy (usually 35 to 41%). The remaining heat is generally wasted. In a combined-cycle power plant (CCPP), or combined-cycle gas-turbine (CCGT) plant, a gas-turbine generator produces electricity and the waste heat is used to make steam for generating additional electricity with a steam turbine. This last step enhances the efficiency for electricity generation to about 60 %, because the temperature difference between the input and output heat [*] Prof. Dr. U. Stimming Physics Department E19, Technical University of Munich James-Franck Strasse 1, 85748 Garching (Germany) Fax: (+ 49) 89-289-12530 E-mail: stimming@ph.tum.de Dr. J. Kunze Physics Department E19, Institute for Advanced Study (IAS), Technical University of Munich James-Franck Strasse 1, 85748 Garching (Germany) Essays 9230  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 9230 – 9237