Energy Storage as Part of a Secure Energy Supply

The current energy system is subject to a fundamental transformation: A system that is oriented towards a constant energy supply by means of fossil fuels is now expected to integrate increasing amounts of renewable energy to achieve overall a more sustainable energy supply. The challenges arising from this paradigm shift are currently most obvious in the area of electric power supply. However, it affects all areas of the energy system, albeit with different results. Within the energy system, various independent grids fulfill the function of transporting and spatially distributing energy or energy carriers, and the demand-oriented supply ensures that energy demands are met at all times. However, renewable energy sources generally supply their energy independently from any specific energy demand. Their contribution to the overall energy system is expected to increase significantly. Energy storage technologies are one option for temporal matching of energy supply and demand. Energy storage systems have the ability to take up a certain amount of energy, store it in a storage medium for a suitable period of time, and release it in a controlled manner after a certain time delay. Energy storage systems can also be constructed as process chains by combining unit operations, each of which cover different aspects of these functions. Large-scale mechanical storage of electric power is currently almost exclusively achieved by pumped-storage hydroelectric power stations. These systems may be supplemented in the future by compressed-air energy storage and possibly air separation plants. In the area of electrochemical storage, various technologies are currently in various stages of research, development, and demonstration of their suitability for large-scale electrical energy storage. Thermal energy storage technologies are based on the storage of sensible heat, exploitation of phase transitions, adsorption/desorption processes, and chemical reactions. The latter offer the possibility of permanent and loss-free storage of heat. The storage of energy in chemical bonds involves compounds that can act as energy carriers or as chemical feedstocks. Thus, they are in direct economic competition with established (fossil fuel) supply routes. The key technology here – now and for the foreseeable future – is the electrolysis of water to produce hydrogen and oxygen. Hydrogen can be transformed by various processes into other energy carriers, which can be exploited in different sectors of the energy system and/or as raw materials for energy-intensive industrial processes. Some functions of energy storage systems can be taken over by industrial processes. Within the overall energy system, chemical energy storage technologies open up opportunities to link and interweave the various energy streams and sectors. Chemical energy storage not only offers means for greater integration of renewable energy outside the electric power sector, it also creates new opportunities for increased flexibility, novel synergies, and additional optimization. Several examples of specific energy utilization are discussed and evaluated with respect to energy storage applications. The article describes various technologies for energy storage and their potential applications in the context of Germany’s Energiewende, i.e. the transition towards a more sustainable energy system. Therefore, the existing legal framework defines some of the discussions and findings within the article, specifically the compensation for renewable electricity providers defined by the German Renewable Energy Sources Act, which is under constant reformation. While the article is written from a German perspective, the authors hope this article will be of general interest for anyone working in the areas of energy systems or energy technology

Effizienzoptimierte CO 2-Abtrennung in IGCC-Kraftwerken mittels Wassergas-Shift-Membranreaktoren

The conversion of solid fuels such as coal and biomass into syngas in the integrated gasification combined cycle (IGCC) process is carried out at elevated pressure. Since,from a thermodynamic point of view, this is a crucial prerequisite for an efficient CO$_{2}$ separation step, IGCC has great potential for incorporating CO$_{2}$ separation with a low energy consumption. However, studies predict efficiency penalties in the range of 6–11 %-points depending on the respective gasification process utilized, thus revealing that the thermodynamic potential is not fully exploited. In this thesis, a specially adapted IGCC power plant concept for the optimized implementation of gas separation membranes was developed and investigated in order to evaluate the extent to which the auxiliary boundary conditions can be advantageously designed. To create a standard of comparison, a reference IGCC power plant as well as a Selexolbased CO$_{2}$ scrubbing process were designed and simulated, resulting in an overall efficiency reduction from 48.0 % to 38.4 %. This corresponds to an increase of 25 % in coal consumption. The analysis of the simulation results revealed that, besides the auxiliary demand of Selexol scrubbing and CO$_{2}$ compression subsequent to the low pressure regeneration of the solvent, the main contributor to the loss is the water-gas shift reaction. To reduce this high efficiency penalty, an integration concept was developed to optimize the use of the gas permeation membrane, with parameters better adapted to its special characteristics and mode of operation. The design process resulted in the use of an H$_{2}$-selective membrane, which was combined with the water-gas shift reaction to create the water-gas shift membrane reactor (WGS-MR), and which was swept with recirculated flue gas at elevated pressure in countercurrent 4-End mode. In addition, the “membrane steam recuperator” was introduced as a new process unit and integrated to enhance the steam utilization within the entire process. By this means, all contributions to the efficiency penalty were decreased significantly: $\bullet$ Reduced CO$_{2}$ compression costs due to maintained elevated CO$_{2}$ pressure $\bullet$ Auxiliary power free separation process thanks to sweep gas$\bullet$ Lower steam demand and enhanced CO conversion thanks to membrane reactor $\bullet$ Utilization of water gas shift reaction heat in the gas turbine process transferred via sweep gas $\bullet$ Minimization of steam demand for WGS-MR thanks to membrane steam exchanger. - The simulation showed that the membrane in combination with its innovative conceptual features is capable of reducing the efficiency loss from 9.6 to 3.6 %-points. The excellent potential of the IGCC process for energy-efficient CO$_{2}$ separation can thus be exploited to its fullest extent if the proposed power plant concept is consistently followed through to technical implementation

Closing the loop: Captured CO2 as a feedstock in the chemical industry

The utilization of ‘captured’ CO2 as a feedstock in the chemical industry for the synthesis of certain chemical products offers an option for preventing several million tons of CO2 emissions each year while increasing independence from fossil fuels. For this reason, interest is increasing in the feasibility of deploying captured CO2 in this manner. Numerous scientific publications describe laboratory experiments in which CO2 has been successfully used as a feedstock for the synthesis of various chemical products. However, many of these publications have focused on the feasibility of syntheses without considering the ancillary benefits of CO2 emissions reduction if the CO2 is sourced from effluent or the potential profitability of this process. Evaluating these environmental and economic benefits is important for promoting the further development of benign CO2 applications. Given the multitude of CO2 utilization reactions in the laboratory context, an initial assessment must be undertaken to identify those which have the most potential for future technical exploration and development. To achieve this, 123 reactions from the literature were identified and evaluated with the help of selection criteria specifically developed for this project. These criteria incorporate both the quantitative potential of reducing CO2 and possible economic benefits of these syntheses. The selected reactions are divided into bulk and fine chemicals. Of the bulk chemicals, formic acid, oxalic acid, formaldehyde, methanol, urea and dimethyl ether, and of the fine chemicals, methylurethane, 3-oxo-pentanedioic acid, 2-imidazolidinone, ethylurethane, 2-oxazolidone and isopropyl isocyanate, mostly fulfil the selection criteria in each category