U.K. Starke Salinity gradient energy storage Quantification of the influence of temperature on salinity gradient flow battery performance. Salinity gradient energy storage Quantification of the influence of temperature on salinity gradient flow battery performance. By Starke, U.K. (1518232) in partial fulfilment of the requirements for the degree of Master of Science in Mechanical Engineering at the Delft University of Technology, to be defended publicly on [To be announced] AM. Supervisors: ir J.W. van Egmond (PhD candidate Wageningen University) Dr. ir D.A. Vermaas Prof. dr. ir. Thijs J.H. Vlugt Thesis committee: This thesis is confidential and cannot be made public. 1 Abstract The combination of desalination technology (Electrodialysis) and power generation from salinity gradients (Reverse Electrodialysis) is a novel electrical energy storage system. The main challenge in the development of this novel energy storage system is to improve the system performance. The performance of the battery is determined by the efficiency, power density and energy density. A strategy to increase the performance of the battery is to increase the temperature of the feed solutions. The main goal of this thesis is to quantify the influence of temperature on the power density [W/m2], energy density [Wh/L] and thermodynamic efficiency [%] of a salinity gradient based energy storage system, charged using electrodialysis and discharged using reverse electrodialysis. In chapter 2 a theoretical framework is developed in order to build an understanding of the fundamental theories that help explain the system characteristics. This is done by studying the literature. Next, chapter 3 deals with the methodology that will be used in order to obtain the experimental data that is needed in order to answer the main research question. After this, the results of the experiments are discussed in chapter 4. In chapter 5 the most important conclusions are summarized and in chapter 6 there is room for a discussion and recommendations. Experimental data showed that: The total electrical resistance decreases if the temperature increases; Osmosis and diffusion increases if the temperature increases. Energy density increases if the operating temperature increases. Power density increases if the operating temperature increases. In case of charging the increase of operating temperature increases the systems thermodynamic efficiency. In case of discharging the increase of operating temperature increases the systems thermodynamic efficiency. In order for the salinity gradient energy storage to be cost competitive the levelized costs should be at most 0.20 euro/kWh. Pumped hydro storage is the cheapest competitor available on the energy storage market today with a levelized cost of about 0.10 euro/kWh . Ways to achieve this competitive price is to decrease the price of the membranes, or increase the performance of the membranes because the membranes are the most expensive component of the system. Another approach is to focus on groups of applications requiring a large capacity to power ratio. The costs of storage capacity of this system are extremely low making it very suitable for the aforementioned group of applications. Irreversible water transport of the system has a large impact on the system performance. Therefore future research should be done in order to decrease this phenomenon. This means that strategies need to be devised in order to reduce the osmotic pressure difference between the concentrate and diluate chambers. Furthermore the internal resistance is an important factor to consider for the overall performance of the system. Currently research is already being done in order to reduce the internal resistance. Increasing the conductivity of the diluate chamber is an important strategy. Lastly the self-discharge of the system is also a parameter to consider and research is also already being done to develop membranes that have high permselectivity. 2 Acknowledgements I would like to thank the whole AquaBattery team: Jan-Willem van Egmond who was also my supervisor at Wetsus, David Vermaas who was my supervisor from the TU Delft, Jan Post, Emil Goosen, Jiajun Cen and Edoardo Cometti. Thanks Thijs Vlugt for making this whole collaboration with Wetsus work. Thanks Michel Saakes for the support! Of course my Spanish Blue Battery teammates at Wetsus deserve a place in this thesis, thanks Cesar and Laura. I am also very grateful to everyone else at Wetsus for making it possible to work in an inspiring environment with the best equipment. Finally I would like to thank my family for supporting me during this thesis and of course the whole period that I studied. 3 Contents Chapter 1 Introduction ........................................................................................................... 9 Background ....................................................................................................................... 9 Salinity gradient based energy storage .............................................................................. 9 Challenges ........................................................................................................................10 Aim ...................................................................................................................................11 Thesis Outline ...................................................................................................................11 Chapter 2 Theoretical framework .........................................................................................12 Principle of electrodialysis and reversed electrodialysis ....................................................12 System components ......................................................................................................12 Driving force electrodialysis ...........................................................................................13 Driving force reverse electrodialysis ..............................................................................13 Membranes ...................................................................................................................14 Relevant phenomena ........................................................................................................14 Mass transport across the ion-exchange membranes ...................................................15 Electrical characteristics of the system ..........................................................................18 Thermodynamic properties ...............................................................................................22 Activity and osmotic coefficient ......................................................................................22 Density ..........................................................................................................................23 Conductivity ...................................................................................................................23 Viscosity ........................................................................................................................23 System performance .........................................................................................................24 Gibbs free energy ..........................................................................................................24 Energy density ..............................................................................................................24 Power density ................................................................................................................25 Thermodynamic efficiency .............................................................................................25 Chapter 3 Materials and methods .........................................................................................26 Methods ............................................................................................................................26 Constant current experiment .........................................................................................26 Single pass experiment .................................................................................................27 Open circuit experiment .................................................................................................28 Materials ...........................................................................................................................30 Stack .............................................................................................................................30 Experimental setup ........................................................................................................31 Chapter 4 Results .................................................................................................................33 Introduction .......................................................................................................................33 4 Single pass experiments ...................................................................................................33 Resistance ....................................................................................................................33 Temperature and the boundary layer resistance ...........................................................34 Temperature and the bulk concentration change resistance ..........................................35