|Title||Concentration Gradient Flow Batteries : salinity gradient energy systems as environmentally benign largescale electricity storage|
|Author(s)||Egmond, Willem Johannes van|
|Source||Wageningen University. Promotor(en): C.J.N. Buisman, co-promotor(en): H.V.M. Hamelers. - Wageningen : Wageningen University - ISBN 9789463438421 - 160|
|Publication type||Dissertation, internally prepared|
The total amount of energy derived from wind turbines and solar panels is rapidly growing. Since these sources of energy are intermittent in nature, supply and demand of energy show an increasing mismatch. To accommodate efficient, large scale use of intermittent renewable energy sources such as wind and sun, energy storage systems are necessary. One of the primary drivers for the increasing use of renewable energy sources is concern about the quality of our environment. Therefore, it is vital that energy storage systems storing sustainable energy, are sustainable themselves. Creating storage systems using abundant, environmentally friendly materials is therefore an important prerequisite for a sustainable energy supply. This thesis aims to explore the potential of the Concentration Gradient Flow Battery (CGFB) as large-scale electricity storage technology. A CGFB stores energy in aqueous solutions of salt (typically NaCl) and uses ion exchange membranes to extract energy from the solutions.
Chapter 1 (Introduction) introduces the need of energy storage. Available energy storage technologies are compared in terms of technical performance but also in terms of safety, environment and political aspects. The CGFB is introduced and explained. Finally, a theoretical background on how a salinity gradient can create a useable voltage across ion exchange membranes is presented.
In Chapter 2 (The Concentration Gradient Flow Battery as electricity storage system: Technology potential and energy dissipation) a working prototype is constructed and tested. This chapter explains how a CGFB works in more detail and the theoretical maximum energy density of the battery is explored (~3.2 kWh m-3 for NaCl). The maximum energy density is shown to vary as function of salt concentrations, volume ratio between salt and fresh solution and salt type. A model is introduced which includes the major dissipation factors; internal resistance, water transport and co-ion transport. Experimental work is performed to validate the model. A wide range of salt concentrations (0-3 m NaCl) and current densities (-49 to +33 A m-2) is chosen. From this work, an optimal working range is identified where the concentrate concentrations preferably do not exceed the 1 m. At higher concentrate concentrations water transport and co-ion transport are found to increase heavily decreasing the energy efficiency of the battery.
In chapter 2 it was shown that the CGFB works best at low (<1 m) concentrations. At low concentrations, internal resistance and water transport are shown to be the most important dissipation factors. In chapter 3 (Energy efficiency of a Concentration Gradient Flow Battery at elevated temperatures), a more specific working range (0-1 m) is explored in more detail. Mass transport is measured accurately and an improved experimental approach allows to determine losses by water transport, internal resistance and co-ion transport in more detail. Chapter 3 shows for both the charge and discharge step the energy efficiency and quantifies the losses at each moment in time. The effect of current density and state-of-charge on power density and energy efficiency is analysed. It is shown that it is not efficient to either completely discharge or charge a CGFB. An optimal working domain is identified (Δm > 0.5 and η > 0.4) where the CGFB delivers best performance in terms of energy efficiency (max. discharge η of 72%) and power density (max. discharge power density, 1.1 W m-2). Tests are also performed at different temperatures (10, 25 and 40 ˚C) to measure the effect of temperature on mass transport, internal resistance and power density. Finally, it is shown that water transport is a major issue in the operation of a CGFB where it causes hysteresis (after discharge the battery does not return to its original state), lower efficiency and leads to decreased energy density.
To improve the performance of a CGFB, it is necessary to decrease water transport across the membranes. Chapter 4 (Tailoring ion exchange membranes to enable low osmotic water transport and energy efficient electrodialysis) introduces modified membranes with a polymer mesh inside with a very small open area (2, 10, 18 and 100% open area). The membranes are prepared by casting an ionomer solution over a polymeric mesh. The material, open area and surface properties of the mesh are changed and the effect on electrical resistance, water transport properties and the efficiency of the charge process are investigated. Comparing a meshed membrane with a homogeneous membrane, the osmotic water transfer coefficient of the meshed membrane is shown to be reduced up to a factor eight. Decreasing the open area of the mesh decreases the water permeability of the membrane but adversely increases electrical resistance. The membranes are tested at different current densities (5-47.5 A m-2). Chapter 4 shows that at low current densities (5-25 A m-2) the meshed membranes outperform the homogeneous membranes in terms of energy efficiency (at a Δc of 0.7 M, maximum energy efficiency η = 67 % for the meshed membranes and η = 50 % for the homogeneous membranes). Also, the meshed membranes outperform the homogeneous membranes in terms of diluate yield across all tested current densities (diluate yield of 78-87% for the meshed membranes, 43-76% for the homogeneous membranes). Using a meshed membrane in a CGFB will lead to less issue with hysteresis. In addition, the relation between material and surface property of the mesh and the ionomer resin is investigated. The type of material (PA or PET) is shown to affect the water permeability of the meshed membrane. It is shown that in some cases, compared to a non-treated mesh, a chemically treated mesh (2 M NaOH treatment) yields lower water permeability membranes. Finally, when optimized ion exchange resin is used it is expected that the water permeability can be reduced even further.
Chapter 2 and chapter 3 show that the CGFB is able to store energy in NaCl solutions which has significant environmental benefits. The measured power density is relatively low and energy density is limited because high concentrations cannot be used. In chapter 5 (Performance of an environmentally benign Acid Base Flow Battery at high energy density) the process is changed to significantly improve power density and energy density while maintaining the environmental benefits. The adjusted system uses three energy storage solutions instead of two and stores most energy in a proton and hydroxyl ion concentration gradient. To create protons and hydroxyl ions (during charge) and to let the ions recombine to pure water again (during discharge) a bipolar membrane is added. Chapter 5 shows that the theoretical maximum energy density of the adjusted system (called Acid Base Flow Battery, ABFB) is over three times higher than the theoretical maximum of the original CGFB (chapter 2, maximum energy density of the CGFB is ~3.2 kWh m-3 and ~11.1 kWh m-3 for the ABFB). In addition, experiments demonstrate that the ABFB reaches a power density which is about a factor four higher compared to the original CGFB (3.7 W m-2 compared to 0.9 W m-2 of membrane area). The main dissipation sources are identified and quantified (energy lost by; co-ion transport 39-65%, ohmic resistance 23-45% and non-ohmic resistance 4-5%). The low selectivity of the membranes to protons and hydroxyls lead to a low coulombic efficiency (13-27 %). The ABFB has potential to be improved significantly. Development of better proton blocking anion exchange membranes and hydroxyl ion blocking cation exchange membranes would increase ABFB performance. Also decreasing the thickness of membranes and compartments would increase ABFB performance as it would lead to lower internal resistance energy losses. In addition, higher current densities would help reduce energy losses by co-ion transport.
Chapter 6 (General discussion and outlook) discusses important aspects of CGFB technology from a societal and commercial point of view. Costs and revenues of energy storage systems are very important drivers and can largely determine the chance of success for a storage technology. First a theoretical background of costs calculations for energy storage systems is presented. Next, the costs of future CGFB systems is calculated and compared to competing technologies. In terms of costs, the ABFB outperforms the CGFB system (0.259 and 0.366 € kWh-1 cycle-1 respectively). Also, possible revenue sources are discussed. Stacking of multiple revenue streams is possible and recommended to increase profitability. Both systems cannot yet generate a profit as costs are too high and single revenue streams are low. However, although difficult, based on the costs calculations, when performance is increased, costs can be reduced and multiple revenue streams are stacked, a commercially viable CGFB/ABFB system is estimated to be feasible. Besides technical and costs aspects, also sustainability of energy storage systems is of major importance. The energy consumption of the production and use of storage systems over their lifetime is analysed and the potential of a CGFB system is discussed. Also, choice in material and system design is discussed. Finally, the size of storage technologies is important. Therefore, the size of a future CGFB system is estimated and discussed with the help of case studies. For diurnal energy storage, the size of a CGFB/ABFB is deemed acceptable given that performance is increased. Seasonal energy storage is not feasible in terms of size without significant technological improvement.
Energy storage with CGFB systems is shown possible. There is a clear need for increased technical performance and reduced costs to create a profitable CGFB. Yet, because of the exciting benefits across different aspects such as safety, environment and politics, CGFB technology is worth continued research.