Aqueous Redox Flow Batteries Enhanced by Solid Boosters

One of the main challenges for certain electricity production technologies (e.g. renewables) is the capability to supply energy according to the demand. It follows that storing energy is a possible solution to the inadequacy between production and demand. Among various technologies, electrochemical energy storage is a particularly good solution. Redox flow batteries, as electrochemical energy storage technology are scalable, durable, robust and quite safe, but as compared to other technologies, they have a quite poor volumetric capacity (10-40 Wh/L) determined by the solubility limit of the redox-active species dissolved in the electrolytes. In this thesis, an approach to overcome the low storage capacity in aqueous redox flow batteries is presented. The approach consists of adding solid-phase materials (herein called solid boosters) into the liquid electrolytes contained in the battery reservoirs. These boosters can receive charge from the redox-active species reacting in the flow cell, if the respective Fermi levels are aligned to those of the liquid electrolyte materials. This concept is denoted as âredox-mediatedâ charge transfer, where solid boosters become the primary storage media, bypassing the solubility limit issue that restraints the storage capacity. This work opens a series of future studies to enable bulk-phase charge storage in the reservoirs of redox flow batteries and it looks promising for industrial demonstrators.

Redox flow battery and indirect water electrolysis

Véronique Amstutz

The progressive integration of renewable energy sources such as wind turbines and photovoltaic pannels in the current electrical network, and the rise of the electrical mobility, provoke a change of paradigm in the sector of energy management. The increasing variability of the electricity production and consumption profiles, together with the requirement for a reliable supply of energy, necessitates the implementation of energy storage means of different scales and for a wide range of applications. Redox flow batteries (RFBs) are well adapted for buffering the fluctuations of solar or wind energy production. They present a fast response time, can withstand a large number of charge-discharge cycles and their ouput power is independant of their energy capacity. The main limitation of RFBs resides in their low energy density. The objective of the present work was to test a mean to overcome this low energy density. A new concept was developed, which rests on the addition of a second pathway to discharge the RFB. This pathway allows to generate hydrogen and oxygen, without affecting the functioning of the RFB. This concept was called dual-circuit RFB and was patented. RFBs are based on two liquids electrolytes, each stored in a reservoir, and flowing through an electrochemical cell for their electrochemical conversion. Each electrolyte contains one redox couple and Ce(IV)/Ce(III) and V(III)/V(II) were selected as positive and negative redox couples in the RFB developed here. Charge-discharge curves of a VâCe RFB were measured for characterisation purposes and for the preparation of the charged electrolytes. The latter ones can be discharged electrochemically in the RFB to generate electricity (electrochemical discharge mode), or they can be directed in a secondary circuit where they are discharged for the production of hydrogen or oxygen (chemical discharge mode). The chemical discharge of the negative electrolyte consists of the reaction of V(II) with protons to produce hydrogen and V(III). Mo2C was selected as heterogenous catalyst for the characterisation of the reaction. A conversion close to 100% suggested no loss of current. A kinetic analysis provided some insights into the catalytic mechanism of this reaction. The chemical discharge of the positive electrolyte aimed at the conversion of Ce(IV) to Ce(III) by the oxidation of water to oxygen and also necessitates a catalyst. Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) were evaluated in terms of conversion and kinetics. The composition of the positive electrolyte was shown to be of importance for this reaction. To show the feasibility of this concept, a larger-scale demonstrator system was designed based on a 10 kW (40kWh) commercially available vanadium RFB. A characterisation of this RFB was first performed. The design a suitable Mo2C catalyst and its corresponding catalytic bed is also discussed. As a conclusion, the concept developed and experimentally tested in the present work leads to a RFB system which is characterised by two discharge modes, increasing its energy storage capacity and energy density. The dual-circuit RFB also represents a crossing between electrical grid and hydrogen mobility as the hydrogen produced by surplus electricity could be delivered to fuel cell cars. The application of this system in a local distribution electrical network, close to a wind or solar source seems a promising approach.

EPFL2015

The Facilitation of Mini and Small Hydropower in Switzerland

Nicolas Crettenand

The electricity sector in Switzerland is undergoing important changes following the liberalisation process and the facilitation of renewable energy technologies. Furthermore, the phasing out of nuclear power will increase the demand for new domestic electricity generation. According to the Federal Energy Strategy 2050, additional generation will have to come from hydropower (currently 57% of Swiss electricity production), including small hydropower (currently 6%, i.e. 3.8 TWh). The small hydropower technology, with an installed capacity between 100 kW and 10 MW (whereby 100 kW till 1 MW is considered to be mini hydropower), is a renewable energy technology (RET) which is well developed. However, the technology still requires further innovation to improve its environmental integration and reduce costs. Small hydropower (SHP) provides electricity with a high energy payback ratio and, generally, with lower production costs than other RETs, aside from large hydropower. SHP can be combined within multipurpose infrastructures such as drinking water and irrigation networks. The institutional framework of SHP is conditioned by multi-level (i.e., Federal, Cantonal and Communal) and cross-sectorial institutions (e.g., within the electricity and water sectors, spatial planning). SHP still has significant potential in Switzerland with the possibility of increasing the production of 2010 by 40-50% by 2050. However, SHP requires appropriate policy instruments for its development within a liberalised electricity market as it is, on average, not yet cost-competitive. In 2009, for example, a Federal feed-in remuneration scheme was introduced. To further develop the SHP potential, the institutional framework has still to evolve. This research was aimed at identifying changes in the institutional framework in Switzerland which can contribute towards developing SHP and increasing the alignment between the technology and its institutions. To this end, the literature on co-evolution and the framework of coherence between institutions and technologies in the case of network industries, such as electricity, were used for the analysis. This thesis contributes towards further development of the coherence framework. Policy instruments are identified that can support the development of SHP. Measures to simplify and harmonise administrative procedures are evaluated, even though their implementation remains difficult. A promising endeavour, however, is to reduce opposition, thus duration of procedures, by developing regional master plans and/or multi-criteria evaluations of projects at the very early project development phase. This enables the pursuit of projects for which all stakeholders are favourable to their realisation. Another required measure is guaranteeing the technical quality of plants which receive the feed-in remuneration by introducing a global efficiency criterion. Finally, other policy instruments are analysed such as green certificates, the feed-in remuneration scheme, and CO2 credits, which will become necessary following the opening of new gas-fired plants. The current institutional facilitation of RETs generating electricity focuses solely on quantity, i.e. kWh. It does not consider the need for flexible production and energy storage to deal with the intermittent generation of some RETs and to align the electricity demand and supply. This is not coherent. The institutional facilitation should take into account flexible production and energy storage, and thus specifically support technologies such as storage and pumped-storage SHP. This research investigated, using an explorative and bottom-up approach, the technical potential of small storage and pumped-storage plants by focusing on existing and planned reservoirs in order to reduce investment costs and environmental opposition. Eleven projects were identified in the Canton of Valais. The potential in Switzerland is evaluated at roughly 200-300 MW for storage SHP plants (today 106 MW is used) and 70-150 MW for pumped-storage SHP plants (today 15 MW is used). In order to further develop this potential, which is complementary to the large storage and pumped-storage hydropower potential, some remuneration instruments are identified. The instruments include adapting the feed-in remuneration scheme to facilitate not only run-of-river plants, introducing requirements for ancillary services from RETs (in addition to large hydropower), and CO2 credits and green certificates depending on the production profile (e.g., peak and off-peak). The identified instruments lead to policy recommendations which would further facilitate the development of SHP in Switzerland, including storage and pumped-storage plants. In summary, the main findings of this research are four-fold: The institutional framework has to further evolve to be aligned with the small hydropower technology. The institutional facilitation of renewable energy technologies must not only focus on the quantity of energy, i.e. kWh, but also on the "quality", such as flexible production and energy storage. Storage and pumped-storage small hydropower could play an important role in producing distributed peak and balancing electricity and in contributing to distributed energy storage. Its technical potential in Switzerland is significant enough to shape the institutional framework adequately. The coherence framework offers a very useful lens to analyse technological and institutional changes in the network industries. However, it still needs to be improved to become more robust and less conceptual.

EPFL2012

Thermoeconomic analysis of a solar enhanced energy storage concept based on thermodynamic cycles

Daniel Favrat, Samuel Henchoz

Large scale energy storage may play an increasingly important role in the power generation and distribution sector, especially when large shares of renewable energies will have to be integrated into the electrical grid. Pumped-hydro is the only large scale storage technology that has been widely used. However the spread of this technology is limited by geographic constraints. In the present work, a particular implementation of a storage concept based on thermodynamic cycles, invented by ABB Switzerland ltd. Corporate Research, has been analysed thermoeconomically. A variant using solar thermal collectors is presented. It benefits from the synergy between daily variations in solar irradiance and in electricity demand. This results in an effective increase of the electric energy storage efficiency. A steady state multi-objective optimization of a 50 MW plant was done; minimizing the investment costs and maximizing the energy storage efficiency. Several types of cold storage substances have been implemented in the formulation and two different types of solar collector were investigated. A storage efficiency of 57% at a cost of 1200 USD/kW was calculated for an optimized plant using solar energy. Finally, a computation of the behaviour of the plant along the year showed a yearly availability of 84.4%.