Modeling and Optimal Control of a Redox Flow Battery

Vanadium Redox Flow Batteries (VRFB) can be used as energy storage device, for example to account for wind or solar power fluctuations. In VRFBs charge is stored in two tanks containing two different vanadium solutions. This approach decouples the storage capacity and the power supply which is dependent only on the number and size of the cells [1]. A control specific model of a VRFB is proposed, which captures the essential dynamic properties of the battery while ignoring all fluid mechanical elements. The model of the battery is a nonlinear DAE comprising a differential equation for the state of charge SoC and algebraic equations based on the Butler-Volmer equation for the current I and the voltage U, the power P = U∙I being set by the operator. The battery typically operates in constant power mode, but the control system limits its operating range by switching to a constant voltage when upper or lower voltage thresholds are reached. In addition, this VRFB contains a secondary flow circuit, where the electrolyte discharge reactions produce hydrogen and oxygen [2]. Model parameters are estimated by minimizing the difference between the measured and modeled SoC over time, the other states (I, U and P) being compared with measured data. The model is validated using independent measurements, which show good fits. Finally, the dynamic model will be used to formulate and numerically solve the problem of optimal battery operation in different scenarios. [1] C. Blanc, A. Rufer, Chapter 18, in Paths to Sustainable Energy, InTech, 2010 [2] V. Amstutz et al., Energy and Environmental Science 7, 2350-2358 (2014)

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

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009

Novel concepts of compressed air energy storage and thermo-electric energy storage

Young Min Kim

The interest in energy storage is currently increasing, especially from the perspectives of matching intermittent sources of renewable energy with customer demand and storing excess nuclear or thermal power during the daily cycle. Technologies to be considered for load leveling for large-scale energy systems, typically in the range of hours to days of discharge time, include pumped-storage hydroelectricity, compressed air energy storage (CAES), sodium sulfur (NaS) batteries, advanced absorbent glass mat lead acid batteries, and flow batteries. CAES is a promising technology because of significant advantages such as its high reliability, economic feasibility, and low environmental impact. Thermo-electric energy storage (TEES), which was recently proposed as a method for large-scale energy storage, is another mechanical storage method based on thermodynamic cycles. This thesis presents a guide to precisely understand each system along with energy and exergy analyses to characterize the key parameters for achieving high efficiency for each of the systems. In addition, some novel concepts for the systems are proposed in order to address some of the current drawbacks and to widen the scope of their applications. Conventional CAES systems are most commonly operated under constant volume conditions with a fixed, rigid reservoir and compressors and turbines that can operate over an appropriate pressure range. These varying pressure ratios can degrade the efficiencies of compression and power generation owing to deviations from design points. Although it is possible to increase the storage volume to reduce the operating pressure range, doing so results in reduced energy density and high construction costs. Therefore, in order to resolve such problems, a novel constant-pressure CAES system combined with pumped hydro storage is proposed. An energy and exergy analysis of the novel CAES system was performed in order to understand the operating characteristics of the system according to several different compression and expansion processes. The effects of the height of the storage cavern and heat transfer between two media (air and water) and the cavern on the performance of the novel CAES system were also examined. Although the large-scale CAES systems are dependent on the right combination of site-dependent geological factors for air storage, a micro-CAES system with man-made air vessels can be a very effective system for distributed power networks, because it provides energy storage, generates electric power using various heat sources, and incorporates an air cycle heating and cooling system. This thesis presents the results of energy and exergy analyses of different types of micro-CAES systems, as well as some innovative ideas for achieving high efficiency of these systems. Recently, another type of mechanical storage, namely, thermo-electric energy storage (TEES) systems, which use heat pumps and heat engines with thermal storage, have been proposed. The advantages of TEES systems are their higher energy densities and independence from geological formations. In particular, a TEES system with transcritical CO2 cycles is considered to be a promising method for large-scale energy storage. This thesis reviews current TEES systems and proposes a novel isothermal TEES system with transcritical CO2 cycles. It is shown that in the case of TEES systems with transcritical CO2 cycles, the roundtrip efficiency and energy density can be increased by isothermal compression/expansion. In addition, novel transcritical or supercritical CO2 cycles using both low-temperature (LT) and high-temperature (HT) heat sources are proposed to maximize the power output of the CO2 power cycle with the given HT heat sources for use in applications such as nuclear power, concentrated solar power, and combustion. Moreover, the previous TEES system with transcritical CO2 cycles can be combined with the proposed transcritical CO2 cycle using both LT and HT heat sources.

EPFL2012