Cyclic degradation analysis of reversible Solid Oxide Cell short-stack

The deployment of renewable energy systems has been increasing steadily in recent years. The intermittency associated with the renewable electricity supply can be addressed through the usage of energy storage and release systems. In this framework, reversible solid oxide cells (rSOC) integrated in power-to-gas-to-power systems can convert excess electricity into multiple useful fuels (solid oxide electrolysis (SOE) mode) and restitute it when its renewable production is too low (solid oxide fuel cell (SOFC) mode). To make rSOC competitive and durable, low degradation for the targeted lifetime must be achieved. In BALANCE project (H2020 GA no:731224), with the purpose of assessing the degradation of rSOC in both modes, a cyclic durability test as shown in the graphical abstract was conducted on a 5-cell short stack (100 cm2 active area each) designed by CEA, using H2- electrode supported Ni-YSZ cells manufactured by DTU. The composition of the inlet gases was H2/N2:50/50 in SOFC mode and H2/H2O:20/80 in SOE mode. Steady-state initial performance revealed an improvement for all the repeatable units (RUs) in SOFC mode. In SOE mode, only RU 4 and 5 showed enhanced performance while RU1-3 degraded. Afterwards, J-V characteristic curves showed loss of performances in both modes all along the experiment. A distribution of relaxation time analysis performed on the electrochemical impedance spectra measured regularly throughout the test revealed that the oxide ion transport through the H2-electrode and the O2-electrode is responsible for the observed degradation, over the 1400 h test duration.

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

Thermoeconomic Analysis of a Solar Enhanced Energy Storage Concept Based on Thermodynamic Cycles

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. Up to now pumped-hydro is the only technology that has been widely used for that purpose. However the spread of this technology is limited by geographic constraints. In the present work, a particular implementation of the storage concept based on thermodynamic cycles, as introduced by ABB Switzerland ltd. Corporate Research, has been analysed thermoeconomically. This concept can be scaled up to a GWh range of stored energy, and is site independent. The underlying idea is to use electricity to power a heat pump transferring heat from a cold source to a hot source (charging mode), while storing the heat and the cold at both sources. Subsequently the energy that has been stored/removed to/from the hot/cold sources can be used to run a heat engine, hence re-delivering the electricity when it is needed (discharging mode). In the implementation presented here, using the ambient temperature as the hot source for both the heat pump and the heat engine allows taking benefit from daily variations of the atmospheric temperature. Indeed, the heat-pump can operate with a higher Coefficient of Performance (CoP) during the night and the heat-engine at higher efficiency during the day. This results in an effective increase of the ratio between the redelivered electricity (discharge) and the stored electricity (charging), compared to a situation where the hot source of both cycles are at the same temperature. The use of solar collectors is a further way to enhance this ratio by increasing the hot source temperature above the ambient temperature during the day. In charging mode, the system consists in an ammonia based Rankine heat pump interfaced at the evaporator with a latent heat cold storage, and at the condenser to the ambient through aircoolers. In discharging mode an ammonia based Rankine heat-engine is interfaced at the condenser with the latent heat cold storage, and the evaporator(boiler) of the thermal engine is interfaced with hot water produced by a field of flat plate solar collectors. A steady state multiobjective optimization of a 50 MW power output plant was carried out, with the objectives of minimizing the investment costs, and maximizing the ratio between electricity output and input. 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 was carried out, reaching a yearly availability of 84.4%.

Pergamon-Elsevier Science Ltd2011

Degradation study of a reversible solid oxide cell (rSOC) short stack using distribution of relaxation times (DRT) analysis

Philippe Aubin, Stefan Diethelm, Suhas Nuggehalli Sampathkumar, Maria del Mar Perez Fortes, Jan Van Herle

Reversible solid oxide cells (rSOC) can convert excess electricity to valuable fuels in electrolysis cell mode (SOEC) and reverse the reaction in fuel cell mode (SOFC). In this work, a five - cell rSOC short stack, integrating fuel electrode (Ni-YSZ) supported solid oxide cells (Ni-YSZ parallel to YSZ vertical bar CGO parallel to LSC-CGO) with an active area of 100 cm(2), is tested for cyclic durability. The fuel electrode gases of H-2/N-2:50/50 and H-2/H2O:20/80 in SOFC and SOEC mode, respectively, are used during the 35 reversible operations. The voltage degradation of the rSOC is 1.64% kh(-1) and 0.65% kh(-1) in SOFC and SOEC mode, respectively, with fuel and steam utilisation of 52%. The post-cycle steady-state SOEC degradation of 0.74% kh(-1) suggests improved lifetime during rSOC conditions. The distribution of relaxation times (DRT) analysis suggests charge transfer through the fuel electrode is responsible for the observed degradation. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.