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In this paper, a theoretical expression for the efficiency of a fuel cell system is derived. The studied system consists of a proton-exchange membrane fuel cell stack coupled at its output to a DC/DC boost converter. A fuel cell stack Larminie and Dicks model along with an efficiency curve for our boost converter are fitted using experimental data. One maximum efficiency point for the fuel cell stack coupled to the DC/DC power converter is shown and lies in the safe zone of the fuel cell stack. Experimental results on the fuel cell efficiency, DC/DC efficiency and their association on a test bench indicate a good agreement between the theoretical estimations and the experimentally obtained results at different values of the DC/DC converter output voltage. The results provide strong theoretical and experimental evidence for a unique maximum efficiency point for the fuel cell system and form a basis for developing maximum efficiency point tracking algorithms. • Experimental verification of theoretical expression on a fuel cell test bench. • Study of maximum efficiency point tracking of fuel cell + DC/DC converter system. • Derivation of a theoretical expression for the total efficiency of a fuel cell system. • Numerical methods for computing theoretical total efficiency. • International collaboration between Australian and French research teams. [ABSTRACT FROM AUTHOR]

Hydrogen is among the energy carriers, which are most often considered for bringing about a sustainable energy system. Yet, so far, hydrogen has not delivered as an energy carrier. The core objective of this paper is to provide a comprehensive analysis of the state-of-the-art and the future prospects of green hydrogen in the European energy system from economic, energetic and CO 2 emissions point-of-view. The analysis shows that there are some increasing opportunities for hydrogen use in industry and in the transport sector, when electrification is not possible or is too expensive, as well as a storage in the European electricity system. However, a hydrogen-based energy system will remain a vision at least over the next decades. The major reason for this is the unfavorable economics mostly due to high investment costs in the whole supply chain. In addition, the overall efficiencies in the hydrogen chains are moderate in general. The full environmental benignity of hydrogen as an energy carrier is only provided when renewable energy sources are used for hydrogen production. However, in Europe, the potentials for green hydrogen are very limited due to the insufficient expansion of renewable electricity generation. For this reason, many European countries are considering options for green hydrogen import. The future of hydrogen is highly dependent on the supporting policy framework. To reduce the risk in the investment in hydrogen infrastructure, as well as to justify the promotion of green hydrogen, it is very important that Europe works out a very clear and realistic long-term implementation strategy. • Hydrogen might have potential in the European energy system, yet a hydrogen-based energy system stays visionary. • Green hydrogen from renewable electricity via electrolyzers is key for environmental benefits and decarbonization. • Hydrogen should be prioritized for uses where electrification is not feasible. [ABSTRACT FROM AUTHOR]

The quantification of the flow irreversibilities is crucial to developing future turbomachinery. Traditionally, design correlations and efficiencies are based on comparisons to isentropic relations to quantify the deviation from an "ideal" case. However, isentropic relations typically assume one-dimensional adiabatic flow, representing a significant departure from the actual situation in turbines operating with large levels of heat transfer. In this paper, the aerothermal losses and power generation in reversible processes are derived for three-dimensional compressible flow considering heat transfer effects. A new definition of the power potential in a reversible process is proposed that can be used to perform detailed budgeting of the losses. The proposed new loss framework can be adopted locally, which enables the precise identification of the loss source. New aerodynamic and aerothermal efficiency expressions are proposed to assess shaft power extraction in combination with heat exchange. The new method enables designers to identify the regions with high loss generation in steady and also in unsteady flow conditions, offering new avenues to evaluate the loss generation mechanisms during the design phase. Therefore, our new tool is essential to developing reduced-order models for the design of future fluid machinery. • Pioneering method for time-resolved loss production in compressible fluid machinery. • A novel approach based on volume integral compliant with heat flux. • First demonstration in transonic turbines with endwall heat flux. • Implementation suitable for experimental control volume assessment. • Proposed new correlations on thermal loss generation. [ABSTRACT FROM AUTHOR]

This paper proposes physical-based, reduced-order electrochemical models that are much faster than the pseudo-2D (P2D) model, while providing high accuracy even under the challenging conditions of high C-rate and strong polarization of lithium ion concentration and potential. In particular, a weak form of equations are developed by using shape functions, which reduces the fully coupled electrochemical and transport equations to ordinary differential equations, and provides self-consistent solutions for the evolution of polynomial coefficients. Results show that the models, named as revised single-particle model (RSPM) and fast-calculating P2D model (FCP2D), give reliable prediction of battery operations, including under dynamic driving profiles. They can calculate battery parameters, such as terminal voltage, over-potential, interfacial current density, lithium-ion concentration distribution, and electrolyte potential distribution with a relative error less than 2%. Applicable for moderately high C-rates (below 2.5C), the RSPM is up to more than 33 times faster than the P2D model. The FCP2D is applicable for high C-rates (above 2.5C) and is about 8 times faster than the P2D model. With their high speed and accuracy, these physics-based models can significantly improve the capability and performance of the battery management system and accelerate battery design optimization. • An approach of shape functions is developed for reducing electrochemical equations. • The integral form provides self-consistent solutions in ODE for fast calculation. • Prediction is an order of magnitude faster with high accuracy even in high C-rates. • The models provide reliable prediction including under dynamic driving profiles. [ABSTRACT FROM AUTHOR]

• A stacked piezoelectric energy harvester inside piezoelectric power generation device is designed considering size-effect. • The harvester has high energy output efficiency under road traffic conditions that match automobile parameters. • The power generation properties of the existing technology is enhanced. In order to further improve the power generation properties of the piezoelectric energy harvester inside the piezoelectric device for road, this paper presents a stacked piezoelectric energy harvester with high electrical output and energy output efficiency for road energy harvesting, which improves the power generation properties of the existing road piezoelectric power technology. Based on the electrical theory, the size parameters related to the electrical properties are clarified. And based on the size effect, the electrical law is analyzed. Subsequently, the output voltage and power of energy harvesters with different sizes are studied under the different traffic conditions. At the same time, considering the processing and testing errors, an energy generation calculation method of energy harvester is deduced, and the energy output efficiency is clarified. The results indicate that the diameter has the greatest influence on the electrical properties, the Φ15-h30 mm energy harvester has the best performance under the random traffic conditions of vehicles with different loads and different speeds. Under the condition of SUV car, the maximum voltage and power are 86 V and 39.2 mW, the generated energy and output electric energy are 69.2 mJ and 14.7 mJ within 2 s, the energy output efficiency reaches 21.4 %. The improvement of the electrical properties of the energy harvester inside the piezoelectric device will be beneficial to the application process of converting road waste vibration energy into clean energy. And the energy generation calculation method will provide a reference for the piezoelectric energy and other energy technologies. [ABSTRACT FROM AUTHOR]

• A data-driven multi-objective energy coordinative management policy for SOFC is proposed. • A new large-scale deep reinforcement learning algorithm is proposed for the strategy. • The TBL-MADDPG algorithm proposed is characterized by superior robustness. • The proposed policy both dynamic OER and FU constraints can be considered. • The proposed method leads to higher net power and prevent constraint violation. In this paper, a data-driven multi-objective energy coordinative management policy is proposed in order to enhance the net output power and efficiency of a solid oxide fuel cell (SOFC) and prevent constraint violations. This study focuses on the optimization agent and controller design for a SOFC power system to maintain stable oxygen excess ratio (OER) and fuel utilization (FU) ratio as well as meet the load demand simultaneously. The optimization agent is responsible for output the reference OER and FU, aiming to achieve maximum net output power and operational efficiency as well as dynamic constraint satisfaction times in terms of OER and FU in real time. By applying reference OER and FU settings, the air and hydrogen flow within the SOFC can be effectively controlled by coordination of the air control agent and hydrogen control agent, respectively. In addition, a triune brain large-scale multi-agent deep deterministic policy gradient algorithm (TBL-MADDPG) is proposed. In order to improve the robustness of the proposed policy, the design of TBL-MADDPG entails curriculum learning, imitation learning and a large-scale multi-agent training framework. The performance of this proposed method is verified by the experiment. [ABSTRACT FROM AUTHOR]

Despite early calls to investigate the application of Vickrey auctions in electricity markets, academic literature still provides little insight into the matter. Similarly, despite general auction-theoretic literature discussing various advantages of employing revealed rather than sealed bidding procedures, publications on electricity market auctions rarely even mention differences between these opposing features. This paper marries these neglected features into a new auction model for electricity markets. We propose a novel open-bid, descending-clock auction capable of handling inter-bidder dependencies such as thermal line limits. A novel set of termination criteria ensures that our proposed auction facing markets with power balance and generator capacity constraints and thermal line limits, preserves truthful bidding as a dominant strategy, which, if employed by all bidders, allocates supply efficiently constituting an ex-post perfect equilibrium. Furthermore, we discuss known disadvantages of Vickrey–Clarke–Groves type auctions in the context of our proposed auction applied in electricity markets. • Proposition of a novel auction design considering electricity grid constraints. • Given quasi-linear utility, truthful bidding is weakly dominant utilising open bids. • Truthful bidding by all bidders results in efficient supply allocation. • Discussion of real-world electricity market design impacting auction properties. [ABSTRACT FROM AUTHOR]

Gasoline and methanol are highly recommended for low temperature combustion (LTC) engines due to their high research octane number. However, engines fuelled with these fuels suffer from high unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions at low loads, which can be improved by controlling the level of mixture stratification. This paper reports on the experimental and numerical comparison of the emission characteristics and engine performance of methanol and iso-octane in a heavy-duty direct-injection compression ignition engine operating in HCCI and PPC regimes. Overall, methanol requires a higher intake temperature, compared with iso-octane, to counter the high heat of vaporization. The onset of ignition is from the relatively fuel-lean regions resulting in lower CO emissions. This is due to the methanol ignition delay times being less sensitive to the fuel/air mixture and the larger temperature gradient of the equivalence ratio. Methanol achieves lower charge stratification because of its low stoichiometric A/F ratio, which extends the SOI window with ultra-low NO x emissions. However, methanol suffers from higher UHC emissions in the HCCI regime due to fuel trapped in the crevice region. Three cases include HCCI, PPC with low and high mixture stratification were detailed investigated. Of these cases, PPC with low mixture stratification case shows the highest engine thermal efficiency and the lowest emissions. Methanol has more advantages operating in the PPC regime in the aspect of emissions. • The overall trend of the intake temperature for both fuels shows 'spoon' shape. • Methanol requires a higher intake temperature to counter the high heat of vaporization. • Ignition of methanol is from the relatively fuel-lean regions resulting in lower CO emissions. • Methanol achieves lower charge stratification because of its low stoichiometric A/F ratio. • Methanol has more advantages operating in the PPC regime in the aspect of emissions. [ABSTRACT FROM AUTHOR]

• The detailed advances in the multi-stack fuel cell system (MFCS) are reviewed. • The advantages of MFCS are presented compared with single fuel cell stack. • The scientific challenges and prospects for MFCS are suggested. • The synergy between ICEs and MFCSs is the core idea to develop the MFCS. To achieve carbon emission reduction and energy structure upgrading, the proton exchange membrane (PEM) multi-stack fuel cell system (MFCS) is an option to meet high-power demands in application scenarios. However, several barriers of the PEM MFCS have not been solved. This paper is aimed to summarize existing information and propose future R&D potentials. Firstly, detailed R&D activities and advances related to PEM MFCSs are reviewed including applications and design principle, architectures (fluidic, electrical, and thermal), power management strategy (PMS), water and thermal management, degraded mode operation, lifetime, efficiency, and hydrogen consumption. Among them, applications and design principle include application scenarios and influencing factors. Moreover, the architecture determination for MFCSs is the core issue, directly affecting the PMS, water and thermal management, degraded mode operation, lifetime, efficiency, and hydrogen consumption. It is noted that a MFCS is better than a single fuel cell system (SFCS) in fields of the lifetime, efficiency, and hydrogen consumption. Furthermore, the synergy between internal combustion engines (ICEs) and MFCSs is proposed to be applied for the MFCS R&D. Secondly, scientific challenges and prospects are presented, including effects of the load power-sharing on the lifetime and efficiency, comparisons of the performance, efficiency and lifetime, and control issues. Finally, the difference of several aspects is discussed, including the architecture, redundancy, lifetime, and PMS between a PEM MFCS and a PEM SFCS. [ABSTRACT FROM AUTHOR]