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The low-temperature cold start has become one of the leading technical bottlenecks limiting the large-scale commercial application of proton exchange membrane(PEM) fuel cells. This paper establishes the 1-D numerical model of the fuel cell stack to study the reactant gas, charge transfer, heat transfer, and water phase transition process. Then the critical parameters in the numerical model of the fuel cell stack are identified based on the cold start experiment data. The identification results show that the cold start model is consistent with the experimental results, and the model's output voltage and average temperature errors are under the acceptable range. Based on the numerical model, with the cold start time as the optimization objective and load current and initial membrane water content as the optimization variable, the cold start strategy is optimized under different ambient temperatures. The results show that the optimized current loading strategy can reduce the cold start time of the fuel cell stack and achieve a fast, successful cold start for −20 °C and − 25 °C conditions. The cold start time of the optimized cold start strategy is reduced by 42.2% compared with the strategy of step stair current under −20 °C. For the lower ambient temperature,-30 °C or below, to realize the successful cold start of fuel cell stacks, two aspects are needed: the redesign of the bipolar plate and endplate structure and the optimization of the cold start strategy. • The supercooled water phase change process is established based on JMAK theory. • Parameters identification and error analysis for the numerical model are conducted. • Initial membrane and segmented currents are optimized for the cold start strategy. [ABSTRACT FROM AUTHOR]

The two-phase expansion inside the expander could be divided into the non-equilibrium flash of superheated liquid and the expansion of the vapor. A higher flash rate would permit higher recovered expansion work. However, it is almost impossible to measure the flash rate inside the expansion chamber. In this paper, an equivalent expansion method s was proposed to study the non-equilibrium flash process for the two-phase expansion. The flash was controlled inside a constant volume flash chamber, which was connected to a vacuum tank to introduce and adjust the flashed vapor rate out of the flash chamber. The flash process inside the chamber was controlled under the working conditions similar to that inside the two-phase expander. By applying the equivalent expansion method, the flash rate could be measured easily. The experiments were carried out with an initial liquid height inside the chamber from 110 to 175 mm, and an initial liquid temperature from 110 to 130 °C with water as the working fluid. The instant flash rate and the P V diagram of the equivalent two-phase expansion were studied. Results showed that the instant flash rate increased and decreased sharply at first, and then the flash became gentle. Thus, the non-equilibrium flash of the two-phase expansion was divided into two stages of the rapid expansion process and the gradual expansion process. It was found that most of the flashed mass was generated during the rapid expansion process. Finally, a non-equilibrium flash model was proposed to simulate the flash process during the two-phase expansion process with the accuracy of ±3.7% predicting the pressure profile and the accuracy of ±2.7% predicting the expansion work. • Instant flash rate of two-phase expansion is obtained by equivalent expansion method. • The sharp jump stage of the instant flash rate was defined as the rapid flash. • Rapid flash is valuable to power recovery ratio considering the expander design. • The new non-equilibrium flash model predicted the flash process with high accuracy. [ABSTRACT FROM AUTHOR]

In future smart thermal grids, decentralized prosumers shall supply each other over the network, optimized by an overarching management system. For this, the substation control must be able to directly process optimal power flow setpoints from the management, while considering the mutual influence of prosumers on the technical level. Addressing these requirements, this paper develops a control approach for bidirectional prosumer substations. Therefore, we identify the objectives and characteristics of the control problem, before relaxing it by reformulation. This enables to keep proportional–integral–derivative controllers (PIDs) for operating the pumps and valves as decentral actuators in the substations. We propose a control approach, exploring the combination of two key aspects: (1) The assignment of temperature objectives to actuators. (2) Weighted error functions that linearly combine temperature and power errors as input for the PIDs, allowing to handle multiple objectives with limited actuators. Through case studies, we validate the suitability of our proposed objective–actuator assignment and demonstrate the necessity and benefits of the weighted error functions. Like this, we conclude with a control approach for bidirectional prosumer substations that aims for the relevant temperature objectives, directly considers heat transfer setpoints, replicates and exploits the coupling within the network, effectively manages conflicting objectives, copes with prosumer interference, and is pragmatic for real-time operation. Our approach can be transferred to different network concepts, bridging the gap between decentralized field-level operation and the management level. • Develop control approach for bidirectional substations in thermal prosumer networks. • Propose weighted error function approach in combination with PID controllers. • Study suitability of different objective-to-actuator assignments for substations. • Bridge management and field level by processing power setpoints in substation. • Anticipate mutual influence of prosumers over the network on the technical level. [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]