• Energy Research

Abstract Metal hydride (MH) is an attractive alternative for thermochemical heat storage. This study proposes an optimal design methodology for MH heat storage reactors (MHHSRs), integrating the optimal design principle (ODP) and a new design procedure. Based on the optimal design methodology, the design of the powder bed with helical heat transfer fluid (HTF) tube is conducted via numerical simulation. A mathematical model is established for the thermal coupling between the powder bed and the HTF tube. The gravimetric exergy-output rate (GEOR) is adopted to evaluate the overall discharging performance. First, the helical HTF tube is optimized and improved based on the ODP, which increase the GEOR from 198.4 to 255.4 W kg−1. The optimal helical diameter is 30 mm and the optimal improved structure is determined as a U-shaped double helical tube. Then, the structural improvement of the reaction bed is supplemented, achieving reductions of material and energy consumption by 12.2% and 11%, respectively. The final design of the powder bed with helical tube based on optimal design methodology improves the GEOR from 198.4 to 306.1 W kg−1, which constitutes a significant increase of 54.3%. This optimal design methodology is validated and efficiently guides the design of advanced MHHSRs.

Abstract Fuel cell power technology has drawn extensive attentions due to its high efficiency, low emission and noise. Solid oxide fuel cell (SOFC) could generate the power by diverse fuels, such as natural gas (NG), while proton exchange membrane fuel cell (PEMFC) only feeds on pure H2. More and more attentions are paid on the combination of SOFC and PEMFC for high efficiency and convenient refueling in the practical applications. To obtain H2 fuel with high purity from SOFC as a reformer, the gas processing subsystem for H2 separation and purification should be applied between SOFC and PEMFC. In this present study, the gas processing subsystem, consisting of water gas shift (WGS) and thermal swing adsorption (TSA), is introduced into the SOFC-PEMFC hybrid system. Then, the SOFC-WGS-TSA-PEMFC hybrid system is modelled to investigate the transient behaviors under different operations. The simulation results show that the SOFC-WGS-TSA-PEMFC hybrid system has an improved energy conversion efficiency of approximately 64%, which is higher than the only-SOFC and the reform-PEMFC. The waste heat recovery for driving the TSA reaction accounts for the higher net electricity efficiency compared with the SOFC-PEMFC hybrid system based on the pressure swing adsorption (PSA) for H2 separation. Since the SOFC and PEMFC have completely different transient responses to the change of the loading, the influences of operating conditions of fuel cell vehicles on the transient behaviors of single SOFC and PEMFC and the overall performance of the SOFC-WGS-TSA-PEMFC hybrid system are further investigated. Through the analysis and discussion based on the dynamic modelling, the operation strategy is unveiled in this paper for the performance optimization of the hybrid system when installed in the fuel cell vehicles.

Abstract This paper proposes a novel autothermal-equilibrium metal hydride reactor as the hydrogen source for the fuel cell power system, which employs phase change material (PCM) to recycle the hydrogen storage heat. A three-dimensional model of the metal hydride reactor coupled with a salt hydrate PCM for heat recovery is developed. Based on the model, the effects of key operating and design parameters on the reactor are investigated for performance optimization, including operating pressure, melting temperature, latent heat and thermal conductivity of PCM. Through the parametric analysis, it is found that increasing the operating pressure is beneficial to accelerate the absorption reaction. The average reaction fraction at 2400 s is increased by 24% with the pressure increasing from 6 to 10 bar. The moderate melting temperature and the thermal conductivity of the PCM that is comparable to that of metal hydride bed contribute to the improvement of hydrogen storage efficiency. Using this kind of hydrogen source reactor in a fuel cell power system, stable hydrogen storage efficiency of approximately 60% in the experiment is presented. In addition, no obvious performance deterioration of the power system occurs after ten cycles.

Abstract Magnesium hydride is one of the most promising materials for hydrogen storage and high-temperature heat storage. The heat and mass transfer processes occurring in magnesium hydride reactors are very complicated. In the present study, a mathematical model for a magnesium hydride reactor is developed to investigate the mass and heat transfer characteristics under various operating conditions. The distributions of gas velocity, gas pressure, temperature and reacted fraction are obtained by solving the rigorous model. Two versions of the model, which are respectively applied on two different computational domains, are investigated. The simplified version of the model is proved capable of predicting the performance of the magnesium hydride reactor, with high computation efficiency and acceptable accuracy. The simulation results indicate that an increase in hydrogen supply pressure ( p in ) accelerates the absorption process due to enhanced absorption reaction kinetics. The hydriding time decreases when heat transfer fluid (HTF) velocity ( u f ) goes up, because the convection heat transfer coefficient between HTF and the magnesium hydride rises with increasing HTF inlet velocity. Moreover, an increase in HTF inlet temperature ( T in ) leads to an increase in the equilibrium pressure of the magnesium hydride which decelerates the hydrogen absorption reaction. The optimal setting of the three operating parameters for the magnesium hydride reactor is determined based on the mathematical model, and the corresponding values of the parameters are as follows: p in = 2.6 MPa, T in = 473 K and u f = 5 m s −1 .

Abstract Advanced biogas power generation technology has been attracting attentions, which contributes to the waste disposal and the mitigation of greenhouse gas emissions. This work proposes and models a novel biogas-fed hybrid power generation system consisting of solid oxide fuel cell, water gas shift reaction, thermal swing adsorption and proton exchange membrane fuel cell (SOFC-WGS-TSA-PEMFC). The thermodynamic, exergetic, and thermo-economic analyses of this hybrid system for power generation were conducted to comprehensively evaluate its performance. It was found that the novel biogas-fed hybrid system has a gross energy conversion efficiency of 68.63% and exergy efficiency of 65.36%, indicating high efficiency for this kind of hybrid power technology. The market sensitivity analysis showed that the hybrid system also has a low sensitivity to market price fluctuation. Under the current subsidy level for the distributed biogas power plant, the levelized cost of energy can be lowered to 0.02942 $/kWh for a 1 MW scale system. Accordingly, the payback period and annual return on investment can reach 1.4 year and about 20%, respectively. These results reveal that the proposed hybrid system is promising and economically feasible as a distributed power plant, especially for the small power scale (no more than 2 MW).

The outstanding performance of DHER is confirmed and the radial projected area of its tubes is determined as the key design factor.

Abstract Although the thermoelectric device (TED) has recently been studied as a sub-cooler of the trans-critical C O 2 cycle, setting the TED to uniquely be an internal heat exchanger (i.e. TED-IHX) is an implementation that is yet to be well studied. Therefore, this paper’s objective is to study the potential of integrating the TED – internal heat exchanger (TED-IHX) component into the trans-critical C O 2 cycle for increasing the cycle’s coefficient of performance (COP). The TED-IHX is studied both as a thermoelectric generator (TEG) to generate additional electricity from the internally exchanged heat, or as a thermoelectric cooler (TEC) to accelerate the internal heat transfer rate. To achieve this analysis, a 1-D finite element thermodynamic model of the TED-IHX is developed, and this is then integrated into the overall cycle model to evaluate the cycle COP. Simulation results demonstrated that although operating the TED-IHX as a TEG offered a COP improvement of up to 5%, the improvement is inferior over the direct IHX. This occurred because the heat transfer effectiveness is lowered by the TED’s thermal resistance. Moreover, while operation as a TEC significantly reduces the compressor power requirement, the increased TEC power consumption negates this benefit.