Hydrogen production via a two-step thermochemical cycle based on solar energy has attracted increasing attention. However, the severe irreversible loss causes the low efficiency. To make sense of the irreversibility, an in-depth thermodynamic model for the solar driven two-step thermochemical cycles is proposed. Different from previous literatures solely focusing on the energy loss and irreversibility of devices, this work decouples a complex energy conversion process in three sub-processes, i.e., reaction, heat transfer and re-radiation, acquiring the cause of irreversible loss. The results from the case study indicate that the main irreversibility caused by inert sweeping gas for decreasing the reduction reaction temperature dominates the cycle efficiency. Decreasing reduction reaction temperature without severe energy penalty of inert sweeping gas is important to reducing this irreversible loss. A favorable performance is achieved by decreasing re-oxidation rate, increasing hydrolysis conversion rate and achieving a thermochemical cycle with a lower equilibrium temperature of reduction reaction at atmosphere pressure. The research clarifies the essence of process irrrversibility in solar thermichemical cycles, and the findings point out the potential to develop efficient solar driven two-step thermochemical cycles for hydrogen production.

The wave rotor technology is an energy exchanging approach that achieves efficient energy transfer between gases without using mechanical components. The wave rotor technology has been successfully utilized in gas turbine cycle systems, gas expansion refrigeration and a variety of other industrial domains, yielding numerous researches and application outcomes. The structure of wave rotor passages inside which the energy exchange between gases is realized has an important impact on the equipment performance. In this study, based on gas wave ejection technology, the first application trials of an expansion wave rotor with curved passages were conducted. Additionally, the performance enhancing effect and mechanism of curved passages on the energy exchanging process were studied precisely by the combination of experimental and three-dimensional numerical simulation methods. 
The experimental results demonstrate that the curved passage rotor (CIR rotor) employed in this research has a maximum isentropic efficiency of 61.6%, and the CIR rotor achieves higher efficiency than the straight passage rotor (STR rotor) on all working conditions in this study. Compared with the STR rotor, the maximum efficiency improving ratio of CIR rotor can exceed 14.2% at each experimental expansion ratio, and the maximum relative increments of ejection rate are more than 5%. In addition, the CIR rotor can also effectively increase the proportion of static pressure in total pressure of the medium-pressure gas, and reduce the device power consumption. The three-dimensional numerical investigations revealed the principle of gas ejection in the wave rotors and explained why the CIR rotor performed better. According to the numerical findings, the curved passages of the CIR rotor may effectively minimize various energy losses created in the processes of high-pressure gas incidence, exhausting flow in nozzle, and high-speed gas flow in the passages.

This work proposes a novel tubular structure of high-temperature proton exchange membrane fuel cell (PEMFC) integrated with a built-in packed-bed methanol steam reformer to provide hydrogen for power output. A two-dimensional axisymmetric non-isothermal model was developed in COMSOL Multiphysics 5.4 to simulate the performance of a tubular high temperature proton membrane fuel cell and a packed bed methanol reformer. The model considers the coupling multi-physical processes, including methanol reforming reaction, water gas shift reaction, methanol cracking reaction as well as the heat, mass and momentum transport processes. The sub-model of the tubular packed-bed methanol reformer is validated between 433 K and 493 K with the experimental data reported in the literature. The sub-model of the high temperature proton exchange fuel cell is validated between 393 K and 433 K with the published literature. Our results show that power output and temperature distribution of the integrated unit depend on methanol flow rates and working voltages. It was suggested that stable power generation performance of 0.14 W/cm² and temperature drop in methanol steam reformer of ≤10 K could be achieved by controlling the methanol space-time ratio of ≥250 kg·s/mol with working voltage at 0.6 V, even in the absence of an external heat source.

Syngas fuel such as hydrogen and carbon monoxide generated by solar energy is a promising method to use solar energy and overcome its fluctuation effectively. This study proposes a combined cooling, heating, and power system using the reversible solid oxide fuel cell assisted by solar energy to produce solar fuel and then supply energy products for users during the period without solar radiation. The system runs a solar-assisted solid oxide electrolysis cell mode and a solid oxide fuel cell mode. The thermodynamic models are constructed, and the energetic and exergetic performances are analyzed. Under the design work conditions, the SOEC mode’s overall system energy and exergy efficiencies are 19.0% and 20.5%, respectively. The electrical, energy and exergy efficiencies in the SOFC mode are 51.4%, 71.3%, and 45.2%, respectively. The solid oxide fuel cell accounts for 60.0% of total exergy destruction, caused by the electrochemical reactions' thermodynamic irreversibilities. The increase of operating temperature of solid oxide fuel cell from 800℃ to 1050℃ rises the exergy and energy efficiencies by 11.3% and 12.3%, respectively. Its pressure from 0.2 to 0.7 MPa improves electrical efficiency by 13.8% while decreasing energy and exergy efficiencies by 5.2% and 6.0%, respectively.

A proper operating strategy is helpful to improve the off-design performance of combined cooling, heating and power (CCHP) systems, providing high efficiency and low emission. The energy level difference graphic analysis method is used to identify energy level as well as exergy destruction of the part-load process. This method illustrates the energy efficiency upgrading mechanism of the flue gas reinjecting (FGR) operating strategy. It is referenced to a reducing turbine inlet temperature (TIT) operating strategy. By comparison, the FGR operating strategy leads to a 2.62% exergy distribution reduction in a gas turbine at an 85% load level due to the decrease of the energy level difference. When the output power is reduced further, the FGR operating strategy is supplanted by the TIT operating strategy with the limit of compressor inlet temperature. However, the opposite results of exergy distribution are presented in the exhaust-heat recovery devices. A heat-driven refrigeration and power cycle is introduced in a typical CCHP system as a solution. Moreover, the results suggest that the operational flexibility of the CCHP system is improved by enlarging the ratio of cooling to electricity.

The main purpose of this study is to analyze the performance of a new system that combines organic Rankine Cycle (ORC) and vapor compression refrigeration cycle (VCRC) for refrigeration and cogeneration. This system uses low-temperature heat sources such as solar energy, geothermal, industrial waste heat and biomass. The novelty of the proposed system manifests itself essentially in: the development of new ORC-VCRC combination architecture, lowering the ORC condensing temperature, the possibility of refrigeration production by the ORC upstream of the pumping phase, preheating of ORC using VCRC fluid and new configurations based on the integration of heat recovery systems to improve the overall system performance. The first part of this study presents the energetic analysis for the basic system using different working fluids and investigation of the operating parameters effect on the system performance (The system performance is described by the ORC thermal efficiency, the VCRC coefficient of performance and the system overall efficiency). Ten working fluids have been selected in order to provide the most adequate candidates for the proposed system. The results showed that the heating temperature and the cooling temperature have a significant effect on the system performance. The choice of fluid was also mentioned; the obtained results confirmed that the best combination for the basic system is R236fa-acetone. Four system configurations are developed and analyzed in the second part of the study. Also in the same part of the study, we will compare these configurations in terms of the performance rate retained. In the last part, we will make a comparison of this new system with another system.

A novel power and cooling cogeneration system which combines a supercritical CO₂ recompression cycle (SCRC), an ammonia-water absorption refrigeration cycle (AARC) and a Kalina cycle (KC) is proposed and investigated for the recovery of medium-temperature waste heat. The system is based on energy cascade utilization, and the waste heat can be fully converted through the simultaneous operation of the three sub-cycles. A steady-state mathematical model is built for further performance study of the proposed system. When the exhaust temperature is 505°C, it is shown that under designed conditions the thermal efficiency and exergy efficiency reach 30.74% and 61.55%, respectively. The exergy analysis results show that the main exergy destruction is concentrated in the heat recovery vapor generator (HRVG). Parametric study shows that the compressor inlet pressure, the SCRC pressure ratio, the main compressor and the turbine I inlet temperature, and the AARC generator pressure have significant effects on thermodynamic and economic performance of the combined system. The findings in this study could provide guidance for system design to achieve an efficient utilization of medium-temperature waste heat (e.g., exhaust heat from gas turbine, high-temperature fuel cells and internal combustion engine).

Oxy-fuel combustion power systems can utilize the cold energy released during the liquefied natural gas (LNG) regasification to reduce the power consumption of CO₂ capture, but the specific LNG cold energy consumption of CO₂ capture is still too large. To recover more CO₂ with the limited LNG cold energy at a low energy cost, a novel natural gas-fired oxy-fuel power system with the cascade utilization of LNG cold energy is proposed in this work, where the LNG cold energy could be sequentially utilized in the air separation unit and the CO₂ recovery process. The new system is evaluated with the Aspen Plus software. The results show that the net electrical efficiency and the specific primary energy consumption for CO₂ avoided (SPECCA) of the new system are comparable to those of the chemical looping combustion cycle, and superior to those of the conventional O₂/CO₂ cycles. Moreover, the specific LNG needed for CO₂ avoided (SLNCC) of the new system is more than 67.2% lower than the existing oxy-fuel power systems utilizing the LNG cold energy. Furthermore, it is found that the O₂ purity of 97.0 mol% and the CO₂ capture ratio of 97.0% are optimal conditions, because the SPECCA, the specific exergy consumption for CO₂ avoided (SECCA) and the SLNCC are at the minimum of 1.87 GJ_LHV·t_CO2⁻¹, 2.60 GJ·t_CO2⁻¹ and 1.88 t_LNG·t_CO2⁻¹, respectively. Meanwhile, the net electrical efficiency and the exergy efficiency of the new system reach 51.51% and 49.23%, respectively.