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In the domain of solar thermal energy utilization, the power cycles that utilize carbon dioxide as the working fluid predominantly encompass the transcritical Rankine cycle and the supercritical Brayton cycle. This study employs MATLAB programming to compute and examine the thermal efficiencies of these two cycles across a spectrum of solar collector temperatures ranging from 200 to 1000°C and carbon dioxide working fluid pressures from 10 to 40 MPa. At elevated temperatures, the thermal efficiencies of both cycles augment with the escalation in working fluid pressure; however, at reduced temperatures, the alterations in thermal efficiencies of the two cycles are converse. Furthermore, beyond a specific temperature, the thermal efficiency of the supercritical Brayton cycle consistently surpasses that of the transcritical Rankine cycle. By means of fitting, the relational expressions for the thermal efficiencies of the two cycles being equivalent were ascertained. In light of practical circumstances, it is proposed that the supercritical Brayton cycle should be adopted when the solar collector temperature exceeds 350°C, whereas the transcritical Rankine cycle is more appropriate below 350°C.

Rice production is sensitive to climate change and significantly affected by warming in recent years. To what extent climate warming shifted rice phenology and varied thermal resource condition were explored across five agro-ecological zones in China, based on up-to-date observations of meteorology and rice cultivation in 1981–2020. It was clearly signaled that there was a general advance of 0.3–3.8 days in observed sowing date and a delay of 0.4–3.5 days in observed maturity date in 2001–2020 relative to 1981–2000 in major zones. A vacant time slice of 2.6–28.8 days between observed sowing date and potential sowing date, and a lag of 15.4–56.7 days in potential maturity date compared to observed maturity date were identified in 2001–2020. Within longer growing season, useful accumulated temperature increased by 76.7–117.6 °C·d in 2001–2020 relative to 1981–2000, while disactive accumulated temperature also increased. In Northeast China, actual rice cultivation was undergoing earlier sowing date than potential sowing date and longer growing duration than potential duration, yet leading to upward disactive accumulated temperature. The decrease in the thermal resource utilization in 2001–2020 relative to 1981–2000 was highlighted at 55.3–78.3% stations in major zones, with a decrement of 0.006–0.018 in average magnitude. The changes in thermal resource utilization unveiled that the shifts in actual rice cultivation still could not compensate for the suitability in thermal resource utilization benefited from climate warming.

To improve the problem of low temperature at night in winter due to the lack of thermal storage in large-span plastic tunnels, an air thermal energy utilization system (ATEUS) was developed with fan-coil units to heat a large-scale plastic tunnel covered with an external blanket (LPTEB) on winter nights. The ATEUS was composed of nine fan-coil units mounted on top of the LPTEB, a water reservoir, pipes, and a water circulation pump. With the heat exchange between the air and the water flowing through the coils, the thermal energy from the air can be collected in the daytime, or the thermal energy in the water can be released into the LPTEB at night. On sunny days, the collected thermal energy from the air in the daytime (Ec) and released thermal energy at night (Er) were 0.25-0.44 MJ/m2 and 0.24-0.38 MJ/m2, respectively. Used ATEUS as a heating system, its coefficient of performance (COP), which is the ratio of the heat consumption of LPTEB to the power consumption of ATEUS, ranged from 1.6-2.1. A dynamic model was also developed to simulate the water temperature (Tw). Based on the simulation, Ec and Er on sunny days can be increased by 60%-73% and 38%-62%, respectively, by diminishing the heat loss of the water reservoir and increasing the indoor air temperature in the period of collecting thermal energy. Then, the COP can reach 2.6-3.8, and the developed ATEUS can be applied to heating the LPTEB in a way that conserves energy.

The utilization of solar thermal energy has shown remarkable growth recently. Due to the development of nanomaterials, suspensions of nanoparticles (nanofluids) have exhibited great performance in the solar thermal systems, especially in direct absorption solar collectors (DASC). The fundamental advantage of DASC by using nanofluids is the minimizing of solar energy transfer steps and reducing thermal losses in converting sunlight, as nanoparticles could harvest the solar energy directly and achieve good photothermal conversion properties. Carbon nanotube (CNT) based nanofluids showed great potential as a working fluid in DASC. Nevertheless, the stability of CNTs in suspensions is the main obstacle for the large use of CNT nanofluids. In this work, small amount of graphene oxide (GO) was introduced to stabilize CNT-water nanofluids without using any other organic surfactants. The GO stabilized CNT nanofluids exhibited long-term and high-temperature stability. The π-π interactions between the GO and CNT played a significant role for the good dispersion and stability of CNT in water. Furthermore, optical characterizations showed that the GO stabilized CNT nanofluids have widely absorbing over the solar spectrum which enabled highly efficient solar energy collections. Eventually, photothermal conversion performance of GO stabilized CNT nanofluids was tested under an irradiation of simulated solar light and the nanofluid demonstrated a high efficiency in DASC. The long-term stability coupled with broadband absorption properties of GO stabilized CNT nanofluids make them ideal candidates as photothermal conversion media for direct solar thermal collectors.

Reducing needs for heating and cooling from fossil energy is one of the biggest challenges, which demand accounts for almost half of global energy consumption, consequently resulting in complicated climatic and environmental issues. Herein, we demonstrate a high-performance, intelligently auto-switched and zero-energy dual-mode radiative thermal management device. By perceiving temperature to spontaneously modulate electromagnetic characteristics itself, the device achieves ~859.8 W m −2 of average heating power (∼91% of solar-thermal conversion efficiency) in cold and ~126.0 W m −2 of average cooling power in hot, without any external energy consumption during the whole process. Such a scalable, cost-effective device could realize two-way temperature control around comfortable temperature zone of human living. A practical demonstration shows that the temperature fluctuation is reduced by ~21 K, compared with copper plate. Numerical prediction indicates that this real zero-energy dual-mode thermal management device has a huge potential for year-round energy saving around the world and provides a feasible solution to realize the goal of Net Zero Carbon 2050. Real-world practical utilization of zero-energy thermal management systems often requires adaptability to dynamic weather. Here, authors demonstrate a zero-energy, self-adapting, dual-mode radiative thermal management device, capable of switching between heating and cooling based on the ambient temperature.

HighlightsAn innovative class of versatile form-stable composite phase change materials (CPCMs) was fruitfully exploited, featuring MXene/phytic acid hybrid depositing on non-carbonized wood as a robust support.The wood-based CPCMs showcase enhanced thermal conductivity of 0.82 W m−1 K−1 (4.6 times than polyethylene glycol) as well as high latent heat of 135.5 kJ kg−1 (91.5% encapsulation) with thermal durability and stability throughout at least 200 heating and cooling cycles.The wood-based CPCMs have good solar-thermal-electricity conversion, flame-retardant, and electromagnetic shielding properties.Phase change materials (PCMs) offer a promising solution to address the challenges posed by intermittency and fluctuations in solar thermal utilization. However, for organic solid–liquid PCMs, issues such as leakage, low thermal conductivity, lack of efficient solar-thermal media, and flammability have constrained their broad applications. Herein, we present an innovative class of versatile composite phase change materials (CPCMs) developed through a facile and environmentally friendly synthesis approach, leveraging the inherent anisotropy and unidirectional porosity of wood aerogel (nanowood) to support polyethylene glycol (PEG). The wood modification process involves the incorporation of phytic acid (PA) and MXene hybrid structure through an evaporation-induced assembly method, which could impart non-leaking PEG filling while concurrently facilitating thermal conduction, light absorption, and flame-retardant. Consequently, the as-prepared wood-based CPCMs showcase enhanced thermal conductivity (0.82 W m−1 K−1, about 4.6 times than PEG) as well as high latent heat of 135.5 kJ kg−1 (91.5% encapsulation) with thermal durability and stability throughout at least 200 heating and cooling cycles, featuring dramatic solar-thermal conversion efficiency up to 98.58%. In addition, with the synergistic effect of phytic acid and MXene, the flame-retardant performance of the CPCMs has been significantly enhanced, showing a self-extinguishing behavior. Moreover, the excellent electromagnetic shielding of 44.45 dB was endowed to the CPCMs, relieving contemporary health hazards associated with electromagnetic waves. Overall, we capitalize on the exquisite wood cell structure with unidirectional transport inherent in the development of multifunctional CPCMs, showcasing the operational principle through a proof-of-concept prototype system.

Electrification of conventionally fired chemical reactors has the potential to reduce CO₂ emissions and provide flexible and compact heat generation. Here, we describe a disruptive approach to a fundamental process by integrating an electrically heated catalytic structure directly into a steam-methane–reforming (SMR) reactor for hydrogen production. Intimate contact between the electric heat source and the reaction site drives the reaction close to thermal equilibrium, increases catalyst utilization, and limits unwanted byproduct formation.The integrated design with small characteristic length scales allows compact reactor designs, potentially 100 times smaller than current reformer platforms. Electrification of SMR offers a strong platform for new reactor design, scale, and implementation opportunities. Implemented on a global scale, this could correspond to a reduction of nearly 1% of all CO₂ emissions.

Energy storage and conservation are receiving increased attention due to rising global energy demands. Therefore, the development of energy storage materials is crucial. Thermal energy storage (TES) systems based on phase change materials (PCMs) have increased in prominence over the past two decades, not only because of their outstanding heat storage capacities but also their superior thermal energy regulation capability. However, issues such as leakage and low thermal conductivity limit their applicability in a variety of settings. Carbon-based materials such as graphene and its derivatives can be utilized to surmount these obstacles. This study examines the recent advancements in graphene-based phase change composites (PCCs), where graphene-based nanostructures such as graphene, graphene oxide (GO), functionalized graphene/GO, and graphene aerogel (GA) are incorporated into PCMs to substantially enhance their shape stability and thermal conductivity that could be translated to better storage capacity, durability, and temperature response, thus boosting their attractiveness for TES systems. In addition, the applications of these graphene-based PCCs in various TES disciplines, such as energy conservation in buildings, solar utilization, and battery thermal management, are discussed and summarized.

Because of the important role of the absorption heat pump in low-grade thermal energy utilization, this paper extends it to micro domain and performs a finite-time thermodynamic modelling for a three-heat-reservoir (THR) thermal Brownian heat pump with heat transfer effect by using an equivalent combined cycle method, which was applied for macro endoreversible THR heat pumps. The working principle and energy transformation rule are studied, and the coefficient of performance (COP) and heating load are derived. With a fixed overall thermal conductance of three heat exchangers, the maximal heating load is determined by optimizing thermal conductance distributions among three heat exchangers and barrier height, and the optimal working temperatures are also obtained. The impact of external heat transfer is elucidated to show the difference between this model and a non-equilibrium thermodynamic one. Results indicate that external heat transfer determines the energy transformation directly, and performance characteristics are closer to reality when external heat transfer is considered. The heating load has a maximal value about thermal conductance allocation ratios. About half the overall heat exchanger inventory needs to be assigned to the heat exchanger of the heating space for maximal heating load. When the cycle is with only heat transfer effect, the net particle numbers are zero, and the cycle fails to pump heat. The research results are expected to offer an idea for thermodynamic optimization and design of micro THR thermal Brownian heat pump devices.

An enhanced dung beetle optimization algorithm is used to solve the comprehensive optimization model of a cogeneration microgrid with hydrogen storage, with the optimization objective of minimizing the microgrid’s overall operation cost. The model is intended to address the issue of low thermal energy utilization efficiency of the hydrogen energy system of a cogeneration microgrid with hydrogen storage. The findings demonstrate that the suggested plan may more effectively satisfy the thermoelectric load requirements of the microgrid and guarantee the microgrid’s steady and independent operation, thereby fulfilling the objectives of energy conservation and emission reduction.