Vol. 1 No. 2 (2023)

In the era of industrial revolution, from powering factories to keeping our digital world humming, energy resources are the hidden engine behind modern economic activity, fueling production, transportation, and every click of a keyboard. However, these commercial endeavors generate contaminants and toxins, which are extremely harmful for the environment and public health. To overcome these concerns, the utilization of clean energy is the main focus to enhance the economic growth and environmental preservation. During this time, many researchers and academicians are pivoted to study and research on clean energy technologies. Their successful research work helped us in publishing one commentary and six review articles, centered around the efficient use of resources, sustainable development, and environmental protection, in issue 2, volume 1 of this journal. It offers readers an overview of the most recent research trends in clean energy technology.

Direct air capture (DAC) of CO₂ is a carbon-negative technology that is not limited by time and geography and can contribute to the realization of the “dual-carbon” goal. In this paper, the current development of direct air capture of CO₂ is reviewed, focusing on four mainstream solid adsorption DAC technologies, namely, metal-organic frameworks (MOFs), solid amine materials, alkali-metal-based adsorbents, and moisture-altering materials, and their advantages and disadvantages in terms of energy consumption, cyclic stability, and adsorption capacity are analyzed. In addition, the engineering applications of solid adsorbent materials are analyzed and the potential of these technologies in practical applications is demonstrated. Finally, the challenges faced by existing DAC adsorbent materials are summarized and the future development direction is put forward.

The potential for utilizing flue gas as a carbon source in microalgal cultivation holds great promise. Incorporating flue gas as a carbon source into microalgae culture processes can accelerate the growth rate of microalgae, consequently enhancing the overall economic viability of the integrated process. There are two key sources of flue gas to consider: flue gas from coal-fired power plants, characterized by a CO₂ concentration of 12%–15% w/w, and flue gas from coal chemical processes, boasting a CO₂ concentration of 90%–99% w/w. Additionally, the choice between an open or sealed microalgae culture system can also influence economic efficiency. Thus, there are four distinct microalgal cultivation routes to assess: in-situ open systems, off-situ open systems, in-situ sealed systems, and off-situ sealed systems. The incorporation of flue gas as a carbon source in microalgae cultivation demonstrates significant potential for reducing both environmental impact and costs, rendering it a highly promising and sustainable approach for economically efficient microalgae cultivation. In this review, the in-situ open route is recommended for systems with a high concentration of flue gas CO₂ with the target product of low-margin commodities, while the off-situ sealed route is suitable for systems with low concentration of flue gas CO₂ with the target product of high-value-added products.

Heat pump technology is an energy-saving technology that can efficiently utilize low-grade energy. It has broad application prospects in building heating, industrial waste heat utilization, new energy, and other fields. However, the refrigerants used in traditional heat pump systems have serious negative impacts on the environment, and there is an urgent need to find a safe, environmentally friendly, and efficient alternative refrigerant. As a natural refrigerant, CO₂ has good physical and chemical properties and is very suitable as a working fluid in transcritical cycles, showing great advantages in the field of heat pump technology. At present, research on CO₂ heat pumps has made certain progress, but there are few reviews of the research status and development trends of CO₂ heat pumps in different applications. Therefore, in this article, the latest research results of transcritical CO₂ heat pumps in different application fields are systematically summarized, pointing out the difficulties such as high pressure and low operating efficiency in system design and operation. The latest optimization research works on system components, cycle structure, mixed refrigerants, and control strategies are also summarized. The results showed that each optimization method can significantly improve system performance, among which mixed refrigerant is the simplest optimization method. Finally, the outlook for CO₂ heat pump technology is put forward. With policy support and technological advancement, a more comprehensive, energy-saving, and intelligent CO₂ heat pump technology will continue to be developed and innovated.

Supercritical hydrothermal combustion technology is a new homogeneous combustion technology with high potential in the fields of efficient removal of organic waste, clean utilization of conventional fossil energy, and efficient recovery of heavy oil. In this paper, the literature related to supercritical hydrothermal combustion in recent years is reviewed, focusing on evaluating the current status of experimental and numerical simulation studies on the characteristics of supercritical hydrothermal combustion, as well as the latest progress in engineering. It has been pointed out that the reduction of ignition temperature and extinction temperature is the key to promoting the application of supercritical hydrothermal combustion technology, and the consideration of the real fluid’s effects and turbulence reaction interactions can correctly reflect the combustion process. In addition, supercritical hydrothermal combustion technology, as a source of heat and reaction medium supply, can realize the efficient removal of highly concentrated organic wastewater, the clean combustion of coal, and in-situ hydrogen production, as well as the thermal recovery of heavy oil by multi-thermal fluids. At present, supercritical hydrothermal combustion forced ignition technology, reactor design guidelines, and corrosion prevention of key equipment are still the focus of future research, which is of great significance to promote the application of supercritical hydrothermal combustion technology.

Carbon nanomaterials are widely used as substrate materials to prepare stretchable conductive composites due to their good stability, strong conductivity, and low price. In response to the demand for optimizing the performance of composite materials, various manufacturing methods for preparing carbon nanomaterial-reinforced stretchable conductive composite materials have emerged. Among them, 3D printing technology has the advantages of flexible processes and excellent product performance and has received widespread attention. This review focuses on the research progress of adding carbon nanomaterials as reinforcing phases to polymer materials using 3D printing technology. The application prospects of conductive polymer composites based on nanocarbon fillers in aerospace, energy storage, biomedicine, and other fields are prospected.

The current impacts of climate change necessitate the promotion and use of renewable energy sources to avert the growing environmental and health concerns emanating from the use of fossil fuels. Lignocellulosic biomass (LCB) is a promising, renewable, and sustainable energy source based on its abundance and feedstock properties. Anaerobic digestion (AD) involves a biochemical process that can convert LCB to biogas through hydrolysis and biomethanation processes through the action of microorganisms, such as methanogens and sulfate-reducing bacteria. The hydrolysis of LCB releases various reducing sugars, which are essential in the production of biofuels, such as bioethanol and biogas, organic acids, phenols, and aldehydes. The resultant biogas can complement energy needs, while achieving economic, environmental, and health benefits. Enhancement of the AD process for converting LCB into bioenergy can be realized through appropriate pretreatment capable of disrupting the complex lignocellulosic structure and freeing cellulose and hemicellulose from the binding lignin for enzymatic saccharification and fermentation. Determining the optimal pretreatment technique for AD is critical for the success of the LCB energy production process. This study evaluated the application of chemical pretreatment techniques for the improvement of LCB digestion for bioenergy production. The study reviewed LCB characteristics, AD processes, and the role of various chemical pretreatment techniques, such as acid, alkali, organosolv, ozonolysis, and ionic fluids. The findings of this study can create an understanding of the action methods and benefits of different LCB chemical pretreatment techniques, while highlighting the outstanding drawbacks that require divergent strategies.

Chromophore molecules that undergo reversible isomerization under excitation by two different wavelengths of light are commonly referred to as photoswitchable molecules. Through the use of light-induced reversible changes combined with precisely spatial and temporal light control, photoswitchable molecules can be applied in areas such as actuators, optics, molecular motors, and photobiology.