• Energy Research

Energy storage is widely recognised as one of the key enablers for higher renewable energy penetration and future energy system decarbonisation. The term Carnot Battery refers to a set of storage technologies with electricity stored in the form of thermal energy, thus making them suitable not only for power balancing, but also for multi-vector energy management as a unique asset. With growing scientific literature on different Carnot Battery technologies and data from ongoing pilot and demonstration projects worldwide, this article aims to provide a review on the most recent developments in the area. More specifically, three complementary aspects are addressed: i) the collection and cross-comparison of quantitative techno-economic performance data of different Carnot Battery systems based on scientific literature findings; ii) the discussion of proposed applications for Carnot Batteries at the energy system scale, including power and thermal service provisions and retrofit opportunities; iii) the discussion of the most recent commercial developments in Carnot Battery technologies. Through this, we present the commonalities and discrepancies between scientific research and system implementation in ongoing projects. Our results show (a) a clear difference in the techno-economics of various Carnot Battery technologies; (b) a wide range of some performance metrics due to the absence of empirical evidence; and, interestingly, (c) a certain discrepancy between the systems and applications most addressed by the scientific community and the projects under development. The harmonisation of these discrepancies and the inclusion of location-specific integration considerations are proposed as a way forward for performance advancement and future deployment of Carnot Batteries.

Abstract Generally in the Chinese iron and steel industry, the electricity consumption of cryogenic air separation unit (ASU) is about 14% of the overall electricity use. To reduce the electricity consumption, the combined variable oxygen (CVO) supply method for ASU is proposed. The exergy calculation program for ASU was developed and the detailed analysis of CVO method was performed. The results show that the general exergy efficiency (GEE) of ASU combined with a liquefaction unit is increased by 11 %–31 %. The consumption of unit oxygen, the total electricity consumption and the overall consumption of unit oxygen (OCUO) was compared. The OCUO is a suitable method to evaluate the energy-saving potential of CVO. Compared with the load regulation method of Automatic Load Control (ALC), the OCUO and the unit consumption of compression of CVO reduced more than 4.47% and 30%, respectively. It means that CO 2 emission of every reduction 1% of gaseous oxygen release in a year in Chinese iron and steel industry will contribute approximately 0.75% to the 2020s CO 2 emission reduction target of China.

Lithium batteries must be connected in series to achieve large capacity and high-power output. Battery management system (BMS), which is designed to protect battery pack from damage and increase battery life, is important in electrical power system. The present equalisation techniques have many disadvantages: The passive balancing wastes energy and generates heat, while active balancing is complex. This study proposes an intelligent BMS with dynamic equalisation (DBMS) which contains active and passive balancing circuit independently per cell. Experimental results indicate that DBMS can reduce the inconsistency among cells. Moreover, the DBMS can assist battery stack to store and release more energy. Besides, the battery stack with DBMS gives an energy efficiency of 96.5% which is 7.7% higher than that without balancing. In addition, the battery stack with DBMS can reduce the maximum state of charge difference of cells from 10.415% to 4.51% after three charge-discharge cycles. What is more, the DBMS is simple and can decrease the auxiliary power level and the system heat. Such a DBMS will help us to provide a high-performance battery pack.

Abstract This paper establishes a thermo-mechanical model considering the liquid density variation to explore the comprehensive energy storage performance of two types of small-sized encapsulated phase change materials (PCMs) as well as effects of shell thickness. The study shows that the varying ranges of internal pressure, melting temperature and latent heat are markedly diminished during melting of PCMs after taking into account the liquid density variation. The decrease of shell thickness leads to a decrease of maximum internal pressure and a larger decrease of critical cracking pressure, which will increase the risk of shell cracking. The decrease in shell thickness slows down the increase in melting temperature and the decrease in latent heat during the melting process, which consequently reduces the melting time and increases the stored latent energy. These results indicate that reducing shell thickness of encapsulated PCMs is favourable for elevating energy charging rate and energy storage capacity while it is harmful to mechanical stability. The Cu/Ni capsule has smaller critical core/shell size ratio to avoid cracking than the salts/SiC capsule, while the former offers a shorter melting period. This implies that physical properties of materials of PCM capsules should be carefully considered for improving mechanical stability and melting dynamics. This study is helpful for selection of appropriate shell thickness and materials to achieve excellent comprehensive energy storage performance of encapsulated PCMs.

The world's energy demand is met mainly by the fossil fuels today. The use of such fuels, however, causes serious environmental issues, including global warming, ozone layer depletion and acid rains. A sustainable solution to the issues is to replace the fossil fuels with renewable ones. Implementing such a solution, however, requires overcoming a number of technological barriers including low energy density, intermittent supply and mobility of the renewable energy sources. A potential approach to overcoming these barriers is to use an appropriate energy carrier, which can store, transport and distribute energy. The work to be reported in this paper aims to assess and compare a chemical energy carrier, hydrogen, with a physical energy carrier, liquid air/nitrogen, and discuss potential applications of the physical carrier. The ocean energy is used as an example of the renewable energy sources in the work. The assessment and comparison are carried out in terms of the overall efficiency, including production, storage/transportation and energy extraction. The environmental impact, waste heat recovery and safety issues are also considered. It is found that the physical energy carrier may be a better alternative to the chemical energy carrier under some circumstances, particularly when there are waste heat sources.

Abstract Possessing nontoxicity, high thermochemical energy storage density, and good compatibility with supercritical CO2 thermodynamic cycles, calcium carbonate (CaCO3) is a very promising candidate in storing energy for next-generation solar thermal power plants featured with high temperature over 700 °C. However, CaCO3 particles are usually white with little absorption of sun light, inhibiting their application in efficient volumetric solar energy conversion systems. In this paper, dark CaCO3 particles are designed by doping with Cu, Fe, Co, and Cr elements based on sol-gel procedures. For particles doped with only Cu elements, the solar absorptance in the visible range is improved prominently while that in the near-infrared does not change so much. By further adding Cr elements, full-spectrum absorption of solar energy is achieved with a value as high as 73.1%, but the energy storage density decreases rapidly with cycling. By incorporating Mn or Al elements, the cyclic stability is enhanced greatly. For binary-doped particles with Cu and Mn, the energy storage density achieving 1952 kJ kg−1 after 20 cycles, which is 84% higher than that of pure CaCO3 particles. Additionally, the average solar absorptance is still considerable with a value of ~60% after 20 cycles. This work guides the design of high-efficiency, large-capacity, and stable thermochemical energy storage particles for simultaneous solar thermal conversion and high-temperature thermochemical energy storage.

Phase change materials (PCMs) are generally integrated into matrix materials to form shape-stabilized composite heat storage materials (HSMs) used for high temperature thermal energy storage applications. The conventional fabrication of composite HSMs is prevalently implemented at quite high temperatures, which is energy-intensive and narrows down the range of applicable PCMs because of thermal decomposition. Therefore, this paper establishes a novel fabrication approach to accomplish highly dense matrix to encapsulate PCMs at extremely low temperatures, based on the recently developed cold sintering process. The feasibility of the proposed approach was demonstrated by a case study of NaNO3/Ca(OH)2 composite HSMs. It was observed that the Ca(OH)2 matrix formed dense microstructure with obvious sintered boundaries and successfully encapsulated NaNO3 as PCM. The HSMs maintained stable macroscopic shape after hundreds of thermal cycles, and exhibited an energy storage efficiency of 59.48%, little leakage of PCM, and good thermal stability. Mechanical tests indicated that the HSMs possessed excellent mechanical properties when the sintering pressure is over 220 MPa. The discharging time of stored heat was presented through infrared thermography, and the heat storage capacity measured for the composite HSMs was over four times as high as those of typical solid storage materials of sensible heat, which demonstrated their excellent heat storage performances. The HSMs can be used in the form of packed bed or parallel channel with multi-layered heat storage, which is beneficial for efficiently utilizing solar heat and improving the performance of current energy storage system. This study therefore provides a novel route for energy-saving and low-carbon fabrication of shape-stabilized composite HSMs.