Abstract:The compressed air energy storage (CAES) system, considered as one method for peaking shaving and load-levelling of the electricity system, has excellent characteristics of energy storage and utilization. However, due to the waste heat existing in compressed air during the charge stage and exhaust gas during the discharge stage, the efficient operation of the conventional CAES system has been greatly restricted. The Kalina cycle (KC) and organic Rankine cycle (ORC) have been proven to be two worthwhile technologies to fulfill the different residual heat recovery for energy systems. To capture and reuse the waste heat from the CAES system, two systems (the CAES system combined with KC and ORC, respectively) are proposed in this paper. The sensitivity analysis shows the effect of the compression ratio and the temperature of the exhaust on the system performance: the KC-CAES system can achieve more efficient operation than the ORC-CAES system under the same temperature of exhaust gas; meanwhile, the larger compression ratio can lead to the higher efficiency for the KC-CAES system than that of ORC-CAES with the constant temperature of the exhaust gas. In addition, the evolutionary multi-objective algorithm is conducted between the thermodynamic and economic performances to find the optimal parameters of the two systems. The optimum results indicate that the solutions with an exergy efficiency of around 59.74% and 53.56% are promising for KC-CAES and ORC-CAES system practical designs, respectively.

Abstract:As intermittent renewable energy is receiving increasing attention, the combination of intermittent renewable energy with large-scale energy storage technology is considered as an important technological approach for the wider application of wind power and solar energy. Pumped hydro combined with compressed air energy storage system (PHCA) is one of the energy storage systems that not only integrates the advantages but also overcomes the disadvantages of compressed air energy storage (CAES) systems and pumped hydro energy storage systems to solve the problem of energy storage in China's arid regions. Aiming at the variable working conditions of PHCA system technology, this study proposes a new constant-pressure PHCA. The most significant characteristics of this system were that the water pump and hydroturbine work under stable conditions and this improves the working efficiency of the equipment without incurring an energy loss. In addition, the constant-pressure PHCA system was subjected to energy and exergy analysis, in expectation of exploring an attractive solution for the large-scale storage of existing intermittent renewable energy.

Bubble behavior is fundamental to the performance of froth flotation operations used in wastewater treatment processes. To fully understand and characterize bubble behavior under flotation-related conditions in wastewater, the high-speed photographic method has been employed to examine the motion of single bubbles and size distribution of bubble swarms with intermediate sizes ranging from 1 to 4 mm in the presence of surfactants in a laboratory scale flotation column. Both distilled water and synthetic municipal wastewater have been used to make solutions as well as two types of common surfactants. The instantaneous bubble motion has been recorded by a high speed camera. Subsequently, bubble trajectory, dimensions, velocity and distribution have been determined from the recorded frames using the image analysis software. The experimental results show that the addition of surfactant into wastewater has similar effects on bubble hydrodynamic behavior as in pure water (e.g., improving trajectory stabilization, dampening bubble deformation, slowing down terminal velocity, reducing bubble size and increasing the specific surface area of bubble swarm) due to the Marangoni effect. However, it is interesting to note that surfactant effects on single bubble hydrodynamics in wastewater are slightly stronger than those in pure water while surfactant effects on size parameters of bubble swarms in wastewater are significantly stronger than those in pure water. This finding suggests that besides surfactant, inorganic salts present in synthetic wastewater have an important influence on bubble dispersion.

The bladeless turbine has a promising future as a new power generation system. To explore the operating characteristics of the turbine, a bladeless turbine experimental platform with an incompressible working medium was designed and built. The relationships among performance parameters were analysed in experiments, and studies were conducted on the flow characteristics of the working medium inside the turbine using numerical simulation software. The causes of entry and exit losses were analysed. The data acquired by simulation were consistent with the result of calculations using the partial loss model developed in this article, which means that this model is capable of calculating the partial loss of a bladeless turbine and is thus suitable for the design and optimization of bladeless turbines.

An adiabatic compressed air energy storage system (A‐CAES) can improve the energy storage potential, compared with conventional CAES systems, by capturing the heat generated during the charge stage. However, a challenge remains in the utilization step of the stored thermal energy due to the energy loss from the thermal energy storage vessel, which has hindered commercialization of A‐CAES. To utilize the thermal energy produced by a compressor during the charge process efficiently, a novel system that couples A‐CAES system with the Kalina cycle (KC) is designed to recover the loss of heat. In the new system, which is characterized by a higher and more stable power output, a Kalina cycle and the expander absorb the heat released from thermal energy storage units to generate electricity. Theoretical thermodynamic analysis shows that the novel combined system can achieve more efficient operation than a single A‐CAES system. By examining the effects of key thermodynamic parameters on the exergy efficiency and overall capital cost, a Pareto frontier obtained shows the tradeoff between exergy efficiency and overall capital cost of the system, where 47.17 % and 1.455 k$ kW−1 are determined as the optimum values for the two objectives from the optimum design solutions.