Shuai Deng

The performance of thermodynamic cycles based on the properties of working fluids

With the increase of global energy demand, themodynamic cycles, such as Rankine cycle, refrigeration or heat pump, have been widely employed to generate work or transfer heat. Since these cycles generally consist of a linked sequence of state points with physical properties variables, the physical properties of working fluid are essential to the cycle analysis. They directly determine the design of components, the cycle performance, the cycle stability and safety. However, in the thermodynamic analysis, the determination of cycle performance and the calculation of required physical properties are usually seperated. The thermodynamic peroperties of working fluids are often obtained from the complex experimental equations, so that the relationship between the cycle performance and the working fluids has not been established yet. What′s more, due to the fact that Carnot efficiency doesn′t contain detailed information on the properties of working fluids, a nature idea emerges how to derive the efficiency limit under the constraint of working fluids. Thus, in this work, the influences of thermodynamic properties on power cycles are investigated and the limiting efficiencys are proposed. In order to derive the cycle performances from the physical properties of working fluids, a general cubic equation of state is employed to obtain the residual properties, such as residual enthalpy, residual entropy and residual internal energy. Thereafter, on the basis of the properties of ideal gas, thermodynamic parameters of working fluids at any state are determined. According to the thermodynamic general relationships, the required heat is obtained for four classical processes, namely isochoric, isobaric, isothermal and isentropic processes. Based on the derived expressions, the output work and cycle efficiency are obtained for power cycles including Carnot cycle, Rankine cycle, Brayton cycle and Sterling cycle. The relationship between the cycle performance and the temperature, the properties of working fluids is developed. As a theoretical upper bound of cycle efficiency, Carnot efficiency is only determined by temperatures of heat source and sink. While the output work of Carnot cycle is a function of temperatures and the properties of working fluids. For other power cycles, it can be concluded that the thermodynamic performances are related with temperatures and working fluids, based on the derived expressions from cubic equation of state. Furthermore, compared with the Carnot efficiency under the same heat source and sink, the thermodynamic perfections of cycles are very low in practical engineering. Therefore, performance limits of thermodynamic cycles are investigated on the basis of the characteristics of working fluids in this paper. Limiting performance is derived from the equation of state and the temperature-entropy diagram of working fluids. For Rankine cycle and Brayton cycle, a maximum isobaric slope in the temperature-entropy diagram is employed to cut out the unexploited area from the enclosed area of Carnot cycle, so that the limiting work and efficiency can be obtained from the cycle areas. For Sterling cycle, when the used working fluid is ideal gas, the cycle efficiency is equal to Carnot efficiency. Although the proposed limiting performance can not be achieved by practical cycles, it can provide some theoretical guidance for the operating condition and the optimal design of thermodynamic cycles.

Evolution of bubbles in decomposition and replacement process of methane hydrate

Abstract Nanobubbles have been proved to play an important role for the extraction kinetics of CH4 hydrates. Existing investigations on nanobubbles mainly focus on the hydrate melting. However, the mechanism and effect of bubble evolution during the replacement of CH4 hydrates by CO2 are still elusive. Therefore, molecular dynamics simulations on melting and replacement of CH4 hydrate are performed with the same initial system size, pressure and amount of CH4 hydrates. It is proved the two processes move layer by layer but with different features. Moreover, despite the different composition of gas phases, behaviours of nanobubbles are similar during the two processes except that the gas film is not formed during the replacement. In addition, a quantitative analysis of molecules in each nanobubble during the decomposition is conducted preliminarily. Also a remarkable effect of mixed nanobubble on the replacement is revealed based on the analysis of bubble behaviour. It is found that CH4 molecules are surrounded by some dense CO2 molecules in the mixed bubble during the replacement. Moreover, CO2 molecules in the solution can assist the bubble formation of CH4 molecules at an initial stage and prevent their further escape and reoccupation by encompassing them afterwards.

The Role of Intelligent Computing in Load Forecasting for Distributed Energy System

The integration of renewable energy into the distributed energy system has challenged the operation optimization of the distributed energy system. In addition, application of new technologies and diversified characteristics of the demand side also impose a great influence on the distributed energy system. Through a literature review, the load forecasting technology, which is a key technology inside the optimization framework of distributed energy system, is reviewed and analyzed from two aspects, fundamental research and application research. The study presented in this paper analyses the research methods and research status of load forecasting, analyses the key role of intelligent computing in load forecasting in distributed energy system, and realizes and explores the application of load forecasting in practical energy system.

Intelligent Control Methods of Demand Side Management in Integrated Energy System: Literature Review and Case Study

Demand side management (DSM) would become an important method to guarantee the stability and reliability of the innovative energy structure model, have received increasing attention. DSM is regarded as an integrated technology solution for planning, operation, monitoring and management of building utility activities. However, there are several problems and technical challenges on the research level of fundamental methodology, which causes difficulties for the practical application of intelligent DSM control strategy. Therefore, optimization would play a vital role in the implementation process of DSM. A real case study was presented to demonstrate how to relieve and solve the existing technical challenges by the application effective optimization strategy and methods of DSM. At last, several possible research directions, that application of intelligent methods in the development of DSM optimization techniques, were presented.

Dynamic match optimzation: Emerging control concept of sustainable distributed energy system

Integration of renewable energy into energy system, which is a reasonable approach for sustainable development, posed a challenge to design, operation and optimization of distributed energy system (DES). The emerging control concept related to DES integrated with renewable energy, namely dynamic match optimization (DMO), is reviewed in this paper. A scoping review, which includes literature research and case introduction, is presented for a preliminary overview on DMO and its two main measures: load forecasting and demand side management. The literature review part contains concept, classification, mathematical tool, research challenges, while case introduction part contains engineering cases and application challenges. The discussion on research and application progress would inform a future picture on development trend of DES.