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Purpose The purpose of this paper is to describe a novel universal error estimator and the adaptive time-stepping process in the generalized single-step single-solve (GS4-1) computational framework, applied for the fluid dynamics with illustrations to incompressible Navier–Stokes equations. Design/methodology/approach The proposed error estimator is universal and versatile that it works for the entire subsets of the GS4-1 framework, encompassing the nondissipative Crank–Nicolson method, the most dissipative backward differential formula and anything in between. It is new and novel that the cumbersome design work of error estimation for specific time integration algorithms can be avoided. Regarding the numerical implementation, the local error estimation has a compact representation that it is determined by the time derivative variables at four successive time levels and only involves vector operations, which is simple for numerical implementation. Additionally, the adaptive time-stepping is further illustrated by the proposed error estimator and is used to solve the benchmark problems of lid-driven cavity and flow past a cylinder. Findings The proposed computational procedure is capable of eliminating the nonphysical oscillations in GS4-1(1,1)/Crank–Nicolson method; being CPU-efficient in both dissipative and nondissipative schemes with better solution accuracy; and detecting the complex physics and hence selecting a suitable time step according to the user-defined error threshold. Originality/value To the best of the authors’ knowledge, for the first time, this study applies the general purpose GS4-1 family of time integration algorithms for transient simulations of incompressible Navier–Stokes equations in fluid dynamics with constant and adaptive time steps via a novel and universal error estimator. The proposed computational framework is simple for numerical implementation and the time step selection based on the proposed error estimation is efficient, benefiting to the computational expense for transient simulations.

In a commercial parabolic trough solar power plant (PTSPP), the solar field (SF) is large-scaled and consists of hundreds of parabolic trough collector (PTC) loops. The PTSPP performance is determined by an appropriate operation strategy of the SF, which is implemented by adjusting the outlet temperature, the flowrate, and the flow distribution through the valves and pump to make the PTSPP output cost-effective. Majority of the published literature study the behavior of the SF with an assumption that all the PTC loops have uniform perfect performance. Due to the lack of appropriate model and algorithm, although operating with nonuniform and degraded PTCs is inevitable for a realistic SF, there is scarcely any literature studies how to develop an optimal operation strategy under this situation. The present paper aims at solving this problem by establishing a PTSPP model and developing an improved operation strategy (IOS). The PTSPP model consists of an improved thermal hydraulic model of the SF and a simplified but effective model of the power block, and the IOS combines the features of both global and distributed operation strategy. Based on the model and the IOS, the mechanism of optimization is illustrated in a uniform case, and then according to the comparison and analysis, the most appropriate strategy for the nonuniform case is determined among three representative strategies. Compared with the traditional operation strategy, the chosen strategy can improve the net electric generation by 3.4% with the low computational cost and good engineering feasibility.

Hot dry rock is a new type of geothermal resource which has a promising application prospect in China. This paper conducted a comparative research on performance evaluation of two eligible bottoming cycles for a hot dry rock power plant in the Gonghe Basin. Based on the given heat production conditions, a Kalina cycle and three organic Rankine cycles were tested respectively with different ammonia-water mixtures of seven ammonia mass fractions and nine eco-friendly working fluids. The results show that the optimal ammonia mass fraction is 82% for the proposed bottoming Kalina cycle in view of maximum net power output. Thermodynamic analysis suggests that wet fluids should be supercritical while dry fluids should be saturated at the inlet of turbine, respectively. The maximum net power output of the organic Rankine cycle with dry fluids expanding from saturated state is higher than that of the other organic Rankine cycle combinations, and is far higher than the maximum net power output in all tested Kalina cycle cases. Under the given heat production conditions of hot dry rock resource in the Gonghe Basin, the saturated organic Rankine cycle with the dry fluid butane as working fluid generates the largest amount of net power.

Advanced adiabatic compressed air energy storage (AA-CAES) has been recognised as a promising approach to boost the integration of renewables in the form of electricity and heat in integrated energy systems. Nevertheless, the uncertainty and variability of renewables spark the consideration of the off-design thermodynamics of AA-CAES, particularly when the heat exchanger is equipped to collect and recycle air-compression heat. Here, the authors propose a general off-design thermodynamic simulation model of AA-CAES incorporating the part-load characteristics of heat exchangers, and validate the effectiveness of the proposed model on a practical AA-CAES demonstration pilot. The results indicate that the simulation model offers an acceptable approximation for the practical operation behaviour of AA-CAES facility and decreases the modelling error from 20.8 to 6.28% compared with the existing model. The proposed simulation model is expected to provide insights into the investigation of the dynamic characteristics of AA-CAES in the future integrated energy system.

The development of renewable is becoming a new trend to cope with the shortage of fossil fuels and environmental pollution, but the inherent uncertainty and volatility pose great challenges to power system operation with high share of renewables. Energy hub which incorporates multiple energy production, conversion, and storage facilities can play a noteworthy role in renewable accommodation and energy saving. This paper proposes a multi-stage robust optimization model for day-ahead scheduling of energy hub composed of wind generation, advanced adiabatic compressed air energy storage (AA-CAES) system, and heat pump in the presence of wind uncertainty. To overcome the computational difficulty, the multistage model is decomposed into two stages. The first stage minimizes the operation cost under the predicted scenario, and the second stage determines real-time dispatch in response to the observed wind power generation. Dynamic lower and upper bound variables for two correlated state-of-charges (SoC) are added as the first-stage decision variables with ramping limit constraints, so that the second-stage decision can be made period-by-period without jeopardizing robust feasibility. This is an important difference compared to the two-stage robust optimization whose second-stage decision is anticipative and thus non-causal. Finally, the day-ahead scheduling problem is reformulated as a mixed-integer linear program (MILP) guaranteeing the robustness and non-anticipativity of real-time dispatch. Case studies verify the effectiveness of the proposed method.

This paper studies the dynamic operation of a residential electric energy system with distributed renewable generation. Energy streams among the power grid, local renewable plant, storage unit, and demand are considered; the temporal volatility of renewable generation is described by an uncertainty set, and the optimal operation problem gives rise to a multi-stage robust optimization. To deal with the non-anticipativity issue, the robust model is solved via robust dual dynamic programming (RDDP) approach. The the cost-to-go function is approximated by iteratively updating the upper and lower bounds, which can be obtained from forward-pass and backward pass procedures. Case studies validate the performance and advantage of the proposed method.

Liquid air energy storage (LAES) is a promising solution for electricity energy storage and grid load shifting. The storage and application of cold energy can significantly affect the performance of LAES systems. A stable and sufficient source of cold energy in the liquefaction process is the key factor for the stable and efficient operation of an LAES system. Hence, a novel hybrid LAES system combined with organic Rankine cycle (ORC) systems based on the utilization of liquefied natural gas (LNG) cold energy is proposed in this paper. In the charging process, the LNG helps cool the compressed air, and the cold energy of the liquid air and excess compression heat are utilized in a two-stage ORC system to generate additional electricity during the discharging process. A mathematical model comprising energy and exergy analyses was developed to analyze the performance of the proposed system and the influence of key parameters. Compared to standalone LAES systems, the cold energy storage system is extremely simplified in the proposed system, and higher electricity storage efficiency and density are obtained. Therefore, the proposed system has a promising prospect in LNG terminals owing to its stability and ease of implementation.

In this paper, the analysis of heat transfer and stress characteristics in pressurized volumetric receiver (PVR) is presented by coupling Monte Carlo Ray Tracing (MCRT), Finite Volume Method (FVM) and Finite Element Method (FEM). Firstly, an integration model coupling MCRT, FVM and FEM applied in PVR is developed. Based on this model, the coupled characteristics of optical-thermal-stress are numerically studied. Meanwhile, the solar flux is homogenized using multi-point aiming strategy for multi-heliostat field zones. Finally, the effects of air mass flow and inlet temperature on the heat transfer and stress characteristics are discussed. The results indicate that the solar flux distributions on aperture and SiC porous absorber are more uniform using multi-point aiming strategy, and the peak flux in porous could be reduced by an order of magnitude compared with single-point. The peak stress of quartz window occurs at the contact position between the steel shell and quartz window, and the damage would not happen because of the peak stress is lower than the ultimate strength. The inlet parameters of air have significantly influences on the heat transfer and stress characteristics of receiver. For example, when the air with higher temperature is adopted, the SiC porous temperature increases by 20%, and the temperature and stress of quartz window increase by 31% and 60%, respectively. The overheating in SiC porous could occur using lower mass flow rates, however, the quartz glass stress exhibits a decreasing tendency when the mass flow decreases. Besides, the temperatures of outlet air and porous absorber increase when the flow rate decreases and inlet temperature increases which would lead to local overheating in absorber and lower stress on quartz window.

In the paper, melting performance of a latent heat thermal energy storage (LHTES) unit with phase change materials (PCMs) locally enhanced by porous media was numerically investigated. The filling ratio of the porous inserts was fixed to a low degree to reduce the material cost. The optimal configuration of the porous inserts was obtained after the discussion about the effects of porous geometry and porosity on the velocity and temperature distribution of melting process. To reveal the superiority of the optimized result, different cases were compared with the help of a new comprehensive criterion about the input-output performance. Then a generalization expression was fitted out with some dimensionless parameters for the rapid calculation of melting fraction of the optimized result in practical application. The results indicated that in the horizontal LHTES unit, the limited porous inserts should be concentrated in the bottom part without interval between the neighboring porous inserts, providing an effective enhancement in the lower part and a low degree of thermal stratification. If the mass of porous inserts is fixed, high porosity is preferred to make the thermal-conduction-dominated area sufficiently covered by the porous inserts. Compared with the non-enhancement case, the selected result in this study can remarkably save more than 80% of the melting time and enhance the melting rate by 5.1 times. More significantly, the selected result has the highest melting rate per cost of the material when the price ratio of the addition to the PCM is larger than 5 based on the results of economic assessment. Therefore, it offers an economical solution to the thermal enhancement problem of the LHTES unit in the practical application. The normalized equation of melting fraction is obtained for the parameter range of 0.187 f = 0.596 X + 0.0438 X 2 - 0.0825 X 3 + 0.0130 X 4 , where X = SteFoRa1/8

The interconnection between initially independent energy infrastructures offers additional system flexibility and efficiency. The integration at distribution level simplifies the implementation of the integrated energy system functionalities. This paper proposes concepts and design principles of a smart micro energy grid (MEG) for accommodating micro-grids, distributed poly-generation systems, energy storage facilities, and associated energy distribution infrastructures. The energy management system is responsible for the smart operation of the MEG while supporting multiple criteria, such as safety, economy, and environmental protection. To realize the vision of the smart MEG, an engineering game theory based energy management system with self-approaching-optimum capability is investigated. Based on the proposed concepts, design principles, and energy management system, this paper presents a prototype of China's first conceptual solar-based smart MEG, established in Qinghai University.

Compressed air energy storage (CAES) is a large-scale energy storage technology that can overcome the intermittency and volatility of renewable energy sources, such as solar and wind energy. Although abandoned mines can be reused for underground CAES of large scale, their feasibility requires further investigations. This study performs a comparative study on the effects of two construction and plugging schemes on the thermo-mechanical responses of an abandoned drift for the 60-MW CAES project in Datong Yunggang Mine, China. The first scheme uses only the drift as the air container, and the shaft is plugged with concrete. The second scheme uses the drift and shaft. A coupled 3D model is established, and the interaction of the temperature field, displacement field, and air state is fully considered. The numerical model is validated via previous analytical and numerical methods. The simulations indicate that air temperature and air pressure evolve “up-down-down-up” in one CAES cycle. The two schemes have roughly consistent air temperature, and relative huge difference exists for the air pressure. During the charging stage, relatively high tensile stress is induced by the air injection into the caverns. The first scheme shows less stress concentration and lower tensile stress, indicating that the concrete plugging at the intersection between Drift 1080 and North-5 Shaft is effective to reduce the stress concentration. This study can serve as reference for CAES design in similar engineering and studies.

The extremely non-uniform solar flux distribution in the solar power tower plant can badly cause some crucial problems for the solar receiver such as the local hot spot, the thermal stress, and the thermal deformation. Homogenization of the solar flux distribution is an effective method to avoid these problems, and has become an important research topic. The objective of the present study is to homogenize the solar flux distribution on the inner surfaces within the cavity receiver while keeping the optics loss as low as possible by replacing the conventional single-point aiming strategy with optimal multi-point aiming strategies. Multi-objective optimizations of the aiming strategy for the solar power tower with cavity receivers are performed by using the non-dominated sorting genetic algorithm. The distribution of the aiming points on the cavity aperture and the allocation of the aiming points for each heliostat are optimized simultaneously. The following conclusions can be made: (1) The uniformity of the solar flux distribution on the aperture does not always signify the uniformity of the solar flux distribution on the inner surfaces, where the later one is what we are truly concerned about. Therefore, the optimization of the aiming strategy should take charge of the solar flux distribution on the inner surfaces rather than on the aperture. (2) The multi-objective optimization can provide the trade-off between the non-uniformity of the solar flux distribution and the optics loss in the form of Pareto optimal fronts. (3) The optimal aiming strategies provided by the multi-objective optimization can significantly homogenize the solar flux distribution on the inner surfaces within the cavity at a minimum cost of optics loss. (4) For the optimal aiming strategies at all time except the noon, there exists a west-east asymmetry of the aiming point distribution on the aperture. Moreover, the asymmetry gets less obvious as the time gets closer to the noon.

Liquid air energy storage (LAES) is a promising large-scale energy storage technology in improving renewable energy systems and grid load shifting. In baseline LAES (B-LAES), the compression heat harvested in the charging process is stored and utilized in the discharging process to enhance the power generation. Due to the low liquid air yield, a large amount of compression heat is wasted. In order to improve the round-trip efficiency (RTE) and extend the application field, a novel combined cooling, heating and power system based on the LAES (LAES-CCHP) is proposed and investigated. In the proposed system, an organic Rankine cycle (ORC) is employed to recover the high-temperature surplus compression heat to generate electricity and an absorption refrigeration system (ARS) is introduced to utilize the low-temperature compression heat to realize district cooling and heating. Based on a mathematical model, performance evaluation and exergy analysis of the system is performed. It is found that the effective and cascaded utilization of the compression heat could significantly improve the efficiency and performance of the system. With optimal operational parameters, the RTE and exergy efficiency of the LAES-CCHP could reach 69.64% and 57.02%, respectively, which are 37.66% and 12.71% higher than those of the B-LAES.

Factories in China are faced with peak-valley electricity prices and carbon reduction policies nowadays. As the adiabatic compressed air energy storage has a potential to store electricity and provide combined cooling, heating and power, in this paper, a cogeneration system based on it is first proposed to meet the comprehensive energy demands of a latex factory. A new assessment method based on daily energy cost savings is also proposed to evaluate its economic performance. Four novel energy supply strategies based on the cogeneration system and two contrast strategies were analyzed and compared. Results show the energy efficiency of the cogeneration system ranged from 89% to 95%. Besides, energy efficiency and exergy efficiency responded oppositely to the heat-to-electric ratio. In addition, the daily energy costs of the novel strategies were significantly lower than those of the contrast ones. Moreover, the air storage pressure affected the economy the most. Given the ceiling storage pressure of 10 MPa, the optimum floor storage pressure for the air storage was 4.5 MPa. Furthermore, when the cogeneration system provided electricity and dry steam only during the peak load period, its assumed internal return rate reached as high as 10.37%. Therefore, this energy supply strategy is suggested as the most promising choice for the latex factory.

Advanced adiabatic compressed air energy storage (AA-CAES) is a scalable storage technology with a long lifespan, fast response and low environmental impact, and is suitable for grid-level applications. In power systems with high-penetration renewable generation, AA-CAES is expected to play an active role in flexible regulation. This paper proposes a state-space set-point control model of AA-CAES for the application in the power tracking mode considering off-design characteristics. The part-load features of the multi-stage turbine and heat exchanger are captured by simplified models, and then tailored for improving computational efficiency in the applications with a timescale of 1 min. The set-point control (power tracking) of AA-CAES entails the coordination of turbine inlet pressure, air mass flow rate and heat transfer fluid (HTF) mass flow rate, while ensuring the secure pressure at the throttle valve linking the air storage tank and the expansion train. The set-point control problem is cast to a differential-algebraic equation (DAE) constrained optimization problem, and is reformulated as a nonlinear program via the simultaneous collocation method. Case studies validate the accuracy and applicability of the proposed AA-CAES model for power tracking under off-design generating conditions.

As a new type of mechanical energy storage, compressed air energy storage (CAES) has attracted wide attention in recent years. This paper studies the optimal sizing problem of CAES in power distribution network (PDN). CAES plays a role in shaving the peak and filling the valley at demand side and thus reduces the operation cost of PDN to purchase electricity from the main grid. A multi-parametric linear programming (MP-LP) model is formulated where the power and energy capacities of CAES are regarded as parameters; the parameterized optimal value function (OVF) provides a graphical tool to describe the impact on operation cost of CAES configuration, which helps determine the planning strategy in a visual manner. The visualized results reveal not only the optimal solution, but also some useful information, like the sensitivity of operation cost to parameters. Case studies conducted on the IEEE 33-bus distribution system verifies the effectiveness of the proposed method.

Cogeneration is becoming increasingly popular in building and community energy systems with demands on electricity and heat, which is suitable for residential and industrial use in remote areas. This paper considers a stand-alone cogeneration energy hub. The electrical and thermal energies are produced by a combined heat and power (CHP) unit, photovoltaic panels, and a solar thermal collector. Since solar units generate no electricity and heat during the night, energy storage units which shift demands over time can promote the usage of solar energy and reduce the fuel cost of the CHP unit. This paper proposes a method to retrieve the optimal operation cost as an explicit function in the capacity parameters of electric and thermal energy storage units, reflecting the value of energy storage in the cogeneration energy hub. The capacity parameter set is divided into a collection of polyhedrons, on each polyhedron, the optimal value is an affine function in the capacity parameters. Furthermore, the optimal sizes of system components are discussed. The capacity of the CHP unit is determined from a linear program, ensuring supply adequacy, the capacities of solar generation and energy storage units are calculated based on the cost reduction and the budget. Case studies demonstrate the effectiveness of the proposed method.

As a promising solution for large-scale energy storage, liquid air energy storage (LAES) has unique advantages of high energy storage density and no geographical constraint. In baseline LAES, the compression heat is surplus because of the low liquefaction ratio, which significantly influences its round-trip efficiency (RTE). In this paper, hybrid LAES systems based on the cascaded storage and effective utilization of compression heat is proposed and analyzed. In order to improve the storage temperature, cascaded-storage of compression heat is proposed. Meanwhile, the organic Rankine cycle (ORC) and Kalina cycle (KC) are considered to utilize the surplus compression heat to generate additional electricity. Based on the same conditions, the performances of the subcritical ORC using dry fluids, supercritical ORC using wet fluids, and KC are calculated and compared. It is found that the cascaded storage of compression heat can significantly increase the storage temperature and further improve the RTE of the system. Moreover, the RTE of the LAES system is increased by 10.9–19.5% owing to the additional power generation. The subcritical ORC using dry fluids is found to be more suitable in utilizing the surplus compression heat for its simple configuration and excellent performance.

The integration and accommodation of the wind and solar energy pose great challenges on todays power system operation due to the intermittent nature and volatility of the wind and solar resources. High efficient large-scale electrical energy storage is one of the most effective and economical solutions to those problems. After the comprehensive review of the existing storage technologies, this paper proposes an overall design scheme for the Non-supplementary Fired Compressed Air Energy Storage (NFCAES) system, including system design, modeling and efficiency assessment, as well as protection and control. Especially, the design principles of the multistage regenerative, i.e. heat recovery system which is used to fully recycle and utilize the waste heat from compression are provided, so as the overall system efficiency evaluation method. This paper theoretically ascertains the storage decoupling rules in the potential and internal energy of molecular compressed air and reveals the conversion mechanism of gas, heat, power, electricity and other forms of energy. On this basis, a 500-kW physical simulation system of CAES system (TICC-500, Tsinghua-IPCCAS-CEPRI-CAES) is built, which passed a system-wide 420-kW load power generation test with less pollution and zero carbon emissions. Besides, the multi-form energy conversion of multi-stage regenerative CAES and storage efficiency is verified, especially its incomparable superiority in solving the uncertainty problem in wind and solar power generation. Finally, the propaganda and application scenario of the CAES system in China is introduced.

In this study, a novel liquid air energy storage system for electrical power load shifting application is introduced. It is a combination of an air liquefaction cycle and a gas turbine power generation cycle without fuel combustion. Thermodynamic analysis is conducted to investigate the performance of this system. The results show that liquid air energy storage systems could be very effective systems for electrical power storage with high efficiency, high energy density and extensive application prospects.

Using multiple energy resources greatly enhances system operating flexibility and efficiency. An energy hub is a vital facility, producing, converting, and storing energy in different forms...

In the view of the complex environment of high altitude areas, a solar chimney power plant experimental setup with a chimney 15 m in height and a collector 15 m in diameter was built. The temperature and velocity of air flow within the collector were tested and analyzed. The results show that the variation of air flow temperature and velocity are similar to the solar radiation. Furthermore, the energy storage layer provides an efficient energy supply for supporting continuous operation at night. The air flow temperatures at different locations inside the collector are different. From the comparison results of air temperature within the collector along each direction, the air temperature in the west side is relatively large and the temperature in the north side is relatively low due to the geographical reason.

The impedance match is important for the efficiency and available acoustic power of thermocompressor (TCP). The filling pressure, hot end temperature, reservoir volume and opening of orifice were changed to test the optimum load impedance for the maximum acoustic power by RC (Resistor and Capacitor) load method. To make use of the acoustic power efficiently, a pulse tube refrigerator (PTR) was designed based on this impedance to match the TCP and relative coupling experiments were conducted for the first time. The refrigerating performance of PTR was observed and the lowest no-load temperature of 70.4 K was obtained at its optimum working frequency 5.6 Hz. Also, the performance characteristics and total impedances of three different PTRs were compared to verify the rationality of the impedance matching method in this paper. The calculation results agree well with the substantial experiments and further optimization methods were also proposed.

The present study proposes a novel refrigerator in theory, which uses 4He as working fluid to directly reach 2.3 K and uses a small amount of 3He to attain the temperature below 1.7 K. The compact and highly efficient new refrigerator works with the Vuilleumier cycle. The novel refrigerator is driven by a thermal compressor which creatively uses mix-refrigerants J-T refrigerator alternative to liquid nitrogen as the power source. Furthermore, the Vuilleumier cycle can be used to achieve temperature below liquid helium with the improvement of the ultra-low temperature regenerator material. A new method of reaching the temperature below 1.7 K is proposed on the regenerative refrigerator, which could be an important breakthrough for the cryogenic science and technology.

DC gas flow in pulse tube cryocooler (PTC) is a crucial problem both in theory and application which considerably affects the refrigeration performance. We have experimentally discovered and verified the third type of DC gas flow in PTC which is formed due to hydrodynamic and thermodynamic asymmetry of the regenerator and other flow channels. This new type of DC gas flow is possible to be identified in other regenerative engines or refrigerators. We also introduced a highlighting method which can suppress this kind of DC gas flow effectively in most cases, with the best result of 30 K temperature drop at the cold end of the PTC.