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Abstract District heating (DH) enables the utilisation and distribution of heating from sources unfeasible for stand-alone applications and combined with cogeneration of heat and power (CHP), has been the cornerstone of Denmark’s realisation of a steady national primary energy supply over the last four decades. However, progressively more energy-efficient houses and a steadily improving heat pump (HP) performance for individual dwellings is straining the competitive advantage of the CHP–DH combination as DH grid losses are growing in relative terms due to decreasing heating demands of buildings and relatively high DH supply temperatures. A main driver for the DH water temperature is the requirements for domestic hot water (DHW) production. This article investigates two alternatives for DHW supply: (a) DH based on central HPs combined with a heat exchanger, and (b) a combination of DH based on central HPs and a small booster HP using DH water as low-temperature source for DHW production. The analyses are conducted using the energyPRO simulation model and are conducted with hourly varying factors; heating demands, DH grid losses, HP coefficient of performance (COP) and spot market prices in order to be able to analyse the relative performance of the two options and their performance over the year. Results are also compared to individual boilers and individual HPs. The results indicate that applying booster HPs enables the DH system to operate at substantially lower temperature levels, improving the COP of central DH HPs while simultaneously lowering DH grid losses significantly. Thus, DH performance is increased significantly. Additionally, performance for the DH HP with booster combination is considerably better than individual boiler or HP solutions.

Abstract With the application of advanced information technology for the integration of electricity and natural gas systems, formulating an excellent energy conversion and management strategy has become an effective method to achieve established goals. Differing from previous works, this paper proposes a peak load shifting model to smooth the net load curve of an integrated electricity and natural gas system by coordinating the operations of the power-to-gas unit and generators. Moreover, the study aims to achieve multi-objective optimization while considering the economy of the system. A dynamic energy conversion and management strategy is proposed, which coordinates both the economic cost target and the peak load shifting target by adjusting an economic coefficient. To illustrate the complex energy conversion process, deep reinforcement learning is used to formulate the dynamic energy conversion and management problem as a discrete Markov decision process, and a deep deterministic policy gradient is adopted to solve the decision-making problem. By using the deep reinforcement learning method, the system operator can adaptively determine the conversion ratio of wind power, power-to-gas and gas turbine operations, and generator output through an online process, where the flexibility of wind power generation, wholesale gas price, and the uncertainties of energy demand are considered. Simulation results show that the proposed algorithm can increase the profit of the system operator, reduce wind power curtailment, and smooth the net load curves effectively in real time.

Abstract In theory, the control mode of a voltage-sourced converter (VSC) within a multi-terminal HVDC (MTDC) transmission system can be represented by using a droop line characteristic in the active power and DC voltage relationship (Pac–Udc) curve. However, operating the droop control as an extension of the Udc control mode with a steep slope was found to be difficult since it introduces instabilities. This also happens in the extended Pac control with a shallow slope. This paper proposes a droop line tracking (DLT) method for mitigating these issues, i.e. by calculating the new operating point of the converter based on the displacement distance from the droop line. The simulation results show that the proposed method enables the converter to operate in different control modes by simply changing the droop line characteristic. Furthermore, the same concept can be extended to implement the advanced droop control, i.e. represented by a multi-slope droop line characteristic such that the converter can be operated in different control modes depending on the DC system disturbance level. Simulation using a three-terminal HVDC system with an offshore wind farm (OWF) station has been performed to demonstrate the functionality of the proposed method in realizing the advanced droop control mode.

How has the contemporary development of sustainable buildings been influenced by the concept of ecological modernization? Ecological modernization is a policy concept describing how environmental considerations are increasingly being integrated into modern society's institutions through, for example, new types of cooperation and new applications of economic and market dynamics. Evidence is based on recent examples from politics and practice in the construction sector in Denmark, where sustainable buildings have gone through great changes – from being primarily isolated cases driven by enthusiasts and grassroots to being a more widespread, generally obtainable, and integrated product. This is considered within the context of ecological modernization, along with the implications of potential benefits and drawbacks. Based on the concepts of governance, standardization, and visibility, it is found that ecological modernization has penetrated in Danish sustainable buildings and has contributed to a positive de...

Abstract When a number of TEMs (thermoelectric modules) are connected in a series–parallel matrix and under mismatched temperature gradients, the overall maximum output power of the thermoelectric generator (TEG) may be lowered by certain TEMs with relatively smaller temperature difference. It is possible to avoid such a performance decrease by the disconnection of these low temperature TEMs, provided that the critical power point can be accurately determined. In this paper, firstly a rigorous and universal formulation is fully detailed to mathematically determine the conceptions and conditions of the critical power point in the series and parallel TEM arrays. Secondly, experiments of a series–parallel hybrid interconnected TEG are presented to clearly quantify the theoretical analyses. Finally, the hierarchical simulation, based on the SPICE (simulation program with integrated circuit emphasis) platform, is applied to estimate the critical power point. By numerically modeling the nonlinear physical processes of the TEG, the simulation can be used as an enabling technique in any model-based controller to dynamically minimize the mismatch power loss within the TEM matrix of any configuration. In experimental and numerical results, a number of critical power points are disclosed for a 2 × 4 parallel–serial hybrid TEM matrix, where the hot temperature mostly ranges from 120 °C to 60 °C.

Abstract A time domain model is applied to a three-dimensional point absorber wave energy converter. The dynamical properties of a semi-submerged hemisphere oscillating around a pivot point where the vertical height of this point is above the mean water level are investigated. The numerical model includes the calculation of the non-linear hydrostatic restoring moment by a cubic polynomial function fit to laboratory test results. Moreover, moments due to viscous drag are evaluated on the oscillating hemisphere considering the horizontal and vertical drag force components. The influence on the motions of this non-linear effect is investigated by a simplified formulation proportional to the quadratic velocity. Results from experiments are shown in order to validate the numerical calculations. All the experimental results are in good agreement with the linear potential theory as long as the waves are sufficiently mild i.e. H / λ ≤ 0.02 . For steep waves, H / λ ≥ 0.04 however, the relative velocities between the body and the waves increase thus requiring inclusion of the non-linear hydrostatic restoring moment to effectively predict the dynamics of the wave energy converter. For operation of the device with a passively damping power take-off the moment due to viscous drag is found to be negligible.

Oxy-fuel combustion of pulverized fuels (PF), as a promising technology for CO2 capture from power plants, has gained a lot of concerns and also advanced considerable research, development and demonstration in the past years worldwide. The use of CO2 or the mixture of CO2 and H2O vapor as the diluent in oxy-fuel combustion, instead of N2 in conventional air–fuel combustion, induces significant changes to the combustion fundamentals, because of the great differences in the physical properties and chemical effects of the different diluents. Therefore, some fundamental issues and technological challenges need to be properly addressed to develop oxy-fuel combustion into an enabled technology. Computational Fluid Dynamics (CFD) modeling, which has been proven to be a very useful and cost-effective tool in research and development of conventional air–fuel combustion, is expected to play a similarly vital role in future development of oxy-fuel combustion technology. The paper presents a state-of-the-art review and an in-depth discussion of PF oxy-fuel combustion fundamentals and their modeling, which underpin the development of this promising technology. The focus is placed on the key issues in combustion physics (e.g., turbulent gas–solid flow, heat and mass transfer) and combustion chemistry (e.g., pyrolysis, gas phase combustion and char reactions), mainly on how they are affected in oxy-fuel conditions and how they are modeled and implemented into CFD simulations. The system performance of PF oxy-fuel combustion is also reviewed. Finally, the current status of PF oxy-fuel combustion fundamentals and modeling is concluded and the research needs in these regards are suggested.

Abstract Over the recent years, the photovoltaic (PV) system generation and integration with utility grid became the most widely used energy resource among other renewable energies worldwide. Thereon, the integration of PV power plants (PVPPs) to the power grid and their dynamics during grid faults had become a critical issue in the new grid codes requirements. In line with this, the fault ride through (FRT) capability control of grid-connected PV power plants (GCPPPs) became the most important issue related to grid codes. In order to fulfill the FRT requirements imposed by grid codes, various approaches have been proposed in the last years. This paper presents an overview and comparison of several FRT capability enhancement approaches during grid fault conditions. A novel feature of this paper is to categorize FRT capability enhancement methods into two main groups depending on the control type and connection configuration including external devices based methods and modified controller based methods and then discuss their advantages and limitations in detail. A comparison between these methods in terms of grid code compliance, controller complexity and economic feasibility are also analyzed in this paper. According to the literature study, the FRT strategies based on external devices can be more effective. However, some of these methods come with significant increased cost. On the other hand, the modified controller-based FRT methods can achieve the FRT requirements at a minimal price. Among various types of control approaches, the modified inverter controller (MIC) is the highly efficient FRT capability approach.