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Abstract Energy storage systems (ESS) are widely used in active distribution networks (ADN) to smoothen the drastic fluctuation of renewable energy sources (RES). In order to enhance the scalability and flexibility of ESS, a virtual energy storage system (VESS), which is composed of battery energy storage system (BESS), RES as well as flexible loads (FL), is developed in this paper to realize the functionalities of ESS in more cost-effective way in ADN. Aiming at achieving voltage regulation, dynamic pricing strategies based on system voltage condition are designed for VESS. A distributed real-time power management model containing dynamic pricing strategies is proposed to accomplish the voltage regulation and economic power sharing in VESS. Moreover, a set of distributed algorithms, over time-varying unbalanced directed networks, are designed for dynamic pricing strategies and optimal power management model. Furthermore, the convergence property, optimality and system voltage stability are explained by detailed mathematical analysis. Three various case studies which were ran on a real time digital simulator (OPAL-RT OP5600) were designed to validate the effectiveness of the strategy. Finally, simulation results show that the economic power dispatch and voltage regulation are achieved among VESS simultaneously, even in the presence of time-varying directed and unbalanced communication networks.

The hydraulic balance of heating network is considered as a pre-condition for the implementation of low temperature district heating (LTDH). Its imbalance result into high energy consumption and heat-losses in the network. In this study, a novel hydraulic model is presented which investigates hydraulic imbalance in the LTDH network, using real weather and hourly monitored operational heating data from an existing boiler based building. Analysis of delta t in space-heating system shows that the delta t is maximum when the outside air temperature is lowest and it decreases with increase in outside air temperature. Furthermore, the hydraulic imbalance is analysed for four different control scenarios with the aim to find an optimum scenario with minimum pumping power, energy consumption and heat-losses in the LTDH network. Results show that the hydraulic imbalance is due to the absence of flow-limiters and balancing valves on the return pipe and thermostatic radiator valves (TRVs) alone are unable to maintain hydraulic balance in the space-heating system of buildings. Moreover, the control scenario with variable flow-rate and fixed supply water temperature from the sub-station is found to be optimum. Compared to the constant flow-rate scenario, the pumping power, energy consumption and heat-losses in the LTDH network are reduced by approximately 2%, 63% and 14%, respectively.

Abstract In this study, we develop indices for the overall technical efficiency (OTE) and energy-saving target ratio (ESTR) using data envelopment analysis (DEA) to calculate the relative efficiency and energy-saving potential of 30 provinces in China from 1997 to 2014. The results are as follows: (1) the OTE of China is 79.187%, indicating that there is 20.813% potential for improvement. The OTE exhibits decreasing efficiency values from the coastal areas to the inland areas and has clear geographical relationships. The average values of OTE in the east, midland and west are 0.932, 0.694 and 0.703. Theoretically, the total energy savings of CE, HE, ME and BE are 11080.60PJ, 5124.71PJ, 4729.24PJ and 6797.39PJ. (2) Regarding CE, HE, ME and BE, the provinces with the highest comprehensive ranks are Henan, Shanxi, Shaanxi, and Gansu, which simultaneously have the greatest energy-saving potentials and energy-saving targets. (3) The HE has the largest average ESTR of 38.357% and the values for BE, CE, and ME are 25.759%, 23.874%, and 22.143%, respectively. The CE category is the greatest in total energy savings (40.171%), which is followed by BE (24.150%), HE (18.384%), and ME (17.293%).

Abstract In this paper, a novel ejector-based multi-evaporator refrigeration system using R134a is proposed to meet different working modes of refrigerated trucks running in tropical regions. The Computational Fluid Dynamics (CFD) simulation is implemented to research the performance of ejectors utilized in both air conditioning - refrigerating mode and air conditioning - freezing mode. Consequently, crucial ejector geometrical parameters such as area ratio (AR), nozzle exit position (NXP), lengths of primary nozzle and diffuser are optimized for each mode. Moreover, a novel approach by taking the deviation of the entrainment ratio or secondary mass flow rate over the deviation of the geometrical parameter into account to evaluate the ejector performance sensitivity to each geometrical parameter is first time presented to the best of the authors’ knowledge. The results show that: 1) Area ratio, NXP, lengths of primary nozzle and diffuser of the ejector in the air conditioning - freezing mode are almost twice as those in the air conditioning - refrigerating mode; 2) The performance of both ejectors is primarily influenced by area ratio and NXP; 3) The proposed sensitivity indicator can better evaluate the sensitivity of the ejector performance to geometrical parameters.

Abstract In view of the current energy demand for miniaturized equipment in extreme environmental fields, such as in deep space exploration. A new fan-shaped radioisotope thermoelectric generator is innovatively presented and designed. Thin-film thermoelectric materials used for miniaturized radioisotope thermoelectric generators are first prepared by electrochemical methods. The prepared fan-shaped radioisotope thermoelectric generator has a volume of 5.75 cm3 and consists of 8 thermoelectric modules and 32 thermoelectric legs. The study finds that when a 1.5 W heat source is loaded, the temperature difference of the device is 54.8 K, the output voltage and the maximum output power is 174.88 mV and 333.20 nW, respectively. On this basis, the number and size of the modules are optimized by the finite element method. When the thermoelectric leg size is optimized to 9 × 2 mm2 and the number of modules is 8, the maximum output power can be up to 369.02 nW. The corresponding experimental verification work is further developed and discussed. This work provides a novel solution for the energy supply problem of small-volume devices in extreme space environments.

The increasing depletion of natural resources, combined with a wider set of pressures on the environment, has, in recent years, highlighted the need for a more efficient use of energy and a development process that involves alternative energy sources. Energy efficiency has received much attention as a solution, implying both monetary and emissions savings. However, the latter may be partially offset by the income and demand effects of the former, both in more efficient sectors and in spreading to the wider economy. This is the problem of rebound effects. Taking Spain as a case study, and introducing an energy-related CGE model that develops the inclusion of renewables, this paper evaluates a combination of efficiency initiatives to deliver both reduced energy use by households and a more sustainable supply of energy. Our findings suggest that a package aimed at improving efficiency in household electricity and petroleum use, combined with a more competitive supply of energy from renewable sources, may be the only way to get reductions in all energy use, and thus benefit the economy. Specifically, we consider how this package may lead to positive economic impacts and associated rebound effects, where the latter are focused on a greener energy supply.

Abstract For a fixed bed gasifier, coal gas is mainly derived from dry distillation zone and gasification zone. The upstream gas from gasification zone will inevitably affect the composition and production of dry distillation gas. Therefore, the composition of upstream gas and the reaction conditions of dry distillation zone were simulated to perform experiments of Yining coal pyrolysis under a series of atmospheres. Some new discoveries are obtained. H2O and the hydrogen-containing gas have a significant impact on the release of dry distillation gas. H2O inhibits the release of CO, CO2 and CH4, but promotes the release of H2 which is not from char−H2O gasification. When the hydrogen-containing gas passes through dry distillation zone, the path of stabilizing radicals is completely changed, resulting in that coal pyrolysis changes from a process of generating H2 under inert atmosphere to a process of consuming a large amount of H2. In addition, water gas shift reaction is the only homogeneous reaction observed obviously in dry distillation zone, which can promote the production increase of CO2, but can not prevent the production decrease of H2 in coal gas.

Abstract The absorption refrigeration system driven by low grade heat sources, especially the waste heat sources, becomes more and more attractive in recent decades. However, most traditional absorption systems cannot achieve a high utilization rate of the waste heat with limited heat capacity. These systems are usually designed to obtain heat in the generator, which means that the waste heat sources cannot be utilized to the temperature lower than the generator temperature. This paper proposed a new structure heated by heat conduction oil in the generator and electric heating rings around the stripping section. This structure can simulate the temperature-distributed heat sources when the electric heating rings work. It can also simulate a traditional generator when the electric heating rings do not work. Influences of different heat distributions are analyzed in detail in this paper. The results show that the heat sources utilization rate will increase with the increase of the heat in the stripping section, while the coefficient of performance will be negatively affected by the increasing heat in the stripping section. By optimizing the heating structure, the coefficient of performance can be similar to that of a traditional system when the heat is just added in the middle and lower part of stripping section. The optimum utilization rate of heat sources in this test model can reach 1.8 times to that of a traditional system. Under this heating model, the lowest temperature required in the heating section is 82 °C when the heat conduction oil inlet temperature is 169 °C. It is much lower than the temperature inside the generator, which is 137.3 °C.