Abstract

Small-scale centrifugal compressors have the potential to offer higher efficiency and operate without oil, compared to positive displacement compressors. With advancements in industrial design and manufacturing technology, the potential for replacing traditional positive displacement compressors with small-scale centrifugal compressors is increasing. While centrifugal compressors have traditionally been favored for large systems with cooling capacities of several
hundred kilowatts or more, their application in smaller refrigeration systems, with capacities in the tens of kilowatts, has not been thoroughly explored and remains under-researched. In this study, a 45 kW refrigeration system test bench was constructed using an R134a small-scale oil-free centrifugal compressor with gas foil bearings. The experiment investigated the impact of key design factors, including refrigerant charge amount and the settings of air-cooled condenser fan, on system performance. After optimization, the system achieved a maximum cooling capacity of 44.54 kW and a highest COP of 3.59 under design conditions (ambient temperature of 35°C). Additionally, control strategies for cooling service were designed and main control parameters were calibrated, with experimental results indicating that the system successfully performed cooling tasks and operated with stable parameters.

Abstract

In the current context where the cost of obtaining pure CO2 is high, using impure CO2 gas for huff-n-puff is a more practical approach. The effects of impure CO2 huff-n-puff and storage in shale reservoirs with complex fracture networks, as well as the characteristics and main controlling factors of injecting CO2 of different purities for huff-n-puff and storage, are not yet well understood. This study considers the fracture extension mechanism in horizontal well fracturing and establishes an unstructured grid numerical model to depict fracture morphology. Using actual physical experiment results, a fluid model is fitted. By analyzing the impacts of huff-n-puff and storage under various production parameters and utilizing statistical methods, the key controlling factors of impure CO2 huff-n-puff and storage in complex fracture well networks are identified and their characteristics are summarized. The results indicate that, compared to injecting pure CO2 for huff-n-puff, injecting impure CO2 can still significantly enhance the recovery rate of shale reservoirs. Furthermore, the effectiveness of huff-n-puff and storage increases with the enhancement of production parameters such as soaking time, injection rate, and cyclic cumulative injection rate. Among these factors, Injection rate has the greatest impact on huff-n-puff effectiveness, while the timing of huff-n-puff has the greatest impact on storage effectiveness. This research provides a robust theoretical foundation and practical guidance for improving the development efficiency of shale oil using impure CO2 huff-n-puff technology.

Abstract

In the development of adsorbents for direct air capture of carbon dioxide, a novel charged-sorbents, enabled by electrochemically inserting hydroxide ions into activated carbon electrodes, has demonstrated enhanced carbon dioxide uptake at low pressures due to chemisorption to form (bi)carbonates. However, the further application of charged-sorbents is limited due to a lack of quantitative studies to material properties of absorption performance, particularly those prepared from inexpensive and readily available commercial activated carbon cloth. In this work, we employed a combined experimental and numerical method to investigate how the carbon dioxide adsorption performance of charged-sorbents is related to structural properties of carbon-based porous absorbent, i.e., charge capacity per unit mass, average pore size, pore volume, and specific surface area. We identified that the average pore size and the mass charging capacity are the two most influential factors. Higher mass charging capacity and smaller pore sizes, in general, lead to higher sorption performance. We observed that a commercial carbon cloth (AC-10-300) with a charing capacity of 1173 C/g and an average pore size of 0.933 nm showed an excellent sorption performance of 0.213 mmol/g with 400 ppm CO2. Our study provides valuable insights into the development and optimization of hydroxide-functionalized adsorbents for direct air capture using commercial activated carbon cloth substrates.

Abstract

Chilled water storage is commonly employed in centralized cooling systems for peak shaving, demonstrating significant potential of load flexibility. However, this cost-effective and accessible flexibility resource has seldom been integrated into domestic air-conditioning systems in response to dynamic electricity tariffs or photovoltaic (PV) generation. This paper focused on capacity design and performance evaluation of air-conditioning systems integrated with chilled water storage for improving PV self-consumption in domestic applications. Operation strategies involving temperature control and flow rate control were both considered. The results show that chilled water storage presents an annual cost saving of over 10% and significantly improves PV self-consumption compared to the baseline case without storage. Furthermore, the chilled water storage shows its additional advantage over the battery system in reducing the capacity of the chiller from 7.5 kW to 6.7 kW and enhancing energy efficiency of the air-conditioning with an average COP increasing from 2.87 to 3.14.

Abstract

Solar PV generation makes a great contribution to promoting renewable energy development. However, the unpredictable cloud cover makes PV generation to become unstable. This study developed a novel method to evaluate the cloud cover with sky images and subaerial meteorology parameters. The improved sun tracking model, RBR threshold model were combined to upgrade the simulation precision of cloud cover in the sky. Validation shows the developed method performs well under the different sky conditions. Furthermore, this study investigated the impact of the sky state and the near-ground visibility on solar irradiance. This study provides a thorough method for evaluating the cloud cover, which may instruct the industry to deal with instable solar irradiance under different sky states and prompt the solar PV application.

Abstract

In this paper, a novel multi-energy complementary power generation system used for natural gas city gate stations (NGCGS) is proposed, which aims at recovering considerable amount of energy (including NG pressure energy and cold energy after expansion) wasted at the pressure regulators in city gate stations (CGS). The proposed system consists of three major subsystems: geothermal water system (GWS), NG expansion power generation system (NGEPGS) and organic Rankine cycle power generation system (ORCGS). GWS works as a heat source to heat the organic working fluid in the ORC’s evaporator and then to heat the NG before it enters the expander for power generation. NGEPGS uses an expander to recover the pressure energy for power generation which replaces the conventional process of NG pressure reduction in the CGS. The ORCGS takes the geothermal energy as heat source and uses the cryogenic NG from the outlet of the expander as ORC’s heatsink for electricity generation. The thermodynamic model of the proposed system is established using EES (Engineering Equation Solver). 9 organic working fluids are selected and compared. Pentane has been chosen as the optimal working fluid in this study because it has the best performance. The proposed system is compared with another system established by previous researchers. The results show that the proposed system in this paper can generate more electricity under the same conditions.

Abstract

Demand response is currently an effective strategy to address the mismatch between electricity supply and demand. In residential buildings, there is a wide variety of household appliances with different usage characteristics. It is necessary to be aware of occupants’ daily appliance usage behavior in order to better manage home energy system. In this study, a home energy consumption tracking application is developed for a longitudinal survey based on self-monitoring, during which household appliance usage data of 166 users in Shenzhen are collected. Then day-ahead prediction model for appliance usage trajectories is established. CNN+GRU is used to identify appliance usage characteristics as well as the logical relationships among different appliances. In addition, due to the limited amount of data, this paper utilizes random forest regression for day-ahead prediction for multi-appliance usage states based on occupants’ social attributes, weather parameters and home awake state. It is shown that the predicted multi-appliance usage trajectories are more accurate and logical to user behavior. The results provide a reference for the incentive recommendation mechanisms of different appliances under demand response.

Abstract

Capturing carbon dioxide (CO2) directly from anthropogenic sources is an essential societal responsibility, particularly in light of the alarming increase in global atmospheric CO2 levels. Achieving net zero emissions and making our industrial clusters carbon-neutral is challenging and cannot be overcome without the development of large-scale carbon capture technology. Although amine-based absorbents have conventionally served this purpose, their efficacy is hindered by challenges such as low-temperature requirements, energy-intensive regeneration, poor recyclability, susceptibility to corrosion, and oxidative degradation. This study presents a novel nano-enhanced formulation derived from amines, aiming to demonstrate a sustainable CO2 absorption process that enhances carbon capture performance and reduces regeneration costs for extensive industrial applications. Experimental trials were conducted in an interfacial contact batch reactor at temperatures of 298 K and 318 K using CO2 with both nanoformulation and conventional amine solutions. The nanoformulation is prepared through a three-step technique involving microgelated nanoparticles and other additives. The SEM, TEM, DLS, and BET characterization were carried out to test nano-enhanced amine solution. Further, thermal analyses such as DSC and TG-DTA were conducted to examine the thermal performance of nanoformulation. Nano-enhanced amines (functionalized amine) exhibit a significant improvement in capture efficiency, approximately 10-12% higher than traditional amine solvents. Additionally, the nanoformulation shows enhanced CO2 absorption rates compared to traditional MEA (30 wt.%) solutions, and nearly 10 and 55 % enhancement has been reported in absorption rates at 320 and 298 K, respectively. When integrated with other strategies, this approach holds promising potential for carbon capture on industrial scales or directly from the atmosphere.

Abstract

Internal short circuit (ISC) is considered one of the main causes of battery failure, making early detection of ISC crucial for battery safety. The charging voltage curve contains abundant information about the battery state, reflecting various conditions, and is easily obtainable during the charging process. Therefore, it serves as an excellent signal for ISC detection. This research employs parallel resistor methods to simulate different leakage current scenarios and analyzes the impact of various charging voltage ranges on ISC. It identifies the optimal voltage range for detecting leakage current as 3.6 V to 3.7 V and determines that the resistance value capable of detecting the slightest ISC is 700 ohms. Furthermore, this method has been successfully applied in various environmental temperatures and series-connected battery packs, demonstrating its versatility across different scenarios.

Abstract

When the high water cut crude oil is collected and transported at low temperature, a large amount of adhesive oil in the traditional stainless steel (SS) pipeline leads to the increase of wellhead back pressure, which seriously threatens the production safety of the oilfield. In this study, the heat transfer characteristics of fiberglass reinforced plastic (FRP) and SS pipe sections were tested with indoor loop experimental equipment, and the temperature drop and pressure drop of oil-water suspension in FRP pipe and SS pipe were studied. Simultaneously, the entire process involving convective heat transfer and heat conduction in FRP and SS pipes is characterized through heat transfer calculations. The experimental and calculation results show that the pressure drop in the SS pipe section is much greater than that in the FRP pipe section at the same temperature drop time, and the FRP pipe is more conducive to safe transportation. In addition, compared with the SS tube section, the temperature of the oil-water suspension in the FRP tube section takes longer to drop from 40 °C to 30 °C. FRP pipeline has better insulation performance, which is more conducive to the implementation of low temperature gathering and transportation (LTGT) technology of high water cut crude oil.