Return Pipe Research Articles

Experimental investigation and numerical modeling of an innovative horizontal coaxial ground heat exchanger (HCGHE) for geothermal heat pump applications

Geothermal is an ideal renewable energy source for building heating and cooling via a ground source heat pump (GSHP) system. However, the high initial cost of installing the ground heat exchanger limits its adoption, especially among owners of residential and smaller non-residential buildings. This study aims to develop a new methodology for installing an innovative horizontal coaxial ground heat exchanger (HCGHE) and to investigate its thermal performance in GSHP applications through field testing and numerical simulation. Horizontal directional drilling and a new manifold design were used to install the new HCGHE, significantly reducing costs by minimizing time, materials, labor, and landscape disruption. The transient numerical model, developed in Ansys Fluent using the finite volume method, was validated against experimental data and employed to predict temperature distributions in the supply pipe, return pipe, and surrounding soil. The study determined the heat transfer rate per unit borehole length and entire thermal resistance of the HCGHE to vary from 23.0 to 35.7 W/m and 0.28 to 0.74 K·m/W in winter, and from 24.3 to 37.4 W/m and 0.12 to 0.52 K·m/W in summer. The short-term soil temperature distributions in both the radial and longitudinal directions of the HCGHE are modeled using second-order polynomial equations, with R2 values ranging from 0.994 to near unity, while the long-term soil temperature distributions follow a power law. This research not only confirmed that the innovative HCGHE design and installation can achieve good thermal performance but also provided valuable insights into continual design improvement. The innovative HCGHE-based GSHP system provides an efficient and sustainable solution for heating and cooling buildings by harnessing the earth’s natural heat, offering the advantages of simplified installation and reduced initial investment.

Comparing Numerical and Analytical Methods for Heat Loss Determination of District Heating Systems

Energy efficiency assessment considers the heat losses in district heating systems. For instance, life cycle assessments of such engineering structures require knowledge of the heat losses during their use phase. Therefore, it is essential to have the most accurate knowledge of the heat losses of a district heating system. In this study, three different methods for the determination of specific heat losses for buried preinsulated steel pipes are compared. The first method involves an analytical calculation in accordance with EN 13941, while the second utilizes an equivalent circuit approach. The third method employs finite element analysis. The objective was to evaluate the accuracy of the methods, the achievable range of results, and the effort required to solve the respective calculation algorithms. Therefore, typical 2-dimensional cross sections including different pipe diameters were selected. In situ measurements were not part of this study. Consequently, the analysis centres on the deviation between the methods. All three methods determine the heat loss in both the supply and return pipes. While the analytical calculation method cannot determine temperatures in the soil, the equivalent circuit method can handle more complex tasks and gives detailed results at predefined points in the model. With the finite element method, a high degree of detail can be achieved, but the requirements for solving the algorithms increase. An emerging trend in district heating involves reducing operational temperatures in both new and existing networks. This will change the relation between heat losses and heat delivered to the customers. Subsequently, an increasing interest in the actual heat losses and the precision of calculation is expected within this development. Therefore, it remains essential to evaluate the performance of different models

Thermomechanical optimization of actively cooled thermal shield in PIP II Cryogenic Distribution System

The PIP-II Cryogenic Distribution System (CDS) connects the cryogenic plant to the superconducting linac consisting of 23 modules. The helium stream flow through the PIP-II CDS is regulated by means of control valves located in the Distribution Valve Box. The cryomodules are supplied with helium from individual Bayonet Cans. The entire CDS process lines are shielded from the thermal radiation from the ambient vacuum jacket by actively cooled thermal shields. High efficiency of the thermal shield has been achieved by state-of-the-art optimization of its thermalization system consisting of thermal bridges connecting the shield to the high temperature helium return pipe nominally at 80 K. The temperature and stress analyses of the thermal shield have been performed numerically. The results obtained for the thermal shield of the PIP-II CDS Distribution Valve Box are compared with the solutions applied in other large science machines.

Decreasing the Return Temperature in District Heating Networks Thanks to a Switch to Alternative Substation Architectures

District heating networks (DHN) are key systems for managing heating and reducing carbon emissions at a large scale. Therefore, increasing their efficiency is a key to reduce climate change. DHNs with centralized production are supplying hot water to building substations (interface between the DHN and the buildings) for domestic hot water (DHW) production and space heating (SH) using heat exchangers. Colder water is returned to the centralized district heating system by the return pipe. One of the crucial factors to improve the efficiency of DHNs is to decrease the return temperature in the network to keep the same amount of energy transferred to the buildings while reducing the thermal losses and the flow rate, leading to more savings. In this paper, we model and simulate two of the most common DHN substation architectures used in a typical third-generation DHN to identify their flaws and propose new district heating substation architectures that can increase the DHN efficiency. The main result from our study is the consistent reduction in the average return temperature across all scenarios when implementing our proposed architectures. The reduction ranged between 3.7 °C and 10.2 °C. We also found that the lower the heating temperature, the greater the reduction. This is particularly beneficial as buildings are increasingly equipped with low-temperature heating systems, especially new constructions. Moreover, these architectures showed optimal efficiency with new buildings that have a high water heating demand compared to heating demand. This is due to the higher impact of cold water preheating.

Corrosion Performance of Epoxy/Sulfur–Selenium Coating on Q235 Steel

Sulfur powder (99.99%) and selenium powder (99.99%) were mixed and heated to approximately 300 °C to obtain an S-Se alloy. It has good flowability at 130 °C and can be applied to Q235 steel to obtain a S-Se coating. Epoxy was used as a filler, and the S-Se alloy was applied as a coating. This combination was utilized to create the composite coatings of epoxy/sulfur–selenium (E/S-Se). To investigate the corrosion resistance of this coating on Q235 steel substrate, we conducted measurements and obtained electrochemical impedance spectra (EIS) and linear polarization curves (LPC). These measurements were performed in a three-electrode cell within an electrochemical workstation using a 3.5 wt.% NaCl aqueous solution. By comparing bare Q235 steel, S-Se, and E/S-Se, the study found that the E/S-Se coating had a higher self-corrosion potential (−0.484 V vs. SCE) and the lowest self-corrosion current density (2.361 × 10−11 A/cm2). The purpose was to simulate the corrosive environment experienced by condensate return pipe walls in petroleum refining equipment. Additionally, experiments were carried out using 0.01 mol/L HCl solution as the corrosion medium at different temperatures (40 °C, 60 °C, 80 °C). The results indicated that the E/S-Se coating exhibited a lower corrosion rate compared to the Q235 steel substrate. Under immersion conditions at 40 °C and 60 °C, no corrosive substances were detected on the surface of the coating. The test results demonstrated that the E/S-Se coating exhibited superior corrosion resistance compared to the Q235 substrate, providing up to 99% protection for the substrate.

Utilization of Surplus Air Thermal Energy by a Water Cycle System in a Chinese-Type Solar Greenhouse

Solar greenhouses are commonly overheated during the day, and the remaining air heat can only be dissipated through ventilation, which is a severe energy waste problem. In order to improve the energy utilization of the greenhouse, this study proposes a water cycle system using surplus air thermal energy, which consists of an air-water heat exchanger, supply and return pipes, a submersible pump, a water tank, and an automatic control system. The proposed system stores the surplus air thermal energy in the greenhouse in the water tank. It releases it into the greenhouse using water circulation, and experimental analyses were carried out using a solar greenhouse in the Shenyang area. The effects of different air and water flow rates on the performance of the surplus air thermal energy water recycling system and the environment inside the greenhouse were analyzed by establishing a CFD model and model validation, and the average difference between the experimental data and the simulated data was 6.98%. The results show that the circulating air flow rate significantly affects the system performance and the environment inside the greenhouse. In the heat collection stage, the water circulation system with an airflow rate of 9 m/s has a minor average temperature difference in the vertical plane of the greenhouse. The water circulation system with an airflow rate of 6.0 m/s collects and releases the most significant heat. The temperature cloud between the vertical and horizontal planes is more uniform. This research provides new ideas for efficient energy use in solar greenhouses.

Effects of different oil return pipe locations on the vortex characteristics of a cylindrical cyclone separator

The optimization of internal components in the oil-gas separator is crucial for enhancing the performance of the compressor system. In this reported study, the effects of three different oil return pipe locations on the vortex characteristics of a cylindrical cyclone separator are investigated by CFD simulation based on the omega method, and the relationship with the separation performance is analyzed. The vortex deformation and breakup near the oil return pipe are evident, with the degree following the order of C > B > A, which is conducive to reducing pressure loss. Cyclone C, with the inclined return pipe, exhibits the lowest pressure drop. The overall separation efficiency follows the sequence of A > B > C. For oil droplets of 5 ?m and larger, the separation efficiency is essentially the same, exceeding 97.5%. Cyclone A exhibits the best separation effect for oil droplets smaller than 5?m. This study provides some references for optimizing the internals of oil separators.

4th generation district heating (4GDH) in developing countries: Low-temperature networks, prosumers and demand-side measures

The aim is locating low-cost renovation measures to support the efficiency of district heating (DH). The topology analysis and GIS Zulu© are applied for hydraulic simulation and to verify the scenario viability. The scenarios are compared by life cycle cost, levelized cost of heat (LCoH), capital and operational costs, energy production and consumption, network length, heat losses, linear heat density, heat pump usage, peak load reduction, nominal power duration or the equivalent full load time. As demand-side measures (DSM), improved control, thermostats with an indoor temperature sensor and variable frequency drive (VFD) pumps reduce heat demand. Prosumer scenario imply installing a base-load heat pump. It means a consumer no longer receives heat from the DH system, because the households are now adapted to lower space heating (SH) supply temperature. Prosumer concept also allows placing a heat pump on the DH return line. The simplest configuration includes only the booster heat pump and domestic hot water (DHW) tank storages enough to cover morning or evening consumption peak. To limit piping cost, only supply and return pipes are considered for low-temperature DH, no additional (service) pipes or mixing valves are assumed. Converting single-family houses to prosumers decreases energy consumption, primary network flows and temperatures. LCoH ranges 19.7-35.4 EUR/MWh, depends on the decommissioned pipe length and its proportional pipe investment cost. LCoH decreases as the share of consumers converted to prosumers increases. Discounted payback time is 10 years for DSM and 41 years for prosumer case. For a base-load heat pump, the payback time is much higher than the reference scenario. It implies the same share of heat losses (16% of heat supplied from the CHP plant) with generally worse environmental and energy saving effect due to higher greenhouse gas (GHG) emissions and higher consumption of primary energy. Energy production is expected to follow the pattern of the useful energy consumption to compensate heat losses. The effects are decreasing residential heat demand (-4%), additional capacity to connect new consumers (+6%) and decreasing the network length.

Testing and evaluation of a smart controller for reducing peak loads and return temperatures in district heating networks

A smart demand response control system aiming towards real-time operational optimisation of district heating (DH) network temperature levels, both in the return and supply pipes, has been developed in the TEMPO project. The return temperature is mainly dependent on the demand side. The controller optimises its value through control of the customers’ heat load. The network supply temperature, however, is directly controllable on the production side. The capabilities of supply temperature control are twofold. On the one hand, lowering the network supply temperature as close as possible to the limits determined by customer thermal demands. On the other hand, activating the intrinsic thermal capacity of the piping to temporarily store heat and thereby shifting the heat load in time. This provides additional energy flexibility potential on top of building demand response.In this study, the two features of the smart control system have been tested in a part of the DH network of Brescia (Italy). A cloud-based platform is used to collect real-time data from various sources and to communicate control signals calculated by the smart controller. The article presents the results of the tests and an evaluation of the controller performance. The analysis indicates that daily flow-weighted average return temperature reductions of almost 1 K on average can be achieved, and up to 15 K instantaneously. Using supply temperature control, the daily peak load energy supply could be reduced by 262 kWh (34%) on average, by shifting the heat load.

Mathematical modelling and model validation of the heat losses in district heating networks

Today the most popular system of district heating systems is based on pre-insulated pipes arranged in parallel or twin-pipe configuration. One of the greatest difficulties with heat distribution through pipelines is thermal loss from the distribution. The most efficient solution to that problem is optimising the insulation wall thickness layer according to the pipe diameter. Heat losses should be minimised at a relatively low investment cost to find the most suitable insulation thickness economically. Numerous studies focus on analytical (1D model) calculations and numerical simulations. However, there is a research gap related to laboratory devices that allow measuring the operation parameters (fluid flow, the temperature of the fluid in the supply pipe and the return pipe). This paper presents an analysis of the heat losses from pre-insulated pipes and twin pipes in the heating system network. This study compares the heat losses in the ground calculated by analytical solution (1D model) with the measurements on the dedicated experimental setup. The calculations have been made for several heating network pipe variants: twin pipes: DN40, DN50, DN65, and their counterparts in a single parallel pre-insulated system. The insulation thickness used in all cases is 30.85 mm for DN40 and 32.00 mm for DN50 and DN65. The insulation is made of rigid polyurethane foam that meets the requirements of the PN-EN 253 standard. During the investigation, the thermal conductivity of insulation material is examined. The obtained thermal conductivity results are used in the calculations. The results from laboratory devices and analytical models have been compared, demonstrating good agreement – with a low error level in the range of approximately 8%, depending on the type of district heating pipe. The validated mathematical model of the heating network is then used to calculate the heat losses in a heating network connecting an underground storage tank with a ground source heat pump. The economic analysis shows that after 5 y, a return on investment is expected when comparing twin-pipe systems and single-pipe pre-insulated heating networks.

Parametric optimisation for the design of gravity energy storage system using Taguchi method

Gravitational energy storage systems are among the proper methods that can be used with renewable energy. However, these systems are highly affected by their design parameters. This paper presents a novel investigation of different design features of gravity energy storage systems. A theoretical model was developed using MATLAB SIMULINK to simulate the performance of the gravitational energy storage system while changing its design parameters. A parametric optimization study was also conducted using Taguchi and analysis of variance (ANOVA) techniques for optimizing the energy storage rate. Six parameters were studied; three are related to the piston design (diameter, height, and material density). The other parameters are the return pipe diameter, length, and charging/discharging time. Results revealed that the piston diameter and height are the two most significant parameters for the system performance compared to the other parameters, as they contributed by 35.11% and 30.28%, respectively. The optimization results indicated that the optimal piston diameter, height, and return pipe diameter were 0.25, 0.5, and 0.01 of the container height. The outcomes of this paper can significantly improve energy storage and power generation from renewable energy systems as it provides a reliable, economical, sustainable, and durable energy storage system.

Technical Feasibility Assessment for a Novel Fifth-Generation District Heating Model of Interconnected Operation with a Large-Scale Building

In this study, a novel fifth-generation district heating (DH) model was proposed that implements the energy-prosumer concept of bilateral heat trading (BHT) process between the DH network and the building. The newly proposed BHT model can be characterized by the feature of using the low temperature of DH return pipe’s water. The technical feasibility of the proposed BHT model was evaluated through operation simulation analysis based on the actual operation data of the hybrid pilot system combined with the fuel cell and heat pump and the annual hourly temperature profile of the existing DH return pipe. The main objective of this study is to examine the technical feasibility of the interconnection operation model with the existing DHN as an alternative to overcome the limitations of the current fuel cell cogeneration model, which suffers from the low production volume caused by the high initial investment cost. From the simulation results, it was confirmed that considerable operational benefit, more than 30% in terms of primary energy savings, can be achieved with the proposed model, and compared to the stand-alone model of the fuel cell cogeneration system for the building, it can provide a more flexible technical environment to improve the system utilization rate by about 40%.

Assessment of the round-trip efficiency of gravity energy storage system: Analytical and numerical analysis of energy loss mechanisms

Emerging large-scale energy storage systems (ESS), such as gravity energy storage (GES), are required in the current energy transition to facilitate the integration of renewable energy systems. The main role of ESS is to reduce the intermittency of renewable energy production and balance energy supply and demand. Efficiency considerations are critical when developing energy storage systems. In this paper, a novel multi-domain simulation tool is employed to determine the round-trip energy efficiency (RTE) of gravity energy storage system. The study considers analytical and numerical simulations to investigate the effect of the flow rate and the pressure on the energy losses i.e., hydraulic losses, mechanical equipment losses, and particularly friction and leakage induced energy losses. Different designs are compared to determine the most efficient storage scenario and to assess the scaling up effect on GES' efficiency of storage. The effect of the internal and external positioning of GES return pipe on the system's round-trip efficiency was also compared. Finally, the overall round-trip efficiency of GES system was calculated and compared to other energy storage technologies. The results obtained from the analytical and numerical models show that the round-trip energy efficiency depends on the pressure inside GES chambers, consequently, the operating scale. The obtained round-trip efficiency of GES system ranges between 65 % and 90 %. The amount of energy lost due to the sealing friction represents the largest share in comparison with other loss mechanisms, with a percentage of 21.1 % and 0.9 % for small-scale and large-scale operations, respectively. This study demonstrates that GES operates more effectively in large-scale applications.

Distributed Fluidic Control Method for Alleviating Rapid Movement of Shock Train

The phenomenon of a shock train experiencing rapid forward movement when climbing upstream in a conventional rectangular isolator with incident shocks has recently been investigated. Although several studies have concentrated on the modeling of this phenomenon, few works have focused on methods to alleviate the problems caused by it. Therefore, a distributed fluidic control method is proposed herein, which combines distributed slot injections at the isolator walls and boundary-layer suctions at the corners of the downstream part of the isolator through return pipes. The control effect of the concept is verified by experiments in a Mach 3 test section. First, a rectangular isolator with incident shocks is tested to reproduce the phenomenon of a shock train accelerating suddenly when surmounting a reflection point in the background waves. Then, different fluidic control methods are proposed to demonstrate that the rapid movement of the shock train in a rectangular isolator can be effectively controlled through distributed slot injections, employing a closed flow cycle built by return pipes. According to the experimental results, when the shock train forms and climbs upstream in the isolator, the secondary flows are ejected toward the core flow from the distributed slots at the upstream part of the shock train, which increases the boundary-layer momentum and decreases the pressure gradient of the shock train head. The distributed injections induced compression similar to that induced at a compression surface. The shape of the slip line changes gradually as the backpressure increases. Moreover, the maximum sustainable backpressure is less affected.

Optimization insulation thickness and reduction of CO2 emissions for pipes in all generation district heating networks.

District heating systems are provided solutions for the increasing energy problems in high-population cities. Energy costs go up depending on increasing heat loss in DHS's distribution network. Heat loss from the network consists of 5-20% of transferred energy, and this loss is higher than the other losses in the heating system. In the study, heat losses from the pipes such as supply-return pipes, hot water and circulation pipes into heat canals are investigated based on energy, exergy, economic and environmental. Optimum insulation thicknesses, energy savings, reduction of CO2 emissions, the first investment costs and payback periods of the pipes in the network of all-generation district heating systems are investigated by using Life Cycle Cost Analysis (LCCA) method for fuel types like natural gas, fuel oil and coal. Optimum insulation thicknesses are calculated for different nominal sizes of pipes and various insulation materials such as glass wool, and rock wool for the different climatic zones. According to the results of the study, the heat losses from pipes in the 4th generation DHS network are decreased between 38.19% and 33.33% from the warmest climate zone to the coldest climate zone according to the 3rd generation. Energy savings, reduction of CO2 emissions, payback periods and optimum insulation thickness values of supply and return pipes in the network are respectively changed between 7.80-98.86 $/m, 39.61-322.32 kg CO2/year, 0.028-0.38 years and 0.025-0.0105 m depending on various fuel types, insulation materials, nominal size pipes, climatic zones and all generation types.

Icreasing the efficiency of district heating supply systems by local heat distribution station modernation

The PURPOSE of the study is to improve the efficiency of district heating systems. To achieve this purpose, the following tasks must be completed. To consider the problems of functioning of an open heat supply system functioning. Perform an analysis of technical solutions for the modernization of local heat distribution and metering station are presented. To develop a technological scheme of local heat distribution station with a hot-water supply system. To determine the main technical, economic and investment indicators of the developed technical solution.MATERIALS AND METHODS. Measurements were made using the local heat metering station. When solving the set tasks, a methodology for assessing efficiency was applied with the calculation of capital and operating costs, including the calculation of the payback period.RESULTS. A new scheme for regulating of hot water supply system recirculation with open connection of consumers to a centralized source is proposed. The results of the implementation of the developed technical solution of local heat distribution and metering station for apartment buildings in Yoshkar-Ola city are presented. An experimental study of the effect of a joint additional installation of a pump and a control valve on the hot water supply system recirculation line before the mixing unit for hot water recirculation with return pipe and before the direct-acting temperature controller was the first to do. As a result, the consumption of district water was reduced by 36 - 39%, the temperature in the return pipeline was reduced by 13.5%; the enthalpy of the hot water was reduced from 0.168 to 0.145 Gcal/m3, the required circulation in all risers of the hot water supply system was ensured.CONCLUSION. The use of the developed technical solution for the modernization of local heat distribution and metering station increases the efficiency of district heating supply systems. Calculations made on the basis of the readings of automatic custody heat transfer meter showed that the payback period for the technical solution is 40 days. The proposed solution is recommended for use in local heat distribution station of open heat supply system in order to reduce water rate, lower the temperature in the return pipeline and reduce heat losses, improve circulation in all riser pipes of the hot water system.

Streamflow droughts aggravated by human activities despite management

Human activities both aggravate and alleviate streamflow drought. Here we show that aggravation is dominant in contrasting cases around the world analysed with a consistent methodology. Our 28 cases included different combinations of human-water interactions. We found that water abstraction aggravated all drought characteristics, with increases of 20%–305% in total time in drought found across the case studies, and increases in total deficit of up to almost 3000%. Water transfers reduced drought time and deficit by up to 97%. In cases with both abstraction and water transfers into the catchment or augmenting streamflow from groundwater, the water inputs could not compensate for the aggravation of droughts due to abstraction and only shift the effects in space or time. Reservoir releases for downstream water use alleviated droughts in the dry season, but also led to deficits in the wet season by changing flow seasonality. This led to minor changes in average drought duration (−26 to +38%) and moderate changes in average drought deficit (−86 to +369%). Land use showed a smaller impact on streamflow drought, also with both increases and decreases observed (−48 to +98%). Sewage return flows and pipe leakage possibly counteracted the effects of increased imperviousness in urban areas; however, untangling the effects of land use change on streamflow drought is challenging. This synthesis of diverse global cases highlights the complexity of the human influence on streamflow drought and the added value of empirical comparative studies. Results indicate both intended and unintended consequences of water management and infrastructure on downstream society and ecosystems.

Research on Fuel Cell Fault Diagnosis Based on Genetic Algorithm Optimization of Support Vector Machine

The fuel cell engine mechanism model is used to research fault diagnosis based on a data-driven method to identify the failure of proton exchange membrane fuel cells in the process of operation, which leads to the degradation of system performance and other problems. In this paper, an extreme learning machine and a support vector machine are applied to classify the usual faults of fuel cells, including air compressor faults, air supply pipe and return pipe leaks, stack flooding faults and temperature controller faults. The accuracy of fault classification was 78.67% and 83.33% respectively. In order to improve the efficiency of fault classification, a genetic algorithm is used to optimize the parameters of the support vector machine. The simulation results show that the accuracy of fault classification was improved to 94% after optimization.

Operational performance characteristics of an axial double baffles three channels classifier for coarse pulverized coal

In order to obtain the working principle of the axial direction classifier and raise its separation efficiency, the flow field and classification performance of the classifier was investigated by numerical simulation and experimental measurement. The particle size distribution at inlet, outlet and return pipe was measured by laser particle size analyzer and coupled with Rosin-Rammler model. The particle trajectory was analyzed using the discrete phase model. The tangential velocities played a dominant role in particles radial distribution mainly in the form of Rankine vortex. The axial velocities determine the vertical distribution of particles. The velocity distributions and turbulence influence the cutting particle size and separation efficiency mostly. The most suitable condition is baffles angle of 20° and inlet velocity of 19.3 m/s considering the load of the mill and the fineness of the pulverized coal at the inlet of the classifier.

Improving a heat supply system by increasing the efficiency of the heat pump unit

The reliability and efficiency of the operation of district heating systems is largely determined by the efficiency of preparation of heating network water. In open heat supply systems, make-up water, among other things, compensates for the water intake in hot water supply systems. A number of technologies have been developed that increase the efficiency of an open heat supply system by reducing the water consumption in the supply pipeline of the heating network, increasing the operating time of the heat pump, and increasing the specific generation of electricity for heat consumption at the CHP plant due to additional cooling of the network water in the return pipe of the heating network.