Part Load Research Articles

In Vivo Investigation of 3D-Printed Calcium Magnesium Phosphate Wedges in Partial Load Defects.

Bone substitutes are ideally biocompatible, osteoconductive, degradable and defect-specific and provide mechanical stability. Magnesium phosphate cements (MPCs) offer high initial stability and faster degradation compared to the well-researched calcium phosphate cements (CPCs). Calcium magnesium phosphate cements (CMPCs) should combine the properties of both and have so far shown promising results. The present study aimed to investigate and compare the degradation and osseointegration behavior of 3D powder-printed wedges of CMPC and MPC in vivo. The wedges were post-treated with phosphoric acid (CMPC) and diammonium hydrogen phosphate (MPC) and implanted in a partially loaded defect model in the proximal rabbit tibia. The evaluation included clinical, in vivo µ-CT and X-ray examinations, histology, energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) for up to 30 weeks. SEM analysis revealed a zone of unreacted material in the MPC, indicating the need to optimize the manufacturing and post-treatment process. However, all materials showed excellent biocompatibility and mechanical stability. After 24 weeks, they were almost completely degraded. The slower degradation rate of the CMPC corresponded more favorably to the bone growth rate compared to the MPC. Due to the promising results of the CMPC in this study, it should be further investigated, for example in defect models with higher load.

Numerical investigation on vortex characteristics in a low-head Francis turbine operating of adjustable-speed at part load conditions

In this paper, the vortex characteristics in a low-head Francis turbine operating of adjustable-speed at part load conditions are numerically investigated. First, the rules of fixed-speed and adjustable-speed operations are introduced in detail, aiming at operating limits extension. Second, comparative analysis on vortex rope is conducted, with emphasis on vortex rope volume, tail water excitation and spontaneous power swing. Finally, inter-blade vortex has been analyzed and successfully demonstrated, and its induced high amplitude pressure fluctuation characteristic is explored. It is found that at deep part load, vortex rope volume and power swing coefficient of adjustable-speed operation decreased by 69 % and 61 % respectively. The amplitude of Rheingans frequency decreased by 17 %, while the amplitude of blade passing frequency increased by 83 %. After decreasing runner speed, inter-blade vortex is stranded at blade suction surface leading-edge. In addition to low-frequency disturbance, pressure fluctuation shows high-frequency wide-band characteristics. It is concluded that adjustable-speed operation has beneficial effects on improvement of cavity cavitation and energy stability. As for hydraulic stability, the Rotor-Stator Interaction effect cannot be ignored. After changing runner speed, the inter-blade vortex becomes more serious, which could provoke low-cycle and high-cycle fatigue of the blades.

A multivariable control strategy based on fuzzy logic interference rule for a solar-driven, direct air-cooled H[formula omitted]O-LiBr absorption chiller

The operation of a solar-driven H2O-LiBr Absorption Chiller (AbC) is subjected to various limitations regarding its ability to satisfy efficiently the required cooling demand while avoiding certain internal conditions, such as H2O-LiBr solution crystallization and water freezing. These circumstances would lead to problematic situations and jeopardize the continuous operation of the system. Thus, the development of accurate control strategies is an issue of interest. A new operational control strategy approach is introduced and tested through simulation studies, by employing a transient model of the entire solar cooling application. A prototype under development is used for the parametrization of the direct air-cooled AbC model used in the simulations. The developed control strategy aims to minimize the electrical consumption of the auxiliary components of the AbC (i.e., fan, pumps) during the partial load operation period, by using a fuzzy logic interference rule for determining the cooling capacity demand and then applying an optimization algorithm for determining the velocity of the auxiliary components. Within the proposed control strategy, a fast-calculation model is embedded, which predicts the chiller’s capacity under variable external conditions and partial load operation. The electrical COP is increased up to 2.2 times with respect to a baseline case, based on a basic two-step control strategy, while allowing the chiller to operate smoother, minimizing the crystallization and freezing risk.

Electrochemical Analysis of an Electrolyte-Supported Solid Oxide Cell-Based MK35x Stack during Long-Term Electrolysis Operation

The currently ongoing scale-up of high-temperature solid oxide electrolysis (SOEL) requires an understanding of the underlying dominant degradation mechanisms to enable continuous progress in increasing stack durability. In the present study, the degradation behavior of SOEL stacks of the type “MK35x” with chromium-iron-yttrium (CFY) interconnects and electrolyte-supported cells (ESC) developed at Fraunhofer IKTS was investigated. For this purpose, the initial electrochemical performance of a 10-cell stack was characterized in various operating conditions in both fuel cell and electrolysis mode. Degradation was evaluated during galvanostatic steady-state steam electrolysis operation for more than 3000 h at an oxygen side outlet temperature of 816 °C and a current density of −0.6 A cm−2 and showed an average voltage evolution rate of −0.3%/kh demonstrating high stability. Initial and final characterization at the part load operating point at −0.39 A cm−2 and 800 °C led to the determination of a positive overall degradation rate of 0.4%/kh showing a considerable impact of the operating conditions on the degradation rate. By means of electrochemical impedance spectroscopy analysis it was shown that the stack’s ohmic resistance increased whereas the polarization resistance decreased most likely due to an enhancement in LSMM’/ScSZ oxygen electrode performance.

Hybrid geothermal-fossil power cycle analysis in a Polish setting with a focus on off-design performance and CO2 emissions reductions

Growing demand for electricity due to economic development contributes to increased greenhouse gas production, especially CO2. However, emissions can be limited by enhancing the efficiency of primary energy conversion, such as integrating geothermal energy into coal-fired power plants. Therefore, this paper proposes replacing conventional feed-water heaters with geothermal preheaters to create a hybridized system. This study was based on a numerical model validated at a selected Polish power unit. The model was subsequently calibrated for off-design conditions to facilitate partial load analysis. The obtained characteristics outperformed those of the non-hybrid unit, generating over 18 MW of electric power output. Such an improvement could potentially boost the unit's net efficiency by more than 2.6 %. This enhancement is significant as power units typically operate under part load for approximately 90 % of the time, hence the need to evaluate the performance characteristics of hybridized units in those states. Furthermore, the research outlines the potential decrease in the plant's CO2 emission factor, with reductions reaching up to 6.5 % under off-design conditions. Based on a gap analysis of the existing literature, this paper's comprehensive partial load evaluation serves as a new addition to research on hybridized systems.

What’s the typical issues influencing operation performance of HVACs in super high-rise buildings? Field tests and optimization analysis

The HVACs in super high-rise buildings commonly consists of variable air volume (VAV) systems, multistage chilled and cooling water systems, primary-secondary chilled water system in chiller plant, and the chillers combination is much more complex, leading to the significantly higher energy consumption than that of normal buildings. This paper contains field tests and comparative studies on the operation performance of HVACs in two super high-rise buildings and summarized representative issues accounting for the high energy consumption. Field test results showed that the annual energy efficiency of the whole HVAC system, before being commissioned, was only 1.79 and 2.15 in two projects. The HVACs, typically VAV systems, chilled and cooling water systems, all suffered from over-supplying and energy wasting. Besides, the chillers commonly operated with a low partial load ratio (PLR), whose energy performance was much lower than expected. Aiming to dig out the typical issues, analyses were carried out from external, internal factors, and their interconnection. Furthermore, optimization methods were put forward with corrected vital indexes. Results showed that proper control strategies should maintain the VAV system, multistage chilled and cooling water system operating with significant distribution temperature differences, tiny distribution pressure drops, and high energy efficiency, thus achieving energy saving and ensuring space cooling effects. Besides, the control strategies, especially the number control of chillers, were influential during the whole cooling season to ensure the chillers operate with optimal PLR and thus achieve better energy performance.

Techno-economic and environmental feasibility study of MILD combustion in domestic utility boilers under partial load operation

This paper aims to achieve the benefits of MILD combustion, such as enhanced thermal efficiency with simultaneous NOx reduction in existing conventional domestic hot water utility boilers which encounter partial heat loads. For this purpose, a system based on waste heat recovery and flue gas recirculation (FGR) is introduced to perform techno-economic and environmental studies under continuous fluctuations of heat load demand in a typical building at various climate conditions. Technical investigations on several types of utility boilers revealed that MILD combustion could be implemented only in modulation boilers, which can serve combustion stability requirements in turndown conditions due to partial load demands. Up to 6 % fuel saving, which aligns with a commensurate decrease in CO2 emissions and a minimum 75 % reduction in NOx emissions, is foreseeable by manipulating optimal FGR for each specific boiler in various climate conditions. Finally, economic investigation demonstrated that the payback period for the proposed revamped system is less than one year for most climates.

Performance optimization of an integrated solar combined cycle for the cogeneration of electricity and fresh water

Many Arabic countries suffer water scarcity while having an energy surplus. The present work investigates and optimizes a proposed plant for the cogeneration of electricity and fresh water. This plant comprises an Integrated Solar Combined Cycle (ISCC) power plant and a Multi Stage Flash (MSF) desalination unit. The steam condenser of the ISCC acts as a brine heater of the MSF desalination unit. Thus, the waste thermal energy from the ISCC is used to desalinate the seawater. The proposed cycle produces fresh water during its part load operating conditions. Simulation programs were developed and validated using real data from the ISCC power plant in Kuraymat-Egypt. The present results show that the proposed plant is very promising for the cogeneration of electricity and fresh water. An optimum top brine temperature (To) of 80 °C was estimated based on the amount of fuel required to desalinate the seawater. The proposed plant produces up to 1140 m3/h of fresh water under the typical operating conditions of Hurghada-Egypt, 50 % of the rated output power, and To = 80 °C. It saves up to 84.7 % of the fuel required for desalination compared to a standalone MSF desalination unit that operates under the same conditions.

Analysis of fouling in domestic boilers fueled with non-woody biomass

The greater sustainability of agricultural and industrial wastes (non-woody biomass) when compared to the widely used high-quality forestry biomass encourages their usage in small-scale thermal devices (residential and domestic applications). However, the typical significant ash and alkali-compounds content of agro-industrial wastes may form undesirable deposits over the heat exchange surfaces that could require additional cleaning systems (thus increasing the cost and complexity of the boilers). This work analyses the fouling tendency of exhausted olive cake and almond shells when used in a biomass boiler (with a nominal thermal capacity of 55 kW) at partial and nominal load, trying to highlight the main mechanism leading to the deposits growth depending on the fuel properties and load. Inertial impaction of coarse fly ash governs deposition at partial load for almond shells, this is attributed to the low K, Cl and S content and the lower bed temperature reached (431 °C vs 528 °C at nominal load). Among the deposition mechanisms associated to K-salts condensates, direct condensation on the incipient layers dominates over thermophoresis of submicron particles at nominal load. Although no additional maintenance procedures seem to be required when using almond shells, it is highly recommended to remove K/Cl components from exhausted olive cake through prior water leaching or by using additives to retain these species in the bottom ash.

Applicability evaluation of a temperature humidity index-controlled ventilation system in livestock using a building energy simulation model

In general, for livestock building, a simple system for controlling the HVAC system that is based only on the indoor air temperature is applied. This study evaluated the applicability of temperature humidity index (THI)-based ventilation systems installed in livestock buildings using a building energy simulation model. Based on the monitoring data of an actual pig house, the integrated predictive performance of the indoor temperature and humidity of the building energy simulation model was calibrated. Additionally, the indoor environment of the existing temperature-based control and THI-controlled ventilation systems was compared through simulations. The average THI and the section exceeding the THI threshold were both lower compared to the existing system. This was because the part load ratio of the fans of the THI-controlled ventilation system was higher than that of the existing system, indicating better ventilation. These results suggest that THI-based ventilation systems can overcome the limitations of existing temperature-based systems and effectively improve the thermal stress management of livestock buildings. This study portrays that the indoor environment can be improved by simply changing control variables, while presenting the field applicability and research value of THI-based control systems in the management of livestock environments.

An optimization of efficient combined cycle power generation system for fusion power reactor

Fusion power plants can meet the energy demands of the world. The high thermal performance of a power generation system is still a challenge and one of the developing trends with a fusion reactor due to the high outlet temperature. A combined cycle system has been optimized to work at high outlet temperature and pressure of fusion power reactor with thermal power of 2500 MWth and its input parameter has been optimized along with impact of partial load to achieve high thermal performance very first time. The combined cycle based on the concept of two stages of expansion in the closed-Brayton-cycle and two reheaters in the Rankine cycle named Schematic-IV with the higher thermal performance of 58.19% as compared to Schematic-I, II and III has been proposed for a fusion power reactor. The increase in more than one expansion stage and reheat stage is not economical because it increased the thermal performance by about half of performance as compared to one. The effect of isentropic efficiency of compressor and heat rate have been investigated to validate the thermodynamic model and calculations. The optimized thermal performance of the combined cycle is 58.19% at a feed water mass flow rate of 592 kgs−1 and steam outlet temperature of 277 °C in the combined cycle. The heat rate decreased and thermal performance increased with the mass flow rate verifying that the thermodynamic model is correct. The thermodynamic analysis of the combined cycle-based nuclear reactor provides insights for better system thermal performance and reveals the effect of key parameters on the system performance.

Optimization of lubrication characteristics of wind turbine’s transmission system based on Newton Raphson method

To improve the reliability and lifespan of wind turbines, this paper takes the two-stage fixed shaft gearbox experimental platform of wind turbines as the research object. Based on Hertz contact theory, the oil film pressure and thickness in the contact area are solved by combining the equations of elastohydrodynamic lubrication and the Newton Raphson method; And the lubrication characteristics of the transmission system were analyzed to verify the correctness of the method; At the same time, in response to partial load phenomenon caused by system coupling deformation, genetic algorithm was selected to modify the gear teeth. The results show that the max unit load on the tooth face and the maximum stress of tooth root decreased by up to 26.48 % and up to 20.35 % respectively after modification which can improve the uneven distribution of oil film and the lubrication performance of the tooth surface.

Subtuberosity anterior closing wedge osteotomy to correct the increased posterior slope of the tibial plateau

Reduction of increased reclination of the tibial plateau (posterior slope) to improve the anterior stability of the knee joint. Increased posterior reclination of the tibial plateau greater than 12° in combination with recurrent instability after anterior cruciate ligament (ACL) reconstruction. Hyperextension of more than 15° (relative). Anterior skin incision approximately 8-10 cm above the tibial tuberosity. Insertion of two converging guidewires directly below the patellar tendon ending obliquely in the area of the posterior cruciate ligament (PCL) insertion. Control of the wire position with the image intensifier core. Oscillating saw osteotomy. Removal of the wedge and closure of the osteotomy. Osteosynthesis with interfragmentary screw and medial angle-stable plate. Partial load with 10-20 kg for 2 weeks, then step by step increase in load. Mobility: free. To date we have operated on 36patients with recurrent instability after ACL reconstruction (20men, 16women, average age 34.4 years) in the manner described in this article. In 25cases, enlarged bone tunnels were filled with allogeneic bone at the same time. The posterior slope of the tibial plateau could be reduced from an average of 14.5° to8.8°. In 28cases another ACL reconstruction was performed after an interval of 4-12months. The Lysholm score significantly increased from 76.3 points to 89.2 points.

Flow Field Analysis of Francis Turbine Draft Tube using POD at Design and Part Load Operating Conditions

Flow Field Analysis of Francis Turbine Draft Tube using POD at Design and Part Load Operating Conditions

Análisis del efecto de cavitación en una turbina hidráulica de tipo francis en condiciones de operación de la C.H. Chaglla

The performance of a hydropower plant depends on several parameters, i.e., the turbine type used, plant design, turbine classification, plant head, flow rate. All turbines are used for their best operation under the best level of efficiency with optimal values for their nominal head, water flow and turbine speed. However, under these conditions, the cavitation phenomenon occurs in the hydraulic turbine. It is only for the reason that the dynamic attribute of liquid flow becomes unstable with high pressure fluctuations. Cavitation is an unavoidable and unavoidable problem in hydraulic turbines. Cavitation destroys the performance of the turbines. When turbines operate at partial loads rather than their highest efficiency point, they experience cavitation to a greater extent. Francis turbines are designed to be operated in various conditions. The research objective was to analyze the cavitation occurrence in a Francis type turbine and how to minimize it, as a result a report was obtained on damages caused by the bubbles implosion on the surfaces of the turbine, especially in the impeller, at the exit waist edge of the intermediate zone in UG-1, present areas with cavitation wear between blade and blade 1-2. 3-4. 8-9. 11-12. 13-1. According to the inspection, the evolution of the cavitated areas is controlled and a slight detachment of material is observed, which does not compromise the useful life of the impeller. Likewise, for the impeller blades in the UG-2, cavitation wear areas occur between blade and blade 3-4. 9-10. 11-12; It is also controlled and slight material release is observed which does not compromise the useful life of the impeller. Regarding the wear of the blades in the impeller, we can indicate that in the UG2 there is 6.47 % accumulated wear, while in the UG1 there is 5.47 %. In the case of the guide vanes, the wear clearance with the highest incidence is in the UG2 with 0.692 mm of accumulated wear, while in the UG1 with 0.2595 mm.

Optimal Operation and Market Integration of a Hybrid Farm with Green Hydrogen and Energy Storage: A Stochastic Approach Considering Wind and Electricity Price Uncertainties

In recent years, growing interest has emerged in investigating the integration of energy storage and green hydrogen production systems with renewable energy generators. These integrated systems address uncertainties related to renewable resource availability and electricity prices, mitigating profit loss caused by forecasting errors. This paper focuses on the operation of a hybrid farm (HF), combining an alkaline electrolyzer (AEL) and a battery energy storage system (BESS) with a wind turbine to form a comprehensive HF. The HF operates in both hydrogen and day-ahead electricity markets. A linear mathematical model is proposed to optimize energy management, considering electrolyzer operation at partial loads and accounting for degradation costs while maintaining a straightforward formulation for power system optimization. Day-ahead market scheduling and real-time operation are formulated as a progressive mixed-integer linear program (MILP), extended to address uncertainties in wind speed and electricity prices through a two-stage stochastic optimization model. A bootstrap sampling strategy is introduced to enhance the stochastic model’s performance using the same sampled data. Results demonstrate how the strategies outperform traditional Monte Carlo and deterministic approaches in handling uncertainties, increasing profits up to 4% per year. Additionally, a simulation framework has been developed for validating this approach and conducting different case studies.

Interaction of a Ventricular Assist Device With Patient-Specific Cardiovascular Systems: In-Silico Study With Bidirectional Coupling.

Ventricular assist devices (VADs) are used to assist the heart function of patients with advanced heart failure. Computational fluid dynamics in VADs are widely applied in the development and optimization, for example, to evaluate blood damage. For these simulations, the pulsating operating conditions, in which the VAD operates, should be included accurately. Therefore, this study aims to evaluate the flow in a VAD by interacting with patient-specific cardiovascular systems of heart failure patients. A numeric method will be presented, which includes a patient-specific cardiovascular system model that is bidirectionally coupled with a three-dimensional (3D) flow simulation of the HeartMate 3. The cardiovascular system is represented by a lumped parameter model. Three heart failure patients are considered, based on clinical data from end-stage heart failure patients. Various parameters of the cardiovascular system and the VAD are analyzed, for example, flow rates, pressures, VAD heads, and efficiencies. A further important parameter is the blood damage potential of the VAD, which varies significantly among different patients. Moreover, the predicted blood damage fluctuates within a single heartbeat. The increase in blood damage is evaluated based on the operating conditions. Both, overload and especially partial load conditions during the pulsating operation result in elevated blood damage.

Hybrid hydropeaking mitigation at storage hydropower plants combining compensation basins with battery energy storage systems (BESS)

Storage hydropower has the ability of flexibly generating electricity to match fluctuating power demand. However, rapid changes of water discharge in river reaches downstream of storage hydropower plants, referred to as hydropeaking, result from this intermittent production mode and may have potentially damaging effects on a river’s ecosystem. The construction of compensation basins usually embedded along the waterway between the powerhouse and the receiving water represents the state-of-the-art for hydropeaking mitigation. In recent studies, the application of battery energy storage systems (BESS) as an alternative mitigation option has been proposed. Instead of buffering the turbine discharge in a basin, electrical energy is stored in and retrieved from a BESS to meet short-term demand requirements, while the hydraulic machinery is ramping up and down according to environmental discharge constraints. The technical and economic feasibility of BESS compared to compensation basins is evaluated based on a newly introduced first-order assessment approach. If environmental discharge constraints are to be strictly followed, the exclusive application of BESS is deemed not feasible, due to the limited operation range of Francis and – to a lesser extent – Pelton turbines, especially for low partial loads. Therefore, hybrid systems combining a BESS with a smaller compensation basin are investigated as existing research focused either on a basin- or a BESS-only approach. Based on production time series of three case study hydropower plants, the sizes of basins and BESS are estimated. Basin sizes can be reduced with a hybrid system by 51% to 96% compared to a basin-only approach, with BESS energy capacities ranging approximately from 10 to 100 MWh. The cost efficiency of hybrid systems is strongly influenced by two factors, namely the future battery costs as well as the overall unit costs for the compensating basin. While the former are subject to considerable uncertainty especially as BESS reach their end-of-life much earlier compared to hydraulic structures and therefore require regular replacements after 10 to 15 years, the latter are highly site-specific as they include land availability and acquisition costs. The case studies show that a hybrid system of basin and BESS may represent a cost-competitive alternative to a basin-only approach when the latter’s investment costs are high or space is limited, even if additional revenue streams including ancillary services and energy arbitrage are not considered.

Investigation of the part load behavior for the R-Graz cycle with pure oxygen and hydrogen fuel

ABSTRACT In order to address the future demands for large-scale and efficient hydrogen storage and utilization, while considering the need of power dispatch and peak shaving, in this article, based on the cycle part load models and the conceptual design of the main components, the R-Graz cycle design parameters have been reoptimized, and subsequently, an analysis of the part load behavior and performance characteristics was conducted. The results show that within the load range discussed in this paper, i.e., 50% to 100%, the components of the R-Graz cycle are well matched, enabling high-efficiency operation. The net cycle efficiencies are 64.4% and 71.0% for 50% and 100% load respectively. Within this wide load range, the performance of the R-Graz cycle surpasses that of the Graz cycle consistently. When the load decreases, key parameters of the R-Graz cycle’s main components, such as rotational speed, pressure, temperature, mass flow rate and split ratio also decrease. Performance analysis under varying load conditions reveals that the maximum temperature in the cycle is a critical parameter affecting the efficiency of the R-Graz cycle. The research findings of this article lay the foundation for the practical application of the R-Graz cycle.

Evaluating the impact of off-design CHP performance on the optimal sizing and dispatch of hybrid renewable-CHP distributed energy resources

The maturation of distributed energy resources (DER) has prompted the exploration of their deployment in commercial building applications due to their potential to supply energy at lower costs and emissions rates compared to centralized generation. While several software tools exist for evaluating the techno-economic potential of integrated renewable energy and combined heat and power (CHP) systems for distributed generation applications, many suffer from poor accuracy in capturing off-design (part load and changes in ambient air temperature and pressure) performance characteristics of microturbines, combustion turbines, or internal combustion engines. Thus, this paper presents a methodology for integrating these off-design characteristics in the mixed-integer linear program within REopt, a hybrid DER screening tool. The economic impact of the CHP off-design performance is observed through several application studies of various hybrid system configurations in different climates. Each study indicates how CHP off-design performance influences optimal sizing and dispatch decisions and therefore overall system economic value. We observe through case studies that modeling without the off-design effects, depending on the CHP prime mover and site, can result in Net Present Value predictions of hybrid systems that can be overoptimistic in frequently hot climates (up to 52 %), too conservative in frequently cold climates (up to 11 %), or unaffected (＋/−1 %) in temperate climates. Cases also highlight several advantages of hybrid systems relative to non-hybrid systems such as total economic value and the systems’ ability to mitigate potentially negative consequences attributed to off-design performance.