Browsing by Author "Jahn, Wolfram"

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    A bidirectional solar thermoelectric generator combining heat storage for daytime and nighttime power generation
    (2023) Montero, Francisco J.; Lamba, Ravita; Ortega, Alfonso; Jahn, Wolfram; Chen, Wei-Hsin; Guzman, Amador M.
    A solar thermoelectric generator (STEG) is a promising technology for harvesting solar energy for standalone applications. However, the STEG cannot generate electricity during nighttime due to unavailability of solar energy. The efficiency of thermoelectric generator (TEG) is also low that limits its application areas. This low efficiency can be improved by partially utilizing the waste heat from its cold side using phase change materials (PCMs). Further, the STEG systems operating during day and nighttime are not proposed so far. Therefore, an experimental test rig of a bidirectional (operative in day and night both) STEG coupled with latent heat storage and cooling system (LHSCS) has been developed in this paper. The LHSCS acts as a sink by storing waste heat from the TEG cold side in a phase change material during the daytime and regulates its temperature effectively. During nighttime, LHSCS acts as a heat source for TEG power generation. This proposed bidirectional model aims to provide non-intermittent electricity generation for 24 h. An experimental setup was tested under laboratory conditions and adjusted using a numerical model previously developed in COMSOL Multiphysics software. Once both models are mutually adjusted and verified, the proven numerical model is used to simulate the prototype in the environmental conditions of the Atacama Desert in Chile. The transient effects of solar radiation, ambient temperature and wind speed of the selected location on the hot and cold side temperatures, voltage, power output and efficiency of STEG have been analyzed. A maximum temperature difference of 120 degrees C is obtained between the TEG hot and cold sides. The experimental results showed TEG efficiency of 5%. The system generated average annual electricity of 5735 Wh. The STEG generated around 0.6 % of the total electricity during the night in the Atacama Desert location. The levelized cost of energy and storage have also been calculated for the proposed system and compared with PV and STEG systems. The LCOE and LCOS of the proposed system are 8850 and 566 USD/MWh respectively. The proposed configuration may provide a reference study for design and development of an efficient and cost-effective STEG coupled LHSCS system.
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    A novel 24-h day-night operational solar thermoelectric generator using phase change materials
    (2021) Montero, Francisco J.; Lamba, Ravita; Ortega, Alfonso; Jahn, Wolfram; Guzman, Amador M.
    To improve the energy matrix using solar energy, the intermittency and variation of the solar resource must be resolved for effective implementation of solar operated electricity generation systems. Solar thermoelectric generators (STEG) can be used for electricity generation in non-grid and grid connected applications. However, to use the STEG systems extensively, the limitations of lower conversion efficiency of around 7%, effective passive thermal management of the thermoelectric generator (TEG) and storage of residual heat of the thermoelectric generator need to be solved. In this study, a conceptual theoretical model of latent heat storage and cooling system (LHSCS) is proposed for the effective thermal management and enhanced electricity generation from the solar thermoelectric generator. The numerical model of the proposed system has been developed in COMSOL Multiphysics software for the climatic conditions of the Atacama Desert, Chile. The effect of phase change material (PCM) volume, heat sink and container geometry on the performance of the system has been studied. It is found that the desert locations are the best geographical locations for operating the solar thermoelectric generator coupled latent heat storage and cooling system due to higher solar radiation resource and favorable environmental conditions. The results showed that with 6 kg of phase change material, a temperature difference of 120 degrees C has been achieved between the hot and cold sides of the thermoelectric generator without using an active cooling system and the stored residual heat in phase change material generated 0.6% more electricity during the off-sunshine hours. Further, it has been found that the natural convection has a relevant impact on the melting of the phase change material and must be considered in the designing of a latent heat container. This proposed numerical model can be used to demonstrate the solar thermoelectric generator coupled latent heat storage and cooling system for any geographical location. (c) 2021 Elsevier Ltd. All rights reserved.
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    A Study on the Reliability of Modeling of Thermocouple Response and Sprinkler Activation during Compartment Fires
    (2022) Krol, Aleksander; Jahn, Wolfram; Krajewski, Grzegorz; Krol, Malgorzata; Wegrzynski, Wojciech
    Real and numerical fire experiments involve temperature measurements with thermocouples, and thus some considerations on numerical modeling of this process are presented and a new approach to thermocouple modeling is introduced. Using ANSYS Fluent software a well-recognized analytical thermocouple model was implemented in each cell of the computational domain, which allows for determination of thermocouple responses as a continuous field. Similarly, sprinklers are key elements of fire-protection systems. Sprinklers activation is one of the breakthrough moments during the course of a compartment fire. Therefore, assumptions on sprinkler activation time are of crucial importance when designing a fire safety system. Just as for thermocouple modeling, virtual sprinklers based on a commonly admitted response time index (RTI) model were placed in all cells. The proposed approach provides data on sprinklers activation or thermocouple response for the whole domain instead of retrieving data point by point only for predefined locations. In this study, experimental data available in the literature were used for the validation of the proposed approach. In addition, the results were compared with those obtained with the commonly used Fire Dynamic Simulator (FDS) software. The outcomes might be of a significant importance for practitioners, who deal with fire experiments and fire protection. Furthermore, some issues on accurate modeling of fire gases flow are discussed extensively. It was found that commonly applied k-epsilon and k-omega turbulence models might fail in the case of modeling of fire plumes in confined spaces.
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    Assessment of the Performance of FireFOAM in Simulating a Real-Scale Fire Scenario Using High Resolution Data
    (2023) Jahn, Wolfram; Zamorano, Rafael; Calderon, Ignacio; Claren, Raimundo; Molina, Benjamin
    An assessment of the performance of FireFOAM in simulating a large-scale compartment fire scenario is presented in this study, using the Edinburgh Tall Building Fire Test I (2017) as the basis for evaluation. Different mesh geometries and sizes are tested, and both theory-based and experiment-based validation approaches are employed. The theory-based validation revealed that the implemented finite volumes method is generally conservative, but unaccounted deviations of up to 20% for the heat release rate were observed due to errors in numerically modelling subgrid phenomena, particularly with tetrahedral meshes. In the experiment-based validation, the simulated data showed good agreement with experimental measurements for flow patterns inside the compartment, neutral plane height, and temperatures outside the ceiling jet. However, there were relatively large errors in incident radiation in the hot gas zone, thermal boundary layer transient temperatures, and compartment inflow/outflow velocities. Systematic errors were attributed to deficient heat transfer boundary conditions and under-estimated air entrainment. The study also identified ways to improve run-time efficiency by implementing parallel processing or reducing solid angles in FVDOM, although using large meshes (30 cm and 40 cm cell size) resulted in faster run-times at the cost of accuracy.
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    Emisiones de motores mosquito y sus impactos
    (Centro de Desarrollo Urbano Sustentable, 2023) Hurtubia, Ricardo; Jahn, Wolfram; Sánchez, Joaquín; CEDEUS (Chile)
    Los motores mosquito, generalmente instalados en bicicletas, son cada vez más numerosos en nuestras ciudades por ser una alternativa de movilidad de muy bajo costo y muy conveniente para quienes trabajan haciendo reparto a domicilio de comida o paquetes pequeños. Estos motores son, además, contaminantes y ruidosos, pero hasta el momento hay poca información respecto a qué tan contaminantes son, comparados con otros vehículos relativamente similares. Esto facilita la desinformación de quienes compran o utilizan estos motores y del público en general, dificultando además la labor fiscalizadora y la implementación de programas para el recambio tecnológico.
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    Energy and exergy analysis of a bidirectional solar thermoelectric generator combining thermal energy storage
    (2023) Montero, Francisco J.; Lamba, Ravita; Singh, Sarveshwar; Jahn, Wolfram; Chen, Wei-Hsin
    In this paper, energy and exergy analysis of a bidirectional solar thermoelectric generator (STEG) coupled to a latent heat storage and cooling system (LHSCS) has been carried out. The effect of various parameters of LHSCS on energy and exergy efficiencies of STEG have been analysed under climatic conditions of Chile's Atacama Desert. It is found that the most relevant design parameter to improve the energy and exergy efficiencies of the thermoelectric generator (TEG) is the container insulation, followed by heat sink at the TEG hot side, fin thickness and the aspect ratio of the container. The results showed that an optimally designed insulation container can improve the energy and exergy efficiencies of LHSCS by 30% and 200%, respectively, and the TEG conversion efficiency by 30% during nighttime. Further, inclusion of heat sink at TEG hot side during reverse operation of TEG at night can improve the TEG efficiency by 20%. The optimal fin thickness can improve the TEG conversion efficiency by 20% during the night and LHSCS energy and exergy efficiencies by 30% and 23%, respectively. The container geometry should have higher aspect ratios. This study may help in optimal design of LHSCS for solar energy conversion applications in the desert locations.
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    Fire Inside the Cavity of a Non-flammable Facade: Step-by-Step Development of Multiphysics Computer Simulations
    (SPRINGER, 2024) Khoo, Benjamin; Jahn, Wolfram; Bonner, Matthew; Kotsovinos, Panagiotis; Rein, Guillermo
    The cavities in a building facade can significantly increase the fire hazard, acting as pathways and accelerators for the vertical spread of flames and smoke, even in non-combustible facades. Ensuring fire safety during facade design requires a thorough understanding of how cavity geometry influences fire dynamics. However, established theories for this phenomenon are lacking. Therefore, in this study, we use the computational fluid dynamics code FireFOAM to develop step-by-step multiphysics simulations incorporating fluid mechanics, heat transfer, buoyancy, and combustion phenomena to investigate the non-linear behaviour in narrow vertical cavities. Four scenarios of increasing complexity are modelled and validated against experimental data from the literature. The simulations predict flow velocities and convective heat fluxes within 20% error and buoyancy-driven flow, radiative heat flux, and flame height predictions within 30% error across a range of cavity widths. The study also highlights the limitations of the models, offering insights for future refinement. The results demonstrate that computer simulations can reliably be used to study critical phenomena of cavity fires and, with future improvements, predict fire behaviour across various facade designs and conditions.
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    Flammability and the heat of combustion of natural fuels: a review
    (Taylor & Francis Group, LLC, 2012) Rivera Agüero, Juan De Dios; Davies, G. Matt; Jahn, Wolfram
    Heat of combustion (HoC) is a key characteristic of fuels when analyzing and modeling wildfire scenarios. Despite significant differences in the structure of fuels from different environments, HoC is frequently considered a constant. This article briefly reviews methods used to describe natural fuels and the various different definitions of HoC. We also summarize measured values of HoC and elemental analyses of 238 plant genera reported in 28 papers since 1973. A statistical analysis of these data provided mean values and standard deviations of HoC for fuels according to six broad plant functional groups. Permutational Multivariate Analysis of Variance (PERMANOVA) demonstrated significant differences in the HoC with ground fuels and softwoods having particularly high values. Net heat of combustion was calculated for four fuel groups and the tabulated data may help to improve wildfire modeling and highlights fuels where further measurements of HoC are required.
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    Influence of Permeability on the Rate of Fire Spread over Natural and Artificial Pinus radiata Forest Litter
    (2019) Figueroa, Sofia; de Dios Rivera, Juan; Jahn, Wolfram
    The rate of fire spread is of paramount interest in order to prevent and control wildland fires. It has been extensively studied under laboratory conditions, but little research has been made regarding the differences between artificially constructed and natural litter. By using a specially designed sampling method, in this study differences in the rate of spread between natural and constructed pine needle litter samples are assessed using a combustion tunnel with controlled wind speed. A total of 68 experiments were conducted with four levels of wind speed and six levels of permeability (five for constructed samples and one for the natural sample). It was found that permeability has a strong influence on the rate of spread when wind speeds are high (2.1-3.6ms-1). It was also found that while wind speed and permeability have a strong influence on the rate of spread, their combined effect is not significant. Thus, both variables may be treated independently from one another. Lastly, it was found that constructed samples with permeability of 1x10-7m2 behave similarly to natural samples in terms of the rate of spread. This suggests that it is possible to use constructed samples in the laboratory by matching their permeability, and, thus, obtain realistic values for the rate of fire spread. This could be of use for medium and large scale laboratory tests, where it is not possible to use undisturbed litter samples.
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    Performance assessment of thermoelectric self-cooling systems for electronic devices
    (2021) Di Capua, M. H.; Jahn, Wolfram
    Due to the development of high-performance electronic devices, there exists a continuous need for effective and efficient cooling systems capable of removing large amounts of heat. A thermoelectric self-cooling system with forced air cooling based on a finned plate heat sink (case-a) and a water cooling based on a microchannel heat sink (case-b) is evaluated through a thermodynamic model. This article investigates the effect of the thermo-element geometry concerning its length and cross-section on the cooling capability of a self-cooling system for a wide range of heat fluxes (10 W to 200 W). Also, the effect of the efficiency related to the DC/DC converter between the thermoelectric generator and pumping devices is included in the analyses. The results show that shorter thermoelements with bigger cross-sections contribute to lower global thermal resistance and lower overheating of the electronic device. However, through the aspect ratio of the thermoelements (delta = cross-section/length), a practical limit was found defining how much shorter the thermoelement must be to satisfy a temperature constraint through the self-cooling condition for different fill factors, which corresponds to delta <= 20. Beyond this limit, the performance of the self-cooling system is affected by the low conversion of heat into electricity used to run the pumping devices. The results also demonstrate for delta = 20 and fill factor equal to 0.95, that the temperature constraint (373 K) is satisfied for a heat flux equal to 124 W (case-a) and 173 W (case-b) when the efficiency of the DC/DC converter is 10%. Instead, a DC/DC converter with the highest efficiency (95%) rises the heat flux to 161 W (case-a) and 200 W (case-b), satisfying the constraint. Finally, this article summarizes the maximum heat flux for different thermoelements aspect ratios (delta <= 20) and fill factors where the self-cooling system for both cases satisfies a temperature constraint.
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    Powerlines and Wildfires: Overview, Perspectives, and Climate Change: Could There Be More Electricity Blackouts in the Future?
    (2022) Jahn, Wolfram; Urban, James L.; Rein, Guillermo
    Overhead powerlines cross extensive areas of forest and grasslands, and these areas are often flammable and can burn. Wildfire is a natural phenomenon important to many ecosystems around the globe, but also capable of considerable damage to people and communities. As a result of human activity in natural spaces, people have altered wildfire regimes over time, and wildfires have become a threat to people, to their property, and infrastructure. For example, Figure 1 shows the thousands of wildfires detected by satellite around the globe during seven days of early September 2021; the image gives an indication of the planetary magnitude of the phenomenon. Powerlines represent both a way in which human activity has changed the natural wildfire regimes (i.e., an ignition source), and vital infrastructure vulnerable to fire.
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    Strategies for fire-fighting in underventilated compartments: Reducing the likelihood and severity of a potential backdraught
    (2025) Majdalani H. , Agustín; Carvel, R.; Calderón Espinoza, Ignacio Alonso; Jahn, Wolfram
    Fires in underventilated compartments remain an unresolved problem for fire brigades. While some brigades have specific guidance in place regarding procedures for approaching and fighting such fires, this guidance is, for the most part, based on anecdotal evidence, having been instigated following historical incidents involving fire-fighter injuries or fatalities. As fire-fighters open a door or window to an under-ventilated compartment fire, there is a risk of a sudden fire development that may come in the form of a rapid flare up or backdraught. A programme of reduced scale fire experiments was designed to obtain deeper insight into this phenomenon. The results of these experiments are presented showing the conditions under which changes in the ventilation conditions of an underventilated compartment were beneficial for firefighting activities and under which conditions this was detrimental. This paper provides further scientific understanding of the fire dynamics in underventilated compartments when changes in ventilation conditions occur. The use of ventilation tactics, preceded by strict safety precautions and comprehensive tactical considerations, appears to be a practical solution that reduces the likelihood and severity of a potential backdraught. The results presented could be used to develop simple guidance, which may be used in fire brigade practice, to decide when and how to intervene in this kind of fires.