The output power and conversion efficiency of the thermoelectric generator (TEG) are closely related to not only the materials properties but also the geometric structure. This paper developed a ...multi-physics, steady-state, and three-dimensional numerical TEG model to investigate the TEG performance, and then the model is compared with the classical thermal resistance model. Bismuth-telluride are used as p- and n-type materials. The comparison reveals that the assumption of constant material properties leads to underestimated inner electrical resistance, and overestimated thermal conductance and Seebeck coefficient, so that the thermal resistance model predicts unrealistically high performance than the present model. The results also indicate that when heat losses exist between the TEG and the ambient, although the output power is slightly elevated, the conversion efficiency is significantly reduced, hence, improvement of the heat insulation effect is critically important for high-temperature TEGs. Furthermore, the TEG geometry also affects its performance significantly: usage of thin ceramic plates increases the junction temperature difference, and hence enhances the TEG performance; there are two optimal leg lengths which correspond to the maximum output power and the maximum conversion efficiency, respectively; when heat losses are not ignorable, a large semiconductor cross-sectional area remarkably reduces the ratio of the heat liberated to the ambient to the heat absorbed from the high-temperature heat source, and hence improves the conversion efficiency.
•Heat losses in smouldering systems promote airflow near walls: non-uniform air flux.•Non-uniform air flux curved the smouldering front in the direction of air flow.•The curved smouldering front ...caused super-adiabatic conditions near the centreline.•The cooling front was also curved because of non-uniform air flux.•Non-uniform air flux inhibited cooling near the centreline.
Smouldering combustion is emerging as a valuable tool for energy conversion purposes. However, the effects of radial/lateral heat losses, while critical to its viability, are not well understood. It is known that heat losses weaken the smouldering reaction near the walls. It is less known that these losses generate non-uniform air flux across the system cross-section, potentially changing conversion rates and quenching limits. This study integrated: (i) highly instrumented smouldering experiments across numerous scales, (ii) a novel method of estimating non-uniform air flux in the experiments, (iii) analytical modelling to predict non-uniform cooling, and (iv) energy balance calculations to quantify the non-uniform heat of smouldering. Altogether, this work demonstrates that heat loss-induced non-uniform air flux is significant, affecting key smouldering propagation and cooling characteristics. The uniform air flux injected at the base became redistributed with a ~50% decrease at the centreline and a ~50% increase at the wall. This was shown to cause a concave (in the direction of air flow) smouldering front and a concave cooling front. The former was shown to cause radial heat transfer inwards, leading to super-adiabatic heating towards the centre of the reactor. The latter was shown to inhibit cooling along the centreline, which progressed ~40% slower than expected during propagation. Altogether, the multiple and integrated analyses used reveal the magnitude and significance of heat losses in smouldering systems. This insight is valuable to better harness smouldering for engineering applications.
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We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of ...bluff body shape on the blow-off limit was investigated with a detailed H2/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.
•Effect of bluff body shape on blow-off limit of a micro-combustor was investigated.•Blowout occurs due to flame stretching for triangular and semicircular bluff bodies.•Triangular bluff body has a lower blow-off limit due to stronger flame stretching.•Heat losses have a negligible effect on the difference of blow-off limit.
The motivation of this study is to explore the changes of heat losses of people who undertake moderate activities and its effect on thermal sensation. Based on heat transfer equations and ...experimental data, heat losses have been calculated. The results have shown that in all conditions, the total ratio of latent heat loss (LHL) and sensible heat loss (SHL) is almost constant ranging from 0.9 to 0.95. With increase of LHL or decrease of SHL, people's mean thermal sensation vote (TSV) will increase regardless of whether they are in thermal balance. Skin wettedness is also related to TSV. Further study has discovered that ratios of LHL (RLHL) and SHL (RSHL) are correlated to TSV separately. By polynomial regression, two predicting equations have been fitted based on RSHL and RLHL, and they are essentially coincident with the sum of RSHL and RLHL being 0.93. The validity of predicting equations has been verified by using independent experimental data. Either of the equations can be used to predict TSV under moderate activities. Different from the current heat balance theory in thermal comfort, attention has been put on the change of ratios of heat losses in this paper, which provides a new perspective to understand thermal comfort under higher level of activities.
•This paper analyzes heat losses of people at moderate activities.•The changes in latent heat loss and sensible heat loss has impact on thermal sensation.•The increase of latent heat loss and decrease of sensible heat loss enhance thermal sensation.•Equations are proposed to predicting thermal sensation at moderate activities.•Skin wettedness is found to be linearly related to thermal sensation vote.
•Quantitative ITT methodology to obtain the thermal bridge heat flow rate in buildings presented.•Methodology suitable for determining Ψ-values of any existing building envelope.•Excellent agreement ...found while testing the methodology in the hot box device.•Relative deviation in Ψ-values from ITT and from hot box measurements varied from −5% to −36%.
A key aspect in assessing the thermal standard of building envelopes is the quantification of the heat loss though thermal bridging, which can be expressed in terms of the linear thermal transmittance Ψ. Values of Ψ may be obtained from tabulated values for standard building details, from numerical modelling or from measurement. Where the internal structure of the building envelope is unknown, which is very often the case, measurement is the only option. This study shows how the infrared thermography technique (ITT) can be used as a non-invasive and easy-to-use method to provide quantitative measures of the actual thermal bridging performance. The novelty of this approach includes evaluation of the actual heat flow rate caused by thermal bridge qTB and Ψ-value by means of the ITT solely, without any supporting methods. Another important aspect of the methodology is that it accounts for the correlation between the surface temperature and the convective and radiative heat transfer coefficients. Values for these coefficients are assessed for the whole range of the surface temperatures recorded on the thermogram resulting in improve accuracy. The qTB and Ψ-value calculated using the presented methodology fully mirrors the real thermal performance of the thermal bridge. The methodology has been tested under laboratory conditions in a steady state in a hot box with excellent agreement.
Large eddy simulation results are presented for a model gas turbine combustion chamber, which is operated with a premixed and preheated methane/air mixture. The off-center position of the high axial ...momentum confined jet burner causes a strong outer recirculation, which stabilizes the flame. Turbulent combustion is modeled by the premixed flamelet generated manifolds (PFGM) technique, which is combined with the artificial thickened flame (ATF) approach. The influence of different heat loss modeling strategies on flame propagation and structure is investigated. Besides the established method of using burner-stabilized flames as basis for the non-adiabatic tabulation, an alternative approach based on freely propagating flames with heat loss inclusion by scaling of the energy equation source term is presented. Different grid resolutions are applied to study the impact of cell size and filter width on the results, the effect of subfilter modeling is also examined. The simulation setup and the modeling approach are validated by comparison of computed statistics against measurements. A good overall agreement between simulation and experiment is observed. However, the length of the flame was slightly under-predicted; it is shown that a simple method for consideration of strain effects on the flame has the potential to improve the predictions here. The effect of heat loss on the combustion process is then characterized further based on probability density functions obtained from the simulation results.
•Design of novel solar receiver for the Dish- Micro gas turbine (MGT) system.•High temperature solar receiver integrated with short term thermal energy storage.•Designed receiver has high inertia to ...reduce the effect of solar flux fluctuations.•Receiver design is based on the 1D thermal model.•Optimum design point of the receiver has been identified.
Small-scale concentrated solar power plants equipped with Micro Gas Turbines (MGTs) could be an effective way for power production in the off-grid areas. One of the most relevant concerns for such no fuel-assisted systems is the solar radiation fluctuations, which reduce system performance and, in the worst cases, can damage the MGT. To overcome this issue, the solar receiver should have sufficient thermal inertia averaging the thermal power inputted in the MGT. In the present study, the design of a novel high-temperature cavity solar receiver is presented for the application in the Dish-MGT system. The receiver has been integrated with a PCM for the short-term thermal energy storage (15–30 min). The design process was based on the main parameters of the solar dish and MGT. The thermal model enables the variation of different geometrical factors and input parameters to identify the optimum receiver design. Moreover, it can be used for the evaluation of heat losses from the cavity surface and temperature distributions in the PCM. A 1D steady-state model has been developed for the heat loss calculation from the cavity and corresponding cavity surface temperature, as well as for the evaluation of number and dimensions of the MGT working fluid tubes. Furthermore, the PCM melting and the transient temperature distribution inside the PCM domain have been determined analytically with the help of a 1D thermal model. The initial design results showed that solar receiver integrated with the high-temperature PCM is a promising option for the stable outlet temperature for MGT.
•New model for temperature estimation in metal casting ladles.•Comparison between physics-based and ANN approaches.•Analyze where and when each approach is more suitable to maximize prediction ...performance.•Limitations of machine learning approaches in extrapolation for this case study.•Optimization strategy for energy savings in foundry applications.
The process and temperature control of metal casting applications is of utmost importance both to guarantee the good quality of the final product and also to pursue an energy saving policy. For this purpose, in this paper two different modelling approaches have been proposed to predict the liquid steel temperature inside a ladle for metal casting, shortly before the casting process. The first is a physics-based grey-box model relying on equations for the characterization of the heat transfer mechanisms inside the ladle structure, whereas the second approach relies on artificial neural networks (ANNs). Both methods have been calibrated with experimental data of a case study plant, and subsequently assessed and compared in terms of prediction accuracy. Results show that the physics-based approach is able to predict the casting temperature with a higher mean absolute error (MAE) of 14 °C, whereas the ANNs predictions result to be better, with MAEs around 6 °C. On the other hand, it has been demonstrated that the ANNs approach may lack of reliability, especially if input data strongly differ from the calibration dataset, whereas the physics-based approach results to be more consistent and trustworthy. Finally, an energy analysis is conducted to demonstrate the feasibility of the model in evaluating the potential energy saving compared with situations in which decisions are taken by operators without the aid of a model predictive control.
On the design of 5GDHC substation control systems Khlebnikova, Elena; Pothof, Ivo; van der Zwan, Sam ...
International journal of sustainable energy,
12/2024, Letnik:
43, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACTThis paper presents challenges in the control of 5GDHC networks and proposes an approach for the robust control design of these networks. Temperatures are low and temperature differences are ...small in 5GDHC networks, in order to minimise heat losses in the distribution network and directly utilise low-temperature heat sources. Therefore, the robust control design of substations is crucial for stable and efficient operation of 5GDHC networks. We proposed the key steps to obtain a consistent control design that can be verified and tested prior to commissioning of the building substations and network. The design approach was elaborated in a case study in an existing neighbourhood in Haarlem, the Netherlands, constructed in the 1930s to 1970s. This neighbourhood will become independent of natural gas using PVT-rooftop panels, individual heat pumps (HP), a 2-pipe, bi-directional energy flow network operating at ultra-low temperatures and ATES system as a seasonal and daily storage facility.
•Optical and heat transfer model of nanofluids as beam splitters in PV/T collectors.•Fabrication of core-shell Ag-SiO2 nanoparticles to act as selective band-pass filters.•Design and fabrication of ...an experimental rig for the ‘on-sun’ testing of nanofluids.•Enhancement of sunlight utilisation in the PV/T collector compared with a CPV system.
Nanotechnology is a rapidly developing field with a wide range of potential applications. One such application is using nanoparticles to selectively absorb and transmit light. By controlling the material, size and shape of nanoparticles it is possible to achieve spectrally selective optical properties. This phenomena can be exploited in solar energy applications where selective absorption of the solar spectrum enables high efficient hybrid photovoltaic/thermal (PV/T) collectors. In this study, we suspend core-shell Ag-SiO2 nanoparticles in water to filter out the ideal spectrum for creating electricity from Si PV cells. A detailed optical and heat transfer model for this approach was developed and validated using the electric and thermal outputs from a prototype system operating in real outdoor conditions. The results of this experimental work indicates that 12% more value (weighted energy output) can be achieved from this PV/T prototype compared with a stand-alone PV system under the same illumination. Since the proposed system inherently decouples the thermal receiver from the PV cells, it is possible to extend this technology to industrial combined heat and power applications.
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