Low Heat Loss Research Articles

Photo-thermal design and analysis of a novel CPC coupled solar air evacuated tube collector

The conventional evacuated tube solar air collector can’t reach a higher temperature due to poor heat transfer between absorber and air. To maintain a low heat loss at medium temperature and enhance heat transfer, a novel evacuated tube solar air collector coupled with a compound parabolic concentrator is proposed. The matching shape is designed and optimized for the concentrator reflecting surface using non-imaging optics, and by ray tracing, the optical performance is obtained. The corresponding photothermal conversion and heat transfer models are established, and numerical simulation is conducted for thermal performance analysis. The results show that the thermal performance is mainly affected by the air flow rate, the inlet temperature, and the heat source energy density. Compared with the existing single-pass evacuated tube collector, the novel collector has advantages in heat output temperature, thermal efficiency, adaptability to flow rate changes, and potential for series temperature rise under most conditions. When concentrated solar power density is 800–1600 W/m2, the maximum outlet temperature of a novel collecting evacuated tube can reach to a limiting value of 181.17 ℃, representing its thermal insulation performance, while that of the contrast one is 153.25℃. Under normal inlet temperature of 25℃, the maximum output temperature of a single tube of the novel collector is 146.05℃ at a flow rate of 1.02 m3/h, and the maximum thermal efficiency of 59.21% can be achieved at a flow rate of 3.58 m3/h.

Analyzing efficiency of solar heat storage unit within a building including trombe wall equipped with phase change material in existence of fins

Trombe wall can be applied for solar energy saving for ventilation purpose and to boost the performance of classic unit, it was combined with paraffin layer and fins in this study. The nanoparticles of alumina have been loaded into pure paraffin (RT18HC). To involve the existence of solar irradiation, two heat generation terms were added in governing equations of glass and absorber layers. Also, radiative and convective heat losses have been involved in boundary condition. Unsteady turbulent air flow within the domain has been simulated and for modeling of various layers, temperature equations with incorporating heat sources have been applied. Two shapes of fins (I and Y) with various numbers have been applied. The impacts of thickness and length of the fins as well as position of paraffin layers have been scrutinized. With assuming equal volume of paraffin, comparison of two configurations showed that system with Y-shaped fins leads to higher liquid fraction. Also, temperature of domain in existence of Y-shaped fins has lower value which leads to lower heat loss. With implement of best configuration, the amount of heat capacity augments about 28.41% at 17:00.

Photothermal conversion performance based on optimized design of multi-section compound parabolic concentrator

Solar energy M-CPC (Multi-section Compound Parabolic Concentrator, M-CPC) has the advantages of no tracking and static concentrating, synchronous collection of beam and diffuse radiation, simple structure and easy construction. Therefore, an integrated system based on the optimized design of M-CPC coupled with ECT (All-glass Evacuated Collector Tube, ECT) for solar photothermal conversion is constructed in present research. Moreover, the stability, reliability and applicability of the solar M-CPC system for energy capturing and photothermal conversion are verified by outdoor experiments and numerical calculations. The results show that the average heat collection efficiency of the integrated M-CPC system is 61.0%, the average output temperature of the fluid could reach 350.9K. In addition, the average relative error between the theoretical calculated value and the experimental measured value of fluid temperature rise is 5.1%, which indicates that the M-CPC system has the characteristic of low heat loss in the process of solar radiation utilization. All the above demonstrate that the M-CPC system has appropriate photothermal conversion as well as industrial manufacturing features.

Experimental investigation of indirect heat transfer through a novel designed lab-scale setup using functionalized MWCNTs nanofluids (MWCNTs-COOH/water and MWCNTs- OH/water)

This study aims to investigate the influence of functionalized multi-walled carbon nanotubes (MWCNTs-COOH and MWCNTs-OH) on the performance of a novel indirect laboratory heater redesigned with a helical tube. Experiments were performed using a bath of water and the two nanofluids mentioned above. The nanofluids were prepared at concentrations of (0.025, 0.05, 0.075, and 0.1) wt%. To ensure the stability of the utilized nanofluids and the structure of the nanoparticles, characterization studies such as dynamic light scattering (DLS), FT-IR, and XRD were carried out. The results indicated that when concentration levels rose, the system's performance rose as well. For instance, the MWCNTs-COOH nanofluid bath had a maximum heat transfer rate of 1709 W, which was 19.04% better than pure water, and a maximum Nusselt number of 15.61, which was an increase of 36.33% over pure water. In summary, heat loss increased with an increment in flow rate, and by increasing the concentration of nanofluids, the lowest heat loss, the highest heat transfer rate, and the best efficiency of the system were achieved. This study can establish the foundation for redesigning indirect heaters to reduce thermal energy consumption and for further research on the use of nanofluids in indirect bath heating.

Performance of sorption thermal energy storage in zeolite bed reactors: Analytical solution and experiment

Sorption thermal energy storage (STES) has the advantage of high energy storage density and low heat loss, which has been considered as one of the promising solutions to achieve carbon neutrality in buildings. An insight into the thermal energy storage performance of reactors enables the optimization of STES systems. In addition, the analytical solution is concise and understandable in physical significance. In the present work, the perturbation theory was introduced to derive the analytical solutions for the fluid and solid temperatures in the reactor during the discharging process. Subsequently, a zeolite 13X STES system was built to conduct charging and discharging experiments. Based on the derived analytical solution, the fluid temperature at different axial positions in the reactor and energy storage density (ED) of zeolite were calculated and compared with the measured data. The results showed that there was a good agreement between the experimental test and the analytical solution, in which the maximum deviation of the energy storage density was less than 4 %. The analytical solution could obtain specific functional expressions for STES performance parameter of ED and further receive the results quickly.

Energy and Exergy Analysis of Solar Air Gap Membrane Distillation System for Seawater Desalination

Air gap membrane distillation (AGMD) is a widely utilized technology for producing drinking water due to its low heat loss, high thermal efficiency, and compatibility with solar energy. The application of the first and second laws of thermodynamics in energy and exergy analyses provides a comprehensive evaluation of the efficiency of thermal processes. This study aims to examine numerically the energy and exergy performance indicators of a solar AGMD system used for seawater desalination. The simulation was carried out using MATLAB 9.7 software. The total thermal efficiency and overall efficiency of each element in the AGMD system were calculated for various solar field energy outputs, and moreover, a parametric study was conducted. The results indicate that the exergetic efficiency of the AGMD system components was the lowest in the solar field, with the concentrator having the lowest energy efficiency. Additionally, the thermal and exergetic efficiency of the entire solar AGMD system decreases along with the raise of ambient temperature. An additional investigation was conducted to better apprehend the sources of exergy destruction in the solar field. The obtained results from this study can be employed as a guide to reduce exergy destruction in the whole solar AGMD desalination system with recognition of the main sources of irreversibility.

Hydraulic Characteristics of Continuous Submerged Jet Impinging on a Wall by Using Numerical Simulation and PIV Experiment

Due to their high efficiency, low heat loss and associated sustainability advantages, impinging jets have been used extensively in marine engineering, geotechnical engineering and other engineering practices. In this paper, the flow structure and impact characteristics of impinging jets with different Reynolds numbers and impact distances are systematically studied by Flow-3D based on PIV experiments. In the study, the relevant state parameters of the jets are dimensionlessly treated, obtaining not only the linear relationship between the length of the potential nucleation zone and the impinging distance, but also the linear relationship between the axial velocity and the axial distance in the impinging zone. In addition, after the jet impinges on the flat plate, the vortex action range caused by the wall-attached flow of the jet gradually decreases inward with the increase of the impinging distance. By examining the effect of Reynolds number Re on the hydraulic characteristics of the submerged impact jet, it can be found that the structure of the continuous submerged impact jet is relatively independent of the Reynolds number. At the same time, the final simulation results demonstrate the applicability of the linear relationship between the length of the potential core region and the impact distance. This study provides methodological guidance and theoretical support for relevant engineering practice and subsequent research on impinging jets, which has strong theoretical and practical significance.

Experimental investigation of double slope solar still integrated with PCM nanoadditives microencapsulated thermal energy storage

Transparent covered slope solar stills are trending but characterised with low productivity, heat losses and high energy consumption, which are setbacks in practice. In this study, double slope solar still (DSSS) integrated with PCM-TES is presented. PCM was microencapsulated with epoxy resin composite using vacuum mould-filled techniques. Conventional DSSS and DSSS-TES data collected have been compared to establish the influence of TES on productivity. Daily average temperature of the glass cover, humid air, saline water, still basin absorber and TES cavity for the DSSS-TES attained are 65.2 °C, 77.5 °C, 82.4 °C, 79.5 °C and 68.4 °C, respectively. DSSS-TES has yielded higher production, with 7.5 Litres of potable water daily and extension in operation period by 3 h has been achieved. In addition, condensation and evaporation rates increased with increase in production by 105%. Integration of TES with the system has reduced the heat losses while leakages from PCM nanocomposite have been prevented by microencapsulated insulator. No trace of metals, bacteria and organic contaminants has been found in desalinated water. A payback period of 0.8 year has been recorded based on all-year-round operations. Findings are in good agreement with existing models. Moreover, sensorial characteristics obtained conform to WHO standards.

Energy and economic performance assessment of a solar-assisted regenerative vacuum membrane desalination system

Vacuum membrane water desalination is a promising technology for freshwater production with the advantages of low conduction heat loss and mass transfer resistance, compared to other membrane technologies. Such systems can be driven by solar thermal collectors for green water production, yet there is a lack of dynamic system simulations in the literature to comprehensively assess the energy and economic system performance and evaluate the impact of brine recirculation as a method of regeneration, especially under hot desert climates where solar resources are abundant, but freshwater resources are scarce (e.g., the northern coast of Egypt, considered as the case study here). In this study, a dynamic system simulation model is developed in TRNSYS, where a new “Type” is introduced for the vacuum membrane module. Initial sizing of the system with ∼20 % solar fraction resulted in annual average gained output ratio and specific thermal energy consumption of 2.51 and 133.5 kWh/m3, respectively, with levelized cost of water and payback period of 14.7 USD/m3 and 1.53 years, respectively. By increasing the solar collectors’ area from 8 to 16 m2, the levelized cost of water increased by 14.54 %. The storage volume was highly influential on the solar fraction, which increased by 101.39 % as the volume increased from 0.3 to 0.7 m3. Increasing the membrane area from 0.3 to 0.5 m2 increased the gained output ratio by 76.75 % while decreasing the levelized cost of water by 14.79 %. Internal heat regeneration through brine recirculation enhanced the system performance at lower recirculation ratios. Without recirculation, the levelized cost of water increased by 117.41 % and the system met the daily distillate water demand for only 90 days of the year.

Analysis of energy flows and emission characteristics of conventional diesel and isobaric combustion in an optical engine with laser diagnostics

In this study, the thermodynamic analysis of energy distribution, exhaust emissions, and particulate characterization was conducted in an optical engine with an all-metal configuration. Additionally, the line-of-sight integrated imaging of combustion luminosity, and OH* chemiluminescence along with planar laser induced fluorescence of formaldehyde (HCHO-PLIF), and planar laser induced incandescence of soot (soot-PLII) were applied in the optical configuration. The experiments were conducted with conventional diesel combustion at λ = 3 (i.e., CDC), isobaric combustion at λ = 3 (i.e., Iso3), and isobaric combustion at λ = 4.2 (i.e., Iso4.2) using n-heptane fuel. Compared to Iso3 and CDC, Iso4.2 yielded higher thermal efficiency and lower heat losses; whilst the exhaust losses were exacerbeted. Isobaric combustion also resulted in lower NOx but increased soot emissions. For all operating conditions, the combustion luminosity and OH* chemiluminescence imaging showed that the signal grows and develops from the jet-axis downstream of the nozzle to the jet-wall impingement point, followed by movement towards the squish region. HCHO-PLIF showed that isobaric combustion leads to a faster transition of low-to high-temperature reactions compared to CDC. Soot-PLII showed increased in-cylinder soot distribution for isobaric combustion due to lesser charge pre-mixing time and spray-flame interaction induced by close-coupled injections.

Neural networks for predicting air gap membrane distillation performance

The hollow fiber air gap membrane distillation (AGMD) has recently attracted tremendous attention for desalination and wastewater treatment due to its high packing density, low conductive heat loss, and latent heat recovery capability. Utilizing fast and accurate modeling tools to predict MD performance can result in the further development of desalination technologies. However, simple and time-saving prediction models to assess the AGMD performance were not abundant. Herein, AGMD performance, including permeate flux (J) and gained output ratio (GOR) was predicted through multiple linear regression (MLR) model, back propagation neural network (BP ANN) and radial basis function neural network (RBF ANN) under different hot temperatures (Th), coolant temperatures (Tc), feed flow rates (F), and feed concentration (c). A total of 30 sets of data were used to train the proposed models, the other 10 external validation datasets not used for training the models were applied to validate the prediction accuracy. The results depicted that RBF ANN (SPREAD = 30, N = 30) showed greatest prediction performance (R2 = 0.99–1) compared with BP ANN and MLR models (R2 = 0.98–0.99; R2 = 0.89–0.97). The computing time consumption of RBF ANN was higher than BP ANN. According to the Mean impact value (MIV) analysis, Th had the strongest effect on J and GOR. Increasing Th and decreasing c both had positive impacts on J and GOR, but increasing Tc or F resulted in a trade-off influence. A genetic algorithm (GA) was employed to optimize J and GOR simultaneously, the optimum J and GOR could reach 6.00 kg/m2·h and 7.70 respectively. In this study, the three prediction models proved their abilities to predict AGMD performance and further provide guidance in the actual membrane distillation water treatment process.

Surface Urban Heat Island In Moscow During The COID-19 Pandemic Lockdown In 2020

The influence of the COronaVIrus Disease 2019 (COVID-19) pandemic lockdown (the period of strict quarantine measures) in the spring of 2020 on the ‘Surface Urban Heat Island’ (SUHI) geographical phenomenon in Moscow has been studied. For this purpose, we used the measurements of the surface temperature TS made by Moderate Resolution Imaging Spectroradiometer (MODIS) radiometer installed on Terra and Aqua satellites. As a result, TS during the 2020 lockdown, both in the city and surrounding rural zone, was found lower than at the same calendar time in the previous 20 years due to the relatively cold spring. The SUHI intensity as the difference between TS inside Moscow and the surrounding rural zone around it during the lockdown was also lower than usual (on average in the previous 20 years), but this decrease is relatively small and nonsignificant. The Normalized Difference Vegetation Index (NDVI) in Moscow and Moscow region during the lockdown was close to its usual values, but the leaf area index (LAI) was significantly lower than its average values in the previous 20 years. Thus, the weakening of the SUHI during the lockdown in 2020 was caused mostly by lower heat loss due to transpiration in the rural zone. This was associated with the slowdown in vegetation development as a result of the cold spring. Besides, an additional possible reason was the reduction of human activity due to the collapse of many anthropogenic heat sources in the city. According to long-term MODIS data, the SUHI intensity in Moscow and the surface temperature in Moscow region, as well as the NDVI and LAI values, do not demonstrate statistically significant long-term trends in the spring season over the past 21 years, despite climate changes. In spring, during faster snow melting in cities, when it still persists in the rural zone, the SUHI intensity can be record high (up to 8 ºC).

Experimental and economic investigation of sweeping gas membrane distillation/pervaporation modules using novel pilot scale device

With the increasing demand for freshwater, the disposal of the highly saline water resulting from industrial processes and seawater desalination plants has become a critical environmental problem. Reverse osmosis (RO) cannot endure high salinity feed streams higher than 75 g/L. Membrane distillation (MD) and pervaporation (PV) techniques have been proving their capability to desalinate high-salinity brines. The sweeping gas MD/PV (SGMD/SGPV) technique has a lower heat loss to the permeate side as air is used to carry the water vapour, but it is rarely evaluated at larger scales. A pilot-scale desalination device was designed and constructed for an MD/PV surface area of 1.17 m2 to analyse the performance of SGMD/SGPV. The resultant fluxes reached 13.2 L m−2h−1 and 9.2 L m−2h−1 at feed salinities of 35 g/L and 125 g/L, respectively, with 99.6 % salt rejection (% SR) at 70 °C. Furthermore, an economic analysis of the overall system shows that it requires minimal maintenance costs to be competitive in water production. Overall, it provides an appealing alternative to small-scale, stand-alone desalination systems capable of generating high-quality drinking water from very salty water.

Application of Paraffin-Based Phase Change Materials for the Amelioration of Thermal Energy Storage in Hydronic Systems

This study aims at investigating the improvement in the thermal performance of energy storage for a hydronic system when it is equipped with evacuated tubes integrated within a hot water tank. The PCM shell in the bottom section is thicker than at the top to maintain a uniform, minimal water temperature difference of 5 °C between the top and bottom sections of the hot water tank. The thermal performance of the system was analyzed in diverse months when the ambient temperature fluctuated. The results have revealed that the thermal performance in December, March, and June was 80%, 81%, and 84%, respectively, meaning that the thermal performance is optimal in warm weather. The results confirmed that the system had boosted the presence of hot water throughout the whole day, including the time of the sun’s absence, due to the release of stored PCM latent heat. The designed system solves the overheating problem and expands the availability of hot water through the cold weather. The system is characterized by lower heat losses because the average water temperature has decreased.

Investigation of vacuum-integrated switchable polymer dispersed liquid crystal glazing for smart window application for less energy-hungry building

The presence of more than one transparency and controllable intermediate transmission between ON and OFF states make the switchable smart glazing more attractive for less energy-hungry building integration than a static transparent window. However, to enable this switchable glazing for cold climatic buildings, overall heat transfer through the glazing needs to be reduced. In this work, the thermal and optical performance of integrated polymer dispersed liquid crystal and low heat loss transparent vacuum glazing was investigated using indoor characterisation. Two systems were developed where PDLC glazing was sandwiched between a vacuum and one acrylic sheet (Vacuum-PDLCA) and vacuum and glass (Vacuum-PDLCG). The employment of acrylic sheets reduced 21% of the overall weight of the system which made it suitable for retrofit building integration. Use of acrylic reduced 35% of solar transmission of PDLC ON state compared to the use of glass. Overall heat transfer coefficient (U-value) or thermal transmittance was below 1.1 W/m2K for all Vacuum -PDLCG and Vacuum-PDLCA ON and OFF states. Solar heat gain coefficient or solar energy transmittance was highest at 0.45 for the Vacuum-PDLCG ON state and lowest at 0.23 for the Vacuum-PDLCA OFF state.

Quantification of heat losses and gains of 5th generation district heating and cooling networks compared to conventional heating networks

AbstractThe fourth and fifth generation of district heating (GDH) networks are among the key elements for the energy optimization of districts. Low heat losses in the distribution lines as well as bidirectional heat flow and the use of low‐temperature waste heat can lead to new business models for energy suppliers. Despite the continuous development of heat networks, significant heat losses still occur in conventional networks, despite well insulated pipelines, with fluid temperatures at 60 °C to 100 °C. Therefore, a study was conducted to investigate heat losses and gains in hot water heating (3GDH), low temperature heating (4GDH) and district heating and cooling networks (5GDHC) based on numerous simulations. The main purpose of the investigations is to deter‐mine the main factors influencing the heat transfer characteristics of underground heat networks. The derived results serve as a basis for the development of future planning tools to determine the heat losses and gains of local heating networks quickly and cost‐efficiently.

Integrated silicon-based spectral reshaping intermediate structures for high performance solar thermophotovoltaics

Solar thermophotovoltaics (STPV) system is a technique that uses absorber and emitter to remold wide-band solar radiation into narrowband infrared emission and then convert it into electric energy in order to get high energy conversion efficiency. In this study, we fabricated an integrated silicon-based intermediate structure to enable the spectral reshaping. The absorber is a hierarchical Si NW structure, which exhibits excellent absorption performance of over 95 % in the spectral range of 220–1100 nm and a low heat loss in the long-wavelength band. The emitter is planar Si-W-SiN/SiNO structure, exhibiting a selective narrowband absorption (i.e., emission) peak in the target wavelength. When the emission peak of emitter is at 1800 nm, the overall efficiency of the STPV system based on this silicon-based intermediate structure can theoretically exceed 29 % with an achievable solar concentration of 1000 and an emitter-to-absorber area ratio of 4. We also test the prepared spectral reshaping intermediate structures on real systems and discuss further optimizations.

Control of the Air‐Sea Oxygen to Heat Flux Ratio During Deep Convection Events

AbstractEarth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O2 inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air‐sea O2 flux to heat flux ratio (O2/heat flux ratio) during deep convection. Our theoretical analysis based on a one‐dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O2/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air‐sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss.

Thermal Spin-Current Generation in the Multifunctional Ferrimagnet Ga0.6Fe1.4O3

In recent years, multifunctional materials have attracted increasing interest for magnetic memories and energy-harvesting applications. Magnetic insulating materials are of special interest for this purpose, since they allow the design of more efficient devices due to the lower Joule heat losses. In this context, ${\text{Ga}}_{0.6}{\text{Fe}}_{1.4}{\text{O}}_{3}$ (GFO) is a good candidate for spintronics applications, since it can exhibit multiferroicity and presents a spin Hall magnetoresistance similar to the one observed in a yttrium-iron-garnet (YIG)/$\mathrm{Pt}$ bilayer. Here, we explore GFO utilizing thermospin measurements in an on-chip approach. By carefully considering the geometry of our thermospin devices we are able to quantify the spin Seebeck effect and the spin-current generation in a GFO/$\mathrm{Pt}$ bilayer, obtaining a value comparable to that of YIG/$\mathrm{Pt}$. This further confirms the promises of an efficient spin-current generation with the possibility of an electric field manipulation of the magnetic properties of the system in an insulating ferrimagnetic material.

A semi-analytical approach for analysis of thermal behaviors coupling heat loss in powder bed fusion

The build-up of the heat losses induced by a high-temperature gradient during laser additive manufacturing process has a significant effect on the thermal behaviors and the formation of parts’ defect. In this paper, considering heat losses caused by conduction, convection and radiation, a semi-analytical model for predicting three-dimensional transient temperature field distributions is established, which is verified by comparison with results from the literature. The temperature distributions coupled with heat losses for typical surface and volumetric heat source models are obtained by using the separation of variables. The effect of process parameters and heat source model on peak temperature, temperature gradient, molten pool dimensions and heat losses is analyzed. In addition, the effect of the surface and volumetric heat source model on formation quality during the powder bed fusion (PBF) process is discussed in detail. The results show that higher average temperature and lower heat losses occur for the volumetric heat source model. Moreover, heat losses increase as the power, energy absorptivity and effective radius of the laser beam, while decrease with a decrease of scan speed. The results could be helpful to enable process optimization of the PBF to improve as-built parts quality.