Low Heat Loss Research Articles

Development of Powerhouse Using Fresnel lens

Solar energy is an alternative source of renewable energy. Sultanate of Oman government showed initiation on utilization of solar energy for domestic and industrial applications. Fresnel lens is one of the methods to collect maximum energy by gathering heat of the sun in the concentrated form (using solar collectors). Earlier research work discloses that Fresnel lens gave better result in terms of power output and produces lower heat loss as compared to linear –parabolic solar collectors. In this work, development of a proto Fresnel lens power house was made to generate electricity. The focused heat from Fresnel lens was used to heat the molten salt in a heat exchanger to produce the steam. The generated steam was used to rotate the steam engine coupled to a generator. In the current work, a maximum power of 30 W was produced. In addition, comparative study was carried out regarding solar salts and heat exchanger materials to understand the Fresnel powerhouse performance. Overall the present study gave valuable information regarding usage of Fresnel lens for electricity generation in Oman.

Exergy analyses of heat supply systems for a building cluster with CARNOT

In this paper, a model to simulate community heating systems is presented and energy and exergy analyses are conducted for a district heating system with three different heat generation alternatives. The alternatives are a gas boiler system, a system assisted by solar thermal collectors with a seasonal thermal energy storage and a gas boiler as backup, and a system with geothermal borehole heat exchangers combined with a heat pump. The heat supply of a building cluster of 11 buildings is dynamically modeled using the MATLAB/Simulink based toolbox CARNOT. The aim is to match the low exergy heating demand with a low exergy heat source. To cover an energy demand of 263.7 MWh/a, the geothermal system needs 174.0 MWh/a of exergy, the solar thermal system 269.2 MWh/a of exergy and the gas boiler system 324.9 MWh/a. A parameter study of the solar thermal system shows better results for lower supply temperatures and a lower heat loss coefficient k , but the results depend strongly on the chosen storage size. It was found that the use of fossil fuel could be reduced by 43.8 % for the geothermal system and by 17.6 % for the solar thermal system compared to the gas boiler system.

Heat transfer enhancement in cylindrical fins through longitudinal parabolic perforations

ABSTRACTIn our previous works, it is clearly addressed that optimisation of fin profile is of vital importance in terms of the rate of heat transfer from a hot surface, and the optimisation procedure depends on several factors. Within the scope of this research, a longitudinal cylindrical fin profile is under interest for the optimisation research. The purpose is to investigate the effects of longitudinal parabolic perforations on the fin parameters such as temperature distribution, effectiveness and efficiency, in which the fin surface is cooled by natural convection and radiation. Different concavity levels are considered to form parabolic perforations. The rate of heat transfer from fin surface is numerically correlated with the fin mass with respect to different concavity levels. According to results, heat transfer from unit fin mass is enhanced with the new designs. The outcome of the study can be used to optimise the needs for particular applications by making a decision between heat loss and weight options. That is, the increase in the concavity level of the perforation results in a lighter and cheaper design, but yielding a lower heat loss. However, heat transfer from unit mass is still enhanced.

On the Seasonal Variability of Sea Surface Temperature and Air–Sea Fluxes in the Lakshadweep Sea

Using satellite-sensed sea surface temperature (SST), winds, and air–sea fluxes, we studied seasonal variability of SST and air–sea fluxes in the Lakshadweep Sea (LS) and quantified their relations. Seasonal variability of SST exhibits a bimodal nature, for that air–sea fluxes and oceanic processes operate differently. Analysis demonstrates that seasonal SST variability due to air–sea fluxes in the LS arises with a time lag of 1 or 2 months. For winter cooling, low solar insolation plays a cardinal role and the effects due to net longwave radiation (NLWR) and latent heat flux (LHF) are surpassed. Although net shortwave radiation (NSWR) is minimum in December, low winter SST with a lag of 1 month occurs in January. All air–sea fluxes are favorable for summer monsoon cooling though cooling is also set by several oceanic processes such as advection, vertical overturning, and turbulent mixing. During pre-monsoon warming months March–April, all air–sea fluxes are favorable. But during intense warming in May, though SST is high (> 29.8 °C) in the LS, air–sea fluxes are not supportive as incoming solar insolation is low and evaporative heat loss is high. This indicates that warming during May is the resultant of the expansion of Indian Ocean warm pool into the LS and westward propagation of the Lakshadweep high (LH) as downwelling Rossby wave. For the post-monsoonal warming during October–November, low NSWR is unfavorable but distributions of LHF and NLWR demonstrate suitable condition. For warming during November, vertical ocean heat convergence and less evaporation act favorably.

Assessing the Relationship between Measurement Length and Accuracy within Steady State Co-Heating Tests

Evidence of a fabric performance gap has underlined the need for measurements of in situ building performance. Steady state co-heating tests have been used since the 1980s to measure whole building heat transfer coefficients, but are often cited as impractical due to their 2–4 week test duration and limited testing season. Despite this, the required conditions for testing and test duration have never been fully assessed. Analysis of field tests show that in 12 of 16 cases, a heat loss estimate to within 10% of the result achieved across a full test period can be achieved within just 72 h. These results are supported by simulated tests upon a wider range of dwellings and across wider environmental conditions. However, systematic errors may still exist, even in cases of convergence and cases with significant uncertainties may never converge. Simulated examples of traditional dwellings and those built in line with current building regulation limits may be tested for more than half the year. However, even when simulated with reduced uncertainties, dwellings with low heat loss and high solar gains, such Passivhaus dwellings and apartments, could be successfully tested for just 22% and 12% of a year respectively, demonstrating the limitations of the co-heating method in assessing such dwellings.

Thermal comfort properties of bifacial fabrics

Bifacial fabrics, with a single jersey on one face and a plain weave on the other, were produced on a purpose-built machine. Thermal comfort properties of bifacial fabrics were compared with conventional woven and knitted fabrics and the effect of weft density and loop length of bifacial fabrics on their thermal comfort properties was investigated. While different fabric structures were produced with the same wool, acrylic, and polyester yarns, the findings confirmed that the bifacial fabric is warmer (lower total heat loss) and more breathable (higher permeability index ( im)) than the corresponding woven and knitted fabrics. Increasing the loop length of bifacial fabrics enhanced evaporative resistance, air permeability, warm feeling, thermal resistance, and water vapor permeability index, yet reduced total heat loss. An increase in the weft density of bifacial fabrics led to higher evaporative resistance, warmer feeling, higher thermal resistance, lower air permeability, and total heat loss. However, the permeability index did not change with an increase in weft density. This study suggests that thermal comfort properties of bifacial fabrics can be optimized by modifying structural parameters to engineer high-performance textiles.

Transition condition and control mechanism of subatmospheric flame spread rate over horizontal thin paper sample

The horizontal flame spread over paper samples was investigated using a subatmospheric cabin with varied O2 concentration. The 25 kPa is found to be a clear turning point for the flame illumination and structure, radiative heat flux and flame spread rate (FSR), which leads to the transition boundary between the extinction limits and power law regions. In the extinction limits (non-linear) region below 25 kPa, the oxygen partial pressure is low with a small Da number. Consequently, the flame spread is gas phase kinetics controlled, resulting in low burning rate, low radiative heat loss and weak buoyancy, and thus the FSR is more sensitive to the oxygen concentration while less sensitive to the ambient pressure. In the power law (linear) region above 25 kPa, in contrast, the oxygen partial pressure is high and the Da number is large, and the flame spread is heat transfer controlled, which weakens the dependence of FSR on oxygen concentration and enhances the dependence on air pressure.

Tubular solar-energy collector integration: Performance enhancement of classical distillation unit

Abstract The current study discusses a new design regarding embedded simple distiller to a solar collector cylindrical shape. Investigating the performance improvement is based on solving transit thermal energy balance equations which can predict temperatures in different locations of the still. Introducing the pre–heater system was also modeled using a third ordinary differential equation for water as a working fluid. In order to validate the obtained findings, Dunkle's model was employed and outputs compared to previous works. Results demonstrated that the collector contributes appreciably in augmenting the basin water temperature higher than a simple still case and both couples of distillate amounts and instantaneous efficiencies were respectively (∼2.77 kg/m 2 , ∼4 kg/m 2 ) which an accorded enhancement of ∼31% and (∼40%, ∼80%) for the passive and active targets. The total cost of the collector materials was estimated to be about 63 US Dollars which represents a cheap price. Moreover, a critical issue was examined around heat losses of the proposed solar heater. At this stage, used empirical correlations available in literature enabling U L prediction (overall heat loss coefficient) of the former and flat–plate collector method has been performed. It was reached that the TSC can perform against a FPC with lower heat loss. Other parameters such as painted receiver, absorbtivity–emissivity effects and environmental circumstances have all influenced the TSC performance were also analyzed.

Comparison-Based Optical Assessment of Hyperboloid and Ellipsoid Reflectors in a Beam-Down Solar Tower System With Linear Fresnel Heliostats

Beam-down concentrating solar tower (BCST) is known for its merits in easy installation and maintenance as well as lower convection heat loss of the central receiver (CR) when comparing to a traditional concentrated solar tower system. A point-line-coupling-focus (PLCF) BCST system using linear Fresnel heliostat (LFH) as the first stage concentrator (heliostat) and hyperboloid/ellipsoid reflector as the tower reflector (TR) is proposed and theoretically analyzed and compared in this paper. Theoretical investigation on the ray concentrating mechanism with two commonly used reflector structures, namely, hyperboloid and ellipsoid, is conducted utilizing Monte Carlo ray-tracing (MCRT) method. The objective of this study is to reveal the achievable optical performance of these types of TRs in the PLCF system considering the effect of LFH tracking errors on TR astigmatism as well as the differences of optical efficiency factors and power transmission in a large-scale biomimetic layout. Results indicate that the ellipsoid system is superior in terms of interception efficiency over the hyperboloid system due to smaller astigmatism at the CR aperture, especially at larger facet tracking error. However, the ellipsoid reflector shows significantly lower TR shading efficiency resulting from the larger TR surface area compared to that of the hyperboloid reflector. The total optical efficiency of the hyperboloid system is always better than that of the ellipsoid system, and this efficiency gap decreases as the ratio ε increases. The hyperboloid TR is proved to be more promising and practical for the PLCF system.

Advances in the linear Fresnel single-tube receivers: Hybrid loops with non-evacuated and evacuated receivers

The work presented in this paper is focused on the single-tube configuration for linear Fresnel receiver. Specifically, this article analyses the feasibility of a new configuration: a hybrid loop in which the first modules, at lower working temperature, use non-evacuated receivers with medium thermal performance, and the remaining, at greater temperature, use evacuated receivers with high thermal performance. The non-evacuated receiver considered in this work consists of a pyromark tube inside a cavity provided with a secondary reflector opened to the ambient. The second one is based on an evacuated tube, similar to those of parabolic trough, and it is also inside an opened cavity with a secondary reflector.In the first place, this work performs a comparative analysis of the heat loss associated to the three single-tube receiver designs proposed: evacuated, non-evacuated and non-evacuated with a glass plate at the aperture. The heat loss is quantified in each case as a function of three parameters: the temperature difference between the tube wall and the environment, the incident concentrated flux on the tube and the wind velocity. Although the evacuated design has the lowest heat loss, the differences with the other two designs decrease as the working temperature is lower. Thus, a hybrid loop is proposed, which consists of non-evacuated modules at the beginning and evacuated modules at the end. The lower investment of the non-evacuated tubes, as well as their robustness and simplicity, can compensate for the lower energy absorbed by these type of receivers. Next, a study of the annual electricity cost produced by Fresnel plants with hybrid loop is performed. The work concludes that there are specific configurations of hybrid loops that minimize the electricity cost, when the different investment, operation and maintenance costs of the receivers are quantified. The maximum percentage of non-evacuated tubes in the loop will depend on the technology and the working temperature range.

Lower critical temperature and cold-induced thermogenesis of lean and overweight humans are inversely related to body mass and basal metabolic rate

It is colloquially stated that body size plays a role in the human response to cold, but the magnitude and details of this interaction are unclear. To explore the inherent influence of body size on cold-exposed metabolism, we investigated the relation between body composition and resting metabolic rate in humans at thermoneutrality and during cooling within the nonshivering thermogenesis range. Body composition and resting energy expenditure were measured in 20 lean and 20 overweight men at thermoneutrality and during individualized cold exposure. Metabolic rates as a function of ambient temperature were investigated considering the variability in body mass and composition. We observed an inverse relationship between body size and the lower critical temperature (LCT), i.e. the threshold where thermoneutrality ends and cold activates thermogenesis. LCT was higher in lean than overweight subjects (22.1 ± 0.6 vs 19.5 ± 0.5°C, p < 0.001). Below LCT, minimum conductance was identical between lean and overweight (100 ± 4 vs 97 ± 3kcal/°C/day respectively, p = 0.45). Overweight individuals had higher basal metabolic rate (BMR) explained mostly by the higher lean mass, and lower cold-induced thermogenesis (CIT) per degree of cold exposure. Below thermoneutrality, energy expenditure did not scale to lean body mass. Overweight subjects had lower heat loss per body surface area (44.7 ± 1.3 vs 54.7 ± 2.3kcal/°C/m2/day, p < 0.001). We conclude that larger body sizes possessed reduced LCT as explained by higher BMR related to more lean mass rather than a change in whole-body conductance. Thus, larger individuals with higher lean mass need to be exposed to colder temperatures to activate CIT, not because of increased insulation, but because of a higher basal heat generation. Our study suggests that the distinct effects of body size and composition on energy expenditure should be taken in account when exploring the metabolism of humans exposed to cold.

Measuring and accounting for solar gains in steady state whole building heat loss measurements

To ensure good thermal performance is delivered consistently and at scale, there is a need to measure and understand the as-built heat loss of dwellings. Co-heating is a steady state, linear regression method, used to measure whole building heat transfer coefficients. This paper assesses the uncertainties in such outdoor, in situ, measurements due to the presence and treatment of solar gains. Uncertainties relating to solar gains are explored through both a number of field test results and simulated co-heating tests. Results demonstrate the potential for fractions of solar gains received on one day to be re-emitted on subsequent days. This dynamic behaviour can lead the steady state analysis to underestimate heat loss. Furthermore, inappropriate measurements of on-site solar radiation are shown to lead to bias in heat loss measurements. In particular, horizontal on-site solar radiation measurements are shown to significantly overestimate heat loss in buildings experiencing high proportions of direct gains through vertical openings. Both forms of uncertainty are dependent upon both the environmental test conditions and the characteristics of a test dwelling. Highly glazed, low heat loss and heavyweight buildings prove to be the most susceptible to such uncertainties, which ultimately limit both when tests can be successfully performed and which buildings can be tested.

Design of Steam Preheating and Ignition Program of In Situ Combustion

The performance of ignition directly affects the success of in situ combustion. Because of the low temperature and heat loss in the formation, it takes a longer time to ignite and the success rate of ignition is relatively low. The steam injection preheating can shorten the ignition time and reduce the ignition cost greatly. However, thus far, there is relatively no complete design method for steam injection. On the basis of the heat conduction equation of formation, the paper evaluates the effect of the preheating process by laboratory experiments and derives a mathematical equation of temperature distribution of steam injection preheating. In addition, considering the ignition threshold value and oxidation characteristics of crude oil, the paper also designs preheating parameters of steam injection. With the mathematical equation and preheating parameters, the performance of ignition can be optimized to establish table combustion rapidly. Finally, on the basis of the field data, the paper presents a pra...

A Solar Reactor Design for Research on Calcium Oxide-Based Carbon Dioxide Capture

An engineering design for a novel 1-kW solar-driven reactor to capture carbon dioxide via the calcium oxide-based two-step carbonation–calcination cycle has been completed. The reactor consists of a downward-facing cylindrical dual cavity. The inner cavity serves as the radiation receiver, while the outer cavity is the reaction chamber that contains a packed- or fluidized-bed of reacting particles. Several aspects have been incorporated in this reactor design, including high flexibility, mechanical rigidity and simplicity, high-temperature and thermal shock resistance, accommodation of thermal expansion, low convective heat losses, uniform gas distribution inside the reaction chamber, and simple reactor assembly. The final reactor design is presented, and the reactor assembly is illustrated.

Modeling of Heat Losses from a PCM Storage Tank for Solar Thermophotovoltaic Systems

This work explores the influence of lateral heat losses from a phase change material (PCM) storage tank on the performance of a storage integrated solar thermophotovoltaic (SISTPV) system by means of an analytical model. The heat losses from the lateral surface of the PCM tank are modeled using Newton’s law of cooling by prescribing a heat-loss coefficient on the lateral surfaces. The results show that at high heat losses, low thermal efficiencies are realized. Correspondingly larger solar concentrations are required to fully melt the PCM tank. At low heat losses, such as can be expected when using thermal insulation on the lateral surfaces, approximately 40% thermal efficiency can be realized. The results also demonstrate that a high absorber area:length of PCM tank squared (SR) ratio enables the system to have a high thermal efficiency. For a high-SR, low-heat-loss design case, having a high taper ratio, high area ratio between absorber area and inlet hole area, and small PCM tank length all achieve higher thermal efficiencies. It is expected that these SISTPV systems will be designed at steady-state to be fully molten in order to maximize thermal energy storage via the latent heat of the PCM. The analytical model developed here can be used to predict the design conditions under which the PCM tank will be fully molten.

Novel in –Situ heating transmission electron microscope holder for atomic resolution

This study presents novel design of “in-situ” TEM heating holder. In-situ experimentations are important because hot deformation, dynamic recovery and dynamic recrystallization are important processes which are not well understood because it is very difficult to assess the nature microstructural processes solely from the traditional post mortem study of samples. We have designed an optimum heating holder that is dynamically stable for accuracy and image processes, capable of elevating the sample temperature up to 2000 K, with minimum controllable drift of the sample position. Sample drift is calculated. Thermal stressed induces in the sample are calculated. The in-depth finite element simulations provide detailed information about thermal and dynamic behavior of the heating holder. Finite element results show that the proposed design outperform the current heating technology. Based on the finite element results, a revised design is proposed that has lower temperature gradient among the sample, lower temperature difference between the sample and the TEM grids, lower thermal stresses in the sample, lower heat losses to the microscope parts. Furthermore, the present analysis provides the basis for the design of optimal heating holder for accurate measurements.DOI: http://dx.doi.org/10.5755/j01.mech.23.2.13544

Study on a high-performance solar thermoelectric system for combined heat and power

In this paper, a high-performance solar thermoelectric system for combined heat and power is developed and investigated. This new system has achieved a higher performance by considering some of the features, including a simplified structure, lower contact thermal resistance, lower convection heat loss, and better heat transfer performance. The mathematical model was built and analyzed. And the models are verified via the experimental prototype test. In addition, the performance of the system under different environmental conditions and operating parameters are evaluated and predicted. The results show that the proposed system can be used for cogeneration of heat water and electric power. And when the solar irradiation is larger than 700W/m2, and water temperature of 13°C, the maximum instantaneous electrical power output was found to be 0.659W at the load resistance 4.2Ω, and the power generation efficiency of the thermoelectric generator (TEG) is 1.956%, which is close to that of a low temperature organic Rankine cycle system. These results indicate the feasibility of the proposed high-performance solar thermoelectric system and give hints for future improvement of the solar thermoelectric system for combined heat and power.

Flexible thermoelectric generator with efficient vertical to lateral heat path films

This paper presents a flexible thermoelectric generator (TEG) with heat path films, which efficiently convert vertical temperature difference (ΔT) into lateral ΔT for thermocouple (TC). The heat path film consists of copper-filled-vias with low thermal resistance and polymer films with high thermal resistance. They were made in two fabrication steps. The first used a flexible printed circuit board with high density copper-filled-vias, while the second saw the deposition of thin film TCs. The combination offers flexibility of application due to its thinness, mass production potential, and low energy heat loss in the device. We demonstrated 54 TCs cm−2 in a 25 cm2 flexible TEG using Bi2Te3- and Nickel-based TCs respectively. The experimental data were in good accordance with a model which was calculated using the finite element method. The prototype flexible TEGs indicated that the proposed structure converted 84% heat flow from vertical into lateral ΔT in each TC, which was two times higher than the non-heat path film. They produced voltage of 11 mV/K/cm2 and power output of 0.1 µW/K/cm2 respectively. These flexible TEGs are ideally suited for harvesting from waste heat emitted from objects with large wavy areas because of their low weight, low cost and high efficiency conversion with flexibility.

Enhancing Performance of Solar Line Concentrator System Using Variable Concentration Ratio

The present line concentrator system with constant concentration ration exhibits rise in temperature of working media, however if the difference between outlet and inlet temperature of working media is large then they exhibit lower efficiency. Also the rate of fall of efficiency with increase in s temperature difference is high. To overcome this problem it is proposed to have a variable concentration ratio concentrator system. The variable concentration ratio is achieved by employing receiver consisting of the pipes having different diameters; with the larger diameter pipe at start followed by small diameter receiver. The present paper describes the theoretical analysis carried out for the newly proposed circular line concentrator with variable concentration ratio. The results obtained from the theoretical analysis are verified experimentally.In the present case a 3-CR receiver system offers better results as compared with single CR receiver system. The heat loss is a function of receiver diameter and receiver temperature. With lower CR used for low temperature gives lower heat loss. As the receiver temperature increases the losses go on increasing and a higher CR by offering lower heat transfer area keeps the losses at optimum level. Thus it is recommended to use variable concentration ratio in case of situation where the difference between the out let and inlet temperature of working fluid is high so as to obtain the higher efficiency.

An experimental study on energy and exergy performance of a cavity receiver for solar parabolic dish concentrator

In this paper, an experimental study is carried out to investigate the thermal performance of a conical frustum cavity receiver for a 16 m2 Scheffler concentrator, using energy and exergy analyses. The experiment is conducted with heat transfer fluid having a flow rate of 2.5 litres per minute (LPM) under an average beam radiation of 608.17 W/m2. The average thermal energy efficiency of the receiver is found to be 57.39% with a peak value of 66.75%. The receiver could attain a very low overall heat loss factor of about 0.027 kW/K. The overall system efficiency of the solar collector is 69.47%. The average exergy efficiency of the receiver is found to be 5.88% with a peak value of 10.35%. A peak exergy factor of 0.152 is obtained for the receiver. Experiments are also conducted with flow rates of 1, 1.5 and 2 LPM in addition to the flow rate of 2.5 LPM and the receiver showed better performance for 2.5 LPM.