Convective Loss Research Articles

Numerical study of a nanofluid-based receiver for linear Fresnel collectors

In concentrated solar technologies, the use of volumetric receivers instead of surface receivers may help to achieve higher solar collection efficiencies. This paper reports a simplified model that computes the concentrated solar radiation absorption and energy transport by advection in the volumetric receiver of a linear Fresnel collector. The proposed linear Fresnel collector is intended for use in industrial solar heat processes. The receiver consists of a rectangular channel whereby flows a nanofluid composed of thermal oil and graphite nanoparticles. The bottom wall of the channel is a glass, while the upper wall is a highly reflective surface that is well insulated. The light absorption into the nanofluid was solved using the Rayleigh dispersion, while the convection was solved with a simplified velocity profile for turbulent flow (power law) and incorporating a radiative source term in the energy equation. All equations were discretized by using finite volumes and were coded in Python language. Receiver efficiencies in the range of 92–96% were found for temperatures of 403–343K. The results show that the volumetric fraction (optical depth) drives the way the radiation is absorpted, and in consequence, the temperature profile and the receiver efficiency. For low optical depths, the solar absorption is low, resulting in high reflected radiation losses. Contrarily, high optical depths mean high solar absorption, consequently, the temperature close to the glass increases, and the convection losses are higher. The results suggest that values of optical depth close to 2.3 are optimum to obtain the highest receiver efficiency.

Analysis of heat transfer characteristics of a sandwich cylindrical shell with a metal-rubber core

This work aims to analyze the heat transfer characteristics of a sandwich cylindrical shell with a metal-rubber core (SCS-MR) using both finite element analysis and experimental techniques. The SCS-MR is a lightweight and high-temperature resistant material that is suitable for construction in environments with varying conditions, such as rocket launchers, aircraft injectors and ships. The heat transfer, heat flux, and thermal convection loss were first analyzed using thermal resistance and Fourier law. Subsequently, a representative volume element of the SCS-MR was established to simulate thermal transfer, followed by a series of comparative experiments that measured the temperature gradients, the boundary conditions, and the density of the SCS-MR. Additionally, three types of metal spirals were examined using differential scanning calorimeter (DSC). The convective thermal conductivity, heat flux, and heat convection loss of the SCS-MR were analyzed experimentally, revealing that the metal spiral exhibited an exothermic peak in the DSC. Furthermore, the heat transfer rate of the SCS-MR was faster than that of the solid cylindrical shell (CS) when the set temperature was reached. The insulation cover was capable of providing boundary conditions to minimize the impact of natural convection. In the high-temperature environment, the thermal conductivity decreased with increasing density of the scaled model SCS-MR.

Effect of argon concentration on thermal efficiency of gas-filled insulating glass flat-plate collectors

Insulating glass flat-plate collectors can save cost by being produced quickly and automatically in insulated glass production facilities, and they can be filled with argon to reduce heat loss. During its lifetime, the collector is likely to lose argon because of gradual material degradation of the sealing. However, information on the influence of the argon concentration on the collector efficiency is limited. Therefore, the objective of this research work was to analyse this effect. A theoretical material property calculation of argon-air mixtures was carried out to determine the convective losses with variable argon concentrations. Thermal collector performance was measured experimentally using an outdoor solar tracker test rig. The results strongly suggest, that the influence of argon concentration on both the convective losses and the thermal efficiency is non-linear. The measurements revealed that an argon concentration of 90 % can increase average thermal performance by 6.7±4.8 percentage points. An increase in argon concentration from 0 % to 50 % has almost twice the effect on average thermal efficiency as an increase from 50 % to 90 %. Concluding from these results, an argon leakage threshold of 2.5 percentage points per year is proposed to avoid disproportionate loss of efficiency over time.

Experimental study of dynamic combustion behavior and heat transfer of heptane pool fire with burning time under thin fuel thickness (2.0 mm–14.0 mm)

A dynamic combustion period occurs after the liquid fuel ignition, but the combustion mechanism is complicated and unclear due to dynamic heat transfer between the fuel and the substrate. This work uses a method of experimental research and theoretical analysis to study the dynamic combustion behaviour and heat transfer of pool fire with burning time (t) under thin fuel thickness (δ). Research contents include the relation of flame height (Z), burning rate (m˙), heat transfer between fuel and substrate with t, and the evolution mechanism of the output composition of total heat feedback with δ. The research found Z and m˙ increase with t until maintaining constant, whereas δ is different. Heat convection loss increases rapidly, then increases slowly, and finally keeps constant because thin-layer fuel is quickly heated. Heat radiation loss firstly surges, then fluctuates due to the combined effect of the flame shape and necking effect. Total heat loss tends from radiation to convection. The heat for heating fuel and the heat radiation reflected from the substrate are considered in the dynamic combustion stage. The fractions of heating fuel, vaporization heat, heat loss and heat reflection were calculated and studied. This work adds theoretical knowledge of fire dynamics

Reduced order 1 + 3D numerical model for evaluating the performance of solar borehole thermal energy storage systems

This study presents a computationally efficient numerical model for solving the fluid flow and heat transfer phenomena in co-axial boreholes heat exchangers arranged on a rectangular grid (N × N) drill hole pattern. Such systems of boreholes are an integral component of a Solar-Borehole Thermal Energy Storage System for seasonal thermal storage application. The numerical model couples a one-dimensional fluid flow and convective heat transfer model in the heat exchanger with a three-dimensional model of conductive heat transfer in the strata. In addition to solving mass and heat transfer in borehole systems, it integrates a solar thermal collector system and building dynamic thermal load to design a Solar- Borehole Thermal Energy Storage system for a 182-unit residential apartment building. Simulation also considers and evaluates the conductive, convective, and radiative thermal losses of the system. The model is validated using experimental data from a coaxial borehole heat exchanger and compared to the results of a commercial finite volume solver for multiple co-axial boreholes. The computational cost of developed numerical model is reduced by a factor of 194 compared to a commercial finite volume solver. Solar- Borehole Thermal Energy Storage system simulation is performed for a period of five years considering hourly fluctuations in solar irradiance and building dynamic thermal energy demand. Parametric study is performed on depth and spacing of boreholes, and mass flowrate in the solar thermal collectors. It is found that a system of 100 boreholes, each 100 m deep, linked to 674 solar thermal collectors can provide the heating for the building with a total thermal demand of 5600 GJ per year.

АСПЕКТИ ПРОГНОЗУВАННЯ ТЕМПЕРАТУРНИХ РЕЖИМІВ АСФАЛЬТОБЕТОННОГО ПОКРИТТЯ В ЦІЛЯХ ПРОЄКТУВАННЯ НЕЖОРСТКОГО ДОРОЖНЬОГО ОДЯГУ

Introduction. The introduction to the paper describes the impact of critical temperatures and temperature loadings on the performance of the asphalt pavement layer and also on the entire flexible road pavement construction. Problem statement. The issues of the paper concern significant aspects of the pavement temperature regimes prediction used in various systems of the flexible road pavement designing. Purpose. The main goal of the paper is to review and compare the flexible road pavement designing approaches based on predicted temperatures of the asphalt pavement layer which may be reached under critical road pavement operating conditions. Results. The application of the theory of thermal diffusion as the framework for the description of establishing the asphalt pavement layer temperature equilibrium was reviewed. As it is assumed, when the maximum asphalt pavement layer temperature is reached exists the equilibrium between outgoing and incoming flows such as outgoing and incoming longwave radiation, shortwave incoming radiation, convection losses and heat transfer. After wide investigations carried out by an SHR program, the maximum asphalt pavement layer temperatures calculated on that heat flows balance were proposed to be used in the flexible road pavement designing. Also, the results of investigations carried out to develop the proposed theory of temperature equilibrium were analyzed and it was stated that under certain meteorological conditions the maximum asphalt pavement layer temperature will not be accompanied by the temperature equilibrium; the subsequent redistribution of heat flows may led to the values of temperature that significantly overcome the calculated maximum.

Multi-objective optimization for comparative energy and economic analyses of a novel evacuated solar collector prototype (ICSSWH) under different weather conditions

This paper aims at optimizing the energy performance of an innovative integrated collector-storage solar water heater (ICSSWH) prototype. The system incorporates a high-vacuum gap, which significantly reduces convective thermal losses, and a serpentine absorber system coated with a low-E layer to minimize radiative thermal losses.The methodology is based on a detailed dynamic simulation model, implemented in the MatLab environment, suitable conceived to accurately predict the energy performance and economic profitability of the ICSSWH under various weather conditions and energy market prices. This approach aims to initially identify the most influential geometrical, thermophysical and optical parameters that impact the energy performance of the system through the assessment of different objective functions. After identifying the most influential parameters, the range of existence with the interval step for each variable is defined, and the optimization procedure is conducted to determine the optimal set of variables that maximize/minimize the selected objective functions.To demonstrate the effectiveness of the proposed methodology, a proof of concept was conducted to optimize the energy performance of novel ICSSWH by considering nineteen parameters with the aim of maximizing/minimizing three different objective functions. The results of this analysis yielded various sets of optimised parameters for the ICSSWH. Therefore, by implementing the set of optimised parameters into the dynamic simulation model, the energy performance of the optimised and non-optimised ICSSWH collectors was evaluated in 42 different weather zones and compared to a reference case scenario.

AC Series Arc Fault Modeling for Power Supply Systems Based on Electric-to-Thermal Energy Conversion

The measured voltage waveform of the arc fault for the 220 V power supply system is continuous but non-smooth which is hard to be reproduced with the traditional arc models. Consequently, this paper proposes an arc model based on electric-to-thermal energy conversion. Both the heat convection loss and the heat conduction loss are considered in this arc model. This proposed arc model is specially used to simulate the arc-circuit interaction during the whole development process of the continuous-arc-fault for the 220 V power supply system. Due to the fact that this proposed arc model is a multivariable-nonlinear differential equation, this paper proposes a computation method based on the current region splitting according to the non-smooth connections of the measured voltage waveform. The model parameters can be adjusted independently in each current region. Then the comparisons between measurements and the arc model results are given which show a good agreement in the all current regions. In addition, the effective estimations for the arc geometrical and electrical parameters are achieved based on this proposed arc model. Finally, experimental results validate that the arc model and the corresponding computation method proposed in this paper are general and effective for the 220 V power supply system.

Thermal analysis of the hybrid HVAC unit using interpolant function

This study aims to cover the thermodynamic aspect of a geothermal site using the Hermite interpolant. The energy distribution, determination of the system exergy and the energy ratio are some of the factors investigated thoroughly. The steady flow energy equation was used to determine the design parameters related to the hybrid heating, ventilation, and air condition (HVAC) system. For the underground heat exchanger, it was assumed that the heat capacity ratio, R, was small in magnitude. The energy ratio estimated in the corridor of the lecture hall increased by 17% as compared to the hall area. The exergy destruction of the system was estimated to be 25%, whereas the maximum exergy gain computed at the secondary side of the geothermal unit, followed by the second fan coil, the first fan coil, and the radiator. The calculated heating rate was found to be 25.273% lower than the required sensible heat gain. The installed unit could attain the maximum energy ratio of 4.031 if the convective losses across the building material are minimised.

Understanding Interfacial Properties for Enhanced Solar Evaporation Devices: From Geometrical to Physical Interfaces

Various materials and device configurations have been reported to enhance the evaporation efficiency of solar interfacial evaporation systems, but it has not yet been revealed how to quantitatively assess the optimal materials and devices to maximize evaporation performance. In this study, the evaporation interface (location and thickness) is identified to quantify the interplay of optical and transport processes for guiding the rational design of materials and devices. We theoretically and experimentally demonstrate that an absorption coefficient of 400 m–1 pinning the interfacial location below the top surface leads to optimal efficiency with reduced radiation and convection losses. A multistage evaporation device based on the optimized interface properties in the transitional region was demonstrated showing an evaporation rate of 5.38 kg m–2 h–1, which is 12% higher than that in the interfacial region. In addition, the optimized device can operate stably with seawater for more than 10 h without salt crystallization.

Investigation of a Compound Parabolic Collector with a Flat Glazing

The compound parabolic concentrator is a promising technology for efficient solar irradiation exploitation at low- and medium-temperature levels. This collector type can be used in a series of applications, such as solar cooling, desalination, and industrial process heat applications. This work presents a novel compound parabolic concentrator that presents satisfying efficiency and low cost due to the use of flat glazing and not an evacuated tube receiver. More specifically, the goal of the present investigation is based on the energy and exergy analysis of a compound parabolic collector with flat glazing, which has a concentration ratio of 2.81. The collector is examined thermally and exegetically, aiming to calculate the efficiency of different operating inlet temperatures. Moreover, the solar unit is studied by a developed computational fluid dynamics model in the SolidWorks Flow Simulation tool. Emphasis is given to the calculation of the convection losses of the receiver tube with the internal air inside the collector. The heat convection coefficient is calculated, and the distribution of the thermal losses, convection, and radiation is presented. Furthermore, the temperature levels of the absorber, the cover glass, and the top thermal loss coefficient are found. The thermal efficiency of the solar unit was 77.4% for inlet temperature at 10 °C and 32.6% for inlet temperature at 110 °C. It was calculated that the maximum exergetic performance of the solar unit is 10.19% for operation at 90 °C, while the thermal efficiency for this case is 41.57%. Additionally, the temperature distributions for different cases are included in the present work.

Performance enhancement of Scheffler concentrator receiver using Conical type Secondary Reflector (CSR) fitted with toughened glass cover

Fixed focus type Scheffler Concentrators and convex receivers are popularly employed for several medium temperature process heat applications. The excessive convection heat loss from the receiver surface and the improper distribution of solar flux at the receiver are few key issues commonly observed for the Scheffler concentrators. The excessive heat lost is attributed to the convection losses at the receiver convex surface due to the wind. Moreover, experimental studies carried out on the receiver of 16 m2 Scheffler concentrator revealed that about 15–20 % of the total radiations received at the receiver are lost due to the focusing and tracking errors of the Scheffler reflector. In order to minimize the heat losses from the receiver convex surface, a Conical type Secondary Reflector (CSR) made up of highly polished mirror finished stainless steel material, in the form of frustum of a cone, and covered with toughened glass was designed and developed. The semi-cone angle of 36° has been estimated for the CSR based on the ray trace diagram analysis. The CSR, fixed over the periphery of the convex type receiver refocuses the lost solar radiations over the receiver surface and enhances the heat flux received at the receiver, while improving the overall efficiency of the concentrator. The toughened glass cover over the CSR, shields the receiver from the wind and thus reduces the overall convection losses due to wind. Experimental analysis carried with the installation of CSR revealed that the convective heat transfer coefficient has been reduced by 10–12 % on average while the overall system efficiency has been increased by at least 3–5 %. The increased heat flux received at the receiver has not only reduced the convection losses but also reduced the initial idle or lag time for the system by almost 15 min based on the solar insolation for the day.

Thermal performance improvement of a heat-sink using metal foams for better energy storage systems

Newly, energy storage systems and harvesting energy equipment are modern and important topics in engineering problems, but they still face the efficiency problem. Using the advantages of micro-scale flow and metal porous materials can be applied to improve the performance of these systems. So, the present investigation provides a numerical study on a heatsink employing various arrangements of metal foams inside a heat sink (in the direction and against the fluid flow). The influences of diverse fluid flow characteristics, metal foams' permeability and thermophysical properties of the coolant (using CuO-MgO/water nanofluids) on the thermal efficiency of the heatsink are examined. Then, the variations of convective heat transfer coefficient and pressure loss for all variables are investigated, and performance evaluation criteria are defined to distinguish the simultaneous impacts of improving heat exchange against incrementing the pressure loss. It indicates the metal foams located in the direction of the flow perform better than those located against the flow, which increases the performance evaluation criteria (PEC) of the heatsink between 1.18 and 1.55 depending upon the permeability of porous media. Also, using the nanofluids in the presence and absence of metal foams decreases the heatsink performance. Investigating the impacts of volumetric flow rate illustrates that thermal performance increases by increasing the flow rate. In addition, outcomes reveal that although adding metal foams and nano-additives and increasing volumetric flow rate enhance the frictional entropy generation rate, the total amount of entropy generation rate decreases due to more contribution of the thermal entropy generation rate, that reduces by those variables.

Global 3D radiation-hydrodynamical models of AGB stars with dust-driven winds

Context. Convection and mass loss by stellar winds are two dynamical processes that shape asymptotic giant branch (AGB) stars and their evolution. Observations and earlier 3D models indicate that giant convection cells cause high-contrast surface intensity patterns, and contribute to the origin of clumpy dust clouds. Aims. We study the formation and resulting properties of dust-driven winds from AGB stars, using new global 3D simulations. Methods. The dynamical stellar interiors, atmospheres, and wind acceleration zones of two M-type AGB stars were modeled with the CO5BOLD code. These first global 3D simulations are based on frequency-dependent gas opacities, and they feature time-dependent condensation and evaporation of silicate grains. Results. Convection and pulsations emerge self-consistently, allowing us to derive wind properties (e.g., mass-loss rates and outflow velocities), without relying on parameterized descriptions of these processes. In contrast to 1D models with purely radial pulsations, the shocks induced by convection and pulsation in the 3D models cover large parts, but not the entirety, of the sphere, leading to a patchy, nonspherical structure of the atmosphere. Since dust condensation critically depends on gas density, new dust clouds form mostly in the dense wakes of atmospheric shocks, where the grains can grow efficiently. The resulting clumpy distribution of newly formed dust leads to a complex 3D morphology of the extended atmosphere and wind-acceleration zone, with simultaneous infall and outflow regions close to the star. Highly nonspherical isotherms and short-lived cool pockets of gas in the stellar vicinity are prominent features. Efficient dust formation sets in closer to the star than spherical averages of the temperature indicate, in dense regions where grain growth rates are higher than average. This can lead to weak outflows in situations where corresponding 1D models do not produce winds. For stars where the overall conditions for dust formation and wind acceleration are favorable, it is unclear whether the resulting mass-loss rates will be higher or lower than in the 1D case. The increased efficiency of dust formation in high-density clumps may be offset by a low volume coverage of the forming clouds. Conclusions. A global 3D approach is essential to make progress in understanding dynamical processes in AGB stars, and, in particular, to solve long-standing problems regarding mass loss.

Gradient-based optimization of micro-scale pressurized volumetric receiver geometry and flow rate

This study focuses on the design optimization of a micro-scale pressurized volumetric receiver by changing geometry and flow rate constrained by the volume, outlet air temperature, and outer surface temperature. The pressurized volumetric receiver model is replicated from an existing model, which assumes constant air pressure and neglects the convection loss from the cavity. The existing model is revised from a solver to a design optimizer.The replicated model is restructured using OpenMDAO (Open-source MultiDisciplinary Analysis and Optimization) framework, and analytical derivatives are implemented for efficient derivative calculation to increase optimization performance. The replicated model is verified, and the maximum outlet air temperature difference is less than 0.05%. Optimization performance, selection of optimizers, the effect of the domain size, and radiative methods are discussed. The combined impact of the design variables is observed by selecting SLSQP (Sequential Least SQuares Programming) and trust-region optimizers. Optimization performance is tested in different domain sizes and compared with a design of experiments analysis. For testing the impact of radiative heat transfer methods to design optimization, the Rosseland approximation, and P1 method are selected. Depending on the design domain, a solution methodology is suggested for future receiver design optimizations applicable for macro-scale pressurized volumetric receivers.

Energy, Exergy and Economic (3E) analysis of evacuated tube heat pipe solar collector to promote storage energy under North African climate

Evacuated tube solar collectors are solar water heating devices known to be technically viable and economically attractive. In contrast, these systems encounter several problems owing to the unavoidable heat and water losses along the manifold, essentially conduction and convection losses. In warm weather, heat pipes become partially filled with water and boil, increasing the need for a pressure vessel. One of the popular strategies used to tackle this technical trouble is integrating Phase Change Materials (PCMs) into the evacuated tube solar collectors. In this paper, this problem was approached with a broader perspective by assembling three collectors technically modified, thermally optimized, and economically profitable. The first configuration presents a normal industrial solar collector (1). While in the second and third configurations, the solar system was improved through several modifications. In the second system, the manifold was re-insulated all around (2). In the third, the manifold was surrounded by PCM filled into a copper cavity (3). To well highlight the performed amendments, a comparative approach between the three vacuum tube heat pipe solar collectors is presented. The established methodology is based on energy and exergy analysis and is supplemented by an economic evaluation. It was found that the energy and exergy efficiencies for the conventional collector were 41 % and 15 % respectively. While the thermal energy performance recorded for the second collector was ranged between 60 % and 80 %. Concerning the third collector with insulation and thermal storage unit, the total solar energy absorbed during a typical day was 39.68 MJ, the total useful energy recuperated during a day was 4.11 MJ and the daily thermal efficiency was 13 %. The efficiency of the improved collector (3) was further improved until 28 MJ of energy storage was recorded of which 4.3 MJ were delivered to the water overnight. Moreover, an economic feasibility study was carried out, in which the economic feasibility for the last new system.showed an amortization period of five years.

Modeling of energy and exergy efficiencies in high vacuum flat plate photovoltaic–thermal (PV–T) collectors

This work deals with the performance evaluation of novel flat photovoltaic–thermal (PV–T) modules under vacuum. Through a 1D (dimensional) steady-state-energy-balance numerical model developed in MATLAB, two different layouts are studied: the first consisting of a photovoltaic (PV) cell installed just below the glass encapsulating the flat panel, and the second where the PV cell is placed on the selective solar absorber (SSA). In both cases the thermal and electrical efficiencies have been evaluated at different SSA operating temperatures, in the range of 323 K to 423 K. The analysis has been conducted at different energy bandgap (Ebg) of the PV cell and assuming a variable transmittance or emittance of the PV cell, depending on the design. The two systems efficiency comparison has been carried out at the same operating temperature. Overall, this work highlights the importance of high vacuum insulation, which guarantees the reduction of convective thermal losses, and shows that the maximum energy is produced for PV cells with Ebg≈1.5-1.7 eV, depending on layout and operating temperature, by including the thermal output in the PV–T​ optimization. The energy and exergy efficiencies obtainable using the proposed PV–T​ systems are considerably improved compared to the results previously reported in the literature.

Understanding dust production and mass loss in the AGB phase using post-AGB stars in the Magellanic Clouds

Context. The asymptotic giant branch (AGB) phase of evolution in low- and intermediate-mass stars is governed by poorly understood physical mechanisms, such as convection, mixing, dust production and mass loss, which play a crucial role in determining the internal structure and the evolution of these stars. The spectra of post-asymptotic giant branch (post-AGB) stars hold critical chemical fingerprints that serve as exquisite tracers of the evolution, nucleosynthesis, and dust production during the AGB phase. Aims. We aim to understand the variation in the surface chemistry that occurs during the AGB phase by analysing results from observations of single post-AGB stars in the Magellanic Clouds. We also aim to reconstruct dust-formation processes, which are active in the circumstellar envelope of AGB stars, occurring towards the end of the AGB phase and during the subsequent course of evolution when contraction to the post-AGB has begun. Methods. We study likely single post-AGB sources in the Magellanic Clouds that exhibit a double-peaked (shell-type) spectral energy distribution (SED). We interpret their SED by comparing with results from radiative transfer calculations to derive the luminosity and the dust content of the individual sources. Additionally, we compare the observationally derived stellar parameters and the photospheric chemical abundances of the target sample with results from stellar evolution modelling of AGB and post-AGB stars. This allows for the characterization of the individual sources in terms of the initial mass and formation epoch of the progenitors. The theoretically derived dust mineralogy and optical depth is used to assess when dust formation ceases and to determine the propagation velocity of the dust-gas system during post-AGB evolution. Results. We find that amongst our target sample of 13 likely single post-AGB stars with shell-type SED, eight objects are carbon stars descending from ∼1−2.5 M⊙ progenitors. Five of the 13 objects are of lower mass, descending from M < 1 M⊙ stars. Based on the dust mineralogy, we find that these five stars are surrounded by silicate dust, and thus failed to become carbon stars. The dust optical depth and the luminosity of the stars are correlated, owing to the faster evolutionary timescale of brighter stars, which makes the dusty layer closer to the central object. From our detailed analysis of the SEDs, we deduce that the dust currently observed around post-AGB stars was released after the onset of the central star contraction and an increase in the effective temperature to ∼3500−4000 K.

Combined Thermal Performance Enhancement of Parabolic Trough Collectors Using Alumina Nanoparticles and Internal Fins

Parabolic trough collectors are the currently dominant technology for concentrated solar power systems, employed to produce thermal energy at low to medium temperatures (up to 400°C). Extensive research has been carried out to enhance the thermal efficiency and reduce the power production costs of these concentrators. However, there is a lack of studies on combined passive performance enhancement using alternative fluids and absorber designs. In this study, the thermal performance of a full-sized parabolic trough collector is analyzed with the presence of internal longitudinal fins in combination with the use of oil-based nanofluid (Al2O3-Syltherm 800) of different volume fractions. The governing equations are numerically solved using ANSYS FLUENT 17.1 software and the Monte-Carlo ray-tracing (MCRT) model was used to apply the non-uniform heat flux profile over the external surface of the solar receiver. The results show that both techniques enhance thermal energy utilization and reduce radiative and convective thermal losses, resulting in higher thermal efficiency, but also larger pressure losses. The thermal performance is enhanced by 0.1-1.16 % with nanofluid, up to 6.8 % with internal fins, and by up to 7.25 % when both techniques are adopted. These enhancements are attributed to the reduced mean circumferential temperature of the absorber tube.

Investigation of a vertical closed‐loop geothermal system for heating an educational building

AbstractThis article is related to the thermal engineering aspect of the hybrid heating unit that can be operated either by shallow geothermal heat or natural gas. The thermal investigation is related to determining the fluctuation in the energy requirement if the unit is coupled with the gas boilers. Therefore, the given system was independently driven by both sources, and the conclusion was based on the comparative merit/demerit of renewable energy over the conventional power supply. The analysis assumed the steady‐state energy flow processes. The heat capacity of the source and sink was assumed to be infinitely large as compared to the working fluid. The leastangle regression algorithm was adopted to establish a mathematical relationship between thermodynamic parameters. The average COP of the closed‐loop geothermal unit was estimated to be 2.07. The overall exergy efficiency of the installed geothermal facility is 3%, whereas it is 33.55% for the gas boilers operated by natural gas. A square meter of the area of the building will get 26.25 W·h of useful convective heat energy from an installed geothermal unit and 28.59% of the total received will be dissipated through the structure. Similarly, the system driven by natural gas provided 21.88 W·h of the net useful convective heat energy, while the convective loss amounted to 23.06%. The net annual energy saving of 502.99 kW·h was expected from the geothermal site.