Analytical and Experimental Investigation of a Triangular-Channeled Solar Water Heater

Utilization of solar energy is increasing in different states of the world, and the sun is regarded as the largest source of continuous and coherent energy. In the present study, a novel configuration of a v-corrugated solar collector with triangular channels for domestic water heating has been analytically investigated. A mathematical model based on effectiveness-NTU method is established to thermally examine the collector. Additionally, the heat losses from the body of the collector, useful energy from the collector and solar efficiency have been calculated analytically over different operating parameters. The effects of mass flow rate and solar heat flux on water outlet temperature are evaluated analytically and compared with the experimental results. Moreover, the study includes the experimental and theoretical investigation of the heat exchange effectiveness and thermal efficiency of the proposed absorber. The study shows that high temperature and high performance can be obtained from this collector as more heat energy can be collected by using triangular channels because all the three sides of these channels are exposed to solar radiations at the same time. Therefore, these channels will enhance the collector exposed surface area and thereby increase the solar efficiency and overall performance of the system.

Numerical Study of Heat and Water Vapour Exchanges Inside a Green Roof Building in a High Irradiation Area for Passive Cooling Purpose

Vegetation cover provides shading and protects the soil beneath them from warming. Vegetation can be used as passive cooling technique that reduces the thermal load of a building. A numerical study has been carried out on laminar double-diffusive mixed convection in a green roof enclosure. The model is equipped with inlet and outlet openings for air removal while the left vertical wall is heated and partially saturated with water for indoor air humidification. The mathematical model is governed by the two-dimensional continuity, momentum, energy and concentration equations. Transfer equations are solved using a finite difference scheme and Thomas algorithm. The model was applied for the simulation of a building with green roof in Togolese climate conditions. Results showed that, the flow structure is a mixed convection type, but the isotherms et iso-concentration distributions reveal a vertical stratification of the temperatures and the relative humidity.To predict heat transfers inside the cavity, a correlation has been established for the estimation of the average Nusselt number as a function of the Leaf Area Index and Reynolds number under solar heat flux of 350 W.m-2, the average in case of Togo. It was found that a larger Leaf Area Index reduces the solar flux penetration and therefore, reduces significantly heat transfer inside the enclosure and then stabilizes it temperature. For the LAI equal to 3, the indoor air fluctuates around 26°C and the relative humidity range is found to be 50% - 60% under solar heat flux of 350 W.m-2.

Numerical investigation of solar collectors as a potential source for sintering of ZrB2

Sintering of ceramics is an energy-consuming process that needs high temperatures, therefore, in the present work; solar energy is used to produce high temperatures for the sintering aim of different materials. Solar energy concentrators increase the intensity of incident energy to the receiver provides high temperatures. Ultrahigh-temperature ceramics (UHTCs) due to their high melting point can also be a good alternative for receiver materials. In the present work, ZrB2 is introduced as an alternative material for solar receivers which can withstand high temperatures of sintering. The governing equations, including heat radiation and conduction ones are solved numerically using the finite element method. Transient heat transfer in the concentrator-collector system is investigated to check the feasibility of high temperatures needs for sintering at the receiver. The highest temperature of 1680 °C was achieved after 15 minutes at the focal point of the concentrator when the solar heat flux of 6.86 w/mm2 used for the location of the city of Ardabil in Iran. The obtained temperature can be used to sintering of some groups of materials.

Heat Transport through Diurnal Warm Layers

Penetration of solar radiation in the upper few meters of the ocean creates a near-surface, stratified diurnal warm layer. Wind stress accelerates a diurnal jet in this layer. Turbulence generated at the diurnal thermocline, where the shear of the diurnal jet is concentrated, redistributes heat downward via mixing. New measurements of temperature and turbulence from fast thermistors on a surface-following platform depict the details of this sequence in both time and depth. Temporally, the sequence at a fixed depth follows a counterclockwise path in logϵ–logN parameter space. This path also captures the evolution of buoyancy Reynolds number (a proxy for the anisotropy of the turbulence) and Ozmidov scale (a proxy for the outer vertical length scale of turbulence in the absence of the free surface). Vertically, the solar heat flux always leads to heating of fluid parcels in the upper few meters, whereas the turbulent heat flux divergence changes sign across the depth of maximum vertical temperature gradient, cooling fluid parcels above and heating fluid parcels below. In general, our measurements of fluid parcel heating or cooling rates of order 0.1°C h−1 are consistent with our estimates of heat flux divergence. In weak winds (<2 m s−1), sea surface temperature (SST) is controlled by the depth-dependent absorption of solar radiation. In stronger winds, turbulent mixing controls SST.

Experimental Investigation of Heat Flux Characteristics on the Thermally Induced Vibration of a Slender Thin-Walled Beam

The spacecraft with large flexible space structures may be subject to the thermally induced vibration (TIV) due to the rapidly changed solar heat flux when it enters and leaves the eclipse, which would lead to certain spacecraft failure. This paper reports a laboratory experiment that aims to study the impact of transient characteristics of heat flux on the ground experiment of TIV. In the experiments on the TIV of a slender thin-walled beam, two different methods of providing transient heat flux were considered, and the process of entering and leaving eclipse was simulated, respectively. The experimental results demonstrate that different transient characteristics of heat flux will have large impact on the TIV of the specimen, and the ideal theoretical estimation of thermal characteristic time has limitations in practical engineering. In addition, it is found that the traditional way of simulating solar heat flux by turning on/off infrared heat lamps is not suitable for the TIV ground experiment. Instead, a transient heat flux simulation method by moving the baffle is recommended.

CFD investigation of temperature distribution, air flow pattern and thermal comfort in natural ventilation of building using solar chimney

Purpose The purpose of this study is to investigate air flow, temperature distribution and thermal confort in natural ventilation induced by solar chimney for different operating. Design/methodology/approach Numerical simulation is performed using a commercial computational fluid dynamics (CFD) package ANSYS CFX software to understand the effects of air temperature, air velocity and solar heat flux on the performance of the solar chimney and thermal comfort. The comfort level was evaluated using the air diffusion performance index (ADPI) according to ASHRAE (55-210). The flow was investigated at inclination angles 45° solar heat flux 550-750 W/m2 and in a solar chimney of 1.4 m length, 0.6 m width and 0.20 m air gab. Findings The numerical results from the present simulation were first validated with experimental data, which was used for the thermal comfort indexes calculation. The obtained results of the analysis showed that the used numerical technique could accurately predict air flow and temperature distribution in natural ventilated building using solar chimney; the air temperature, air velocity and solar heat flux have a significant impact on thermal comfort; the temperature of 19°C with velocity of 0.15 m.s−1 gives the best effective draft temperature (EDT) satisfy ASHRAE (55-210) criteria that V = 0.35 m.s−1 and EDT range between −1.7 and 1.1. Originality/value In the present paper, air flow, temperature distribution and thermal comfort inside a room equipped with inclined solar chimney were numerically investigated and analyzed. The commercial CFD package (CFX 15) is used. Calculations are carried out in an empty room without any human or mechanical activity and the numerical results are compared with measurement points.

Influences of Optical Factors on the Performance of the Solar Furnace

In this paper, an optical structure design for a solar furnace is described. Based on this configuration, Monte Carlo ray tracing simulations are carried out to analyze the influences of four optical factors on the concentrated solar heat flux distribution. According to the practical mirror shape adjustment approach, the curved surface of concentrator facet is obtained by using the finite element method. Due to the faceted reflector structure, the gaps between the adjacent mirror arrays and the orientations of facets are also considered in the simulation model. It gives the allowable error ranges or restrictions corresponding to the optical factors which individually effect the system in Beijing: The tilt error of heliostat should be less than 4 mrad; the tilt error of the concentrator in the orthogonal directions should be both less than 2 mrad; the concentrator facets with the shape most approaching paraboloid would greatly resolve slope error and layout errors arising in the concentrator. Besides, by comparing the experimentally measured irradiance with the simulated results, the optical performance of the facility is evaluated to investigate their comprehensive influence. The results are useful to help constructors have a better understanding of the solar furnace’s optical behavior under conditions of multiple manufacture restrictions.