A transient model for the energy analysis of indoor spaces

Using a finite-difference procedure, the dynamic energy response of indoor spaces under the influence of indoor energy pulses is analyzed. The method of analysis is simple and explicit and is based on the indoor surface thermal capacitance and heat-loss coefficient C_s and L_s respectively. It is demonstrated that these parameters characterize fully any specified indoor space, as far as its energy behaviour is concerned. Their values are calculated for an extended variety of indoor spaces, i.e. for various floor areas, floor dimensions ratios, indoor surface materials of envelope, partitions and furnishings, fenestration and indoor partitions areas. The range of validity of the present method of analysis is also defined and the corresponding deviations are quantified with reference to rigorous finite-difference solutions. The provided values of indoor space characteristics C_s and L_s may be used in a wide range of technological building applications, including comparisons and classifications of indoor spaces, design and selection of construction materials and furnishing as well as the investigation of effects from electric equipment, windows or doors opening, short-time ventilations, brief stay of visitors, etc.     

On the dynamic thermal behaviour of indoor spaces

A new model is presented for predicting the dynamic thermal response of indoor spaces to indoor heat pulses. The model is based on the concept of the “indoor surface thermal capacitance”, C_s, which characterizes the thermal inertia of an indoor space and expresses the heat stored within indoor air and surface layers of walls and furnishings, per degree of mean temperature difference between indoor air and building envelope. Extensive comparisons with measurements and rigorous finite-difference solutions show that the accuracy of the proposed model is satisfactory for a wide range of practical applications. Comparison with other indoor space simulations of the same class, characterized as “simplified approaches”, show that the present one may provide considerably increased accuracy.     

A new look at the correlation of Kd and Kt ratios and at global solar radiation tilt models using one-minute measurements

One-minute measurements of horizontal diffuse, D_h, and global, G_h, solar radiation components were used to calculate K_d and these ratios were compared with values determined from previously published functions of the independent parameter K_t. The spectrum of the 9906 ratios, used in the comparison, was taken from representative days with percent sunshine ranging from 10 to greater than 90 per cent and from different seasons. The results of this comparison show that K_d is not accurately determined by these functions of K_t alone and from functions fitted to our data, and suggest that other parameters might be needed. The calculation of global radiation on a tilted surface from models which depend on D_h were also analyzed by comparisons with global tilt radiation measured at 45° and 90° tilt angles at each of the four cardinal directions. The poor determination of K_d causes large scatter in calculated global tilt radiation values because of the corresponding poor value of D_h. However, if a good value of D_h is used then the global tilt models replicate measured global tilt radiation values to within 10 per cent.     

Mass effects in residential energy consumption

The objective of this study was to determine the effect of mass on both the annual energy consumption and the maximum rate of energy consumption. There have been recent references in the literature to such effects as ‘mass resistance’, ‘capacity insulation’, and ‘thermal inertia’, suggesting that mass can actually resist the flow of energy through a wall. In this study it is shown that the annual energy comsumption is dependent on the R-factor and the heat capacity per unit area. An example compares two buildings, one with walls that are 400 times as massive as those of the other, and yet the annual energy consumption is identical; hence, the idea of ‘mass resistance’ is to be avoided. The real effect of increased mass is typically to increase the heat capacity. The effect of increased heat capacity in a building is shown to lead to a slight decrease in the annual energy consumption and a significant decrease in the maximum rate of energy consumption.     

Analytical optimization of interior PCM for energy storage in a lightweight passive solar room

Lightweight envelopes are widely used in modern buildings but they lack sufficient thermal capacity for passive solar utilization. An attractive solution to increase the building thermal capacity is to incorporate phase change material (PCM) into the building envelope. In this paper, a simplified theoretical model is established to optimize an interior PCM for energy storage in a lightweight passive solar room. Analytical equations are presented to calculate the optimal phase change temperature and the total amount of latent heat capacity and to estimate the benefit of the interior PCM for energy storage. Further, as an example, the analytical optimization is applied to the interior PCM panels in a direct-gain room with realistic outdoor climatic conditions of Beijing. The analytical results agree well with the numerical results. The analytical results show that: (1) the optimal phase change temperature depends on the average indoor air temperature and the radiation absorbed by the PCM panels; (2) the interior PCM has little effect on average indoor air temperature; and (3) the amplitude of the indoor air temperature fluctuation depends on the product of surface heat transfer coefficient h_in and area A of the PCM panels in a lightweight passive solar room.     

Inverse design methodology to optimize sensible thermal energy storage systems working as rectifiers

The main goal of this paper is to present a methodology to achieve the optimal design of a sensible thermal energy storage system (T3S) working as a thermal rectifier. The system was composed by the heat storage material (HSM), distributed in a set of flat parallel plates, and the working fluid, both modeled by a simplified lumped element model (LEM). The ratio of operational outlet temperature range to source temperature oscillation is defined as the designed rectifying duty. Optimization procedure combines a one‐factor‐at‐a‐time (OFAT) and line search strategies in order to find optimal T3S design that satisfies the designed rectifying duty with the minimum HSM mass. The inverse design philosophy is applied to the optimal results to generalize the T3S dynamic behavior as functions fitting curves of the number of transfer unit (NTU) and the time constant τ . These fitting curves can be used to identify T3S geometric parameters, HSM thermal properties, fluid inlet conditions, among others, which guarantee the outlet fluid temperature to be found within the operational range with the minimum HSM mass. A three step‐by‐step sequence design methodology is presented and detailed, based on design charts from the NTU and τ correlations. The proposed design methodology is able to find the optimal plate length L, plate thickness e_s , and plate distance e_f that satisfies the designed rectifying duty for three test cases. These optimal T3S designs are simulated in a computer fluid dynamics (CFD) code, with deviations below 1.5% between the designed rectifying duty and the one simulated. With the proposed approach, several design solutions or configurations can be found for T3S operating as a thermal rectifier based on NTU and τ fitting curves submitted to a sinusoidal cyclic temperature input and with constant and uniform HSM and fluid properties.     

Theoretical estimations of the efficiency of thermophotovoltaic systems using high-intensity silicon solar cells

Efficiencies have been calculated for (a) heat-mirror filter and (b) rear-mirror thermophotovoltaic systems using actual materials values for source emissivity ((λ)), mirror-filter transmissivity (τ_M(λ)) and intensity-dependent cell parameters V_oc and FF. The ratios of heat-mirror and rear-mirror efficiencies to direct sunlight impingement efficiency are estimated to be 1.6 and 1.9, respectively (2500°K thermal source), if mirror and other extraneous absorption are taken to be zero in both cases. The more realistic inclusion, for the first case, of 2 per cent light absorption in the filter/mirror and, in the second case, of 2 per cent absorption of low energy light in the cell and 8 per cent absorption at the rear mirror, plus a 2 per cent thermal loss directly from the hot source (for both), reduces the advantage of both to about a factor of 1.2.     

Oxide semiconductors in photoelectrochemical conversion of solar energy

A wide range of oxides is examined for use as anodes in photoelectrochemical cells for the conversion of solar energy into electrical power or hydrogen. The Schottky barrier model of the semi-conductor-electrolyte interface is used throughout. Type (a) oxides, not containing partly-filled d-levels, are found to conform to the relationship between flat band potential, V_fb(SHE), and band gap, E_g, V_FB(SHE) = 2.94 − E_g which essentially rules out the possibility of finding type (a) oxides with simultaneously the small band gap and large negative flat band potential required for efficient operation in the unbiased photoelectrolysis of water.Incorporation of this realationship into the Schottky barrier formula for photocurrent enables the calculation of efficiencies of conversion for type (a) oxides. For air mass 1 radiation, the predicted maximum efficiency of conversion to hydrogen is 3.4 per cent for E_g = 3 eV for unbiased photoelectrolysis; 6.3 per cent for E_g = 2.4 eV for series voltage biased photoelectrolysis; and 4.7 per cent for E_g = 2.2 eV for pH biased photoelectrolysis. For power cells with redox operation, type (a) oxides are predicted to give 5–6 per cent efficiency for E_g = 2.4 eV, with a redox couple having standard potential not less than 0.8 V. High efficiency operation of photoelectrochemical cells with type (a) oxide anodes appears to be possible only in some special cases.Comparisons of the observed stabilities for a wide range of oxides with their calculated thermodynamic stabilities towards photoanodic dissolution indicate that oxygen overpotentials at the oxide anodes in photoelectrolysis may be small, and that the thermodynamic method of estimation of long term stability in oxide anodes is useful.The prospects of obtaining satisfactory efficiencies with oxides containing partly-filled d-levels are also examined, and found to be severely limited by the simultaneous requirements of stability, flat band potential, and band gap. Some suggestions for further research are made.     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.     

Comparative study of natural solar drying of cassava, banana and mango

This paper is about a comparative study of some parameters related to natural solar drying of cassava, ripe(I) or raw(II) plantain banana and mango. The comparative study was carried out by investigating the constant rate period, the falling rate period and the transition between these two periods. For the constant rate period, the product temperature T_p and the ambient temperature T_a were analysed in relation with the initial moisture content W_i. Then the maximum temperature difference DT_max between T_a and T_p was investigated, and found to decrease exponentially with W_i. In the constant rate period, parameters such as h_c/λ, dm/dt and the mass transfer coefficient h_m were reported and discussed. A new parameter was then introduced and termed the specific rate R_sp, which strongly correlated to W_i and DT_max. The critical point W_c was also taken into account. As for the falling rate period, the study was carried out by means of the overall resistance to diffusion R_ds. The next step was the analysis of the drying rhythms. Finally, the investigation about the rewetting behaviour of the four products was made by using the desorption and sorption velocities (V_des and V_sor), and the rewetting rate, too.     

Auto-ignition of toluene-doped n-heptane and iso-octane/air mixtures: High-pressure shock-tube experiments and kinetics modeling

Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times τ_ign were measured behind reflected shock waves in mixtures with air at φ = 1.0 and 0.5 at p = 40 bar, over a temperature range of T = 700–1200 K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on τ_ign on the base fuels as function of temperature. For typical toluene tracer concentrations ?10%, the ignition delay time τ_ign changes by less than 10% in the relevant pressure and temperature range.     

Salt diffusion in a temperature field: Application to salinity profiles in solar ponds

In the present investigation, the process of diffusion of salt in a vertical column of liquid, subjected to temperature variations of the types T(x) = constant, linear ( = a + bx) and parabolic ( = a + bx - cx²); with a constant concentration difference between the top and the bottom (0 and 25 per cent, respectively) is studied. It is seen that a linear temperature gradient, T(x) = a + bx, leads to a near convex parabolic salt concentration profile with maximum deviations increasing from 13.5 per cent (at 40°C) to 14.8 per cent (at 70°C) and eventually to 15.7 per cent (at 90°C) with respect to the linear concentration value of 12.5 per cent (by weight) at the midpoint. Conversely, the parabolic temperature profile as well as the modified profile due to the Soret effect leads to near cubic salt profiles which differ only by 2–3 per cent in the upper half of the pond. However, they show a point of inflexion at larger depths near the bottom around which the convex profiles change over and become concave. Subsequently, these studies have been extended to compute the salinity profiles of thermal configurations of the operational solar pond.     

Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra‐lean methane–air combustion in some precious metal‐based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas‐phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very‐low emission stable combustion were found to be significantly lower than for conventional gas‐phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NO_x and unburnt hydrocarbons were detected. Thus, the operating conditions verified ‘near‐zero’ pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO₂ via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible. Copyright © 2000 John Wiley & Sons, Ltd.     

Investigation of heat transfer augmentation in a horizontal tube using different rectangular cut ring inserts

Experimental and computational investigations have studied the heat transfer, friction factor, and enhancement of heat transfer in a horizontal tube equipped with rectangular cut ring inserts and different diameter ratios (D/d) and pitch-to-tube diameter ratios (p/d_t). In the present study, air having a Reynolds no. range of 6700–20,100 was used as a working fluid. Three diameter ratios (D/d) were considered experimentally and numerically as 1.2, 1.25, and 1.3, and the pitch-to-tube diameter ratio (p/d_t) was (1, 0.625, and 0.5). Air was forced as working fluid through the tube and a uniform heat flux of 2000, 3500, and 5000 W/m² was applied through the tube's exterior surface. On the basis of the turbulence model k–ɛ with various parameters, three-dimensional numerical simulations using the ANSYS Fluent software 17.2 were investigated. Under the same working conditions, the results manifested a higher heat transfer rate and friction factor as compared to the plain tube. The results evinced that the Nusselt number for a horizontal tube equipped with rectangular cut ring inserts having various pitch ratios and diameter ratios is discovered to be higher than that for the plain tube. With the increased ring spacing, the overall improvement in heat transfer occurred. And, with a rise in Re, the total enhancement ratio decreased. Consequently, the greatest overall improvement attained was 38% at Reynolds number (Re = 12,860) with the pitch ratio (p/d_t = 1). The three diameter ratios (D/d) of 1.3, 1.25, and 1.2 gives in this study the average thermal performance factor in the value of 1.6, 1.5, and 1.4, respectively. Using the Nusselt number and friction factor, the results are correlated as a function of the Reynolds number, diameter ratio, and pitch ratio.     

The diffuse fraction of hourly solar radiation for Amman/Jordan

This paper presents various correlations of the hourly clearness index K_t with the hourly diffuse fraction K_d on a horizontal surface for Amman/Jordan. The first correlation is based on Orgill and Hollands method, but using weighted averages. The second correlation is based on hourly integrated values of K_t's and K_d's without any grouping. A refinement of the latter correlation is attempted by including a random shock.     

Thermal performance of an architectural window with chemically deposited SnS---CuxS solar control coating

A mathematical model enabling the prediction of the thermal performance of solar control glazings employing chemically deposited solar control coatings with or without a transparent protective polymer coating is presented. Differential energy balance for the glazing is set up assuming one-dimensional steady state case for normal incidence of air mass 2 solar radiation and by considering conductive heat transfer within the glazing and convective and radiative heat transfer into the interior and exterior of the building. Using the specific example of the optical properties of the already reported SnS---Cu_xS solar control coatings, the redistribution of the absorbed component of the solar radiation is evaluated for constant convective heat transfer coefficient and temperature in the interior and for exterior temperatures in the 0–50°C range. The results yield shading coefficient versus exterior temperature curves for two specific SnS---Cu_xS coatings without and with a protective transparent varnish and offering transmittance in the visible region of 27 and 21%.     

Effect of Blocked Core-Tube Diameter on Heat Transfer Performance of Internally Longitudinal Finned Tubes

Three-dimensional turbulent flow and heat transfer in an internally finned tube with a blocked core-tube have been numerically studied by the realizable k ? ε turbulence model with the wall-function method. The numerical method is validated by comparing the calculated results with experimental data. The range of ratio of blocked core-tube outside diameter to outer-tube inside diameter (d ₀/D _i ) is from 0.25 to 0.75. The computational results demonstrated that there exists an optimal ratio of (d ₀/D _i ) under both identical mass flow rate and identical pressure drop. The optimal ratio of (d ₀/D _i ), which is reduced with the increase of mass flow rate, is approximately 0.5 to 0.625 at given mass flow rate for both constant wall temperature and uniform wall heat flux. The optimal ratio of (d ₀/D _i ) at a given pressure drop is from 0.44 to 0.50, which is also slightly reduced with the increase of pressure drop. Furthermore, the optimal ratio of (d ₀/D _i ) is not sensitive to the number of cross-section wavy fins of an internally longitudinal finned tube, in the range of a fin wave number of 15–25.     

Alloy-Like Behavior of the Thermal Conductivity of Nonsymmetric Superlattices

In this work, we show a phenomenological alloy-like fit of the thermal conductivity of (A)_d1:(B)_d2 superlattices with d₁ ≠ d₂; that is, a nonsymmetric structure. The presented method is a generalization of the Norbury rule of the summation of thermal resistivities in alloy compounds. Namely, we show that this approach can be also extended to describe the thermal properties of a crystalline and ordered system composed of two or more elements and has a potentially much wider application range. Using this approximation, we estimate that the interface thermal resistance depends on the period and the ratio of materials that form the superlattice structure.     

ONE MINUTE kb AND kd PROBABILITY DENSITY DISTRIBUTIONS CONDITIONED TO THE OPTICAL AIR MASS

In this work we have analysed and modelled one-minute probability distribution function of solar direct and diffuse irradiance conditioned to the optical air mass. For this purpose, we have used one-minute data acquired in a radiometric station located in southeastern Spain (37.13° N, 3.63° W, 687 m a.m.s.l.). The study has been made over the dimensionless indices k_b and k_d. We have found marked bimodality in the k_b distribution function and asymmetry associated with the unimodal function of one-minute k_d values. In spite of these differences, we have modelled both distribution functions using a functional form based on Botlzmann's statistics. These functions have been used in a previous study devoted to modelling the clearness index for global radiation, k_t. In order to model the asymmetry that is evident in the k_d and k_b distribution functions, the functional forms have been modified by the inclusion of an additional parameter. The adjustable parameters included in the model equations present a dependence on the optical air mass.     

Transient heating of an evaporating droplet with presumed time evolution of its radius

New solutions to the heat conduction equation, describing transient heating of an evaporating droplet, are suggested, assuming that the time evolution of droplet radius R_d(t) is known. The initial droplet temperature is assumed to be constant or allowed to change with the distance from the droplet centre. Since R_d(t) depends on the time evolution of the droplet temperature, an iterative process is required. Firstly, the time evolution of R_d(t) is obtained using the conventional approach, when it remains constant during the timestep, but changes from one timestep to another. Then these values of R_d(t) are used in the new solutions to obtain updated values of the time evolution of the distribution of temperatures inside the droplet and on its surface. These new values of droplet temperature are used to update the function R_d(t). This process continues until convergence is achieved, which typically takes place after about 15 iterations. The results of these calculations are compared with the results obtained using the previously suggested approach when the droplet radius was assumed to be a linear function of time during individual timesteps for typical Diesel engine-like conditions. For sufficiently small timesteps the time evolutions of droplet temperatures and radii predicted by both approaches coincided. This suggests that both approaches are correct and valid. Similarly to the case when droplet radius is assumed to be a linear function of time during the timestep, the new solutions predict lower droplet temperature and slower evaporation when the effects of the reduction of R_d are taken into account. It is shown that in the case of constant droplet initial temperature, models both taking and not taking into account the changes in the initial droplet temperature with the distance from the droplet centre predict the same results. This indicates that both models are correct.