Performance of trombe walls and roof pond systems

This paper describes an analysis of the periodic heat transfer through thermal storage walls and roof pond systems subjected to periodic solar radiation and atmospheric air on one side and in contact with room air at constant temperature (corresponding to air-conditioned rooms) on the other. A one-dimensional heat conduction equation for temperature distribution in the walls and roof has been solved using the appropriate boundary conditions at the surfaces; explicit expressions for the periodic heat flux through storage walls and the roof have been derived. Numerical calculations for the periodic heat flux into the room have been made in order to assess the relative thermal performance of storage walls and roof pond systems in both winter and summer. It is found that a thermal storage mass wall is preferable for longer heat storage times while a water wall is suitable for rapid heat dissipation into the living space. For New Delhi, a roof pond system comprised of water-concrete-insulation, in ascending order of thickness, in the summer and in descending order of thickness in the winter, is found to be most desirable, whereas a combination with an ascending order of thickness is more appropriate for a typical cold climate like that of Boulder, Colorado, USA.     

Thermal load levelling in a multilayered wall/roof

An explicit expression for the periodic variation of thermal flux through a multilayered insulated hollow wall/roof, whose one face is exposed to solar radiation and ambient air and the other is in contact with room air at constant temperature, has been derived. to obtain optimum placement of insulation and airgap, numerical calculations for the heat flux through a multilayered insulated hollow wall/roof have been made for a typical hot day (26 May 1978) in Delhi. It is seen that for a given total thickness of concrete, best load levelling is achieved when the thickness of the outer layer is as small as possible. the effect of a water film on heat flux into the buidling has also been discussed.     

Thermal load levelling of heat flux through an insulated thermal storage water wall

This communication presents an investigation of the thickness distribution of a given total thickness of the insulation inside and outside a thermal storage water wall for acheiving the maximum load levelling of the heat flux entering through the wall. Analysis is based on the solution of the heat conduction equation for the temperature distribution in the insulated wall subjected to periodic solar radiation and atmospheric air on one side and in contact with room air at constant temperature (corresponding to air-conditioned rooms) on the other side. an explicit solution for a temperature distribution satisfying the apporpriate boundary conditions at the surface has been derived to obtaing a periodic heat flux through the storage water wall. It is found that for a given total thickness (cost) of insulation the thicknesses of outside and inside insulation must be equal for best load levelling. Moreover, more load levelling is achieved when the whole of the insulation is outside rather than inside the thermal storage water wall.     

Effect of inhomogeneous thermal conductivity on steady periodic heat flow through semi-infinite media

The paper presents an analysis of the periodic temperature distribution in a semi-infinite medium with continuous inhomogeneity in thermal conductivity, and evaluates its effect on the heat flux at the surface. This analysis has been used to calculate an effective heat transfer coefficient between the air and the ground. For two inhomogeneous thermal conductivity profiles (linearly decreasing and exponentially increasing), the inhomogeneity factor for the effective heat transfer coefficient has been determined.     

Solar thermal modelling of a non-airconditioned building: Evaluation of overall heat flux

This paper presents an investigation of the thermal behaviour of a non-airconditioned building with walls/roof being exposed to periodic solar radiation and atmospheric air while the inside air temperature is controlled by an isothermal mass, window and door in the walls of the room. The effects of air ventilation and infiltration, the heat capacities of the isothermal storage mass inside air and walls/roof, heat loss into the ground, and the presence/absence of the window/door have been incorporated in the realistic time dependent periodic heat transfer analysis to evaluate the overall heat flux coming into the room and the inside air temperature. A numerical computer model using typical weather data for Delhi has been made to appreciate the analytical results quantitatively. It is found that the heat fluxes through different walls have different magnitudes and phase lags w.r.t. the corresponding solair temperatures. The overall heat flux coming into the room as well as the room air temperature are sensitive functions of the number of air changes per hour, closing/opening of the window and the door ventilation. The effects of the heat capacity of the isothermal mass and the basement ground are found to reduce the inside air temperature swing and the presence of a window is found to increase the inside air temperature even when the window area is much smaller than the wall/roof area. The model presented would be an aid to a building architect for good thermal design of non-airconditioned buildings.     

An experimental study of axial conduction through a thermosyphon pipe wall

The effect of the axial conduction through the pipe wall on the performance of a thermosyphon was experimentally investigated in this study. Two 2-phase closed thermosyphons were tested; each had the same dimensions, materials and partially filled with R134a. The only difference between them was that one had a thermal break within the adiabatic section that resisted axial conduction between the evaporator and the condenser sections. The thermosyphons were heated by a constant-temperature hot bath and cooled by water via a concentric heat exchanger. The experiments were performed for different bath temperatures and different fill ratios. It was found that the axial conduction through the pipe wall caused an increase in the overall heat transfer coefficient, evaporation heat transfer coefficient and condensation heat transfer coefficient of the thermosyphon. However, the fraction of heat transfer associated with axial conduction decreased as the heat flux increased. For small heat flux (T_b = 30 °C), the increment of the evaporation and condensation heat transfer coefficient contributed by axial conduction reached 100% and 25%, respectively. For high heat flux (T_b = 60 °C), the increment was negligible (less than 1%).     

Periodic heat transfer through inhomogeneous media. Part 1. Slab

The paper presents exact analytical solutions of one-dimensional periodic heat conduction through an inhomogeneous slab for a certain class of thermal conductivity profiles (including linear and exponential). The exact analytical solutions for some of these profiles have been compared with those obtained by considering the slab to be made up of a number of homogeneous layers with different thermal conductivities varying from layer to layer and using the layered structure (or matrix multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. This gives confidence in the application of the layered-structure method to periodic heat conduction through inhomogeneous slabs. The numerical results have been presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and heat flux on the two sides of the slab.     

Thermal analysis of constant flow partitioned solar pond

This paper presents the thermal analysis of the process of heat extraction by circulating water layer through the convective zone of a partitioned solar pond. The observed variation of atmospheric air temperature and solar intensity is assumed periodic. Explicit expressions for the transient rate and temperature at which heat can be extracted by circulation of water at constant flow rate, are derived. Numerical computations corresponding to solar heat flux and atmospheric air temperature measurement at New Delhi during the year 1974 have been made, and the optimization of the flow rate as well as the depth of the convective-non-convective zones in the pond have been investigated. The optimum heat retrieval efficiency of 27.5%, 34% and 40% corresponding to heat retrieval temperatures of 97°C, 60.5°C and 45.5°C, respectively, are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s.m², respectively. The load levelling in retrieved heat flux improves as flow rates are lowered, and the non-convective zone is oversized. With the non-convective zone depth near optimum, an increase in the depth of the heat extraction zone considerably influences the retrieved heat flux; it shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the extraction zone and larger depths of non-convective zone.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

Heat transfer in a conical cavity calorimeter for measuring thermal power of a point focus concentrator

A theoretical study of the heat transfer process that takes place in a special calorimeter of conical cavity named CAVICAL is presented. This instrument is used to measure the thermal power of a point focus solar concentrator system named DEFRAC, developed at the Center for Energy Research of the National University of Mexico. The DEFRAC concentrator has a power of 1.3 kWth and a very fine optical system. The calorimeter has a cavity opening of 8.24 cm². A detailed heat transfer study was done using FLUENT code. The heat transfer processes taken into account for the analysis were the radiative energy absorbed by the inner cavity wall, the energy transfer from the outer cavity wall to the air by natural convection, the energy transferred by conduction through the inner metallic wall of the calorimeter, and by forced convection to the fluid in the cooling system. The calorimetric information gathered allowed determining the thermal power that the concentrator is able to capture. Temperature and velocity fields have been calculated for each of the thermal fluids considered inside of the calorimeter. The analysis gave thermal losses and measured the thermal efficiency of the device. The information generated is useful to further optimize the design of the calorimeter.     

Winter temperature control in a non-airconditioned building with non-convective roof pond

This paper presents an investigation of the internal thermal environment of a building having a non-convective roof pond. A self consistent periodic heat transfer analysis has been developed to evaluate the indoor heat flux and the resultant inside air temperature. The effects of air ventilation/infiltration, furnishings (assumed to be isothermal masses) and the basement ground heat storage have been incorporated in the analysis. Numerical computations, corresponding to a typical winter day at New Delhi, have been made to illustrate the analytical results. It is found that the placement of a non-convective roof pond of about 10 cm depth considerably enhances the heat flux entering the living space and increases the indoor temperature to comfortable conditions in mild winter climates.     

Thermal modeling of integrated roof air heater for passive solar space heating of a non-air-conditioned building

This communication presents the periodic heat transfer analysis for solar space heating of an unconditioned building with an integrated roof air heater. The system consists of an air duct within the roof such that the air is continuously or intermittently forced to circulate the cooler room air through the inlet of the air duct. Time dependence of the air flow is represented by a step function of time for daily operation and, hence, has been expressed as a Fourier series in time. The analysis takes into account air ventilation, ground heat conduction and furnishings. The effects of depth of the air duct from the outer surface of the roof and the magnitude and duration of air flow rate on indoor air temperature have been studied for a typical cold winter day in Delhi. It is seen that a time dependent air flow through the duct is desirable from the point of view of increasing the indoor air temperature in the case of a bare roof. However, in the case of a blackened and glazed roof, continuous air flow is needed for increasing the room air temperature. The results are desirable from the point of view of efficient space heating of solar passive buildings.     

Efficiency analysis of radiative slab heating in a walking-beam-type reheating furnace

The thermal efficiency of a reheating furnace was predicted by considering radiative heat transfer to the slabs and the furnace wall. The entire furnace was divided into fourteen sub-zones, and each sub-zone was assumed to be homogeneous in temperature distribution with one medium temperature and wall temperature, which were computed on the basis of the overall heat balance for all of the sub-zones. The thermal energy inflow, thermal energy outflow, heat generation by fuel combustion, heat loss by the skid system, and heat loss by radiation through the boundary of each sub-zone were considered to give the two temperatures of each sub-zone. The radiative heat transfer was solved by the FVM radiation method, and a blocked-off procedure was applied to the treatment of the slabs. The temperature field of a slab was calculated by solving the transient heat conduction equation with the boundary condition of impinging radiation heat flux from the hot combustion gas and furnace wall. Additionally, the slab heating characteristics and thermal behavior of the furnace were analyzed for various fuel feed conditions.     

Effect of phase change material on passive thermal heating of a greenhouse

In this study, a periodic analysis of a greenhouse with combination of phase change material (PCM) and insulation as a north wall has been developed for thermal heating. The thermal model is based on Fourier analysis. Effect of distribution of PCM thickness on plant and room air temperature has been studied in detail. The plant and room air temperature have been evaluated with and without north wall. Numerical computations have been carried out for a typical winter day of New Delhi. On the basis of numerical results, it is inferred that (i) there is a significant effect of PCM north wall and heat capacity of plant temperature during off‐sunshine hour due to storage effect and (ii) the rate of heat flux inside greenhouse from north wall is maximum for least thickness of PCM. Copyright © 2005 John Wiley & Sons, Ltd.     

Periodic heat flux through a three layered slab

This communication presents a derivation of an explicit expression for the periodic variation of thermal flux through a composite slab (insulation-concrete-insulation or concrete-insulation-concrete), with one face exposed to solar radiation and atmospheric air and the other face in contact with room air at constant temperature. It is seen that for maximum or minimum load levelling of the periodic heat flux the thicknesses of the outer layers of the composite slab are equal.     

Periodic analysis of a ventilated trombe wall

A periodic analysis of a ventilated Trombe wall is presented. The analysis is able to take into account the storage effects and predict the daily or long term performance of the system. The thermal flux entering an air-conditioned space (maintained at 20°C) through the wall has been evaluated. Numerical calculations have been performed corresponding to the meteorological data on 19 January 1981 at New Delhi, a typical cold winter day. Subsequent parametric studies using the simulation indicated that the critical parameters were the mass flow rate of air, depth of plane of heat retrieval and the length of the network of pipe.     

Thermal load levelling of heat flux in PCCM collection/storage walls/roof system

In this communication, a steady state analysis of a phase change component material (PCCM) collection-cum-storage walls/roof system has been presented by incorporating the effect of water flow through a parallel plate placed at the liquid and solid interface. The effect of various parameters, i.e. fraction of melted region, walls/roof thickness, flow rate and latent heat of fusion of PCCM on heat flux entering the air conditioned room has been studied in detail. Numerical calculations have been made for two typical days at Delhi (18 June 1983 and 18 January 1984). The following conclusions have been made: (i) thermal load levelling decreases with an increase of the fraction of melted region and vice versa for total thickness of walls/roof; and (ii) phase shift of 10 h is observed for 40–50% fraction of melted region for walls/roof thickness of 0.10 m.     

Periodic heat transfer through inhomogeneous media part 2: Hollow cylinder

The paper presents exact analytical solutions for periodic radial heat conduction through an inhomogeneous hollow circular cylinder for a certain class of thermal conductivity profile. The exact analytical solutions for some of these profiles (including linear and quadratic) are compared with those obtained by considering the cylindrical medium to be made up of a number of homogeneous layers with different thermal conductivities, varying from layer to layer, and using the layered-structure (or matrix-multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. This gives confidence in the application of the layered-structure method to periodic heat conduction through an inhomogeneous hollow cylinder. Assuming the inhomogeneous hollow cylinder to be made up of a number of cylindrical layers with a linear profile of thermal conductivity has also been shown to be a more effective alternative method of considering any type of inhomogeniety; it saves computation time, as the rate of converegence is much higher than for the homogeneous-layer structure method. Numerical results are presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and the heat flux on the two sides of the cylinder.     

Enhancement of solar gains through a roof using a thermal trap

This paper investigates the viability of increasing solar gains through the roof of an air-conditioned room using the thermal trap effect. In solving the heat conduction equations through the thermal trap and the concrete slab, the finite difference method has been employed and the initial conditions are derived from the assumption that, initially, the ceiling of the room and the top of the trap material are in equilibrium with constant room air temperature and the ambient air temperature, respectively. The effect of the thickness of the thermal trap and that of the concrete slab on the thermal flux transferred through the roof have been studied.     

Periodic heat transfer through inhomogeneous media part 3: Hollow sphere

The paper presents the exact analytical solutions for periodic radial heat conduction through an inhomogeneous hollow sphere for a certain class of thermal-conductivity profile. The exact analytical solutions for some of these profiles, (including linear and quadratic) have been compared with those obtained by considering the spherical medium to be made up of a number of homogeneous layers with different thermal conductivities, varying from layer to layer, and using the layered-structure (or matrix-multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. It strengthens the confidence in applying the layered-structure method to the case of periodic heat conduction through an inhomogeneous hollow cylinder. Considering the inhomogeneous conducting medium to be made up of a number of spherical layers with a linear profile of thermal conductivity has been shown to be a more effective alternative method of accounting for any type of inhomogeniety; and it saves computation time, as the rate of convergence is much higher than for the homogeneous-layered structure method. The numerical results have been presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and heat flux of the two surfaces of the hollow sphere.