A generalized methodology for determining the total heat gain through building envelopes

We present a generalized methodology for determining the annual total heat gain through external walls and proofs of large air-conditioned buildings. The methodology is based on the concept of the overall thermal transfer value (OTTV). Respective OTTV equations for building envelopes and roofs are developed through parametric simulations using the DOE-2 computer code. The equations are valid for buildings having different aspect ratios and wall masses. Appropriate coefficients for heat conduction through fenestrations and opaque walls and solar correction factors for wall facades of different orientations are computed from local weather data. The equations allow building designers to make accurate estimates of the total heat gain for the purpose of evaluating energy-efficient building envelope components and air-conditioning systems and plant options. The methodology is validated using DOE-2 computed heat gain results and can be applied to different classes of buildings, construction types and locations.     

Accounting for rain effects in building energy estimation

We demonstrate a methodology to account for rain effects on wall heat gain using the DOE-2 building energy estimation software as a reference. The methodology involves modifying the boundary conditions used for estimating heat flow through exterior wall components. Information on boundary conditions during rain is obtained from a classification of rain patterns from local weather data. The DOE-2 computer code, with the appropriate modifications, is calibrated with experimental results obtained for both dry and wet outdoor conditions. Finally, the calibrated code is used to estimate the reduction in heat gain due to rain by a porous building wall for different rain patterns. The annual heat gain reduction is also estimated for a typical tropical climate. © 1998 John Wiley & Sons, Ltd.     

Comparison of vacuum glazing thermal performance predicted using two- and three-dimensional models and their experimental validation

Thermal performance of vacuum glazing predicted by using two-dimensional (2-D) finite element and three-dimensional (3-D) finite volume models are presented. In the 2-D model, the vacuum space, including the pillar arrays, was represented by a material whose effective thermal conductivity was determined from the specified vacuum space width, the heat conduction through the pillar array and the calculated radiation heat transfer between the two interior glass surfaces within the vacuum gap. In the 3-D model, the support pillar array was incorporated and modelled within the glazing unit directly. The predicted difference in overall heat transfer coefficients between the two models for the vacuum window simulated was less than 3%. A guarded hot box calorimeter was used to determine the experimental thermal performance of vacuum glazing. The experimentally determined overall heat transfer coefficient and temperature profiles along the central line of the vacuum glazing are in very good agreement with the predictions made using the 2-D and 3-D models.     

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.     

Experimental validation of a numerical model for heat transfer in vacuum glazing

Flat vacuum glazings consisting of a narrow evacuated space between two glass panes separated by an array of small support pillars have been fabricated. A guarded hot box calorimeter was designed and constructed to measure their heat transfer coefficients. Experimental measurements of temperatures and rates of heat transfer were found to be in very good agreement with those predicted using a developed finite element model. A method for determining the heat transfer coefficient of the evacuated gap has been established and comparisons are made between the measured and predicted glass surface temperature profiles of the exposed glass area and the heat transfer coefficients of the total glazing system in order to validated the model.     

Nonequilibrium Gas Flow and Heat Transfer in a Heated Square Microcavity

The flow of a rarefied gas in a square enclosure with one wall at high temperature and the other three walls at the same low temperature is investigated. The flow, characterized by the reference Knudsen number and ratio of the cold over the hot temperatures, is simulated both deterministically, using the nonlinear Shakhov kinetic model, and stochastically, using the DSMC method. Excellent agreement between the two approaches is obtained. It is found that along the side walls the gas velocity, depending on the flow parameters, may be either from cold to hot or from hot to cold regions. Furthermore, it is confirmed that the average heat flux departing from the hot plate exhibits a nonmonotonic behavior with regard to the temperature ratio, deducing a maximum heat flux at a temperature ratio of about 0.3. The flow and heat transfer characteristics are explained by computing the ballistic and collision parts of the total bulk quantities and by investigating the contribution of each part to the overall solution.     

Comparison of experimental and theoretical results for the transient heat flow through multilayer walls and flat roofs

This study deals with comparison of experimental and theoretical results of transient temperature variations in multilayered building walls and flat roofs, and heat flow through the building structures. Experimental and theoretical models are presented to find the transient temperature variations in these structures and heat flow through these elements, which depends on inside surface and room air temperatures. Instantaneous inside and outside air temperatures, and surface temperatures of each wall and roof layers are measured by using the experimental model consisted of two rooms, cooling units, measuring devices and computers. A computer program based on the theoretical model is developed to perform numerical calculations. Hourly temperature variations of the nodal points are computed numerically over a period of 24 h by using the hourly measured ambient air temperatures and solar radiation flux on a horizontal surface for the city of Gaziantep (37.1°N), Turkey, and also by using thermophysical properties of the structures. Results obtained from the experimental and theoretical models are compared with each other, and validation of the theoretical model is verified in this paper. Computations for various multilayer building walls of briquette, brick, blokbims, and autoclaved aerated concrete (AAC), which are commonly used in Turkey are repeated for finding heat gain through these structures, and results are compared to determine suitable wall material. It is observed that AAC and blokbims are more suitable wall materials than briquette and brick due to heat flow through these elements.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

Evaluating rammed earth walls: a case study

The following research has been undertaken as a response to the recent controversy regarding the suitability of rammed earth wall construction as an effective building envelope in regard to its thermal performance. The R-value for rammed earth walls is low hence they might be expected to conduct heat into a building during summer. However the large mass of these walls and the associated thermal lag in heat transfer from outside to inside may result in the walls performing satisfactorily in a building which is only occupied during working hours. Internal rammed earth walls may act as moderators of large diurnal temperature swings helping to produce an even comfortable temperature within a building. Empirical (in situ) measurements of temperature and heat flux were taken on the walls of an existing rammed earth office building in New South Wales, Australia during the summer. An analysis was performed which established a methodology to measure the heat flow associated with the walls, floor, ceiling, windows and infiltration for one office during occupied hours and the net energy transferred between the office and these elements was established. During this time the earth walls performed well. External walls were found to transmit comparatively little heat to the office and the internal walls absorbed heat during this time. Diffuse sky radiation transmitted by the window and infiltration are both likely to be important factors in the summer heat load.     

TWO-DIMENSIONAL AND UNSTEADY NATURAL CONVECTION IN A HORIZONTAL ENCLOSURE WITH A SQUARE BODY

A two-dimensional solution for unsteady natural convection in an enclosure with a square body is obtained using an accurate and efficient Chevyshev spectral collocation method. A spectral multidomain methodology is used to handle a square body located at the center of the computational domain. The physical model considered here is that a square body is located at the center between the bottom hot and top cold walls. To see the effects of the presence of a body on natural convection between the hot and cold walls, we considered the cases that the body maintains the adiabatic and isothermal thermal boundary conditions for different Rayleigh numbers varying in the range of 103 to 106. When the Rayleigh number is small, the flow and temperature distribution between the hot and cold walls shows a symmetrical and steady pattern. At the intermediate Rayleigh number, the fluid flow and temperature fields maintain the steady state but change their shape to the nonsymmetrical pattern. When the Rayleigh number is high, the flow and temperature fields become time dependent, and their time-averaged shapes approach the symmetric pattern again. The Rayleigh number for the fluid flow and temperature fields to become nonsymmetrical and time dependent depends on the thermal boundary conditions of a body. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls and at the body surfaces for different Rayleigh numbers and thermal boundary conditions are also presented to show the overall heat transfer characteristics in the system.     

Analysis of exergy demand for air heating of an air handling unit

In modern buildings, an increasing amount of the consumed energy falls on ventilation systems. The amount of energy needed for ventilation depends on weather fluctuations, wind, interaction between natural gravity and air tightness of the building, heat exchangers used in ventilation systems, efficiency of other ventilation equipment, and operating mode of ventilation systems in the building. Ventilation systems are comprised of a variety of elements that facilitate processes using energy of different types. The main elements that use energy in ventilation systems are fans, heat exchangers, and heaters. They have a significant effect on both energy needs of a public building and the exergy efficiency of a system. In order to achieve a more efficient use of exergy in heat exchangers, it is recommended to execute processes under as little temperature difference as possible; however, this increases the area of heat exchangers. Results of the analysis show that it is recommended to design ventilation systems based on the temperature that corresponds with the maximum demand of exergy in order to use the heat recovery unit as much as possible in the system.     

Effect of passive techniques on interior temperature in small houses in the dry, hot climate of northwestern Mexico

A simple lumped parameter model is used to represent the time variations of internal temperature of a simple house, under hot, extreme weather variation, characteristic of northwestern Mexico. Results are validated by experimental work in a physical model without ventilation, with materials and building techniques typical of low-income family housing in this region. Energy balance in the present work is achieved by means of a system of three simultaneous differential equations, each depicting energy equilibrium in one of the basic building elements: the window glazing, the building materials, and inside air. With the mathematical model properly calibrated, heat transfer coefficients between walls, ceiling and windows are calculated. Passive techniques, such as window shading, orientation and thermal inertia, are evaluated by a normalized temperature index. Results show that interior temperature in the house can be reduced resulting in comfort increase. Then economical pertinence of studied passive elements can be evaluated.     

An online adaptive optimal control strategy for complex building chilled water systems involving intermediate heat exchangers

This paper presents an adaptive optimal control strategy for online control of complex chilled water systems involving intermediate heat exchangers to enhance operation and energy performances. This optimal control strategy determines the optimal settings of the heat exchanger outlet water temperature (hot side) and the required operating number of heat exchangers and pumps in order to minimize the total energy consumption of pumps under varying working conditions. Adaptive method is utilized to update the key parameters of the proposed models online. A simulated virtual platform representing a chilled water system in a super high-rise building was established to validate and evaluate the proposed optimal strategy. Test results show that the strategy has enhanced control robustness and reliability, particularly in avoiding deficit flow problem. Significant energy of chilled water pumps is saved when compared with conventional methods.     

Heat transfer enhancement in dense suspensions of agitated solids. Part II: Experiments in the exchange limit

We test the theory of Part I in the exchange limit by vibrating acrylic and aluminum spheres in a box consisting of two flat, vertical isothermal walls, two bumpy, horizontal, insulated walls, and two flat vertical insulated surfaces. The steady heat flux through the thermally-guarded hot wall is recorded at different temperatures of the opposite wall cooled by thermoelectric modules, and enhancements of suspension conductivity are calculated using a lumped-parameter model of the box. To compare results and theory, we also predict vertical profiles of agitation and solid volume fraction in the box using granular dynamics.     

Effect of Heat Transfer on Natural Convection in a Square Cavity With Two Source Pairs

In this paper, the influence of a small heating source, positioned in the lateral walls of a square cavity, is investigated. Numerical and experimental analyses are performed to investigate natural convection heat transfer in a square cavity heated by hot strips in the side walls. The H side square cavity is filled with air and heated by two hot strips with heights of H/4. The effect of placing the hot strips at two different positions is evaluated. The temperature distribution and the Nusselt numbers at different Rayleigh numbers are experimentally measured using both real-time and double-exposure holographic interferometry. The isothermal patterns obtained through the holographic interferometry are compared with the temperature and velocity fields from a numerical study performed using the finite-volume code Fluent.     

Mathematical model of the thermal performance of box-type solar cookers

A mathematical model of the heat transfer processes involved with a box-type solar cooker, containing food, was developed. Solar radiation, including that from a flat reflector, enters the box and heat transfer within and out of the box was considered. Using small increments of time, the computer model finds the temperature of the air, food, interior walls and top cover as the solar flux varies throughout the cooking period. Sample results are presented, giving the food temperature as affected by latitude, month, wind, clouds, mass of food, thermal resistance of the box walls and adjustment of the box while cooking.     

LARGE BUILDING COOLING LOAD AND ENERGY USE ESTIMATION

We present a methodology for developing a set of cooling load and energy estimating equations for large commercial buildings. The methodology is developed from parametric simulations on the DOE-2 computer code for any given generic building type and annual weather data file of the location. The equations estimate the annual cooling energy use of a building, its peak cooling load and the sensible cooling load. Correlations are obtained using the concept of the overall thermal transfer value of building envelopes and the number of cooling degree-days in a year. Other key variables include space internal loads, design outdoor-indoor temperature difference, and the global efficiency of chiller plants. The set of simple equations are validated using results obtained from DOE-2 simulations of audited actual buildings. © 1997 by John Wiley & Sons, Ltd.     

Exergy transfer effectiveness on heat exchanger for finite pressure drop

In this paper, exergy transfer effectiveness is defined to describe the performance of heat exchangers operating above/below the surrounding temperature with/without finite pressure drop. It is discussed systemically that the effects of heat transfer units number, the ratio of the heat capacity of cold fluids to that of hot fluids and flow patterns on exergy transfer effectiveness of heat exchangers. Furthermore, the results of exergy transfer effectiveness with a finite pressure drop are compared with those without pressure drop when different objective media, such as ideal gas and incompressible liquid, etc. are applied. The detailed comparisons of the exergy transfer effectiveness with heat transfer effectiveness are also performed for the parallel flow, counter flow and cross flow heat exchangers operating above/below the surrounding temperature.     

Study on performance of solar assisted air source heat pump systems for hot water production in Hong Kong

This paper reports the investigation results on application of the solar assisted air source heat pump systems for hot water production in Hong Kong. A mathematical model of the system is developed to predict its operating performance under specified weather conditions. The optimum flow rate from the load water tank to the condenser is proposed considering both the appropriate outlet water temperature and system performance. The effect of various parameters, including circulation flow rate, solar collector area, tilt angle of solar collector array and initial water temperature in the preheating solar tank is investigated, and the results show that the system performance is governed strongly by the change of circulation flow rate, solar collector area and initial water temperature in the preheating solar tank.     

Simulation studies of the expected performance of Kerava solar village

The Kerava solar village is the first regional building complex in Finland with a combined solar heating and heat pump system using seasonal heat storage. The village will be completed at the beginning of 1983. In this study we describe the heating system of Kerava solar village. The heat demand was estimated by computer simulations. Different weather conditions and operational situations were considered. Under normal weather and operating conditions approximately 60 percent of the total heat demand of the Kerava solar village is estimated to be obtained from solar energy.