Thermal performance of a non-air-conditioned building with PCCM thermal storage wall

This paper presents a time-dependent periodic heat transfer analysis of a non-air-conditioned building having a south-facing wall of phase-changing component material (PCCM). A rectangular room (6 × 5 × 4 m) based on the ground is considered. The effects of heat transfer through walls and roof, heat conduction to the basement ground and furnishings, heat gain through window and heat loss due to air ventilation have been incorporated in the periodic time-dependent heat transfer analysis. The time-dependent heat flux through the PCCM south-facing wall has been obtained by defining the effective thermal properties of the PCCM for a conduction process with no phase change. Numerical calculations are made for a typical mild winter day (7 March 1979) at New Delhi for heat flux entering through the wall and inside air temperature. Further, a PCCM wall of smaller thickness is more desirable, in comparison to an ordinary masonry concrete wall, for providing efficient thermal energy storage as well as excellent thermal comfort in buildings.     

Experimental studies on a novel roof pond configuration for the cooling of buildings

A new evaporation based passive cooling technology was tested. The technology is based on the exposure of “floating” wetted cloth to the ambient air. It was compared to various other passive cooling techniques, with very favorable results. Two identical shallow ponds were constructed. One of them was covered with white cotton towels stretched on a densely perforated PVC panel supported by pieces of waterproof polystyrene keeping it just floating on the water surface. Five comparable experiments of different cooling techniques have been carried out. The results indicate that the new cooling technique performed slightly better than the pond with movable insulation, which is widely considered as one of the best roof cooling techniques based on evaporation. It seems that the higher efficiency of the tested technique is due to the thermal stratification created in the water inside the pond, which more effectively resists the transfer of heat gains from the sun and ambient air into the deep water of the pond. In turn, the water temperature near the floor of the pond is lower, thus heat flow from the building to the pond is increased. During the experiment, all the ponds which were compared were ranked according to performance (from best to worse): shaded pond with towels floated on it, pond with towels floated on it and pond with movable insulation, shaded open pond, open pond, covered pond.     

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.     

Non-convective roof pond for passive space heating

This paper reports on an investigation of a passive system comprising a non-convective pond placed on the roof of a building in order to heat the building. Periodic analysis of the solar heat transfer process in the system, which is exposed to solar radiation and atmospheric temperature on one side and is in contact with room air at constant temperature (corresponding to an air-conditioned room) on the other, indicates that this system provides better thermal storage than a thick concrete roof or the convective roof pond system. In sunny winter climates, such a system can provide 100% of all heating needs. A viscosity stabilised non-convective pond of shallow depth (10–15 cm) is envisaged to be the most suitable for the present application.     

Cooling performance of roof ponds with gunny bags floating on water surface as compared with a movable insulation

A roof pond with gunny bags floating on water surface (RPWGB) has been proven to be an efficient evaporative based cooling technique in the previous studies of the authors due to the creation of thermal stratification inside the pond. In this paper, a mathematical model is developed for the investigation of its cooling performance as compared to a roof pond with a movable insulation (RPWMI) under hot dry climatic conditions. This one-dimensional model is based on the new empirical correlations of water evaporation rate from a wetted surface and a free water surface to the ambient air proposed by the authors, and takes into account the response of buildings as a whole to evaporative cooling. Results by simulations indicated that RPWGB performed slightly better than RPWMI, which is widely considered as the most efficient evaporative based roof cooling techniques, except when they are applied to a building with poorly insulated light walls and roof.     

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.     

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 behavior of curved roof buildings exposed to solar radiation and wind flow for various orientations

In this study air flow, solar radiation and heat transfer from a two dimensional curved roof with north-south and east-west faced are determined and results are compared with flat roof for the same size and orientation. Comparison are performed for their corresponding roof surface temperature, and heat flow for several roof rim angles and also for various wind flow velocities, as well as for different wind directions. Turbulence is modeled by RNG k–ε method and solar radiation distribution over the roof is determined based on an appropriate model applicable to hot arid regions of Iran. Solar radiation is calculated based on the summation of beam and diffuse radiation and ground reflected radiation. For certain inside roof temperature, over all heat transfer to the building is determined with day time for various wind flows and arc shapes and results are compared with flat roof. It was found that various wind flow condition over the vaulted roof makes substantial difference on the convection heat transfer coefficient and finally on the rate of heat transfer to the building with respect to flat roof. Based on heat transfer simulation, roof temperature, heat transfer convection coefficient and heat flow though the vault for different roof arrangement and flat roofs have been determined and advantages of specific vault orientation and wind direction are specified.     

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.     

Total and local heat transfer from a smooth circular cylinder in cross-flow at high reynolds number

The total and local heat transfer from a smooth circular cylinder to the cross flow of air has been measured over the Reynolds number range 3 × 10⁴ < Re < 4 × 10⁶. The interaction between flow and heat transfer is discussed. In particular, the boundary-layer effects on the heat transfer, such as transition from laminar to turbulent flow or boundary-layer separation, are considered in conjunction with the distributions of local static pressure and skin friction.     

空调建筑外墙和屋顶经济绝热厚度的计算

该文介绍了种计算空调建筑外墙和屋顶经济绝热厚度的模型。该模型根据热流通过不同组件时间的相关分析并运用Lagrange乘子法，计算了不同增初投资额条件下外墙和屋顶的经济绝热厚度。运用该模型，依据上海、南京、武汉和重庆的气候参数对一典型住宅建筑进行了数值计算。计算结果表明，从经济评价角度来看，外墙保温隔热宜采用各热材料价格和气候因素对外墙和屋顶绝热材料厚度取值的影响。     

Analytical model of a solarium for cold climate—A new approach

In this present communication, a new approach has been evolved for analysing the thermal performance of a solarium useful in a cold climate. So as to trap the maximum solar radiation, the concept of glass windows (with movable insulation) in both the east and west walls of the sun space, besides in its south facing will and with the roof made of glass, has been introduced. Considering the ground temperature constant over a day, an overall heat transfer coefficient has been incorporated for the estimation of the heat flux transferred from the floor of the solarium to the ground. The introduction of this heat transfer coefficient eliminates the need of solving the conductivity equation for the heat flux conducted from the floor of the solarium to the ground. On account of the almost insulating behaviour of wood, an overall heat transfer coefficient has been taken for an all wooden structure of the solarium. Carrying out a transient analysis, explicit expressions for the temperatures of the sun space, the blackened absorbing surface of the water wall, the water itself, the living space and the isothermal masses have been developed as a function of time. These temperatures are required for the evaluation of the thermal energy taken in by the water wall, the heat flux entering the sun space and living space and the thermal energy distributed over the isothermal masses lying in the living space.     

International aspects of air conditioning with movable insulation

Natural air conditioning, involving heating with solar energy and summer cooling to the night sky, is accomplished with ceiling ponds and movable insulation. Pertinent international aspects of climate, economies, mores and needs are related to industrialized and to developing countries. Tested throughout 15 months, in Phoenix, Arizona, when temperatures varied from below freezing to 115°F days, extensive data were obtained for very hot-dry and hot-humid climates. The Sky Therm system met these weather types with six means for modulating ambient conditions: solar heating, heat capacity, nocturnal radiation, water evaporation; radiation, evaporation and fan-coil operation; and evaporation, fan-coil and pond-blower use. Typical results are presented with a chart indicating that average daily room temperatures between 70° and 80°F can be maintained throughout a normal year, in Phoenix, without supplementary heating or cooling. During low dewpoint periods, water evaporation is not required until maximum temperatures exceed 100°; until 105° is exceeded, no electricity is needed. With dewpoints above 50° and dry-bulb maxima over 100°, both evaporation and electricity were used. In most developing countries nearer the equator, ample heating with horizontal pond collectors can be expected at normal altitudes. Acclimatization of people to somewhat higher summer temperatures may make the use of electricity and water evaporation unnecessary. The required plastics are being increasingly produced in industrializing countries, but cost may restrict use of the new method to buildings of higher priority than those to which the system would be applied in industrialized countries. Dual effects obtainable simultaneously with the same movable insulation include keeping one building or room warm and another one cold. The efficiency of a solar water-heater (or a solar still) can be improved by covering it at night with insulation moved from over a container in which water is cooled (or frozen) by nocturnal radiation; this water is maintained cold throughout the day when the insulation is positioned over it.     

Effect of Floor Geometry on Building Heat Loss Via the Ground

This paper analyzes the heat loss from an insulated slab on the ground, focusing on the influence of floor geometry on thermal processes in the ground. The calculation model includes the vertical and horizontal structures of the building; the foundation is also included. A building with a rectangular floor is considered; the ratio between the sides of floor (defined as aspect ratio) changes from 0 to 1. The thermal analysis is carried out resorting to a finite element code, validated in accordance with the requirements of International Standard ISO 10211. Numerical results show that floor geometry has a significant influence on steady-state ground global heat transfer coefficient; ranging from a narrow rectangular floor to a square one the steady-state ground global heat transfer coefficient decreases by about 15%. The effects of the perimeter insulation are also investigated; depending on the insulating layer thickness, the decrease of the heat transfer coefficient ranges from 8% to 13%. A comparison with the results obtained by applying the International Standard method ISO 13370 is also presented.     

室内空调条件下孝感市建筑物屋顶、墙体的隔热节能指标的选取

根据建筑物热工气候分区,对夏热冬冷地区的孝感市运用“建筑热环境与建筑节能设计标准相关控制法”中所提出的简化公式及孝感市的气象参数,计算出了室内空调条件下建筑物屋顶、墙体的隔热控制和节能控制指标,为我国夏热冬冷地区及其他地区建筑的隔热和节能控制设计提供参考依据。     

Heat losses from a salt-gradient solar pond

A theoretical approach for the calculation of heat losses from a cylindrical flat-bottomed salt-gradient solar pond is discussed. Steady-state heat losses from the sides and bottom of the pond have been estimated, when the pond is uninsulated as well as insulated. The insulating materials considered in the present study are dry sand, mud powder, dry cement, marble dust and mica powder. The effective insulation is varied by varying the thickness and interstitial air pressure of the insulating materials. We find that the losses are reduced to a minimum with a marble dust wall of thickness 0·20 m at an interstitial air pressure of 0·5 mm of mercury.     

Heat transfer of a plate in a liquid flow

The paper presents results of the experimental investigation of heat transfer of a plate in a flow of air, water and transformer oil at different directions of heat flow over the range of Re_f numbers from 2 × 10⁴ to 3 × 10⁷ and that of Pr_f—from 0.701 to 380. Formulae were obtained to calculate heat transfer of a plate in a liquid flow.     

Optimal roof structure with multilayer cooling function materials for building energy saving

Both cool roof and phase change thermal storage are promising technologies in decreasing building energy consumption. Combining these two technologies is likely to further enhance the thermal comfort of the building as well as reduce air condition loads. In this paper, the cooling performance and energy-saving effects of four types of roof (normal roof, phase change material [PCM] roof, cool roof, and cool PCM roof [cool roof coupled with PCM]) were investigated under a simulated sunlight. Experimental results indicate that compared with normal roof, the other three roofs are able to narrow the indoor temperature fluctuation and decrease the heat flow entering into the room. Among them, cool PCM roof gave the best energy-saving effect that can lower the indoor temperature and heat entering into rooms by 6.6°C and 52.9%, respectively. Besides, the PCM location, PCM thickness, and insulation thickness exerted great impacts on the cooling performance of the roof. Placing the PCM on the internal layer beneath the extruded polystyrene (XPS) insulation board can make the indoor temperature 1.2°C lower than that on the middle layer. Although thicker PCM panels or insulation boards can provide a better thermal insulation, 5 mm in PCM thickness and 20 mm in insulation thickness are enough to guarantee the indoor temperature of cool PCM roof system at a comfortable range (22°C-28°C) for a whole day. These findings will give guidance in designing buildings with a light and compact roof structure to decrease energy consumption and improve comfort level.     

Natural convection in an inclined rectangular channel at various aspect ratios and angles—experimental measurements

Rates of heat transfer were measured for laminar natural convection in silicone oil and air in a long rectangular channel. The aspect ratio (width/height) of the cross-section of the channel was varied over 1, 2, 3, 4·2, 8·4 and 15·5, and the Rayleigh number from 3 × 10³ to 10⁵. The channel was heated from below and cooled from above while the other two sides were insulated. The channel was then rotated about the long axis in steps through 180 degrees. The effect of inclination and of the aspect ratio on the rate of heat transfer was measured experimentally.A minimum and a maximum rate of heat transfer occurred as the angle of inclination was increased from 0 to 180 degrees. The angle of inclination at these critical conditions was found to be a strong function of the aspect ratio and a weak function of the Rayleigh number. A transition in the mode of circulation occurred at the angle corresponding to the minimum rate of heat transfer.     

Field thermal performance of naturally ventilated solar roof with PCM heat sink

For decades, residential and commercial roofs have been considered a prime location for installation of building integrated solar systems. In climatic conditions of East Tennessee, USA, an experimental solar roof was tested during 2009/2010, by a research team representing Metal Construction Association (MCA), and a consortium of building insulation companies, photovoltaic (PV) manufacturers, and energy research centers. The main objective was to thermally evaluate a new roofing technology utilizing amorphous silicon PV laminates integrated with the metal roof panels. In order to mitigate thermal bridging and reduce roof-generated thermal loads, this novel roof/attic assembly contained a phase change material (PCM) heat sink, a ventilated air cavity over the roof deck, and thermal insulation with an integrated reflective surface. During winter, the experimental roof was expected to work as a passive solar collector storing solar heat absorbed during the day, and increasing overall attic air temperature during the night. During summer, the PCM was expected to act as a heat sink, reducing the heat gained by the attic and consequently, lowering the building cooling-loads.In this paper, field thermal performance data of the experimental PV-PCM roof/attic system are presented and discussed. Performance of the PV-PCM roof/attic is evaluated by comparing it to a control asphalt shingle roof. The test results showed about 30% heating and 50% cooling load reductions are possible with the experimental roof configuration.