Solar thermal modelling of a building with a roof pond and ventilation control systems

The paper proposes and presents thermal modelling of a ventilation-controlled, non-air-conditioned building with evaporative cooling (e.g. open water pond) over the roof for passive solar air conditioning. The ventilation rate, expressed in terms of number of air changes per hour, is assumed to be time-dependent, as should be the case in normal practice. A self-consistent periodic heat transfer analysis for a non-air-conditioned building with roof cooling and ventilation control systems, furnishing (assumed isothermal mass), windows, door and basement ground heat storage effects has been developed to assess the feasibility of the proposed passive space air-conditioning. It is shown that for no-ventilation summer nights the inside air temperature remains higher than the ambient air temperature even with an effective roof cooling system, and hence the windows should be opened to lose the internal heat and to introduce cool and fresh outside air. It is found that for a ventilation-controlled building with a roof pond the passive solar air conditioning can be achieved more effectively.     

光热建筑一体化Trombe墙体系统传热性能

为了改善建筑围护结构的保温隔热性能和利用太阳能,提出了一种光热建筑一体化Trombe墙体系统,建立了实验墙体和模拟计算模型,并对墙体系统的热传递性能进行了实验测试和模拟分析。研究结果表明:实验工况下集热板、主墙层外侧和内侧最高温度测量值分别为91.3、57.9、23.4℃,模拟值为88.4、58.3、17.2℃,墙体系统在冬季具有较好的保温性能;太阳辐射作用下,墙体系统的各材料层均产生竖向温度差,实验工况下竖向温度差为集热板17.9℃、主墙层外侧31.7℃、主墙层内侧2.2℃,模拟值为集热板17.2℃、主墙层外侧21.9℃、主墙层内侧1.2℃;墙体系统各材料表面的竖向温度差随太阳辐射照度增加而增大,随空气夹层厚度增大而减小。     

Passive control methods of Kerala traditional architecture for a comfortable indoor environment: Comparative investigation during various periods of rainy season

The traditional architecture of Kerala, a state in India lying along its southwest coast, is known for its use of natural and passive methods for a comfortable indoor environment. Although there have been attempts to analyze the traditional architecture of Kerala, they were focused only on qualitative approach. An investigation was thus initiated by the authors to understand the passive environment control system of Kerala traditional architecture in providing better thermal comfort, by continuously monitoring thermal comfort parameters of a typical traditional residential building over a period of time. The inferences of the first phase of the investigation carried out during winter and summer seasons, lasting about half of the year have already been published. This paper illustrates the inferences of the second phase of the investigation that is carried out during the rainy season of the year. A comparative analysis with the results of the winter and summer periods is also incorporated. The investigation has revealed that, when the outside ambient temperature is below normal, the building system tries to maintain the indoor air temperature at a higher but comfortable level and when the outside temperature is above normal, the indoor is kept at a lower but comfortable level. It is found that a continuous gentle wind flow is maintained inside the building irrespective of the wind outside. The required level of thermal comfort is achieved by maintaining a balanced level of temperature and relative humidity along with a continuous and controlled airflow inside the building irrespective of seasons.     

Hysteresis effects on the thermal performance of building envelope PCM-walls

This work presents a numerical study of the combined effects of the hysteresis temperature difference, peak melting temperature, and thickness of a building envelope PCM-wall on its thermal performance in air-conditioning and non-air-conditioning conditions. The study was carried out considering complete melting-freezing daily cycles of the PCM in a climate exhibiting both hot and cold thermal discomfort. A time-dependent one-dimensional heat conduction code, which uses the effective specific heat method to simulate the heat transfer through the PCM was developed. Insights into the effects of the hysteresis phenomenon were obtained; it was found that hysteresis improves the thermal performance of PCM-walls. The higher the hysteresis temperature difference the better the thermal performance, but there is a limit in the improvement of the thermal performance, which is achieved when the entire phase change process takes place at temperatures outside of the thermal comfort zone. Maximum improvements from 4% to 29% for air-conditioning and from 4% to 30% for non-air-conditioning, for a BioPCM wall with thicknesses from 6 mm to 18 mm, were found. Suggested criteria to achieve the maximum possible thermal performance of PCM-walls given a thickness and use condition were obtained. This work proposes the basis of a methodology to optimize simultaneously any pair of variables of a PCM-wall for different use conditions (AC, nAC, or a combined use of AC and nAC).     

In situ measuring and evaluating the thermal resistance of building construction

This paper introduces an in situ measuring method for the thermal resistance of buildings, including the test chamber, measuring points’ arrangement, and measurement results, in Nanjing during 2000 and 2001. Three methods for the analysis of in situ data are also presented to determine the thermal resistance of buildings although the R-values evaluated by these methods have smaller values than those of design due to the limitation of field conditions. The synthetic temperature method only requires measuring the heat flow rate on the inside surface of the building construction and both the synthetic indoor and outdoor temperatures. And the surface temperature method just requires testing the heat flow rate on the inside surface of building envelopes and both the inside and outside surface temperatures of building construction. However, the frequency response method introduced in this paper only relates to the mean synthetic indoor and outdoor temperatures and the average inside surface temperatures of the building envelopes. In other words, it is not involved with the heat flow rate, which is difficult to measure. Thus, on this point, the frequency response method is better than the other two methods to evaluate the in situ R-value of buildings.     

基于遗传算法的大体积混凝土热力学参数反演分析

为准确快速确定混凝土热力学参数中难以确定的绝热温升、导热系数、表面放热系数及反应速度,以云南普立大桥散索鞍支墩基础大体积混凝土施工实测温度为基础,采用遗传算法进行混凝土热力学参数的反演分析,并根据反演参数建立三维有限元模型预测后续混凝土施工中的温度场,然后通过混凝土内部实测温度及应力验证预测结果。最后依据预测结果,在混凝土浇筑早期采用表面降温,内部布设冷却水管的措施有效减小了内外温差并防止了裂缝产生。结果表明:混凝土内部温度达到峰值时表面拉应力最大值为1.5 MPa,出现表面裂缝的可能性较小;混凝土浇筑3 d后,抗裂指数都在1.5以上,一般不会产生裂缝;基于反演参数的温度场计算值与实测值吻合良好。     

A simple procedure for selection and sizing of indirect passive solar heating systems

Equivalent sol-air temperatures have been defined for four indirect gain passive solar heating concepts, namely, mass wall, water wall, Trombe wall and solarium. Steady state thermal efficiencies have also been defined as a measure of the ability of each system to deliver heat into the living space.

Design curves have been developed which relate the average instantaneous solar radiation incident on the passive element to thermal efficiency for different values of ambient temperature. These curves are useful in selection of an appropriate passive heating concept for a particular location.

It is inferred that a solarium is most effective at very low levels of incident radiation and low ambient temperature. Water walls and Trombe walls are most efficient at higher levels of incident radiation.

A simple procedure has been developed for a first approximation of sizing the selected system using these design curves and a minimum of meteorological information, namely, monthly average of daily global solar radiation, monthly average maximum and minimum ambient temperatures.     

Influence of thermal insulation position in building envelope on the space cooling of high-rise residential buildings in Hong Kong

At the present time, thermal insulation is almost not used in fabric of tall residential buildings in Hong Kong, as their fabric slabs usually comprise concrete layer covered on each side by plaster layers. This study investigates into the influence of an existence of the thermal insulation layer in the outside walls on the yearly cooling load and yearly maximum cooling demand in two typical residential flats in a high-rise residential building by employing HTB2, detailed building heat transfer simulation software. During the investigations, the thermal insulation layer up to 15 cm thick was placed either at the inside, or at the outside, or at the middle part of the outside wall structure. Then, the concrete layer was up to 40 cm thick. The simulation predictions indicate that the highest decrease in the yearly cooling load of up to 6.8% is obtained when a 5 cm thick thermal insulation layer faces the inside of the residential flat. The highest decrease in the yearly maximum cooling demand of 7.3% is recorded when a 5 cm thermal insulation layer faces either the outside or the inside of the flat; this depends on the flat orientation. However, much weaker reductions in the yearly cooling load and yearly maximum cooling demand are found when the thickness of thermal insulation is increased above 5 cm and the thickness of concrete is increased above 10 cm.     

Model predictive control of a building heating system: The first experience

This paper presents model predictive controller (MPC) applied to the temperature control of real building. Conventional control strategies of a building heating system such as weather-compensated control cannot make use of the energy supplied to a building (e.g. solar gain in case of sunny day). Moreover dropout of outside temperature can lead to underheating of a building. Presented predictive controller uses both weather forecast and thermal model of a building to inside temperature control. By this, it can utilize thermal capacity of a building and minimize energy consumption. It can also maintain inside temperature at desired level independent of outside weather conditions. Nevertheless, proper identification of the building model is crucial. The models of multiple input multiple output systems (MIMO) can be identified by means of subspace methods. Oftentimes, the measured data used for identification are not satisfactory and need special treatment. During the 2009/2010 heating season, the controller was tested on a large university building and achieved savings of 17–24% compared to the present controller.     

Harmonic analysis of building thermal response applied to the optimal location of insulation within the walls

The analysis of heat transfer through building walls using Fourier transforms and the matric method are briefly reviewed. The formalism is applied to a simple one-room building. By making a few simplifying assumptions and by considering only one- or two-layer walls and roofs, the equations are kept sufficiently short to preserve the insight of the reader into the effects of a few construction features upon the building's thermal response. Such construction features, mainly the placement of insulation inside or outside the main wall mass, are extensively discussed, with an eye on their potential energy savings.The results are: (1) the placement of insulation outside the wall masonry reduces the amplitude of the internal temperature swing caused by weather conditions and by internal heat gains. If the inside temperature is left free to oscillate within a few degrees, the amplitude of the heating or cooling load is greatly reduced, allowing for substantial energy savings. However, the building is thermally sluggish and inefficient durign thermostat setbacks because of its large wall heat storage. (2) Inside placement of insulation increases the room temperature response to weather conditions and to internal heat gains. Thus, heating or cooling is needed for temperature peak-shaving. In return the building's response to a thermostat setting change is quick and the heat stored in the walls, lost during a setback, is relatively small.     

Passive cooling in hot,arid regions in developing countries by employing domed roofs and reducing the temperature of internal surfaces

Natural ventilation due to wind effects through buildings employing domed roofs was estimated by a flow network analysis. The dome was assumed to have an opening at its crown. When compared with flat roofs, the domed roofs always increase the air flow rate through the building. The increase in natural ventilation becomes significant in buildings with doors and windows all in one wall, or whenever the wind effects on the building envelope do not produce large pressure differences at the openings.The large air flow rate in the buildings with domed roofs may be utilized to store night air coolness in the structure more effectively and keep the mean radiant temperature of the interior surfaces low for thermal comfort in summer. The lowest internal surface temperatures can be obtained when the surfaces are kept moist and evaporatively cooled.Through a one-dimensional energy analysis the inside surface temperature of a horizontal slab was estimated for various slab materials and thicknesses and external and internal conditions. The inside surface temperature was compared with the case of employing a roof pond. It was found that lower temperatures can be obtained by evaporatively-cooled moist internal surfaces than that which can be obtained by unshaded roof ponds: For a building whose internal surfaces (walls and ceiling) are kept moist a large ventilation rate is needed to prevent water vapor build-up in the space. A domed roof with a hole in its crown can produce the necessary ventilation for such a building.     

Distribution of wind- and temperature-induced pressure differences across the walls of a twenty-storey compartmentalised building

An experimental investigation has been made of the distribution of pressure differences across the walls of a 20-storey student residence building at the University of Ottawa. The wind velocity at the test building as well as the temperature distributions both inside and outside the building were measured simultaneously.

While pressure differences are caused by all three of the factors investigated, namely the temperature gradient (stack effect), the wind and the mechanical ventilation system installed in the building, the first two effects are predominant for this particular building during the winter season.

The stack effect is found to be linearly proportional to the difference of the reciprocal outside and inside (absolute) temperatures, and varies almost linearly with height. The neutral pressure level occurs at a height of 40 m, or 70% of the height of the building.

The wind-induced pressure difference under relatively strong wind shows a good conformity with previous knowledge for typical bluff sections such as a rectangular prism.     

A method for the experimental evaluation in situ of the wall conductance

A method for the experimental determination of the in situ building's wall conductance is presented, based on the measurement of wall inside heat flux and inside and outside surface temperatures and on the use of a finite differences calculation code.By applying the method to a test wall, in different periods of time, the conductance values proved to be in agreement with values obtained by the mean progressive method suggested by the EN 12494 pre-regulation. This method has limits in its use since the thermal energy accumulated in the wall has to be negligible compared to the thermal energy passing through the wall during the testing period, whereas the method proposed by the authors can be used more generally and also allows the value of equivalent thermal capacity to be obtained.     

Coupling of thermal mass and natural ventilation in buildings

The coupling of thermal mass and natural ventilation is important to passive building design. Thermal mass can be classified as external thermal mass and internal thermal mass. Due to great diurnal variation of ambient air temperature and solar radiation intensity, heat transfer through building envelopes, which is called external thermal mass, is a complex and unsteady process. Indoor furniture are internal thermal mass, affecting the indoor air temperature through the process of absorbing and releasing heat. In this paper, a heat balance model coupling the external and internal thermal mass, natural ventilation rate and indoor air temperature for naturally ventilated building is developed. In this model, the inner surface temperature of building envelopes is obtained based on the harmonic response method. The effect of external and internal thermal mass on indoor air temperature for six external walls is discussed of different configurations including lightweight and heavy structures with and without external/internal insulation. Based on this model, a simple tool is developed to estimate the indoor air temperature for certain external and internal thermal mass and to determine the internal thermal mass needed to maintain required indoor air temperature for certain external wall for naturally ventilated building.     

Five-year data of measured weather,energy consumption,and time-dependent temperature variations within different exterior wall structures

This paper presents coherent 5-year measured data that have been gathered for analyses of building energy consumption and thermal performance of exterior walls. The data is also very suitable for calculations and simulations of heating and cooling energy need of buildings. The data was collected from six identical test buildings, having exterior walls that are constructed of different building materials. The data include the following: indoor–outdoors temperatures; temperatures at various depths within the northern, southern, eastern, and western exterior wall facades; indoor–outdoors relative humidity, heating energy, wind speed and direction; air tightness, infiltration, and horizontal global solar radiation. A computer system (data logger) was used to monitor, check, calculate, integrate, and save the data acquired from approximately 520 sensors in each test building. Measurements were taken with a time interval of 20 s. The 20 s values were then integrated over a time interval of 30 min and the minimum, maximum, and mean values were subsequently stored to a computer database. Analyses of the results indicated that temperatures within the buildings’ exterior walls are constantly changing and, that occasionally the flow of conduction heat is reversed (i.e. outside–inside) due to solar radiation. For accurate results of temperature distribution and the actual heat losses through building envelopes, none steady-state calculations are essential. Depending on the intensity of solar radiation and the material characteristics of the walls, temperature gradient at the inner surfaces of exterior walls may become milder compared to that of the outer surfaces.     

Assessment of green roof thermal behavior: A coupled heat and mass transfer model

Green roofs have a positive effect on the energy performance of buildings, providing a cooling effect in summer, along with a more efficient harnessing of the solar radiation due to the reflective properties found inside the foliage. For assessing these effects, the thermodynamic model was developed as well as the thermo-physical properties of the green roof components were characterized. Its typologies and vegetation styles should also be studied. The proposed model is based on energy balance equations expressed for foliage and soil media. In this study, the influence of the mass transfer in the thermal properties and evapotranspiration were taken into account. We then added the water balance equation into our model and performed a numerical simulation. By assuming the outdoor conditions, the roof support temperature and the drainage water as inputs, the model evaluates the temperatures evolution at foliage and soil ground levels. A parametric study was performed using the proposed model to classify green roofs depending on the considered climate condition. Comparisons were undertaken with a roof slab concrete model; a significant difference (of up to 30 °C) in temperature between the outer surfaces of the two roofs was noticed in summer. The model was experimentally validated according to green roof platform, which was elaborated. The mass transfer effect in the subtract was very effective in reducing the model errors. Simulation results show that the use of vegetation in the roof building improves not only thermal comfort conditions, but the energy performance of a building.     

热质墙体在我国的热适应性研究

室外大气综合温度通过热质墙体传入室内,波动大大衰减,波峰延迟,在节能降耗中具有重要意义。本文首先建立了热质墙体的一维非稳态传热模型,对应用在一栋零能耗建筑中的热质墙体传热性能进行了模拟,模拟结果与实际检测的墙体温度和热流率吻和良好。而后进一步利用该模型分析了热质墙体在我国5类典型气候区域代表城市的适应性,结果表明在昼夜温差大、室内外平均温差大的地区较适宜采用厚重围护结构,而在冬暖夏凉地区现象相对不明显。     

Study of FDS simulations of buoyant fire-induced smoke movement in a high-rise building stairwell

Numerical simulations of fire-induced smoke movement in the stairwell of a high-rise building are conducted using FDS, version 6.0.1, with default settings. Twelve scenarios are considered. The required fineness of the grid has been determined in earlier work by considering both the fire source and the vent flow, and by assessing the velocity profile at the bottom opening and the vertical distribution of temperature in the stairwell. In the present study, the results including the airflow velocity at the bottom opening, vertical distribution of temperature, the temperature at the middle opening, pressure distribution, and neutral plane height in the stairwell, are compared to experimental data. For the average velocity through the bottom opening, a maximum deviation of 16.23% is obtained. Good agreement is achieved for the vertical temperature inside the stairwell (maximum relative deviation of 12.3%). By analyzing the temperature at the middle opening, it is found that the smoke moves faster than in the experiment. The influence of the staircase on the pressure distribution is demonstrated by comparing two cases: one with and one without staircase. The difference between the pressure inside the stairwell and the pressure outside increases with height, due to fire-induced buoyancy. However, the pressure difference evolution is non-monotonic when there are staircases inside the stairwell. The neutral plane height value, as obtained by post-processing the simulation results, is too high in the simulations, compared to experimental data and the corresponding analytical expression. Finally, the influence of the turbulence model is shown to be negligible.     

浅谈大体积混凝土温度检测及裂缝控制

首先指出裂缝是大体积混凝土的主要质量缺陷,而内外温差过大是产生裂缝的主要原因之一,通过对大体积混凝土中温度裂缝产生的过程和预防措施进行探讨,以期对大体积混凝土的测温及温度裂缝控制起到一定指导作用。