Performance of a solarium: An analytical study

In this communication, a combination of phase change component material (PCCM) and water wall as a link wall in a solarium have been proposed and analysed for load levelling in the living space of the solarium. The choice of the aforesaid combination as a link wall has been made because of relatively high heat transfer coefficient of water and greater thermal energy storage potential of the combination. The relatively greater thermal storage potential of the proposed combination reduces the temperature swings in the living space and provides, thereby a comfortable living environment. Analytical expression for living space temperature as a function of climatic and system parameters as well as various heat transfer coefficients have been derived. Numerical calculations have been carried out for a typical day of North America (Boulder) for zero ventilation rate. It is concluded that the proposed combination as a link wall gives best thermal load levelling.     

Investigation of the most suitable location of insulation applying on building roof from maximum load levelling point of view

In this study, the maximum load levelling of periodic heat flux entering a room through a composite roof consisting of different insulation layer has been evaluated for different insulation positions in the roof. A numerical model based on implicit finite difference scheme was applied for 12 different roof configurations during typical winter and summer days. For this purpose, total insulation thickness was kept constant and insulation was placed as equally two pieces and as equally three pieces in different locations within the roof thickness. Then, insulation layers were moved 1 cm at a time across the roof thickness for 12 different configurations. Maximum and minimum values of periodic heat fluxes for each sweeping process in the roof were calculated for achieving the maximum levelling of the heat flux entering through the roof. It was found that the best load levelling was achieved in the case where three pieces insulation of equal thickness were placed one at the outdoor surface of the roof, the second piece of insulation was placed in the middle of the roof and third piece of insulation was placed at the indoor surface of the roof.     

Reduction of heat flux by a flowing water layer over an insulated roof

This communication presents an investigation of the reduction of heat flux by the flow of water over an insulated roof; the water surface is exposed to periodic solar radiation and atmospheric air while the bottom of the insulation is in contact with the room air at a constant temperature (corresponding to an air conditioned building). The heat conduction equation characterizing the temperature distribution in the roof/insulation, has been solved using appropriate periodic boundary conditions. It is seen that as the flow velocity increases, the heat flux coming into the room decreases while the heat taken away by the flowing water increases. It is concluded that the maintenance of a flowing water layer over an insulated roof is to a great extent more effective than a roof pond system; to some extent it is also more effective than a water film spray system over the roof for reduction of the cooling load of a building.     

The relative performance of different approaches to the passive cooling of roofs

This communication presents a discussion of the relative performance of five different approaches to the reduction of heat flux through the roof. These are roof shading by plants, use of removable canvas, evaporative cooling, a roof garden and use of earthen pots over the roof. Explicit expressions have been obtained for the time-dependent heat flux entering into the living space through the roof; the living space is assumed to be at constant temperature (corresponding to air conditioning). Results of numerical calculations corresponding to a typical hot day (16 May 1981) at New Delhi have been presented. For optimum thermal load levelling and least average heat flux into the room, a shaded roof (due to a vegetable pergola) with a water film is the best choice.     

Thermal performance investigation of a PCM-enhanced wall/roof in northern Morocco

Energy efficiency improvement in building sector has become a real challenge in Morocco, especially in the northern region evolving a rapid urban growth. In this context, using the phase change materials (PCM) in the construction is presented as one of the promising solutions to enhance the thermal behaviour of building envelope. This work aims to investigate the thermal performance of a PCM integrated external wall and roof under the summer climate conditions of northern Morocco. Dynamic thermal characterization methodology is adopted through the calculation of decrement factor (DF) and time lag (TL) parameters. For that, a one-dimensional numerical model based on resistance–capacitance (RC) approach is developed and implicitly solved to simulate the heat transfer process through a wall/roof structure. The model is validated through a new small-scale experimental device. Different qualities of PCM, regarding its peak phase change temperature, have been evaluated. Moreover, the PCM layer emplacement within the wall/roof structure was evaluated considering two possible configurations. The obtained results show a significative enhancement of the thermal performance of different wall facades and roof through the use of the suitable PCM quality. An appropriate selection of PCM layer emplacement yields an evidenced enhancement in the case of the roof. As for the walls, the integration of PCM from the interior side increase the TL period but has a negative impact on the DF parameter.     

建筑热桥三维非稳态传热数值模拟分析

本文以上海地区冬季典型年室外温度为计算参数,建立外墙与屋面连接处建筑热桥三维非稳态导热偏微分方程。计算了不同保温形式下墙体内表面温度及热流分布,并分析了热桥对负荷及结露的影响。     

新型节能墙体传热研究

建筑墙体负荷计算是建筑能源系统设计的基本依据，也是建筑能耗分析和能源管理的基础。根据反应系数法编程计算，得到适合工程技术应用的新型节能墙体冷负荷温度计算用表和动态传热特性基础数据，从而将节能建筑墙体改革成果应用于空调冷负荷计算，并为建筑新型节能墙体热工性能的深入研究和综合性能评价提供参考依据。     

Periodic solution of transient heat flow through multilayer walls and flat roofs by complex finite Fourier transform technique

An analytical solution method for the estimation of space heat gain through multilayer walls and flat roofs submitted to periodic boundary conditions is developed. Formulation of the transient heat transfer problem consisting of differential equation and the initial and boundary conditions is converted into dimensionless form and it is solved by the application of complex finite Fourier transform (CFFT) technique. This analytical solution is used to determine hourly variation of heat flow into the space through the walls and the temperature at the inner surface of the walls. A computer program is developed based on the analytical model to perform numerical calculations considering six different wall and two different roof constructions, characteristics to the buildings in Turkey. The CFFT technique provides itself as an alternative to other analytical and numerical methods and it appears to be useful in that it allows the calculations to be performed for various multilayer wall and roof constructions and for various climatological locations with varying ambient air temperatures and solar heat inputs.     

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.     

Parametric space distribution effects of wall heat capacity and thermal resistance on the dynamic thermal behavior of walls and structures

The present work aims at the investigation of the combined space distribution effects of heat capacity and thermal resistance on the transient thermal behavior of a wall, seen as a continuum of distributed parameters. The physical system is compared with the idealized wall lumped parameter model and its thermal time constant is related to its effective wall heat capacity, defined as the fraction of the wall heat capacity which participates in a transient thermal process. The effect of the space distribution of heat capacity and thermal resistance on the effective wall heat capacity is investigated for a wide range of homogeneous and multilayer thermally insulated walls. It is derived that the decrease of thermal resistance in homogeneous walls leads to an increase of their effective heat capacity. However, the effects are remarkably stronger on the effective heat capacity of thermally insulated multilayer walls, in which when the thermal insulation layer is at the ambient side, it leads to a significant increase of effective heat capacity, although when it is installed at the room side it leads to very low effective heat capacity, irrespective of the wall thermal resistance. Based on the first order results from a simplified room model, it was subsequently found that the influence of these parameters on the effective heat capacity of the building envelope leads to significant effects on the transient thermal behavior, thermal time constant and stability of structures.     

变温热源不可逆制冷机分析

利用有限时间热力学分析了变温热源制冷循环性能，计入了工质与高低温侧换热器的热阻损失，热源之间的热漏损失及工质内部耗散的不可逆损失。在给定制冷率条件下，推导出最大性能系数和高低温侧循环工质温度的解析式，通过数值计算，分析了各不可逆损失对循环性能的影响。     

Weekly storage of coolness in heavy brick and adobe walls

Weekly storage of coolness in heavy walls (walls with large thermal inertia or large characteristic time constants or low Fourier numbers) was investigated numerically by considering one-dimensional heat conduction through the walls. The study consisted of first analyzing the heat flow through a single wall and considering various boundary conditions on the inside. The boundary conditions were: constant inside air tempeture throughout the day, variable inside air temperature on a 24-hour cycle, and variable inside air temperature on a 48-hour cycle. Next, the heat flow through walls was studied through a thermal network analysis of a simple building. In this case it was assumed that the ambient air temperature (following a periodic distribution) was increased suddenly and followed this new distribution for many days thereafter.It was concluded that walls with high thermal inertia or time constant can store coolness for several days. The larger the time constant of the wall (for example adobe as compared with brick) the longer it takes for the wall temperature to reach a steady periodic distribution after the change has occurred. However, because of low thermal conductivity of adobe, the retrieval of the stored coolness in these walls is slow, and the mean daily temperature of the room air in the adobe building does not change appreciably beyond seven days after the change. Increase of the wall thickness beyond 50 cm does not improve the thermal performance of the building significantly.     

Determination of transient two-dimensional heat transfer in ventilated lightweight low sloped roof using Fourier series

In this paper, the analysis of transient two-dimensional (2D) heat transfer in low sloped roof with forced ventilated cavity made from lightweight building elements (LBE) is presented. For the heat transfer analysis the 2D numerical model, which was verified with experiments, was used. Forced ventilated cavity was configured in two different ways. In the first case the cavity was configured with coloured thin metal sheet and in the second case with thin metal sheet with added layer of thermal insulation and radiation barrier. Beside the influence of the ventilated cavity configuration on the transient 2D heat transfer in the LBE and on the cavity outlet air temperature also the influence of the LBE thickness, specific air flow rate through the cavity, inner air temperature and wind velocity was analysed. Multi-parametric equations for determination of Fourier series coefficients were formed. These coefficients were used for evaluation of transient 2D heat transfer on the inner side of the roof and cavity outlet air temperature for a clear day.     

Effects of coefficients of solar reflectivity and infrared emissivity on the temperature and heat flux of horizontal flat roofs of artificially conditioned nonresidential buildings

Nonresidential buildings such as shopping centers, supermarkets and factories are constructions characterized as having large roof surfaces in comparison to their external wall surfaces. When conditioned artificially, these buildings become great consumers of electricity. To reduce this consumption, insulation materials are usually used. Notwithstanding the effectiveness of such a procedure, both the cost and environmental issues (the heat-island effect) have been considered as limiting factors for its usage. In this study, we analyze the effect of the application of selective coatings (with very high reflectance for solar radiation and high emittance for infrared radiation) on the surface temperature of the roof and the heat flux that crosses it. Two roof concepts were considered, namely metal roofs both with and without conventional mass-insulation, i.e. an insulating board on the internal side or between two metal foils. The methodology adopted to solve the proposed problem was based on the heat transfer analysis of a roof composed of multiple layers. External air temperature and solar radiation are considered time-varying. The results obtained show the benefits of selective coatings in terms of reductions in both roof temperature and heat flux and, consequently, alleviation of the urban heat-island effect and reduction of electricity consumption.     

能源地下连续墙温度应力的离心模型试验研究

为研究能源地下连续墙在换热过程中温度应力的变化及其对结构物的影响,通过离心机模型模拟地下连续墙在砂土地基中的换热过程。在50g的离心加速场内模拟地下连续墙对砂土地基的传热升温,并对砂土地基中的温度场及地下连续墙体应变进行了监测,计算得到墙体所受温度应力。试验结果表明:能源地下连续墙的砂土地基温度场分布与空间有关,温度的差异是能源地下连续墙上温度应力产生的主要原因。随着埋深的增加,模型受到的约束作用越强,其温度应力也越大,即实际中埋深越深的地下连续墙受到的温度应力越大。在温差相近的条件下,地下连续墙开挖侧的温度应力沿埋深方向上的增幅明显大于未开挖侧。     

采暖建筑结构屋面雪荷载实测研究

为考虑采暖建筑热环境对屋面雪荷载取值的影响,参考国外规范ASCE/SEI 7-10和ISO 4355:2013雪荷载章节中用以量化该影响的热力系数Ct的取值,设计一种装配式可变屋顶热阻及室内采暖温度的建筑实测模型,并完成对屋面自然降雪（低含水率）及人工造雪（高含水率）表观密度与雪荷载值在融雪期（日最高气温大于0℃）与非融雪期（日最高气温不大于0℃）的实测研究。结果表明,融雪期屋面积雪经历从下部融化湿润、湿润区上迁与融雪水外排的过程,其表观密度与环境温度呈正相关关系,而雪荷载值变化呈现出明显的“三段式”形式,即湿润段、质量损失段及再结晶段。相比而言,采暖建筑屋面积雪在非融雪期变化远小于融雪期的,但二者屋面雪荷载值均反映出随屋顶热量损失的增加而减小的规律。最后依据GB 50189—2015和JGJ 134—2010,计算出各组工况下用以量化计算建筑采暖影响的热力系数Ct值,并对室内需维持在0℃左右的建筑建议该系数取值1.2。     

Modeling the heat diffusion process in the abiotic layers of green roofs

Green roofs have been increasingly installed to alleviate some common environmental problems. The thermal benefit of living vegetation on rooftop has been extensively studied. The individual and joint contribution of the non-living green roof layers, namely soil, rockwool (water storage) and plastic drainage layers, to thermal performance of green roof has seldom been assessed. This study evaluates the insulating and cooling effects of these abiotic materials. A one-dimensional theoretical model was developed to assess the heat diffusion process in the layers. The model was validated with empirical results from three experimental plots. A calibration procedure was successfully applied to determine key model parameters. The model can capture the most critical features of temperature variations and thermal performance of common abiotic green roof materials. The appreciable water-retention capacity of rockwool plays the dual role of supplying water to the soil to enhance evaporative cooling, and increasing the specific heat capacity of the green roof. The plastic drainage sheet with ample air spaces serves as an excellent thermal insulator. The model remains robust despite seasonal and weather variabilities. Our research findings contradict with some researches in the temperate region that the thermal dissipation in green roofs with dense vegetation is lower than thermally insulated bare roofs. The theoretical model could be used to simulate the micro-environmental conditions and predict the thermal performance of different materials to improve green roof design.     

Effect of roof solar reflectance on the building heat gain in a hot climate

The effect of the roof solar reflectance on the thermal performance of a building is often ignored. However, there are significant differences in heat gain from light and dark-coloured roof surfaces. In this paper an equation for the average daily downward heat flow of a sunlit roof is derived. Using building simulation, it is first shown that the thermal mass of the roof does not significantly affect the overall daily heat gain (although it causes a time lag and reduction in peak heat flow). As a consequence the daily heat gain from the roof may be estimated by integrating the equation for the steady-state downward heat transfer over the day. For north Australia the derived equation suggests that a light-coloured roof has about 30% lower total (air temperature difference and solar-driven) heat gain than a dark-coloured one. The effect of aging (change in solar reflectance with time) is considered in the calculations and a relationship between the solar absorptance of new and aged material is suggested. A classification of roof colours with respect to their solar absorptance (dark, medium, light and reflective) is proposed to enable a quick and simple assessment of the effect of roof colour on the heat gain and R-value.     

相变材料应用于外墙表面隔热的研究

根据表面隔热机理,通过对普通外墙的理论计算分析,设计了新型的相变墙体。相变材料应用于外墙体外表面,夏季能有效的改善建筑物的外表面热环境,降低传入室内的热量,缓解室内空调冷负荷。同时,提出隔热相变材料的相变温度、掺量在不同气候环境下的选择方法。     

ECOTECT能耗模拟下既有住宅建筑围护结构节能改造的热环境分析研究

目前,既有住宅建筑节能改造主要有围护结构改造和供热计量改造两方面。围护结构节能改造主要包括:外墙节能改造、外窗节能改造、屋面节能改造等技术措施的研究;建筑物围护结构节能改造除了能够降低建筑能耗之外,对建筑物室内热环境也有很大影响。采用ECOTECT能耗模拟软件,对西安市某住宅建筑围护结构不同节能改造方案的热环境进行模拟,深入分析不同节能改造方案的能源消耗、不舒适度、围护结构得热、温度分布和热舒适度情况,以热舒适为前提、节能为目的选择最优的节能改造方案。为既有住宅建筑节能改造方案优选提供依据。