Apparent and effective thermal capacitance of buildings

The apparent thermal capacitance of a building is obtained by adding the distributed specific heats of all building elements. It differs considerably from the effective thermal capacitance, which is calculated in the present study by forcing the solution of a lumped-system differential equation to follow the experimentally validated, finite-difference solution of a rigorous set of coupled differential equations describing the heat transport and energy balance in buildings. The effective thermal capacitance is calculated for various characteristic cases of Greek buildings and provides a simple procedure for approximating the transient thermal behaviour of buildings.     

公共建筑节能设计中几个问题的探讨

本文对执行《公共建筑节能设计标准》(GB 50189-2005)中几个常见的重要问题进行探讨,包括围护结构热工性能参数分类、建筑朝向判别、外墙平均传热系数计算、地面和地下室热阻计算。     

Thermal shielding of multilayer walls with phase change materials under different transient boundary conditions

A numerical model is presented to determine the thermal shielding performance of an exterior wall (e.g., building envelope) containing layers of PCMs. The model is exploited to perform a parametric study to assess the influence of the position and melting temperature of one PCM layer. Results showed that benefits are to be expected when the interior and exterior temperatures are close. Then, the wall composition has been optimized with a genetic algorithm based on a yearly analysis with the possibility of including several PCM layers. Idealized weather conditions and measured weather conditions (including solar radiation) have been considered. Results showed that for Québec City, optimal south-facing wall includes one PCM layer when real weather data are considered. Its effect is to shield the heat transfer in the summer. This paper provides a fundamental understanding of multilayer walls with PCMs.     

河南地区村镇住宅屋面保温隔热性能浅析

屋面作为建筑外围护结构保温隔热体系的一个重要组成部分,其保温隔热性能直接影响到建筑保温隔热性能及其能耗。本文根据国家"十一五"科技支撑计划项目"村镇建筑节能及改善室内热环境关键技术研究"中对河南地区村镇建筑围护结构的调查结果,归纳总结了不同时期、不同材料、不同构造做法的村镇建筑的屋面形式的保温隔热性能,依据试验数据对比得出不同材料及构造做法的保温隔热性能的优劣,从农村建筑的经济、技术、材料、节能意义等方面分析了屋面保温隔热技术在我国广大农村推广实施的重要性和必要性,并对建筑材料的发展方向和趋势提出了建议和意见。     

On-site experimental estimation of thermal conductivities and heat capacities in multilayer walls under arbitrary transient conditions using explicit and implicit finite difference schemes

It is often required to estimate the thermal properties of the layers of a multilayer wall, which is already part of an existing building. Such cases are encountered when an ex post check is required in order to find out if the design specifications have been followed, or if air conditioning loads have to be calculated in old buildings, the walls of which are composed of layers of unknown materials and thermal properties. In the present study, a method is proposed for estimating the thermal conductivities and heat capacities of the layers a multilayer wall is composed of. The method is based on explicit and implicit finite difference schemes and uses on-site temperature measurements at various locations within the wall. It is applicable to multilayer walls which are already parts of buildings. The outdoor and indoor conditions may be arbitrary, i.e. transient, nonperiodic, with solar radiation. The accuracy of the method, which has been verified by numerical and experimental applications, depends on the available number of temperature values in space. For example, in a 10-cm thick wall layer, measurement at five locations gives satisfactory accuracy, which is considerably improved by increasing the number of values in space using fourth-order polynomial interpolation.     

Finite-difference prediction of transient indoor temperature and related correlation based on the building time constant

A numerical procedure for the calculation of the transient indoor temperature in buildings is developed. The procedure is based on an implicit finite-difference solution of a closed set of differential equations, which express the indoor energy balance and the transient heat conduction in all elements of the building envelope. Using the above procedure, calculations are carried out for 21 types of buildings with 18 and 10 different kinds of wall and roof constructions, respectively, i.e. the total number of building cases examined is 21 × 18 × 10 = 3780 with floor areas ranging from 30 m² to 300 m². It is found that buildings of different construction characteristics and sizes but with the same time constant, respond in a similar way under the same outdoor temperature variation. Based on this similarity, a correlation is developed which expresses, under periodic conditions, the indoor temperature variation in terms of the building time constant and the outdoor temperature characteristics. The correlation contains nine coefficients, the values of which are different for different ranges of the building time constant.     

The adjoint-solution technique for the calculation of the thermal properties of layers in multilayer walls under transient heat conduction

The thermal properties of the layers of a wall, whether or not exposed to solar radiation, are calculated provided that the boundary conditions and some values of the transient temperature field within the wall are known. The developed procedure is based on the adjoint-solution technique and is applicable both to walls in operation and to the design of walls that are required to meet certain temperature specifications. In the former case, temperature measurements are needed. Theoretical and experimental tests have proved the accuracy of the method. Applications may be found in energy management and thermal storage in buildings, in the improvement of passive systems and in the design of multilayer slabs forming parts of heat-transfer equipment. © 1997 by John Wiley & Sons, Ltd.     

Experimental and numerical assessment of using coconut oil as a phase-change material for unconditioned buildings

The efficacy of integrating organic coconut oil (co-oil) phase-change material (PCM) into an unconditioned building with a lightweight envelope is explored experimentally and numerically for heat gain reduction. In what we think is the first test of its kind for co-oil PCM, twin side-by-side single-room buildings (with and without PCM) are constructed and studied experimentally under ambient weather conditions. The effects of the co-oil on the thermal performance of the buildings are investigated with respect to the window orientation (east, west, north and south). Furthermore, numerical simulation of the buildings is carried out to evaluate the contribution of the co-oil to their thermal performance and to determine the effect of the co-oil layer thickness on the heat storage capacity. Moreover, by employing a simplified heat transfer analysis, an approximate relation for the optimal thickness of co-oil PCM layer is developed. Experimental and numerical results show that co-oil PCM can be a promising solution to improve the indoor thermal environment. It is found that with a south-facing window equipped with co-oil PCM, the indoor temperature is lowered by 23.8% compared to the case without PCM, when an optimal PCM layer of ~4 cm in thickness is embedded in the wall.     

On the inverse transient heat-transfer problem in structural elements exposed to solar radiation

The inverse transient heat-transfer problem in walls, whether or not exposed to solar radiation, i.e. the estimation of the thermal properties of a wall if the transient temperature and/or heat flow fields are known, is interesting both from the theoretical and practical point of view. An attempt is made to analyse and solve this problem. Methods are developed for estimating the thermal properties of structural elements which are already parts of existing buildings, i.e. under real transient, non-periodic conditions. The finite-difference and experimental examples presented show that the thermal diffusivity, the thermal conductivity and the overall heat transfer coefficient may be estimated with very good accuracy by taking on-site temperature and heat flow measurements.     

多工况运行下围护结构动态热响应的评价方法

建筑运行工况会影响围护结构的热响应,为研究自然通风和空调2种居住建筑运行工况下围护结构动态热响应的差异,通过对比2种工况围护结构的边界条件与传热过程,提出围护结构动态热响应的评价指标——墙体温变条件指数WTCI,该指标兼顾2种工况下墙体热性能不同的评价内涵。以夏热冬暖地区为例,选取18种墙体构造来验证WTCI在多工况下对墙体动态热响应评价的有效性,结果表明该指标计算简便、评价有效,能够评价围护结构在多工况动态传热过程中的热响应快慢,可区分墙体在不同工况下的隔热表现。深入分析WTCI在多工况下的适用性,对WTCI指标的应用给出建议。     

太阳能富集地区采暖居住建筑节能构造研究

针对太阳能富集地区居住建筑外墙构造特点和室外综合温度条件,以拉萨地区为例进行研究,提出了采暖居住建筑非平衡保温节能墙体构造,其中南墙不做保温、东西墙和北墙采用外保温。在对当地典型居住建筑形式和围护结构现状调研基础上,采用南墙兼顾延迟时间和传热系数,东西墙、北墙单位面积等净失热量的分析方法,确定了非平衡外墙的传热系数,并对以此设计的非平衡保温构造总净热损失和内表面温度进行了分析。给出了非平衡保温构造的计算与分析方法,为同类地区非平衡保温构造设计提供了方法和理论依据。     

A model of the thermal transient state of a wall of a room during the heating by a heating system

After turning on a room heating system (e.g. central heating) a thermal transient phenomenon takes place on the wall–room system, until it reaches a final thermal equilibrium state. The temperature profiles on the wall cross‐section, starting from an initial profile, corresponding to the initial thermal equilibrium state, come gradually through successive intermediate temperature profiles, to a final temperature profile corresponding to the final thermal equilibrium state. These intermediate, nonlinear and time‐dependent temperature profiles characterize the wall thermal transient state and describe the dynamic thermal behaviour of the wall–room system. The mass of the air in the room is negligible, compared to the mass of the surrounding walls, so the dynamic behaviour of the room–wall system is imposed by the corresponding thermal dynamic behaviour of the walls. The influence of this thermal transient state is important for the room heating behaviour because it acts as a thermal flywheel attenuating and smoothing the room temperature variations. In the present work, using the integral method, analytical expressions yielding the temperature profiles, and the duration of the transient state as a function of thermal and structural characteristics have been developed. Conclusions were drawn on the dynamic thermal behaviour of the room–wall system. Copyright © 2000 John Wiley & Sons, Ltd.     

Analysis on exergy consumption patterns for space heating in Slovenian buildings

Problem of high energy use for heating in Slovenian buildings is analyzed with exergy and energy analysis. Results of both are compared and discussed. Three cases of exterior building walls are located in three climatic zones in winter conditions. Results of energy analyses show that the highest heating energy demand appears in the case with less thermal insulation, especially in colder climate. If the comparison is made only on the energy supply and exergy supply, the results of exergy analysis are the same as those of energy analysis. The main difference appears, if the whole chain of supply and demand is taken into consideration. Exergy calculations enable us to analyze how much exergy is consumed in which part, from boiler to building envelope. They also reveal how much energy is supplied for the purpose of heating. Results show that insulation has much bigger effect than effect of boiler efficiency. However, the most effective solution is to improve building envelope together with boiler efficiency. Better thermal insulation also makes an important contribution to the improvement of thermal comfort conditions. It causes higher surface temperatures resulting in a larger warm radiant exergy emission rate and consequently better thermal comfort.     

Analytical solution of the inverse unsteady wall heat conduction problem and experimental application

Based on the analytical solution of the unsteady heat conduction differential equation, a solution procedure is presented for the inverse unsteady wall heat conduction problem, i.e. for the calculation of the thermal properties of structural elements of existing buildings under real transient conditions, using on-site temperature measurements. Previous procedures, which were based on the finite-difference method, required a considerable number of temperature measurements in space and time within the wall. The advantage of the present analytical procedure is that it requires only two temperature measurements, apart from some information on the outdoor and indoor temperature variations. The two temperature measurements may be taken on the outdoor and indoor wall surfaces at the same time level, or on one of these surfaces at two different time levels. The proposed analytical procedure provides the values of the thermal conductivity and heat capacity of structural elements, and therefore it may be used in practice for ex post checking of the materials used by the constructor, or for load calculation when heating or cooling systems are to be installed in old buildings of unknown wall properties. Experimental examples are presented which show that the proposed analytical procedure may be applied in practice with very good accuracy.     

Determination of the economical insulation thickness of building envelopes simultaneously in energy-saving renovation of existing residential buildings

When the energy saving rate of existing residential buildings renovation is determined, the thermal performances of external walls, windows, and roof interact with each other. Therefore, it is necessary to study the determination of economical insulation thickness of building envelopes considering the interaction among building envelope performances. The objective function and its bound of envelope thermal performance optimization in the energy-saving renovation of existing residential buildings in severe cold and cold zones in China were established. It is the conditional extremum problem and can be solved through Lagrange’s method of multipliers to determine the economical insulation thickness of external walls and roofs simultaneously. The method is proved to be feasible by an existing residential building in Beijing. When the same window types are selected, the energy-saving renovation program of the building envelope determined by the Lagrangian optimization method can produce the minimum investment in insulation, minimum investment payback period, and the largest net present value (NPV) of the life cycle.     

Effect of indoor mass on the time constant and thermal delay of buildings

A numerical procedure is presented for predicting the transient thermal behaviour of buildings and estimating time constant τ and thermal delay t_d. The proposed procedure is used to examine the effect of indoor mass on the above characteristics. The predicted effect of these is considerable; for example, the usual indoor mass in houses increases τ and t_d by up to 40 per cent, which is analysed to 25 per cent for interior partitions and 15 per cent for furnishings. Both τ and t_d are found to be linear functions of the mass of interior partitions M_p and furnishings M_f. These functions are given for typical buildings. A simple ‘time constant and thermal delay model’ is also proposed, which is in satisfactory agreement with the developed rigorous numerical solution. Components of τ and t_d are introduced, which express the contribution of any indoor construction element or piece of furniture to the ‘total’ time constant and thermal delay of buildings. The analysis in components, which are related to the corresponding ones of the ‘effective thermal capacitance’, provides useful information for the maximization of heat or cool storage in buildings. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

Energy efficiency of a dynamic glazing system

The reduction of air-conditioning energy consumptions is one of the main indicators to act on when improving the energy efficiency in buildings.In the case of advanced technological buildings, a meaningful contribution to the thermal loads and the energy consumptions reduction could depend on the correct configuration and management of the envelope systems. In recent years, the architectural trend toward highly transparent all-glass buildings presents a unique challenge and opportunity to advance the market for emerging, smart, dynamic window and dimmable daylighting control technologies (Lee et al., 2004).A prototype dynamic glazing system was developed and tested at ITC-CNR; it is aimed at actively responding to the external environmental loads. Both an experimental campaign and analyses by theoretical models were carried out, aimed at evaluating the possible configurations depending on different weather conditions in several possible places. Therefore, the analytical models of the building-plant system were defined by using a dynamic energy simulation software (EnergyPlus).The variables that determine the system performance, also influenced by the boundary conditions, were analysed, such as U- and g-value; they concern both the morphology of the envelope system, such as dimensions, shading and glazing type, gap airflow thickness, in-gap airflow rate, and management, in terms of control algorithm parameters tuning fan and shading systems, as a function of the weather conditions.The configuration able to provide the best performances was finally identified by also assessing such performances, integrating the dynamic system in several building types and under different weather conditions.The dynamic envelope system prototype has become a commercial product with some applications in façade systems, curtain walls and windows.The paper describes the methodological approach to prototype development and the main results obtained, including simulations of possible applications on real buildings.     

Simulation of indoor temperature and humidity conditions including hygrothermal interactions with the building envelope

The hygrothermal behaviour of the building envelope affects the overall performance of a building. Numerous tools exist for the simulation of the heat and moisture transfer in the building envelope and whole building simulation tools for energy calculations. However, working combinations of both models for practical application are just about to be developed. In this paper such a combined model, that takes into account moisture sources and sinks inside a room, input from the envelope due to capillary action, diffusion and vapour absorption and desorption as a response to the exterior and interior climate conditions as well as the well-known thermal parameters will be described. The new model is validated by performing a series of field experiments and the moisture buffering capacity of the building envelope is investigated. In the conclusions the possible range of future applications of hygrothermal building performance models is addressed and the needs for further research are identified.