PCM thermal insulation in buildings

This paper presents the results of a numerical and experimental study of phase change material (PCM) filled walls and roofs under real operational conditions to achieve passive thermal comfort. The numerical part of the study was based on a one-dimensional model for the phase change problem controlled by pure conduction. Real radiation data was used to determine the external face temperature. The numerical treatment was based upon using finite difference approximations and the ADI scheme. The results obtained were compared with field measurements. The experimental set-up consisted of a small room with movable roof and side wall. The roof was constructed in the traditional way but with the phase change material enclosed. Thermocouples were distributed across the cross section of the roof. Another roof, identical but without the PCM, was also used during comparative tests. The movable wall was also constructed as is done traditionally but with the PCM enclosed. Again, thermocouples were distributed across the wall thickness to enable measurement of the local temperatures. Another wall, identical but without the PCM, was also used during comparative tests. The PCM used in the numerical and experimental tests was composed of a mixture of two commercial grades of glycol in order to obtain the required fusion temperature range. Comparison between the simulation results and the experiments indicated good agreement. Field tests also indicated that the PCM used was adequate and that the concept was effective in maintaining the indoor temperature very close to the established comfort limits. Further economical analysis indicated that the concept could effectively help in reducing the electric energy consumption and improving the energy demand pattern. © 1997 by John Wiley & Sons, Ltd.     

Optimum thermal design of office buildings

Proper design and selection of building components at the early stages of the design process can greatly help in achieving thermal comfort with minimum reliance upon HVAC systems and, therefore, minimum energy requirements. Given today's complexities in building design as well as advances in computer technology, optimization techniques can be used as an aid to building designers in their decision making process. Office buildings are characterized by being ‘internal-load’ dominated with internal heat generation determining the need for energy to air-condition such buildings. This paper presents the results of applying an optimization model to the design of energy conserving office buildings in different climatic regions to test the impact of mainly envelope related parameters on the thermal performance of offices. Optimum sets of building design variables for three different sizes office building in four U.S. and two Saudi Arabian cities are presented with the objective of minimizing annual energy consumption for those buildings. © 1997 John Wiley & Sons, Ltd.     

Modelling of a domestic-scale co-generation plant thermal capacitance considerations

The following paper describes the development of a computer model to simulate the operation of a domestic-scale co-generation plant incorporating a heat pump. Thermal capacitance considerations are critical to the model's accuracy, especially as the plant design requires two heat-exchangers with differing transient characteristics. The paper compares model results with those obtained experimentally in order to assess the modelling assumptions and methods employed. Comparative analysis of the two sets of data, for individual heat-exchangers, shows that the modelling techniques and assumptions used were correct. The general economic validity of the model is additionally proven.     

Lightweight thermal storage for solar heated buildings

A new building element is introduced that greatly improves the thermal and architectural peformance of passively heated structures. Insolated ceiling tiles charged with thin layers of phase change material are used to stabilize room temperatures and store large quantities of heat without using weighty materials which are expensive to support in multiple story structures. Experimental and analytical results are presented which detail the tile performance.     

A correlation for the thermal delay of buildings

A correlation is developed which expresses the thermal delay of buildings in terms of mean indoor temperature increase over mean outdoor temperature, heating power density including solar radiation, and size of building, for various building types. The correlation is based on a finite-difference solution of a system of coupled differential equations describing the transient heat transfer and energy balance in buildings.     

Cheap effective thermal solar-energy collectors

A light-weight flexible solar-collector, with a wavelength-selective absorption surface and an insolation-transparent thermal-insulation protector for its aperture, was built and tested. Its cheapness and high performance, relative to a conventional flat-plate solar-collector, provide a prima-facie case for the more widespread adoption of its design.     

被动式太阳房热工计算设计软件发展

太阳房是太阳能热利用的一个重要领域,它的推广有利于节约常规能源,减少环境污染,改善生态环境,和谐人与自然的关系,具有重要的社会、经济效益。甘肃省科学院自然能源研究所研制的《被动式太阳能采暖房（动态）热工计算软件》可用来计算国内外最常用的直接受益式、集热墙式（含集热蓄热墙和对流环路式集热墙）、附加阳光间式三种典型太阳房及其组合型太阳房的动态热工性能,预测其在不同地点、不同时间及不同气象条件下的逐时室温;计算房间在控温运行时所需的逐时辅助热量;优选建筑热工设计参数及设计方案,提高太阳房和节能房的适用性和经济…     

Nonlinear coupling between thermal mass and natural ventilation in buildings

An ideal naturally ventilated building model that allows a theoretical study of the effect of thermal mass associating with the non-linear coupling between the airflow rate and the indoor air temperature is proposed. When the ventilation rate is constant, both the phase shift and fluctuation of the indoor temperature are determined by the time constant of the system and the dimensionless convective heat transfer number. When the ventilation rate is a function of indoor and outdoor air temperature difference, the thermal mass number and the convective heat transfer air change parameter are suggested. The new thermal mass number measures the capacity of heat storage, rather than the amount of thermal mass. The analyses and numerical results show that the non-linearity of the system does neither change the periodic behaviour of the system, nor the behaviour of phase shift of the indoor air temperature when a periodic outdoor air temperature profile is considered. The maximum indoor air temperature phase shift induced by the direct outdoor air supply without control is 6 h.     

Energy analysis of buildings employing thermal mass in Cyprus

In this paper the effects on the heating and cooling load resulting from the use of building thermal mass in Cyprus are presented. This is achieved by modelling and simulation with the TRNSYS program of a typical four-zone building with an insulated roof in which the south wall of one of the zones has been replaced by a thermal wall. Despite the fact that the diurnal temperature variations in Cyprus are ideal for the application of thermal mass, no such application is presently available. Therefore the main objective of this paper is to investigate the possible benefits resulting from such an application. The results of the simulation show that there is a reduction in the heating load requirement of the zone by about 47%, whereas at the same time a slight increase of the zone-cooling load is exhibited. Optimisations of the various construction parameters have also been carried out. The optimum overhang size is found to be equal to 1.2 m with minor variations in the range of 1 to 1.5 m. The effect of the air gap size between the glazing and the thermal wall is insignificant. The optimum value of wall thickness obtained is equal to 25 cm. The effect of roof insulation is investigated and it is found that insulation is a must for better comfort conditions. Also, the effect of applying ventilation whenever the ambient temperature is lower than the indoor temperature during summertime is investigated. A reduction of 7.5% is obtained when air at 3 air changes per hour is directed into the house. In conclusion it can be said that the thermal wall offers some advantages and should be used whenever buildings are erected with south-facing walls.     

Synergic effects of thermal mass and natural ventilation on the thermal behaviour of traditional massive buildings

The energy policies about energy efficiency in buildings currently focus on new buildings and on existing buildings in case of energy retrofit. However, historic and heritage buildings, that are the trademark of numerous European cities, should also deserve attention; nevertheless, their energy efficiency is nowadays not deeply investigated. In this context, this study evaluates the thermal performance of a traditional massive building situated in a Mediterranean city. Dynamic numerical simulations were carried out on a yearly basis through the software DesignBuilder, both in free-running conditions and in the presence of an air-conditioning (AC) system. The results highlight that the massive envelope of traditional residential buildings helps in maintaining small fluctuations of the indoor temperature, thus limiting the need for AC in the mid-season and in summer. This feature is highly emphasised by exploiting natural ventilation at night, which allows reducing the building energy demand for cooling by about 30%.The research also indicates that, for Mediterranean climate, the increase in thermal insulation does not always induce positive effects on the thermal performance in summer, and that it might even produce an increase in the heat loads due to the transmission through the envelope.     

Apparent thermal conductivity of glass-fibre insulant: effects of compression and moisture content

The line source thermal probe technique was validated and used to measure the apparent thermal conductivities of commercially available dry and moist glass-fibre blankets under various compressions. For the dry insulant, a minimum apparent thermal conductivity of 0·039 W m^?1 K^?1 occurred at ～ 20°C for optimal values of bulk density—and consequently of volume voidage—of 0·45 kg m^?3 and 0·985, respectively. Observations for the apparent thermal conductivities of moist insulants did not agree with theoretical predictions but were corroborated by the experimental results of other investigators.     

Axial effective thermal conductivities of packed beds

Experimental investigations have been carried out to measure axial effective thermal conductivities of packed beds for a number of particles and catalyst pellets. Measurements were made for three gases (air, nitrogen and carbon dioxide) in beds packed with ball bearings, copper chromite, chromia alumina, alumina hollow cylinders and alumina spheres. A glass vacuum vessel was employed for most measurements, but a thin wall stainless steel vessel was used in a few experiments.

Empirical correlations to predict the axial effective thermal conductivity of packed bed reactors have been derived from the experimental results.     

Estimating effective thermal conductivity of two-phase materials

In this paper, correlations are proposed to estimate the effective thermal conductivity of two-phase materials. For any α, Maxwell equation for 0.0 < c ⩽ 0.10 and phase inverted Maxwell for 0.9 ⩽ c ⩽ 1 are considered. For concentrations between 10% and 90%, and low α (<20), an equation based on the unit-cell approach (constant isotherms) is proposed. For α > 20, three correlations are proposed based on field solution approach which includes three α ranges viz. medium (20 ⩽ α ⩽ 100), high (100 ⩽ α ⩽ 1000) and very high (1000 < α). The predicted effective thermal conductivity of two-phase system is compared with well-established models. Comparison of the predicted values of the correlations with experimental results is also made. The predictions of effective thermal conductivity of two-phase materials match well with the experimental values.     

The effective thermal conductivity of wire screen

A combined experimental and theoretical investigation was conducted to examine the thermal conductivity of layers of wire mesh screen. Employing a carefully controlled sintering process, layers of sintered wire screen were examined experimentally. The experimental results indicate that the effective thermal conductivity in the z-direction (i.e., normal to the plane of the wire screen layers) for single layer, inline structures and staggered multilayer wire screens is approximately 4–25% and 6.4–35% times that of the solid metal thermal conductivity, respectively. In addition to the conductivity of the base material, the contact conditions between the individual wires, as well as between the individual layers were found to be the most important factors in the determination of the effective thermal conductivity. In parallel with the experimental investigation, an analytical model was developed and shown to accurately predict the effective thermal conductivity of these layers as a function of the contact conditions between the wires, between the individual layers, and between the solid heating surface and the first layer of screen. In addition to the contact conditions, this new model is strongly dependent on three geometric parameters: the mesh number, M, the wire diameter, d, and the compression factor, cf. Experimental data from this and other investigations were used to verify the accuracy of the model, which was shown to accurately predict the effective conductivity as a function of the above mentioned parameters. In addition, a comprehensive literature review was performed and analyzed in order to further substantiate the validity and accuracy of this new model. The resulting conclusions highlight the importance of the contact conditions between the wires in the individual layers, and between the layers and the solid surface.     

Permeability and effective thermal conductivity of bisized porous media

In the region of minimum porosity of particulate binary mixtures, heat exchange and permeability were found to be higher than the ones obtained with a mono-size packing built with the same small size particles used in the binary packing. This effect was noticed in the range of the particles size ratio 0.1–1.0.The obtained improvement on thermal performance is related to the increase of effective thermal conductivity (ETC) in the binary packing and to the increase in transversal thermal dispersion due to the porosity decrease and tortuosity increase.Permeability can increase by a factor of two, if the size ratio between small and large spheres of a loose packing stays in the range 0.3–0.5.     

Scattering of thermal waves and non-steady effective thermal conductivity of composites with coated particles

In this study, thermal wave method is proposed to predict the non-steady effective thermal conductivity of composites with coated particles, and the analytical solution of this problem is obtained. The Fourier heat conduction law is introduced to analyze the propagation of thermal waves in the particular composite. The scattering and refraction of thermal waves by a coated particle in the matrix are analyzed, and the results of the single scattering problem are applied to the composite medium. The wave fields in different material zones are expanded by using the spherical wave functions and Legendre polynomial, and the expanded mode coefficients are determined by satisfying the boundary conditions of the coating layer. The theory of Waterman and Truell is employed to obtain the effective propagating wave number and the non-steady effective thermal conductivity of composites. As an example, the effects of the material properties of the particles, coating and matrix on the effective thermal conductivity of composites under different wave frequencies are graphically illustrated and analyzed. Analysis shows that the non-steady effective thermal conductivity under higher frequencies is quite different from the effective thermal conductivity under lower frequencies. In the region of lower frequency, the effect of the properties of the coating on the effective thermal conductivity is greater. Comparisons with the steady effective thermal conductivity obtained from other methods are also presented.     

PCM thermal storage in buildings: A state of art

A comprehensive review of various possible methods for heating and cooling in buildings are discussed in this paper. The thermal performance of various types of systems like PCM trombe wall, PCM wallboards, PCM shutters, PCM building blocks, air-based heating systems, floor heating, ceiling boards, etc., is presented in this paper. All systems have good potential for heating and cooling in building through phase change materials and also very beneficial to reduce the energy demand of the buildings.     

Experience on integration of solar thermal technologies with green buildings

The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings, which are for the demonstration of flat and villa, respectively. Here, a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply was designed and constructed for the office building. However, only solar hot-water systems were installed for the flat and villa. All the three solar thermal systems have continuously run for 2 years. Two different integrated approaches have been put into practice in the two green residential buildings. It is shown that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. The solar-powered integrated energy system has the advantage of high utilization ratio with different functions according to different seasons. It is testified to be capable of taking on about 70% of the yearly building load regarding the involved space under the weather condition of Shanghai.     

A dynamic model for the thermal energy management of buildings

The presented mathematical model needs, as input data, only the geometrical-physical properties of the building in a very aggregated form and the daily values of the climatic variables. The model is designed to obtain an accuracy of 95%. For this reason, the building is considered as an accumulation of many blocks. To obtain a short computational time, each block is simulated in the simplest way compatible with the target precision. The model is able to solve many problems in energy management.