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Active pipe-embedded building envelope is a new kind of pipe-embedded building external wall/roofs. Usually, pipes are embedded in the wall slab or roof slab to allow water circulating in these pipes for heat transfer. Low-grade energy sources can be used for this structure since the enlarged heat transfer surface between the slab mass and water in the pipe allowing for substantial heat flow for relatively small temperature difference. This structure may reduce building cooling/heating load and improve indoor thermal comfort by intercepting the heat/coolth from the ambient air to indoor space. This paper presents a semi-dynamic model of this structure for thermal performance evaluation for system design or indoor environment control etc. This semi-dynamic model is a coupled model. The first part is a simplified dynamic RC model (i.e., resistance and capacitance model). The heat transfer along the width of this structure can be predicted easily by the RC model. The second part is a classical NTU model for evaluating the heat transfer along the pipe and total heat transfer on both surfaces of this structure. A CFD model is developed to act as a virtual experimental test rig to simulate the thermal characteristics of this structure. The simulated results are used to validate this semi-dynamic model and its performance prediction. [ABSTRACT FROM AUTHOR]

Abstract: Active pipe-embedded building envelope is a new building envelope, which is an external wall or roof with pipes embedded in it. This structure has the advantages to utilize directly low-grade energy sources for reducing building cooling/heating load and improving indoor thermal comfort. This structure may also use some phase change materials (PCM) to be pasted as a thin layer for further enhancing these benefits. This paper presents a dynamic simplified thermal model of this structure with the thermal network structure of lumped thermal mass, and the parameter identification of the simplified model based on frequency characteristic analysis. These resistances and capacitances are identified in frequency domain by using generic algorithm (GA) by comparing the frequency characteristics of the simplified model with the theoretical frequency characteristics of this structure obtained with Frequency-Domain Finite Difference (FDFD) method. Firstly, the FDFD model of this structure is established, and the theoretical frequency characteristics under various disturbances are calculated for the reference of parameter identification. Then, an equivalent dynamic simplified thermal model with lump thermal network structure is developed, and its frequency characteristics are also deduced and calculated. Finally, GA estimator is used to identify these parameters of the simplified model for allowing the frequency responses of the simplified model to match the theoretical frequency responses by using FDFD method. Various case studies are presented to validate the accuracy of the simplified models and the effectiveness of the parameter identification for the model. [Copyright &y& Elsevier]

Abstract: An alternative simplified building model is developed to describe existing building system aiming at providing performance benchmark for performance evaluation and diagnosis at building level and performance prediction for air-conditioning system optimal control. This model combines detailed physical models of building envelopes and a thermal network model of building internal mass. The detailed physical models are the CTF (Conduction Transfer Function) models of building envelopes based on the easily available detailed physical properties of exterior walls and roof. The thermal network model is the 2R2C model, and its parameters are estimated and optimized using genetic algorithm with short-term monitored operation data. The parameter optimization of the simplified building internal mass model (2R2C) and the simplified dynamic building model (i.e., model) are validated in a high-rising commercial office building under various weather conditions. This model is an alternative modeling approach for simulating the overall building dynamic thermal performance when CTF model is chosen to model the building envelope. [Copyright &y& Elsevier]

Abstract: Building simple and effective models are essential to many applications, such as building performance diagnosis and optimal control. Detailed physical models are time consuming and often not cost-effective. Black box models require large amount of training data and may not always reflect the physical behaviors. In this study, a method is proposed to simplify the building thermal model and to identify the parameters of the simplified model. For building envelopes, the model parameters can be determined using the easily available physical details based on the frequency characteristic analysis. For the building internal mass involving various components, it is very difficult to obtain the detailed physical properties. To overcome this problem, the building internal mass is represented by a thermal network of lumped thermal mass and the parameters are identified using operation data. Genetic algorithm (GA) estimators are developed to identify these parameters. The simplified dynamic building energy model is validated on a real commercial office building in different weather conditions. [Copyright &y& Elsevier]

The energy efficiency of traditional PCM wall is low in summer since the PCM storing heat in the daytime will become a secondary heat-source and release the heat into the room again at night. In this paper, a novel pipe-encapsulated PCM wall system for self-activated heat removal is proposed. It integrates PCM, nocturnal radiation cooling and gravity heat-pipe technology, and can realize automatic heat insulation and self-activated removal for buildings. Three simplified models for pipe-encapsulated PCM are developed to predict its thermal characteristics. Model Parameters are determined and identified by using genetic algorithm. The pipe-encapsulated PCM is prepared by RT28 and expanded graphite. An experiment setup is established for model validation. Results shows that the 4R2C PCM model are preferable since the identified parameters are relatively less and the accuracy is also acceptable. Furthermore, a simple application about pipe-encapsulated PCM wall is proposed based on the coupled wall model with 4R2C PCM model. Compared with conventional wall, the internal surface accumulated heat transfer of the pipe-encapsulated PCM wall with night active heat removal in 7 days can be reduced by 74.5% in hot climate. It shows great application potential for energy-saving in buildings. [ABSTRACT FROM AUTHOR]

Abstract: The use of phase change materials (PCM) to enhance the building energy performance has attracted increasing attention of researchers and practitioners over the last few years. Thermodynamic models of building structures using PCMs are essential for analyzing their impacts on building energy performance at different conditions and using different control strategies. There are few PCM models of detailed physics providing good accuracy in simulating thermodynamic behavior of building structures integrated with PCM layers. However, simplified models with acceptable accuracy and good reliability are preferable in many practical applications concerning computation speed and program size particularly when involving large buildings or models are used for online applications. A simplified physical dynamic model of building structures integrated with SSPCM (shaped-stabilized phase change material) is developed and validated in this study. The simplified physical model represents the wall by 3 resistances and 2 capacitances and the PCM layer by 4 resistances and 2 capacitances respectively while the key issue is the parameter identification of the model. The parameters of the simplified model are identified using genetic algorithm (GA) on the basis of the basic physical properties of the wall and PCM layer. Two GA-based preprocessors are developed to identify the optimal parameters (resistances and capacitances) of the model by frequency-domain regression and time-domain regression respectively. Validation results show that the simplified model can represent light walls and median walls integrated with SSPCM with good accuracy. [Copyright &y& Elsevier]

Abstract: Short time step heat flow calculation of building constructions is often needed for practical applications. Conventional methods such as state-space method and root-finding method may produce unstable conduction transfer function (CTF) coefficients at short time steps, and thus result in unstable heat flow calculation through building constructions. Frequency-domain regression (FDR) method is a newly developed method for computing CTF coefficients efficiently by representing the real building construction system with equivalent polynomial s-transfer functions. Previous studies on this method mainly addressed CTF coefficients at the conventional time step of 3600s and the performance of heat flow calculation using these coefficients. This paper presents the investigation on the performance of CTF coefficients at various short time steps based on FDR method, and the performance of the heat flow calculation using these coefficients. The results show that FDR method can produce stable CTF coefficients at various time steps for most building constructions, and the calculated heat flows using these coefficients are of high accuracy. [Copyright &y& Elsevier]

Active pipe-embedded building envelope can allow for substantial heat flow for relatively small temperature difference due to the embedded pipes in the external wall/roof. This structure can directly utilize the low-grade sources for reducing building cooling/heating load and improving indoor thermal comfort by intercepting the heat gain/loss through the building envelope due to the circular water inside the embedded pipe. This paper presents the experimental validation of a semi-dynamic simplified model of the active pipe embedded building envelope. The experiment test rig consists of two environment chambers with one for simulating the ambient environment and one for indoor environment with a pipe-embedded building envelope sample separating both chambers. A small chiller and a buffer tank are installed to provide stable water temperature for the test sample. The temperatures and heat fluxes on both sides of the test sample, the inlet and outlet water temperature as well as flow rate are measured. The measured boundary conditions are used as the inputs of the semi-dynamic simplified model for calculating the thermal performance of the active structure. The results show the semi-dynamic simplified model can predict the semi-steady or pure dynamic thermal performance of the pipe-embedded building envelope very well by comparing with the measurement. [ABSTRACT FROM AUTHOR]