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The Pipe-encapsulated PCM (PEPCM) is a new type of thermal storage and insulation structure used in the building envelopes to improve the performance of buildings. CFD models are often used to investigate the thermal characteristics of the PEPCM. However, the CFD model is usually complicated and computationally intensive. In this paper, a fast and reliable quasi-steady-state simplified model of PEPCM is developed to improve the effectiveness while maintaining the accuracy. A numerical model of the PEPCM is built to validate the developed model. Thermal characteristics in the phase transition process obtained from the simplified model and the CFD model are compared. Results show that the mean temperature difference of the working fluid is less than 0.5 °C and the mean difference of the heat fluxes through the PEPCM is less than 10% while the computation time can be significantly reduced. It proves that the quasi-steady-state simplified model is highly efficient and reliable. [ABSTRACT FROM AUTHOR]

Pipe-embedded concrete radiant floor system has gained much popularity due to its higher energy efficiency and better thermal comfort. Simplified yet accurate heat transfer models are essential for energy simulation of this system. This study presents the development and validation of a simplified RC-network model for the pipe-embedded concrete radiant floors. A concrete radiant floor mainly consists of three parts which are the cover layer, the pipe-embedded concrete slab and the insulation layer. The cover layer and the insulation layer are simplified as one-dimensional and represented by RC models with two resistances and one capacitance (2R1C). Within the pipe-embedded concrete slab, the complex heat transfer is represented by a coupled RC model which includes two parts describing the heat transfers of the concrete slab and the water loop respectively. In the case studies, the coupled RC model for the pipe-embedded concrete slab has been validated in both numerical and experimental ways. The experimental validation results show that the average relative errors of the coupled RC model in predicting the upper and bottom surface heat fluxes are all less than 5.5%. Validation has also been conducted to demonstrate the accuracy and effectiveness of this developed simplified RC-network model in predicting the dynamic thermal performance of a multi-layer pipe-embedded concrete radiant floor. The developed simplified RC-network model can be beneficial for design and optimal control of the pipe-embedded concrete radiant floor system. [ABSTRACT FROM AUTHOR]

Hollow blocks or bricks are widely used due to the good thermal insulation, lightweight, acoustic insulation, etc. This paper presents the development of a simplified heat transfer model of hollow block for simple and efficient heat flow calculation. This model is developed based on parameter identification in frequency domain by using finite element method in frequency domain. Firstly, the frequency domain finite element model (FDFEM) of the hollow block is developed by using finite element method in frequency domain to obtain the theoretical frequency characteristics of this structure. Then, the heat transfer function model in the form of s -polynomial (i.e., the s -polynomial heat transfer function model) is identified by using a parameter identification procedure based on the calculated theoretical frequency thermal characteristics. Finally, the simplified heat transfer model, i.e., the Conduction Transfer Function (CTF) coefficients, of this hollow block are derived from the identified s -polynomial heat transfer function model. A case study is presented for the prediction of the dynamic heat transfer of this hollow block by using the simplified heat transfer model. The predicted dynamic heat transfer of this block is validated by experimental data. [ABSTRACT FROM AUTHOR]

Active pipe-embedded building envelope is a new kind of external building wall or roof which usually has pipes embedded in it to let water circulate in these pipes for heat transfer. This structure may effectively intercept the heat/coolth from the ambient environment to indoor space and provide extra space conditioning for the indoor space. Frequency-domain finite-difference (FDFD) model can predict the frequency thermal response of active pipe-embedded building envelope directly and provide some important guidelines for system control and system sizing. This paper presents the experimental validation of this model in time domain by using Fourier series analysis. An experiment test rig was developed for the thermal response measurements of the pipe-embedded building envelope under pre-defined conditions. First, the measured time series of surface temperatures are transformed into complex Fourier series by using Discrete Fourier Transform and applied into the FDFD model as the boundary condition for calculation. Then, the calculated frequency thermal responses by FDFD model can be easily transformed into time series by using Inverse Discrete Fourier Transform. Finally, the time-domain thermal responses of FDFD model are obtained. The results show that the calculated time-domain thermal responses of the active pipe-embedded building envelope by using FDFD model agree well with the experimental measurements. [ABSTRACT FROM AUTHOR]

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: Low-grade energy sources such as geothermal energy, favorable ambient air and industrial waste heat etc. exist widely. Sufficient utilization of these low-grade energy sources may reduce our daily dependence on high-grade energy sources such as electricity resulting in reduced emission of green house gas for environmental conservation. Active pipe-embedded structure as floor/ceiling usually with water as the medium to carry heat or coolth may utilize these low-grade energy sources for providing space air-conditioning. Compact arrangement of pipes in the structure may significantly enlarge heat transfer surface between the slab mass and water in the pipe allowing substantial heat flows even for relatively small temperature differences. Application of the heat or coolth storage capacity of this structure for preheating or pre-cooling is also one among the advantages of this structure for shifting load and exploiting the nighttime cheap electricity tariff in some regions. This paper presents the technology of the active pipe-embedded structure for utilizing widely existing low-grade energy sources following by a comprehensive review on the heat transfer calculation models of this structure and its practical applications in real building systems for space air-conditioning. This review shows that more works on the active structure, especially simple and transient models for dynamic and accurate performance prediction and easy integration with existing building energy simulation packages, are worthwhile for further promoting the practical application wherever the low-grade energy sources are favorable. [ABSTRACT FROM AUTHOR]

A decoupled radiant cooling unit (DRCU), which uses an infrared-transparent and conductive resistant (ItCr) layer to cover the radiant cooling surface and prevent it from contacting with air, reduces the condensation risk and simultaneously increases the cooling capacity. The cooling capacity could be enhanced using a low radiant cooling temperature (e.g., <15.0 °C), which is lower than conventional cooling capacity (e.g., 18.3 °C). However, one interesting problem under low radiant cooling temperatures is whether the thermal environment and thermal comfort built by the DRCU will be significantly affected. The present study developed a computational fluid dynamic model to investigate the thermal environment created by a DRCU inside a small chamber. Four cases with the radiant cooling temperatures of 18.3 °C, 14.7 °C, 9.5 °C and −2.3 °C and radiant cooling areas of 9.0 m2, 7.2 m2, 5.4 m2 and 3.6 m2 were investigated. The thermal environment was evaluated using temperature and velocity distributions, and the thermal comfort was evaluated through predicted mean vote (PMV), predicted percentage dissatisfied (PPD) and percentage dissatisfied (PD). The results indicated that the thermal environment and thermal comfort created by the DRCU under low radiant cooling temperatures satisfied the comfort criteria of the ASHRAE Standard. • Thermal environment created by decoupled radiant cooling unit (DRCU) was studied. • Heat transfer process inside DRCU was validated using experimental data. • Computational fluid dynamics was used to study the thermal performance of DRCUs. • Thermal comfort built by DRCU was evaluated using general & local comfort indices. • Thermal comfort built by DRCU with low cooling radiant temperatures is satisfactory. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a simple time domain calculation method to derive thermal response factors and conduction transfer conduction (CTF) coefficients of finite differential models for estimating transient heat transfer through building structures. It is developed on the basis of converting the matrix exponential function, which is a part of the solution of the state equation established from the finite differential equations of building finite differential models, to matrix polynomial. The thermal response factors and CTF coefficients can be easily derived from the matrix polynomial with simple arithmetic and integral in time domain. This method avoids the time-consuming root-finding process of conventional methods and the computation of all the internal temperature of the finite differential model, while utilizing the advantage of the thermal response factors/CTF coefficients which relate the desired outputs at a moment to the previous inputs through a set of coefficients. Various case studies were conducted to validate the performance of this time domain calculation method in calculating the thermal response factors and CTF coefficients of various order finite differential models. [Copyright &y& Elsevier]

Abstract: Simple and effective building energy models are essentially needed for many applications, such as building performance diagnosis and optimal control, etc. The energy model involves two important parts of building components, i.e., building envelopes and building internal mass. This paper presents a methodology for parameter optimization of 3R2C thermal network model of building envelopes (composed of three resistances and two capacitances) based on frequency domain regression using genetic algorithm (GA). First, the theoretical frequency characteristics of heat transfer through building envelope are calculated using detailed physical description within the frequency range of concern. Second, the frequency characteristics of the simplified 3R2C model are calculated with random values of individual resistances and capacitances which constrain to total thermal resistance and capacitance. Then, the errors between the theoretical frequency characteristics and the frequency characteristics of the simplified model are calculated. Finally, GA estimator is developed to optimize the parameters of the simplified model, allowing the frequency responses of the simplified model match the actual heat transfer through building envelope the best. Various case studies are conducted also to validate the parameter optimization method of the simplified 3R2C model. The accuracy of simplified models for constructions of different weights is studied. [Copyright &y& Elsevier]

Abstract: Building thermal transfer models are essential to predict transient cooling or heating requirements for performance monitoring, diagnosis and control strategy analysis. Detailed physical models are time consuming and often not cost effective. Black box models require a significant amount of training data and may not always reflect the physical behaviors. In this study, a building is described using a simplified thermal network model. For the building envelope, the model parameters can be determined using easily available physical details. For building internal mass having thermal capacitance, including components such as furniture, partitions etc., it is very difficult to obtain detailed physical properties. To overcome this problem, this paper proposes to present the building internal mass with a thermal network structure of lumped thermal mass and estimate the lumped parameters using operation data. A genetic algorithm estimator is developed to estimate the lumped internal thermal parameters of the building thermal network model using the operation data collected from site monitoring. The simplified dynamic model of building internal mass is validated in different weather conditions. [Copyright &y& Elsevier]

Self-activated phase change material (PCM) wall integrated with radiative sky cooling (RSC) is a novel wall system that uses natural energy directly for low-energy buildings to support carbon peaking and neutrality goals. A separated gravity heat pipe (SGHP) is an effective heat transfer component for heat transport from the wall body to the radiative cooler without using mechanical energy. Its heat transfer characteristics affect the thermal performance of the wall system. In this study, a numerical Volume of Fluid (VOF) model of the SGHP is established. The thermal and flow characteristics under the small temperature difference boundary of building scenarios are simulated and analyzed. Results show that the average temperature of the working fluid inside the SGHP in the "steady stage" is about 26.3 °C when the boundary temperature of the evaporation and condensation sections are respectively 28 °C and 20 °C. The heat exchange can reach 356 W/m2 and the flow velocity of the working fluid is about 0.1 m/s. Influences of different evaporation/condensation boundary temperatures on the heat transfer effect are further studied. Compared to increasing the evaporation section temperature, decreasing the condensation section temperature is a better strategy for improving the heat exchange capacity of the SGHP. • A numerical VOF model of the SGHP is established. • Thermal and flow characteristics of the SGHP are simulated. • Two parameters are proposed for estimating the heat-transfer capability of SHGP. • Thermal impacts of evaporation/condensation boundary temperatures are studied. [ABSTRACT FROM AUTHOR]

Radiant cooling has been well acknowledged as high-performance space cooling technique owing to the advantages of thermal comfort and energy efficiency. However, there are two problems (viz., condensation risk, and insufficient cooling capacity) which hinders their application in hot and humid climate zones. Herein, a condensation-free radiant cooling was demonstrated by covering cooling source with double-skin infrared-transparent membranes (DIMs). The DIMs' surface instead of the cooling source surface served as the air-contact surface while maintaining infrared radiant transfer from cooling source to cooling load. Therefore, the DIMs' surface temperature was maintained above the dew point for condensation-free safety while the cooling source temperature could be lower for improving cooling capacity. A heat transfer model was presented to predict the cooling capacity. The calculated results were validated with the measured data on a reduced-scale radiant cooling setup by using commercial polyethylene for the DIMs with an error smaller than 5%. The results indicated that the cooling capacity with DIMs at cooling source temperature of 7 °C was 104.0 W/m2, 48% improved compared to conventional radiant cooling. It will be of great guidance for high-performance radiant cooling design without condensation and improved cooling capacity. Concerning size difference of the reduced scale demonstration in this work with real scale application more accurate and detailed investigations (e.g. cooling capacity, air-tightness and mechanical strength) on the condensation-free radiant cooling should be carried out in further work. • A radiant cooling with double-skin infrared-transparent membranes was demonstrated. • Cooling capacity with condensation-free DIMs at 7 °C was 104 W/m2, 48% improved. • Both heat transfer model and reduced scale radiant cooling setup were established. • Effect of infrared characteristics on cooling capacity was explored. [ABSTRACT FROM AUTHOR]

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]

The hollow block ventilated wall is a new kind of ventilated wall, the exhaust air of HVAC system is used as the supply air to flow through the cavity of this wall in this paper. The heat or cool storage in the wall is reduced by using the heat recovery technology; the temperature of internal surface is lowered in summer and increased in winter, and the human thermal comfort is enhanced. A semi-dynamic heat transfer model of the ventilated wall is presented for thermal performance prediction. The dynamic optimized resistance-capacitance (ORC) model and the number of transfer units (NTU) model are joined together to develop the semi-dynamic heat transfer model of this ventilated wall. The experimental study is carried out for validating the semi-dynamic heat transfer model. The maximum relative error is 4.3% for mean internal and external surface temperatures, and it is 2.6% for the outlet air temperature of the cavity. The heat transfer model established for hollow block ventilated wall is fairly reasonable and possesses high accuracy. This model can be used to predict the surface temperature of the hollow block ventilated wall and the heat flux removed by the airflow in the cavity for performance evaluation. [ABSTRACT FROM AUTHOR]

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]

Radiant cooling in buildings, as an energy-saving and comfort cooling method, is very promising in creating a green and healthy indoor thermal environment in buildings. However, radiant panels are prone to dew condensation and have low cooling capacity; thus, their application in hot and humid regions is limited. Covering the surface of the radiant cooling panel with a high transmittance low conductivity material (HTLCM), and, separating the air contact surface and the radiating surface, to form a new decoupled radiant cooling technology is an effective way to address the deficient radiant cooling capacity and easy condensation. Radiation heat exchange occurs between the radiant cooling surface and the indoor heat source surface through the HTLCM, while the indoor humid air undergoes convective heat exchange with the higher temperature HTLCM surface. The optical, and thermal properties of the material determine the cooling capacity and condensation resistance (thermal performance) of the radiant cooling air conditioning. This review provides a summary of the research advances in HTLCM-assisted radiant cooling air conditioning (RCAC) in recent years. First, the classification and optical and thermal properties of the currently used HTLCMs for radiant cooling are presented. Second, the heat transfer mechanism of RCAC with HTLCM is introduced. Finally, the research advances in the application of HTLCM-assisted radiant cooling are summarized, and the challenges and future development in this field are provided. The practical engineering application of HTLCM-assisted radiant cooling units in hot and humid regions is highlighted. • Material, mechanism, application of HTLCM-assisted radiant cooling are discussed. • Structural design, thermal performance, and cooling effective of RCAC are presented. • HTLCM improves condensation resistance, cooling capacity, temperature range of RCAC. • An updated review and discussion of HTLCM-assisted RCAC is provided. [ABSTRACT FROM AUTHOR]

Renewable energy is becoming increasingly popular in the development of low-carbon societies. Radiative sky cooling (RSC) technology utilizes long waves in the atmospheric window to radiate heat into outer space; it promises a good cooling effect and wide applicability. This paper reviews RSC technology in terms of its background, materials, heat transfer and influences, and applications in buildings. High-performance radiative cooling materials have been proposed to realize RSC during both the day and night. Using these materials, sky radiative coolers and cooling systems have been fabricated for different scenarios. Radiative and non-radiative heat transfer and cooling performances can be calculated or measured using the radiation law. The correlations between the sky effective radiative temperature and effective emissivity are summarized. Environment conditions affect the RSC performance, and the influences are systematically analyzed. Owing to its advantages of cleanliness, accessibility, and extensive resources, RSC technology can be applied to building envelopes and heating, ventilation, and air conditioning systems, to develop low-carbon and zero-energy buildings. Passive and active applications are also discussed. Finally, challenges of applying RSC technology to building systems are briefly discussed. This review should help promote the practice and implementation of RSC technology. • Spectrally selective radiative cooling materials and cooler forms are reviewed. • Heat-transfer parameters, performances and influences of RSC are summarized and analyzed. • Applications of RSC in building envelopes and HVAC systems are presented. • Challenges on RSC applied to buildings are discussed. [ABSTRACT FROM AUTHOR]

Pipe-encapsulated PCM wall system with self-activated heat removal by nocturnal sky radiation cooler is a feasible means for building insulation and heat removal by using natural energy. In this paper, a system simulation platform including the simplified pipe-encapsulated PCM wall model, heat pipe heat-transfer model and nocturnal radiation model are established to investigate the thermal performance of the pipe-encapsulated PCM wall system with self-activated heat removal by a nocturnal sky radiation cooler. The temperature and heat flow of this system used in light-weight, medium-weight and heavy-weight walls under a dynamic boundary condition are respectively simulated on this platform. Results show that about 55.6%–82.8% of heat from the outdoor can be resisted by the pipe-encapsulated PCM wall, and 54.7%∼81.0% of that can be removed by the nocturnal radiation cooler at night. Comparing with the common wall, the proposed system can reduce by 32.4%∼55.5% of the accumulated heat entering into indoor environment. Good energy-saving potential can be achieved by this pipe-encapsulated PCM wall system. Besides, the result further reveals that the reduction of the accumulated internal surface heat transfer of this system used in medium-weight and heavy-weight walls is obvious larger than that of this system used in light-weight wall. • System platform of a pipe-encapsulated PCM wall system with heat removal. • Thermal performance of three walls (light, medium, heavy) with the PCM wall system. • 55.6%–82.8% of heat from the outdoor can be resisted by the PCM wall system. • Reduction of heat into the indoor is lager for the heavy wall with the PCM system. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] Sensitivity analysis of energy performance is performed for residential envelope. [•] EETP method is introduced to calculate cooling and heating energy use. [•] Shading coefficient and window to wall ratio are the most vital factors in summer. [•] Wall heat transfer coefficient and shape coefficient have crucial effect in winter. [•] Wall heat transfer coefficient and window to wall ratio weight the most in a year. [ABSTRACT FROM AUTHOR]

The coupled pipe-encapsulated PCM wall system with a nocturnal sky radiator is a novel type of building envelope structure for low energy buildings. It absorbs outdoor heat and stored it by using the pipe-encapsulated PCM inside the wall in the daytime, and the stored heat is automatically discharged into space through the sky radiator by the effect of the gravity heat pipe at nighttime. It makes full use of the natural cooling source, which can effectively reduce indoor heat gain through the envelope and improve the utilization efficiency of PCM. In this paper, an experiment platform integrating the pipe-encapsulated PCM wall module, the nocturnal radiator module, the heat pipe circulation module and measurement system together is established. The heat transfer characteristics of the wall system in four cases under the dynamic boundaries are tested, and the performances are contrastively discussed. Comparing to the wall without nocturnal radiation, the coupled wall system can well reduce the indoor cooling load and the wall internal surface temperature. The wall internal surface average heat flux can reduce 10.2 W/m2, and the cumulative indoor heat gains can reduce 29.7%. The results show the good thermal performance and energy-saving potential of this coupled pipe-encapsulated PCM wall system with a nocturnal sky radiator. With the development of new materials for enhancing the capability of radiative cooling such as the glass-polymer hybrid metamaterial, the performance of the coupled wall system may be significantly improved when these materials are used in the sky radiators. [ABSTRACT FROM AUTHOR]