Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 2, ventilated concrete slab

This paper is the second of two papers that describe the modeling and design of a building-integrated photovoltaic-thermal (BIPV/T) system thermally coupled with a ventilated concrete slab (VCS) adopted in a prefabricated, two-storey detached, low energy solar house and their performance assessment based on monitored data. The VCS concept is based on an integrated thermal-structural design with active storage of solar thermal energy while serving as a structural component - the basement floor slab (∼33 m²). This paper describes the numerical modeling, design, and thermal performance assessment of the VCS. The thermal performance of the VCS during the commissioning of the unoccupied house is presented. Analysis of the monitored data shows that the VCS can store 9-12 kWh of heat from the total thermal energy collected by the BIPV/T system, on a typical clear sunny day with an outdoor temperature of about 0 °C. It can also accumulate thermal energy during a series of clear sunny days without overheating the slab surface or the living space. This research shows that coupling the VCS with the BIPV/T system is a viable method to enhance the utilization of collected solar thermal energy. A method is presented for creating a simplified three-dimensional, control volume finite difference, explicit thermal model of the VCS. The model is created and validated using monitored data. The modeling method is suitable for detailed parametric study of the thermal behavior of the VCS without excessive computational effort.     

双层壁幕墙传热特性的数值模拟

建立了双层壁中层空间的流体动力学和传热学二维稳态数学模型。将内、外层玻璃幕墙的传热模型、遮阳板的传热模型相耦合计算出双层壁的热力性能。给出了不同高度上的温度和速度分布数据。计算结果与相关文献中的实验数据较吻合。     

含相变材料双层玻璃窗光热分析模型

建立了含相变材料双层玻璃窗的光热传输模型,考虑相变材料和玻璃半透明特性,采用有限差分求解方程。在通过实验数据验证模型准确的基础上,分析了相变材料的融化温度对含相变材料双层玻璃窗光热性能的影响。结果显示:建立的模型可模拟相变材料双层玻璃窗的光热传输过程;随融化温度升高,含相变材料双层玻璃窗温度衰减因子则逐渐增大,相变材料融化时间延后,相变材料呈液态的时间变短,但温度滞后值、热流密度和太阳透射能则呈不规则变化。     

Design and overall energy performance of a ventilated photovoltaic façade

In this study, a theoretical ventilated photovoltaic (PV) façade, which functions as a pre-heating device in winter and a natural ventilation system in summer and reduces PV module temperatures, was analysed. The interrelationship between an optimum proportion of transparent window (and an opaque PV module) to the total façade area, and the variables relevant to the energy performance was assessed. The design parameters under consideration have been categorised according to climate, building characteristics, façade configurations and PV system elements. One outcome of this investigation is a new index, effectiveness of a PV Façade (PVEF), that has been developed to evaluate the overall energy performance of a PV façade with regard to the proportion of useful daylight that may displace the use of electric lighting, and the electricity generated by the PV modules to the heating and cooling energy consumption within a building. In conclusion, the electricity generation and the factors, affecting the ventilation performance of a ventilated PV façade, have been presented.     

Evaluation of the effect of nanocoated window glazing films on the thermal behavior of buildings

In this study, different coating samples of antimony tin oxide (ATO) films on clear glass were prepared and their optical and thermal properties were evaluated. Performance parameters such as glass U-factor, solar heat gain coefficient, temperature distribution, and net heat transfer gain using the glazed walls inside the test room were determined through numerical simulations. The results obtained indicate that the nanocoating thickness and doping level concentration of ATO films have a pronounced effect on the thermal insulation of the window glass. When compared with normal window clear glass, the transmittance of solar radiation for ATO thin-film coating of thickness 1140 nm reduced from 0.881 to 0.114 in the visible (VIS) region and 0.817 to 0.012 in the infrared (IR) region, whereas radiation in the ultraviolet (UV) region was almost completely blocked. Similarly, for the ATO films with doping level 15%, the transmittance value reduced from 0.742 to 0.432 in the VIS region and from 0.718 to 0.114 in the IR region, and radiation in the UV region was almost totally blocked. Finally from the analysis of results, it was observed that the ATO thin films show exceptional optical and thermal properties and exhibit improved solar blocking behavior when compared with commercially available glazing (eg, from Saudi Arabian glass companies).     

An analytical and experimental analysis of a very fast thermal transient

According to some international standards, some products, developed for use under heavy thermal conditions, have to be tested by subjecting them for a short time to a particular heating and cooling thermal stress to allow them an acceptable future operative life. It is possible to obtain these fast thermal gradients in confined environments, called climatic chambers where the air is heated by an electrical resistance and is cooled with a finned evaporator which is linked to a vapour compression system subjected to a particular control system of the refrigerating power. In particular, in this paper the air and object tested thermal transients are studied from an analytical and experimental point of view. The study of the mathematical model is realized assuming simplified hypotheses about the air, the object and the air cooled evaporator temperature. The most complex circumstances are related to a very fast temperature decrease because under this working condition the mathematical model is characterized by a nonlinear differential system. The nonlinear term is represented by the refrigerating power that varies in a definite range with the evaporator temperature according to a sinusoid trend. For this power a suitable analytical expression, derived by the control system performance and by the compressor characteristic, has been found. The analytical–experimental comparison during a cooling thermal stress of typical products subjected to international standard tests as the electronic boards, has been carried out showing acceptable results. The model presented is useful to foresee the climatic chamber performances in the presence of a specific refrigerating power trend; this is the start‐point for the design of the vapour compression plant and its control system. Copyright © 2001 John Wiley & Sons, Ltd.     

Numerical evaluation and the effects of geometrical and operational parameters on thermal performance of the shell and double coil tube heat exchanger

Heat exchangers are extensively used in various industries. In this study, the impact of geometric and flow parameters on the performance of a shell and double helical coil heat exchanger is studied numerically. The investigated geometric parameters include external coil pitch, internal coil pitch, internal coil diameter, and coil diameter. The influences of considered geometrical parameters are analyzed on the output temperature of the hot and cold fluid, convective heat transfer coefficient, pressure drop, and average Nusselt number. Water is considered as working fluid in both shell and tube. As an innovation, double helical coils are used instead of one in the heat exchanger. To compare the obtained results accurately, in each section, the heat transfer area (coil outer surface) is kept constant in all models. The results show that the geometrical parameters of double helical coils significantly affect the heat transfer rate.     

轻质双层纺织基太阳能空气集热器的集热性能试验研究

纺织基太阳能空气集热器作为轻质柔软的太阳能集热器,在中低温集热领域具有潜在的应用前景。文章设计了新型双层纺织基太阳能空气集热器,并进行户外试验,探究单双层结构、间隔丝密度、空气质量流量对集热器性能的影响。研究结果表明:在空气质量流量为0.023 kg/(m²·s)时,双层纺织基太阳能空气集热器热转移因子和总热损系数分别为0.97和5.87 W/(m²·℃),集热性能优于同条件下单层结构集热器;进出口空气温差和集热效率随间隔丝密度的增加而增加;集热效率随空气质量流量的增加而增加,当空气质量流量为0.037 kg/(m²·s)时,间隔丝密度为36根/cm²的集热器集热效率为0.64。     

On the performance of a vertical helical coil heat exchanger. Numerical model and experimental validation

A numerical model was developed in order to predict the heat transfer process and pressure drop in a vertical helical coil heat exchanger (HCHE) located inside a fluid storage tank in which water is used as inner and outer fluid. Natural convection was considered as boundary condition for the HCHE outer surface. The model was validated with experimental data obtained from an own facility with two HCHEs tested under several operating conditions. The model developed was used to evaluate the main HCHE representative geometrical parameter's influence on the overall heat transfer coefficient and pressure drop. The results show that by increasing the tube diameter causes an increase of the Nusselt number and a larger heat transfer rate to pressure drop ratio is obtained.     

Theoretical and experimental study of a direct contact thermal screw dryer for biomass residues

In the present study a one-dimensional model has been developed for the calculation and design of a direct contact thermal screw dryer for biomass residues. The model input data are inlet temperatures and the flow rates of gas and biomass (washed grape pomance). An experimental facility has been developed in order to measure the temperature inside the dryer along time. The results obtained indicated that Frössling correlation used for the development of the model provided a satisfactory fitting of the experimental data. An optimal dryer length of approximately 3 m has been calculated for an inner diameter of 0.22 m.     

Theoretical formulation and experimental validation of the input-output modeling approach for large solar thermal systems

In this study, a theoretical and experimental investigation of a new modeling method for solar thermal processes, based on the input-output (I/O) approach is presented.This approach is characterized primarily by the consideration of the solar thermal system, irrespective of size, as a functionally integral set, the daily yield of which is expressed as a function of the total daily radiation, the average ambient temperature and the energy level of the storage tank at the beginning of the day.The study focuses first on the theoretical foundation of the method, based on the analysis of the evolution of the main energy quantities during a daily cycle, with emphasis on the energetically most important phase of the pseudo-steady state. By integrating the instantaneous energy balances over the entire duration of this phase, analytical expressions for the coefficients of the characteristic I/O equation arise, as a function of the main characteristics of the solar system.Subsequently, the experimental validation of the proposed approach is presented, based on the comparison of theoretical results with extensive measurements on two typical large-scale solar systems, one with a fully-stratified and the other with a fully-mixed thermal storage tank, respectively.     

Finite difference thermal model of a latent heat storage system coupled with a photovoltaic device: Description and experimental validation

The use of photovoltaic (PV) systems has been showing a significant growth trend but for a more effective development of this technology it is essential to have higher energy conversion performances. Producers of PV often declare an higher efficiency respect to real conditions and this deviation is mainly due to the difference between nominal and real temperature conditions of the PV. To improve the solar cell energy conversion efficiency many authors have proposed a methodology to keep lower the temperature of a PV system: a modified PV system built with a normal PV panel coupled with a Phase Change Material (PCM) heat storage device. In this paper is described a thermal model analysis of the PV–PCM system based on a theoretical study using finite difference approach. The authors developed an algorithm based on an explicit finite difference formulation of energy balance of the PV–PCM system. To this aim, a forward difference at time t and a first-order central difference for the space derivative at position x was used. Two sets of recursive equations were developed for two types of spatial domains: a boundary domain and an internal domain .The reliability of the developed model is tested by a comparison with data coming from a test facility. Results of this experience confirm the performed numerical simulations and show that the proposed model is valid and can be used to determine the thermal behaviour of a solar cell coupled with a PCM heat storage device.     

A sensitivity analysis for the determination of unknown thermal coefficients through a phase-change process with temperature-dependent thermal conductivity

In Tarzia, Int. Comm. in Heat and Mass Transfer, 25 (1998), 139-147, explicit formulas for the simultaneous determination of unknown thermal coefficients of a semi-infinite material through a phase-change process with temperature-dependent thermal conductivity were obtained. Moreover, ten different cases were studied: four cases of free boundary problems (i.e. Stefan-like problems) and six cases of moving boundary problems (i.e. inverse Stefan-like problems).The goal of this paper is to obtain a numerical sensitivity analysis of the mentioned ten cases for the simultaneous determination of unknown thermal coefficients and to determine the coefficients which are more sensitive with respect to the given parameters. We show numerical result for the aluminum.     

The dynamic characteristics of a thermal control system using latent heat: Comparison between analytical and experimental results

The two‐phase flow thermal control system, using latent heat of the internal fluid, has received a great deal of research interest as a method for heat removal on the space station and the Space Solar Power System (SSPS). The system has a much lower weight than the single‐phase flow, and the temperature can be accurately controlled by changing the saturated pressure inside the loop. To date, this system has not been put into practical use. Numerical analyses were therefore used to investigate the dynamic responses of the loop and to investigate the operational characteristics of the thermal control system. A simulation model was constructed, and the results of the numerical analysis were compared with the experimental results. Good agreement was obtained between analytical and experimental results. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 564–578, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20090     

Experimental investigation and simulation analysis of the thermal performance of a balcony wall integrated solar water heating unit

A balcony wall type solar water heater system was designed and constructed in a high-rise building. The U-type evacuated glass tube solar collector is fixed vertically on the balcony wall. The water, heated in the solar collector, flows through the exchanger coil in the water tank and then flows back to the solar collector. With regard to the hot water supply system, the cold water, heated by the heat exchanger, is sent to the point of use. Considering storeys and water consumption pattern, four apartments are selected for testing. Meanwhile, the theoretical analysis with TRNSYS was presented. According to the experimental results, mean daily collector efficiency is about 40%. Solar fraction is high in summer and autumn for the relative high radiation and high ambient temperature. Under given conditions, the annual energy extracted from tank is 2805.3 MJ/m², and the annual solar fraction is 40.5%. When the tank volume-to-collector area ratio is decreased to 37.5 L/m², the solar fraction can be increased to 50%. The results show that the family to use water all day round gets higher solar fraction than the family using hot water mostly in the morning and night.     

An experimental study on the thermal performance of a solar chimney without and with PCM

The thermal performance of a solar chimney without and with phase change material (PCM) has been experimentally studied in this paper. For the case of solar chimney with PCM, three different modes (closed-fully charging mode, open-partly charging mode and open-fully charging mode) were developed. The closed mode was designed to maximize the use of the solar energy when the heating is not required. Whilst the open mode was designed for delivering the heated air to the living space during charging period. The results showed that the inclusion of PCM to a solar chimney would reduce the air flow during charging period but increase it during discharging period compared with the solar chimney without PCM. For the open-partly mode, the mean air flow rate during phase change period was only 0.036 kg/s, which was lower than that for closed-fully charging mode (0.041 kg/s). Regarding the open-fully charging mode, the melting time of the PCM was almost 11 h, which was 57% longer than that for closed mode. The mean air flow rate during phase change period was 0.04 kg/s, which was higher than that for open-partly mode but lower than that for closed mode.     

Effects of nonlinear Boussinesq approximation and double dispersion on a micropolar fluid flow under convective thermal condition

This article investigates the effect of double dispersion on the natural convective flow of a micropolar fluid along an inclined plate in the presence of the convective thermal condition. In addition, the nonlinear convection is considered to analyze the heat and mass transfer phenomena of thermal systems, which are performed at moderate‐ and high‐level temperatures. A combination of local nonsimilarity and successive linearization techniques is used to evaluate the associated complicated nondimensional governing equations. This study discusses the impact of relevant factors on the fluid characteristics through graphs. The influence of nonlinear convection parameters on the heat and mass transfer rates seems to be more in Darcy porous media compared with that in non‐Darcy porous media.     

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H₂ flux of 28–36 m³/(m² h) and H₂/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H₂ flux (∼30 m³/(m² h)) and H₂/He selectivity (∼2000) over the 1500 h test period.     

Numerical and experimental assessment of thermal performance of vertical energy piles: An application

A district space heating and cooling system using geothermal energy from bearing piles was designed in Shanghai and will be installed in two years before 2010. This paper describes the pile-foundation heat exchangers applied in an energy pile system for an actual architectural complex in Shanghai, 30% of whose cooling/heating load was designed to be provided by a ground-source heat pump (GSHP) system using the energy piles. In situ performance tests of heat transfer are carried out to figure out the most efficient type of energy pile and to specify the design of energy pile system. Numerical investigation is also performed to confirm the test results and to demonstrate the medium temperature variations along the pipes. The averaged heat resistance and heat injection rate of different types of energy piles are calculated from the test and numerical results. The effect of pile type, medium flow rate and inlet temperature on thermal performance is separately discussed. From the viewpoint of energy efficiency and adjustability, the W-shaped underground heat exchanger with moderate medium flow rate is finally adopted for the energy pile system.     

An analytical model to predict the thermal performance of a novel parallel flow packed bed solar air heater

A design of a parallel flow solar air heater with packed material in its upper channel and capable of providing a higher heat flux compared to the conventional non-porous bed double flow systems is presented. An analytical model describing the various temperatures and heat transfer characteristics of such a parallel flow packed bed solar air heater (PFPBSAH) has been developed and employed to study the effects of the mass flow rate and varying porosities of the packed material on its thermal performance. The model employs an iterative solution procedure to solve the governing energy balance equations describing the complex heat and mass exchanges involved. To validate the proposed analytical model, comparisons between theoretical and experimental results showed that good agreement is achieved with reasonable accuracy. Also, PFPBSAH is found to perform more efficiently than the conventional non-porous double flow solar air heaters with 10–20% increase in its thermal efficiency. Furthermore, the effect of the fraction of mass flow rate in the upper or lower flow channel of PFPBSAH device on its performance, has also investigated theoretically. The fraction of the mass flow rate in the respective channels of the PFPBSAH is shown to be dominant parameter in determining the effective thermal efficiency of the heater.