A comprehensive review of solar facades. Transparent and translucent solar facades

In the first paper of a series of two publications entitled “A comprehensive review of solar facades. Opaque solar facades” an exhaustive review of scientific studies carried out during the last decade on opaque solar facades was proposed. The paper dealt with facades that absorb and reflect the incident solar radiation but cannot transfer directly solar heat gain into the building. This article offers a complementary survey of studies conducted during the same period of time on transparent and translucent solar facades, highlighting the categories of ventilated facades and semi-transparent building-integrated photovoltaic facades.     

Investigation of temperature distribution on a new linear Fresnel receiver assembly under high solar flux

A linear Fresnel collector design with an operation temperature of 300°C or above typically requires a solar flux concentration ratio of at least 20 on the surfaces of the receiver assembly. For the commercial linear Fresnel collector design in this work, the receiver assembly includes a secondary reflector and an evacuated receiver tube. The high‐concentration solar flux may impose additional operating‐temperature requirements on the secondary reflector and receiver tube. Thus, a careful heat‐transfer analysis is necessary to understand the operating temperature of the receiver assembly component surfaces under design and off‐design conditions to guide appropriate material selections. In this work, a numerical heat‐transfer analysis is performed to calculate the temperature distribution of the surfaces of the secondary reflector and receiver glass envelope for a commercial collector design. Operating conditions examined in the heat‐transfer analysis include various wind speeds and solar concentration ratios. The results indicate a surface temperature higher than 100°C on the secondary reflector surface, which suggests that a more advanced secondary reflector material is needed. The established heat‐transfer model can be used for optimization of the other types of linear Fresnel collectors.     

Consistent heat transfer analysis for performance evaluation of multichannel solar absorbers

The numerical evaluation of solar absorber performance must be based on the coupling between solar flux modeling and heat transfer modeling. We have developed a ray-tracing method to model the solar flux distribution absorbed at SiSiC multichannel absorber surfaces under a solar furnace, and solved one-dimensional heat transfer governing equations that import the solar flux modeling results. By consistently dealing with changes in properties or dimensions of absorbers for the two modeling processes, we are able to evaluate absorber performance with the balance of radiation loss. It turns out that the diffuse irradiation assumption is applicable for performance evaluation on multichannel absorbers although it may differ from a real solar flux distribution. A sensitivity analysis demonstrates that the increase of absorptivity is most effective to performance enhancement because the increase of reflection loss dominates the decrease of emission loss. The proposed consistent approach provides a better understanding of heat transfer in volumetric solar absorbers and thus helps the improvement of their performance.     

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: II—Modeling and analysis

The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test was conducted to determine the thermal, chemical, and mechanical performance of a commercial-scale, dish-mounted, direct catalytic absorption receiver (DCAR) reactor over a range of steady state and transient (cloud) operating conditions. The focus of the test was to demonstrate “proof-of-concept” and determine global performance such as reactor efficiencies and overall methane conversion. A numerical model was previously developed to provide guidance in the design of the absorber. The one-dimensional, planar, and steady-state model incorporates the following energy transfer mechanisms: solar and infrared radiation, heterogeneous chemical reaction, conduction in the solid phase, and convection between the fluid and solid phases. Improvements to the model and improved property values are presented here. In particular, the solar radiative transfer model is improved by using a three-flux technique to more accurately represent the typically conical incident flux. A spatially varying catalyst loading is incorporated, convective and radiative properties for each layer in the multilayer absorber are determined, and more realistic boundary conditions are applied. Considering that this test was not intended to provide data for code validation, model predictions are shown to generally bound the test axial thermocouple data when test uncertainties are included. Global predictions are made using a technique in which the incident solar flux distribution is subdivided into flux contour bands. Reactor predictions for anticipated operating conditions suggest that a further decrease in optical density (i.e., extinction coefficient) at the front of the absorber inner disk may improve absorber conditions. Code-validation experiments are needed to improve the confidence in the simulation of large-scale reactor operation.     

Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator

In this paper, thermal performance analysis of 20 m² prototype fuzzy focal solar dish collector is presented. The focal image characteristics of the solar dish are determined to propose the suitable design of absorber/receiver. First, theoretical thermal performance analysis of the fuzzy focal solar parabolic dish concentrator with modified cavity receiver is carried out for different operating conditions. Based on the theoretical performance analysis, the total heat loss (conduction, convection and radiation heat losses) from the modified cavity receiver is estimated. It is observed that the maximum theoretical efficiencies of solar dish collector are found to be as 79.2% for no wind conditions and 78.2% and 77.8% for side-on and head-on winds speed of 5 m/s respectively. Latter, real time analysis of parabolic dish collector with modified cavity receiver is carried out in terms of stagnation test, time constant test and daily performance test. From stagnation test, the overall heat loss coefficient is found to be 356 W/m² K. The time constant test is carried out to determine the influence of sudden change in solar radiation at steady state conditions. The daily performance tests are conducted for different flow rates. It is found that the efficiency of the collector increases with the increase of volume flow rates. The average thermal efficiencies of the parabolic dish collector for the volume flow rate of 100 L/h and 250 L/h are found to be 69% and 74% for the average beam radiation (I_bn) of 532 W/m² and 641 W/m² respectively.     

A model for improved solar irradiation measurement at low flux

Accurate measurement of solar radiation heat flux is important in characterizing the performance of CSP plants. Thermopile type Heat Flux Sensors (HFSs) are usually used for this purpose. These sensors are typically reasonably accurate at high heat fluxes. However measurement accuracy drops significantly as the measured radiation is below 1 kW/m², this often leads to underestimation of the actual flux. At the Masdar Institute Beam Down Solar Thermal Concentrator (BDSTC), measurement of fluxes ranging from 0 kW/m² to more than 100 kW/m² is required. To improve the accuracy of the sensors in the lower range around 1 kW/m², we have performed a test under ambient (not-concentrated) sunlight. Such low irradiation levels are experienced in characterizing the concentration quality of individual heliostats. It was found during the test that the measurement at this low range is significantly affected by ambient conditions and transients in the HFS cooling water temperature. A Root Mean Square Error (RMSE) of more than 100 W/m² was observed even though we kept the transients in water temperature to a minimum. Hence we devised a model to account for this measurement error at this flux range. Using the proposed model decreased the RMSE to less than 10 W/m². The application of the model on existing heat flux measurement installations is facilitated by the fact that it only employs easily measurable variables. This model was checked by using a test data set and the results were in good agreement with the training data set.     

A proposal for “correction values” for winter outdoor design temperatures

This study aims to find a correlation between winter outdoor design temperature (WDT) and mass of the building envelope. The daily variations of the inside surface temperatures and heat fluxes of the walls under various climatic conditions and different wall constructions have been calculated by a computer program based on the response factor technique, which uses variable outside air temperature and solar radiation and constant inside air temperature values as input climatic data. The analysis of the relation between mass of the walls and inside surface heat fluxes resulted with the correction values for winter design temperature (WDTCV) depending on the mass of the wall and on the direction of facades for different climatic zones.     

Combined nongray radiative and conductive heat transfer in multiple glazing taking into account specular reflection and absorption

The problem of combined nongray radiative and conductive heat transfer in multiple glazing subjected to solar irradiation is analyzed. A spectral solar model proposed by Bird and Riordan is used to calculate direct and diffuse solar irradiance. The radiation element method by ray emission model, REM², is used to analyze the spectral dependence of radiative heat transfer. Specular reflection at boundary surfaces is taken into account. The spectral dependence of radiation properties of glass such as specular reflectivity, refraction angle, and absorption coefficient is taken into account. The steady‐state temperature and heat flux distributions in the glass layer are obtained and the insulating efficiency of multiple glazing is examined. The overall heat transfer coefficients predicted by the present method are compared with those based on the JIS method. The values obtained by the present method are slightly lower than those obtained by the JIS method. To investigate the spectral variation of radiative heat flux attenuated in the glass layer, the spectral heat flux at the room‐side surface and incident radiation are compared. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 712–726, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10125     

Numerical Prediction of Natural Convection Occurring in Building Components: A Double-Population Lattice Boltzmann Method

In this article, a double-population thermal lattice Boltzmann method is proposed to solve the problem of the heated cavity with imposed temperatures. This family of problems can be considered as a test model for building physics application. A Taylor series expansion- and least-square-based lattice Boltzmann method (TLLBM) has been implemented in order to use a non-uniform mesh. This allowed us to investigate, at reasonable computational cost, the laminar and transitional flow fields (10³ ≤ Ra ≤ 10⁸). The numerical results, concerning the heat and mass transfers in the cases tested, are in good agreement with those from the literature. In order to demonstrate the possibilities of the method described in the article, applications are described covering double-skin facades and solar collectors or local heaters.     

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.     

Experimental and theoretical evaluation of the overall heat loss coefficient of vacuum tubes of a solar collector

The overall heat loss coefficient (U-value) of a vacuum tube solar collector is investigated experimentally and theoretically with regard to the pressure of the remaining gas inside the evacuated glass envelope. A number of collector tubes of same geometry are randomly selected from an installation of a solar based air-conditioning system and tested individually in the laboratory for the determination of the U-value. Measurement results indicate that most of the examined collector tubes have higher overall heat loss coefficients than expected corresponding to a significant amount of gas inside the glass envelope.For the same conditions, an approximate theoretical model is developed for the evaluation of the U-value. The theoretical model is validated against the experimental results for a collector tube having air inside the glass cover at atmospheric pressure and found to be in close agreement. Then, the influence of gas pressure is studied for various gases. Possible presence of air, hydrogen, helium and argon is discussed.     

Solar energy powered Rankine cycle using supercritical CO2

A solar energy powered Rankine cycle using supercritical CO₂ for combined production of electricity and thermal energy is proposed. The proposed system consists of evacuated solar collectors, power generating turbine, high-temperature heat recovery system, low-temperature heat recovery system, and feed pump. The system utilizes evacuated solar collectors to convert CO₂ into high-temperature supercritical state, used to drive a turbine and thereby produce mechanical energy and hence electricity. The system also recovers heat (high-temperature heat and low-temperature heat), which could be used for refrigeration, air conditioning, hot water supply, etc. in domestic or commercial buildings. An experimental prototype has been designed and constructed. The prototype system has been tested under typical summer conditions in Kyoto, Japan; It was found that CO₂ is efficiently converted into high-temperature supercritical state, of while electricity and hot water can be generated. The experimental results show that the solar energy powered Rankine cycle using CO₂ works stably in a trans-critical region. The estimated power generation efficiency is 0.25 and heat recovery efficiency is 0.65. This study shows the potential of the application of the solar-powered Rankine cycle using supercritical CO₂.     

A review on applying ventilated double-skin facade to buildings in hot-summer and cold-winter zone in China

The need to energy conservation and sustainable development in buildings is causing a new interest towards passive solar systems. Among them, double-skin facade (DSF) proves to be extremely attractive and promising. DSF is building envelope formed by two layers of different glazing facades which are separated by a ventilated air cavity. The cavity of DSF is used to collect or evacuate the solar radiation absorbed by the facades, thereby improving the thermal comfort and the indoor air quality while conserving energy for heating and cooling. Being a technique developed for colder climates, DSF has been widely applied in commercial buildings across Europe. Nowadays buildings with DSF also appear in the hot-summer and cold-winter zone in China where the weather conditions in summer seem to be not so good for the application. In fact, the thermal analysis of the DSF system is essential to its application in such hot-summer zone. This paper seeks to describe the existing main research methods on the thermal performance of DSF and the shading devices. Problems and possibilities are concomitant. Applying ventilated DSF with controlled shading device system would be a new efficient way for the commercial buildings in the hot-summer and cold-winter zone to meet the task of sustainable building design in China.     

Analytical Simulation of the Passive Solar System

The computer simulation is a useful tool for heat transfer analysis of the passive solar systems especially in preliminary performance evaluation, implementation, and/or optimization of the system configurations in the course of the development and constructive design.

The present article describes advantages and conditions of an application of the simulation scheme to the passive solar system analysis, along with design criterion and requirements for analytical heat transfer models for the passive solar system simulations.

A state-of-the-art program, ESPAR, used for the simulations, has been developed for the elementary heat transfer analysis of the passive solar systems constructed as a part of a single room residential building, and formulated around a finite differential heat conductive model combined with typical passive solar system submodels.

Analytical results by ESPAR for the direct gain system with sensible or latent heat storage wall and Trombe wall system are presented comparatively with measured results.     

热流计测量精度影响因素的数值分析

本文通过建立二维非稳态导热的数学模型 ,模拟了瞬态热流计测量低温金属表面与高温流体换热时影响测量精度的五种主要因素。计算得出 ,由于热流计测头的出现 ,对流换热系数的变化会给热流测量带来较大误差。其次 ,热流计测头自身因素的变化也会给测量带来误差。测头越薄 ,测量误差越小 ,稳定越迅速 ;测头边长越长 ,测量误差越小。但当被测物表面积较大时 ,测头边长存在一个最优值 ,能达到测量精度与测头尺寸的最佳结合 ,这个值约为 2 0mm ;测头与被测物粘贴越紧密 ,误差越小 ,稳定越迅速 ;被测物导热系数越小 ,则测量误差越小。模拟计算的结果能够为热流计的设计制作以及实际应用提供指导和参考     

Numerical analysis of the thermal behaviour of ventilated glazed facades in Mediterranean climates. Part I: development and validation of a numerical model

Ventilated glazed facades are formed by two layers of different materials, opaque or transparent, that are separated by an air channel, used to collect or evacuate the solar radiation that is absorbed by the facade. For architectonic reasons, the outer layer is usually made entirely of glass, while the indoor layer may be partially opaque. This allows direct solar gains to be reduced and increases the thermal inertia of the building. This paper is a presentation of a code for the numerical simulation of ventilated and conventional facades. It is based on time-accurate, one-dimensional discretizations for the channel and the different solid zones, and allows heat fluxes and temperature distributions in the facade to be obtained over the course of one year. The numerical code allows advanced elements to be integrated into the facade, such as phase change materials, selective surfaces and improved glasses. The code has been validated by comparing it with analytical solutions where possible, with reference situations and with experimental measurements obtained in real-site test facilities in different climatic conditions. The numerical code is a useful tool for optimising the design of facades so as to take advantage of different materials, orientations, geometries and to address different climatic conditions.     

Investigation of solar cell degradation using electrochemical impedance spectroscopy

The performance of solar cells reduces annually due to various unavoidable phenomena of thermal cycling, damp heat, UV exposure, and mechanical stress, etc. Generally, I-V characteristic is used to check the performance of the solar cell, but the minor stress conditions mentioned above are difficult to characterize by I-V measurement. Impedance spectroscopy is a widely used method in fuel cell and a battery that can be used to detect minor degradation in the solar cell by analyzing the change in equivalent circuit parameters. In this work, commercially available polycrystalline silicon solar cell is investigated under the condition of hotspot, mechanical stress, and disconnection of interconnection ribbon and then characterized by impedance measurement, Fourier transform (Bode plot) as well as I-V characteristic. The results show noteworthy decrease in parallel resistance (Rp) which is clearly visible in Nyquist plot in compare to the I-V characteristic. The Rp decreases in EIS from 283.60 to 234.80 Ω for mechanical stress test, from 273.0 to 187.10 Ω for hotspot and from 352.80 to 345.20 Ω for disconnection of interconnection ribbon test. The results confirm potential application of impedance measurement for solar cell characterization due to noteworthy change in equivalent circuit parameters after test conditions.     

EXPERIMENTAL STUDIES ON A SOLAR AIR COLLECTOR WITH METAL MATRIX ABSORBER

The present contribution describes the development and testing of an efficient single-glazed solar matrix air collector. This collector was designed in order to overcome the physical problems of conventional flat-plate air collectors as well as the technical problems of matrix air collectors, in particular. The absorber of the collector consists of two parallel sheets of black oxidized or black galvanized industrial woven, fine-meshed wire screens made of copper. The new collector can be readily produced industrially at acceptable costs. A test collector was developed and tested indoors by varying design features and operating conditions using a solar simulator as a radiation source. The new collector is very durable and flexible regarding mass flow rate and collector duct height, and yields high thermal performances at very low pressure losses. High outlet temperatures are obtainable, thus improving the quality of the gained heat. This type of collector can be used for drying and heating applications and – due to its light-weight design and the possibility to serve as a sunshade (anti-glare device) which can be unrolled if there is dazzle by the sunlight inside of the building – it can be easily integrated vertically into double facades of buildings.     

Experimental investigation of pulsating heat pipe performance with regard to fuel cell cooling application

A pulsating heat pipe (PHP) is a closed loop, passive heat transfer device. Its operation depends on the phase change of a working fluid within the loop. Design and performance testing of a pulsating heat pipe was conducted under conditions to simulate heat dissipation requirements of a proton exchange membrane (PEM) fuel cell stack. Integration of pulsating heat pipes within bipolar plates of the stack would eliminate the need for ancillary cooling equipment, thus also reducing parasitic losses and increasing energy output. The PHP under investigation, having dimensions of 46.80 cm long and 14.70 cm wide, was constructed from 0.3175 cm copper tube. Heat pipes effectiveness was found to be dependent upon several factors such as energy input, types of working fluid and its filling ratio. Power inputs to the evaporator side of the pulsating heat pipe varied from 80 to 180 W. Working fluids tested included acetone, methanol, and deionized water. Filling ratios between 30 and 70 percent of the total working volume were also examined. Methanol outperformed other fluids tested; with a 45 percent fluid fill ratio and a 120 W power input, the apparatus took the shortest time to reach steady state and had one of the smallest steady state temperature differences. The various conditions studied were chosen to assess the heat pipe's potential as cooling media for PEM fuel cells.