Trends of solar radiation effects on the temperature of vertical surfaces of a modern terrace house

This is a study done to investigate the effects of solar radiation on the wall surface temperature of a modern terrace house. It involves the analysis of the thermal fluctuations of common building materials used for walls or vertical surface areas such as the bricks, glass of the window, and metal doors. A thermal imager and data logging system was used in collecting the data for the Southeast and Northwest façades of the house. The imager gave resourceful data on the thermal heat trend of the walls and their surface temperature. The results show that a lighter wall surface color reduces the temperature of the surface. Furthermore, the bricks, which have a higher density hence a higher absorptivity due to their high capacity for storage of heat, decrease the flow of heat. However, the use of tinted glass on windows increases the surface temperature of the glass area of the wall surface. The metal also shows a high similarity with glass in terms of its thermal performance. In conclusion, the types of material used on wall surfaces have a significant impact on the wall temperature.     

隔热涂层降低建筑空调负荷效果的参数分析

从降低建筑空调负荷的角度出发,对外壁面有隔热涂层的房间建立了稳态传热模型,用数值方法逐一分析了涂层的各种参数,包括导热系数、厚度、表面太阳吸收率、发射率以及地区因素等对空调负荷的影响。研究发现：隔热涂层能显著降低空调负荷及能耗；仅当涂层热阻与墙体热阻之比大于0．2时,涂层导热系数的降低才会对空调负荷产生显著影响；涂层表面太阳吸收率和发射率对空调负荷的影响非常大,是起主导作用的因素；隔热涂层用在太阳辐照密度越大、日最高气温越低、日较差越大的地区节能率越高,越有利于发挥其降低空调负荷及日累计能耗的功能；决定着空调运转时间长短的当地室外气温分布状况对于评价隔热涂层节能效果也有重要影响。     

Study of the natural convection phenomena inside a wall solar chimney with one wall adiabatic and one wall under a heat flux

Four wall solar chimneys have been constructed and put at each wall and orientation of a small-scale test room so as to be used for the evaluation and measurement of their thermal behavior and the certification of their efficiency. At this stage, research focuses on the study of the buoyancy-driven flow field and heat transfer inside them. A numerical investigation of the thermo-fluid phenomena that take place inside the wall solar chimneys is performed and the governing elliptic equations are solved in a two-dimensional domain using a control volume method. The flow is turbulent and six different turbulence models have been tested to this study. As the realizable k–ε model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, it has been selected to be used in the simulations. This is also confirmed by comparing with the experimental results. Predicted velocity and temperature profiles are presented for different locations, near the inlet, at different heights and near the outlet of the channel and they are as expected by theory. Important parameters such as average Nusselt number are also compared and calculated at several grid resolutions. The developed model is general and it can be easily customised to describe various solar chimney’s conditions, aspect ratios, etc. The results from the application of the model will support the effective set-up of the next configurations of the system.     

Effect of solar absorptivity and emissivity of exterior surfaces on the thermal performance of a building

The effects of the solar absorptivity and the thermal emissivity of exterior building surfaces on the indoor air temperature of a one room building are evaluated in terms of the discomfort index. The thermal performance of the building has been investigated for four different climates prevalent in India, namely, composite (New Delhi), hot and dry (Jodhpur), cold and humid (Srinagar), and cold and dry (Leh). The results confirm the common-sense view that the building surface should have low solar absorptivity and high thermal emissivity in hot climates and high solar absorptivity and low thermal emissivity in cold climates for indoor thermal comfort conditions.     

Solar collector surfaces with wavelength selective radiation characteristics

The use of porous materials through which a coolant is forced for the protection of surfaces exposed to high temperature gas streams has been discussed in a number of recent papers. The knowledge of the radiation properties of these materials is required if the designer is to be able to predict the porous wall temperatures to determine whether metallurgical limitations have been exceeded. With this application in mind, measurements were previously reported for the absorptivity for solar radiation of a number of porous surfaces.¹ Since that time total normal emissivity data have been obtained for the same surfaces. In viewing these results, it was discovered that these surfaces combined high absorptivity for solar radiation with a low emissivity value, and consequently their use as solar collectors is suggested. It is the purpose of this paper to describe the experimental apparatus used to obtain the radiation data and to compare the recent emissivity measurements with the solar absorptivity data for the same surfaces. For specified conditions, an efficiency is defined which allows a quantitative comparison of these with other surfaces as solar collectors.     

Prediction on effects of absorptivity and emissivity for surface temperature distribution using an improved thermal network model

The conventional thermal network model for satellite surface temperature distribution is directly solved by a new solution method on the basis of the Monte Carlo Ray Tracing (MCRT) method. The solar direct incidence area, the solar radiation transfer coefficient, the infrared radiation transfer coefficient, and the network conduction and radiation coefficients are calculated by Monte Carlo statistical analysis rather than the Gebhart approach. The advantage of the MCRT method is that the surface material characteristics are taken into account in the solving process. The effect of absorptivity and emissivity for temperature distributions is analyzed in detail. Numerical simulation is carried out for the temperature distribution of the satellite surface with different solar incidences of round angle and zenith angle. In the steady stage, the maximum temperature difference reaches up to 200 K and increasing the absorptivity‐emissivity ratio will lead to a rapid temperature rise. In the unsteady stage, the cooling velocity of the main body surface is always smaller than that of the solar array surface and the maximum temperature of the main body is larger than the solar arrays. Under the same absorptivity‐emissivity ratio, the solar incidence angle has a great effect on the uniform character of temperature distribution. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res, 39(7): 539–553, 2010; Published online 16 July 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20316     

Mathematical modeling and the thermal performance of floating cum tilted-wick type solar still

In this paper, mathematical modeling for the thermal performance of floating cum tilted-wick type solar still has been presented. Explicit expressions for the temperatures of various components of the proposed system and its efficiency have been developed. The effect of mass flow rate due to capillary action of jute wick has been investigated for evaporative, convective, and radiative heat transfer from the evaporating surface to the condensing surface.It has been found that the mass flow rate of 2.5 x 10^?3 kg/s is optimum for effective distillation. Also, the effect of absorptivity of the wick surfaces (floating and tilted-wick) on the productivity of the solar still has been studied and found that α_w1 (absorptivity of the tilted-wick surface) and α_wf(absorptivity of the floating-wick surface) of 0.85 have given higher productivity.For enunciation of the analytical results, numerical calculations have been made using meteorological parameters for a typical winter and summer day in Coimbatore.The theoretical results are in good agreement with the experimental results.     

Compact Linear Fresnel Reflector solar thermal powerplants

This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear receivers elevated on tower structures that are close enough for individual mirror rows to have the option of directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror inclination can be set up such that shading and blocking are almost eliminated while ground coverage is maximised. Preferred designs would also use secondary optics which will reduce tower height requirements. The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the development of specific raytrace and thermal models to simulate the new concepts.     

Multiobjective optimisation of integrated energy systems for remote communities considering economics and CO2 emissions

The methods for designing, planning and managing integrated energy systems, while holistically considering the major economic and environmental factors, are still embryonic. However, the first phase of the design is often crucial if we want to manage resources better and reduce energy consumption and pollution. Considering integrated energy systems implies dealing with complex systems in which the synergy between the various components is best exploited (for example the thermal energy of a diesel engine produced during the night is complimented by the Rankine organic cycle of a solar thermal plant). The context of isolated communities further increases the difficulties when considering the long distance of transport required to supply fossil fuels. These sites are often located in very precarious environments, with limited or nonexistent resources except for solar energy, and with frequent additional needs for desalination (in arid zones).This paper illustrates a holistic method to rationalize the design of energy integrated systems. It is based on a superstructure (collection of models of all envisaged technologies) and a multi-objective optimisation (resources, demand, energy, emission, costs) using an evolutionary algorithm. The approach proposed allows the identification of more complete and more coherent integrated configurations characterizing the most promising designs (also taking into account the time dependency aspects). It also allows to better structure the information in view of a participative decision approach. The study shows that the economic implementation of renewable energy (solar) is even more difficult, compared to diesel based solutions, in cases of isolated communities with high load variations. New infrastructure or retrofit cases are considered.     

Evaluation of exergy and energy efficiencies of photothermal solar radiation conversion

In this paper, a new thermodynamic model for photothermal solar radiation conversion into mechanical through a heat engines is proposed. The developed equations allow for the energy and exergy contents of solar radiation to be found, as well as the energy and exergy efficiencies corresponding to concentration type solar-thermal heat engines operating under a range of conditions. The calculation method remains accurate to other published models when their assumed conditions are imposed to the newly developed model. The heat flux absorbed by the receiver (which is assumed to be a grey body and is placed in the focal point of the solar concentrator) depends on the hemispherical absorptivity and emissivity, concentration ratio and receiver temperature. The model is used to conduct a parametric study regarding the energy and exergy efficiencies of the system for assessing its performance. The use of a selective grey body receiver (having a reduced emissivity and a high absorptivity) for enhancing the conversion efficiency is also studied. If the absorptivity approaches one and the emissivity is low enough the photothermal conversion efficiency becomes superior to the known black body receiver limit of 0.853. It is found that in the limit of receiver emissivity tending to zero and absorptivity lending to one, the present model gives the exergy content of solar radiation because the work generated reaches its maximum. In this situation the energy efficiency approaches the exergy efficiency at 1-ITTIN0/TINS where T_S and T₀ are the sun and ambient temperatures, respectively. The influence of the ambient temperature on the exergy and energy efficiencies becomes apparent, with effects of up to 15%, particularly for high absorptivity and low emissivity. The heat transfer conductances at sink and source of the heat engine have a considerable impact on the efficiency of solar energy conversion. The present model is developed in line with actual power system operations for better practical acceptance. In addition, some irreversibility parameters (absorptivity, emissivity, heat transfer conductivity, etc.) are studied and discussed to evaluate the possible photothermal solar radiation conversion systems and assess their energy and exergy efficiencies.     

An alternative method for calculation of semi-gray radiation heat transfer in solar central cavity receivers

This paper describes a general method to calculate the semi-gray radiation heat transfer that occurs within an enclosure comprised of diffuse surfaces. The method is implemented in an existing solar power tower cavity receiver model in the System Advisor Model (SAM, NREL, 2011). The semi-gray radiation model is used to find an optimal distribution of emissivities for the thermal- and solar radiation wavelength bands for surfaces that comprise the solar central cavity receiver. The optimal distribution of emissivities maximizes the overall thermal efficiency of a cavity receiver. The model shows an effective way to reduce heat loss from the cavity is to minimize the temperatures of the passive surfaces through manipulation of their radiative surface properties.For the cavity receiver design considered, an optimal emissivity distribution for the active absorber surfaces of the cavity is a selective surface with high absorptivity in the solar wavelength band and low emissivity in the thermal wavelength band. Passive surfaces within the cavity should be highly reflective for radiation over the full spectrum. For absorber surfaces with solar absorptivity of 0.95, thermal emissivity of 0.1 and reflective passive surfaces with emissivities of 0.1, for the full spectrum, the thermal efficiency of the receiver can be increased by about 0.7% in comparison to gray surfaces having an emissivity of 0.95 for all wavelengths.     

A novel integrated simulation approach couples MCRT and Gebhart methods to simulate solar radiation transfer in a solar power tower system with a cavity receiver

An integrated simulation approach, which couples Monte Carlo ray tracing (MCRT) and Gebhart methods, is proposed to simulate solar radiation transfer in a solar power tower system with a cavity receiver. The MCRT method is used to simulate the solar radiation transfer process from the heliostat field to interior surfaces of the cavity receiver, and the Gebhart method is used to simulate the multiple reflections process of solar radiation within the cavity. This integrated simulation method not only reveals the cavity effect on receiver performance but also provides real-time simulation results. Based on this method, the reflection loss of the cavity receiver and solar flux distributions are discussed in detail. The results indicate that the cavity effect can significantly reduce the reflection loss and homogenize the concentrated solar energy distributed on interior surfaces to some extent. Moreover, the surface absorptivity has less effect on the reflection loss when cavity effect is considered. The cavity effect on homogenizing solar flux distributions is greater with lower surface absorptivity. In addition, although the concentrated solar energy is distributed on the cavity aperture with similar shapes at different times, the shape of the solar flux distribution on interior surfaces varies greatly with time.     

Thermal performance assessment of recyclic double-pass flat and V-corrugated plate solar air heaters

Double-pass solar air heaters occupy an important place among solar air heating systems, because of minimal heat loss and maximum thermal efficiency with marginal heater size and cost. In the present work, investigations related to the thermal performance predictions have been carried out for double-pass flat and V-corrugated absorber plate solar air heaters under recycle operation. The mathematical models proposed herein are solved using an analytical approach that uses an iterative solution procedure. Furthermore, based on simulation results obtained from the analytical study, the optimum value of the recycle ratio, the mass flow rate, the absorptivity and the emissivity at which the heaters yield the maximum value of the thermal efficiency have been identified and presented using response surface methodology (RSM). The results of RSM revealed that the mathematical models are significant. In addition, results of the present study are validated and compared with previous studies. A reasonable agreement and significant improvement have been achieved.     

Effects of tracking errors on the performance of point focusing solar collectors

An important parameter in the design of point focusing solar collectors is the intercept factor which is a measure of efficiency and of energy available for use in the receiver. Using statistical methods, and expression for the expected value of the intercept factor is derived for various configurations and control law implementations. The analysis assumes that a radially symmetric flux distribution (not necessarily Gaussian) is generated at the focal plane due to the sun's finite image and various reflector errors. The time-varying tracking errors are assumed to be uniformly distributed within the threshold limits and allows the expected value calculation.     

On the transient thermal behaviour of structural walls — the combined effect of time varying solar radiation and ambient temperature

The aim of the present work is to investigate the transient conduction heat transfer in structural walls. The developed finite difference numerical simulation code, which is suitable to run in conventional microcomputers, has the flexibility to incorporate a wide range of boundary and initial conditions and time-dependent forcing functions for the investigation of the transient heat transfer in walls of any design. The analysis was employed for the prediction of the dynamic thermal behaviour of two wall designs under the effect of a step temperature change and a combined time varying ambient temperature and solar radiation corresponding to typical local weather conditions. The results allow the prediction of the transient duration, the transient temperature fields as well as the quasi-steady state heat fluxes. They also allow the investigation of the dynamic effects of realistic time varying meteorological forcing functions and the strong influence of the wall surface absorptivity, something which confirms its importance as a handy instruction and research tool in the renewable and rational use of energy fields.     

几种集热-贮热墙式太阳房的动态模拟及热性能比较

被动式太阳房的结构对其热性能具有重要影响,采用一维热网络模型对多种结构的太阳房进行了动态模拟,研究了吸热面的热辐射性质,厚墙墙体材料的热物性,不同的透明覆盖以及在透明覆盖与厚墙之间增设金属吸热板等不同结构对太阳房热性能的影响,计算结果为厚墙式被动太阳房的结构设计与材料选取提供了有益的参考。     

The summer thermal behaviour of “skin” materials for vertical surfaces in Athens, Greece as a decisive parameter for their selection

This paper analyses the thermal behaviour of the materials, which are widely used on the vertical surfaces of Greek cities. This analysis is based on surface temperatures measurements, which were carried out both in situ in various buildings of Athens, Greece and experimentally on samples of building materials exposed to solar radiation on a building’s flat roof. The study includes surfacing materials, which are usually applied on building facades around Greece.The study leads to a number of conclusions concerning the effect of colour and orientation on the summer surface temperatures of materials, used on vertical city surfaces. These conclusions indicate how surfacing materials should be chosen in order to help mitigate the urban heat island and improve thermal comfort conditions in the outdoor spaces of Greek cities during the overheated summer period.     

不同吸热涂层平板集热器的热性能衰减研究

为分析不同吸热涂层平板集热器的热性能衰减,以蓝膜、阳极氧化和黑铬集热器为研究对象,基于太阳集热器热性能测试平台,对集热器热性能及空晒老化性能进行测试。分别测试吸热涂层样品的吸收比和发射比,分析空晒前后平板集热器吸收比、发射比和瞬时效率的变化情况及影响因素。研究结果表明:在温度约为18℃的工况下,蓝膜集热器热性能为75.5%,黑铬为73.4%,阳极氧化为69.3%,吸热涂层的光学性能是影响平板集热器热性能的主要因素。平板集热器瞬时效率、吸收比和发射比变化情况的依存关系为Δη≈9.553Δα-1.213Δερ,该式可衡量平板集热器的热性能衰减度。通过空晒老化性能实验可知,提高平板集热器吸热涂层的抗腐蚀、抗氧化能力,可延长集热器的使用寿命。     

On the improvement of annual performance of solar thermal power plants through exergy management

Advancing in the learning curve of solar thermal power plants (STPP) requires detailed analysis for reducing exergy losses in the energy conversion chain. This requirement should be applied to any configuration proposed for the solar field and the power block. The aim of this work is to perform this type of analysis for two ways of structuring the power plant. The first plant structure consists of a subdivision of the solar collector field into specialized sectors with specific goals conveying different requirements in temperature. The second plant structure is based on a dual thermal energy storage system with a defined hierarchy in the storage temperature. The subdivision of the solar field into different sectors reduces the exergy losses in the heating process of the working fluid. Moreover, the average temperature of the heat transfer fluid in the solar field decreases when it is compared to the conventional solar field, reducing this way the exergy losses in the collectors. The dual thermal energy storage system is devised for keeping the exergy input to the power block at its nominal level for long periods of time, including post‐sunset hours. One of the storage systems gathers a fluid heated up to temperatures above the nominal value and the second one is the classical one. The combination of both allows the manager of the plant to keep the nominal operation of the plant for longer periods than in the case of classical system. Numerical simulations performed with validated models are the basis of the exergy analyses. The configurations are compared to a reference STPP in order to evaluate their worth. Furthermore, the behaviour of the configurations is analysed to study the irreversibility of the included devices. Special attention is paid to the storage systems, as they are a key issue in both plant structures. Copyright © 2013 John Wiley & Sons, Ltd.     

Vernacular technologies applied to modern architecture

Various technologies as can be seen in vernacular architecture especially in Japanese traditional buildings are reviewed and the ways and means to have those technologies applied to the design of modern architecture are discussed with some examples. It is stressed that the vernacular technologies have been devised uniquely to the region where people lived to cope with the severe climate by inventing various devices without resorting to fossil fuels, thus the form of vernacular architecture representing regionalism of their own. For example earth sheltered buildings corresponds to thatched roof houses where evaporation from the wet surface gives rise to cooling effects in hot and humid climate. The optimum shape of sun shades is devised to a given orientation so that it allows the air to flow through for making ventilative cooling effective. Direct solar heat gain as representing a passive solar system is no less than fundamental for both vernacular and modern architecture with most appropriate design of geometry associated with wall orientation. Various suggestions are given for architects to consider how to apply vernacular technologies to the design of modern architecture.