Recommendation on modelling of solar energy incident on a building envelope

It is important to know how to design a building to meet seasonally varying energy needs. In high latitude countries in winter the demand for space heating is high and a building envelope should receive maximum incident solar energy. On the other hand, in summer, walls and roofs exposed to incident solar radiation usually require shading to avoid too much solar gain. Data on solar energy availability are crucial for good building design. However, it is important how the availability of solar radiation is determined. An important aim of the paper presented is to give some results of a comparative analysis of two basic sky models, isotropic: Hottel–Woertz–Liu–Jordan and anisotropic: the HDKR, Hay–Davies–Klucher–Reindl, to recommend one of these models for determination of solar energy availability on a building envelope and to formulate the energy balance of a building. Differences between results obtained from both models increase with the slope of exposed surfaces. The biggest differences (12–15%) are evident for vertical south surfaces, especially in summer. The simplified isotropic sky model is not recommended for evaluation of solar radiation availability on the building envelope. Underestimation of solar gains can lead to the selection of an unsuitable concept and construction of a building and result in poor indoor thermal comfort, i.e. overheating of rooms in summer.     

Investigations into the functioning of a supply air window in relation to solar energy as determined by experiment and simulation

‘Supply air windows’ under optimum flow conditions function as an efficient heat reclaim device. Heat escaping from the room, through the inner glass pane, is entrained in the air flow between the inner and outer sashes and returned to the room. A low-E coating to the inner glass acts as a barrier to radiation heat loss across the window so very low U-values can be achieved. These same characteristics enable the window to function as a passive solar component. Its efficiency is inferior to that of a dedicated passive solar device due to its transparency, but even so at modest levels of incident solar gain a worthwhile proportion is entrained into the air flow and supplied to the rooms as pre-heated ventilation air supply. These characteristics have been established by laboratory, test cell investigations, and simulations using computational fluid dynamics and ESP-r, a whole building dynamic thermal modelling tool.     

Effect of modelling solar radiation on the cooling performance of radiant floors

When modelling buildings, solar radiation has a large impact on the thermal balance because it usually heats the rooms. In radiant systems that are used for heating and cooling buildings, solar radiation has a large influence both on indoor temperatures and on the efficiency of the radiant system.Many analyses have already been carried out in order to study how beam and diffuse radiation can be distributed in a room. One of the most difficult issues, when modelling room thermal balance, is how to simulate the solar radiation when it enters the room, which in turn depends on the reflectance characteristics of the surface finishing elements.In this study, four different radiation models have been applied in order to solve an overall detailed, dynamic thermal balance in a room with pipes embedded in the floor. Two of the models are detailed; the other two consider the radiation entering the room to be diffuse radiation. As for the behaviour of the impinging solar radiation on the covering materials in a room, measurements have been carried out to determine the reflectance coefficients, which will be used in simulations for characteristic materials used in buildings.Results of the simulations show that a simplified model, which considers solar radiation as uniformly distributed in a room, cannot be used for a detailed comfort analysis; however, when looking at the cooling output of a radiant floor system at the design stage, a simplified model can predict energy transfer to a certain level of accuracy. Moreover, results coming from combined measurements and simulations show that the reflectance characteristic of the covering materials does not affect the cooling capacity of the radiant floor systems, since the most important parameter for cooling performance is the thermal conductivity of the covering layer.     

Solar heat gains and operative temperature in attached sunspaces

Solar heat gains obtainable from attached sunspaces to air-conditioned rooms are evaluated by means of the solution to the optical problem of incident solar radiation absorption through the windows and of the temperature field in the shell separating the sunspace from outdoors and adjacent spaces. The effective absorption coefficient of the sunspace was used for these evaluations as well as the ratio of the absorbed energy of the internal surfaces to the solar energy entering, and the utilization factor of the solar contributions that represent the fraction of the absorbed energy supplied to the indoor air. With reference to a pre-established geometry and to a system of windows made up of clear double-glazing, the solar gains of the sunspace and the adjacent spaces are calculated for some Italian localities at variation of exposure, optical properties and thermal capacity of the opaque surfaces, the amount of ventilation and of the shading device. Finally, the operative temperature was determined for an estimate of comfort acceptability conditions in the sunspace.     

Assessment of fixed shading devices with integrated PV for efficient energy use

The use of external fixed shading devices to adjust solar influx radiation and to save energy is well known. However, fixed shading devices can reduce daylight availability, increase artificial light needs and block the beneficial winter solar radiation.This paper is part of a research on the characteristics of the optimum shading device. The aim is to investigate the balance between the energy needs for heating and cooling the space that the shading device is used for and the energy that is used for lighting the same space and the energy that the shading device can produce.In order to investigate the balance between the above mentioned parameters, thirteen types of fixed shading devices have been studied and categorized according to their energy performance, for a single occupant office room. The same office room is tested for two different Mediterranean latitudes in Athens and in Chania, Crete in Greece and for two different south facing windows’ sizes.The thermal behavior of the devices is assessed through computer simulation application and the daylight analysis is assessed with both computer simulation and physical modeling. Stable parameters were the internal loads in the office room, the south orientation of the façade and the type of glazing. Variable parameter was the type of the fixed shading device.The study shows that all shading devices with integrated south facing PV can efficiently produce electricity which may be used for lighting. The study highlights the fact that shading devices such as Surrounding shading, Brise–Soleil full façade and Canopy inclined double work efficiently against thermal and cooling loads and may be used to produce sufficient electricity and control daylight. The study defines the geometrical parameters that will be incorporated to the overall characteristics of the optimum fixed shading device and proposes new fields of development for the BIPV technologies.     

The reaction coefficient of an enclosure to the solar energy collection

A dimensionless reaction coefficient R_a is defined which gives a measure of the solar energy collectivity of a room with opaque external walls. This number is defined as the ratio between the absorbed solar energy by the wall and the correspondent thermal gain of the room, and depends on the thermal and structural characteristics of the room and its environment. The reaction coefficient R_a is reduced to a simple expression and provides a simple way of estimating the amount of the net radiation energy gain by the room, when the amount of radiation, which is falling or absorbed by the wall, is given. This analytical relation has been confirmed experimentally especially with respect to the influence of the wall total thermal conductance on the reaction coefficient R_a.     

PCM glazing systems

This paper presents a different approach for thermal effective windows, i.e. windows that reduce the energy transmitted into or out of a room. The idea is to use a double sealed glass filled with a phase change medium (PCM) whose fusion temperature is determined by solar–thermal calculations. The PCM used is polypropylene glycol. The investigation includes modelling of the heat and radiation transfer through a composite window and optical investigation of conventional and PCM filled windows, testing of the window and comparison with numerical simulations. A one-dimensional model for the composite glass window is developed to predict the thermal performance as a function of the geometrical parameters of the panel and the PCM used. Optical measurements were realized using photo-spectrometry to determine the transmittance, reflectance and absorptance. The specimens used include single glass of different thicknesses, double glass of different gap spacing and thicknesses filled with air or PCM, and finally coloured PCM. The results indicate big reductions in the energy transmitted, specially in the infra-red and ultraviolet regions, while maintaining a good visibility. © 1997 by John Wiley & Sons, Ltd.     

Optical and thermal characterization of multiple glazed windows with low U-values

The optical reflectance and transmittance spectra of complete windows for near normal and oblique angles of incidence are calculated from spectra of the individual panes taking multiple reflections into account. Calculated and experimental spectra are compared for triple and quadruple glazed windows with different combinations of low-e coatings. For the annual energy balance of a window the total solar transmission at oblique incidence is more relevant than the near normal performance. It is shown that, owing to the experimental difficulties involved in optical measurements at oblique incidence, great care must be taken when evaluating the annual performance. A simple equation for the annual energy balance of the window taking solar radiation and thermal heat losses into consideration is presented. Annual meteorological data for the insolation and outside temperature are used together with the optical performance to evaluate the net energy heat flow through a window. This can be performed for the complete heating season to evaluate the heat load needed for the building as well as for the warm season to evaluate the cooling load needed owing to solar overheating. This model provides a simple way of comparing the thermal performance of windows with different combinations of advanced glazings in both cold and hot climates, and makes it possible to estimate the cost efficiency of such windows.     

Lighting and cooling energy consumption in an open-plan office using solar film coating

In subtropical Hong Kong, solar heat gain via glazing contributes to a significant proportion of the building envelope cooling load. The principal fenestration design includes eliminating direct sunlight and reducing cooling requirements. Daylighting is an effective approach to allow a flexible building façade design strategy, and to enhance an energy-efficient and green building development. This paper studies the lighting and cooling energy performances for a fully air-conditioned open-plan office when solar control films together with daylight-linked lighting controls are being used. Measurements were undertaken at two stages including the electricity expenditures for the office using photoelectric dimming controls only (first stage) and together with the solar control film coatings on the windows (second stage). Electric lighting and cooling energy consumption, transmitted daylight illuminance and solar radiation were systematically recorded and analysed. The measured data were also used for conducting and validating the building energy simulations. The findings showed that the solar film coatings coupled with lighting dimming controls cut down 21.2% electric lighting and 6.9% cooling energy consumption for the open-plan office.     

建筑窗体玻璃节能研究

本文对建筑中的外门窗在太阳辐射热的传递与利用,对几种玻璃光在不同波段情况有效光的传递等方面作了分析研究,得出了不同的太阳高度角下、建筑不同朝向的窗户应选用的建筑门窗玻璃。并特别指出Low-E玻璃对建筑节能有着突出的优势。     

Evaluation of the thermal performance indices of a ventilated double window through experimental and analytical procedures: Uw-values

Simulations to evaluate energy demand for heating and cooling and thermal comfort are becoming more and more common place in the building design process, at least in the most complex cases. In all detailed or simplified calculations, to analyse heat transfer to and from a building, several input parameters are needed. The inputs for the simulation of a whole building are at least the building geometry, the building envelope thermal indices (like thermal transmittance or the solar heat gain coefficient) and typical local climatic data. In a ventilated double window, the air flow through the channel between the two windows makes its thermal performance highly dynamic and dependent on the air flow characteristics. For a whole building simulation, single coefficients or easily calculated coefficients are needed for each facade system, including ventilated systems. In this paper, equivalent thermal transmittance coefficients for a ventilated double window are assessed and presented. For that, experimental measurements in the absence of solar radiation (night period) were used to identify tendencies and validate calculations. Furthermore, simulations were done in order to estimate the U_w-values of the ventilated double window under different windows configuration and different air flow rates. These values can then be used in whole building simulation programmes.     

Artificial neural networks in renewable energy systems applications: a review

Artificial neural networks are widely accepted as a technology offering an alternative way to tackle complex and ill-defined problems. They can learn from examples, are fault tolerant in the sense that they are able to handle noisy and incomplete data, are able to deal with non-linear problems and, once trained, can perform prediction and generalisation at high speed. They have been used in diverse applications in control, robotics, pattern recognition, forecasting, medicine, power systems, manufacturing, optimisation, signal processing and social/psychological sciences. They are particularly useful in system modelling such as in implementing complex mappings and system identification. This paper presents various applications of neural networks mainly in renewable energy problems in a thematic rather than a chronological or any other order. Artificial neural networks have been used by the author in the field of solar energy; for modelling and design of a solar steam generating plant, for the estimation of a parabolic trough collector intercept factor and local concentration ratio and for the modelling and performance prediction of solar water heating systems. They have also been used for the estimation of heating loads of buildings, for the prediction of air flow in a naturally ventilated test room and for the prediction of the energy consumption of a passive solar building. In all those models a multiple hidden layer architecture has been used. Errors reported in these models are well within acceptable limits, which clearly suggest that artificial neural networks can be used for modelling in other fields of renewable energy production and use. The work of other researchers in the field of renewable energy and other energy systems is also reported. This includes the use of artificial neural networks in solar radiation and wind speed prediction, photovoltaic systems, building services systems and load forecasting and prediction.     

Integration of smart windows into building design for reduction of yearly overall energy consumption and peak loads

The purpose of this work is to investigate the potential of diminishing the energy consumed by typical low thermal mass office buildings for heating, cooling and lighting by using smart windows. The windows considered consisted of a double pane glazing unit in which a controllable absorbing layer is added on the interior surface of the exterior glass pane. This absorbing layer allows to change the optical properties of the window, resulting in a direct potential of control of the incident solar heat flux entering the building through the windows. A corresponding numerical model is developed showing that optimizing the solar heat flux absorption rate of the absorbing layer in regard of the necessary heating, cooling and lighting needs helps reducing significantly the total yearly energy consumption, and cooling peak loads. The simulations were done considering a building located in Quebec City, Canada.     

Thermal restoration of a wall after the interruption of solar radiation flux

The transient phenomenon of the thermal restoration of an opaque wall of a room after the interruption of solar radiation flux was analysed. The wall, was initially, in thermal equilibrium under solar radiation. When the solar radiation flux was interrupted, the temperature distribution on the wall cross-section went through a transient state until it reached a final equilibrium state. During the thermal restoration of the wall, a thermal gain is maintained for the room as a result of the exploitation of a part of the thermal energy stored in the wall. Analytical expressions have been obtained for the duration of the thermal restoration, for the thermal gain of the room during this transient state and for the coefficients of storage and exploitation of the solar radiation by the wall.     

广州地区建筑围护结构能耗分析

分析总结出广州地区建筑围护结构中不透明部分、半透明部分(玻璃窗)和朝向三个因素对建筑整体能耗的影响规律:围护结构中半透明部分比不透明部分的建筑整体能耗大,东西朝向比南北朝向的建筑能耗大。     

Novel glazing technologies to mitigate energy consumption in low‐carbon buildings: a comparative experimental investigation

Buildings play a key role in total world energy consumption as a consequence of poor thermal insulation characteristics of facade materials. Among the elements of a typical building envelope, windows are responsible for the greatest energy loss because of their notably high overall heat transfer coefficients. About 60% of heat loss through the building fabric can be attributed to the glazed areas. In this respect, novel cost‐effective glazing technologies are needed to mitigate energy consumption, and thus to achieve the latest targets toward low/zero carbon buildings. Therefore in this study, three unique glazing products called vacuum tube window, heat insulation solar glass and solar pond window which have recently been developed at the University of Nottingham are introduced, and thermal performance analysis of each glazing technology is done through a comparative experimental investigation for the first time in literature. Standardized co‐heating test methodology is performed, and overall heat transfer coefficient (U‐value) is determined for each glazing product following the tests carried out in a calibrated environmental chamber. The research essentially aims at developing cost‐effective solutions to mitigate energy consumption because of windows. The results indicate that each glazing technology provides very promising U‐values which are incomparable with conventional commercial glazing products. Among the samples tested, the lowest U‐value is obtained from the vacuum tube window by 0.40 W/m²K, which corresponds to five times better thermal insulation ability compared to standard air filled double glazed windows. Copyright © 2015 John Wiley & Sons, Ltd.     

Solar XXI building: Proof of concept or a concept to be proved?

Solar XXI building is a low energy office building where passive and active solar strategies have been applied to reduce the use of energy for heating, cooling and lighting, combining also an extensive photovoltaic façade for electricity production. Solar XXI opened in 2006 and is considered a high efficient building, close to a net zero energy building (NZEB), where the difference between the energy consumed and that produced is 1/10^th of the energy consumed by a Portuguese standard new office building. Its design includes many energy efficiency concepts, such as a high insulated envelope, south sun exposure, windows external shading, photovoltaic panels heat recovery, ground-cooling system, daylighting, stack effect and cross ventilation. The solar gains of the windows and the effectiveness of shading devices were evaluated in order to correlate solar radiation, external and indoor air temperatures. It was also verified that amplitude-dampening of ground-cooled air ranged between 5 and 8 °C, following the trend of the analytical solution for heat diffusion of a cylindrical air/soil heat-exchanger.     

Reversible low solar heat gain windows for energy savings

Summer cooling loads in buildings can be reduced with windows of low solar heat gain coefficients (SHGC). Such windows are often double glazed with the exterior pane tinted or selectively absorbing. They reject part of the absorbed solar radiation to the environment, reducing the solar heat gain. This effect is undesirable in the cold season. However, the same window installed in reverse, i.e. turned by 180°, has a significantly higher SHGC. Thus, windows that can be reversed according to the season will both reduce summer heat gains and collect much of the beneficial solar radiation in winter. This paper investigates the energy savings achievable by reversing equator-facing windows for the duration of the cold season as opposed to leaving them in the “summer position”. Candidate climates in which these savings may be significant are identified. By means of a computer simulation, seasonal energy savings are predicted for a model room with reversible, low SHGC, windows. The results indicate that for suitable climates, significant savings are achievable.     

Reducing energy use in the buildings sector: measures, costs, and examples

This paper reviews the literature concerning the energy savings that can be achieved through optimized building shape and form, improved building envelopes, improved efficiencies of individual energy-using devices, alternative energy using systems in buildings, and through enlightened occupant behavior and operation of building systems. Cost information is also provided. Both new buildings and retrofits are discussed. Energy-relevant characteristics of the building envelope include window-to-wall ratios, insulation levels of the walls and roof, thermal resistance and solar heat gain coefficient of windows, degree of air tightness to prevent unwanted exchange of air between the inside and outside, and presence or absence of operable windows that connect to pathways for passive ventilation. Provision of a high-performance envelope is the single most important factor in the design of low-energy buildings, not only because it reduces the heating and cooling loads that the mechanical system must satisfy but also because it permits alternative (and low-energy) systems for meeting the reduced loads. In many cases, equipment with significantly greater efficiency than is currently used is available. However, the savings available through better and alternative energy-using systems (such as alternative heating, ventilation, cooling, and lighting systems) are generally much larger than the savings that can be achieved by using more efficient devices (such as boilers, fans, chillers, and lamps). Because improved building envelopes and improved building systems reduce the need for mechanical heating and cooling equipment, buildings with dramatically lower energy use (50–75% savings) often entail no greater construction cost than conventional design while yielding significant annual energy-cost savings.

Comparison of experimental and theoretical results for the transient heat flow through multilayer walls and flat roofs

This study deals with comparison of experimental and theoretical results of transient temperature variations in multilayered building walls and flat roofs, and heat flow through the building structures. Experimental and theoretical models are presented to find the transient temperature variations in these structures and heat flow through these elements, which depends on inside surface and room air temperatures. Instantaneous inside and outside air temperatures, and surface temperatures of each wall and roof layers are measured by using the experimental model consisted of two rooms, cooling units, measuring devices and computers. A computer program based on the theoretical model is developed to perform numerical calculations. Hourly temperature variations of the nodal points are computed numerically over a period of 24 h by using the hourly measured ambient air temperatures and solar radiation flux on a horizontal surface for the city of Gaziantep (37.1°N), Turkey, and also by using thermophysical properties of the structures. Results obtained from the experimental and theoretical models are compared with each other, and validation of the theoretical model is verified in this paper. Computations for various multilayer building walls of briquette, brick, blokbims, and autoclaved aerated concrete (AAC), which are commonly used in Turkey are repeated for finding heat gain through these structures, and results are compared to determine suitable wall material. It is observed that AAC and blokbims are more suitable wall materials than briquette and brick due to heat flow through these elements.