Energy analysis of buildings employing thermal mass in Cyprus

In this paper the effects on the heating and cooling load resulting from the use of building thermal mass in Cyprus are presented. This is achieved by modelling and simulation with the TRNSYS program of a typical four-zone building with an insulated roof in which the south wall of one of the zones has been replaced by a thermal wall. Despite the fact that the diurnal temperature variations in Cyprus are ideal for the application of thermal mass, no such application is presently available. Therefore the main objective of this paper is to investigate the possible benefits resulting from such an application. The results of the simulation show that there is a reduction in the heating load requirement of the zone by about 47%, whereas at the same time a slight increase of the zone-cooling load is exhibited. Optimisations of the various construction parameters have also been carried out. The optimum overhang size is found to be equal to 1.2 m with minor variations in the range of 1 to 1.5 m. The effect of the air gap size between the glazing and the thermal wall is insignificant. The optimum value of wall thickness obtained is equal to 25 cm. The effect of roof insulation is investigated and it is found that insulation is a must for better comfort conditions. Also, the effect of applying ventilation whenever the ambient temperature is lower than the indoor temperature during summertime is investigated. A reduction of 7.5% is obtained when air at 3 air changes per hour is directed into the house. In conclusion it can be said that the thermal wall offers some advantages and should be used whenever buildings are erected with south-facing walls.     

Innovative economic options for low-energy housing in northern climates

In a joint project, the authors have investigated several possible options to reduce the heating energy demand in residential dwellings in northern climates. The single measures investigated include improved thermal insulation of walls, doors, and windows, heat recovery systems and transparent insulation (TI). The investigations were carried out using the simulation programme TRNSYS by Klein et ea. (1990) with the TI extension for the building model treated in Sick and Kummer (1992). Climate data basis are hourly irradiation and temperature values from Jyväskylä, Finland (latitude: 62.16° North). The results show that (a) even during the winter months with very few hours of sunlight per day, the irradiation gains through the TI walls can compensate the heat transmission losses to a large extent. The average night U-value of the TI wall may be twice as high as an opaque wall U-value to lead to the same heating demand in November, December, and January; (b) the efficiency of a south-facing TI facade may exceed 60% in the winter months and still reach more than 15% in July and August; and (c) seasonal shading is sufficient to prevent overheating in the summer months. An analysis of the TI behaviour is given in the paper. The economics of the TI solution are compared to conventional measures for reducing the heating energy demand. Due to the simple seasonal shading control, it will be competitive when the building is optimized for TI use and the TI material production volumes increase to a realistic extent.     

Towards green buildings: Glass as a building element—the use and misuse in the gulf region

The recent economic growth in the Gulf region notably in Dubai, United Arab Emirates (UAE) has led to a colossal number of buildings that has been constructed in the past 5 years. In the last decade, a total glazed building's façades became the icon of Dubai. This large area of glazing in each façade needs protection against overheating and sun glare in summer. According to leader in energy and environmental design (LEED) glass selection becomes a main element in this equation to contribute towards achieving a green building. The aim of this paper is to investigate the problems associated with misuse of glass, as a building element in UAE particularly in Dubai. Inadequate design with ill-selected glass/glazing type may lead not only to poor daylighting in building interiors but also contribute significantly to fatigue, insomnia, seasonal affective disorder (SAD) and above all increase CO₂ emission.The purpose of the study is to specify the required improvements to permit natural, ‘free’ daylight to filter through the building façade into interior space, especially with the right type of glass. This paper examines the status of buildings in Dubai in terms of glass type, visible light transmittance, reflection (out/in) and relative heat gain. A quantitative analysis is conducted to assess the impact of glass on the building users' performance in terms of daylight environment. A recent built high-rise office building was selected in the investigation to asses whether selected glass provide the recommended daylight factor (DF) and daylight level (DL) according to IES standards. The results revealed that most the glass/glazing was misused in 70% of buildings in intermediate and low performance groups. The DF and DD in the selected office building were unexpectedly tremendous and found far beyond the recommended level due to the use of spectrally selective glazing (clear on both sides).     

A study of the U-factor of a window with a cloth curtain

Heating and cooling energy calculations and building energy simulations need the overall heat-transfer coefficient (U-factor) of windows. This study focuses on the U-factor of a window with a cloth curtain indoors. Two side-by-side hot boxes are built in a laboratory for testing the window U-factors. The tested window systems are 100% glass area without frames which are equivalent to the center of glass of practical windows. The two types of tested window systems are the single glazing and the double glazing with a cloth curtain, respectively. The effect of curtain edge sealing conditions on U-factors is also studied. The empirical equations of the center-of-glass U-factors are derived from the experimental data. The corrections for the effects of window frames and outdoor wind velocity are made thereafter. The corrected empirical equations can be easily used to estimate the U-factor of a practical window with a cloth curtain.     

Numerical investigation on using inclined partitions to reduce natural convection inside the cavities of hollow bricks

Hollow bricks are widely used to build facades because of their light weight and high thermal resistance. The air-filled voids within the brick configuration elevates the block’s resistance to heat penetration. Suppressing the natural convection inside the voids increases the blocks thermal resistance. This paper presents a new approach to suppress the convection currents inside brick’s cavities by inserting cell dividers. A numerical study on the effect of inserting a folded sheet inside the cavities of the hollow buildings brick is presented. The folded sheet divides the cavity into several triangles producing a string of small convection cells.The mass, momentum and energy equations are solved for a section of a masonry brick represented by a partitioned squared cavity. The inclined partitions are conductive and are in perfect contact with the walls of the cavity. The two outer vertical sides of the brick section are assumed to be isothermal and the two horizontal sides are insulated. Air, the fluid filling the voids, is modeled as a Newtonian fluid with density defined based on Boussinesq approximation.The heat flux transported across the partitioned voids depends on the number of partitions used and their thermal conductivity. The heat flux through the cavity decreases as the number of partitions increases. Moreover, the convection is nearly diminished when the number partitions becomes six. One day simulation for a typical summer conditions shows that inclined partitions can reduce the heat flux by 37–42% depending on the number of partitions and their material.     

On minimizing heat transport in architectural glazing

Significant reductions in the heating energy demand of buildings are achievable through minimization of the thermal transmittance of glazing. This paper reviews all the heat transport processes occurring in gas-filled and evacuated insulating glazing. The heat transfer mechanisms in gas-filled glazing cavities include radiative exchange between the glass sheet surfaces, convection and gaseous conduction. The application of two low-emissivity coatings (0.04) lowers the thermal conductance due to radiation between the glass pane surfaces to roughly 0.1 W m⁻² K⁻¹. At the same time, even where fill gases such as krypton and xenon are used, thermal conductance due to convection and conduction cannot be reduced to much below 1 W m⁻² K⁻¹. Heat transfer by convection and gaseous conduction only becomes negligible where the cavity is evacuated to approximately 10⁻² Pa. Heat transfer is then determined by radiation and, even more importantly, conduction in support pillars required to bear the atmospheric load on the external glass sheet surfaces. The fact that the average centre-of-glazing heat transfer rates achievable by evacuation of the cavity are some two to five times lower than those of gas-filled cavities increases the significance of heat transfer in the glazing edge regions. Consequently, in addition to the heat transfer in the cavity, the impact on glazing thermal transmittance of the edge seal and different frame constructions was also quantified. The possibilities and limitations of reducing total heat transfer in evacuated glazing are discussed on the basis of analytical and numerical methods. The results suggest that this concept offers significant advantages over current glazing technology in terms of overall thermal transmittance.     

Solar and thermal energy gain through windows in Jordan

Solar gain and thermal energy transfer through windows is studied for three different sites in Jordan using the TRNSYS computer program. Solar and thermal energy is calculated using the monthly average daily data for the above-mentioned three regions. Calculation of hourly radiation on a vertical plane is presented, and also the method of determination of the amount of radiation transmitted through the glazing layers is given. The effect of window orientation on the total solar gain is analysed. It is found that for all directions, solar gain is season-dependent, and this dependency varies from one direction to another. Calculations are carried out for two cases of glazing location: case 1, glazing flush with the outside of the wall; and case 2, glazing recessed by 15 cm from the outside wall, which represents a window with overhang and sidewalls. The number of glazing layers is taken as 1, 2 and 3 to observe the effect on solar gain as well as on the thermal energy exchange between the inside and outside of the building. During the calculations, the temperature of the inside is fixed at 22°C for the entire year. The results are tabulated to serve as a database for solar and thermal energy in Jordan.     

An experimental and analytical evaluation of the insulative properties of double glazing and thick curtains

Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.     

Energy efficient building strategies for school buildings in Oman

In this paper hour-by-hour computer simulations of cooling load for a public building were carried out under local weather conditions using TRNSYS building computer simulation software. Different passive measures to reduce the cooling load were investigated. These include the envelope insulation, space ventilation, shading, glazing, artificial lighting variation, and evaporative cooling of the structure. The results show as high as 43% reductions in peak cooling load can be achieved using a combination of well-established passive cooling techniques and technologies. The significance of these results stems from the fact that they were obtained under local weather conditions, a matter of importance to building architects, designers, contractors, and builders as well as air-conditioning equipment manufacturers. Although this work was undertaken to improve the thermal performance of school buildings the results were extended to cover the summer school vacation months so that they will benefit public buildings as well. Copyright © 2002 John Wiley & Sons, Ltd.     

Holographic optical elements: various principles for solar control of conservatories and sunrooms

Holographic optical elements (HOE) can be used to reflect the direct (beam) sunlight incident on a window whilst allowing the diffuse light to pass through. This is achieved with a semi-transparent hologram, which enables solar control of a building without reducing the benefit of a window or façade to the occupants. Domestic conservatories represent an interesting potential market for such a product. Conservatories and sunrooms by their very nature have large expanses of glass and so are prone to periods of unacceptably high solar gain during the summer months. Currently, blinds or other opaque shading devices are used to reduce the solar gain but this is at the expense of daylighting. This paper describes the potential application of solar control HOE applied as either a fixed plate or tracked solution. The performance of such systems for the UK climate is predicted using transient thermal analysis simulation of commonly used UK conservatories. The benefit of selective HOE glazing of specific elevations within a conservatory is considered to achieve ‘peak clipping’ of daytime temperatures to an acceptable level. The simulations predict that reductions of up to 17° are possible with fully tracked HOE applied to all the elevations of a typical conservatory design. Selective use of fixed plate HOE glazing, can achieve a temperature reduction of 6.1° when only 62% of the glazing within a conservatory is replaced.     

Improvements of building envelope using passive cooling techniques to reduce the cooling load in hot‐dry regions

In developed countries, the buildings are responsible for massive energy consumption. When the construction is based on the use of nonsustainable methods, the buildings were dependent on the active systems, for ensuring a maximal indoor comfort. This has increased energy consumption and related greenhouse gas emissions. In this study, a reduction technique of cooling load and power consumption in buildings is proposed. This technique involves the combination of three passive cooling techniques, thermal insulation, phase change materials (PCMs), and electrochromic double glazing. These techniques are applied through simulation software “DesignBuilder” to a building envelope model in various areas like walls, roofs, and glazing. For this purpose, a standard building model is compared with 16 other cases that incorporated the passive cooling techniques for determining the effectiveness of the proposed method. The results show that a combination of PCMs, thermal insulation, and double glazing can reduce the cooling load from 70.37 to 50.53 kW and the energy consumption from 1.51 to 0.90 kWh/m ².     

Comparison of the periodic solution method with TRNSYS and SUNCODE for thermal building simulation

Based on periodic solutions of the governing heat conduction equations in a single zone building, computer software ADMIT has been developed for thermal simulation of buildings. Standard computer software, namely TRNSYS and SUNCODE, have also been used to simulate the same building under similar conditions. Simulations have been performed for three different climatic zones in India for light and heavy constructions under conditions of glazed/unglazed areas and ventilation rates. The results are presented in terms of the hourly variation of the room temperature. For insulated heavy construction, the results of different models are significantly different. This difference is due to the use of different approaches to solve the heat conduction equations. SUNCODE depends on the RC network approach and underestimates the heat losses. TRNSYS uses the transfer function approach, which is sensitive to the initially assumed value of the room temperature. ADMIT represents a quasi-steady-state periodic variation and is not suitable for transient variations. For insulated light buildings, the heat transfer mechanisms used in the mathematical models are not the governing factors. The models also differ in treating the penetration of solar radiation through a glazed window and the subsequent heat-transfer mechanism. For a south window and air changes in an insulated building, the results obtained by SUNCODE and ADMIT are in good agreement, but the results obtained by TRNSYS are considerably different. The reason for this needs detailed analysis.     

Some aspects of modelling the energy balance of a room in regard to the impact of solar energy

The architecture of a building is crucial in determining its thermal energy balance and indoor comfort conditions. Knowledge of solar radiation availability and its transmission through a building envelope to the interior of the building helps an architect to design the building in an energy efficient way. Nowadays, in highly populated urban areas, attics are used as living spaces and the building envelope includes inclined external walls and windows in roofs. This paper presents some aspects of modelling the energy balance of rooms with different orientations and with vertical or inclined surfaces of building envelope, with stress on the impact of solar energy. The dynamics of energy flow through windows is analysed in more detail. One dimensional energy flow through the centre of glass area (based on a thermal resistance model), two-dimensional energy flow through the edge of glass area and two-dimensional heat flow through the opaque frame are analysed. The third dimension is also considered in a simplified way by taking into account the specific perimeter of the edge or frame. Stress is put on modelling the solar energy input. Solar radiation is modelled as short wave radiation that is transmitted directly to the room through glazing and as energy absorbed by the building envelope (glass panes, frame and opaque external walls) that becomes internal heat sources and is transferred indirectly to the room. The model developed has been used for numerical simulation using MATLAB as the programming language. This model predicts (amongst other things) the solar energy impact on the energy balance of a room in a building. It allows many cases of rooms and their envelopes to be run and evaluated and as a result both general and detailed conclusions can be drawn. Some results are presented in both graphical and tabular form.     

Central electric thermal storage (ETS) feasibility for residential applications: Part 1. Numerical and experimental study

Detailed models have been developed and integrated into a TRNSYS calculation tool to evaluate the optimal storage capacity of central electric thermal storage (ETS) units for residential applications. The tool uses hourly weather data to establish the energy use profile of residential buildings under different heating system configurations and control strategies. The reliability of the system for each configuration is determined as the percentage of time the heating system meets the heating requirement of the building. The level of reliability is used to evaluate the appropriate capacity of the system. This study investigates the theoretical development and modelling of a multi‐zone building equipped with a central ETS in TRNSYS. The TRNSYS simulation results are then compared and validated against experimental data obtained during the project. Furthermore, the effect of various parameters such as system configuration, charging capacity, control strategies, heat losses from the building, and electric utility time‐of‐use schedules are analysed. Copyright © 2001 John Wiley & Sons, Ltd.     

Natural convection in partially divided square enclosures: Effects of thermal boundary conditions and thermal conductivity of the partitions

Natural convection in partitioned square enclosures filled with air is numerically studied, trying to characterize these enclosures mainly in what concerns its overall heat transfer performance. Two partitions of finite thickness are considered, placed in the enclosure following an ordered arrangement, which position, length and thermal conductivity are varied for some values of Rayleigh number and for different thermal boundary conditions. Study starts considering the simplest enclosures with two adiabatic partitions, after the more realistic enclosures of heat conductive walls and partitions are considered, and finally the even more realistic situation of enclosures with heat conductive partitions and walls subjected to cyclic thermal boundary conditions in the vertical direction is also considered. Position and length of the enclosures’ effects depend on the thermal boundary conditions prescribed for the enclosure, and different thermal boundary conditions (corresponding to the heating or cooling operations or seasons) are considered to capture this effect. Fluid flow field, thermal field and heat transfer are analyzed for some particular situations through the streamlines, isotherms, and heatlines. The overall thermal performance of the enclosure is analyzed through the overall Nusselt number, and many data are compactly presented for different placements and lengths of the partitions, for different thermal conductivity of the walls and partitions of the enclosure, for different Rayleigh numbers and for different thermal boundary conditions imposed to the enclosure. Considered boundary conditions and the enclosure walls and partitions of finite thickness and finite thermal conductivity are much more realistic conditions than simply the single cavity without walls and with perfectly adiabatic partitions usually considered in many studies of this kind.     

Performance and economic analysis of air flow windows in a tropical climate

An experimental study was conducted in Thailand to determine the thermal performance of twin glazed windows with dynamic insulation. The effects of blinds situated either between or outside the glazing were analysed. With an external blind, the heat transfer coefficient was 1–25 W/m²°C with natural convection and 0–6 W/m²°C for a flow of 20m³/h (glass area = 2–16m²). The solar flux transmitted was evaluated analytically and experimentally depending on the blind's position. An economic study was performed on a six-sotrey air-conditioned building in Thailand. It showed in tropical countries that it may be more economical to use air flow windows than to have tinted single or double glass windows.     

Calcul des flux aux parois présentant des singularités

Nous présentons une étude du transfert thermique dans des domaines possédant des singularités en flux aux frontières. Après une mise un évidence des deux types de singularités que l'on peut rencontrer, une méthode de calcul des flux aux parois utilisant des volumes de contrôle aux frontières est proposée. Des exemples numériques en conduction et en convection nous permettent d'illustrer l'importance des erreurs commises d'une part et les gains en précision que la méthode proposée permet d'obtenir d'autre part. Nous terminons par une discussion sur la zone d'influence de la singularité qui conditionne le choix de la taille des volumes de contrôle.This paper deals with the calculation of heat fluxes at singular boundaries. We show that accurate evaluation of these fluxes can be made by using a heat balance on boundary control volumes. Numerical examples in heat diffusion and natural convection problems are considered to assess the correctness of the proposed method. Finally, the region of influence of the singularity is discussed.     

Experimental evaluation of ventilated glazing performance in Hong Kong

Window glazing affects much the indoor environment and the energy use in buildings. While double glazing has better thermal performance than single glazing, the airflow window options carry additional advantage of directly removing the absorbed solar energy in glass panes. This paper reports an experimental study in Hong Kong in evaluating the thermal/energy performance of the above‐mentioned glazing systems. A new approach of using numerical simulation technique to improve the quality of experimental analysis was introduced. Our findings show that the natural‐ventilated glazing system has a better thermal performance than the double‐glazing system since the latter received 13.6% more convective heat gain. The main advantage of the exhaust‐ventilated glazing system lies in the decrease of convective heat transfer to 34.3% of the double glazing, and 19.4% of the single‐glazing types. The results showed that the ventilated glazing schemes in association with daylight utilization could lead to substantial electricity savings in the office environment. Copyright © 2008 John Wiley & Sons, Ltd.     

THE PERFORMANCE OF A COLLECTOR-STORAGE WALL IN ALEXANDRIA,EGYPT

This investigation presents an analysis of the performance of a collector-storage wall composed of concrete using the weather data of Alexandria, Egypt. An energy balance on the collector-storage wall is used to predict the energy flows into and out of the wall. The numerical model is then incorporated into TRNSYS to determine the thermal performance of the wall. The effect of the wall thickness, the number of glazing, the building capacitance, and the night insulation resistance on the performance of the collector-storage wall in Alexandria, Egypt are studied.