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This paper reports the measured solar spectral properties and the thermal performance of 10 prototype cool colored coatings, developed at the National and Kapodistrian University of Athens, using near-infrared reflective color pigments in comparison to color-matched, conventionally pigmented coatings. These coatings are developed to be used in the urban environment to fight the heat island effect. The spectral reflectance and the infrared emittance were measured and the solar reflectance of the samples was calculated. The surface temperature of the coatings when applied to concrete tiles was monitored, using surface temperature sensors and a data logging system, on 24 h basis from August to December 2005 in an effort to investigate the ability of the cool colored coatings to maintain lower surface temperatures than conventionally pigmented color-matched coatings. The data obtained has been extensively analysed and indicate significant success in the development of these cool colored coatings. It was found that all the coatings containing infrared reflective pigments have solar reflectance values higher than those of standard coatings. Furthermore, it was demonstrated that cool colored coatings maintain lower surface temperatures than color-matched conventionally pigmented coatings. This temperature difference is mainly due to differences in solar reflectance. These cool colored coatings can be used on buildings (roofs and walls) and other surfaces in the urban environment. Thus, at building scale, the use of cool colored coatings with increased solar reflectance can improve building comfort and reduce cooling energy use, and at city city-scale it can contribute to the reduction of the air temperature due to the heat-transfer phenomena and therefore improve outdoor thermal comfort and reduce the heat-island effect. Keywords: Cool colored coatings: Solar spectral reflectance: Infrared emittance; Thermal performance; Passive cooling; Ageing

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2005.12.003 Byline: S. Karatasou, M. Santamouris, V. Geros Keywords: Flat plate solar collector; Utilizability Abstract: The utilizability or [phi]-curve method is a design method for some types of solar energy systems that use flat-plate solar collectors. It can be a very useful design tool, for the preliminary design or the design of small systems, cases where methods easy to use and apply are required. In this paper, a simple method to calculate monthly average hourly and daily flat plate collector utilizability is presented. It is based on empirical correlation of [phi] with independent variables that separate meteorological and collector parameters, making possible quick evaluations of changes in collector design and collector inlet temperature. In general, the method reduces the calculations required to determine [phi], and gives good results compared to long-term hourly simulations as well as to existing hourly and daily utilizability calculations methods. Author Affiliation: Group Building of Environmental Physics, University of Athens, Building 5, Physics University Campus, 15784 Athens, Greece Article History: Received 5 October 2005; Revised 2 December 2005; Accepted 2 December 2005 Article Note: (miscellaneous) Communicated by: Associate Editor Brian Norton

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2005.08.005 Byline: A. Synnefa, M. Santamouris, I. Livada Keywords: Passive cooling; Reflective coatings; Building envelope; Urban fabric; Weathering; Heat island Abstract: This paper presents the results of a comparative study aiming to investigate the effect of reflective coatings on lowering surface temperatures of buildings and other surfaces of the urban environment, and thus test their suitability to lower ambient temperatures and fight the heat island effect. In total, 14 types of reflective coatings, selected from the international market were studied, from August to October 2004, on a 24h basis. These coatings are used in buildings and some of them are used or could be used in the future in other surfaces of the urban environment (sidewalks, parking lots, etc.). In order to investigate the thermal performance of the reflective coatings, surface temperature sensors and a data logging system as well as infrared thermography procedures were used. The spectral reflectance and the infrared emittance of the samples were also measured. The collected data have been extensively analyzed. It was demonstrated that the use of reflective coatings can reduce a white concrete tile's surface temperature under hot summer conditions by 4[degrees]C and during the night by 2[degrees]C. It can be warmer, than the ambient air by only 2[degrees]C during the day and cooler than the ambient air by 5.9[degrees]C during the night. 'Cool' coatings present superior thermal performance even compared to other 'cool' materials. This study can assist in choosing more appropriate coatings for building envelopes and other surfaces of the urban environment, and thus contribute to the mitigation of the heat island effect as well as the reduction of cooling loads and electricity consumption of buildings. Author Affiliation: Group Building Environmental Studies, National and Kapodistrian University of Athens, Department of Applied Physics, Physics Department, Build. Physics 5, 15784 Athens, Greece Article History: Received 19 April 2005; Revised 4 August 2005; Accepted 4 August 2005 Article Note: (miscellaneous) Communicated by: Associate Editor John Reynolds

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2005.07.007 Byline: V.D. Assimakopoulos (b), C. Georgakis (a), M. Santamouris (a) Keywords: Natural ventilation; Urban street canyon; Microscale modelling; Experimental measurements Abstract: Natural ventilation is one of the most efficient passive cooling techniques for buildings. Knowledge of the wind speed in street canyons is the necessary condition for the application of such a technique in dense urban configurations. Thus, prediction techniques to evaluate the microclimate and dispersion parameters in street canyons, has become a subject of intense scientific research in recent years. In most of cases, wind flow and pollutant dispersion characteristics have been studied numerically and experimentally with a view to provide an insight in urban dispersion. The microscale model MIMO was employed in order to perform a three-dimensional modelling of the wind field within three typical deep street canyons, located in the centre of Athens. Computational results were compared to field data collected during consecutive three-day experimental campaigns that took place in the summer period. Results from the computations have shown that the wind field in urban areas is quite complex, presenting areas of very low wind speeds and convergence of vortices. The model underestimated the measured wind speed intensities, which may be partly explained by the uncertainty of specific input parameters, the necessary simplifications for the application of such models and finally the geometrical complexity of the area modelled. Finally, computations were performed for a reference velocity of 2m/s, which is considered to be a threshold value. The wind field developed departed from the one observed in the previous cases. Author Affiliation: (a) Group of Building Environment Studies, Department of Applied Physics, Faculty of Physics, University of Athens, Building Physics 5, University Campus, 157 84 Athens, Greece (b) Institute for Environmental Research and Sustainable Development, National Observatory of Athens, I. Metaxa and V. Pavlou Str., 152 36 Athens, Greece Article Note: (miscellaneous) Communicated by: Associate Editor Dr. D. Yogi Goswami

This paper presents the development of a method to control the light output from a prototype hybrid lighting system. This system transports daylight from a heliostat with a concentrating fresnel lens to a luminaire in a windowless room, via a large core liquid fiber optic. The main artificial lighting system is located outside of the building without the possibility for dimming due to the lamp type chosen. Consequently a control strategy is needed in order to minimize the switching cycles of the lamps (severe reduction in lamp life) while at the same time maintain the lighting levels in the interior. Keywords: Sunlight; Heliostats; Liquid fiber optics

This paper presents the results of a comparative study aiming to investigate the suitability of materials used in outdoor urban spaces in order to contribute to lower ambient temperatures and fight heat island effect. The study involved in total 93 commonly used pavement materials outdoors and was performed during the whole summer period of 2001. The thermal performance of the materials was measured in detail using mainly infrared thermography procedures. The collected data have been extensively analysed using statistical techniques. Comparative studies have been performed in order to identify the major advantages and disadvantages of the materials studied. Materials have been classified according to their thermal performance and physical properties into 'cool' and 'warm' materials. The impact of color, surface roughness and sizing has been analysed as well. The study can contribute to selection of more appropriate materials for outdoor urban applications, and thus assist to fight the heat island effect, decrease the electricity consumption of buildings and improve outdoor thermal comfort conditions. Keywords: Passive cooling; Pavement materials; Outdoor comfort

An average monthly variable, called the Obstruction Illuminance Multiplier (OIM), defined as the ratio of the illuminance received on a vertical surface due to light received from the sky, ground and the obstruction to the illuminance on the same surface without the presence of the obstruction, is proposed as a simple tool to investigate the potential of perfectly diffuse vertical south-oriented facades to reflect daylight onto the opposing facades.

Passive cooling techniques applied to thermostatically controlled buildings can contribute to reducing their cooling load significantly. Ground and night ventilation techniques appear to be the more promising among the proposed dissipation techniques. Calculation of the specific contribution of night ventilation and ground cooling techniques requires appropriate algorithms to account for the coupling of the passive with the active cooling system. The present article deals with the development of a new, integrated method to calculate the energy contribution of ground and night ventilation dissipation techniques to the cooling load of thermostatically controlled buildings. The method is based on the principle of 'balance point temperature' and succeeds in estimating the hourly value of the balance temperature of a building. In addition, it calculates the cooling load of the building as well as the global contribution of the ground and night ventilation techniques. An extensive validation procedure has been followed using data from an extended version of TRNSYS including detailed algorithms to simulate dynamically the thermal performance of night ventilation and earth-to-air heat exchangers. Very good agreement between the results predicted by TRNSYS and the present method is found.

A complete model is used in this study for the prediction of the daily and annual variation of ground surface temperature. This model is based on the transient heat conduction differential equation using as boundary condition the energy balance equation at the ground surface. The energy balance equation involves the convective energy exchange between air and soil, the solar radiation absorbed by the ground surface, the latent heat flux due to evaporation at the ground surface as well the long-wave radiation. The model is validated against various extensive sets of measurements for bare and short-grass covered soil in Athens and Dublin and its results are compared with the corresponding results of models based on Fourier analysis. Furthermore, a sensitivity investigation was carried out in order to evaluate the impact of various factors involved in the energy balance equation at the ground surface on the soil temperature distribution. The overall analysis is useful for the prediction of the thermal performance of buildings in direct contact with the soil as well as for the prediction of the energy efficiency of earth-to-air heat exchangers.

Several statistical characteristics of the soil temperature in Athens/Greece are determined through Fourier analysis of a 74 year record (1917-1990) of soil temperatures at the surface and at various depths and for both bare and short-grass-covered areas. Specifically the soil temperature patterns are presented along with the Fourier amplitudes and phases of the most important harmonics. It was found that the first three harmonics can reproduce most of the observed patterns in soil temperatures at the surface and at various depths. Finally, the results of the present analysis are compared with some existing data from other known sets of measurements.

A new, integrated method to calculate the energy contribution of night ventilation techniques to the cooling load of a building is presented in this paper. The method is based on the principle of 'Balance Point Temperature' and permits the calculation of the energy required to cool a building to acceptable comfort conditions when night ventilation techniques are used. It also permits the calculation of the energy contribution of night ventilated buildings compared to conventional air conditioned buildings. The proposed method is successfully validated with data from an extended and detailed simulation procedure using the TRNSYS simulation programme to calculate dynamically the thermal performance of buildings using night ventilation techniques. It is found that the method is of sufficient accuracy and can be used during the predesign as well as the design phase of a building to access the performance of night ventilated buildings.

A new parametrical model for the prediction of the thermal performance of the earth to air heat exchangers is presented. The system consists of an earth tube, buried in the ground, through which ambient or indoor air is propelled and cooled by the bulk temperature of the natural ground. The proposed model has been developed by analysing temperature data of the circulated air at the pipe's outlet using a systematic parametrical process. Four variables influencing the thermal performance of the earth to air heat exchangers were taken into account: pipe length (L), pipe radius (r), velocity of the air inside the tube (V) and depth of the buried pipe below earth surface (D). The developed algorithm is suitable for the calculation of the exit air temperature, and therefore of the cooling potential of the system. The model was validated against experimental data as well as a data set which was obtained by an accurate numerical model describing the thermal performance of the earth tube system.

Earth-to-air heat exchangers can be used for energy conservation in cooling of agricultural greenhouses. A parametric analysis performed for a typical glass greenhouse illustrates the effect of pipe length, pipe diameter, and air velocity inside the pipes on the performance of the system. Measured data of indoor air temperature collected from a 1000 [m.sup.2] fiberglass covered greenhouse, equipped with four buried pipes, are compared with calculated data and are found to be in very good agreement.

A new expression for the integral transmission of atmospheric water vapor has been developed. This expression is based on the latest known water vapor spectral absorption data, on a new vertical atmospheric profile and finally, on the Neckel and Labs model (1981; 1984), incorporating the most recent corrections of the extraterrestrial solar spectrum. The proposed expression can be easily used by solar radiation models in order to predict the beam, diffuse, and global solar radiation, especially in regions where the vertical atmospheric profile is similar to that in Athens.

Results from a recently completed study of 95 solar greenhouses from around the world, representing the state-of-the-art in this field, are presented and are classified. Relevant technical information on the utilised heating systems using alternative energy sources are identified and the existing applications are grouped according to the characteristics of the implemented type of technology. Five categories of passive solar greenhouses are reviewed, according to the characteristics of the heat storage system, namely, water, latent heat material, rock bed, buried pipes, and some other type of systems. Available information on the performance of each system is also included. The suitability of the technologies for each climatic zone and type of cultivation is estimated and evaluated. This up-to-date information on the state of the art in this field can be of great importance to potential users for the successful development of future applications.

A new complete numerical model for the prediction of thermal performance of the earth-to-air heat exchangers is presented. The model describes the simultaneous heat and mass transfer inside the tube and into the soil accounting for the soil natural thermal stratification. The model is validated against an extensive set of experimental data and it is found accurate. The proposed algorithms are suitable for the calculation of the temperature and humidity variation of the circulating air and for the temperature and humidity distribution inside the ground. The presented model was developed within the TRNSYS environment and can be easily coupled with building or greenhouse simulation codes in order to describe the impact of the earth-to-air heat exchangers to indoor environments.

The design, construction, and operation of a prototype 1000 m squared passive solar agricultural greenhouse are described. The greenhouse has been designed in order to reduce heat losses and increase useful solar gains on a daily and seasonal basis. The passive elements of the greenhouse are a mass storage wall located on the north side and a network of earth-to-air heat exchangers buried in the greenhouse. Monitoring of the greenhouse for a 2-year period has shown that the passive systems have offered energy equal to 35% of the heating requirements of an identical conventional greenhouse. Also, the cooling potential of the earth-to-air heat exchangers has been found to be very important. This project is a Solar Demonstration Programme of the European Communities.

The present paper reports the findings of a monitoring campaign in 186 office buildings in Greece. The specific energy consumption of the buildings for heating, cooling, and lighting purposes, as well as the consumption of the office equipment is reported. The impact of the used systems, techniques, and components is investigated. The potential and the limitations of various energy conservation systems and alternative techniques is assessed. The present study provides useful information for efficient energy planning, as well as appropriate design and equipment selection, in office buildings.

The necessary analysis of the summer ambient temperature data for evaluation of passive and hybrid cooling techniques and components is discussed. A complete analysis using data from Athens, Greece, is presented. Daily, monthly, and annual distributions of the more important parameters, as well as statistical indexes, are estimated and discussed. Correlation techniques are used, and expressions to predict accurately the necessary parameters are proposed. The presented analysis aims to define an appropriate format to report on the summer ambient temperature data used for cooling purposes.