D. N. Asimakopoulos

Passive Cooling of Buildings

Cooling in Buildings * Passive Cooling of Buildings * The Mediterranean Climate * Microclimate * Urban Design * Thermal Comfort * Cooling Load of Buildings * Heat Attenuation * Natural Ventilation * Solar Control * Ground Cooling * Evaporative Cooling * Radiative Cooling * Simplified Methods for Passive Cooling Applications

Potential for energy conservation in apartment buildings

Abstract The potential for energy conservation in apartment buildings is investigated following the epiqr methodology and software. Several scenarios are evaluated for various apartment buildings located in three climatic zones of Hellas. The proposed retrofit actions concentrate on space heating and cooling, domestic hot water production and lighting. Energy savings for the corresponding actions in each building are accounted for in order to identify the most effective scenarios, suitable for different building constructions, installations and system characteristics.

A field study of the wake behind a 2 MW wind turbine

Abstract The wake behind the 2 MW wind turbine situated on flat country at Nasudden, Sweden has been mapped in a field experiment where several measuring techniques were employed. A high resolution sodar appeared to be a very powerful tool which enabled detailed studies of both mean and turbulence wake profiles to be carried out. Measurements with this system were made with three antennae in three different modes of operation and at several distances in the range 2 D –3.6 D from the turbine ( D being the rotor diameter, 75 m). Additional detailed data were obtained with turbulence instruments at three levels at a 145 m tower, situated 3 D downwind, during conditions with the wind blowing from the turbine towards the tower. Finally good quality data of the centre line velocity deficit and of the longitudinal turbulence intensity were obtained with Tala kites at distances of up to 10.5 D . The measurements show good agreement with data on the thrust coefficient derived from turbine characteristics. The centre line velocity deficit was found to vary with distance as x −1.1 , in general agreement with wind tunnel simulations, but the constant of proportionality was found to be about 40% lower than in those simulations. Detailed data on the turbulence structure in the wake have been obtained. For downwind distances of a few diameters, the vertical profile of turbulence intensity is found to resemble the corresponding profiles found in the laboratory, with two sharp peaks at about rotor radius distance from the centre. The general level of turbulence intensity as an average at the centre of the wake is found to be about a factor of two higher than found in wind tunnel simulations. Turbulence spectra measured in the wake with the sodar and with tower mounted instruments show significantly increased fluctuation levels for frequencies above about 0.004 Hz and decreased energy at lower frequencies.

On the performance of buildings coupled with earth to air heat exchangers

The use of earth to air heat exchangers has gained an increasing acceptance during the recent years. However, there is a lack of calculation models coupling the performance of the exchangers with the building. The present paper deals with the development of a new integrated method to calculate the contribution of the earth to air heat exchangers to reduce the cooling load of the buildings. The method is based on the principle of balance point temperature and permits the calculation of the hourly value of the balance temperature of the building as well as the daily cooling load of the building and the contribution of the buried pipes. An extensive validation procedure has been followed using data from an extended version of TRNSYS including detailed routines to simulate dynamically the performance of earth to air heat exchangers. It is found that the method is of sufficient accuracy and, therefore, can be used during the predesign and design phase for the dimensioning of the buried pipes.

A quantitative comparison between acoustic sounder returns and the direct measurement of atmospheric temperature fluctuations

Results are presented which show that reasonably accurate estimates of the atmospheric temperature structure paparmeter CT2 can be derived from the returns of a monostatic acoustic sounder in both stable and convective atmospheric conditions.

Photosynthetically active radiation in Athens

Abstract Hourly measurements of solar radiation values in the intervals 295–385 nm, 285–2800 nm ( R s ), and 695–2800 nm recorded simultaneously at the National Observatory of Athens from January 1990 to December 1992 have been utilized to determine the distribution of radiant energy in the visible (385–695 nm) region of the spectrum, which is often referred to as photosynthetically active radiation ( R P ). These data were used to evaluate the temporal variability of R P and its dependence on sky conditions and air pollution in Athens. The seasonal variation of the ratio R P / R S obtained from hourly data ranges from 0.419 (during Winter) to 0.446 (during Summer) with an annual mean value of 0.436. The ratio R P to R S increased from 0.428 to 0.444 (hourly values) as sky conditions changed from ‘sunny’ to ‘overcast’. The effect of air pollution on the ratio R P / R S does not seem to be noticeable in the present investigation.

Energy conservation strategies for sports centers: Part B. Swimming pools

This paper summarizes the results from a number of energy audits and the analysis performed in order to improve indoor conditions and optimize energy use, in Hellenic indoor swimming pools, performed for the European Commission, in the framework of the SAVE program. The aim of this work was to investigate the technical, functional and administrative obstacles for energy conservation in sports centers (including sports halls) and to propose practical and cost-effective solutions for improving their energy efficiency, indoor thermal and visual comfort throughout the year. The work concentrated on retrofitting of existing buildings, although the proposed design and management principles could also be followed in new projects in the area of sports and recreation facilities.

On the ground temperature below buildings

Abstract A transient, numerical model for the prediction of the ground temperature at various depths below buildings is presented in this paper. The proposed model was developed by calculating the heat flow to the ground from a building, which depends on the complicated three-dimensional thermal process in the ground. The main difficulties in obtaining manageable solutions of the heat flow problem were: The three-dimensionality of the thermal process, the strong temporal variability of the outdoor temperature as well as the large number of parameters involved in describing the building foundation geometry as well as the thermal insulation. The techniques of superposition and numerical analysis were used to cope with these difficulties. The model was validated against experimental data and it was found that it could accurately predict the ground temperature under a building.

Quantitative low-level acoustic sounding and comparison with direct measurements

In this paper, measurements of the first 150 m of the atmospheric boundary layer obtained by a high-frequency acoustic mini-sounder are compared with measurements obtained by a full complement of instruments including sonic anemometers mounted on the Boulder Atmospheric Observatory tower. The acoustic mini-sounder, starting as low as 6 m from the ground, measures in the monostatic mode the profiles of the vertical wind speed, w, and of the temperature structure parameter, CT2 with enhanced height resolution of the order of 1 m and time resolution of the order of 30 s. The results of the comparison show that the high-frequency mini-sounder is an effective atmospheric boundary-layer profiler that is also portable and relatively inexpensive.Measurements of the spectrum of CT2 are presented that provide information on the local isotropy of the temperature field. Statistics of the variability of CT2 in both stable and unstable conditions are also given.The sounders capabilities are further demonstrated by some detailed observations of the structure and time evolution of a thermal plume root at noon and of a nocturnal, stably stratified layer in which a dynamic instability develops. The plume starts at a height of less than 5 m, possesses substantial internal structure, and includes vertical velocities in excess of 2 m s-1.

Urban aerosol and clear skies spectra for global and diffuse photosynthetically active radiation

Abstract From a data archive of spectral energy distribution of global and diffuse solar irradiances measured in Athens during a field experiment, the influence of atmospheric turbidity caused by urban aerosol on spectral composition of visible radiation and transparency of urban atmosphere has been determined for cloudless conditions. The effect of urban aerosol is significant in the PAR part of solar spectrum, where attenuation of global PAR irradiance surpasses 18% on very polluted atmospheric conditions. The increase in diffuse PAR irradiance caused by urban aerosol ranges in the interval 7%–51%. The ratio of global PAR to total global irradiance was found to depend slightly on air pollution whereas ratios dependencies on site elevation were not identified clearly. Nevertheless, this ratio was found to be in good agreement with those reported in the literature. The ratio of diffuse-to-global PAR irradiance was found to depend on both, air mass and turbidity. For very turbid atmosphere and large air masses the 40% of PAR irradiance results from diffuse radiation. Finally, some biological implications of the results are briefly discussed.