Kazunobu Sagara

LATENT HEAT STORAGE MATERIALS AND SYSTEMS: A REVIEW

The use of a latent heat storage system using Phase Change Materials (PCM) is an effective way of storing thermal energy (solar energy, off-peak electricity, industrial waste heat) and has the advantages of high storage density and the isothermal nature of the storage process. It has been demonstrated that, for the development of a latent heat storage system, choice of the PCM plays an important role in addition to heat transfer mechanism. The information on the latent heat storage materials and systems is enormous and published widely in the literatures. In this paper, we make an effort to gather the information from the previous works on PCMs and latent heat storage systems. This review will help to find a suitable PCM for various purposes a suitable heat exchanger with ways to enhance the heat transfer, and it will also help to provide a variety of designs to store the heat using PCMs for different applications, i.e. space heating & cooling, solar cooking, greenhouses, solar water heating and waste heat recovery systems. Measurement techniques of thermophysical properties, studies on thermal cycles for long term stability, corrosion of the PCMs and enhancement of heat transfer in PCM are discussed. New PCM innovations are also included for the awareness of new applications. This paper contains a list of about 250 PCMs and more than 250 references.

Stream Tube based Analysis of Problems in Prediction of Cross-Ventilation Rate

Abstract The airflow rate of a building ventilated by wind is usually predicted by using the wind pressure coefficients obtained for a sealed building and discharge coefficients based on measuring the airflow characteristics through an opening in a sealed chamber (chamber method). This can result in the underestimation of wind driven flow through large openings located on opposite sides of a room. In this paper, the discharge coefficient, based on the chamber method, and the actual condition of cross-ventilation are calculated and compared with each other by means of stream tube analysis. The driving pressure based on wind pressure coefficients obtained from a sealed building are also compared with those based on pressures inside the stream tube of the actual flow field representing a porous rather than sealed building. A building model of dimensions 120 mm (width)×120 mm (height)×180 mm (length) was used for the analyses. The size of openings, expressed as the porosity (opening area divided by façade area), was 11.6 %, 20.7 % and 46.5 %. These models were analyzed by CFD simulation and the stream tubes caught by the opening were determined. From the analysis the errors in discharge coefficient and wind pressure coefficient were identified. Finally, the flow rate based on these discharge coefficients and driving pressures were calculated and compared. It is shown that the effect of the underestimation of the discharge coefficients by the chamber method is significant for all cases of porosity studied in this paper. Moreover, it is shown that the use of wind pressure coefficients is not appropriate for the case of extremely large openings.

Wind Driven Flow Through Openings - Analysis of the Stream Tube

Abstract Wind approaching a building provided with openings on the windward and leeward sides has a choice, either it flows through the openings or flows around and above the building. This choice gives rise to a dominant stream tube containing the fluid flowing through the openings. In this paper the stream tube is analysed based on wind tunnel measurements and CFD simulation. A house model with dimensions 120 mm (Width)×120 mm (Height)×180 mm (Length) was provided with rectangular openings of equal size located opposite each other. The end walls were thin giving rise to a sharp edged opening. The size of the openings expressed as the porosity (opening area divided by the façade area) was 1.3 %, 5.2 %, 11.6 %, 20.7 % and 46.5 %. In the wind tunnel, velocity including velocity fluctuations and pressure were measured along the centre line through the openings. In the CFD prediction it was possible to visualize the stream tube by the method of “flying particles”. This made it possible to explore the change in shape of the stream tube and to calculate the cross-sections of the stream tube at different positions and to know the total pressure distribution within the stream tube cross section. Finally, the discharge coefficient based on stream tube analysis was compared to that from a conventional chamber method.

Design, Development of a Solar Chimney with Built-in Latent Heat Storage Material for Natural Ventilation

A prototype of a solar chimney with a built-in latent heat storage system for prolonging ventilation system operation until evening/night or even 24 hours was designed and developed. Sodium Sulfate Decahydrate “Na2SO4.10H2O” (melting point 32°C, latent heat of fusion 126 kJ/kg) was used as a Phase Change Material (PCM) for latent heat storage. Experiments have been carried out with the air gap of 0.20 m and 45 degree inclination angle of solar chimney. Results showed that integration of PCM storage inside the solar chimney is positive and supplies a constant airflow rate of 155 m3/h in evening and night periods. Description of the fabricated prototype solar chimney with PCM storage, thermal analysis to predict the air flow rate, and temperatures of the component of the system with experimental results are presented.

Effects of ceiling induction diffusers on indoor environmental quality of sickroom under the cooling air supply condition

ABSTRACT A series of experiments were carried out in a full-scaled model room to verify the characteristics of the air-conditioning system with ceiling induction diffusers. By the step-up tracer gas method and decay method, 10 cases with three varying parameters (the positions of exhausts, the heights of the exhaust ports and the states of the curtains around the beds) were designed to examine the influence on the indoor environment. It can be found that the exhaust ports had obvious influence on the normalised concentration distributions. The curtains made the indoor temperature higher than that of in without curtains and prevent the contaminant escaping out when in patients breathed. In terms of the local mean age of air, the ventilation efficiency presented an optimum result on a specific position of the exhaust port. However, there was minor influence on the air age caused by the height of the exhaust ports.

PIV measurement and CFD analysis of airflow around building roof with various building installations

ABSTRACT The air-cooled packaged air-conditioner, widely used in small and medium buildings, needs the large number of outdoor units to apply various room conditions. Its operating efficiency can be changed by the high distribution density of the units and some other installations. This study aims to propose an optimum configuration of building installation at the rooftop by providing a better wind environmental condition for the energy saving. Thus, it is necessary to predict the airflow field around the rooftop. First, the airflow field around the rooftop was investigated by the Particle Image Velocimetry using scale models. The positional relation between the devices showed a sensible effect on the airflow characteristics. Moreover, the experimental data was compared with that of the Computational Fluid Dynamics (CFD) analysis to evaluate the accuracy of the calculation. They are similar to be said that CFD can represent the real situation approximately.

C&C efficiency of canopy hood exposed to horizontal air stream in commercial kitchen calculated by CFD analysis

ABSTRACT As one of the commercial kitchen ventilation researches, the mechanism of capture phenomenon and containment one of a canopy hood has to be investigated. Several experiments intended to change the Capture and Containment (C&C) efficiency of combustion gas and cooking effluence were conducted under the conditions of different suction wind velocity at the bottom of a hood, sometimes called face velocity, which is caused by the different hood size. The overall C&C efficiency was analyzed by means of computational fluid dynamics (CFD). The C&C efficiency from the experiment and CFD analysis showed good agreement, and it was assumed that the disturbing ambient airflow around the hood can influence on the C&C efficiency. Therefore the kitchen space was assumed which includes the disturbing airflow over the counter between a dining hall and a kitchen. As a result, the C&C efficiency of the larger hood decreased more under the disturbing airflow.

THERMAL ENVIRONMENT IN A SCHOOL BUILDING WITH NATURAL VENTILATION CHIMNEY USING STAIRCASE IN WINTER

Recently, as increase of eco-awareness, the number of building adopted with natural ventilation is increasing. Naturally ventilated building is expected to effectively reduce the building’s cooling load, and also expected to make people feel comfortable and familiar with nature, especially in spring and autumn. Few studies are found about the actual thermal condition of naturally ventilated building in winter. The purpose of this study is to figure out the influence of the ventilation chimney using staircase on the actual thermal environment and occupants’ evaluations in winter. The result shows that closing the opening at the top of chimney decreases the air change rate at room. It is also found that the cold draft at the window side and air leakage through the switched-off ventilation fan also influence on the thermal environment, except for that of ventilation chimney.

TEMPERATURE AND CONTAMINANT CONCENTRATION DISTRIBUTION AND ENERGY SAVING INSIDE AN OFFICE ROOM WITH HYBRID AIR-CONDITIONING SYSTEM WITH WIND-FORCED VENTILATION

In this paper, the influence of combination of the introduction method of wind-forced ventilation and the supply method of conditioned air on the indoor thermal environment and airflow characteristics are examined by means of CFD calculation. In CFD, two types of supply location of the air conditioned by AHU were set : (1) supply from ceiling, (2) supply from floor, and four locations of outdoor air intake for wind-forced ventilation were set ; (1) ceiling, (2) high position of outer wall close to ceiling, (3) low position of outer wall close to floor and (4) floor. From the calculated data, indoor environment and energy efficiency were evaluated by the distribution of air temperature, contribution ratio of indoor climate No.3 (CRI3) and CO2 concentration emitted from occupants.