Mohammed M. Farid

A review on energy conservation in building applications with thermal storage by latent heat using phase change materials

Energy storage in the walls, ceiling and floor of buildings may be enhanced by encapsulating suitable phase change materials (PCMs) within these surfaces to capture solar energy directly and increase human comfort by decreasing the frequency of internal air temperature swings and maintaining the temperature closer to the desired temperature for a longer period of time. This paper summarizes the investigation and analysis of thermal energy storage systems incorporating PCMs for use in building applications. Researches on thermal storage in which the PCM is encapsulated in concrete, gypsum wallboard, ceiling and floor have been ongoing for some time and are discussed. The problems associated with the application of PCMs with regard to the selection of materials and the methods used to contain them are also discussed.

Adsorption kinetics for the removal of chromium(VI) from aqueous solution by adsorbents derived from used tyres and sawdust

The batch removal of hexavalent chromium (Cr(Vl)) from wastewater under different experimental conditions using economic adsorbents was investigated in this study. These adsorbents were produced from the pyrolysis and activation of the waste tyres (TAC) and from the pyrolysis of sawdust (SPC). The performance of these adsorbents against commercial activated carbon F400 (CAC) has also been carried out. The removal was favoured at low pH, with maximum removal at pH = 2 for all types of carbon. The effects of concentration, temperature and particle size have been reported. All sorbents were found to efficiently remove Cr(VI) from solution. The batch sorption kinetics have been tested for a first-order reversible reaction, a first-order and second-order reaction. The rate constants of adsorption for all these kinetic models have been calculated. The applicability of the Langmuir isotherm for the present system has been tested at different temperatures. The thermodynamic parameters (AGO, K,) obtained indicate the endothermic nature of Cr(Vl) adsorption on TAC, SPC and CAC

Solar desalination with a humidification-dehumidification cycle: performance of the unit

Abstract The closed air cycle humidification-dehumidification process was used for water desalination using solar energy. The circulated air by natural or forced convection was heated and humidified by the hot water obtained either from a flat plate solar collector or from an electrical heater. The latent heat of condensation was recovered in the condenser to preheat the saline feed water. Two units of different sizes were constructed from different materials. The productivity of these units was found to be much higher than those of the single-basin stills. Moreover, these units were capable of producing a large quantity of saline warm water for domestic uses other than drinking. No significant improvement in the performance of the desalination units was achieved using forced air circulation at high temperatures. While at lower temperatures, a larger effect was noticed. This can be related to the low heat and mass transfer coefficients at low temperatures and to the non-linear increase in the water vapor pressure with temperature.

MICROWAVE VACUUM DRYING OF BANANA SLICES

ABSTRACT Vacuum drying of banana slices was studied in a domestic microwave oven. The results show that banana temperature rises uniformly and rapidly to the saturation water vapor temperature corresponding to the vacuum used then rises slowly until most of the free moisture is lost. The thermal and drying efficiencies were found to drop from almost 100% at the beginning of the drying (high moisture content) to as low as 40% and 30% respectively at the end of drying. Both efficiencies were found to increase with the use of vacuum, especially at low moisture content.

A new approach to modelling of single droplet drying

A model was developed to predict the change in droplet mass and temperature when it is exposed to hot air, as in spray drying of droplets containing solids. The droplet was assumed first to undergo sensible rapid heating with no mass change. Then the droplet experiences some shrinkage, with no temperature change but rapid mass losses, followed by a period of crust formation with a significant change in droplet mass and temperature and finally a short period of sensible heating of the dried particle. The model, unlike previous models, accounts for shrinkage and for the temperature distribution in the droplet. It provided a good prediction for the change in droplet temperature and mass for some of the experimental measurements available in the literatures.

Numerical simulation of natural convection heating of canned food by computational fluid dynamics

Abstract Natural convection heating within a can of liquid food during sterilization is simulated by solving the governing equations for continuity, momentum and energy conservation for an axisymmetric case using a commercial Computational Fluid Dynamics (CFD) package ( PHOENICS ). Transient flow patterns and temperature profiles within model liquids (sodium carboxy-methyl cellulose (CMC) and water) have been predicted. The model liquids, CMC and water, were assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). It has been shown that the action of natural convection forces the slowest heating zone (SHZ) to migrate towards the bottom of the can as expected. The shape and the size of the SHZ area are different for CMC and water. The magnitude of the axial velocity was found to be in the range of 10 −5 –10 −4 m/s for CMS and of 10 −2 –10 −1 m/s for water, these magnitudes of course vary with time and position in the can. The time required for the SHZ to reach the sterilization temperature of 100°C was 1800 s for CMC and only, 150 s for water.

Thermal Performance of a Heat Storage Module Using PCM’s With Different Melting Temperatures: Mathematical Modeling

In the present study, the performance of a heat storage unit consisting of number of vertical cylindrical capsules filled with phase change materials, with air flowing across them for heat exchange has been analyzed. Earlier theoretical models did not consider temperature distribution in the radial direction within the capsules, an assumption that limits their applications for small diameter capsules. The mathematical model developed in this work is based on solving the heat conduction equation in both melt and solid phases in cylindrical coordinates, taking into account the radial temperature distribution in both phases. Heat flux was then evaluated at the surface of the first row of the capsules to determine the temperature of the air leaving that row by a simple heat balance. It was found that such computation may be carried out for every few rows rather than for a single row to minimize computer time. The simulation study showed a significant improvement in the rate of heat transfer during heat charge and discharge when phase change materials with different melting temperatures were used. Air must flow in the direction of decreasing melting temperature during heat charge, while it must be reversed during heat discharge.

Solar desalination with a humidification-dehumidification technique — a comprehensive technical review

Major desalination processes consume a large amount of energy derived from oil and natural gas as heat and electricity. Solar desalination, although researched for over two decades, has only recently emerged as a promising renewable energy-powered technology for producing fresh water. Solar desalination based on the humidification-dehumidification cycle presents the best method of solar desalination due to overall high-energy efficiency. This paper provides a comprehensive technical review of solar desalination with a multi-effect cycle providing a better understanding of the process. Discussion on methods to improve system performance and efficiency paves the way towards possible commercialisation of such units in the future.

Solar desalination based on humidification process—I. Evaluating the heat and mass transfer coefficients

Abstract Solar desalination with a humidification–dehumidification process has proven to be an efficient means of utilizing solar energy for the production of fresh water from saline or sea water. The process used in this work is a closed air cycle type, in which air is circulated in the unit by natural draft between the humidifier and condenser. In order to scale up a unit of this type, it is necessary to obtain sufficient information on the process of heat and mass transfer in the unit. The humidifier and the condenser had to be specially designed to maintain minimum pressure drop in the unit. The mass transfer coefficient in the humidifier was found to be affected mostly by the water flow rate due to its effect on the wetting area of the packing. In natural draft operation, the air circulation rate was found to increase with water flow rate, causing a further increase in the mass transfer coefficient. It was possible to predict and correlate the mass and heat transfer coefficients in the humidifiers and condensers, having different designs, in the three units constructed by us in Jordan and Malaysia.

Underfloor heating with latent heat storage

Abstract Underfloor heating is known for its comfort due to the low and favourable temperature gradient in the heated space above the large floor area available for heating. This paper addresses the idea of improving the existing underfloor heating system by using an encapsulated phase change material (PCM) that melts and solidifies at about 28 °C, placed in the concrete floor during construction. The objective is to minimize the fluctuation in the floor surface temperature and to have sufficient heat storage so that heating can be done during the off-peak electricity period only. Two concrete slabs (0.5 m × 0.5 m × 0.095 m) were constructed with a hot water pipe embedded in both of them to provide the required heating. Nodules having a diameter of 75 mm and containing CaCl26H2O were placed in one of the concrete slabs prior to its moulding. Unlike the plain concrete slab, the concrete-PCM slab showed a much lower surface temperature fluctuation and maintained an acceptable surface temperature during the whole day even though the heating process was done for only 8 h. A simple one-dimensional model was developed to simulate the underfloor heating system, so that it can be used to optimize its design. Comparison between the measured and predicted performance of the two slabs for 3 days showed a reasonable agreement.