Hassan E.S. Fath

Solar distillation : a promising alternative for water provision with free energy, simple technology and a clean environment

Abstract Solar distillation presents a promising alternative for saline water desalination that can partially support humanitys needs for fresh water with free energy, simple technology and a clean environment. The development of solar distillation systems has demonstrated their suitability for the desalination process when the weather conditions are favorable and the demand is not too large, i.e., less than 200 m 3 /d. The problem of low daily productivity of the solar stills triggered scientists to investigate various means of improving still productivity and thermal efficiency in order to reduce water production cost. This paper presents an overall review and technical assessment of the various and up-to-date developments in single and multi-effect solar stills. The development in still configurations, the problems encountered with units during the course of operation as well as the impact on the environment are addressed.

Solar desalination using humidification dehumidification processes. Part I. A numerical investigation

A numerical investigation of a humidification dehumidification desalination (HDD) process using solar energy is presented. The HDD system consists mainly of a concentrating solar water heating collector, flat plate solar air heating collector, humidifying tower and dehumidifying exchanger. Two separate circulating loops constitute the HDD system, the first for heating the feed water and the second for heating air. A mathematical model is developed, simulating the HDD system, to study the influence of the different system configurations, weather and operating conditions on the system productivity. The model validity is examined by comparing the theoretical and experimental results of the same authors. It is found that the results of the developed mathematical model are in good agreement with the experimental results and other published works. The results show also that the productivity of the unit is strongly influenced by the air flow rate, cooling water flow rate and total solar energy incident through the day. Wind speed and ambient temperature variations show a very small effect on the system productivity. In addition, the obtained results indicate that the solar water collector area strongly affects the system productivity, more so than the solar air collector area.

Thermal-economic analysis and comparison between pyramid-shaped and single-slope solar still configurations

Abstract This paper presents an analytical study as well as thermal and economic comparisons between two solar still configurations: the pyramid and the single slope. A mathematical model was developed to simulate the two configurations and study their thermal performance. The meteorological data of Aswan City (south of Egypt) were used, and the daily total energy received by each still basin was calculated. The main performance parameters such as still productivity and efficiency are presented for the whole year. In addition, an economic assessment of the distilled water production cost has been carried out. On the basis of yearly performance results, the single slope still was found to be slightly more efficient and economical than the pyramidal one.

Technical assessment of solar thermal energy storage technologies

Solar energy is recognized as one of the most promising alternative energy options. On sunny days, solar energy systems generally collect more energy than necessary for direct use. Therefore, the design and development of solar energy storage systems, is of vital importance and nowadays one of the greatest efforts in solar research. These systems, being part of a complete solar installation, provide an optimum tuning between heat demand and heat supply. This paper reviews the basic concepts, systems design, and the latest developments in (sensible and latent heat) thermal energy storage. Parameters influencing the storage system selection, the advantages and disadvantages of each system, and the problems encountered during the systems operation are highlighted.

Solar desalination using humidification—dehumidification technology

Abstract A numerical study has been carried out to investigate the performance of a simple solar desalination system using humidification—dehumidification processes. The desalination system consists of a solar air heater, humidifier, dehumidifier and a circulating air-driving component. The study covers the influence of different environmental, design, and operational parameters on the desalination system productivity. Environmental parameters include solar intensity, ambient temperature and wind speed. Design parameters include the solar heater base insulation, the humidifier and the dehumidifier effectiveness. Operational parameters include air circulation flow rate, feed water rate and temperature. The results indicated that the solar air heater (energy source) efficiency significantly influences system productivity. Increasing the solar intensity and ambient temperature and decreased wind velocity increases system productivity. Increasing the air flow rate up to 0.6 kg/s increases the productivity, after which it has no significant effect. The feed water flow rate has an insignificant influence on system productivity. The surprising result is that the dehumidifier effectiveness has an insignificant influence on system productivity, which has a very important implication for the systems economy. The physical explanation of this finding is given.

Thermal performance of a simple design solar air heater with built-in thermal energy storage system

Abstract The thermal performance of a simple design solar air heater is presented. The conventional flat plate absorber is replaced by a set of tubes filled with a thermal energy storage material. The proposed integrated system heat transfer area and heat transfer coefficient are increased, and the heat loss is reduced. Based on a simple transient analysis, explicit expressions for the heater absorber and glass cover temperatures, effective heat gained, outlet air temperature, and the heater efficiency have been developed as a function of time. The integrated system performance curves are presented, and a marked improvement on the system performance is noticed over the conventional flat plate heater system.

Effect of adding a passive condenser on solar still performance

Abstract A theoretical and experimental study was conducted to investigate the effect of adding a passive condenser on the performance of the single slope, basin type solar still. A theoretical model based on the Dunkle (Int. Devs Heat Transfer 5, 895 (1969); Ref. [1]) mass transfer (evaporation) rate was developed. The model assumes that the transfer of water vapour from the still to the condenser is due to one or more of the following mass transfer modes: (i) diffusion, (ii) purging, and (iii) natural circulation. The theoretical results indicate that the diffusion contribution is relatively small. The contribution through purging represents the fraction [condenser volume/(condenser volume + still volume)] of the still yield. About 75% of the still yield is contributed through natural circulation. An experimental study that simulates the purging mass transfer mode was investigated. The experimental results show good agreement with the theoretical predictions and an increase of 50% in the still efficiency.

Heat exchanger performance for latent heat thermal energy storage system

Abstract An experimental and theoretical study was carried out to investigate the heat transfer in a component of a low temperature thermal energy storage (TES) system using the latent heat of fusion of a phase change material (PCM). The heat storage container is a double pipe heat exchanger where the energy source fluid (hot water) flows through the inner tube, while the heat sink (storage PCM) fills the annulus gap. The tested phase change material is the paraffin wax of melting temperature of 50°C. Measurements were made of the temperature change of the hot water as well as of the paraffin wax. The heat transfer rate and the accumulative energy stored as a function of time is presented for different hot water flow rates and inlet temperatures. An analytical model is developed for the prediction of the storage heat transfer rate and accumulative energy. The predictions of the model are compared with experimental results and show good agreement. The developed model is also used to study the effect of convertion in the molten zone, initial PCM subcooling, and heat exchanger dimensions on heat transfer rate and accumulative energy.

Transient analysis of thermosyphon solar air heater with built-in latent heat thermal energy storage system

The thermal performance of a thermosyphon solar air heater with built-in latent heat thermal energy storage system is presented. Phase change materials (PCM) of different melting temperatures (Tm) of 61, 51, 43 and 32°C are studied and compared with the system with no storage material. The results are presented for two limiting values of the maximum (noon) solar intensities of 700 and 900 W m−2 and the average ambient temperature of the city of Alexandria, Egypt. It was found that the heaters with Tm = 51 and 43°C show the best performance. For Tm = 43°C, the heater thermal load of minimum outlet air temperature of 8°C above ambient, and minimum air flow rate of 0.01 kg s−1 (30 m3 h−1), is delivered for the whole day. The hot air flow rate varies from 0.006 kg s−1 (18 m3 h−1) for 24 h, to 0.036 kg s−1 (111 m3 h−1) for 10 h, depending on the system flow resistance. The proposed air heater is simple in design and adds no operational or maintenance complexities over conventional air heaters.

Transient analysis of naturally ventilated greenhouse with built-in solar still and waste heat and mass recovery system

A transient analysis of a naturally ventilated greenhouse with built-in solar still and waste heat and mass recovery system (WHMRS) is presented. Explicit expressions for the transient temperature of the greenhouse inside air, still basin-water, and WHMRS have been developed so as to study the thermal performance of the integrated system. The effects of the system flow resistance, thermal chimney height, still basin-water heat capacity, WHMRS heat capacity and effectiveness, and environmental conditions have been studied. On the basis of the numerical results, it was found that the integrated system is able to ventilate the greenhouse for the 24 h with an air flow rate which varies from 0.1 kg/s at midnight to 1.0 kg/s at midday. The corresponding ventilation air change rate ACR varies from 1.2 to 12 ACR/h, respectively. The ventilation air rate increases with increasing chimney height or decreasing system flow resistance. The system daily yield of fresh water varies from 1.26 to 1.82 kg/m2 and increases with increasing basin-water and WHMRS heat capacities and WHMRS effectiveness or decreasing flow resistance. The integrated system is simple, passive and presents an exciting possibility for support of agricultural activities in hot and rural regions.