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.     

Efficiency investigation of a new-design air solar plate collector used in a humidification–dehumidification desalination process

The aim of this research is to experimentally study the efficiency of a new-design plate collector used to heat air in a new desalination humidification–dehumidification process. In fact, in such processes, the air solar collectors work at unusual experimental parameters (forced convection, elevated air humidity, high solar irradiation…). At these stressed experimental conditions, few published works are available in literature. Furthermore, the comparison of the efficiency of collectors running with normal air humidity content (about 10–20 g kg⁻¹) and air of elevated humidity (20–50 g kg⁻¹) were not yet published as our knowledge. In the present investigation, a new air solar plate collector was designed and developed for its use in a desalination process. Moreover, a characterization of such collector was performed under different experimental conditions. The effect of different parameters, namely: the solar radiation, the wind velocity, the ambient temperature, the air mass flow rate, the inlet air humidity and temperature, on the collector efficiency was also investigated. The collector was optimized for its use in a new solar desalination process. In fact, the air solar collector was designed in order to lower its economic cost making them applicable for water desalination.     

Modeling and experimental validation of a humidification–dehumidification desalination unit solar part

This paper presents the modeling and the experimental validation of air and water solar collectors used in humidification-dehumidification (HDH) solar desalination unit. The solar desalination process is currently operating under the climatological conditions of Sfax (34 N, 10 E), Tunisia. To numerically simulate the air and water solar collectors, we have developed dynamic mathematical models of the solar collectors. The resulting distributed parametric systems of equations are transformed into a system of ordinary differential equations (ODEs) using the orthogonal collocation method (OCM). A comparison between numerical and experimental data was conducted. It was found that the two-temperature mathematical model describes more precisely the real behaviour of the water solar collector than the one-temperature mathematical model. It was also shown that the developed mathematical models are able to predict accurately the trends of the thermal characteristic of the water and air solar collectors. As a result, the proposed models can be used to size and test the behaviour of such a type of water and air solar collectors.     

Solarthermal seawater desalination systems for decentralised use

The performance of a pilot solar Multi Effect Humidification (MEH) Desalination system in Fuerteventura, Canary Islands, had been measured and analysed in detail by the ZAE Bayern since 1992. The investigated distillation units showed constant performance over several years without extensive maintanance. However the efforts towards further efficiency improvement by economic means pointed out the need of supplementing the system with a thermal storage tank. The distillation unit being the most expensive part of a desalination system has to be run 24 hours a day in order to be economic. A cost estimation for storage implementation yields the result, that cost reduction for the produced water by more than a half is possible. In April 1997 a desalination system with 24 hour thermal storage was built up in Sfax / Tunisia. The results of a short term measuring campaign at this site are presented here.For optimizing the ratio of sizes of thermal storage, collector field and distillation module a simulation tool for collector field and storage is developed at the ZAE Bayern. The simulation results of a combined laboratory distillation unit and storage system are presented.     

Collector selection for solar ejector cooling system

The performance of a solar ejector cooling system is simulated using three different collectors: a conventional flat plate collector, a high efficiency flat plate collector and a vacuum-tube collector. It is shown that with the proper selection of the generating temperature an optimum COP can be achieved. The solar ejector cooling system using the single-glazed solar collector with selective surface and an enhanced air insulating layer can be most economical when operated at the optimum generating temperature of the ejector cooling machine. In this case, the solar system cost is around 1 USD per watt of cooling capacity for air conditioning applications.     

Study of a solar water heater using stationary V-trough collector

There are various types of solar water heater system available in the commercial market to fulfill different customers’ demand, such as flat plate collector, concentrating collector, evacuated tube collector and integrated collector storage. A cost effective cum easy fabricated V-trough solar water heater system using forced circulation system is proposed. Integrating the solar absorber with the easily fabricated V-trough reflector can improve the performance of solar water heater system. In this paper, optical analysis, experimental study and cost analysis of the stationary V-trough solar water heater system are presented in details. The experimental result has shown very promising results in both optical efficiency of V-trough reflector and the overall thermal performance of the solar water heater.     

Heat Transfer Analysis of Solar Air Heating System for Different Tilt Angles

Solar Air Heater is a simple, cheap and most widely used for various applications such as textile industries, agricultural, desalination and space heating. Generally collectors are tilted so as to absorb maximum radiation, so it is necessary to calculate the optimum tilt angle to maximize the solar radiation falling on the collector area to gain maximum useful energy. The maximum solar radiation can be collected by using a tracking mechanism. Tracking systems are expensive and complicated in construction. The working operation of solar integrated tracking system is difficult. This paper presents the mechanism of evaluating the overall heat transfer coefficient of the solar air-heater at variable intensities and inlet velocities. The experimental setup is integrated with blower at inlet to the solar air heater in order to pump air at different velocities. The work focus on comparative study of solar air heating system for different tilt angles ranging from 250 to 600 and determines the overall heat transfer coefficient so as to find the optimum tilt angle of a solar flat plate collector.     

Experiments of a solar flat plate hybrid system with heating and cooling

Based upon the prior research of the solar hybrid water heater and refrigerator, a new flat plate solar hybrid system with heating and cooling was proposed and experimental prototype device was constructed. With this new hybrid system, the heat and mass transfer can be improved effectively both in desorption process and adsorption process. The conventional flat plate solar water heater collector absorber is immersed inside adsorbent bed in the new hybrid system. The experimental results show that not only the cooling effect can be obtained, but also both the sensible heat of the adsorbent bed and the adsorption heat can be recovered effectively to produce hot water for domestic use. The COP of this new flat plate hybrid system can reach 0.11 and the heat efficiency is about 0.45, this achievement has demonstrated an efficient way of the application of solar energy.     

Modeling of the CSU heating/cooling system

This paper resents a thermal simulation of the Colorado State University solar house. A computer model of the solar energy system was developed and computer runs were made using one year of meteorological data to determine the important design features. The system consists of a flat plate solar collector, main storage tank, service hot water storage tank, auxiliary heater, absorption air conditioner with cooling tower and heat exchangers between the collector and storage, storage and service hot water tank and storage and residence. This system very closely models the CSU house in operating mode one.The results are in the form of monthly integrated values for the pertinent energy quantities. In addition, results are presented which show the effect on the system performance of the collector tilt, collector area and number of covers.     

A comparison of solar absorption air conditioning systems

A computer simulation of solar powered absorption air conditioning systems is discussed. The results of simulations of various systems composed of conventional flat plate or evacuated tube collectors, wet or dry cooling towers, lithium bromide-water or aqua-ammonia working fluids and hot water, chilled water or refrigerant storage alternatives are obtained over a common operating cycle. Performance of the lithium bromide-water working fluid is shown to be superior to aqua-ammonia. Relative performance gains realized with the evacuated tube collector and relative performance losses associated with the dry cooling tower are presented. Chilled water storage is shown to be advantageous for an evacuated collector, dry cooling tower, lithium bromide-water system.     

Performance evaluation of v-trough solar concentrator for water desalination applications

The working principle and thermal performance of a new v-trough solar concentrator are presented in this paper. Compared with the common parabolic trough solar concentrators, the new concentrator has two parabolic troughs which form a V-shape with the focal line at the bottom of the troughs. This is beneficial for the installation and insulation of the receiver, and the shadow on the reflective surface is avoided. The new v-trough collector does not require high precision tracking devices and reflective material. And therefore the cost of the system could be significantly reduced. Various experimental tests were carried out both outdoor and indoor using different types of receiver tubes. The results show that the collector system can have thermal efficiency up to 38% at 100 °C operating temperature. System modelling was used to predict the rate of fresh water produced by four different solar collector systems which include both static and one-axis solar tracking technologies. Comparison of the solar collectors at different temperature ranges for humidification/dehumidification desalination process using specific air flow rate were considered. At each temperature range, suitable solar collectors were compared in the aspect of fresh water production and area of solar collector required. Results showed that the new v-trough solar collector is the most promising technology for small to medium scale solar powered water desalination.     

Simulation of a solar heating and cooling system

This paper presents thermal and economic analyses of a solar heated and air conditioned house in the Albuquerque climate. The system includes the following components: water heating collector, a water storage unit, a service hot water facility, a lithium bromide-water air conditioner (with cooling tower), an auxiliary energy source, and associated controls. The analysis of the thermal performance indicates the dependence of output on collector area (considered as the primary design variable) and shows, for example, the manner in which annual system efficiency decreases as collector area increases. Based on the computed thermal performance, cost estimates are made which show variations in annual cost as functions of collector area and costs of collector and fuel.     

Effect of Buoyancy on the Perfomance of Solar Air Collectors with Different Structures

In order to study the effect of buoyancy on the performance of solar air collector, the theoretical analysis and experimental tests of four solar air collectors with different structures under natural convection and mixed convection are carried out. The results show that the air temperature rise of the protrusion-corrugated plate air collector is the highest in the natural convection, which is 9.17 Chigher than that of the flat plate collector, and the air outlet velocity is 0.19 m/s, increasing by 16.88% than that of the flat plate collector. Observing the effects on the heat transfer performance of mixed convection, it can be found, in addition to the protrusion-corrugated plate air collector, the buoyancy plays a positive role on the other three solar air collectors in the upward flow, while the buoyancy plays a negative role on the other three solar air collectors in the downward flow, and the enhanced degree of the buoyancy to the corrugated plate air collector is the largest, while the enhancement degree of the flat plate collector is the least.     

Performance studies of a double glazed augmented integrated rock system

This paper describes the design details of and experimental investigations on a prototype augmented integrated rock system (AIRS) with an intention of using solar energy for crop drying and space heating. The system investigated is a flat plate solar collector of conventional design connected in series with rock storage and a collection unit. For the evaluation of the 24 hour performance of this system, outdoor calorimetric tests have been performed. The experimental results for varying inlet air flow rate have been analysed and the final results have been included. AIRS' overall daily efficiency is determined by testing the system outdoors and monitoring the output energy and the incident solar radiation. Taking into consideration the total cost of the system it has been designed so that it can be fabricated on site using, wherever possible, readily available tools and materials.     

A solar drier supplemented with auxiliary heating systems for continuous operation

A prototype solar fruit and vegetable drier was developed. This comprises a glass covered flat plate collector containing metal chips, a drier with translucent walls, and an insulated tunnel, joining the two.Bell peppers and sultana grapes were dried to commercially acceptable moisture levels in various kinds of weather conditions and at various air velocities. The quality of the dried product as well as the drying times were found to be in favor of the solar drier as compared with open-air drying.Likewise, an economic analysis was undertaken to investigate the possibility of using various heat sources for an auxiliary heating system. Such a system is being developed and will be coupled to the prototype to enable all-weather operation.     

Net energy analyses of eight technologies to provide domestic hot water heat

Net energy analyses have been carried out for eight trajectories which convert an energy source into heated domestic water. In addition, economic and metals consumed estimates have been made for three methods of producing heated domestic water. While some of the trajectories producing electrically heated water showed a slightly higher ratio of energy produced to energy consumed than a flat plate solar collector system, when all factors were considered (i.e. cost, quality of energy, and metals consumed), the flat plate solar collector system appears to be the system our energy policy should encourage for heating domestic water, even in the cloudy inter-Lakes area of the United States. Based on only net energy factors, it seems clear that the synthetic fuels program should not be encouraged as a means of providing a fuel source for heating domestic water.     

Experimental study of the performance of a solar collector by closed-end oscillating heat pipe (CEOHP)

An experimental flat plate solar collector operating in conjunction with a closed-end oscillating heat pipe (CEOHP) offers a reasonably efficient and cost effective alternative to conventional solar collector system that use heat pipes. The CEOHP system described in this study relies on the natural forces of gravity and capillary action and dose not require an external power source. The flat plate collector consisted of a 1 mm thick sheet of black zinc covered by a glass enclosure with a collecting area of 2.00 × 0.97 m² , an evaporator located on the collecting plate, and a condenser inserted into a water tank. A length of 0.003 ID copper tubing was bent into multiple turns at critical points along its path and used to channel the working fluid throughout the system. R134a was used as the working fluid. Efficiency evaluations were conducted during daylight hours over a two month period and included extensive monitoring and recording of temperatures with type-K thermocouples placed at key locations throughout the system. The results confirmed the anticipated fluctuation in collector efficiency dependant on the time of day, solar energy irradiation, ambient temperature and flat plate mean temperature. An efficiency of approximately 62% was achieved, which correlates with the efficiency of the more expensive heat pipe system. The CEOHP system offers the additional benefits of corrosion free operation and absence of freezing during winter months.     

Investigation of rock bed solar collector cum storage system

This paper presents an experimental investigation of a prototype rock bed solar collector. This collector consists of rocks in a galvanized iron box; the rocks are painted dull black and suitably glazed. The heat can be extracted by forced air convection. This system can act as a storage system, as well, when suitably constructed. The heat decay characteristic of the bed is also studied. A preliminary economic analysis of this air heater is presented.     

Influence of the design parameters on the overall performance of a solar adsorption refrigerator

This paper describes a parametric improvement study of a solar adsorption refrigerator, using a flat plate collector with silica-gel and water as the adsorption pair. Based on a set of experimental results obtained with a prototype, used mainly for model validation purposes, a numerical study is conducted to determine the influence of the most relevant parameters and to improve the overall performance.The main parameters considered in this study are: the mass of silica-gel, the number of metallic fins in the silica-gel bed, the orientation of the solar collector (azimuth angle), the improvement of the collector's cooling during the night, the thermal contact resistance between silica-gel and the collector plate, the condenser surface area, the evaporation surface area, the radiant properties of the collector plate, and the thermal insulation of the refrigerated cabinet. The influence of each individual parameter is analyzed, and its optimum value is determined. The refrigeration system with all the individually obtained optimum parameters has an overall performance considerably higher than that corresponding to the analyzed prototype, providing useful information for a better integrated understanding of the solar adsorption refrigeration systems, and for a better design of such systems looking for their maximum overall performance.     

一种新型太阳能海水淡化系统的实验研究

介绍了一种新型的太阳能海水淡化方法,即结合太阳能空气集热器和太阳能热管、利用空气增湿除湿来实现海水淡化。分别进行了电吹风模拟太阳能空气集热器的蒸发器实验,以及结合3m2太阳能空气集热器和热管集热器的实际装置实验。结果表明,影响蒸发量的主要因素为热空气温度、热空气流量、初始水量、水温、出气孔直径和数量。实验结果表明,装置可获得的最大冷凝量为790g/h,计算出系统的产水率和热力学效率分别为5.59×10-5kg/kJ和12.4%。