| Energy and freshwater resources are the basic needs of social life. In the aspect of freshwater, one possible way to fill in the gap between demanded and available freshwater is the desalination of brackish water and seawater. In the aspect of energy, solar energy, which is freely available, is environmentally friendly and has no hazardous emissions. If seawater desalination could be driven by solar energy, it will solve the problem of fresh water shortage in remote areas, where not only potable water, but also conventional energy, for instance heat and electricity, is in great need. There are two categories of solar thermal applications, concentrating collector and non-concentrating collector. For collector, the purpose of solar concentrator device is to absorb solar beam radiation. Most concentrator devices are made of reflection mirrors or Fresnel lens, which are not easy to be manufactured and are thus very costly. Also, the system tracking device is complicated and consumes a lot of energy. For non-concentrating collectors, there are two types, flat plate collectors and evacuated tube collectors. The stationary solar collector is mainly applied to only water heating or only seawater desalination. There is currently no combined system of hot water supply and freshwater production. What’s more, conventional desalination methods, such as Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), and Reverse Osmosis (RO), are suitable for large and medium capacities of fresh water production. But most remote areas, where technique and economic are the most common problem, small capacity desalination systems are significantly desired, which are cost less and easy to be run. Therefore, the objective of this research is to propose and test two novel solar seawater/brackish water desalination systems based on the adsorption of low-medium temperature resources. One is double vacuum tube solar hot water and pure water co-production system, and the other is humidification-dehumidification desalination system using double-vacuum tube low temperature heating and fixed mirror solar concentrator medium temperature heating. Both solar heating systems have been run at an optimum temperature. Thermal properties were then optimized and improved by recycling and rearranging heat distribution. This research is expected to provide a feasible technology of seawater/brackish water desalination for remote areas, such as coastal areas, islands, brackish water areas with strong wind.System of fixed linear mirror solar concentrator has been built here. Optical and thermal performance has been studied as well. It is necessary to calculate solar beam radiation on each of tilted mirror for the design of solar collector system. Fixed mirror solar concentrator (FMSC) can be considered as a discrete smooth linear optical reflector. As the direction of solar radiation changing throughout the year, the incidence angle and illuminate area of each mirror is varied consequently, making the calculation of the beam radiation quite complex. The equation of incidence angle, illuminating area and geometric concentration ratio, had been obtained by vector analysis. The instantaneous solar beam radiation of the whole mirror field was then computed. Finally, the accumulated radiation of the overall system was obtained in the duration of work time. Furthermore, two kinds of concentrator are compared, including installing on the south facing with slope of latitude angle and adjusting tilt angle each year. Radiation from concentrator system with open width of 1800 mm would increase 65.78GJ/m and 6.53% each unit length by adjusting slope angle once a year.Three different absorbers, which are trapezoidal groove absorber, glass-metal vacuum tube absorber and cavity style of glass-metal vacuum tube absorber, have been proposed and then investigated experimentally. A 3D model, with a fixed linear mirror solar concentrator, was built for the cylindrical cavity glass-metal vacuum tube absorber and. The Monte Carlo ray tracing method is applied to investigate the concentrating characteristics of concentrator. The flux distribution on the receiver is simulated and drawn with software of TracePro, while ray trace analysis at different transverse angles determines optical efficiencies, optical loss and flux distribution on the absorber. The results show that an overall ray’s acceptance of 98%-74.08% and optical efficiency of 73.65%-56.9% is obtained from transverse angles of 0° to 40° at a mirror reflectance of 0.92 and a receiver absorbance of 0.9. As the angle increased, optical efficiency of the system is decreased and distribution of energy flow on the absorber tends to be uniform. In addition, thermal performance of the trapezoidal cavity absorber and cylindrical cavity absorber has been investigated experimentally at the different transverse angles and output temperature. The cylindrical cavity glass-metal vacuum tube absorber had a significant advantage in terms of superior thermal performance as compared to trapezoidal cavity absorber, thermal efficiency had higher than trapezoidal cavity absorber by 2%-3% at the low environment temperature and irradiation condition. Thermal efficiency of the cylindrical cavity glass-metal vacuum tube absorber decreased from only 46.93% to 39.98% while the output temperature increased from 76.7℃ to 99.6℃, confirming that concentrating temperature has little influence on the thermal property of the system.In this research, a detailed themodynamic analysis was conducted to assess the performance of humidification dehumidification desalination system with a fixed mirror solar concentrator. The HDH system is considered as an open air, open water, air heated system in which a FMSC was used as air heater. Two different configurations were considered for the HDH system. In the first configuration, the solar air heater was placed before the humidifier, whereas in the second configuration the solar air heater was placed between the humidifier and the dehumidifier.The theoretical case study of Jinan revealed that FMSC works well for air heated HDH systems using solar radiometer meteorological environment. The comparison between the two HDH configurations demonstrates that the gained output ratio (GOR) of the first configuration is, on average, about 1.293, whereas for the second configuration the GOR increases to an average value of 3.454. The study demonstrates that the HDH configuration with the air heater placed between the humidifier and the dehumidifier has a better performance and a higher productivity. GOR of the system for the time period of from November to the next February is smaller than the GOR in the time period between March and September.A theoretical and experiment investigation of a three-stage multi-effect desalination system based on the HDH of air is performed. The system is based on an open cycle for water and a closed cycle for air stream, in which water is heated using a double vacuum tube collector and a FMSC water heater. Air is circulated by forced circulation. The effect of operating parameters on the system characteristic has been investigated. An experiment system was built. The system was equipped with measuring and controlling devices which control electric heater to gradually raise second and third humidifier spray water temperature. Detailed experiments have been carried out at various operating conditions of electric heater, humidifier and dehumidifier. The results of the investigation have shown that the system productivity increases with the increase of water mass flow rate through the unit and the water temperature at the humidifier inlet. The higher water temperature at humidifier inlet or water flow rate, the higher is the air temperature and humidity ratio at condenser inlet and exit. The fresh water production in first stage is small. No signification improvement in the performance of the first desalination unit has been achieved at low temperature and humidity air. The fresh water production in second stage increased from 1.5882 kg/h to 11.964 kg/h as the secondary humidifying temperature increased from 60℃ to 76℃, water flow rate increased from 1.5 kg/min to 3 kg/min. The fresh water production in third stage increased from 4.11 kg/h to 62.43 kg/h as the humidifying temperature increased from 80℃ to 96℃, water flow rate increased from 1.5 kg/min to 3 kg/min. Production of distillate water is increased upon the increase of hot water temperature when the water mass flow rate is matching with hot water temperature.In addition, detailed experiments have been carried out at various operating conditions of solar collector, humidifier and dehumidifier. The effect of the three stages of humidification dehumidification on the unit performance is considered. The results show that the temperature of double-vacuum tube collector was obtained from 55.1℃ to 80.9℃, collection efficiency from 46.71% to 57.32%. At the same time, the temperature of FMSC was reached from 64.3℃ to 93.4℃, collection efficiency from 37.78% to 46.74%. Collector thermal performance has been improved and optimized using hierarchical arrangement in heating. The results also show that when the system operates 6 hours in a day from 9:30 am to 15:30 pm, the fresh water of second stage reaches a maximum of 1.13 kg/10 min, the fresh water of the third stage reaches a maximum of 2.08 kg/10 min, and the gained output ratio (GOR) reaches 2.2383 when solar radiation is largest at 13:00. The total production of fresh water of 74.23 kg was obtained a day. The ratio of fresh water production rate to the inlet salt water reaches 10.31%. Experiment studies indicate that the production rate of distillate water increases with the increase of hot water temperature. On the other hand, the production rate of the distillate water exhibit a maximum upon a increase of air and water flow rates. In all cases, increasing air or water flow rates would enhance mixing within the system and thus increase the humidification rate. At higher water or air flow rates the evaporation efficiency is decreased because of the increase in the sensible heat load of the system. This would reduce the water evaporation rate, humidification efficiency and gained output ratio.The solar hot water and pure water co-production system has been built and the mathematical model of energy conversion and transmission has been established, expecting to improve the thermal performance and water productivity of the system experimentally and theoretically. The double evacuated tube solar collector was integrated into the desalination stills to ensure the continuity production of distillate. The evaporation-condensation tank and the heat storage water tank have been designed and built with a φ58 mm×1.8 m×24 double evacuated heat pipe. The governing energy balance equations were solved analytically with Matlab software and compared with the experimental results. The results indicated that water productivity and performance coefficient increased firstly and then decreased with the increase of temperature at a certain temperature range (50-70℃). It was found that the productivity of water and the coefficient of performance increased to a maximum at 60℃. The constant temperature (60℃) heat storage had a significant advantage in terms of superior performance as compared to quantitative heat storage at 60℃. The increments of the distilled water productivity, the total coefficient of performance and the productivity of water were 950 ml,0.102 and 0.056, respectively. Furthermore, the system performance has been conducted without heat storage. The distilled water productivity, the overall system coefficient of performance and the productivity of water were 5978.4 ml,1.2498 and 0.468, respectively. The decrement of the coefficient of performance and the productivity of water were 0.1979 and 0.219 compared with constant temperature heat storage at 60℃.The transformation rules of GOR are obtained under different heat storage temperatures. The results show that GOR of system is increase firstly, and then decreased upon heat storage temperature. The constant temperature heat storage had a significant advantage in terms of superior performance as compared to quantitative heat storage under same temperature.GOR is smaller because of small temperature difference of the evaporator and condenser at no heat storage conditions setting.Finally, the water quality of the distilled water has been tested according to quality standard of drinking water (CJ 3020-1993) and the national standard "drinking water health standards" (GB 5749-2010). It was found that each parameter in the water quality test has reached the drinking water standard, including the sensory index, chemical index, bacteriological index, toxicology index. A 100% pass rate was obtained for the system tested. |
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