Performance analyses of solar distillation units having vertical evaporating surfaces | |
| Author | Tanongkiat Kiatsiriroat |
| Call Number | AIT Diss. no. ET-87-01 |
| Subject(s) | Mass transfer Solar stills |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Abstract | This study presents a method of predicting mass transfer in solar stills from a modification of mass transfer theory developed by Spalding. To estimate the mass transfer rate, the modified theory requires the information of the temperatures of the evaporating surface and the condensing surfaces. The latter temperature can be more easily and accurately measured than the temperature of the bulk state which is needed in the original model. The modified method has been applied to estimate the mass transfer rates in a horizontal solar still, an inclined solar still and a vertical solar still; agreement between the theoretical and the reported experimental results has been found to be very good. Theoretical and experimental performances of a single effect vertical solar still, a multiple-effect vertical solar still and a distillation unit coupled with a flat plate solar collector as heat source have been studied. The theoretical model for each of the solar stills have been developed by using the solar radiation, the ambient temperature and the wind speed as the input and the output are the temperature- time history of the evaporating and the condensing surfaces in the distillation unit and the distilled water production. The theoretical model has been used to find the effects of the total heat capacity of the absorbing/evaporating plate, of the gap between the absorbing/evaporating surface and the cover, and of the still orientation on the performance of a singleeffect solar still. It is found that with lower total heat capacity and smaller gap, better performance of the still is obtained. The suitable orientation for maximum distilled water yield is the direction in which the highest average incident solar radiation is obtained. For Bangkok, the suitable orientation for a single-effect two-sided still in which solar radiation is absorbed on both sides of the absorbing/evaporating plate, is in east- west direction. The average daily efficiency of the still is about 44 %. In case of a single-effect one-sided still, the absorbing surface should face either east or west. The average daily efficiency of the still is about 40 %. A multiple-effect one- sided vertical solar still consisting of a black anodized aluminum sheet as an absorber was tested with the absorbing surface facing east and south . The theoretical model developed can predict the still performance with good agreement with experiments. The simulation also shows that a higher number of evaporating plates increases the still performance and when the plate number is over 3, the productivity increases only slightly. For Bangkok, the orientation for maximum distilled -iii- water yield is in the east or west direction. The still performance can be improved by covering the back side of the distillation unit with a wetted cloth. For the three-effect vertical solar still facing the east, the average daily efficiencies of the still, with and without the wetted cloth, are about 61.7 and 51.1 %, respectively. In the case of a two- sided multiple effect vertical solar still, the performance is found to be slightly better than the two one-sided stills oriented in the same direction. A locally made flat plate solar collector of area 1.4 m2 was used to supply heat to a multiple-effect distillation unit. The unit was an enclosure having two effects of distillation. The front side of the first plate was insulated and the back side of the last condensing plate was exposed to ambient. The theoretical model developed can predict the system performance with good agreement with the experiments. From the model, it is found that as the ratio of the evaporating surface area to the solar collector area increases, the distilled water output also increases. When the ratio is over about 1 , the productivity increases only slightly. The system performance can be improved by about 15 - 34 % when the last condensing plate is covered with a wetted cloth. In this study, the feasibility of coupling a reversed absorption heat pump for upgrading solar heat from a solar collector and supplying to the multiple-effect distillation unit has been theoretically investigated. The working fluid pair of the absorption cycle is LiBr- water. To get high heat ratio of the absorption cycle, a R- 11 vapor compression h eat pump has been proposed to absorb heat rejected from the condenser of the absorption cycle and supply it back to the evaporator. The system has been compared with the distillation unit having only a flat plate solar collector as a heat source. It is found that the daily distillation efficiency of the first system is higher than that of the latter system by about 10 %. The distilled water yielded by the two-sided vertical solar still is the most economic, the cost being about 0.76 baht/liter at the interest rate of 14 % and useful life of 10 years. For the multiple-effect vertical solar still, the optimum number of the evaporating plates inside the distillation unit is 3. In the case of the multiple-effect distillation unit having a flat plate solar collector as a heat source, when the ratio of the area o f each evaporating surface to the solar collector area is about 1, the optimum number is 5. For the system having the reversed absorption heat pump assisted distillation, the cost of the distilled water yielded by the system is rather high. |
| Year | 1987 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Other Field of Studies (No Department) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Bhattacharya, Sribas C. ;Prida Wibulswas |
| Examination Committee(s) | Exell, Robert H.B. ; Huynh, Ngoc Phien ;Veziroglu, T. N. |
| Scholarship Donor(s) | The Government of Japan; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1987 |