Effect Of Porous Media On Performance Of Solar Pond

With the population and economy growing, exploitation of renewable energy was noticeably accelerated. Solar energy, one kind of rich and clean alternative energy, is one of perfect energies. Salt gradient solar pond is a simple and cheap device to collect and reserve solar energy. At present, solar pond is a hot research topic.In the purpose of studying salt gradient solar pond, we researched the effect of porous media on solar pond performance by combing experimental measurement, numerical simulation with theoretical analysis. The dissertation is composed of the following two parts. One is that effective thermal conductivity of porous media was studied through experimental measurements and numerical simulations, which was the preliminary work to investigate the effect of porous media on thermal performance of solar pond. The other part is that the effect of porous media on thermal performance of solar pond was considered using simulation and experiments in laboratory and at the seaside.Effective thermal conductivity of mixture of porous media and concentrated brine is needed when investigating effect of porous media on thermal performance of solar pond. Considering that instruments used to measure thermal conductivities of solid and fluid are not suitable for measuring those of brine layer with slag or pebble, special experimental apparatus was designed and set up. Thermal conductivity and thermal diffusivity of mixtures of slag or pebble and water or concentrated brine was measured based on the unsteady heat conducting principles of a semi-infinite body. Fractal pore network model composed of fractal theory and pore network model was selected to simulate the structure of porous media which is applicable for fractal theory. Effective thermal conductivity of porous media was calculated by using two-dimensional steady state heat conduction equation without heat source. The numerical results of fractal pore network model agree well with experimental ones, which indicated that the fractal pore network model is suitable for researching effective thermal conductivity of porous media. However, fractal theory is not applicable to simulate granular porous media. We adopted two-dimensional cylinder model to calculate effective thermal conductivity of granular porous media in which cylinder is used to represent particle. The simulation results correspond well with the experimental results. This confirms that this two-dimensional model is suitable to calculate effective thermal conductivity of granular porous media.In order to study effect of porous media on thermal diffusion and salinity diffusion of solar pond, experiment in laboratory has been performed. Temperature distribution of systems with and without porous media was tested. Heat carried away by cooling water was calculated. Results show that porous media does play insulation role. To discuss the effect of porous media on salinity diffusion, we tested the salinity in our experimental container which was filled with several layers of solution with different salinity. The experiment on salt diffusion indicates that the upward diffusion of salt is slowed down when porous media are added, which helps maintain salt gradient.Two experimental and small-scaled solar ponds with same structure were built to research the effect of porous media on performance of solar pond. For both solar ponds, the aperture area is 2.5×2.5m2. The depth of water is 1.0m. Temperature, salinity and turbidity of solar ponds with and without porous media were measured and compared. Total heat storage quantities, salinity gradient and stability coefficient were calculated. We concluded that porous media would be in favor of enhancing temperature and total heat of solar pond, maintaining bigger salinity gradient and stability coefficient, keeping steady running. The inhibitory effect of porous media on turbidity during the replenishment of salt to lower convective zone (LCZ) was investigated through measuring turbidity. It was confirmed that porous media is beneficial to the process of concentrated brine replenishment.Experimental research is time-consuming and material-consuming and several experimental conditions are hard to realize, therefore numerical simulation was used to analyze the effect of porous media on heat and mass transfer of solar pond. Because of twists and turns of porous media layer, convection is not easy to emerge in this layer. Traditional solar ponds consist of three main zones-the upper convective zone (UCZ), non-convective zone (gradient zone, NCZ) and LCZ. This three layers model is not suitable for solar pond with porous media. Porous media layer should be added. In other words, original model should be replaced by four layers model, which is UCZ, NCZ, LCZ and non-convection porous media layer. The numerical model was calibrated by data of experimental solar pond. The agreement between the measured and calculated results is fairly good. Based on Beijing's long-time average meteorological data, effect of thickness and porosity of porous media layer on temperature, total heat and extraction rate of solar pond was researched on different ground thermal conductivity. Simulation results indicate when ground thermal conductivity is big, it is necessary to add porous media. Oppositely, when ground thermal conductivity is small, brine layer with porous media could not improve thermal performance of solar pond. LCZ maximum temperature increases with the increment of thickness of brine layer with porous media. However total heat storage quantity of solar pond increases and then decreases with the increment of thickness of brine layer with porous media. There is an optimum thickness of brine layer with porous media which make total heat storage quantity of solar pond reach its maximum. Therefore both LCZ maximum temperature and total heat storage quantity of solar pond should be considered when thickness of brine layer with porous media is selected. The effect of porosity of porous media on performance of solar pond was also investigated which was vary depending on the ratio of solid's thermal conductivity and liquid's thermal conductivity. Because of plenty varieties and different physical parameters of porous media, alternative scope of porous media was studied which could play heat insulation role. The consequences confirm that alternative scope of porous media is broad. Unsaturated porous media is formed after adding porous media to outside of solar pond. The effect of unsaturated porous media on performance of solar pond was considered. Results show that unsaturated porous media helps to enhance temperature and total heat of solar pond. However, temperature and total heat of solar pond decreased with increasing percentage of liquid.