Thermal Performance Study On Seawater-bittern Solar Pond And Dynamic Stability Analysis On NCZ

The need for energy is increasing sharply owing to the rapid increase in the world's population and developing technologies. Solar energy is one of the most important energy sources. Salt-Gradient Solar Pond is a salt-water pond, which is capabale of both absorption and storage solar energy. It is not olny the complementary of the tranditional energy currently, but also one of the main energy soureces in the future.The paper focused on the thermal performance of salt gradient solar pond both experimentally and numerically. The main works and conclusions are summaried as follows:A solar pond with surface area of 2.8×2.3m~2 and depth of 0.8m was constructed. A series of small-scale laboratory and outdoor experiments were carried out. The first experiment in the solar pond was started in August 2006. The maximum temperatue is 45.7℃on the 17th day. Compare to the analogical experiment, this experimental solar pond has favorable thermal performance. In this paper, the solar pond was filled with bittern and seawater, in order to investigate the turbidity evolution in the bittern filled solar pond, two laborlatory experiments were carried out, one is for the regrowth of the aegal in the treated bittern, and another is for the turbidity variation in bittern and salt water after rainfall. The results shows that it controls the growing of the bittern's algae to some extend by using flocculants to reduce the turbidity of the bitten, but after sometime, it is possible that the algae may greatly propagate, and the turbidity of the solar pond greatly increases; the dust can subside quickly when it falls in the pure salt pond, but it subsides slower in bittern pond, so the water of bittern can't be clarified easily.In order to enhance LCZ temperature, according to a series of small-scale experiments, a novel and feasible method to increase LCZ temperature of solar pond was devoloped, and it has been conducted in the experimental solar pond. The studies include three facts: the small-scale experiments, the results show that the porousity of the adding material has a better effect than the solid one, and coal cinder has a better characteristic in this thesis compared to pebble and porous pebble; one dimensional temperature simulation considered the influence of the porosity and thickness of the porous material on the thermal performance, and the result shows that it is benefit for increasing LCZ temperature with a small porosity and thick porous layer; finally, a one month experiment was conducted in the experimental solar pond.Heat extraction and apply to some heat progress is the ultimately aim of building a solar pond. In the adding coal cinder solar pond, a short period experiment of heat extraction from LCZ to heat external fresh water was conducted in August 2007.The result shows that the maxium temperature of the fresh water is 38.4℃on the first day. For the certain solar pond, the paper gives the numerical method to calculate the optimum volume of the external water box.Gradient zone is the main layer of a solar pond. The thermal performance of solar pond is largely depended on the stability of this layer. The analysis of linear stability on the salt gradient layer suggests that the stability of nonconvective layer is not much susceptible to the turbidity of the water, and a turbidy pond tends to a decreasing Ra~c. Heat extraction from LCZ is benifit for the stability of nonconvective layer. Combined the influnce of turbidity and heat extraction, it is indicated that for a good transparency pond, the influence of heat extraction on the marginal stability Raleigh number Ra~c is much obvious than the turbidy pond. Nonlinear osillations cannot be studied analytically and we have studied it using a fourth-order Runge-Kutta-Merson scheme. Two-dimensional field of temperature, salinity and stream function obtained by the numerical study suggests that the marginal stability of Ra~c derived from linear stability analysis properly define the stability of the nonlinear system. The marginal stability Raleigh number Ra~N, which is caculated according to Nusselt number, is a little greater than Ra~c. When salt Raleigh number Rs is in the order of magnitude of 105 and 106, the difference between Ra~N and Ra~c is only 1%.