Experimental study on achieving salt from seawater by. using solar chimney and porous materials

Yiping Wang*, Lin Chen, Li Zhu, Zhiyong Yang, Lei Zhang (School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,China) Tel.:+86-22-27404674, Fax:+86-22-27404771,E-mail: xinxing@tju. edu. cn Abstract: In this paper, a technique to achieve salt from seawater by using a solar chimney combined with porous materials was introduced. The solar chimney could enhance the ventilation above the seawater surface, and the greenhouse effect of the solar collector could increase the seawater temperature, both of which could promote significantly the mass transfer during the seawater evaporation. When one side of porous materials was dipped into the seawater, the seawater rose along the materials based on the capillary phenomena, which increased the evaporation surface and decreased obviously the occupied area. In this paper, wind tunnel was used to simulate the situation in the solar chimney dryer. Effects of experimental conditions, such as wind velocity and water temperature on the evaporation of seawater were studied preliminarily. Additionally, influences of the chimney height and the diameter of the solar collector were investigated on the drying process. Experimental results show that the new technique is more efficient than the traditional method.

Key words:

Solar chimney, porous material, evaporation, capillary phenomena

1. Introduction

Solar energy is a renewable and clean energy resource. It sets off no greenhouse effect gases and harmful wastes when it is utilized. Among many solar energy techniques, solar chimney seems to be one of the most attractive. The thought of "solar chimney” was carried out by a Germany professor, named J Schlain, in 1978. In the following days, this technique was used mainly in the fields of electricity generation and improvement of indoor air quality.

The Germany government and a Spain electricity factory set up the first solar chimney electricity generator factory at Manzanares (Spain) in 1982[1].Being the main component, the solar chimney was 200m in height, 10.3m in diameter. And the solar collector was 250m in diameter. The power output was 100KW in the daytime (Revolution: 1500rpm) and 40KW in the nighttime (Revolution: 1000rpm). The other solar chimney electricity factory, which had a solar chimney of 900m high, was brought into the plan of Spain Government. The solar chimney was also used for electricity generation by America[2-3], Turkey[4], Canada[5], China[6] and Germany[7].

The solar chimney technique can lower the costs of exchange air between indoor and outdoor, and it is much cheaper than traditional methods. So the use of solar energy, namely through solar chimney, could improve indoor air quality by increasing ventilation in the building, which was benefit for human health[8] This method was used by Italy[9], etc.

Besides the utilizations mentioned above, there is another important usage of solar chimney, which is drying process. Drying is an important step in many industrial fields, such as wood[10], fruit[11],medicine[12], and so on. And many materials needed drying for usage or storage. Compared with other drying technique, the solar chimney technique could quicken

this process. This technique used solar chimney combined with porous materials to accelerate the drying process. And we choose seawater as an example. Salt is necessary of our day life. Traditionally, the seawater was disposed directly to the sunshine to achieve the salt. Although the costs were low, it wasted a lot of time. So we used solar chimney combined with porous materials to quicken this process. We sought to reduce thermal energy required to dry seawater by controlling airflow velocity, temperature and evaporation area. Fig.1 shows the model of solar chimney dryer. The chimney was surrounded by a solar collector and the air flowed through the open rim of the collector and up the chimney. The solar collector increased the air temperature, and then the heated air went upward in the solar chimney. The porous materials increased the evaporation area.

In this paper, we set up wind tunnel equipment to simulate the situation of seawater in the solar collector with a solar chimney. By using the wind tunnel, the airflow, which was affected by environment to be out-of-order was accelerated and directed.

2. Experiment

Fig.2 shows the model of wind tunnel. The aperture of the hole in the flow-meter was

0. 05m, and the cross-sectional area facing the airflow is 0.125×0.180mm2. Seawater used in this paper is achieved from Bohai Bay (East of Tianjin, P. R.CHINA), and the two types of porous materials are respectively gauze and non-woven fabric band.

During the experiments, seawater is transfused into a round plastic plate having the diameter of 100mm to be placed in the wind tunnel. The situation in the wind tunnel is similar with that of solar collector. Since in the solar chimney dryer, the temperature and airflow velocity varies with the height, diameter and type of solar chimney and also with the area, material and shape of solar collector, we change the temperature and airflow velocity in the wind tunnel to investigate the evaporation efficiency of seawater.

The airflow in the wind tunnel is generated by a blower fan. Being heated by the electric heater located in the entrance of the tunnel, the airflow temperatures can be changed. There are two papilionaceous valves, which are respectively in the end of tunnel and in the end of blower fan pipe to the air to control the airflow velocity. During the experiments, the temperature of environment is 20 ‘C and at the same time the thermometer in dew point has a display of 14.1 C.