Solar ponds are bodies of water in which circulation is incomplete and there is a very high salt concentration that increases with depth. This vertical change in salinity serves to trap heat because concentrated brine in the lowest water level acts as a collector and storage area for solar heat, while the less saline, lighter water at the upper levels provides insulation. Heat is thus retained in the depths.

An artificial pond of this type has been constructed on the western shore of the Dead Sea in Israel in order to test its suitability as a source of low-grade heat for conversion into electricity. An immediate threat to the success of the venture was the growth of algae. Water in solar ponds must be kept maximally transparent to allow penetration of light to the deep storage area. Therefore, any particles of matter in the water, such as algae cells, that scatter or absorb light will interfere with the collection of heat.

One proposed method of controlling the algae was the application of an algicide. However, the Dead Sea is a closed body of water without any outlet and as such is very easily contaminated. Extensive use of chemicals in numerous future full-scale solar ponds would lead to such contamination of the Dead Sea, which now enjoys a lucrative tourist trade.

A recent experiment has supplied a more promising method for controlling the algae. To repress the algae cells' capacity for accommodating themselves to environmental changes, the water in the solar pond was first made more saline through evaporation and then diluted by a rapid inflow of fresh water. This shock reduced the cells' ability to regulate the movement of water through their membranes. They rapidly absorbed water, resulting in distortions of shape, increase in volume, and impairment to motility. Their buoyancy adversely affected, the cells sank to the bottom of the pond, where they encountered the hot waters of the storage layer and were destroyed. This method allows for effective control of nuisance algae while leaving solar ponds as one of the cleanest technologies providing energy for human use.

The primary purpose of the passage is to

discuss ways of solving a problem that threatens to limit the usefulness of an energy source

explain the mechanisms by which solar heat may be converted into energy

detail the processes by which algae cells colonize highly saline bodies of water

report the results of an experiment designed to clean contaminated bodies of water

describe the unique properties of a solar pond on the edge of the Dead Sea






主旨(Main idea)   



A选项:Correct. 讨论解决威胁限制能源来源的方法。其实“考点“中讲的已经比较明确了。作者在第一段解释什么是太阳能湖,第二段提出问题可能会影响太阳能湖的发展,后面讨论怎么解决问题。所以整篇文章集中的论点在于讨论怎么解决太阳能湖的藻类问题。






登录注册 后可以参加讨论