雅思阅读：浅析雅思阅读新话题（二）

　　外延扩大型
　　这种新题是在保留原有核心内容的基础上作一个新的延伸。总的来说，调整不算太大，即在原有的知识基础上再补充一些相关的信息。以环保节能型的新能源开发的话题为例。在07年12月8日出现的英国潮汐发电的文章，主要内容就是讲解潮汐发点的装置及相关内容。但是，在08年9月27日再次考察这一内容的时候，则是以“海洋发电”的形式出现，介绍了四种主要方法的海洋发电的方法。这样一来就要求考生在掌握潮汐发电内容的同时，对于其他海洋发电类型也有所了解才能应付相关的文章。
　　以下是摘自“维京百科”的关于潮汐发电及海洋热能发电的一些背景知识，
　　Tidal power, sometimes called tidal energy, is a form of hydropower that converts the energy of tides into electricity or other useful forms of power.
　　Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Historically, tide mills have been used, both in Europe and on the Atlantic coast of North America. The earliest occurrences date from the Middle Ages, or even from Roman times.
　　Tidal power can be classified into three main types:
　　Tidal stream systems make use of the kinetic energy of moving water to power turbines, in a similar way to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.The largest tidal energy is the bay of Fundy
　　Barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages are essentially dams across the full width of a tidal estuary, and suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.
　　Tidal lagoons, are similar to barrages, but can be constructed as self contained structures, not fully across an estuary, and are claimed to incur much lower cost and impact overall. Furthermore they can be configured to generate continuously which is not the case with barrages.
　　Ocean thermal energy conversion (OTEC or OTE) is hydro energy conversion system which uses the temperature difference that exists between deep and shallow waters to run a heat engine. As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. This temperature difference generally increases with decreasing latitude, i.e. near the equator, in the tropics. Historically, the main technical challenge of OTEC was to generate significant amounts of power, efficiently, from this very small temperature ratio. Changes in efficiency of heat exchange in modern designs allow performance approaching the theoretical maximum efficiency.
　　The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth's surface; this temperature difference contains a vast amount of solar energy which can potentially be harnessed for human use. If this extraction could be made cost effective on a large scale, it could provide a source of renewable energy needed to deal with energy shortages, and other energy problems. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency. Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6 and 7%). Current designs under review will operate closer to the theoretical maximum efficiency. The energy carrier, seawater, is free, although it has an access cost associated with the pumping materials and pump energy costs. Although an OTEC plant operates at a low overall efficiency, it can be configured to operate continuously as a Base load power generation system. Any thorough Cost-benefit analysis should include these factors to provide an accurate assessment of performance, efficiency, operational and construction costs and returns on investment.
　　The concept of a heat engine is very common in thermodynamics engineering, and much of the energy used by humans passes through a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one to the other, the engine converts some of the heat energy to work energy. This principle is used in steam turbines and internal combustion engines, while refrigerators reverse the direction of flow of both the heat and work energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.
　　The only heat cycle suitable for OTEC, is the Rankine cycle, using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use the water heat source as the working fluid.
　　考生在备考过程中应针对一个核心内容尽可能多的去了解相关的外延信息，这样才能保证在考试的时候遇见这样的新题也能够游刃有余。