The second category of marine energy that I'll mention is tidal energy.
我要提到的第二种海洋能源是潮汐能源。
One major advantage of using the tide, rather than waves, as a source of energy is that it's predictable:
利用潮汐能而不是波浪能的一个主要优势是，它的能源的源头可预测性较好：
we know the exact times of high and low tides for years to come.
我们知道接下来很多年的高潮和低潮的时间。
For tidal energy to be effective, the difference between high and low tides needs to be at least five metres,
要让潮汐能有效，高潮和低潮的差值需要至少是五米，
and this occurs naturally in only about forty places on Earth.
而这只会在全球大概四十个地方自然发生。
But the right conditions can be created by constructing a tidal lagoon, an area of sea water separated from the sea.
但是通过建造潮汐泻湖，也就是区别于大海的一个海水区域能够创造合适的条件。
One current plan is to create a tidal lagoon on the coast of Wales.
现在的一个计划是在威尔士的海岸建造潮汐泻湖。
This will be an area of water within a bay at Swansea,
这会建在斯旺西海湾的一块水域
sheltered by a U-shaped breakwater, or dam, built out from the coast.
，用在海岸建造的U形防波堤或者是大坝进行保护。
The breakwater will contain sixteen hydro turbines, and as the tide rises, water rushes through the breakwater,
防波堤会有16个水轮机，当涨潮的时候，水就会冲向防波堤，
activating the turbines, which turn a generator to produce electricity.
激活水轮机，而这会让发动机产生电力。
Then, for three hours as the tide goes out,
然后，在落潮的三个小时，
the water is held back within the breakwater, increasing the difference in water level,
水会退回到防波堤中，这会增加水平面的差值，
until it's several metres higher within the lagoon than in the open sea.
直到泻湖里面的水位比海域的水位高很多米。
Then, in order to release the stored water, gates in the breakwater are opened.
再然后，为了释放存储的水，防波堤的门会打开。
It pours powerfully out of the lagoon, driving the turbines in the breakwater in the opposite direction
它会强有力地冲出泻湖，反方向驱动防波堤的水轮机，
and again generating thousands of megawatts of electricity.
然后再次产生几亿瓦特的电力。
As there are two high tides a day, this lagoon scheme would generate electricity four times a day, every day,
因为每天有两次涨潮，泻湖体系每天能够四次发电，
for a total of around 14 hours in every 24 -- and enough electricity for over 150,000 homes.
这大概占据了每天24小时的14个小时--这能够为超过十五万家庭供给足够的电力。
This system has quite a lot in its favour: unlike solar and wind energy it doesn't depend on the weather;
这个系统有很多优势：不同于太阳和风力资源，它并不受气候所左右；
the turbines are operated without the need for fuel, so it doesn't create any greenhouse gas emissions;
水轮机不需要燃料驱动，所以这并不会产生任何的温室气体排放；
and very little maintenance is needed.
而且不怎么需要维护。
It's estimated that electricity generated in this way will be relatively cheap,
据估计，通过这种方式产生的电力相对来说比较便宜，
and that manufacturing the components would create more than 2,000 jobs, a big boost to the local economy.
生产部件能够创造超过两千个工作岗位，这会极大地促进当地的经济发展。
On the other hand, there are fears that lagoons might harm both fish and birds,
在另一方面，有很多人害怕泻湖可能会危害鱼类和鸟类，
for example by disturbing migration patterns, and causing a build-up of silt, affecting local ecosystems.
比如说会扰乱迁徙路线，导致泥沙堆积，影响当地的生态系统。
There are other forms of tidal energy,
有很多其他形式的潮汐能，
but I'll go on to the third category of marine energy: ocean thermal energy conversion.
但是我会继续介绍第三种海洋能源：海洋热转换能源。
This depends on there being a big difference in temperature between surface water and the water a couple of kilometres below the surface,
这主要依靠的是表面的海水和海面以下几千米水域海水的温度差，
and this occurs in tropical coastal areas.
这一般出现在热带海岸区域。
The idea is to bring cold water up to the surface using a submerged pipe.
主要原理就是使用浸入管将冷水带到海面。
The concept dates back to 1881, when...
这个概念可以追溯到1881年，那时候...