The working theory of a hydropower station built on a river is demonstrated by this flow chart. The operating mode of it varies from the daytime to the night.（开头段先是改写题目，再一句点出本题其实介绍的是白天黑夜两个不同的工作原理。）
By daylight, water stored in a high-level reservoir initially flows downstream through the intake under the dam, and then drives turbines, and finally arrives at a low-level reservoir. These turbines, powered by the water flow, can correspondingly operate agenerator in the power station. The electricity produced, in the last step, will be transferred to the National grid through power lines.（主一段着重于白天工作原理的描写。）
By night, water in the low-level reservoir can be pumped upstream to the high-level reservoir through the same riverway by turbines designed to be able to operate both clockwise and anticlockwise. In thisprocess, these turbines spin in the opposite direction as the daytime. The intake under the dam is a non-return one, which is closed at night in order to ensure that water can flow into the high-level reservoir, but not out.（主二段自然是聚焦于夜晚工作原理的描写。本段需要注意的是“反转的涡轮”和“闭合的intake”。这些都是图中细节，不应该忽视。）
To sum up, the power station functions in totally opposite ways in the daytime and at night. In the whole process, water is utilized as the energy carrier, which at first releases its gravitational energy to produce electricity and then helps transform electricity into gravitational energy. （结尾段总括全文，揭露出这个工作原理的本质。）
The working theory of utilizing geothermal energy to produce electricity is demonstrated by this flow chart.
Initially,cold water will be pumped into the injection well, which is underground at the depth of around 4.5 kilometers. After being injected, cold water will flow through the geothermal zone, where it will be heated by hot rocks. After becoming hot water, it will be kept in the production well. Thereafter, through pipes, the hot water will be pumped again up to a condenser on the ground.
Then,the hot water in this condenser will emit steam with high temperature. Being powered by the steam, the turbine begins spinning.Subsequently, the generator will be driven by the spinning turbine,thus producing electricity. Thereafter, the electricity from the generator will be transported to the grid for all users.
To sum up, the flow chart reveals how geothermal energy is used for generating electricity in five sequential stages. During the whole process, cold water will be heated and release steam which is able to power the generator. This is how the whole process functions.