📚 4.3.4 Electricity Generation and the National Grid | 发电与国家电网
For IGCSE Edexcel Science, understanding how we produce electricity from different energy resources and deliver it to homes and industries is essential. This topic covers the science behind thermal power stations, nuclear reactors, renewable sources, environmental issues, and the operation of the National Grid – including why high voltage transmission is so efficient.
在IGCSE Edexcel 科学课程中，理解如何从不同能源资源产生电力并将其输送到家庭和工业是至关重要的。本主题涵盖热电站、核反应堆、可再生能源背后的科学原理、环境问题以及国家电网的运作——包括为什么高压输电如此高效。
1. Energy Resources and the Demand for Electricity | 能源资源与电力需求
Electricity is a secondary energy source – it must be generated from primary resources such as fossil fuels, nuclear fuel or renewables. The global demand for electricity is growing, and we rely on a mix of resources to meet both baseload (constant demand) and peak demand.
电力是一种二次能源，必须由一次能源（化石燃料、核燃料或可再生能源）产生。全球电力需求持续增长，我们依靠多种资源组合来满足基荷（恒定需求）和峰值需求。
Energy resources can be classified as non-renewable (coal, oil, natural gas, nuclear) or renewable (wind, solar, hydroelectric, tidal, wave, geothermal). Each resource has different implications for the environment, cost and reliability.
能源资源可分为不可再生能源（煤、石油、天然气、核能）和可再生能源（风能、太阳能、水力发电、潮汐能、波浪能、地热能）。每种资源对环境、成本和可靠性的影响各不相同。
2. Fossil Fuels in Thermal Power Stations | 热电站中的化石燃料
In a coal, oil or gas-fired power station, the fossil fuel is burned to release heat energy. This heat is used to produce high-pressure steam, which drives a turbine connected to a generator. The generator transforms kinetic energy into electrical energy via electromagnetic induction.
在燃煤、燃油或燃气发电站中，化石燃料被燃烧释放热能。这些热量用于产生高压蒸汽，驱动连接发电机的涡轮机。发电机通过电磁感应将动能转化为电能。
The energy transfers in a fossil fuel power station can be summarised as: chemical energy of fuel → thermal energy of steam → kinetic energy of turbine → electrical energy. However, a lot of energy is wasted as heat in the exhaust gases and cooling water.
化石燃料发电站的能量转换可概括为：燃料的化学能 → 蒸汽的热能 → 涡轮机的动能 → 电能。然而，大量能量以废气和冷却水的形式浪费掉。
Burning fossil fuels also produces carbon dioxide (a greenhouse gas) and sulfur dioxide (which causes acid rain). Gas-fired stations emit less CO₂ per unit of electricity than coal, but still contribute to climate change.
燃烧化石燃料还会产生二氧化碳（温室气体）和二氧化硫（导致酸雨）。燃气电站每单位电力排放的二氧化碳比燃煤少，但仍会加剧气候变化。
3. Nuclear Power Stations | 核能发电站
Nuclear power stations use nuclear fission of uranium-235 or plutonium-239 to generate heat. The reactor core releases a huge amount of thermal energy, which is used to make steam and drive a turbine–generator set, just like in a fossil fuel plant. The key difference is the source of heat.
核电站利用铀-235或钚-239的核裂变产生热量。反应堆堆芯释放巨大的热能，用来产生蒸汽并驱动涡轮发电机组，与化石燃料电厂类似。关键区别在于热源。
The energy chain is: nuclear energy stored in atomic nuclei → thermal energy of coolant → kinetic energy of turbine → electrical energy. No carbon dioxide or sulfur dioxide is produced during operation, but nuclear waste is radioactive and must be stored safely for many years.
能量链为：原子核储存的核能 → 冷却剂的热能 → 涡轮机的动能 → 电能。运行过程中不会产生二氧化碳或二氧化硫，但核废料具有放射性，必须安全储存多年。
Nuclear fuel has a very high energy density: 1 kg of uranium can produce the same energy as thousands of tonnes of coal. However, the risk of accidents, high decommissioning costs and long-lived radioactive waste are major disadvantages.
核燃料的能量密度非常高：1千克铀可以产生相当于数千吨煤的能量。但事故风险、高昂的退役成本和长寿命放射性废物是主要的缺点。
4. Renewable Energy: Wind and Solar Power | 可再生能源：风能与太阳能
Wind turbines use the kinetic energy of moving air to turn blades and spin a generator. The energy transfer is kinetic energy of wind → kinetic energy of blades → electrical energy. No fuel is burned, so there are no greenhouse gas emissions during operation.
风力涡轮机利用流动空气的动能转动叶片并带动发电机。能量转换是：风的动能 → 叶片的动能 → 电能。不燃烧燃料，因此运行中不排放温室气体。
Solar photovoltaic (PV) cells convert light energy directly into electrical energy. They contain semiconductors (like silicon) that generate a direct current when sunlight photons excite electrons. Solar thermal power stations use mirrors to focus sunlight and produce steam for a turbine.
太阳能光伏电池将光能直接转化为电能。它们含有半导体（如硅），当阳光光子激发电子时会产生直流电。太阳能热电站使用镜子聚焦阳光产生蒸汽驱动涡轮机。
Wind and solar are intermittent – they depend on weather. To match demand, they often need backup from reliable sources or energy storage systems. Despite this, their environmental footprint is low once installed.
风能和太阳能是间歇性的，取决于天气。为了匹配用电需求，通常需要可靠的备用电源或储能系统。尽管如此，一旦安装，它们的环境足迹很小。
5. Renewable Energy: Hydroelectric and Tidal Power | 可再生能源：水力与潮汐能
Hydroelectric power stations use water stored behind a dam. The water has gravitational potential energy; when released, it flows downhill and transfers this to kinetic energy, driving a turbine and generator. The energy transfer is: gravitational PE of water → kinetic energy → electrical energy.
水力发电站利用大坝后储存的水。水具有重力势能；释放时水流向低处，将势能转化为动能，驱动涡轮机和发电机。能量转换：水的重力势能 → 动能 → 电能。
Hydroelectricity is reliable and can be used for baseload or peak shaving. However, building dams floods valleys, displaces communities and alters river ecosystems. Silt retention behind dams can also reduce farmland fertility downstream.
水力发电可靠，可用于基荷或调峰。然而，修建大坝会淹没山谷、迫使社区搬迁并改变河流生态系统。大坝后方的淤泥淤积还会降低下游农田的肥力。
Tidal power uses the gravitational pull of the moon and sun. Tidal barrages trap water at high tide and release it through turbines at low tide, much like a dam. Tidal stream generators capture kinetic energy from moving water in a similar way to wind turbines.
潮汐能利用月球和太阳的引力。潮汐拦河坝在高潮时拦截水流，在低潮时通过涡轮机释放，类似于大坝。潮汐流发电机以类似于风力涡轮机的方式捕获流水的动能。
Wave power devices are another form of marine energy, using the up-and-down motion of waves to drive generators. All these energy resources produce no CO₂ during operation, but their construction can have environmental impacts.
波浪能装置是另一种海洋能形式，利用波浪的上下运动驱动发电机。所有这些能源在运行过程中均不产生二氧化碳，但其建设可能对环境造成影响。
6. Environmental and Socio-economic Considerations | 环境与社会经济考虑
When choosing energy resources, we must weigh factors such as carbon emissions, pollution, fuel costs, decommissioning costs, reliability and visual impact. The table below compares these aspects for common resources used in large-scale electricity generation.
选择能源资源时，必须权衡碳排放、污染、燃料成本、退役成本、可靠性和视觉影响等因素。下表比较了用于大规模发电的常见资源在这些方面的差异。
| Resource | CO₂ emission | Reliability | Cost |
| Coal | Very high | High | Low fuel cost |
| Natural gas | Medium | High | Moderate |
| Nuclear | None (operation) | High | High build/decommission |
| Wind | None | Low (intermittent) | Moderate (falls after installation) |
| Solar PV | None | Low (daylight only) | Moderate (falling) |
In addition, fossil fuels are finite and will eventually run out, while renewables are essentially inexhaustible. The use of sustainable resources also helps to reduce the greenhouse effect and limit global warming.
此外，化石燃料是有限的，终将耗尽，而可再生能源基本是取之不尽的。使用可持续资源也有助于减少温室效应并控制全球变暖。
7. The National Grid: Distributing Electricity | 国家电网：电力分配
The National Grid is a network of power lines and substations that carries electricity from power stations to consumers. It connects many different generators and allows regions to share power, matching supply and demand across the country.
国家电网是一个由输电线路和变电站组成的网络，将电力从发电站输送到用户。它连接众多不同的发电机组，使各地区共享电力，在全国范围内匹配供需。
Electricity is generated at a relatively low voltage (typically 25 kV in large power stations). To transmit it over long distances efficiently, step-up transformers increase the voltage to 275 kV or 400 kV (sometimes even 765 kV).
电力在大电站中以相对较低的电压（通常25千伏）产生。为了长距离高效输送，升压变压器将电压升高至275千伏或400千伏（有时甚至765千伏）。
Near cities and consumers, step-down transformers reduce the voltage in stages to safer levels: first to 33 kV for heavy industry, then to 230 V (UK) for households. This ensures safety and compatibility with appliances.
在城市和用户附近，降压变压器逐级将电压降低到更安全的水平：先降至33千伏供重工业使用，再降至230伏（英国）供家庭使用。这确保了安全性以及与电器的兼容性。
8. Transformers and High-Voltage Transmission | 变压器与高压输电
A transformer is a device that changes the voltage of an alternating current (a.c.). It consists of two coils wound around a common iron core. When a.c. flows in the primary coil, it creates a changing magnetic field, which induces a voltage in the secondary coil.
变压器是一种改变交流电电压的装置。它由两根绕在同一铁心上的线圈组成。当初级线圈中有交流电时，它产生变化的磁场，从而在次级线圈中感应出电压。
The relationship between primary voltage (Vₚ), secondary voltage (Vₛ), primary turns (Nₚ) and secondary turns (Nₛ) is: Vₚ / Vₛ = Nₚ / Nₛ. In a step-up transformer, Nₛ > Nₚ so Vₛ > Vₚ; in a step-down transformer, Nₛ < Nₚ so Vₛ < Vₚ.
初级电压（Vₚ）、次级电压（Vₛ）、初级匝数（Nₚ）和次级匝数（Nₛ）之间的关系为：Vₚ / Vₛ = Nₚ / Nₛ。在升压变压器中，Nₛ > Nₚ，所以Vₛ > Vₚ；在降压变压器中，Nₛ < Nₚ，所以Vₛ < Vₚ。
Transformers are nearly 100% efficient; the power in the primary coil roughly equals the power in the secondary coil: Pₚ = Pₛ, or VₚIₚ = VₛIₛ. This means that when voltage is stepped up, current is stepped down proportionally.
变压器效率接近100%；初级线圈的功率大致等于次级线圈的功率：Pₚ = Pₛ，即 VₚIₚ = VₛIₛ。这意味着当电压升高时，电流会按比例降低。
9. Why High Voltage Reduces Energy Losses | 为什么高电压能减少能量损失
When electric current flows through a transmission cable with resistance R, it produces heat according to the formula P_loss = I²R. Reducing the current dramatically decreases the power wasted as heat, even though resistance remains the same.
当电流流过有电阻R的传输电缆时，根据公式 P_loss = I²R 产生热量。即使电阻不变，减小电流也能显著降低以热量形式浪费的功率。
For a given power demand P = IV, if the voltage is increased 10 times, the current becomes one-tenth, and the resistive losses fall to (1/10)² = 1/100 of the original value. This is why the National Grid uses high voltages for long-distance transmission.
对于给定的功率需求 P = IV，如果电压升高10倍，电流变为十分之一，电阻损耗会降至原来的 (1/10)² = 1/100。这就是国家电网采用高压进行长距离输电的原因。
Step-down transformers near the end user bring the voltage back to safe, usable levels. The entire system minimises energy waste while ensuring a stable supply. Typical overall grid losses in developed countries are around 6–8%.
靠近用户的降压变压器将电压恢复到安全、可用的水平。整个系统最大限度地减少了能源浪费，同时确保稳定供电。发达国家的典型电网总损耗约为6%到8%。
10. Key Equations and Calculation Examples | 关键公式与计算示例
Two equations are fundamental to this topic: electrical power P = IV and heat loss in cables P_loss = I²R. For example, if a power station generates 500 MW at 25 kV, the current I = P/V = 500,000,000 W ÷ 25,000 V = 20,000 A. If this current passed through a cable of resistance 0.5 Ω, the power lost would be (20,000)² × 0.5 = 200 MW – an enormous 40% loss.
两个公式是本主题的基础：电功率 P = IV 和电缆热损耗 P_loss = I²R。例如，如果一个电站以25千伏发出500兆瓦，电流 I = P/V = 500,000,000 W ÷ 25,000 V = 20,000 A。如果该电流通过电阻为0.5 Ω的电缆，功率损失为 (20,000)² × 0.5 = 200兆瓦，高达40%的损失。
If the same power is transmitted at 400 kV, the current falls to 500,000,000 ÷ 400,000 = 1,250 A. The loss becomes (1,250)² × 0.5 ≈ 0.78 MW, which is negligible relative to 500 MW. This clearly illustrates why high-voltage transmission is essential.
如果同样的功率以400千伏传输，电流降至500,000,000 ÷ 400,000 = 1,250 A。损失变为 (1,250)² × 0.5 ≈ 0.78兆瓦，相对于500兆瓦可忽略不计。这清楚地说明了高压输电为何至关重要。
When solving problems, remember that transformers maintain power (ideally VₚIₚ = VₛIₛ) and that resistive loss depends on the square of the current. Always check units – convert MW to W and kV to V before calculation.
解题时，要记住变压器维持功率不变（理想情况下VₚIₚ = VₛIₛ），而且电阻损耗取决于电流的平方。务必检查单位，计算前将兆瓦转换为瓦，千伏转换为伏。
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