📚 Energy Sources for AQA International A-level Physics | AQA 国际 A-level 物理：能量来源
Energy supply is a central topic in AQA International A-level Physics because every power station, solar panel or wind turbine is a physical system that converts one store of energy into electrical energy. This article reviews the main energy sources, the calculations used to compare them, and the environmental and economic factors that influence energy choices. It also covers the definitions, unit conversions and worked examples that examiners reward most often.
能源供应是 AQA 国际 A-level 物理的核心主题之一，因为每一座发电站、每一块太阳能电池板或每一台风力机，本质上都是将一种能量储存转化为电能的物理系统。本文梳理主要能量来源、用于比较它们的计算方法，以及影响能源选择的环境与经济因素，同时归纳考官最看重的定义、单位换算与典型例题。
1. Energy demand and primary energy sources | 能源需求与一次能源
Primary energy sources are natural stores such as coal, oil, natural gas, uranium, sunlight, wind and water that have not yet been converted for human use. A power station converts a primary source into electricity, which is a secondary energy carrier because it must be generated from another source. Electricity is convenient but cannot be mined or pumped directly from the ground.
一次能源是指尚未被人类转化使用的天然储存，例如煤、石油、天然气、铀、太阳光、风和水。发电站将一次能源转化为电能，而电能属于二次能源载体，因为它必须由其他能源产生。电能使用方便，但它无法直接从地下开采或抽取出来。
Global energy demand is usually quoted in terawatt-hours per year (TW h y⁻¹) or exajoules per year (EJ y⁻¹). The power output of any source is given by P = E / t a power of 1.0 × 10¹⁰ W, i.e. 10 GW, which is roughly the output of a large power station.
全球能源需求通常以每年太瓦时（TW h y⁻¹）或每年艾焦耳（EJ y⁻¹）计量。任何能源输出的功率均为 P = E / t。因此若某个能源在一小时内提供 3.6 × 10¹³ J 的能量，其输出功率为 1.0 × 10¹⁰ W，即 10 GW，约相当于一座大型发电站的功率。
P = E / t
0¹⁸ J. Many exam errors come from mixing up energy in joules with power in watts, so always write the unit next to every number.
必须记住的换算关系：1 kW h = 3.6 × 10⁶ J，1 TW h = 3.6 × 10¹⁵ J，1 EJ = 10¹⁸ J。许多考试失分源于把能量的焦耳与功率的瓦特混淆，因此每个数字后面都要写上单位。
2. Renewable and non-renewable sources | 可再生与不可再生能源
A renewable source is one that is replenished on a timescale comparable to or shorter than the rate at which it is used. Solar, wind, hydroelectric, geothermal, tidal and sustainably managed biofuel are usually classed as renewable. Fossil fuels and nuclear uranium are non-renewable because they take millions of years to form or are finite on human timescales.
可再生能源是指在与其使用速率相当或更短的时间尺度内可以得到补充的能源。太阳能、风能、水力、地热、潮汐能和可持续管理的生物燃料通常归为可再生能源。化石燃料和核燃料铀属于不可再生能源，因为它们的形成需要数百万年，或在人类时间尺度上是有限的。
- Renewable and continuous: solar, wind, tidal, wave, geothermal, hydro. | 可再生且持续：太阳能、风能、潮汐能、波浪能、地热能、水能。
- Renewable but limited by land or processing: biomass and biofuels. | 可再生但受土地或加工限制：生物质能与生物燃料。
- Non-renewable: coal, oil, natural gas, uranium. | 不可再生：煤、石油、天然气、铀。
Note that renewable does not automatically mean clean or reliable. A large hydroelectric reservoir floods land and can release methane, and a wind farm produces no electricity on a still day. Renewable means only that the input is replenished; it says nothing about intermittency or environmental impact.
要注意，可再生并不自动等于清洁或可靠。大型水库会淹没土地并可能释放甲烷，而在无风的日子里风电场不发电。"可再生"只说明输入能源会得到补充，并不说明它是否间歇、是否环保。
3. Fossil fuels: coal, oil and natural gas | 化石燃料：煤、石油与天然气
tore of steam → kinetic store of the turbine → electrical energy. Pulverised coal or gas heats water in a boiler; the high-pressure steam drives a turbine which turns a generator. In a combined cycle gas turbine (CCGT) plant the hot exhaust gases also drive a steam cycle, raising the overall efficiency.
化石燃料发电站通过一系列能量转移，把燃料的化学能转化为电能：燃料的化学储存 → 蒸汽的热储存 → 汽轮机的动能储存 → 电能。煤粉或天然气在锅炉中加热水，高压蒸汽推动汽轮机，汽轮机带动发电机。在联合循环燃气轮机（CCGT）电厂中，高温废气还要再驱动一个蒸汽循环，从而提高整体效率。
Typical efficiencies are 33% to 40% for coal and older oil plants, and up to about 60% for modern CCGT plant. Advantages include high power density, output that can be ramped up on demand (dispatchability) and low fuel cost per kW h. Disadvantages include carbon dioxide emissions, sulfur dioxide and nitrogen oxides that cause acid rain, particulates, mining damage and the fact that reserves are finite.
煤和较老燃油电厂的典型效率为 33% 至 40%，而现代联合循环燃气电厂可达到约 60%。其优点包括功率密度高、可按需快速调节输出（可调度性）以及每千瓦时燃料成本低。缺点包括二氧化碳排放、造成酸雨的二氧化硫和氮氧化物、颗粒物污染、采矿对环境的破坏，以及储量有限。
Efficiency = (useful energy output ÷ total energy input) × 100%
4. Nuclear fission power | 核裂变发电
aller nuclei plus two or three further neutrons, releasing about 200 MeV (≈ 3.2 × 10⁻¹¹ J) per fission. The energy comes from the mass defect and is predicted by E = Δm c². The released energy heats a coolant, which raises steam to drive the same type of turbine and generator used in fossil fuel plant.
核电站利用铀-235 的裂变。一个慢中子被 ²³⁵₉₂U 原子核吸收后，核变得不稳定并分裂成两个较小的核，同时放出两到三个中子，每次裂变释放约 200 MeV（≈ 3.2 × 10⁻¹¹ J）的能量。这部分能量来自质量亏损，可由 E = Δm c² 计算。释放的能量加热冷却剂，产生蒸汽推动与化石燃料电厂相同类型的汽轮机和发电机。
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n
nt to about 3000 tonnes of coal), reliable baseload output, small fuel volume. Disadvantages: highly radioactive waste requiring long-term storage, very high capital cost and long build times, risk of accident and meltdown, thermal pollution of cooling water, public opposition, and decommissioning costs.
优点：运行过程中不产生二氧化碳；能量密度极高（1 kg 铀-235 约释放 8 × 10¹³ J，相当于约 3000 吨煤）；可稳定提供基荷电力；燃料体积小。缺点：产生需要长期储存的高放射性废物；建设成本极高、周期很长；存在事故与堆芯熔毁风险；冷却水造成热污染；公众反对；退役成本高昂。
Uranium is classed as non-renewable, although breeder reactors can extend the fuel supply by converting fertile ²³⁸₉₂U into fissile plutonium. In the exam, always state that fission is non-renewable on a human timescale.
铀被归为不可再生能源，尽管增殖反应堆可以通过把可增殖的 ²³⁸₉₂U 转化为易裂变的钚来延长燃料供应。在考试中应指出：在人类时间尺度上，核裂变属于不可再生能源。
5. Solar energy: photovoltaic and thermal | 太阳能：光伏与光热
Photovoltaic (PV) cells use the photoelectric effect in a semiconductor junction. Photons with energy greater than the band gap excite electrons across the junction, producing a potential difference and a direct current, which an inverter converts to alternating current. Commercial panels reach 15% to 22% efficiency, while laboratory multi-junction cells exceed 40%. The solar constant at the top of the atmosphere is about 1.36 kW m⁻², and the irradiance at the Earth's surface at noon on a clear day is roughly 1.0 kW m⁻².
光伏（PV）电池利用半导体结中的光电效应。能量大于禁带宽度的光子把电子激发越过结区，产生电势差和直流电，再由逆变器转换为交流电。商用组件效率为 15% 至 22%，实验室多结电池可超过 40%。大气层顶的太阳常数约为 1.36 kW m⁻²，晴天正午地球表面的辐照度约为 1.0 kW m⁻²。
Solar thermal systems absorb radiation to heat water directly, while concentrating solar power (CSP) uses mirrors to heat molten salt, which can store heat and generate electricity after sunset. Advantages: no fuel, no emissions in operation, modular and silent, low maintenance, ideal for remote locations. Disadvantages: intermittent and weather dependent, capacity factor typically only 10% to 25%, large land area, high manufacturing cost, and battery or thermal storage needed for night-time supply.
光热系统直接吸收辐射来加热水，而聚光太阳能发电（CSP）用反射镜加热熔盐，可以储存热量并在日落后发电。优点：无需燃料；运行时不排放污染物；模块化、无噪声；维护简单；适合偏远地区。缺点：间歇且依赖天气；容量因子通常只有 10% 至 25%；占用土地面积大；制造成本高；夜间供电需要电池或储热装置。
Efficiency = (electrical power output ÷ (irradiance × area)) × 100%
6. Wind power | 风能
A wind turbine extracts kinetic energy from moving air. The available power in the wind is proportional to the cube of the wind speed: P = ½ ρ A v³, where ρ is air density, A is the swept area and v is the wind speed. Doubling the wind speed therefore increases the available power eight times, which is why wind farms are sited on exposed coasts and hills.
风力机从流动的空气中提取动能。风中的可用功率与风速的三次方成正比：P = ½ ρ A v³，其中 ρ 为空气密度，A 为扫风面积，v 为风速。因此风速加倍，可用功率增大到八倍，这就是风电场多建在开阔海岸和山丘上的原因。
P = ½ ρ A v³
Because the air leaving the rotor still has to move away, no turbine can extract all of this power. The Betz limit states that the maximum theoretical fraction is 16/27, about 59.3%; real turbines achieve 40% to 45%. A modern turbine starts generating at a cut-in speed of about 3 to 4 m s⁻¹, reaches rated power at 12 to 14 m s⁻¹ and shuts down above a cut-out speed of about 25 m s⁻¹ for safety.
由于离开叶轮的空气仍必须流走，任何风机都不可能提取全部功率。贝兹极限指出理论最大比例为 16/27，约 59.3%；实际风机可达 40% 至 45%。现代风机在约 3 至 4 m s⁻¹ 的切入风速开始发电，在 12 至 14 m s⁻¹ 达到额定功率，超过约 25 m s⁻¹ 的切出风速时会为安全而停机。
Advantages: no fuel, no carbon dioxide during operation, cheap electricity once built, land between turbines can still be farmed. Disadvantages: intermittent and unpredictable over short timescales, noise and visual pollution, danger to birds and bats, high capital cost, and the need for back-up generation or storage.
优点：无需燃料；运行时无二氧化碳排放；建成后电价低廉；风机之间的土地仍可用于耕作。缺点：在短时间尺度上间歇且难以预测；噪声与视觉污染；对鸟类和蝙蝠有危害；建设成本高；需要备用电源或储能。
7. Hydroelectric power | 水力发电
Hydroelectric schemes store water at height behind a dam. The gravitational potential energy of the water is converted to kinetic energy as it falls through penstocks, and the turbine and generator convert this to electrical energy. The energy released per kilogram is g Δh, so for a volumetric flow rate Q the available power is P = ρ Q g Δh. Efficiencies of 85% to 90% make hydro the most efficient of the common large-scale sources.
水力发电工程把水蓄在坝后的高处。水经压力管道下落时，重力势能转化为动能，再由水轮机和发电机转化为电能。每千克水释放的能量为 g Δh，因此体积流量为 Q 时的可用功率为 P = ρ Q g Δh。其效率达 85% 至 90%，是常见大规模能源中效率最高的一种。
P = ρ Q g Δh
Worked example: a scheme with Q = 100 m³ s⁻¹, Δh = 50 m, ρ = 1000 kg m⁻³ and g = 9.81 N kg⁻¹ gives P = 1000 × 100 × 9.81 × 50 = 4.9 × 10⁷ W, i.e. 49 MW. If the overall efficiency is 90%, the electrical output is 0.90 × 49 ≈ 44 MW.
例题：某工程 Q = 100 m³ s⁻¹，Δh = 50 m，ρ = 1000 kg m⁻³，g = 9.81 N kg⁻¹，则 P = 1000 × 100 × 9.81 × 50 = 4.9 × 10⁷ W，即 49 MW。若总效率为 90%，则电输出为 0.90 × 49 ≈ 44 MW。
Advantages: dispatchable within seconds, ideal for meeting sudden peaks, long service life, no fuel and no direct emissions, and pumped storage can store surplus energy from other sources. Disadvantages: very high capital cost, flooding of valleys and displacement of communities, loss of habitat, methane and carbon dioxide from decaying vegetation in reservoirs, and dependence on reliable rainfall.
优点：可在数秒内调节输出，非常适合满足尖峰负荷；使用寿命长；无燃料、无直接排放；抽水蓄能还能储存其他能源的富余电量。缺点：建设成本极高；淹没河谷、迫使居民搬迁；破坏栖息地；水库中植被腐烂产生甲烷和二氧化碳；依赖可靠的降水量。
8. Tidal, wave and geothermal energy | 潮汐、波浪与地热能
Tidal barrages use the potential energy of water trapped in an estuary. As the tide rises and falls twice a day, water is allowed through turbines, giving a highly predictable output. Tidal stream devices instead use the kinetic energy of fast-moving currents. Wave power devices such as oscillating water columns use the rise and fall of waves to push air through a turbine. Geothermal plant taps hot rocks, using dry steam, flash steam or a binary cycle with a low-boiling-point fluid.
潮汐坝利用河口中被拦蓄水的重力势能。潮水一天涨落两次，让水通过水轮机发电，输出非常可预测。潮流装置则利用快速流动海流的动能。波浪能装置（如振荡水柱式）利用波浪的升降把空气推过汽轮机。地热电站则开采高温岩层，采用干蒸汽、闪蒸或使用低沸点流体的双循环方式。
- Tidal: predictable and long-lived, but very few suitable estuaries, huge civil engineering cost, and serious effects on intertidal ecosystems. | 潮汐能：可预测、寿命长，但适合的河口极少，土木工程成本巨大，且严重干扰潮间带生态系统。
- Wave: large theoretical resource, but devices must survive storms and salt water, so it remains the least developed option. | 波浪能：理论资源丰富，但装置必须经受风暴和海水腐蚀，因此仍是开发程度最低的选项。
- Geothermal: provides constant baseload power independent of weather, but is site specific, releases some hydrogen sulfide, and has high drilling costs. | 地热能：不受天气影响、可稳定提供基荷电力，但受地点限制，会释放少量硫化氢，钻井成本高。
In a comparison question, emphasise that tidal and geothermal output is predictable or constant, whereas solar and wind are intermittent, and that predictability is valuable for matching supply to demand on the national grid.
在比较类题目中应强调：潮汐和地热的输出可预测或恒定，而太阳能和风能是间歇性的；可预测性对电网中供需匹配具有重要价值。
9. Biofuels and biomass | 生物燃料与生物质能
Biomass such as wood, straw, crop residues and animal waste can be burned directly, or processed by anaerobic digestion to give biogas that is mainly methane, or fermented to give ethanol and biodiesel. In principle biomass is carbon neutral because the carbon dioxide released during combustion was absorbed by the plant during photosynthesis. In practice the balance depends on replanting, fertiliser use, harvesting and transport.
木材、秸秆、作物残余和动物粪便等生物质可以直接燃烧，也可以通过厌氧消化产生以甲烷为主的沼气，或经发酵制得乙醇和生物柴油。从原理上讲，生物质是碳中性的，因为燃烧释放的二氧化碳在植物光合作用时已被吸收。实际上，这一平衡取决于是否重新种植，以及化肥使用、收割和运输带来的排放。
Advantages: renewable if managed sustainably, uses waste material, can be stored and burned on demand, and provides a use for agricultural residues. Disadvantages: low power density requiring large areas, competition with food crops for land and water, deforestation when poorly regulated, air pollution from combustion, and emissions of methane during digestion if not captured.
优点：若管理可持续则属可再生；可利用废弃物；可储存并按需燃烧；为农业残余物提供用途。缺点：功率密度低、占用大量土地；与粮食作物争夺土地和水资源；监管不当时导致森林砍伐；燃烧造成空气污染；若未收集，消化过程会排放甲烷。
10. Comparing sources: power, efficiency and capacity factor | 比较各能源：功率、效率与容量因子
Comparing energy sources requires more than one number. Rated power tells you the maximum output, efficiency tells you what fraction of the input becomes electricity, and the capacity factor tells you how much of that rated power is actually delivered on average over a year. The capacity factor is defined as the actual energy generated divided by the energy the plant would generate at full rated power.
比较能源来源不能只看一个数字。额定功率表示最大输出；效率表示输入能量中有多大比例变成电能；容量因子表示一年中实际平均输出占额定功率的比例。容量因子定义为实际发电量与满额定功率运行时的发电量之比。
Capacity factor = actual energy generated ÷ (rated power × time)
| Source | 能源 | Typical efficiency | 典型效率 | Capacity factor | 容量因子 | Key property | 关键特性 |
| Coal / 煤 | 33% - 40% | 50% - 70% | Dispatchable, high CO₂ / 可调度，高碳排放 |
| Gas CCGT / 燃气联合循环 | up to 60% | 40% - 60% | Flexible, lower CO₂ than coal / 灵活，排放低于煤 |
| Nuclear / 核能 | 33% - 40% | 80% - 90% | Baseload, radioactive waste / 基荷，放射性废物 |
| Hydro / 水能 | 85% - 90% | 40% - 60% | Dispatchable, site limited / 可调度，地点受限 |
| Wind / 风能 | 40% - 45% | 25% - 40% | Intermittent, cheap running cost / 间歇，运行成本低 |
| Solar PV / 光伏 | 15% - 22% | 10% - 25% | No fuel, weather dependent / 无燃料，依赖天气 |
Notice that a source can have a high efficiency but a low capacity factor, or vice versa. Nuclear has a modest efficiency but the highest capacity factor because it runs almost continuously; solar has a low capacity
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