📚 Energy Sources | 能源来源
Energy sources are the natural or artificial reservoirs that provide usable power for modern society. In IB Physics, the study of energy sources links thermodynamics, nuclear processes, electromagnetic induction, and environmental science. We classify sources as non‑renewable (fossil fuels, nuclear fission) and renewable (solar, wind, hydro, biomass, geothermal, tidal, wave). Energy transformation, efficiency, power density, and specific energy are central concepts, often illustrated with Sankey diagrams. This article revisits the fundamental physics behind each major energy source, compares their energy densities, and examines sustainability from a quantitative standpoint.
能源来源是为现代社会提供可用动力的天然或人工资源。在 IB 物理中，能源来源的学习将热力学、核过程、电磁感应和环境科学联系在一起。我们将能源分为不可再生（化石燃料、核裂变）和可再生（太阳能、风能、水力、生物质、地热、潮汐、波浪）两类。能量转换、效率、功率密度和比能是核心概念，通常借助桑基图展示。本文回顾每种主要能源背后的基础物理，比较它们的能量密度，并从定量角度讨论可持续性。
1. Energy Sources and Their Physical Origin | 能源来源及其物理起源
Every energy source ultimately derives from a fundamental physical process. Fossil fuels store chemical energy from ancient photosynthesis; nuclear energy comes from mass‑defect and binding energy; solar radiation originates from thermonuclear fusion in the Sun; wind and hydro are driven by solar heating and gravity; tides arise from the Moon’s gravitational pull; geothermal energy flows from radioactive decay inside Earth. Recognising these origins helps us analyse the maximum theoretical efficiency of conversion.
每一种能源最终都源自一个基本的物理过程。化石燃料储存的是远古光合作用留下的化学能；核能来自质量亏损和结合能；太阳辐射源自太阳内部的热核聚变；风能和水能由太阳加热和重力驱动；潮汐能来自月球的引力；地热能来自地球内部的放射性衰变。认识这些起源有助于我们分析能量转换的最大理论效率。
2. Primary vs Secondary Sources | 一次能源与二次能源
A primary energy source is found in nature before any conversion: crude oil, raw wind, direct sunlight. A secondary energy source is a carrier produced from a primary source, such as electricity or refined gasoline. Physics emphasises that every conversion step introduces losses dictated by the second law of thermodynamics. For example, a coal‑fired power station converts about 35–45 % of the primary chemical energy into electrical energy; the rest becomes low‑grade heat. Therefore, comparing sources at the point of end‑use requires tracking these transformations.
一次能源是在任何转换之前存在于自然界的能量，如原油、自然风、直射阳光。二次能源是由一次能源制成的载体，例如电能或精炼汽油。物理强调，每一步转换都会引入由热力学第二定律决定的损耗。例如，燃煤电厂将约 35–45% 的一次化学能转化为电能，其余成为低品位的热。因此，在终端使用点比较能源需要追踪这些转换过程。
3. Non‑Renewable Energy: Fossil Fuels | 不可再生能源：化石燃料
Coal, oil, and natural gas store chemical potential energy in hydrocarbon bonds. Combustion releases this energy through exothermic reactions, principally:
C + O₂ → CO₂ + energy
The specific energy of coal is roughly 30 MJ kg⁻¹, while natural gas (methane) releases about 55 MJ during complete combustion. Energy density by volume is significantly higher for oil (≈35 GJ at atmospheric pressure. In power stations, the heat generates steam that drives a turbine‑generator set. The maximum Carnot efficiency depends on the temperature difference: ηCarnot = 1 – Tcold/Thot. Real plants operate well below this limit because of friction, heat loss, and boiler inefficiencies.
煤、石油和天然气在碳氢键中储存化学势能。燃烧通过放热反应释放这些能量，主要是：
C + O₂ → CO₂ + 能量
煤的比能约为 30 MJ kg⁻¹，而天然气（甲烷）完全燃烧时释放约 55 MJ kg⁻¹。石油的体积能量密度（≈35 GJ m⁻³）远高于常压下的气体。在发电站，热量产生蒸汽驱动涡轮‑发电机组。最大卡诺效率取决于温差：ηCarnot = 1 – Tcold/Thot。实际电厂由于摩擦、热损失和锅炉低效，运行效率远低于这一极限。
4. Non‑Renewable Energy: Nuclear Fission | 不可再生能源：核裂变
Nuclear power exploits the binding energy of heavy nuclei. A thermal neutron induces fission of uranium‑235:
n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3n + energy
s The mass defect Δm is converted to energy via E = Δm c². Each fission event liberates roughly 200 MeV (3.2 × 10⁻¹¹ J). Consequently, the specific energy of enriched uranium fuel is around 80 × 10⁶ MJ kg⁻¹, millions of times greater than fossil fuels. In a pressurised‑ coolant loop transfers heat to a secondary steam cycle. The thermal efficiency is about 33–37 %, limited by the lower steam temperature compared to fossil‑fuel plants. Safety, radioactive waste disposal, and proliferation risks are the main non‑physical constraints.
核能利用重核的结合能。热中子诱发铀‑235 裂变：
n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3n + 能量
质量亏损 Δm 通过 E = Δm c² 转化为能量。每次裂变释放约 200 MeV（3.2 × 10⁻¹¹ J）。因此，浓缩铀燃料的比能约为 80 × 10⁶ MJ kg⁻¹，比化石燃料高出数百万倍。在压水反应堆中，控制棒调节链式反应，一次冷却剂回路将热量传递给二次蒸汽循环。热效率约为 33–37%，受限于比化石燃料电厂更低的蒸汽温度。安全性、放射性废物处置和扩散风险是主要的非物理约束。
5. Renewable Energy: Solar Power | 可再生能源：太阳能
Solar irradiance reaching Earth’s surface averages about 1000 W m⁻² at sea level on a clear day. Photovoltaic (PV) cells use semiconductor p‑ junctions to convert photons directly into electricity. A typical silicon cell has a band‑gap energy that limits theoretical efficiency to about 33% (Shockley‑Queisser limit); commercial modules achieve 18–22% fill factor describes the quality of the cell. Concentrated solar power (CSP) uses mirrors to focus sunlight, heating a fluid to drive a turbine, achieving temperatures high enough for higher Carnot efficiencies but requiring direct beam radiation and large land areas.
晴天海平面处抵达地球表面的太阳辐照度平均约为 1000 W m⁻²。光伏（PV）电池利用半导体 p-n 结将光子直接转换为电能。典型的硅电池带隙能量将理论效率限制在约 33%（肖克利-奎瑟极限）；商用组件达到 18–22%。输出功率遵循 P = IV，填充因子描述了电池的品质。聚光太阳能（CSP）用镜子聚焦阳光，加热流体驱动涡轮，温度更高可获得更高的卡诺效率，但需要直射辐射和广阔的土地。
6. Renewable Energy: Wind and Hydropower | 可再生能源：风能与水力发电
Wind turbines convert kinetic energy of moving air into rotational mechanical energy and then electricity. The power available in wind is:
P = ½ ρ A v³
where ρ is air density (≈1.2 kg nd speed. The Betz limit caps the fraction of kinetic energy that can be extracted at 59.3%. Modern turbines achieve 40–50% of that limit. Hydropower, whether from a dam or run‑of‑river, converts gravitational potential energy of water: ΔEp : P = η ρ Q g h, where Q is volume flow rate. Large hydropower installations can exceed 90% mechanical‑to‑electrical efficiency, making them one of the most efficient conversion technologies.
风力发电机将运动空气的动能转换为旋转机械能再转换为电能。风中的可用功率为：
P = ½ ρ A v³
其中 ρ 是空气密度（≈1.2 kg m⁻³），A 是扫掠面积，v 是风速。贝兹极限将可提取的动能比例限制在 59.3%。现代风机的实际提取比例达到这一极限的 40–50%。水力发电不论是来自大坝还是径流式，都是将水的重力势能转化：ΔEp = mgh。功率取决于流量和水头高度：P = η ρ Q g h，其中 Q 是体积流量。大型水电站的机械‑电能转换效率可超过 90%，是其最高效的转换技术之一。
7. Other Renewables: Biomass, Geothermal, Tidal, Wave | 其他可再生能源：生物质、地热、潮汐、波浪
Biomass stores chemical energy produced by recent photosynthesis. Combustion or anaerobic digestion releases it; the specific energy of dry wood is about 15–20 MJ kg⁻¹. Geothermal plants use Earth’s internal heat, either directly as steam or to vaporise a secondary fluid, with typical thermal efficiencies of 10–20% because of low source temperatures (150–300 °C). Tidal barrages exploit the gravitational potential of sea water trapped at high tide, while tidal stream devices convert kinetic energy akin to underwater wind turbines. Wave energy converters capture the oscillatory motion of the sea surface; their power per metre of wave front scales with the square of wave height and the period.
生物质储存近期光合作用产生的化学能。燃烧或厌氧消化可释放出来；干木柴的比能约为 15–20 MJ kg⁻¹。地热电厂利用地球内部热量，或直接利用蒸汽，或使二次流体汽化，由于热源温度较低（150–300 °C），典型热效率为 10–20%。潮汐坝利用高潮时拦蓄的海水的重力势能，而潮汐流装置类似于水下风力发电机，转换动能。波浪能转换器捕获海面的振荡运动；单位波前长度的功率与波高平方和周期成正比。
8. Energy Density and Specific Energy | 能量密度与比能
Two critical quantities allow fair comparison of sources: specific energy (energy per unit mass, J kg⁻¹) and energy density (energy per unit volume, J m⁻³). The table below lists representative values for several fuels and storage systems.
两个关键量允许公平比较能源：比能（单位质量能量，J kg⁻¹）和能量密度（单位体积能量，J m⁻³）。下表列出几种燃料和储存系统的代表值。
| Fuel / System | Specific energy (MJ kg⁻¹) | Energy density (GJ m⁻³) |
|---|---|---|
| Natural gas (CH₄) | 55 | 0.04 (gas, STP) |
| Gasoline (petrol) | 46 | 34 |
| Coal (bituminous) | 30 | 20–25 |
| Uranium‑235 (fission) | 80 000 000 | 1 500 000 000 |
| Lithium‑ion battery | 0.5–0.8 | 1.8–2.5 |
The enormous specific energy of nuclear fuel explains its appeal for base‑load electricity, while the low figures for batteries highlight why energy storage remains a challenge for intermittent renewables.
核燃料极高的比能解释了它作为基荷电力的吸引力，而电池的低数值则突显了为什么储能仍然是间歇性可再生能源面临的挑战。
9. Sankey Diagrams and Efficiency Calculations | 桑基图与效率计算
A Sankey diagram represents energy flow with arrows whose widths are proportional to the amount of energy. It vividly shows useful output and losses. For any device or power plant, efficiency is defined as:
η = Euseful / Einput or η = Pout / Pin
Typical efficiencies: coal 35–40%, nuclear 33–37%, combined‑cycle gas turbine 55–60%, wind 40–50% (of Betz), hydro >90%, PV modules 18–22%. Total system efficiency for a solar charging station would be the product of PV efficiency, battery round‑trip efficiency, and inverter efficiency, illustrating cumulative losses. Sankey diagrams also help visualise energy degradation according to the second law of thermodynamics, where a portion of energy becomes unavailable for work.
桑基图用箭头表示能量流，箭头的宽度与能量大小成正比。它清晰地展示有用输出和各种损失。对于任何设备或电厂，效率定义为：
η = Euseful / Einput 或 η = Pout / Pin
典型效率：煤电 35–40%，核电 33–37%，联合循环燃气轮机 55–60%，风电 40–50%（相对于贝兹极限），水电 >90%，光伏组件 18–22%。太阳能充电站的系统总效率将是光伏效率、电池往返效率和逆变器效率的乘积，体现了累积损失。桑基图也有助于按照热力学第二定律可视化能量降级，其中一部分能量变得无法做功。
10. Environmental Impact and Sustainability | 环境影响与可持续性
Every energy source has an environmental footprint beyond carbon emissions. Fossil fuel combustion releases CO₂, SO₂, NOₓ, and particulate matter, contributing to the enhanced greenhouse effect and acid rain. Nuclear waste remains hazardous for millennia. Biomass burning can be carbon neutral over a life‑cycle but may compete with food production. Large hydro dams alter river ecosystems and displace populations. Wind and solar are clean during operation but require mining for rare‑earth metals and have land‑use implications. A source is considered sustainable if it can meet present needs without compromising future generations. Quantitatively, this connects to the ratio of energy returned on energy invested (EROI): an EROI significantly below 5 often makes a source economically and physically unviable in the long term.
每种能源除了碳排放之外还有环境足迹。化石燃料燃烧释放 CO₂、SO₂、NOₓ 和颗粒物，加剧温室效应和酸雨。核废料的危险性持续数千年。生物质燃烧在整个生命周期内可以是碳中性的，但可能与粮食生产竞争。大型水坝改变河流生态系统并迁移人口。风能和太阳能在运行中清洁无污染，但需要开采稀土金属并涉及土地利用。如果一种能源能满足当代需求而不损害后代，则被认为是可持续的。定量上，这涉及能源投资回报率（EROI）：EROI 远低于 5 的能源在长期内往往在经济和物理上不可行。
11. Power Density and Capacity Factor | 功率密度与容量因子
Power density (W m⁻²) measures the rate of energy delivered per unit area of the source or plant footprint. For example, a wind farm delivers 2–3 W m⁻², while a nuclear plant site can exceed 500 W m⁻². Capacity factor is the ratio of actual energy output over a period to the maximum possible output if the plant ran at full rated power continuously. Fossil and nuclear plants achieve capacity factors of 85–95% (limited by maintenance), while solar PV typically sits at 15–25% owing to day‑night cycles and weather, and onshore wind at 25–40%. These figures determine how much backup or storage is needed in a grid dominated by variable renewables.
功率密度（W m⁻²）衡量单位面积来源或电厂占地面积的能量输送速率。例如，风电场提供 2–3 W m⁻²，而核电厂场地的可超过 500 W m⁻²。容量因子是一段时间内实际能量输出与电厂持续满发运行时最大可能输出之比。化石和核电厂因维护限制达到 85–95% 的容量因子，而太阳能光伏由于昼夜周期和天气通常为 15–25%，陆上风电为 25–40%。这些数据决定了在波动性可再生能源主导的电网中需要多少备用或储能。
12. Key Equations and Concept Checklist | 关键方程与概念清单
IB Physics assessment may expect you to recall and apply the following relationships:
Efficiency: η = Eout / Ein or η = Pout / Pin (may multiply by 100% for percentage).
效率：η = Eout / Ein 或 η = Pout / Pin（可乘以 100% 求百分比）。
Wind power: P = ½ ρ A v³; Betz limit 0.593.
风能功率：P = ½ ρ A v³；贝兹极限 0.593。
Hydropower: P = η ρ Q g h.
水力发电功率：P = η ρ Q g h。
Solar cell: P = IV, fill factor = (Vmp Imp)/(Voc Isc).
太阳能电池：P = IV，填充因子 = (Vmp Imp)/(Voc Isc)。
Mass‑energy equivalence: E = Δm c².
质能等价：E = Δm c²。
Carnot efficiency: ηCarnot = 1 – Tcold/Thot (temperatures in kelvin).
卡诺效率：ηCarnot = 1 – Tcold/Thot（温度用开尔文）。
Energy density and specific energy: ρE = E/V; e = E/m.
能量密度和比能：ρE = E/V；e = E/m。
Sankey diagrams: width proportional to energy; useful for tracking energy degradation.
桑基图：宽度与能量成比例；用于追踪能量降级。
Understanding how these equations connect to real‑world data enables you to analyse unfamiliar energy scenarios critically and quantitatively, a skill central to the IB Physics approach.
理解这些方程如何与现实数据相连接，使你能够批判性和定量地分析不熟悉的能源情景，这是 IB 物理方法的核心技能。
Published by TutorHao | Physics Revision Series | aleveler.com
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