📚 Common Energy Sources and Their Utilisation | 常见能源及其利用
Energy is the capacity to do work, and the way we harness and convert energy from primary sources into useful forms underpins every aspect of modern civilisation. In IB Physics, understanding the characteristics, advantages, and limitations of various energy sources is essential for tackling questions on energy production, efficiency, and sustainability.
能量是做功的能力，我们如何从一次能源中获取能量并将其转化为有用的形式，是现代文明各个层面的基础。在IB物理中，理解各种能源的特性、优势与局限，是解答能源生产、效率与可持续性相关问题的关键。
1. Primary and Secondary Energy Sources | 一次能源与二次能源
Primary energy sources are those found naturally in the environment, such as coal, crude oil, natural gas, uranium, sunlight, wind, and flowing water. Secondary energy sources are those produced from primary sources through a conversion process; the most common example is electricity, which must be generated from a primary source before it can be used.
一次能源是自然界中天然存在的能源，如煤炭、原油、天然气、铀、阳光、风力和流水。二次能源则是通过转换过程从一次能源生产而来的能源；最常见的例子是电能，它必须先由一次能源发电产生，随后才能被使用。
All energy sources ultimately derive from one of three fundamental origins: nuclear fusion in the Sun (solar, wind, hydro, biomass, fossil fuels), nuclear fission or radioactive decay within the Earth (geothermal, nuclear), or gravitational interactions (tidal energy). This classification helps physicists trace the ultimate origin of any energy source.
所有能源最终都源自三种基本起源之一：太阳内部的核聚变（太阳能、风能、水能、生物质能、化石燃料）、地球内部的核裂变或放射性衰变（地热能、核能），或者引力相互作用（潮汐能）。这一分类有助于物理学家追溯任何能源的最终起源。
2. Renewable and Non-Renewable Energy | 可再生能源与不可再生能源
A renewable energy source is one that is replenished naturally on a human timescale, such as solar, wind, hydroelectric, biomass, geothermal, and tidal energy. These sources are generally considered sustainable because their utilisation does not deplete the resource for future generations.
可再生能源是在人类时间尺度上自然补充的能源，如太阳能、风能、水能、生物质能、地热能和潮汐能。这些能源通常被认为是可持续的，因为对它们的利用不会耗竭子孙后代的资源。
A non-renewable energy source is one that exists in finite quantities and cannot be replenished on a human timescale once consumed. Fossil fuels (coal, oil, natural gas) and nuclear fuels (uranium) fall into this category. The rate of consumption far exceeds the rate of natural formation, making these resources ultimately exhaustible.
不可再生能源是储量有限、一旦消耗便无法在人类时间尺度上补充的能源。化石燃料（煤、石油、天然气）和核燃料（铀）都属于这一类。其消耗速率远超过自然形成的速率，因此这些资源最终会枯竭。
| Energy Source | Renewable? | Origin | Typical Use |
|---|---|---|---|
| Coal | No | Ancient biomass | Electricity generation |
| Solar | Yes | Nuclear fusion in Sun | Electricity, heating |
| Uranium-235 | No | Supernova nucleosynthesis | Nuclear fission power |
| Wind | Yes | Solar-driven atmospheric circulation | Electricity generation |
3. Fossil Fuels: Coal, Oil and Natural Gas | 化石燃料：煤、石油与天然气
Fossil fuels are the stored chemical energy of ancient marine organisms and terrestrial plants, transformed over millions of years by heat and pressure deep underground. When burned, the carbon-hydrogen bonds release energy through exothermic combustion reactions, producing carbon dioxide and water as the primary products.
化石燃料是古代海洋生物和陆地植物的化学能，在地下经数百万年的高温高压作用转化而成。当它们燃烧时，碳氢键通过放热燃烧反应释放能量，主要产物是二氧化碳和水。
For example, the complete combustion of methane can be written as CH₄ + 2O₂ → CO₂ + 2H₂O, releasing approximately 50 MJ per kilogram. Coal has a lower energy density of roughly 24–35 MJ/kg, while crude oil sits between 42–45 MJ/kg. The choice of fossil fuel depends on availability, cost, and the required application.
例如，甲烷的完全燃烧可写为 CH₄ + 2O₂ → CO₂ + 2H₂O，每千克约释放 50 MJ 的能量。煤的能量密度较低，约为 24–35 MJ/kg，原油则在 42–45 MJ/kg 之间。化石燃料的选择取决于可得性、成本和应用需求。
Fossil fuels dominate global energy supply because they are energy-dense, relatively inexpensive to extract, and compatible with existing infrastructure. However, their combustion releases greenhouse gases and particulate pollutants, and their reserves are finite, driving the global transition toward cleaner alternatives.
化石燃料之所以主导全球能源供应，是因为它们能量密度高、开采成本相对较低，且与现有基础设施兼容。然而，其燃烧会释放温室气体和颗粒污染物，且储量有限，推动着全球向更清洁的替代能源转型。
4. Nuclear Energy: Fission and Fusion | 核能：裂变与聚变
Nuclear fission involves the splitting of a heavy nucleus, such as uranium-235 or plutonium-239, when it absorbs a neutron. The reaction produces two smaller nuclei, 2–3 free neutrons, and a substantial amount of energy due to the mass defect, as described by Einstein’s mass-energy equivalence, E = mc².
核裂变是指重原子核（如铀-235 或钚-239）在吸收一个中子后发生分裂的过程。反应产生两个较小的原子核、2–3 个自由中子和大量能量，这些能量源于质量亏损，由爱因斯坦的质能等价方程 E = mc² 描述。
A typical fission reaction of uranium-235 is:
²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n + energy
One kilogram of uranium-235 can release approximately 80,000,000 MJ – over two million times the energy of the same mass of coal. This extraordinary energy density makes nuclear power a compelling low-carbon option, though it produces radioactive waste that requires careful long-term management.
一千克铀-235 可释放约 80,000,000 MJ 的能量，是同等质量煤炭释放能量的两百多万倍。如此惊人的能量密度使核能成为一种极具吸引力的低碳选择，但同时也会产生需要长期妥善管理的放射性废物。
Nuclear fusion, the process that powers the Sun, combines light nuclei (such as deuterium and tritium) to form helium and release energy. Despite decades of research, sustained, net-energy-positive fusion remains technologically challenging, though projects like ITER aim to demonstrate its feasibility.
核聚变是为太阳提供能量的过程，它将轻原子核（如氘和氚）结合形成氦并释放能量。尽管经过数十年的研究，可持续、净能量为正的聚变在技术上仍具挑战性，但 ITER 等项目正致力于验证其可行性。
5. Solar Energy: Photovoltaic and Thermal | 太阳能：光伏与光热
Solar energy originates from nuclear fusion reactions in the Sun’s core, where approximately 600 million tonnes of hydrogen are converted to helium every second. The Earth receives about 1.36 kW/m² of solar irradiance at the top of the atmosphere, known as the solar constant.
太阳能源自太阳核心的核聚变反应，太阳每秒钟约有 6 亿吨氢转化为氦。地球位于大气层顶部时接收到的太阳辐照度约为 1.36 kW/m²，即太阳常数。
Photovoltaic (PV) cells convert sunlight directly into electricity via the photoelectric effect in semiconductor materials. When photons strike the p-n junction of a silicon cell, they excite electrons across the band gap, creating a potential difference and driving a current through an external circuit.
光伏电池通过半导体材料中的光电效应，将太阳光直接转化为电能。当光子照射硅电池的 p-n 结时，会将电子激发跨越带隙，产生电势差，并在外电路中驱动电流。
Solar thermal systems, by contrast, use mirrors or lenses to concentrate sunlight onto a receiver, heating a working fluid that drives a turbine. The overall efficiency of commercial PV cells typically ranges from 15% to 22%, while concentrated solar power plants can achieve efficiencies of 30–40% under optimal conditions.
相比之下，太阳能热系统利用镜子或透镜将阳光集中到接收器上，加热工作流体驱动涡轮机。商用光伏电池的整体效率通常在 15% 至 22% 之间，而聚光太阳能电站在最佳条件下可实现 30–40% 的效率。
6. Wind Energy | 风能
Wind energy is a secondary form of solar energy, arising from the uneven heating of the Earth’s surface, which creates pressure differences that drive air movement. Wind turbines convert the kinetic energy of moving air into rotational kinetic energy of the blades, and then into electrical energy via a generator.
风能是太阳能的间接形式，源于地球表面受热不均所产生的气压差，驱动空气运动。风力涡轮机将运动空气的动能转化为叶片旋转的动能，再通过发电机转化为电能。
The power available in the wind is given by the equation:
P = ½ρAv³
where ρ is the air density (about 1.2 kg/m³ at sea level), A is the swept area of the rotor blades, and v is the wind speed. The cubic dependence on wind speed means that doubling the wind speed increases power output by a factor of eight, making site selection crucial for wind farm viability.
其中 ρ 是空气密度（海平面约为 1.2 kg/m³），A 是转子叶片的扫掠面积，v 是风速。功率对风速呈三次方依赖意味着风速加倍可使功率输出增大八倍，因此风电场选址至关重要。
Real turbines cannot extract all available wind power; the Betz limit sets a theoretical maximum efficiency of about 59.3%. Practical turbines achieve 35–45% efficiency. Wind power is clean and renewable, but its intermittency and dependence on geographic conditions remain significant challenges.
实际涡轮机无法提取全部可用的风能；贝兹极限设定了约 59.3% 的理论最大效率。实际涡轮机的效率在 35–45% 之间。风电清洁可再生，但其间歇性和对地理条件的依赖仍是重大挑战。
7. Hydroelectric Power | 水力发电
Hydroelectric power harnesses the gravitational potential energy of water stored at elevation. In a typical scheme, water flows from a high reservoir through a penstock to a turbine, converting potential energy to kinetic energy, then to rotational energy, and finally to electrical energy.
水力发电利用高处储存水的重力势能。在典型的方案中，水从高位水库通过压力管道流向涡轮机，将势能转化为动能，再转化为旋转能，最终转化为电能。
The power output of a hydroelectric station is given by:
P = ηρgQh
where η is the overall efficiency, ρ is the density of water (1000 kg/m³), g is the gravitational field strength (9.81 m/s²), Q is the volumetric flow rate in m³/s, and h is the effective head (vertical drop) in metres.
其中 η 是整体效率，ρ 是水的密度（1000 kg/m³），g 是重力场强度（9.81 m/s²），Q 是体积流量（m³/s），h 是有效水头（垂直落差，单位米）。
Hydroelectricity is highly efficient (80–90%), reliable, and capable of rapid response to demand changes. However, large dams can cause significant environmental disruption, including habitat destruction, methane release from flooded vegetation, and displacement of communities.
水力发电效率高（80–90%）、运行可靠且能快速响应需求变化。然而，大型水坝可能造成严重的环境破坏，包括栖息地破坏、被淹没植被释放甲烷以及社区迁移。
8. Biomass and Biofuels | 生物质能与生物燃料
Biomass energy is derived from organic materials such as wood, agricultural residues, and dedicated energy crops. The stored chemical energy originates from photosynthesis, where plants convert solar energy into chemical energy stored in carbon-carbon and carbon-hydrogen bonds.
生物质能来源于木材、农业残留物和专用能源作物等有机材料。其储存的化学能源于光合作用，植物通过光合作用将太阳能转化为储存在碳-碳键和碳-氢键中的化学能。
Biomass can be burned directly for heat and power, or converted into liquid biofuels such as ethanol and biodiesel through fermentation or transesterification. For example, the combustion of glucose (a basic biomass component) releases approximately 15.6 MJ/kg:
生物质可以直接燃烧产生热和电力，也可以通过发酵或酯交换转化为液体生物燃料，如乙醇和生物柴油。例如，葡萄糖（生物质的基本成分）的燃烧释放约 15.6 MJ/kg 的能量：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Bioenergy is considered carbon-neutral in principle because the CO₂ released during combustion is roughly equivalent to the CO₂ absorbed during the plant’s growth. However, land use changes, fertiliser production, and transport can introduce significant indirect emissions, complicating the net carbon balance.
生物能在原理上被认为是碳中性的，因为燃烧释放的 CO₂ 大约等于植物生长期间吸收的 CO₂。然而，土地利用变化、化肥生产和运输可能引入显著的间接排放，使净碳平衡变得复杂。
9. Geothermal and Tidal Energy | 地热能
At this point, we examine two more specialised sources. Geothermal energy originates from the radioactive decay of isotopes deep within the Earth, as well as residual heat from planetary formation. This heat can be tapped by circulating water through hot rock formations, producing steam to drive turbines.
在这里，我们考察两种更为特殊的能源。地热能来源于地球深部同位素的放射性衰变，以及行星形成时留下的残余热。通过使水在高温岩层中循环，可以获取这种热量，产生蒸汽驱动涡轮机。
The temperature gradient of the Earth is approximately 25–30°C per kilometre of depth. Geothermal plants are highly reliable and have high capacity factors, but are limited to regions with accessible geothermal gradients, such as Iceland, New Zealand, and parts of California. The efficiency of geothermal power is relatively low (10–20%) because of the modest temperatures involved.
地球的温度梯度约为每千米 25–30°C。地热电站高度可靠、容量系数高，但仅限于地热梯度可达的地区，如冰岛、新西兰和加利福尼亚部分地区。由于涉及的温度较低，地热发电效率相对较低（10–20%）。
Tidal energy harnesses the gravitational interactions between the Earth, Moon, and Sun. Tidal barrages capture potential energy during high tide and release water through turbines during low tide. While tidal energy is predictable and clean, suitable sites with large tidal ranges are scarce, and construction costs are substantial.
潮汐能利用地球、月球和太阳之间的引力相互作用。潮汐坝在高潮时捕获势能，在低潮时通过涡轮机放水发电。虽然潮汐能可预测且清洁，但具有大潮差且合适的地点是稀缺的，且建设成本高昂。
10. Energy Density and Specific Energy | 能量密度
In IB Physics, energy density and specific energy are key quantities for comparing energy sources. Specific energy is the energy released per unit mass, measured in MJ/kg, while energy density is the energy stored per unit volume, measured in MJ/L or MJ/m³.
在IB物理中，能量密度是比較能源的关键量。比能（质量能量密度）是单位质量释放的能量，单位为 MJ/kg；能量密度（体积能量密度）是单位体积储存的能量，单位为 MJ/L 或 MJ/m³。
| Fuel | Specific Energy (MJ/kg) | Energy Density (MJ/L) |
|---|---|---|
| Hydrogen (liquid) | 142 | 10 |
| Gasoline | 46 | 34 |
| Coal | 24–35 | — |
| Wood | 16 | — |
| Lithium-ion battery | 0.5–0.7 | 1–2 |
These metrics explain why fossil fuels and hydrogen remain attractive for transport: their high specific energy allows long-range travel with relatively low mass. Batteries, despite improving, remain an order of magnitude less energy-dense, which is why electric vehicles carry significant battery weight compared to fuel tanks of equivalent range.
这些指标解释了为什么化石燃料和氢气在交通领域仍具吸引力：其高比能允许以较低质量实现长距离行驶。尽管电池在不断改进，但其能量密度仍低一个数量级，这就是为什么电动汽车相对于同等续航的燃油车需要携带更重的电池。
11. Efficiency of Energy Conversion | 能量转换效率
The efficiency of an energy conversion process is defined as the ratio of useful output energy to total input energy:
能量转换过程的效率定义为有用输出能量与总输入能量之比：
η = Euseful / Einput × 100%
In electricity generation, thermal power plants (coal, gas, nuclear) typically operate at 33–45% efficiency because of the inherent limitations of heat engines, described by the Carnot efficiency η = 1 − Tc/Th, where Tc and Th are the cold and hot reservoir temperatures in kelvin.
在发电领域，火力发电厂（燃煤、燃气、核能）的效率通常在 33–45% 之间，原因是热机固有的局限性，由卡诺效率 η = 1 − Tc/Th 描述，其中 Tc 和 Th 分别是冷端和热端热库的温度（单位开尔文）。
Renewable sources without heat engines – such as hydroelectric and wind – achieve much higher conversion efficiencies because they convert mechanical energy directly. Hydroelectric plants reach 80–90%, while wind turbines achieve 35–45% of the Betz-limited maximum. Understanding these efficiencies allows physicists to evaluate the true cost and environmental impact of different energy choices.
不使用热机的可再生能源——如水力发电和风力发电——由于直接转换机械能，可实现高得多的转换效率。水电站可达 80–90%，风力涡轮机则可实现贝兹极限最大值的 35–45%。理解这些效率有助于物理学家评估不同能源选择的真实成本和环境影响。
12. IB Exam Focus: Energy Calculations and Evaluation | IB考试重点：能量计算与评估
IB Physics examinations frequently ask students to calculate energy output, power, efficiency, and to compare energy sources using quantitative data. A typical question might provide the mass of fuel consumed, the calorific value, and the efficiency of a power plant, requiring calculation of the useful electrical energy output.
IB 物理考试经常要求学生计算能量输出、功率、效率，并使用定量数据比较不同能源。典型题目可能提供燃料消耗质量、热值和电厂效率，要求计算有用电能输出。
For example: A coal-fired power station with 35% efficiency burns 2000 tonnes of coal per day with a specific energy of 28 MJ/kg. The useful electrical energy output per day is:
例如：一座效率为 35% 的燃煤电厂每天燃烧 2000 吨煤，其比能为 28 MJ/kg。每天的有用电能输出为：
Einput = 2,000,000 kg × 28 MJ/kg = 5.6 × 10⁷ MJ
Euseful = 0.35 × 5.6 × 10⁷ MJ = 1.96 × 10⁷ MJ ≈ 5.44 × 10⁶ kWh
When evaluating energy sources in extended-response questions, consider the following criteria: specific energy and energy density, availability and geographic constraints, environmental impact (particularly CO₂ emissions and waste products), economic cost including construction and decommissioning, and reliability (whether the source is intermittent or dispatchable). A balanced evaluation that weighs both quantitative data and qualitative factors earns top marks.
在扩展作答中评估能源时，请考虑以下标准：比能与能量密度、可得性和地理限制、环境影响（特别是 CO₂ 排放和废物）、经济成本（包括建设和退役成本），以及可靠性（能源是间歇性的还是可按需调度的）。平衡权衡定量数据和定性因素的回答能获得高分。
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