本文深入探讨了地球上和人类历史中的能量流动，揭示了太阳能作为根本驱动力。文章详细阐述了地球如何通过温室效应维持适宜生命生存的温度，分析了植物光合作用的效率及其在全球生物质生产中的作用。同时，文章回顾了人类历史上的能源利用，从古代农业到工业革命，对比了不同能源的能量密度和效率，并指出现代文明本质上仍是太阳能的延伸。最后，文章聚焦于日常生活中的能源消耗，例如建筑保温、照明效率及“幽灵功耗”，并量化了不同物品的能量成本，强调了能源理解的重要性。

☀️ 地球温度的维持与温室效应：文章指出，如果没有温室气体对太阳辐射的“缓冲”作用，地球平均表面温度将为-18°C，所有水体都会冻结。当前15°C的平均表面温度，主要归功于温室气体对辐射的缓冲，其中水蒸气是最主要的温室气体，地球被形象地描述为被包裹在“水茧”中，这使得地球能够避免过快地变冷。

🌱 光合作用的能量效率与生物质生产：文章解释了植物能量来源仅限于自养生物通过光合作用或化能合成。尽管光合作用的能量效率较低，仅能利用可见光中约60nm的波段，理论最大效率为4%，全球平均仅0.33%（大陆）至0.2%（生物圈），但全球大陆和海洋每年仍分别生产约1200亿吨和1100亿吨植物质量。

⏳ 人类历史的能源演变与农业效率：文章追溯了人类能源利用的演变，指出农业效率在古代埃及为每公顷一人，欧洲（前工业化）则从未超过每公顷两人。这受到作物能量转换效率低和缺乏氮元素的影响。同时， manuel 农业劳动（如耕作）的能量消耗巨大，远超采集活动。

💡 现代文明的能源本质与生活中的能量成本：文章强调，所有文明本质上都是太阳能驱动的，化石燃料是储存的太阳能。现代文明与前工业化文明的区别在于能源来源是近期的生物质还是远期的化石燃料。文章还列举了生活中的能量成本，如建筑保温、屋顶颜色对温度的影响，以及“幽灵功耗”等，并量化了不同材料和食物的能量成本。

Published on August 18, 2025 8:49 PM GMT

Thought it would be cool to extract some interesting notes & share here. Please correct if you spot any mistakes.

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This is a good book because it gives you numbers.

It's important to understand the quantities of things around you. I think thatthere's a sort of deep understanding that you can have when you know the exactfigures (or ranges) for many physical phenomena. This is something I hint at inbeing good at the basics. Smil's book is a good introduction to suchthinking.

Energy in the biosphere

On celestial matters:

If the Earth didn't have a "buffering" mechanism for absorbing solar radiation, andit re-radiated all the solar radiation back immediately (black bodyradiator), the planet wouldstay at -18°C. So, all water would be permanently frozen. The mean surfacetemperature is 15°C, which is all caused by greenhouse gases buffering theradiation. Buffers = life!The power of the sun is 3.9x10^26 W. So, each second that the sun shines, it sends13 orders of magnitude more energy than all fuels and all primary electricity onEarth in a given year.Solar flow on topmost atmosphere is 1376W/m^2, or a little more than 1 kW per metersquared. However, due to the Earth being round, in any given second, some partshave more light, some fewer (towards the poles), and some none (nighttime); theaverage is 342W/m^2. Then the light goes through a bunch of filtering mechanisms,such as the ozone layer, different cloud layers, and so on. So, roughly half ofthe radiation that hit the uppermost atmosphere hits the surface; averaging it outover the entire surface of the earth, that's around 170W/m^2.
This adds up to 2.7x10^24 J every year, around 5000 thousand times more than theworld spends each year.
About two thirds of global albedo is due to cloud tops.Many equatorial places are poorly insolated due to large amounts of cloudspreventing the sun from accessing the earth. So, "tropical" doesn't necessarilymean "good for placement of pV systems".All the inbound radiation is eventually re-radiated back to space, but taking intoaccount the "buffering" effects mentioned above. For example, 95% percent ofsurface level re-radiation doesn't end up in space directly, but rather getsbuffered by the atmosphere. The most dominant greenhouse gas is by far water vapor.The Earth is essentially wrapped in a water cocoon, which protects it from gettinghow too fast, and getting cold too fast.Planetary water cycles moves around 585,000 km^3 of water every year. Around 46 PWof power is required to vaporize this water; this is half of total insolation.Geothermal power is comparatively small; about 44TW (~90mW/m^2 on average).

On plants:

External biodiversity obscures the fact that there are very few metabolicpathways in organisms. That is, energy comes into the biome only throughautotrophs: either phototrophs converting solar energy into mass, orchemotrophs converting inorganic compounds into mass. Literally everything elsewhich is alive is downstream of these two producers.Photosynthesis reminder:

6CO2 + 6H2O = C6H12O6 + 6CO2

Energy efficiency of photosynthesis is quite low.Chlorophylls have narrow absorption bands: 420-450 nm and 630-690 nm. Meaningthat all the light outside of these bands is useless for photosynthesis. Forreference, the human eye can see from 380 to 750 nm. So, around 370 nm worth ofradiation. Plants can convert around 60 nm worth, so around 6 times less thanwhat the human eye can see (and much much less if we count in the other radiationexcept visible light).Maximum theoretical efficiency is around 4% of insolation, but in practice, theglobal continental average is 0.33%; the average for the entire biosphere is0.2%. In other words, if 500 photons hit the surface, only one of those photonsgets converted into plant mass.Despite the inefficiency, continents produce around 120 billion tonnes of plantmass a year; oceans around 110 billion tonnes. Different forests producedifferent amounts, but e.g. continental forests produce around 0.5 kg to 2.5 kgper m^2 every year. These rates are roughly equal for grown crops.

Energy in human history

Human caloric needs are roughly around 2-3 thousand kcal a day. There aredifferences across age, gender, size, as well as level of activity. The basalmetabolic rate: 50-80 W for females, 60-90 W for males (again, given averagesizes).Energy in food: pure fat gives 39 MJ/kg, protein 23 MJ/kg, carbs 17 MJ/kg. However,the body processes carbohydrates primarily, and the protein comes last (at leastfor energy needs).Total amount of energy available at higher trophic levels is reduced. For example,all the prey in the world combined has less energy than all the plant food of thatprey. This explains historical dominance of vegetarianism in foraging societies(besides, meat is difficult to get, in addition to not being as abundant asplants). The energy return for foraging (the amount of joules from foraged food vs.the amount of joules expended for foraging) is around 10x. So for each joule ofenergy expended foraging, you'd, historically, get around 10x more, sometimes 30xmore.Agricultural efficiency in ancient Egypt was 1 person per hectare (the cultivationof one hectare of land would energetically meet the needs of one person). Thatnumber grew very slow; some areas reached up to 5 persons per hectare. Europe(pre-industrial) never went above 2 persons per hectare.
There are two reasons for this: growing non-improved crops (where the conversionof energy is inefficient, and relatively little is converted into usable food,and the rest is converted into inedible phytomass) and lack of nitrogen.And an additional reason: it's energetically very costly to perform agriculturalmanual labor (tilling a hectare of land takes up much much more than foraging ahectare of forest).
Humans can work at around 60-80W; draft animals yield 300-500W (and also don'tneed specialized feed, as they can eat grass).Dry wood has around 16 MJ/kg. That's pretty low energy density. Compare:
Coal: 24 MJ/kgDiesel: 45 MJ/kgGasoline: 46 MJ/kgNatural gas: 55 MJ/kg
The low energy density of wood is not a problem if the source is dense (and you canharvest a lot from a small acreage), but that's not the case: there's usually nomore than 200t per hectare (20 kg/m^2), the yield is 300 MJ/m^2. Since it takes50-100 years to return to pre-harvest state, you need to divide the harvestedenergy by the time to regenerate. For example, dividing by 50 years, you get 0.2W/m^2 (of sustainable wood energy).Traditional charcoaling gives 0.04 W/m^2 (again, by taking sustainable harvestingfigures). Pretty wasteful.Energy needs in pre-industrial cities: 10-30 watts per m2 (that's for heating,cooking, metallurgy, and so on). This means that if they relied solely on wood,cities would need forests 50-150 times their size nearby (for sustainableharvesting).

Energy in the modern world

Every civilization is fundamentally solar: fossil fuels are essentially batteriesfor solar energy. The difference between pre-industrial and industrialcivilizations is whether they're locally solar (recent phytomass) or remotely solar(old phytomass=fossil fuels).Coals are basically just plants in swamps, subjected to high temperatures andpressures for up to 350 million years. Many of these plants are still around, insmaller form. For example, mosses up to 30 meters in height vs. today's moss."Peak oil" is unjustifiably pessimistic; there is little chance of hitting asupermassive oil field like Saudi Arabia's Ghawar field, but the total oil reservesare still pretty large.A third of the world's electricity is not generated by the combustion of fossilfuels.

Energy in everyday life

The Mediterranean diet has switched to much more meat, fish, butter, and cheese;originally it was much more heavy on bread, fruit, potatoes, and olive oil. Thetrue (original) Mediterranean diet now survives only among the rural elderlypopulation.American walls (wood frame, drywall, insulation) provide around 4x more insulationthan sturdier European brick walls.Dark roofs get up to 50°C hotter than the air temperature in summer; white or lightroofs are just 10°C hotter.One lumen of US electric light is three orders of magnitude more affordable in 2015than in 1900."Phantom loads" (vampire power; use of electricity even when everything is turnedoff) is around 50 W per household in the US; combined, that's more electricity thanSingapore's use in a year.Energy costs of various items, in GJ/t (for reference, one tonne of crude oil is 42GJ/t):
excavating sand: 0.1 GJ/tquarrying stone: 1 GJ/textracting construction wood: 1.5-3 GJ/tcement: 3.5 GJ/tadding steel to concrete makes it 3x more expensiveinsulation: 10 GJ/ta three-bedroom house costs around 500 GJ (around 12 tonnes of crude oil)wheat, corn, fruits: 4 GJ/trice: 10 GJ/tpeppers, tomatoes, greenhouse veggies: 40 GJ/twheat contains four times as much energy as was used to produce it; greenhousetomatoes can have up to 50x less energy than what was used to produce them
however, food's total energy cost is dominated by packaging & transport

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