UPPSC 2022 Prelims GS Paper-I · Question 4 of 150

Correct — A, Nuclear energy. Nuclear energy is released from the binding energy of atomic nuclei — the fission of heavy nuclei such as uranium-235 and plutonium-239, or the fusion of light ones. Those heavy elements were forged in stellar processes that long predate our own Sun and were already present in the cloud from which the solar system condensed; nothing about the energy released when a uranium nucleus splits was ever put there by sunlight. Nuclear is therefore the clearest possible case of an energy source with no solar ancestry, and that is why the examiner marks it. Be honest about the item's defect, though: option (b), geothermal energy, is equally not derived from the sun. The Earth's internal heat comes from the primordial heat of accretion and core formation plus the ongoing radioactive decay of uranium, thorium and potassium-40 in the mantle and crust — again, not sunlight. On a strict reading this question has two scientifically correct answers, (a) and (b), and it has been noted as such. The commission's key marks (a), and (a) is the option to carry into the exam; the examiner's intent is plainly that nuclear energy, which is not even a natural surface flow, is the least solar of the four.

• (b)Geothermal energy — Marked wrong by the official key, but it is not scientifically wrong: geothermal heat comes from the Earth's own interior — leftover heat from planetary accretion and core formation, plus the continuing radioactive decay of uranium, thorium and potassium-40 — and not from the sun. Treat this as a defect in the item rather than as a fact about geothermal energy. The examiner's reading is that nuclear energy, being a laboratory-scale release of nuclear binding energy rather than a natural planetary flow, is the intended 'not from the sun' option.
• (c)Biomass energy — Genuinely solar. Biomass is plant matter built by photosynthesis, in which green plants use sunlight to convert carbon dioxide and water into carbohydrates; burning firewood, crop residue, dung or biogas simply releases chemical energy that sunlight stored. Fossil fuels are the same story on a geological timescale.
• (d)Wind energy — Genuinely solar. Winds exist because the sun heats the earth's surface unevenly — equator against poles, land against sea, day against night — producing pressure differences that air flows down. No solar heating means no pressure gradients and therefore no wind, so wind energy is stored solar energy at one remove.

Almost every energy source on Earth traces back to one of three ultimate reservoirs. The first and largest is solar radiation, which drives photosynthesis (hence biomass and, over geological time, fossil fuels), the atmospheric circulation (wind), the hydrological cycle (hydropower) and ocean thermal gradients. The second is the Earth's own internal heat — primordial heat of accretion plus radiogenic heat from decaying uranium, thorium and potassium-40 — which surfaces as geothermal energy. The third is gravitational and rotational: the Earth–Moon–Sun system's gravitational interaction with the Earth's rotation produces the tides, and so tidal energy. Nuclear fission and fusion sit outside all three as an engineered release of nuclear binding energy from elements formed in stars long before the solar system existed.

The question is asking you to sort four sources into 'solar-derived' and 'not solar-derived'. Wind and biomass are unambiguously solar, so the real contest is between nuclear and geothermal — and that is precisely where the item breaks down, because neither is solar. Two lines of reasoning point to the key's option. First, three of the four options are natural energy flows at the earth's surface while nuclear energy is a manufactured release of energy from the nucleus, so it is the odd one out in kind as well as in origin. Second, the elements that fuel fission were made in stars other than our sun, so nuclear energy is not merely non-solar but pre-solar. Neither argument makes geothermal solar, and you should not tell yourself that it is. Learn the correct three-reservoir classification and remember the key's option for this particular paper.

• Nuclear energy comes from the binding energy of atomic nuclei — fission of uranium-235 or plutonium-239, or fusion of light nuclei — and the fissile elements were formed in stellar processes predating the solar system.
• Geothermal energy comes from the Earth's interior: residual heat from accretion and core formation, plus the continuing radioactive decay of uranium, thorium and potassium-40. It is not derived from the sun.
• Biomass energy is stored sunlight — photosynthesis converts solar radiation into the chemical energy of plant tissue, which is then released by burning or by digestion into biogas.
• Wind energy is stored sunlight too — uneven solar heating of land, sea and latitude creates the pressure gradients that drive winds.
• Tidal energy is the third non-solar source, arising from the gravitational interaction of the Moon and the Sun with a rotating Earth rather than from solar radiation.

• Convincing yourself that geothermal heat is solar in order to justify the key. It is not — the item simply has more than one defensible option, and the key marks nuclear.
• Confusing 'renewable' with 'derived from the sun'. Geothermal and tidal energy are renewable but not solar; fossil fuels are solar in origin but not renewable.
• Forgetting that hydropower is solar-derived — the sun evaporates the water that rain and rivers return to the reservoirs.

This area is asked as a one-line classification — 'which is non-conventional', 'which is non-renewable', 'which is not derived from the sun' — and both UPPSC and UPSC like the NOT-format, so read the stem twice before choosing.

• practice — not a real PYQ

  Which one of the following energy sources does NOT ultimately depend on solar radiation?

  • (a)Hydropower
  • (b)Tidal energy
  • (c)Wind energy
  • (d)Biogas

  Answer(b) Tidal energy — it arises from the gravitational interaction of the Moon and Sun with a rotating Earth, while hydropower, wind and biogas all trace back to solar heating or photosynthesis.

• practice — not a real PYQ

  The heat that sustains geothermal energy resources is chiefly attributed to

  • (a)solar radiation absorbed by the earth's crust
  • (b)frictional heating caused by the earth's rotation
  • (c)residual heat of planetary accretion together with radioactive decay of uranium, thorium and potassium
  • (d)chemical reactions between crustal rocks and groundwater

  Answer(c) residual heat of planetary accretion together with radioactive decay of uranium, thorium and potassium — the two recognised sources of the Earth's internal heat budget.