Defence, nuclear & energy technology

Answer: (d) All four

Distributed energy resources (DER) are small generating or storage units placed close to where power is used, usually on the consumer's side of the meter, instead of large central power stations. Rooftop solar, home or grid batteries, small biomass generators and fuel cells are all standard examples.

• ✓ 1. Battery storage, including electric-vehicle batteries that can send power back to the grid, is listed by Australia's energy market rule-maker (AEMC) as a distributed energy resource.
• ✓ 2. Small biomass generators running on waste gas or farm and industrial by-products are a DER; the US EPA also lists biomass combustion among distributed generation options.
• ✓ 3. Fuel cells, run on natural gas, hydrogen or biomass-derived gas, produce electricity on site and are counted as distributed generation.
• ✓ 4. Rooftop solar PV is the most common DER: power is produced on the building that uses it, and surplus can be fed into the distribution grid.

Remember · DER = small, local generation or storage near the point of use: rooftop solar, small wind, batteries (incl. EVs), biomass generators, fuel cells, CHP units, mini-hydro.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) 2 and 3 only

An RTG is not a reactor: nothing is split in a chain reaction. It simply turns the steady heat given off by the natural decay of plutonium-238 into electricity using thermocouples, which is why it powers deep-space probes and rovers such as Voyager, New Horizons, Curiosity and Perseverance. The US made its plutonium-238 at the Savannah River nuclear-materials plant, part of its weapons complex.

• ✗ 1. RTGs have no fission or chain reaction. Heat from the spontaneous radioactive decay of plutonium-238 is converted to power by thermocouples, with no moving parts.
• ✓ 2. They supply electricity and warmth to spacecraft where sunlight is weak or dust blocks solar panels; the current US model, the MMRTG, gives about 110 watts at launch.
• ✓ 3. Plutonium-238 is not weapons-grade plutonium, but the US produced it at its Savannah River plant, a site built to make nuclear-weapons materials, until the late 1980s; it is the standard RTG fuel.

Remember · RTG = heat from natural decay of Pu-238 → electricity via thermocouples (Seebeck effect). No fission. Powers Voyager, Cassini, New Horizons, Curiosity, Perseverance.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (d) None

A fifth-generation fighter is built around stealth (low-observable shaping, weapons carried inside the body), fused sensor data and network-centric operations; Lockheed Martin describes the F-35 this way, and ADA lists the same features for India's own fifth-generation project, the AMCA. UPSC's key counts none of the three listed aircraft as fifth generation, and ADA's Tejas Mk-1 page lists fly-by-wire controls, a glass cockpit and a multi-mode radar, with external stores and drop tanks, and no stealth features.

• ✗ 1. The Rafale (printed 'Rafael') is a French twin-engine multirole fighter. UPSC's official key treats it as not fifth generation; we could not confirm its generation label from an official source, so we do not explain it here.
• ✗ 2. The MiG-29 is a Soviet-designed fighter. UPSC's official key treats it as not fifth generation; we could not confirm its generation label from an official source, so we do not explain it here.
• ✗ 3. ADA's page for Tejas Mk-1 lists relaxed static stability, fly-by-wire control, a glass cockpit and a multi-mode radar, with external stores and drop tanks; the low-observability shaping and internal weapons that ADA lists belong to the AMCA, India's own fifth-generation project.

Remember · Fifth-gen = stealth shaping + internal weapons + sensor fusion + networking (F-35, and India's AMCA). Tejas Mk-1 has none of the stealth features.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (d) Water vapour

A hydrogen fuel cell combines hydrogen from the tank with oxygen from the air to produce electricity for the motor. The only chemical product of that reaction is water (H2O), so the tailpipe of a fuel-cell car releases just water vapour and some warm air, with no smog-forming pollutants.

• ✓ (d) At the cathode, the hydrogen ions, the electrons that have passed through the circuit and oxygen join to form water, which leaves as vapour.
• ✗ (c) Oxygen is consumed by the fuel cell, not given out; it is taken in from the air as a reactant.
• ✗ (a) Hydrogen peroxide (H2O2) is not a product of a fuel cell's hydrogen–oxygen reaction; the cell is designed to yield plain water.

Remember · Hydrogen fuel cell: 2H2 + O2 → 2H2O + electricity + heat. FCEV tailpipe emits only water vapour and warm air.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) Long duration energy storage

Pumped-storage hydropower is a way of storing electricity, not of watering crops. When power is cheap or surplus (for example sunny afternoons with lots of solar), water is pumped from a lower reservoir to an upper one; when demand peaks, it is released back down through turbines. It can hold energy for many hours, which is why it is discussed as long-duration storage for a grid with more solar and wind.

• ✓ (c) Two reservoirs at different heights work like a giant rechargeable battery; pumped storage is the dominant form of grid-scale energy storage worldwide.
• ✗ (b) Lift irrigation also pumps water uphill, but only to deliver it to fields; nothing flows back through turbines to generate electricity.
• ✗ (d) Rainwater harvesting collects and stores rainwater for use or groundwater recharge; it has no power-generation or storage role.

Remember · Pumped-storage hydro = pump water up with surplus power, release it through turbines at peak demand. The biggest grid-scale storage technology; key for integrating solar and wind.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (c) 1, 2, 3 and 4

Sustainable Aviation Fuel is made from renewable, non-petroleum feedstocks, and the US Department of Energy's list of them names all four materials: agricultural residues, corn grain, wet waste such as wastewater treatment sludge, and wood mill waste.

• ✓ 1. Agricultural residues are named on the DOE list of SAF feedstocks. They are a typical waste-based biomass source.
• ✓ 2. Corn grain is also on the list. It is a starch crop, which is why it feeds ethanol-based routes to jet fuel. ICAO's India study notes that maize supplies part of India's ethanol, and that ethanol can be turned into jet fuel by the alcohol-to-jet route.
• ✓ 3. Wet wastes, specifically manures and wastewater treatment sludge, are listed as feedstocks. ICAO's India study also mentions sewage sludge as a wet waste that can be turned into biocrude for SAF.
• ✓ 4. Wood mill waste is on the list. ICAO's India study lists sawdust and bark from sawmills among forest-based SAF feedstocks.

Remember · SAF feedstocks are broad: crops such as corn, crop and forest residues, wood mill waste, municipal solid waste, wet waste such as sludge, and fats, oils and greases.

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error