Science & Technology
Energy Resources
Renewable Energy Technology: Solar, Wind, Bioenergy, Geothermal and Ocean
How solar cells, wind turbines, biogas plants, biofuels, geothermal and ocean energy work, with India's 2026 status, for CGPSC Prelims and Mains GS-II.
Prerequisites
- Conventional Energy Technology: energy conversion, capacity vs generation, capacity factor.
- Light and Sound: light as energy carried by photons.
- Non-conventional Energy (Geography): where India’s solar parks, wind farms and geothermal sites are, plus targets and schemes. This page explains the technology.
The Hook: A Solar Pump in a Mahasamund Paddy Field
Connect
Under Chhattisgarh’s Saur Sujala Yojana (सौर सुजला योजना), launched in 2016 and run through CREDA, thousands of farmers now irrigate their paddy with solar pumps. There is no diesel, no grid line and no bill. A few kilometres away, a rice mill may burn husk in its boiler to make steam and electricity.
How does a dark blue panel turn sunlight into a current that runs a motor? Why does a wind turbine produce eight times more power when the wind speed only doubles? And how does cow dung become cooking gas? This page explains the science behind the energy transition.
Why it matters: Prelims tests principles (photovoltaic effect, semiconductor used, Betz limit, gas in biogas, generations of biofuels, OTEC conditions). Mains GS-II asks how these technologies work and what limits them.
Where India Stands (July 2026)
| Source | Installed capacity |
|---|---|
| Solar | ~164.6 GW, the largest renewable source |
| Wind | ~58 GW |
| Hydro (large and small) | ~57 GW |
| Bio-power | ~11.8 GW |
| Nuclear | ~8.8 GW |
| Total non-fossil | ~300.5 GW, over 54% of ~552 GW total |
Non-fossil capacity crossed 50% of the total in June 2025, meeting the updated NDC target five years early. The 2030 target is 500 GW of non-fossil capacity (COP26, Glasgow, 2021). Details by state and scheme: Non-conventional Energy.
Solar Energy (सौर ऊर्जा)
The Sun’s energy comes from nuclear fusion of hydrogen into helium in its core. Two families of technology capture it.
1. Solar photovoltaic (PV): light → electricity directly
Photovoltaic effect (प्रकाश-वोल्टीय प्रभाव): when photons of light strike a semiconductor, they knock electrons free. A p–n junction inside the cell creates an internal electric field that pushes these electrons one way, producing a direct current (DC).
| Point | Detail |
|---|---|
| Main material | Silicon (a semiconductor), made from purified quartz sand |
| Output | DC; an inverter converts it to AC for the grid or home |
| Typical module efficiency | Around 20% or more for modern commercial silicon modules |
| Theoretical limit | A single-junction silicon cell cannot exceed about 33% (the Shockley–Queisser limit); tandem cells that stack two materials can go higher |
| No moving parts | Low maintenance; output falls slightly as panels heat up and gather dust |
Types of cells:
| Type | Feature |
|---|---|
| Monocrystalline silicon | Cut from a single crystal; most efficient silicon type; uniform dark colour |
| Polycrystalline (multicrystalline) silicon | Many crystals; cheaper, slightly less efficient; bluish, speckled look |
| Thin-film | Very thin layers of cadmium telluride (CdTe), CIGS (copper indium gallium selenide) or amorphous silicon; flexible, lower efficiency |
| Perovskite | New crystal-structure materials; cheap and high efficiency in the lab, but durability is still being solved |
| Bifacial modules | Capture light on both faces, including light reflected from the ground |
Ways to deploy PV:
- Utility-scale solar parks (Bhadla in Rajasthan, 2,245 MW; the Khavda park in Gujarat, under construction).
- Rooftop solar with net metering: surplus power is exported to the grid and set off against the bill (PM Surya Ghar: Muft Bijli Yojana, 2024).
- Floating solar on reservoirs: saves land and cuts evaporation; panels stay cooler and work better.
- Agrivoltaics: panels raised above crops so that land produces food and power together.
- Solar pumps for irrigation (PM-KUSUM, 2019; Saur Sujala in Chhattisgarh).
Manufacturing chain: polysilicon → ingot → wafer → cell → module. India assembles many modules but has depended heavily on imports for polysilicon, wafers and cells, which is why the PLI scheme for high-efficiency modules and the ALMM (Approved List of Models and Manufacturers) exist.
2. Solar thermal: light → heat
| Technology | How it works | Use |
|---|---|---|
| Flat-plate collector | A black absorber plate heats water flowing through tubes | Solar water heaters |
| Solar cooker | Box or dish concentrates sunlight | Cooking |
| Concentrated solar power (CSP) | Mirrors focus sunlight to heat a fluid, which makes steam to drive a turbine | Power plants |
| Parabolic trough | Curved mirrors focus on a pipe along their length | CSP |
| Solar power tower | A field of mirrors (heliostats) focuses on a receiver at the top of a tower | CSP |
| Molten-salt storage | Hot salt stores heat so a CSP plant can keep running after sunset | Storage |
PV vs CSP in one line
PV turns light straight into electricity using a semiconductor. CSP first makes heat, then uses a conventional steam turbine. CSP can store heat cheaply, but PV has become far cheaper to build, which is why almost all of India’s solar capacity is PV.
Wind Energy (पवन ऊर्जा)
Wind is moving air, driven by unequal heating of the Earth by the Sun. A turbine converts the kinetic energy of wind into rotation and then into electricity.
Power in the wind:
$$P = \tfrac{1}{2} \, \rho \, A \, v^{3}$$- \(\rho\) is air density, \(A\) is the area swept by the blades, \(v\) is wind speed.
- Power rises with the cube of wind speed: double the speed, eight times the power (\(2^3 = 8\)). A small gain in average wind speed matters enormously, which is why turbines are placed on ridges, coasts and mountain passes.
- Power rises with swept area, so longer blades and taller towers (stronger, steadier winds higher up) raise output.
Betz limit
No turbine can capture all the wind’s energy, because the air must keep moving past it. The theoretical maximum fraction is 16/27, about 59.3%, known as the Betz limit. Real turbines achieve less.
| Component | Job |
|---|---|
| Rotor blades | Shaped like aircraft wings; wind creates lift that turns them |
| Nacelle | Housing on top of the tower: gearbox, generator, controls |
| Gearbox | Raises the slow rotor speed to the high speed a generator needs (some designs are direct drive, without a gearbox) |
| Yaw system | Turns the nacelle to face the wind |
| Cut-in / cut-out speed | Turbine starts at a minimum wind speed and shuts down in storms to protect itself |
- Horizontal-axis turbines (three blades) dominate; vertical-axis designs suit small, turbulent sites.
- Onshore vs offshore: offshore winds are stronger and steadier but foundations, cables and maintenance cost much more. India has a target of 30 GW offshore wind by 2030; in June 2024 the Cabinet approved viability gap funding of ₹7,453 crore for India’s first 1 GW of offshore wind off Gujarat and Tamil Nadu. No large offshore wind farm was operating as of 2026.
- Repowering: replacing old, small turbines at the best windy sites (such as early Tamil Nadu and Gujarat clusters) with larger, taller ones.
- India’s installed wind capacity was about 58 GW (July 2026); Gujarat leads, followed by Tamil Nadu.
- NIWE (National Institute of Wind Energy) is at Chennai.
Bioenergy (जैव ऊर्जा)
Biomass is organic matter: crop residue, wood, dung, husk, bagasse, municipal waste. It stores solar energy captured by photosynthesis, so burning it releases carbon that plants absorbed recently. It is broadly considered carbon-neutral if regrown sustainably.
Routes to energy
| Route | Process | Product |
|---|---|---|
| Direct combustion | Burn biomass in a boiler | Steam and electricity (rice-husk and bagasse plants, sugar-mill cogeneration) |
| Gasification | Heat with limited air | Producer gas (CO, H₂, CH₄) for engines |
| Anaerobic digestion | Bacteria break down wet waste without oxygen | Biogas |
| Fermentation | Yeast converts sugars to alcohol | Ethanol |
| Transesterification | Vegetable oil or fat reacts with an alcohol (methanol) | Biodiesel (by-product: glycerol) |
Biogas (गोबर गैस / बायोगैस)
- Made by anaerobic digestion of cattle dung, kitchen waste or sewage by methanogenic bacteria (methanogens).
- Composition: mainly methane (about 50–70%) and carbon dioxide (about 30–40%), with traces of hydrogen sulphide and moisture.
- The leftover slurry is a good organic manure.
- Compressed biogas (CBG / Bio-CNG): biogas purified to remove CO₂ and H₂S, raising methane to over 90%, then compressed. It can replace CNG in vehicles.
- Schemes: SATAT (2018) to set up CBG plants and buy their gas; GOBARdhan (2018) for village cattle-dung and organic waste.
Generations of biofuels
| Generation | Feedstock | Issue |
|---|---|---|
| First (1G) | Food crops: sugarcane juice and molasses, maize, surplus grain (ethanol); edible oils (biodiesel) | Food vs fuel conflict; water use |
| Second (2G) | Non-food lignocellulosic biomass: crop residue (rice straw, wheat straw), bagasse, wood | Harder to break down; costlier. Indian Oil’s 2G ethanol plant at Panipat (2022) uses rice straw |
| Third (3G) | Algae | High yield per hectare; still costly |
| Fourth (4G) | Genetically engineered organisms, often with carbon capture | Experimental |
- Ethanol Blending Programme: E20 (20% ethanol in petrol) was reached in 2025, five years ahead of the original 2030 target (National Policy on Biofuels, 2018, amended 2022). Ethanol cuts crude imports and gives farmers another market.
- Biodiesel sources include non-edible oilseeds such as Jatropha and used cooking oil.
Chhattisgarh angle: as a major paddy state, Chhattisgarh produces large volumes of rice husk and straw. Husk-fired power plants already run in the state, and 2G ethanol and CBG plants could turn straw into fuel instead of it being burnt in the field.
Geothermal Energy (भूतापीय ऊर्जा)
Heat inside the Earth comes from the decay of radioactive elements (uranium, thorium, potassium) and heat left over from the planet’s formation. Temperature rises with depth (the geothermal gradient), roughly 25–30 °C per kilometre in typical continental crust.
| Plant type | Resource |
|---|---|
| Dry steam | Steam straight from the ground spins a turbine (rare) |
| Flash steam | High-pressure hot water “flashes” into steam when pressure drops |
| Binary cycle | Moderate-temperature water heats a second fluid with a lower boiling point, which drives the turbine. Suits most Indian sites |
| Direct use / ground-source heat pumps | Space heating, greenhouses, drying, bathing |
- Indian sites: Puga (Ladakh), Manikaran (Himachal Pradesh), Tattapani (तातापानी) in Balrampur, Chhattisgarh, and the Cambay basin (Gujarat).
- The National Policy on Geothermal Energy was notified by MNRE on 15 September 2025. India had no commercial geothermal power plant as of 2026.
- Advantage: steady, round-the-clock output. Limits: suitable sites are few and remote; drilling is costly; hot water can carry dissolved gases and minerals.
Ocean Energy
| Form | How it works | Condition / India |
|---|---|---|
| Tidal (ज्वारीय) | A barrage traps water at high tide and releases it through turbines at low tide; or tidal stream turbines sit in fast currents | Needs a large tidal range. Gulf of Khambhat (highest on India’s coast), Gulf of Kachchh, Sundarbans. Projects remain at the study stage |
| Wave | Floating or shore devices convert the up-and-down motion of waves | Early pilot at Vizhinjam (Kerala) in the 1990s |
| Ocean thermal energy conversion (OTEC) | Uses the temperature difference between warm surface water and cold deep water to run a low-boiling-point fluid cycle | Needs a difference of about 20 °C, so works only in tropical seas. India’s pilot at Kavaratti (Lakshadweep) uses the same principle for low-temperature thermal desalination (LTTD), producing drinking water |
Analysis for Mains: Strengths and Limits of Renewables
| Strengths | Limits |
|---|---|
| No fuel cost and no fuel imports once built | Intermittency: solar stops at sunset; wind is seasonal (strongest in the south-west monsoon) |
| Very low lifecycle emissions | Low capacity factor compared with coal or nuclear |
| Modular: from a single pump to a gigawatt park | Evening peak problem: demand peaks after sunset, when solar output is zero, so storage or other sources must fill the gap |
| Falling costs; solar is among India’s cheapest new power | Land: large parks compete with farms, grazing land and habitats (the Great Indian Bustard in Rajasthan) |
| Jobs in rural areas (pumps, biogas, maintenance) | Import dependence for polysilicon, cells and wind components; end-of-life waste: old solar panels are covered under the E-Waste (Management) Rules, 2022 |
Way forward: storage (batteries, pumped hydro), green energy corridors for transmission, hybrid solar-wind parks, domestic manufacturing (PLI, ALMM), agrivoltaics and floating solar to save land, and bioenergy from crop residue. See Energy Storage and Hydrogen.
Memory Tricks
Wind power: 'Double the breeze, eight times the ease'
\(P \propto v^3\). Double the wind speed, power rises \(2^3 = 8\) times. And remember the Betz limit = 59.3% as “Betz says 59”.
Biofuel generations: 'Food, Field-waste, Algae, Altered genes'
1G food crops · 2G field waste (straw, bagasse) · 3G algae · 4G altered (engineered) organisms.
Biogas recipe: 'M-C 60-40'
Biogas is roughly 60% Methane, 40% Carbon dioxide, made by bacteria without oxygen (anaerobic).
Don’t Confuse
| This | vs | That |
|---|---|---|
| Solar PV: light → electricity (semiconductor, DC) | vs | Solar thermal / CSP: light → heat → steam turbine |
| Monocrystalline: single crystal, more efficient | vs | Polycrystalline: many crystals, cheaper |
| Biogas: anaerobic digestion, methane + CO₂ | vs | Producer gas: biomass gasification, CO + H₂ |
| Ethanol: fermentation of sugar, blended with petrol | vs | Biodiesel: transesterification of oils, blended with diesel |
| Tidal: needs high tidal range (Khambhat) | vs | OTEC: needs ~20 °C temperature difference (tropical deep sea) |
| Geothermal: Earth’s internal heat (Tattapani) | vs | Solar thermal: Sun’s heat |
| Small hydro: up to 25 MW | vs | Large hydro: above 25 MW, also counted as RE since 2019 |
Common Mistakes
MCQ traps
- “Solar panels produce AC that can be fed straight into the grid.” False. PV gives DC; an inverter is needed.
- “A well-designed turbine can capture 80–90% of wind energy.” False. The Betz limit caps it at about 59.3%.
- “Wind power is proportional to wind speed.” False. It is proportional to the cube of wind speed.
- “Biogas is mainly hydrogen.” False. It is mainly methane, with CO₂.
- “Second-generation biofuels are made from sugarcane juice.” False. That is first generation; 2G uses non-food residue such as straw.
- “India has an operating tidal power station.” False (as of 2026); projects are at the study stage.
- “Geothermal heat comes from the Sun.” False. It comes mainly from radioactive decay inside the Earth.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The semiconductor most widely used in solar cells is ________.
- The maximum fraction of wind energy any turbine can theoretically capture is about ________ %.
- Biogas is produced by ________ (aerobic / anaerobic) digestion and is mainly ________.
Answers
- Silicon.
- 59.3% (Betz limit, 16/27).
- Anaerobic; methane (with carbon dioxide).
Level 2: Prelims MCQs
Q1. Consider the following statements about solar photovoltaic (PV) cells:
- A PV cell converts sunlight directly into alternating current.
- PV cells work on the photovoltaic effect at a p–n junction of a semiconductor.
- Bifacial modules can generate power from light falling on both of their faces.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
(c) 2 and 3 only. PV cells produce direct current; an inverter converts it to AC, so statement 1 is wrong. Statement 2 states the principle correctly. Bifacial modules also use light reflected from the ground onto their rear face.
Q2. If the average wind speed at a site doubles, the power available in the wind becomes, other things remaining the same:
- (a) Two times
- (b) Four times
- (c) Six times
- (d) Eight times
Answer and explanation
(d) Eight times. Power in the wind is \( \tfrac{1}{2}\rho A v^3 \), so it varies with the cube of speed: \(2^3 = 8\). This is why siting on windy passes and coasts matters so much.
Q3. Match List I (Biofuel generation) with List II (Typical feedstock):
| List I | List II |
|---|---|
| A. First generation | 1. Algae |
| B. Second generation | 2. Genetically engineered organisms |
| C. Third generation | 3. Sugarcane molasses and grains |
| D. Fourth generation | 4. Rice straw and bagasse |
- (a) A-3, B-1, C-4, D-2
- (b) A-3, B-4, C-1, D-2
- (c) A-4, B-3, C-1, D-2
- (d) A-3, B-4, C-2, D-1
Answer and explanation
(b) A-3, B-4, C-1, D-2. 1G uses food crops; 2G uses non-food lignocellulosic residue such as straw and bagasse; 3G uses algae; 4G uses engineered organisms, often with carbon capture.
Q4. Ocean thermal energy conversion (OTEC) is feasible mainly in tropical seas because it requires:
- (a) A tidal range of more than 5 metres
- (b) A temperature difference of about 20 °C between surface and deep water
- (c) Continuous strong onshore winds
- (d) Volcanic activity on the sea floor
Answer and explanation
(b). OTEC runs a heat engine between warm surface water and cold deep water; only tropical seas offer a difference of about 20 °C. A large tidal range is needed for tidal power, not OTEC.
Q5. Tattapani in Chhattisgarh is associated with which source of energy?
- (a) Tidal energy
- (b) Wind energy
- (c) Geothermal energy
- (d) Small hydropower
Answer and explanation
(c) Geothermal energy. Tattapani in Balrampur district has hot springs and is one of India’s identified geothermal sites, alongside Puga (Ladakh) and Manikaran (Himachal Pradesh). There was no commercial geothermal power plant in India as of 2026.
Level 3: Mains Answer Writing
Q1. (60 words) Distinguish between solar photovoltaic and concentrated solar power technologies.
Model answer outline
- PV: photovoltaic effect in a semiconductor (silicon); light → DC electricity directly; no moving parts; modular from rooftop to park.
- CSP: mirrors (troughs, heliostats on a tower) concentrate sunlight to make heat → steam → turbine; can store heat in molten salt.
- India: almost all solar capacity is PV because it is cheaper; CSP’s advantage is cheap thermal storage.
Q2. (150–200 words) Bioenergy can address both the energy and the agricultural-waste problems of a paddy state like Chhattisgarh. Discuss.
Model answer outline
- Intro: Biomass stores solar energy through photosynthesis; in a paddy state, husk and straw are abundant residues.
- Technologies: husk-fired power and cogeneration; gasification (producer gas); anaerobic digestion to biogas and CBG (SATAT, GOBARdhan); 2G ethanol from straw (Panipat model).
- Benefits: extra farm income; less stubble burning and air pollution; lower crude imports (E20 reached in 2025); organic slurry as manure; rural jobs.
- Challenges: seasonal and scattered supply of residue; collection and storage costs; 2G technology costs; food-vs-fuel concerns for 1G ethanol from grain.
- Chhattisgarh angle: large paddy output; existing rice-husk plants; cattle dung for village biogas under GOBARdhan; CREDA as the nodal agency.
- Conclusion: With assured collection chains and offtake prices, bioenergy can turn waste into fuel and income.
Quick Revision Table
| Fact | Detail |
|---|---|
| Solar capacity | ~164.6 GW (July 2026), largest RE source |
| PV principle | Photovoltaic effect at a p–n junction; silicon; DC output; inverter |
| Single-junction limit | About 33% (Shockley–Queisser) |
| Thin-film materials | CdTe, CIGS, amorphous silicon |
| CSP | Mirrors → heat → steam turbine; heliostats; molten-salt storage |
| Wind power | \( P = \tfrac{1}{2}\rho A v^3 \); cube of speed |
| Betz limit | 16/27 ≈ 59.3% |
| Wind capacity | ~58 GW (July 2026); Gujarat leads; NIWE Chennai |
| Offshore wind | 30 GW target by 2030; VGF ₹7,453 crore for 1 GW (June 2024) |
| Biogas | Anaerobic digestion; methane ~50–70% + CO₂ |
| Biofuel generations | 1G food, 2G residue, 3G algae, 4G engineered |
| Ethanol | E20 reached in 2025 |
| Geothermal | Radioactive decay; binary cycle; Tattapani (Balrampur, CG); policy 15 Sep 2025 |
| Tidal / OTEC | Large tidal range (Khambhat) / ~20 °C difference (Kavaratti LTTD) |
Key Takeaways
Key Takeaways
- Solar PV turns light into DC electricity in a semiconductor; CSP turns light into heat for a turbine.
- Wind power varies with the cube of speed, and the Betz limit (59.3%) caps any turbine.
- Biogas (methane + CO₂) comes from anaerobic digestion; biofuels move from food crops (1G) to residues (2G) and algae (3G).
- Geothermal (Tattapani) and ocean energy are steady but largely untapped in India.
- The main limits of renewables are intermittency, land and imports, which is why storage matters.
What’s Next
- Energy Storage and Hydrogen: how India plans to store solar and wind power
- Nuclear Energy: the other large source of low-carbon base load
- Agricultural Science: crop residue, biomass and farm technology
- Power Generation in Chhattisgarh: CREDA and solar in the state
Advertisement