Renewable energy

Environmental Science (B.Sc) · Environmental management · free study notes for exams, placements and interviews

In short

Renewable energy is energy drawn from flows that are replenished on a human timescale - sunlight, wind, flowing water, biomass, geothermal heat and ocean energy - as against fossil fuels, which are stocks. For an environmental manager it is the main supply-side mitigation lever: it cuts CO₂ and local air pollutants per unit of useful energy and is the cheapest new generation in India today. India has crossed 50% of its installed electricity capacity from non-fossil sources (2025) against a target of 500 GW non-fossil capacity by 2030, so project appraisal, resource assessment and compliance work is growing fast. The topic is examined in B.Sc Environmental Science (energy resources and environmental management papers), in UGC-NET/GATE-ES style questions on CUF, Betz limit and energy payback, and is asked in interviews at solar/wind developers, EPC firms, ESG consultancies and state energy development agencies.

Explained

Definitions, classification and the resource base

Renewable: replenished naturally at a rate comparable to use. Non-conventional: the Indian policy term (MNRE's old name was the Ministry of Non-Conventional Energy Sources) covering solar, wind, small hydro, biomass, waste-to-energy and new technologies.
Non-fossil ≠ renewable: India's 500 GW "non-fossil" target includes large hydro and nuclear; MNRE's "renewable" statistics include large hydro since 2019 but nuclear never.
Classes: (i) *solar* - PV and solar thermal; (ii) *wind* - onshore and offshore; (iii) *hydro* - large (>25 MW, under the Ministry of Power) and small hydro (≤25 MW, under MNRE: micro ≤100 kW, mini 100 kW-2 MW, small 2-25 MW); (iv) *bioenergy* - combustion, gasification, biogas/CBG, bagasse cogeneration, ethanol; (v) *geothermal* - India has hot springs (Puga in Ladakh, Tattapani in Chhattisgarh) but negligible installed power; (vi) *ocean* - tidal, wave, OTEC, all at demonstration stage in India.
Firm vs infirm: solar and wind are variable/intermittent (infirm); biomass, biogas, small hydro with pondage and geothermal are dispatchable. This distinction drives storage, balancing and grid-integration questions.
Energy quality terms: *installed capacity* (kW/MW, a rating), *generation* (kWh/MWh, the output), *capacity utilisation factor* (CUF, India's usual term) or *capacity factor*.

Solar energy - resource, devices and systems

Solar constant ≈ 1,361 W/m² outside the atmosphere; at the ground, clear-sky peak is about 1,000 W/m² (the STC test condition, with AM 1.5 spectrum and 25 °C cell temperature).
India receives about 4-7 kWh/m²/day of global horizontal irradiance over 300+ clear days; Rajasthan, Gujarat and Ladakh are best. Resource data comes from NIWE/NISE and the MNRE solar radiation atlas.
Photovoltaic effect: light of photon energy above the band gap creates electron-hole pairs in a p-n junction; the junction field separates them and drives a current. Silicon band gap ≈ 1.1 eV.
Cell technologies: monocrystalline silicon (module efficiency ~20-23%), polycrystalline (~16-18%, now being displaced), thin film (CdTe, CIGS, a-Si, ~10-18%), and newer PERC/TOPCon/HJT and bifacial modules; perovskites are lab/pilot.
I-V curve terms: short-circuit current Isc, open-circuit voltage Voc, maximum power point (Vmp, Imp), fill factor FF = Pmax/(Voc·Isc). Output rises with irradiance and falls with cell temperature (power temperature coefficient about −0.3 to −0.45 %/°C) - so hot deserts lose some of their irradiance advantage.
System types: grid-connected rooftop (net/gross metering), ground-mounted utility scale, off-grid with battery, solar pumps (PM-KUSUM), floating solar (Ramagundam, Omkareshwar), agrivoltaics.
Balance of system: mounting structure, string/central inverter with MPPT, cables, junction boxes, protection, earthing, lightning arrester, and for off-grid a charge controller and battery.
Solar thermal: flat-plate collectors and evacuated tube collectors for water heating (domestic and industrial), solar cookers (box, dish), solar dryers, solar stills; concentrating solar power (CSP) - parabolic trough, linear Fresnel, central receiver (tower), parabolic dish - raises steam for a Rankine cycle and can add thermal storage (molten salt). India's CSP fleet is small, e.g. Dhursar and the NTPC/ONGC pilots.
Tilt and orientation: in India, modules face true south with tilt roughly equal to the latitude (flatter for summer bias, steeper for winter bias); row spacing is set to avoid inter-row shading on 21 December.
Standards: IS 14286 (crystalline silicon terrestrial PV modules - design qualification and type approval, aligned to IEC 61215) and the IS 16221 series on PV module safety; inverters, cables and earthing follow the relevant IS/IEC and CEA safety regulations. ALMM (Approved List of Models and Manufacturers) List-I governs which modules may be used in government-supported projects; quote scheme-specific rules as they are revised often.

Wind, hydro and ocean energy

Wind power in a stream of air: P = ½ρAv³. The cube law is the key idea - a 10% higher wind speed gives about 33% more power, so site selection and hub height dominate economics.
Betz limit: no turbine can extract more than 16/27 ≈ 59.3% of the kinetic energy, because the air must keep enough velocity to leave the rotor. Modern three-bladed turbines reach Cp ≈ 0.45-0.50 at best operating point.
Turbine types: horizontal-axis (HAWT, almost all commercial machines; upwind, 3 blades, yaw control, pitch control) and vertical-axis (Darrieus lift-type, Savonius drag-type - omnidirectional, used in small applications).
Operating points: cut-in speed ~3-4 m/s, rated speed ~11-14 m/s, cut-out ~25 m/s; above rated speed, blades pitch to shed power so output is flat.
Indian wind: resource mapped by NIWE at 100 m and 120 m hub heights; best in Tamil Nadu (Muppandal), Gujarat, Karnataka, Rajasthan, Maharashtra, Andhra Pradesh. Onshore CUF is typically 20-35%; repowering old low-hub-height machines is policy-supported. Offshore wind is being developed off Gujarat and Tamil Nadu under the National Offshore Wind Energy Policy (2015) with a viability-gap scheme announced in 2024.
Hydro: P = ρgQHη. Run-of-river schemes (little storage, low submergence, so a smaller environmental footprint), storage, and pumped storage (net consumer of energy, used as storage for balancing). Turbine choice by head: Pelton (high head, impulse), Francis (medium head, reaction), Kaplan/propeller (low head, high flow).
Ocean: tidal barrage/stream (Gulf of Kutch and Khambhat, Sundarbans creeks have potential; no commercial plant), wave energy (NIOT demonstration off Kerala/Tamil Nadu), OTEC needing ~20 °C surface-to-deep temperature difference.
Geothermal: dry steam, flash steam, binary cycle plants; ground-source heat pumps for space conditioning. India's provinces are mostly low-to-medium enthalpy, suited to direct heat use.

Bioenergy and green hydrogen

Biomass routes: direct combustion (bagasse cogeneration in sugar mills, rice-husk boilers), gasification (partial oxidation to producer gas - CO + H₂ + CH₄, CV ~4-6 MJ/m³), pyrolysis (bio-oil, char), anaerobic digestion (biogas, 55-70% CH₄, CV ~18-23 MJ/m³), transesterification (biodiesel from non-edible oils such as jatropha and used cooking oil), fermentation (ethanol from sugarcane molasses, grain, and 2G cellulosic).
Biogas plants: floating-drum (KVIC) and fixed-dome (Janata/Deenbandhu) designs; hydraulic retention time ~30-55 days; C:N ratio ~20-30:1; mesophilic ~35 °C; slurry pH 6.8-7.2. Digested slurry is a fertiliser - this co-benefit is examined.
Compressed biogas (CBG) is promoted through SATAT; ethanol blending reached the 20% (E20) target for petrol around 2025 under the national biofuel policy.
Waste-to-energy: incineration with energy recovery, RDF, landfill gas, biomethanation - judged by calorific value of the waste, and controlled under air-emission and solid-waste rules rather than renewable-energy law.
Green hydrogen: water electrolysis (alkaline, PEM, solid oxide) run on renewable power; about 50-55 kWh of electricity per kg of H₂ at realistic system efficiency; lower heating value of H₂ = 120 MJ/kg. The National Green Hydrogen Mission (2023, outlay ₹19,744 crore) targets 5 MMT/yr production by 2030 with the SIGHT incentives for electrolysers and hydrogen.
Carbon-neutral, not carbon-free: biomass combustion releases CO₂ that was recently fixed by photosynthesis, so net emissions are near zero only if the biomass is regrown and transport/processing energy is small.

Indian policy, law and institutions

Institutions: MNRE (nodal ministry); SECI (central auction/trading agency); IREDA (financing); NIWE (wind resource and turbine type certification), NISE (solar testing); state nodal agencies (e.g. GEDA, TEDA, ANERT, MEDA); CERC/SERCs (tariff and regulation), CEA, Grid-India (system operation).
Electricity Act, 2003: sections empowering SERCs to promote renewables, specify Renewable Purchase Obligation (RPO) and permit open access; renewable generation has must-run status under the grid code.
Energy Conservation (Amendment) Act, 2022: introduced a carbon credit trading scheme and a non-fossil energy consumption obligation on designated consumers, extending obligations beyond electricity distribution licensees.
Market instruments: Renewable Energy Certificates (RECs), green term-ahead and green day-ahead markets on power exchanges, Electricity (Promoting Renewable Energy Through Green Energy Open Access) Rules, 2022, and ISTS transmission-charge waivers for RE (being phased down).
Schemes: PM Surya Ghar: Muft Bijli Yojana (2024) for residential rooftop - central financial assistance of ₹30,000/kW for the first 2 kW and ₹18,000 for the third, capped at ₹78,000; PM-KUSUM (solar pumps and feeder solarisation); PLI for high-efficiency solar modules; solar park scheme; National Bioenergy Programme; VGF schemes for battery storage and offshore wind. Scheme rates are revised - verify the current MNRE notification before quoting.
Net metering: the Electricity (Rights of Consumers) Rules allow net metering for rooftop systems up to a prescribed size (500 kW in the central rules, with state variations); above that, net billing or gross metering applies.
Environmental clearance interface: solar PV and wind projects are generally not listed in the EIA Notification 2006 schedule, but still need land, forest (Forest (Conservation) Act), wildlife and CRZ clearances as applicable, and hydro above threshold capacity does need EIA - the detail belongs to the EIA note.

Environmental and social impacts, and limitations

Land use: utility solar needs roughly 2-2.5 hectares per MW (about 5-6 acres); competition with grazing, commons and agriculture is the main social issue; agrivoltaics and rooftop reduce it.
Wind: bird and bat collision (Great Indian Bustard habitat in Rajasthan/Gujarat is a live legal issue), noise, shadow flicker, blade disposal (thermoset composites are hard to recycle).
Hydro: submergence, displacement, sediment trapping, fish migration (needs fish ladders), methane from reservoirs, seismic and landslide risk in the Himalaya.
Solar PV end-of-life: modules are now covered as e-waste under the E-Waste (Management) Rules, 2022 amendments - EPR applies to PV panels, with recycling obligations deferred on a notified timeline.
Water: CSP and PV module cleaning consume water in arid regions; dry/robotic cleaning is the mitigation.
Technical limits: variability and forecasting error, duck curve and evening ramp, curtailment, low inertia and need for grid-forming inverters, must-run conflicts with thermal flexibility, and the cost of storage. Energy payback time of silicon PV in Indian insolation is about 1-2 years against a 25-year life, so the net energy balance is strongly positive.

Key points

Renewable - replenished on a human timescale; non-fossil - includes nuclear and large hydro, a wider policy category.
Solar constant - 1,361 W/m² extraterrestrial; STC - 1,000 W/m², 25 °C, AM 1.5.
kWp vs kWh - kWp is a nameplate rating at STC; kWh is what the system actually delivers.
CUF - actual generation ÷ (rated capacity × hours); ~16-21% for Indian solar, 20-35% onshore wind, higher offshore.
Performance ratio - actual output ÷ output expected from measured irradiance; a quality index, typically 0.75-0.85.
Betz limit - maximum 16/27 (59.3%) of wind kinetic energy extractable; real Cp ≈ 0.45-0.50.
Cube law - wind power varies as v³, so speed and hub height matter more than rotor size alone.
Hydro power - P = ρgQHη; turbine by head: Pelton > Francis > Kaplan.
Small hydro - ≤25 MW in India, under MNRE; above that, Ministry of Power.
Biogas - anaerobic digestion gives 55-70% CH₄, CV ~20 MJ/m³; slurry is a fertiliser.
Gasification - partial oxidation to producer gas, CV ~4-6 MJ/m³, one-tenth that of natural gas.
Green hydrogen - electrolysis on RE power, ~50-55 kWh/kg H₂; National Green Hydrogen Mission, 2023.
RPO - obligation on discoms and designated consumers to buy a notified share of renewable power; met directly or via RECs.
Must-run status - RE generation cannot be curtailed except for grid security.
PM Surya Ghar - residential rooftop subsidy, maximum ₹78,000 (₹30,000/kW for first 2 kW + ₹18,000 for the third).
ALMM - approved module list for government-supported projects; IS 14286 is the PV module qualification standard.
Land intensity - about 2-2.5 ha/MW for ground-mounted solar; PV waste now sits under the E-Waste Rules, 2022.
Energy payback time - ~1-2 years for silicon PV in India against a 25-year design life.

Formulas and examples

PV energy output: E = Pp × H × PR, where Pp = installed peak capacity (kWp), H = annual *peak sun hours* (kWh/m²/day × 365, equivalent full-sun hours), PR = performance ratio (dimensionless)
CUF (%) = [E (kWh) / (P (kW) × 8,760)] × 100 - use 8,784 for a leap year
Fill factor FF = (Vmp × Imp) / (Voc × Isc); module efficiency η = Pmax / (A × 1,000 W/m²)
Wind power P = ½ ρ A v³ Cp, ρ = air density (≈1.225 kg/m³ at 15 °C, sea level), A = swept area = πD²/4 (m²), v = wind speed (m/s), Cp = power coefficient (≤16/27)
Hydro power P = ρ g Q H η, ρ = 1,000 kg/m³, g = 9.81 m/s², Q = discharge (m³/s), H = net head (m), η = overall efficiency
Biogas energy = V (m³) × CV (MJ/m³); typical cattle dung yield 0.03-0.04 m³ of gas per kg of fresh dung
Levelised cost of energy LCOE = (CRF × Capex + annual O&M) / annual energy, with CRF = i(1+i)ⁿ / [(1+i)ⁿ − 1], i = discount rate, n = life in years
Simple payback (years) = net capital cost / annual saving; CO₂ avoided = E (MWh) × grid emission factor (≈0.71-0.73 tCO₂/MWh for the Indian grid, CEA database - revised yearly)
Degradation: output in year n = Year-1 output × (1 − d)ⁿ⁻¹, d = annual degradation (~0.5%/yr for crystalline silicon)

1. Rooftop solar - generation, subsidy and payback. A 5 kWp rooftop system in Delhi, CUF 16%, retail tariff ₹8/kWh, installed cost ₹2.5 lakh.

Annual generation = 5 × 8,760 × 0.16 = 7,008 kWh.
Annual bill saving = 7,008 × 8 = ₹56,064.
PM Surya Ghar subsidy = 30,000×2 + 18,000 = ₹78,000; net cost = 2,50,000 − 78,000 = ₹1,72,000.
Simple payback = 1,72,000 / 56,064 = 3.07 years.
CO₂ avoided = 7.008 MWh × 0.71 = ≈5.0 tCO₂ per year.
Output in year 25 at 0.5%/yr = 0.995²⁴ = 0.887 → about 88.7% of year-1 output.

2. Wind turbine power and the Betz limit. Rotor diameter 100 m, wind 8 m/s, ρ = 1.225 kg/m³.

A = π/4 × 100² = 7,854 m²; v³ = 512 m³/s³.
Available power = 0.5 × 1.225 × 7,854 × 512 = 2,463 kW.
Betz maximum (Cp = 0.593) = 1,461 kW; at a real Cp = 0.40 → 985 kW.
If the same machine is rated 2 MW and runs at CUF 25%: annual energy = 2,000 × 8,760 × 0.25 = 4.38 million kWh.
If the site wind were 9 m/s instead of 8 m/s, power rises by (9/8)³ = 1.42 times - a 12.5% speed gain for a 42% power gain.

3. Small hydro scheme. Q = 2 m³/s, net head 40 m, turbine-generator efficiency 85%.

P = 1,000 × 9.81 × 2 × 40 × 0.85 = 667 kW (a "mini hydro" plant).
At CUF 45%, annual energy = 667 × 8,760 × 0.45 = 2.63 million kWh.
Head 40 m with moderate flow → Francis turbine.

4. LCOE of a utility solar plant. 100 MW, capex ₹400 crore, O&M ₹4 lakh/MW/yr, i = 9%, n = 25 years, CUF 20%.

(1.09)²⁵ = 8.623 → CRF = 0.09 × 8.623 / 7.623 = 0.1018.
Annualised capex = 400 × 0.1018 = ₹40.72 crore; O&M = 100 × 4 lakh = ₹4 crore; total ₹44.72 crore/yr.
Annual energy = 1,00,000 kW × 8,760 × 0.20 = 17.52 crore kWh.
LCOE = 44.72 / 17.52 = ₹2.55 per kWh - below the variable cost of much new coal power, which is why solar wins auctions.

5. Family biogas plant. Five cattle giving 50 kg of fresh dung a day, yield 0.04 m³/kg, biogas CV 20 MJ/m³.

Gas = 50 × 0.04 = 2 m³/day; energy = 2 × 20 = 40 MJ/day.
1 m³ of biogas substitutes roughly 0.43 kg of LPG → 2 × 0.43 × 30 = ≈26 kg LPG a month, about 1.8 domestic cylinders (14.2 kg) - a saving of roughly ₹1,500 a month at prevailing cylinder prices.
Plant size: at 40 days retention and 1:1 dilution, digester volume ≈ 100 litres/day × 40 = ≈4 m³.

Source	Typical Indian CUF	Land / resource need	Dispatchable?	Main environmental concern
Solar PV (utility)	17-21%	2-2.5 ha/MW	No (unless stored)	Land use, module waste, cleaning water
Wind (onshore)	20-35%	Low (turbine pads only)	No	Birds/bats, noise, blade disposal
Small hydro	35-50%	Site-specific	Partly	Flow alteration, fish passage
Biomass / bagasse	50-70%	Large fuel catchment	Yes	PM and NOx emissions, fuel supply
Biogas / CBG	60-80%	Feedstock dependent	Yes	Slurry and odour management
Large hydro	30-45%	Reservoir submergence	Yes	Displacement, sediment, methane

Where it is used

Developers and IPPs (Adani Green, Tata Power Renewables, ReNew, NTPC Green, Suzlon, Vestas): resource assessment, yield estimation, DPR preparation, land and clearance management.
EPC and O&M roles: site survey, shadow analysis, string sizing, commissioning tests, PR and CUF monitoring, module soiling and degradation audits.
Consultancy and ESG: renewable energy audits for corporate RE100 and GHG Scope 2 targets, green open access structuring, RPO compliance filings before SERCs.
Government and regulatory: MNRE, SECI, IREDA, NIWE/NISE, state nodal agencies - scheme implementation, PM Surya Ghar and PM-KUSUM rollout, inspection and empanelment.
Rural and decentralised energy: solar pumps, solar home systems, biogas plants and improved cookstoves with KVIC, NGOs and CSR programmes; mini-grids in Bihar, Jharkhand and Ladakh.
Industry: captive solar and wind for cement, textiles and IT campuses; solar process heat and solar drying in food processing; green hydrogen pilots in refineries and fertiliser plants.

Interview questions

Q: What is the difference between kWp and kWh in a solar quotation? A: kWp is the module nameplate power at standard test conditions; kWh is the energy actually generated over time, which depends on irradiance, temperature, losses and performance ratio.
Q: Define CUF and give a realistic value for Indian solar. A: CUF is annual generation divided by rated capacity times 8,760 hours; Indian utility solar plants typically achieve 17-21%.
Q: Why is the Betz limit 16/27 and not 100%? A: If all kinetic energy were extracted the air would stop at the rotor and no further flow could pass; the optimum slows the wind to one-third of upstream speed, giving a maximum of 59.3%.
Q: A site's wind speed estimate rises from 7 to 8 m/s. What happens to the energy? A: Power scales as v³, so the available power rises by (8/7)³ ≈ 1.49 times - roughly 49%, which is why anemometry accuracy is critical.
Q: Why does PV output fall on a very hot day even in full sun? A: Cell voltage drops with temperature - about −0.3 to −0.45% of power per °C above 25 °C - so high module temperature offsets high irradiance.
Q: What is the difference between net metering and gross metering? A: Net metering credits only the surplus exported after self-consumption against imported units; gross metering sells all generation to the discom at a feed-in tariff and buys all consumption at retail tariff.
Q: What is RPO and how can an obligated entity comply? A: The Renewable Purchase Obligation is a notified minimum share of renewable power; it is met by buying RE directly, through open access, or by purchasing Renewable Energy Certificates.
Q: Is biomass power carbon neutral? A: Only approximately - the CO₂ released was recently fixed by the crop, but emissions from cultivation, transport, processing and any land-use change must be counted, and particulate emissions remain a local problem.
Q: Which turbine would you choose for a 300 m head and a small flow? A: A Pelton wheel - an impulse turbine suited to high head and low discharge.
Q: Why are solar modules now covered by e-waste rules? A: The E-Waste (Management) Rules, 2022 brought solar PV panels, cells and modules under extended producer responsibility, so producers must account for and recycle end-of-life panels on the notified timeline.
Q: What is the "duck curve" and why does it matter? A: Heavy midday solar pushes net demand down and creates a steep evening ramp when solar falls while demand peaks, requiring storage, flexible thermal plants or demand shifting.
Q: How much electricity does a kilogram of green hydrogen need? A: Roughly 50-55 kWh of renewable electricity at realistic system efficiency, plus about 9 litres of purified water stoichiometrically.

Common mistakes

Treating installed capacity as generation - quoting "500 GW" as though it were energy, or forgetting to multiply by CUF and 8,760.
Using the wind speed linearly instead of cubed, or omitting Cp and assuming the Betz value is achievable in practice.
Confusing performance ratio with efficiency, or with CUF - PR normalises for actual irradiance, CUF does not.
Calling nuclear and large hydro "renewable" in an exam answer, or forgetting that MNRE counts large hydro while nuclear is never renewable.
Mixing units in hydro and biomass sums - m³/s with litres/s, MJ with kWh (1 kWh = 3.6 MJ), kcal with kJ.
Quoting an outdated subsidy or RPO figure; these are revised by notification, so state the scheme and say the rate should be verified.
Claiming renewables have zero environmental impact, instead of discussing land use, biodiversity, water for cleaning and end-of-life waste.