Small hydro, geothermal and ocean energy

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In short

These are the "other" renewables: energy taken from falling water in small streams, from heat stored in the Earth's crust, and from the tides, waves and thermal layers of the sea. In India small hydro (up to 25 MW, the MNRE definition) is the only one of the three that is commercially built at scale — roughly 5.1 GW installed against an assessed potential of about 21,133 MW at some 7,100 sites (AHEC, IIT Roorkee database; MNRE updates the installed figure monthly) — while geothermal power and ocean power remain at pilot and feasibility stage. The topic is examined in GATE (EE/ME/CE "non-conventional energy sources"), in university renewable-energy and hydraulic-machines papers, and in interviews at NHPC, SJVN, THDC, state hydro corporations (KPCL, HPPCL, UJVNL), Jyoti/Flovel/Voith/Andritz turbine makers, ONGC and GSI (geothermal), and NIOT Chennai (ocean). It matters because small hydro is a dispatchable, decades-long asset in Himalayan and Western Ghat states, geothermal is a baseload option for Ladakh, and ocean energy is predictable even when wind and sun are not.

Explained

1. Small hydro: classification, schemes and civil works

Indian size classes (MNRE): micro up to 100 kW; mini 101 kW to 2 MW; small above 2 MW up to 25 MW. Anything above 25 MW is "large hydro" — and since the Cabinet decision of March 2019 large hydro is also counted as renewable, with a separate Hydro Purchase Obligation.
Head classes (as used in Indian SHP practice): ultra-low below about 3 m; low up to about 30 m; medium about 30–100 m; high above 100 m. Low head means large discharge and bulky machines; high head means small discharge and long penstocks.
Scheme types: (a) run-of-river — a diversion weir, no storage, output follows the stream; (b) canal-based / canal-fall — a drop on an irrigation canal, cheapest and least intrusive; (c) dam-toe — uses an existing dam's release; (d) penstock-on-existing-conduit (water supply mains, tailraces).
Water-conductor system, in order: diversion weir/trench intake → intake gate and trash rack → desilting tank → power channel or low-pressure pipe → forebay (with spillway) → penstock → powerhouse (turbine, generator, governor, valves) → draft tube → tailrace back to the river.
Surge tank / forebay absorbs water-hammer pressure rise on sudden load rejection; essential when the penstock is long.
Hydrology: build a flow duration curve (FDC) from 10 years of daily or ten-daily discharge; the design discharge is usually the flow available 30–50% of the year, with mandatory environmental (minimum) flow deducted first. Firm power corresponds to 90–95% dependable flow.
Indian guidance: the AHEC–MNRE Standards/Guidelines for Small Hydropower Development (series covering hydrology, civil works, electro-mechanical equipment, SCADA); IS 12800 series for selection of turbines and preliminary dimensioning and layout of small hydro; IEC 61116 for electro-mechanical equipment in small hydro; IS 11625 for penstock design criteria. Where unsure of an IS number, cite the subject.
Clearances: no EIA clearance is needed below 25 MW under the EIA Notification, 2006 (river-valley projects of 25 to under 50 MW are Category B, 50 MW and above Category A); state water-use/lease and forest clearance still apply, and tariff is determined by the SERC or by competitive bidding under the Electricity Act, 2003.

2. Small hydro: turbines, generators and performance

Impulse turbines (atmospheric pressure in the runner, energy converted to a jet in the nozzle): Pelton (high head, 60 m to >1,000 m), Turgo (medium-high head, more flow than Pelton for the same diameter), Cross-flow / Banki–Mitchell (low to medium head, very flat efficiency curve, cheap, widely used in Indian micro hydro).
Reaction turbines (pressure drops through the runner, needs a draft tube to recover exit kinetic energy): Francis (medium head 10–350 m, spiral casing, adjustable guide vanes), Kaplan/propeller (low head 2–30 m, adjustable blades; bulb, tubular, S-type and siphon layouts for ultra-low head and canal falls), and pump-as-turbine (PAT) — a standard centrifugal pump run in reverse, cheap but with no flow regulation.
Selection is by specific speed and head; part-load behaviour matters more in SHP than peak efficiency because the stream varies. Double-regulated Kaplan and cross-flow hold efficiency down to ~25% flow; propeller and PAT fall off sharply.
Cavitation: if local pressure falls to vapour pressure, bubbles collapse and pit the runner. Controlled by limiting the setting level (suction head) using Thoma's cavitation coefficient σ; reaction turbines are the ones at risk, impulse turbines are not.
Runaway speed: the no-load speed if the generator trips — about 1.8× rated for Pelton, 2–2.2× for Francis, up to 2.5–3× for Kaplan; all rotating parts must survive it.
Generators: synchronous machines with AVR for grid-connected and isolated plants; squirrel-cage induction generators (cheap, need grid or capacitors) for small grid-tied units; electronic load controllers (ELC) replace mechanical governors in micro hydro by dumping surplus power into a resistive load.
Advantages: 30–50 year life, high plant load factor where flow is perennial, dispatchable within the day, no fuel, low land and no reservoir submergence in run-of-river schemes, suitable for isolated mini-grids.
Limitations: site-specific and non-transferable, high civil cost share (often 60–70% of capex), seasonal and monsoon-dependent output, silt abrasion (a serious problem in Himalayan rivers), trash and freezing at high altitude, fish passage and minimum-flow obligations.

3. Geothermal energy

Source: radioactive decay plus primordial heat; the average geothermal gradient is about 25–30 °C/km, rising to 100 °C/km or more in volcanic and rift zones. A usable field needs heat, a reservoir fluid, permeability and a cap rock.
Resource types: hydrothermal (vapour-dominated or liquid-dominated), hot dry rock / EGS, geopressured, and magma (research only). Classified by temperature: high (>180 °C, power generation), medium (100–180 °C, binary power and industry), low (<100 °C, direct use).
Power-plant types:
- Dry steam: dry saturated steam from the well drives the turbine directly (Larderello, The Geysers).
- Single/double flash: hot pressurised brine is flashed in a separator; steam to the turbine, brine reinjected. Double flash recovers a second, lower-pressure steam fraction and adds 15–25% output.
- Binary (ORC/Kalina): brine heats an organic working fluid (R-245fa, isobutane, pentane) or an ammonia–water mixture in a heat exchanger; the geothermal fluid never touches the turbine. The only option below about 150 °C, and the one relevant to India.
- Flash–binary hybrid: binary bottoming cycle on the separated brine.
Direct (non-power) use: space and greenhouse heating, aquaculture, crop and timber drying, balneology, industrial process heat, and ground-source heat pumps (GSHP) that use the stable shallow ground temperature (about 25–30 °C in much of India) as a sink in summer and a source in winter.
Efficiency: low, because the source temperature is low — utilisation efficiency (actual work / exergy of the fluid) is 25–45%, while first-law conversion is typically 10–13% (binary) to 15–20% (flash/dry steam).
Indian position: GSI has mapped about 340 hot springs in seven geothermal provinces — Puga and Chumathang (Ladakh), Tattapani (Chhattisgarh), Cambay graben (Gujarat), Manikaran/Parvati (Himachal), Surajkund and Tapovan, Bakreshwar (West Bengal), Rajgir (Bihar). Estimated potential is often quoted as about 10,600 MWe, but this is a resource estimate, not a proven reserve. India has no commercial geothermal power plant yet; a pilot at Puga in Ladakh (ONGC/LAHDC) and exploratory work at Tattapani are the leading efforts, and MNRE circulated a draft geothermal policy in 2022 — status of any final policy should be checked rather than assumed.
Limitations: drilling cost and dry-hole risk dominate; scaling and corrosion from dissolved silica, chloride and H₂S; H₂S and CO₂ emissions (small but not zero); induced seismicity in EGS; reservoir pressure decline unless brine is reinjected; remote, high-altitude, ecologically sensitive sites. Steam-side pressure parts fall under the Boilers Act, 2025 and the Indian Boiler Regulations, 2026.

4. Ocean energy: tidal range and tidal stream

Origin: gravitational pull of the Moon (dominant) and Sun; two highs and two lows a day in semi-diurnal regimes, with spring and neap variation. Tides are predictable years ahead — their key advantage over wind and solar.
Tidal range (barrage) schemes: a barrage across an estuary with sluices and low-head bulb turbines.
- Single-basin, ebb generation: fill on flood through sluices, hold, generate on the falling tide. Most common.
- Single-basin, flood generation: generate while filling; lower energy and worse for the estuary.
- Single-basin, two-way (double-effect): generate on both flood and ebb; smaller range per cycle but more hours of output.
- Double-basin: one high and one low basin give near-continuous output at higher cost.
- Economic viability needs a mean range of roughly 5 m or more and a large basin with a narrow mouth.
Tidal stream (current) turbines: horizontal- or vertical-axis rotors in a fast channel, working exactly like wind turbines in a 1,000× denser fluid, so a 2.5 m/s current is energetically comparable to a strong wind. Need ~2–2.5 m/s peak spring velocity; outputs scale with v³.
Indian resource and projects: tidal potential is commonly assessed at about 12,455 MW, concentrated in the Gulf of Khambhat (~7,000 MW), Gulf of Kutch (~1,200 MW) and the Sundarbans creeks. A 50 MW Kutch project and the 3.75 MW Durgaduani tidal scheme in the Sundarbans were sanctioned but not commissioned; India has no operating tidal power station.
Limitations: very high civil cost, long payback, intermittency tied to the tide clock (four short generation windows a day for single-effect), sediment trapping, changes to the tidal prism and intertidal ecology, navigation and fish migration, severe corrosion and biofouling.

5. Ocean energy: waves, OTEC and salinity gradient

Wave energy is wind energy concentrated by the sea surface; power is quoted as kW per metre of wave crest. India's west coast averages roughly 5–15 kW/m (monsoon peaks far higher) against 40–70 kW/m on the Atlantic coasts of Europe — so Indian wave resource is modest; the often-quoted ~40,000 MW figure is a theoretical coastline estimate.
Wave energy converter (WEC) classes: oscillating water column (OWC) — a partly submerged chamber whose air column drives a bidirectional Wells turbine (the NIOT/IIT Madras OWC caisson at Vizhinjam, Kerala, is India's demonstration plant); point absorber (heaving buoy); attenuator (Pelamis-type articulated raft); oscillating wave surge converter (bottom-hinged flap); overtopping device (ramp into a reservoir plus low-head turbine); submerged pressure differential.
Classified by location as shoreline, nearshore and offshore — offshore has the most energy and the hardest moorings and cabling.
OTEC (Ocean Thermal Energy Conversion): uses the temperature difference between warm surface water (26–29 °C) and deep water at 1,000 m (4–7 °C). Needs ΔT ≥ 20 °C, which the Indian Ocean around Lakshadweep, the Andamans and the southern tip provides year-round.
- Open (Claude) cycle: flash-evaporate warm sea water in a vacuum, expand the low-pressure steam, condense — gives desalinated water as a by-product.
- Closed (Anderson) cycle: warm water boils ammonia or R-134a in a heat exchanger; vapour drives a turbine; cold water condenses it.
- Hybrid cycle: closed cycle for power plus flash evaporation for fresh water.
- Carnot efficiency is only about 6–8% and net efficiency 2–3% after the enormous cold-water pumping load, so cold-water pipe cost dominates. NIOT (Ministry of Earth Sciences) runs the Indian programme, including LTTD (low-temperature thermal desalination) plants at Kavaratti, Minicoy and Agatti in Lakshadweep and an OTEC-powered desalination plant under the Deep Ocean Mission.
Salinity gradient (osmotic) power: PRO (pressure-retarded osmosis) drives a turbine with the osmotic pressure built across a membrane between river and sea water; RED (reverse electrodialysis) generates directly across ion-exchange membranes. Theoretical energy of mixing is about 0.7–0.8 kWh per m³ of fresh water; membrane fouling keeps it at laboratory and pilot scale.
Regulatory note: MNRE clarified in 2019 that ocean energy — tidal, wave and OTEC — qualifies as renewable energy for RPO purposes. Ocean projects also need CRZ clearance under the CRZ Notification, 2019 and shipping/navigation concurrence (the Merchant Shipping Act, 2025 replaced the 1958 Act from 15 March 2026, and DG Shipping is now the Director General of Maritime Administration).

Key points

Small hydro (India) — up to 25 MW; micro ≤100 kW, mini 101 kW–2 MW, small 2–25 MW; nodal ministry MNRE, technical centre AHEC, IIT Roorkee.
Assessed SHP potential — about 21,133 MW at ~7,100 sites; about 5.1 GW built, led by Karnataka, Himachal Pradesh, Maharashtra and Uttarakhand.
Hydro power equation — P = ρgQHη; output is linear in both head and flow, unlike wind's cube law.
Net head — gross head minus friction and local losses in channel, penstock and trash rack; always use net head for power.
Flow duration curve — ranks flows by exceedance; design discharge at 30–50% exceedance, firm power at 90–95%.
Specific speed — Nₛ = N√P/H^1.25 selects the runner: Pelton low, Francis medium, Kaplan high.
Impulse vs reaction — impulse works at atmospheric pressure with a jet (no cavitation, no draft tube); reaction has a pressure drop across the runner and needs a draft tube.
Cross-flow turbine — flat efficiency curve from 25% to 100% flow, cheapest choice for Indian micro hydro.
Runaway speed — up to 3× rated on Kaplan; the mechanical design case on load rejection.
Geothermal gradient — about 25–30 °C/km average; a useful reservoir needs heat plus permeability plus a cap rock.
Plant types — dry steam, single/double flash, binary (ORC/Kalina), hybrid; binary is the only route below ~150 °C.
Reinjection — returning spent brine sustains reservoir pressure and avoids surface pollution; it is standard practice, not optional.
Indian geothermal — Puga/Chumathang, Tattapani, Cambay, Manikaran, Bakreshwar; ~10,600 MWe resource estimate; no commercial plant yet.
Tides are predictable; waves are forecastable for days; OTEC is nearly constant — all three contrast with solar and wind variability.
Tidal energy per cycle — E = ½ρgAR²; it scales with the square of the tidal range, so range matters far more than basin area.
Tidal stream — P = ½ρAv³Cₚ, same as wind but with ρ ≈ 1,025 kg/m³.
Wave power — P ≈ 0.49 Hₛ²Tₑ kW per metre of crest; India's coast is a modest 5–15 kW/m resource.
OTEC — needs ΔT ≥ 20 °C; Carnot ~7%, net 2–3%; open (Claude) cycle also yields fresh water; NIOT leads in India.

Formulas and examples

Hydro power: P = ρ g Q H_net η_o — P in W; ρ = 1,000 kg/m³ (water); g = 9.81 m/s²; Q = discharge (m³/s); H_net = net head (m); η_o = overall efficiency = η_turbine × η_generator × η_drive (0.70–0.85 for SHP).
Net head: H_net = H_gross − h_f − h_minor, with pipe friction h_f = f L V² / (2 g D) (Darcy–Weisbach) — f friction factor, L and D penstock length and diameter (m), V velocity (m/s).
Specific speed (metric, turbines): Nₛ = N √P / H^1.25 — N in rpm, P in kW, H in m. Pelton ≈ 10–60; Francis ≈ 60–300; Kaplan ≈ 300–1,000.
Synchronous speed: N = 120 f / p — f = 50 Hz, p = number of poles.
Annual energy: E = P_rated × 8,760 × CF (kWh); capacity factor CF = annual energy / (rated power × 8,760).
Penstock shell thickness: t = p D / (2 σ_allow) + corrosion allowance — p = design pressure including water-hammer rise (Pa), σ_allow = allowable hoop stress (Pa).
Geothermal heat extracted: Q̇ = ṁ c_p (T_in − T_out) (kW); ORC output W = Q̇ × η_cycle; Carnot limit η = 1 − T_cold/T_hot (kelvin); depth for a target temperature z = (T − T_surface) / gradient.
Heat-pump COP: COP_heating = Q_delivered / W_input; Carnot ceiling = T_hot / (T_hot − T_cold) in kelvin.
Tidal barrage energy per cycle (single-effect): E = ½ ρ g A R² (J) — A = basin area (m²), R = tidal range (m), ρ ≈ 1,025 kg/m³ (sea water). Average power = E × cycles per day × η / 86,400.
Tidal stream / current turbine: P = ½ ρ A v³ Cₚ — A = swept area (m²), v = current speed (m/s), Cₚ ≤ 0.593 (Betz).
Regular wave power per metre of crest: P = ρ g² H² T / (32π) (deep water, H = wave height, T = period).
Irregular sea state: P = ρ g² Hₛ² Tₑ / (64π) ≈ 0.49 Hₛ² Tₑ kW/m — Hₛ = significant wave height (m), Tₑ = energy period (s).
OTEC net power: P_net = P_turbine − P_pumps(cold water + warm water + working fluid); the cold-water pump alone can take 20–30% of gross.

Example 1 — Sizing a run-of-river SHP unit. Design discharge 4 m³/s after deducting environmental flow, gross head 22 m, total hydraulic losses 2 m, turbine 0.90, generator 0.95, drive 0.97.

H_net = 22 − 2 = 20 m; η_o = 0.90 × 0.95 × 0.97 = 0.829.
P = 1,000 × 9.81 × 4 × 20 × 0.829 = 784,800 × 0.829 ≈ 650.6 kW → install a 650 kW unit.
At CF = 0.45 (monsoon-fed stream): E = 650.6 × 8,760 × 0.45 ≈ 2.564 million kWh/year.
At a tariff of ₹4.50/kWh, revenue ≈ ₹1.15 crore/year.

Example 2 — Turbine choice from specific speed. Same plant, P = 650 kW, H = 20 m, generator at 750 rpm direct-coupled (8-pole, 120 × 50 / 8 = 750 rpm).

√650 = 25.50; 20^1.25 = 20 × 20^0.25 = 20 × 2.115 = 42.29.
Nₛ = 750 × 25.50 / 42.29 ≈ 452 → firmly in the Kaplan range; a Francis would need a much lower speed or a gearbox.

Head available	Discharge	Preferred turbine	Nₛ (metric)
2–30 m	Large	Kaplan / bulb / tubular	300–1,000
10–350 m	Medium	Francis	60–300
5–100 m	Small–medium	Cross-flow (Banki)	20–100
50–250 m	Small	Turgo	30–100
60–1,000 m+	Small	Pelton	10–60

Example 3 — Binary geothermal plant. Brine at 160 °C, 150 kg/s, cooled to 90 °C before reinjection; c_p = 4.2 kJ/kg·K; ORC first-law efficiency 11%; parasitic load 17% of gross.

Q̇ = 150 × 4.2 × (160 − 90) = 44,100 kW thermal.
Gross electrical = 0.11 × 44,100 = 4,851 kW; net = 4,851 × 0.83 ≈ 4.03 MWe.
Carnot check with sink at 25 °C: 1 − 298.15/433.15 = 31.2%, so the cycle achieves about 35% of the Carnot limit — typical for ORC.
Depth needed for 160 °C at a 30 °C/km gradient from a 25 °C surface: z = (160 − 25)/30 = 4.5 km.

Example 4 — Tidal barrage, single-effect ebb generation. Basin area 20 km², mean range 5 m, two tidal cycles a day, overall conversion (turbine + generator + sluice and head losses) 35%.

E per cycle = 0.5 × 1,025 × 9.81 × 20 × 10⁶ × 5² = 2.514 × 10¹² J = 2.514 × 10¹²/3.6 × 10⁶ ≈ 698 MWh gross.
Usable = 698 × 0.35 ≈ 244 MWh per cycle; per day = 489 MWh; per year ≈ 178.5 GWh.
Average power = 178.5 × 10⁶ / 8,760 ≈ 20.4 MW — note a 10 m range would give four times this, showing the R² dependence.

Example 5 — Wave and tidal-stream comparison at an Indian site. Sea state Hₛ = 2.0 m, Tₑ = 8 s; separately, a channel with peak spring current 2.5 m/s and a 12 m diameter rotor at Cₚ = 0.40.

Wave: P = 0.49 × 2.0² × 8 = 15.7 kW/m. A 20 m wide WEC with capture-width ratio 0.30 and PTO efficiency 0.70 → 15.7 × 20 × 0.30 × 0.70 ≈ 65.9 kW.
Tidal stream: A = π × 6² = 113.1 m²; P = 0.5 × 1,025 × 113.1 × 2.5³ × 0.40 = 0.5 × 1,025 × 113.1 × 15.625 × 0.40 ≈ 362 kW at peak; because the current varies sinusoidally the annual average is roughly 35–40% of this, about 130 kW.

Attribute	Small hydro	Geothermal	Tidal range	Wave	OTEC
Predictability	Seasonal, known	Baseload, constant	Exact, years ahead	Days ahead	Near constant
Typical capacity factor	0.35–0.60	0.80–0.95	0.22–0.30	0.20–0.35	0.70–0.90
Conversion efficiency	70–90%	10–20%	25–40%	15–30%	2–3% net
Indian status	~5.1 GW commercial	Pilot (Puga)	None operating	Demonstration (OWC)	R&D / desalination
Main cost driver	Civil works	Drilling	Barrage civil work	Mooring, survivability	Cold-water pipe

Where it is used

Hydro utilities and developers: NHPC, SJVN, THDC, NEEPCO and state bodies (KPCL, HPPCL, UJVNL, Kerala SEB) for canal-fall and run-of-river SHP; independent power producers under SERC tariffs and REC/green open access.
Turbine and equipment manufacture: Jyoti, Flovel, Voith Hydro, Andritz Hydro, BHEL and Kirloskar supply Pelton, Francis, Kaplan, cross-flow runners, governors and SCADA; roles in hydraulic design, model testing, site erection and commissioning.
Design consultancy and hydrology: AHEC (IIT Roorkee), WAPCOS, CWC and private consultants prepare DPRs — FDC, head loss, penstock and powerhouse layout, cost per MW, EIA where applicable.
Isolated mini-grids and rural electrification: micro hydro with electronic load controllers in Himachal, Uttarakhand, Arunachal, Sikkim and the North-East; watermill (gharat) upgrades.
Geothermal and heat pumps: GSI and ONGC in exploration and reservoir assessment at Puga and Tattapani; ground-source heat pumps and direct-use drying in building services and HVAC consultancy.
Ocean energy and marine R&D: NIOT Chennai (OWC at Vizhinjam, LTTD desalination in Lakshadweep, OTEC under the Deep Ocean Mission), IIT Madras Ocean Engineering, shipyards and offshore contractors for moorings, corrosion protection and subsea cabling.

Interview questions

Q: What is "small hydro" in India, and how is it sub-divided? A: Hydro plants up to 25 MW, divided by MNRE into micro (up to 100 kW), mini (101 kW to 2 MW) and small (2 to 25 MW).
Q: Why does hydro power scale linearly with head and flow while wind scales with the cube of speed? A: Hydro extracts potential energy, P = ρgQH, where Q is the mass flow already given by the site; wind extracts kinetic energy where the flow rate itself rises with velocity, giving ½ρAv³.
Q: How do you pick between a Pelton, Francis and Kaplan turbine? A: By specific speed Nₛ = N√P/H^1.25 — low Nₛ with high head and small flow gives Pelton, medium gives Francis, high Nₛ with low head and large flow gives Kaplan.
Q: Why is a draft tube needed on a Kaplan turbine but not on a Pelton? A: A reaction turbine discharges water with significant pressure and velocity, and the diverging draft tube recovers that kinetic energy and allows the runner to sit above tailwater; a Pelton's jet already leaves at atmospheric pressure.
Q: What is a flow duration curve used for? A: To choose the design discharge from exceedance probability and to estimate firm power and annual energy; SHP design flow is typically at 30–50% exceedance after deducting environmental flow.
Q: Why is a surge tank or forebay spillway provided? A: To absorb the water-hammer pressure rise and mass oscillation when the turbine gates close suddenly on load rejection, protecting the penstock and conduit.
Q: Why is geothermal conversion efficiency so low? A: The source temperature is low (often 120–200 °C), so the Carnot ceiling is only 25–35% and real cycles reach 10–20% of the heat input; the right measure is utilisation (exergy) efficiency, not first-law efficiency.
Q: When would you choose a binary (ORC) plant over a flash plant? A: Below roughly 150 °C, or where the brine is highly scaling or corrosive, because the geothermal fluid stays in a closed loop and only exchanges heat.
Q: Does India have a commercial geothermal power station? A: No — exploration and pilot work at Puga/Chumathang in Ladakh and Tattapani in Chhattisgarh is the current stage; the ~10,600 MWe figure is a resource estimate.
Q: Why does tidal energy depend on the square of the tidal range? A: The stored potential energy of the trapped water is ½ρgAR²; both the mass impounded and the average head through which it falls are proportional to R.
Q: What minimum temperature difference does OTEC need, and why is net output so small? A: About 20 °C between surface and deep water, giving a Carnot limit near 7%; pumping the huge cold-water flow up a 1,000 m pipe consumes most of the gross output, leaving 2–3% net.
Q: Which ocean resource would you develop first on the Indian coast, and why? A: Tidal stream and tidal range in the Gulfs of Khambhat and Kutch, because the resource is large and exactly predictable; Indian wave power at 5–15 kW/m is too weak for present WEC economics.
Q: Is ocean energy eligible for RPO in India? A: Yes — MNRE clarified in 2019 that tidal, wave and OTEC generation counts as renewable energy for renewable purchase obligation purposes.

Common mistakes

Using gross head instead of net head in P = ρgQH, or forgetting to subtract the mandatory environmental flow from the available discharge.
Omitting the generator and drive efficiency and reporting turbine shaft power as plant output.
Mixing units — Q in litres/s with ρ in kg/m³, or P in W where the specific-speed formula needs kW.
Using fresh-water density for tidal and wave problems; sea water is about 1,025 kg/m³.
Treating a tidal barrage's installed capacity as its average output — single-effect schemes generate for only a few hours per tide, giving capacity factors near 0.25.
Quoting the Carnot efficiency of a geothermal or OTEC plant as its actual efficiency, and ignoring parasitic pumping loads (decisive in OTEC).
Claiming India has operating geothermal or tidal power plants, or that run-of-river SHP needs no clearances — state water-use, forest and CRZ approvals still apply.
Confusing specific speed with synchronous speed, or forgetting that runaway speed (up to 3× rated for Kaplan) is the mechanical design case.