9. Biomass, Geothermal, Tidal & Hydrogen

Part B covered the six technologies that produce almost all of the world’s electricity. This module sweeps up what remains — three genuine but geographically limited sources, and one topic that generates more confusion than any other in energy.

Biomass: burning this year’s sunshine

Coal is fossilised sunlight from 300 million years ago. Biomass is the same idea, minus the geology: burn plant matter grown this year.

The appeal is a neat piece of accounting. A plant builds itself out of atmospheric CO₂ while growing; burning it releases that CO₂ back. Grow a replacement and the cycle closes. Biomass is therefore described as carbon neutral — not because burning it is clean, but because the carbon was borrowed from the air recently rather than dug out of the ground.

That argument is sound only if you keep two promises:

• The crop must actually be regrown. Cutting a forest and calling it renewable is accounting fiction — the carbon debt takes decades to repay.
• The collection must not cost more than it saves. Biomass is bulky and low in energy density (Module 1’s table: dry wood is a third of petrol per kilogram), so trucking it far quickly erodes the benefit.

Burning it also produces local air pollution — particulates especially — so “renewable” does not automatically mean “clean to stand next to”. India’s installed biomass-and-bagasse capacity is around 11.6 GW (FY 2024-25), a small but real slice of the fleet.

The sugar mill that powers itself

Here is the Indian biomass story worth telling properly.

Crushing sugarcane leaves an enormous quantity of dry fibrous pulp called bagasse — roughly a quarter to a third of the cane’s weight. For most of history it was a nuisance to dispose of.

But a sugar mill needs two things: process heat for boiling cane juice, and electricity for its machinery. And it is sitting on a mountain of free fuel it produced itself. So mills burn bagasse in a boiler, use the steam first to spin a turbine and then for process heat — extracting both from one fire. That is cogeneration.

It is exactly the trick suggested at the end of Module 3’s Carnot discussion: you cannot beat the second law, but you can decline to waste the heat it forces you to reject.

Mills run during the crushing season, power themselves, and export surplus electricity to the grid — turning a disposal cost into a revenue line. Maharashtra, Uttar Pradesh and Karnataka have thousands of megawatts of it.

Stubble, and the Delhi problem

Every autumn, after the paddy harvest in Punjab and Haryana, farmers face a brutal timetable: the rice is cut and the wheat must go in within weeks. Clearing the leftover straw by hand is slow and expensive. Burning it takes an afternoon.

The result is a vast pall of smoke that drifts southeast and settles over Delhi, combining with vehicle exhaust, industrial emissions and cool, still winter air to produce some of the worst urban air quality on Earth.

This is where biomass power stops being an abstraction. That straw is fuel. Collecting and burning it in a power station — or converting it to pellets or biogas — replaces an open field fire with a controlled boiler fitted with filters. India has pushed hard on this, including requirements for thermal plants to co-fire biomass pellets alongside coal.

Whether it works comes down to unglamorous logistics: someone must pay to collect, bale, store and haul low-density straw within a three-week window, at a price that beats a free box of matches. That, not the physics, is why the problem persists.

Waste-to-energy is the same idea applied to cities: burn municipal rubbish, generate electricity, and shrink landfill volume dramatically. It is genuinely useful and genuinely contested — plants have faced local opposition over emissions, and Indian municipal waste is wet and mixed, which makes it a poor fuel unless properly segregated. An incinerator is not a substitute for not creating the rubbish.

Geothermal: the Earth’s own boiler

Dig down anywhere and it gets hotter — the Earth’s interior is heated by primordial heat and radioactive decay in the rock. Geothermal power taps that: drill into hot rock, bring up steam or hot water, and run the exact steam cycle from Module 3. The furnace is simply the planet.

Its advantages are excellent. It runs day and night regardless of weather, giving capacity factors that beat almost everything else — a genuine base-load renewable, which solar and wind are not.

The catch is geology. You need hot rock close to the surface, which means volcanic zones and plate boundaries. Iceland, sitting astride the Mid-Atlantic Ridge, heats most of its buildings geothermally and generates a large share of its electricity that way. The Philippines, Indonesia, Kenya, New Zealand and the United States have substantial capacity.

India, geologically, is mostly out of luck. It sits in the stable middle of a continental plate, not on a volcanic belt. Promising sites have been studied for decades — Puga valley in Ladakh is the best known, along with Tattapani in Chhattisgarh — and Ladakh’s hot springs are visibly promising. ONGC has drilled exploratory wells at Puga and, as of 2026, is developing a 1 MW pilot plant there with the Ladakh administration — India’s first attempt at a demonstration-scale geothermal station, after decades of studies with nothing built. Even if it succeeds, geothermal will be a regional niche for India, not a pillar of the grid the way it is for Iceland.

Tidal and wave: perfectly predictable, brutally difficult

Tides are the one renewable resource that is completely predictable. They are driven by the Moon’s gravity, and we can print a timetable for the next century. Compared with “will it be windy on Thursday”, that is enormously valuable.

Two ways to harvest them:

• Tidal barrage — a dam across an estuary. Water fills at high tide, and is released through turbines as the tide falls. La Rance in France, running since the 1960s at around 240 MW, is the pioneer; Sihwa Lake in South Korea, at 254 MW, is the largest.
• Tidal stream — underwater turbines in fast tidal currents, like wind turbines in water. Water is roughly 800 times denser than air, so a slow current carries serious energy.

So why are there so few? Three reasons, and they compound.

The engineering is savage. Salt water corrodes, marine growth fouls, and maintaining machinery on a seabed under a tidal race is dangerous and expensive.

The geography is rare. You need an unusually large tidal range — a small handful of places worldwide qualify.

The ecology objects. A barrage across an estuary changes the sediment, salinity and tidal flats that migratory birds and fish depend on. Estuaries are among the most biologically productive habitats there are.

India’s best sites are the Gulf of Kachchh and Gulf of Khambhat in Gujarat, plus the Sundarbans. Projects have been proposed repeatedly since the 1980s; none has been built. Meanwhile solar’s price collapsed by a factor of five (Module 7). Tidal did not lose on physics — it lost a race.

Hydrogen: the great misunderstanding

Now the topic that deserves the most care, because more nonsense is spoken about it than any other in energy.

Hydrogen is not an energy source. It is an energy carrier.

Coal, uranium, sunlight and wind are sources — they exist in nature and we take energy from them. There is no hydrogen mine. Free hydrogen is almost absent from Earth’s atmosphere, because it is so light it escapes to space and so reactive it is already bonded into water and hydrocarbons.

So every kilogram of hydrogen must be manufactured, using energy from somewhere else. Which makes it an energy carrier — the same category as electricity, and the same category as a battery. The honest analogy: hydrogen is a battery you can burn. And like every battery, it returns less energy than it took to fill. It can never be a net source of energy, no matter how good the technology gets. This is Module 1’s conservation law, not a temporary engineering limitation.

Once that is clear, the useful questions become: how is it made, and what is it for?

The colour chart

• Grey hydrogen — made from natural gas by steam reforming, releasing CO₂. This is the overwhelming majority of hydrogen made today. Hydrogen is already a huge global industry; it is simply a chemical industry, and a fairly dirty one.
• Blue hydrogen — grey, but with the CO₂ captured and stored. Cleaner in principle; entirely dependent on the capture rate actually achieved and on the CO₂ staying put. Contested for exactly that reason.
• Green hydrogen — made by electrolysis: run renewable electricity through water and split it into hydrogen and oxygen. Emissions at the point of production: essentially none. Cost: still much higher than grey, which is the entire problem.

Where it genuinely helps

The strongest case for green hydrogen is not futuristic at all — it is replacing the grey hydrogen the world already uses.

• Fertiliser. Ammonia for nitrogen fertiliser is made from hydrogen, and India makes and imports enormous quantities. That hydrogen is grey today. Making it green cuts real emissions without inventing any new demand.
• Refining. Oil refineries consume hydrogen in bulk. Same argument.
• Steel. Steelmaking traditionally uses coal to strip oxygen from iron ore, releasing CO₂ by chemical necessity. Hydrogen can do that job instead, producing water. This is one of the few credible routes to low-carbon steel.
• Long-distance shipping, possibly, via ammonia or methanol made from hydrogen — because batteries heavy enough to cross an ocean are not remotely practical.

Notice the pattern: hydrogen wins where you need a chemical reducing agent or a very energy-dense fuel, and where direct electrification is genuinely hard.

Where it is mostly hype

Your car. Compare the two chains honestly:

• Battery EV: grid electricity → battery → motor. Roughly 70–80% of the electricity reaches the wheels.
• Hydrogen car: electricity → electrolysis (~70%) → compression and transport (losses) → fuel cell back to electricity (~50–60%) → motor. Round trip lands somewhere around 30%.

You need roughly two to three times as much clean electricity to drive the same distance. Add that hydrogen refuelling stations barely exist while electrical sockets are everywhere, and the market has already delivered its verdict: battery EVs outsell hydrogen cars by orders of magnitude, and several manufacturers have abandoned hydrogen passenger cars entirely.

India’s bet

India approved its National Green Hydrogen Mission on 4 January 2023, targeting 5 million tonnes of green hydrogen production per year by 2030, with an initial outlay of ₹19,744 crore (a crore is ten million) and an ambition to make India an export hub.

The strategic logic is coherent. India has some of the world’s cheapest solar electricity, and electricity is the dominant input cost of electrolysis. If green hydrogen becomes an internationally traded commodity, cheap sunshine becomes an export industry.

The honest caveats: the target is ambitious against a base near zero; electrolysers must be manufactured at enormous scale; electrolysis consumes a lot of purified water in a country with real water stress; and the economics only work if green hydrogen approaches the cost of grey. Worth attempting, and far from guaranteed — which is exactly how a serious industrial bet should be described.

🔥Why the round trip loses so much — and what that means for storageAdvanced · optional

Chain the efficiencies for storing electricity as hydrogen and getting it back:

Step	Typical efficiency
Electrolysis (electricity → hydrogen)	~65–75%
Compression / liquefaction / transport	~85–90%
Fuel cell (hydrogen → electricity)	~50–60%

Multiply: roughly 0.7 × 0.87 × 0.55 ≈ 0.33. About a third of what you put in comes back out.

Compare with lithium-ion at 85–95% and pumped hydro at 70–80% (Modules 5 and 11). Hydrogen loses badly, and no plausible improvement closes that gap — each step is bounded by its own physics.

So why consider it for storage at all? Because efficiency is not the only variable. Duration matters too. A battery is superb for hours and hopeless for months: it is expensive per unit of energy stored and it self-discharges. Hydrogen, once made, can sit in a salt cavern for a season at negligible cost.

That points at the one storage niche hydrogen may genuinely own: seasonal storage — banking a summer solar surplus for a winter shortfall. For the daily duck curve, batteries win comfortably. For the annual cycle, nothing else obvious exists. Whether that niche is big enough to justify the infrastructure is still an open question, and anyone who tells you they know for certain is selling something.

🔥🔥Why splitting water costs 1.23 volts — and why hot cells get away with lessDeep · optional

Electrolysis has a precise thermodynamic price, and deriving it explains almost every design decision in the electrolyser industry.

The two energies. Splitting one mole of liquid water into hydrogen and oxygen requires:

• ΔH = +286 kJ/mol — the total energy (enthalpy) needed to break the bonds.
• ΔG = +237 kJ/mol — the work that must be supplied as electricity, at 25 °C.

The gap between them, TΔS ≈ 49 kJ/mol, is entropy’s share — energy that can legitimately arrive as heat rather than electricity.

Converting work to voltage. Each H₂ molecule requires 2 electrons, so per mole we move 2F coulombs, where F is Faraday’s constant (96,485 C/mol). Voltage is work per unit charge:

E_rev = ΔG / (nF) = 237,100 / (2 × 96,485) ≈ 1.23 V

That is the reversible potential — the thermodynamic floor for liquid water at 25 °C and 1 bar. Below 1.23 V, no amount of engineering will split room-temperature water. It is Carnot’s role from Module 3, played by electrochemistry.

That qualifier is not pedantry, and it comes back at the end of this block. E_rev is ΔG/(nF), and ΔG = ΔH − TΔS, so the floor falls as the cell gets hotter: the entropy term grows with temperature and eats into the work you have to supply electrically. Split steam rather than liquid water and the reversible potential drops to roughly 1.01 V at 700 °C, 0.98 V at 800 °C and 0.96 V at 850 °C (re-derived here from standard NIST thermochemical data for H₂O(g) → H₂ + ½O₂; the vaporisation heat has by then already been paid separately, as heat). There is no single universal 1.23 V wall — there is a floor for every temperature, and hot cells legitimately sit below the room-temperature one.

The thermoneutral voltage. Feed the whole enthalpy in electrically and you get:

E_tn = ΔH / (nF) = 285,800 / (2 × 96,485) ≈ 1.48 V

Between 1.23 V and 1.48 V the cell absorbs heat from its surroundings while running (it cools itself). Above 1.48 V it produces waste heat. Real commercial low-temperature cells run at roughly 1.8–2.0 V, so efficiency on a higher-heating-value basis is around 1.48/1.9 ≈ 78% — matching the 65–75% range quoted in the 🔥 block once system losses are included.

Note what temperature does to that pair of numbers. E_tn hardly moves — for steam it sits between 1.28 and 1.29 V right across 700–850 °C, because ΔH is nearly flat — while E_rev keeps falling. So the heat-absorbing window between the two widens with temperature, from about 0.25 V at room temperature to about 0.31 V at 800 °C. That widening window is the entire prize of high-temperature electrolysis.

Where the extra volts go. In a low-temperature cell, the gap between 1.23 V and ~1.9 V is overpotential, in three parts:

1. Activation — the reaction’s own sluggishness, especially oxygen evolution at the anode. This is what catalysts (iridium, platinum) are fighting, and why their cost and scarcity matter so much for scaling.
2. Ohmic — plain resistance in the electrolyte, membrane and metalwork. Module 2’s I²R, in a cell.
3. Concentration — gas bubbles physically blocking the electrode surface.

Why heat is the frontier. Notice that TΔS term. If you supply it as heat instead of electricity, you need less electricity per kilogram of hydrogen — and since electricity is the dominant cost, that matters enormously. Raising temperature also increases the entropy term. This is the entire case for solid-oxide electrolysers running at 700–850 °C: they trade cheap high-grade heat for expensive electricity.

This is exactly the qualification made after the 1.23 V derivation above, now cashed out. An SOEC at 800 °C is not cheating thermodynamics by running below 1.23 V — it is obeying a different, lower floor, its own E_rev of about 0.98 V, because part of the job is being paid for in heat. At the limit that is roughly a fifth less electrical work per kilogram of hydrogen. So never apply the room-temperature 1.23 V figure to a hot cell: doing so overstates its minimum electricity demand by about a quarter. Pair one with a nuclear or concentrated-solar heat source (Modules 6 and 7) and green hydrogen’s economics change materially — which is why the idea keeps resurfacing despite the materials challenges of running an electrochemical cell at furnace temperatures.

What comes next

That completes the menu. You now know every significant way humanity turns something into electricity — and, more usefully, why each one is limited: by geology, by geography, by thermodynamics, or by arithmetic.

Part D changes the question entirely. Not “how do we make electricity?” but “how do hundreds of these machines, scattered across a subcontinent, stay in perfect balance every second of every day?” It is the part almost nobody teaches, and it contains the single most alarming fact in this whole course.