The blueprint of the paper

The paper walks the syllabus from front to back, so the question number tells you the chapter before you read it.

• Questions 3 to 6 are renewables: solar, hydro, wind, geothermal, biomass, fuel cells and hydrogen, with one calculation in most recent papers.
• The last questions are short notes: smart grid, hybrid vehicles, supercapacitors, batteries, and a hazard.

What to study first

These topics were asked in eight or more of the 22 sittings. Learn them before anything else; each link opens its card.

1. Appropriate technology and its criteria TOP 12/22
2. Biomass and bio-energy TOP 12/22
3. The smart grid TOP 12/22
4. Electrochemistry and fuel cells TOP 11/22
5. Technology transfer: methods, importance, opportunities and challenges TOP 10/22
6. Hydrogen energy TOP 10/22
7. Greenhouse effect, global warming and climate change TOP 9/22
8. The Human Development Index and energy consumption TOP 8/22
9. Energy trends, demand and supply, world and Nepal TOP 8/22
10. Kyoto Protocol and the Clean Development Mechanism TOP 8/22
11. Hydropower: principle and layout TOP 8/22
12. The solar PV cell TOP 8/22
13. Wind turbines, wind parks and power control TOP 8/22
14. Energy storage: need, forms and challenge TOP 8/22
15. Supercapacitors TOP 8/22
16. Hybrid and electric vehicles TOP 8/22

Appropriate technology and its criteria TOP 12/22

82 Bh · 82 Ba · 81 Ba · 79 Bh · 75 Ch · 73 Shr · 72 Ch · 72 Ka · 71 Shr · 70 Ch · 70 Asa · 69 Ch81+31+2+2

The idea behind the phrase. A technology can be excellent engineering and still be the wrong answer in a particular place. A combine harvester is superb on a thousand hectares of flat land and useless on a terraced hill farm of half a hectare, where it cannot turn, cannot be repaired and costs more than the land earns in a decade. The economist E. F. Schumacher made that point in Small is Beautiful with the term intermediate technology: between the hand hoe and the combine there is a middle level a poor community can afford, run and mend itself, and that level does more good than either extreme. Appropriate technology is the same argument turned into engineering practice, which is why the definition insists on judging a solution socially, environmentally, politically and economically as well as technically.

• Characteristics, seven, each with its reason: fewer resources and light capital, so a village can afford it; it meets the actual needs of the people, because it was chosen with them; lower cost to buy and to run; renewable energy, since imported fuel is the first thing to fail; less environmental impact; and labour intensive, which is a virtue where labour is plentiful and capital is scarce, because the money stays in the village as wages.
• Development goals it serves: more output and consumption goods, economic growth, less unemployment, regional development, a smaller balance of payment deficit, fairer income distribution, political development, better quality of life.
• Six criteria:
  • Technical: mature, suits local geography and climate, local materials and energy, built on locally known technology.
  • Economic: low investment and maintenance cost, little hard currency, benefits stay local.
  • Social: local people decide, existing skills and local labour, gradual impact, rural development.
  • Cultural: culturally sensitive, ongoing dialogue with the community.
  • Environmental: local and global damage minimised, renewable energy used.
  • Political: no reliance on outside support, strengthens the local area, the poor benefit.
• Role in transforming society, by sector: building (natural ventilation, green materials), agriculture (compost, animal power), water (SODIS, ceramic filters, rainwater harvesting), sanitation (composting toilets), energy (micro and pico hydro, solar collectors, biogas), transport (bicycles, animal carts), health (herbal medicine), finance (micro finance, cooperatives), ICT (low cost computers).

Appropriate against indigenous, with the Nepali case. Indigenous technology grew in a place over generations and is carried by custom: the ghatta, the wooden water mill that grinds grain on a hill stream, is the classic one. It is local, cheap and understood by everybody, but it wastes most of the energy in the water. The improved water mill keeps the same idea and the same stream and adds a better runner, metal bearings and a proper drive, so the same flow grinds faster and can also hull rice or turn a small generator. That is the difference: indigenous technology is inherited, appropriate technology is chosen or designed, often by improving an indigenous one with a little modern science. All indigenous technology is local, but not all of it is appropriate, and a technology can be appropriate without being indigenous.

• For Terai agriculture, where the land is flat and fertile, the groundwater shallow and the sunshine strong: solar PV and treadle irrigation pumps, drip and sprinkler irrigation, power tillers rather than large tractors, biogas on cattle dung, briquettes from rice husk and straw, solar dryers and small agro processing mills.
• For transport, where the country is steep and every litre of fuel is imported: electric vehicles and electric public transport on domestic hydroelectricity, ropeways and cable cars where roads slide, bicycles and rickshaws in Terai towns, mass transit in the valley, suspension trail bridges, and animal carts for rural haulage.

Micro hydro is the textbook case: see why it is sustainable in a rural area, and how local ownership is built in chapter 6.

Technology transfer: methods, importance, opportunities and challenges TOP 10/22

82 Bh · 82 Ba · 80 Bh · 79 Bh · 78 Bh · 76 Ash · 74 Ch · 71 Ch · 70 Asa · 69 Ch1+31.5+1.51+4

What actually moves. Hardware is the easy part. One half of a technology is codified, in drawings, patents and manuals, and that half travels in a crate or a file. The other half is tacit: the judgement of the operator, the maintenance habits, the small corrections a workshop learns over years. Only the codified half can be sold; the tacit half has to be rebuilt in the receiving country through training and practice. That is why a transfer succeeds when it comes with training of local people, adaptation to local conditions and local manufacture of spare parts, and why a transfer that is only a purchase order ends as an idle machine.

• Push and pull: supply pushes when a laboratory has a result looking for a use, and demand pulls when a user has a need looking for a solution. Three critical events: idea, prototype, product.
• It is a process, with stages, at many levels from national policy down to the individual scientist, among stakeholders who have to keep talking, so it is at bottom a communication process.
• Ways it is transferred: consulting, where experts advise and commission; moving heads, as graduates and faculty carry knowledge into industry; collaborative research between universities, industry and foreign partners; patenting and licensing of the right to use; service and outreach (extension, training and demonstration); spin off companies formed to commercialise a result; and, between countries, foreign direct investment, joint ventures, turnkey projects, franchising and aid programmes.
• Importance, and both sides gain: for the receiver, economic development and diversification, since a proven technology can be borrowed instead of invented; for the donor, future markets once its home market is saturated, and cheaper research and testing whose feedback returns to it; and for both, the well being, world order and peace that come when nations are busy with infrastructure and welfare instead of conflict.
• Opportunities for a developing country: leapfrogging straight to the current generation, as Nepal went to mobile telephones without ever building a dense landline network; proven technology at lower cost than inventing it; higher productivity and new jobs; local capacity building in skills, research and industry; foreign investment and export competitiveness; and less import dependence once the technology is made locally.
• Challenges: shortage of skilled manpower and weak capacity to absorb what arrives; a weak research and development base; high licensing and intellectual property cost with scarce foreign exchange; inadequate power, roads and internet; technology unsuited to the local climate, scale or culture; dependence on the donor for spares, maintenance and upgrades; and weak policy, weak institutions and social resistance.

The Clean Development Mechanism in chapter 2 is technology transfer with a carbon price attached.

Impact of technology on society and the environment HOT 7/22

81 Bh · 80 Ba · 78 Bh · 76 Ch · 71 Shr · 70 Ch · 69 Ch41.5+1.52+3

The same technology fills both columns. The mobile phone reached Nepali villages the landline never did, and with it came banking, remittance, weather forecasts, market prices and a way to call for help. The same device takes hours out of the day, moves money faster than judgement, exposes private data, and ends as electronic waste containing lead and lithium. The impact is not a property of the machine but of the choice: which technology is adopted, on what terms, and with what rules around it.

• On the environment, positive: renewable energy technologies replacing fossil fuel, cleaner and more efficient production, pollution control by filters, scrubbers, catalytic converters and treatment plants, monitoring by remote sensing and sensors, recycling and waste treatment, and electric vehicles that remove tailpipe emission.
• On the environment, negative: air pollution and greenhouse gases from fossil fuel use, water and soil pollution from industry and chemicals, e-waste and battery waste, depletion of fossil fuels and minerals, deforestation and habitat loss, and noise and radioactive waste from some technologies.
• The balance: technology is neutral; its impact depends on which technology is chosen and how it is used, which is exactly the case appropriate technology makes.

Energy and Maslow's hierarchy of needs HOT 7/22

82 Bh · 82 Ba · 81 Bh · 75 Ch · 74 Ch · 74 Ash · 71 Ch1+442+4

Why a psychologist's pyramid belongs in an energy course. Every need on it is met by a service, not by a wish: cooked food, a warm room, a lit street, a phone call, a school lesson. Every one of those services is delivered by energy, and the higher levels are not reached until the lower ones stop taking the whole day. A household that spends four hours collecting firewood has no hours left for schooling, which is the pyramid working in reverse. That is why access to modern energy, and not energy in the abstract, is treated as a development indicator.

• Physiological: air, food, water, shelter, warmth, sleep. Energy for cooking, water pumping and heating.
• Safety: security, order, law, stability. Street lighting, refrigerated vaccines, early warning systems.
• Love and belonging: family, friends, community. Phones, internet, transport.
• Esteem: achievement, status, respect. Electricity for schools, computers and productive work.
• Self actualisation: full potential. ICT, research, travel, creative work.
• As a computer engineer: smart metering and smart grid software, demand side management and load forecasting, monitoring of energy use, controllers for solar mini grids and micro hydro, efficient data centres, energy saving apps.

The energy ladder, the Nepali version of the same idea. As income rises a household climbs from dung and crop residue to fuelwood, then kerosene, then LPG, and finally to electricity, and each rung is cleaner, more efficient and more controllable than the one below. Most rural households in Nepal are still on the lower rungs for cooking while already using electricity for light, so they sit on two rungs at once. Moving cooking up the ladder is the single change that would do most for health, for forests and for the trade deficit.

The Human Development Index and energy consumption TOP 8/22

82 Bh · 81 Bh · 79 Bh · 76 Ch · 76 Ash · 71 Ch · 71 Shr · 69 Ch41+2+21+4

What the index is trying to fix. Income alone is a poor measure of how well people live: a country can have a respectable average income and short lives and empty schools. The HDI answers that by scoring three dimensions and combining them as a geometric mean, so a country cannot buy its way to a high score with income while neglecting health or education. A zero in any one dimension pulls the whole index down, which is the point of using a product rather than an average.

• Factors: health and life expectancy, schooling, GNI per capita, and behind them energy access, jobs, gender equality, inequality, governance and environment.
• Relation with energy: positive and saturating. HDI rises steeply with energy per person at low use, then flattens: above about 4,000 kWh of electricity (2,400 kgoe of energy) per person a year, extra energy adds almost nothing.

Why the curve bends over. It is diminishing returns, one service at a time. The first few hundred kilowatt hours a person gets buy the things that save lives: a pumped water supply, a vaccine refrigerator, light to read by, a mill that ends hours of pounding grain. The ten thousandth kilowatt hour buys a second television. Nothing forbids using more energy, but the health and education it can still buy have already been bought, so the index stops rising. The policy conclusion follows directly: a country on the steep part should expand access, and a country on the flat part should improve efficiency rather than consumption.

• Why development depends on energy: clinics and clean water raise life expectancy, lighting and computers raise schooling, industry and irrigation raise income.
• Nepal against Norway: HDI about 0.60 against 0.97; electricity about 400 kWh against over 20,000 kWh per person a year. Nepal sits on the steep part of the curve, so a small rise in modern energy buys a large rise in human development.

Energy trends, demand and supply, world and Nepal TOP 8/22

82 Ba · 78 Bh · 76 Ch · 76 Ash · 75 Ch · 75 Ash · 72 Ch · 70 Ch2+282.5+2.5

• World: about 81 percent of energy from fossil fuels (2005). Primary supply in 2015 (IEA 2017): oil about 32 percent, coal 28, gas 22, biofuels and waste 10, nuclear 5, the rest hydro and other renewables.
• World trends: oil price fluctuation, fast growth of solar, reduction of coal in developed countries, research in nuclear power, growth of storage.

Read Nepal's energy balance as one sentence: most of the energy is burnt in kitchens. The residential sector takes about 80 percent of all energy and traditional biomass supplies roughly two thirds of the total, because cooking on a wood fire is inefficient, so it swallows a large quantity of energy to deliver a small amount of useful heat. That single fact explains why fuelwood still leads the balance sheet in a country famous for hydropower, and why electric cooking changes the picture more than any new plant.

• Nepal's supply: traditional biomass (fuelwood, residue, dung) is the largest share; all petroleum is imported; hydro potential about 83,000 MW (45,000 technically, 42,000 economically feasible), installed over 3,000 MW by 2024; plus micro hydro, solar, biogas.
• Nepal's demand by sector (WECS 2014): residential 80.36 percent, industrial 7.89, transport 7.12, commercial 3.43, agriculture 1.17.
• Nepal's trend: biomass share falling, petroleum imports rising with vehicles, electricity fastest growing; load shedding ended in 2018, wet season surplus now exported to India and Bangladesh, dry season still short.
• Not environmentally friendly: petroleum (CO₂ and urban pollution), and fuelwood in open stoves (indoor smoke, deforestation).
• Avoiding the crisis: storage hydro, transmission, solar and wind for the dry season, electric cooking and vehicles, loss reduction, demand management.

Three potentials, and they are not the same number. Theoretical potential counts every drop of water falling down every river: about 83,000 MW. Technically feasible potential keeps only what machinery could actually capture at real sites: about 45,000 MW. Economically feasible potential keeps only what would repay its cost: about 42,000 MW. Quoting the largest of the three as though it were available capacity is the commonest error in this chapter, and the installed figure, past 3,000 MW, shows how far apart potential and reality still are.

The crisis of 2072 BS, when load shedding ran for over twelve hours a day, is worked as a case study in chapter 6.

Greenhouse effect, global warming and climate change TOP 9/22

81 Bh · 81 Ba · 79 Bh · 78 Bh · 76 Ash · 75 Ash · 74 Ash · 72 Ka · 71 Shr41+2+22×2.5

The mechanism in one paragraph. Sunlight arrives as short wave radiation, passes through the atmosphere almost unhindered and warms the ground. The warm ground radiates that energy back upward as long wave infrared. Greenhouse gas molecules are transparent to the first and opaque to the second: they absorb the infrared and re-radiate it in all directions, including downward. The surface therefore settles at a higher temperature than it would with no atmosphere, about 33 degrees Celsius warmer, which is what makes the planet habitable. Adding more of those gases does not add a new effect; it strengthens this one, and the surface warms further until the balance is restored. That is the difference between the natural and the human enhanced greenhouse effect, and it is the whole answer to "cause and mechanism".

• Gases: CO₂, CH₄, N₂O, CFCs, water vapour, tropospheric ozone. Methane traps far more heat per molecule than carbon dioxide but leaves the air within decades, while carbon dioxide is weaker per molecule and stays for centuries, which is why it dominates the long run.
• Causes: fossil fuels, deforestation, paddy and livestock, fertilisers, cement and refrigerants, landfills.
• Weather against climate: weather is what happens this week, climate is the statistics of decades. One cold winter is not evidence against warming, and one flood is not proof of it; the trend is the claim.

Why Nepal is exposed out of all proportion to what it emits. Nepal's emissions are negligible in world terms, but mountains warm faster than the global average, and the country stores its water as ice and as monsoon flow. Warming therefore attacks the thing the economy runs on: glaciers retreat and fill lakes that can burst, the monsoon becomes shorter and fiercer so the same rain arrives as floods and landslides, springs dry in the hills, and dry season river flow falls, which is also the flow every hydropower plant depends on.

• Impacts in Nepal: glacier retreat and GLOF (Tsho Rolpa, Imja), erratic monsoon, floods and landslides, drying springs, crop loss, biodiversity loss, lower dry season hydro.
• Nepal's plan: Climate Change Policy 2019, NDC and a net zero target for 2045, NAPA and LAPA, community forestry and REDD+, renewables through AEPC, electric cooking and vehicles.
• Mitigation against adaptation: mitigation cuts the emissions that cause warming (renewables, efficiency, forests); adaptation lives with the warming that is already coming (early warning, drainage, drought resistant crops). A country that emits little and suffers much, as Nepal does, needs mostly the second and argues for the first internationally.
• Slowing it, three examples: renewable electricity for cooking, electric transport, and forests; also efficiency and biogas.
• UNFCCC and COP: Rio 1992; COP3 Kyoto 1997; COP21 Paris 2015 (well below 2 degrees, NDCs); COP26 Glasgow 2021; COP27 loss and damage fund; COP28 Dubai 2023 (transition away from fossil fuels, triple renewables); COP29 Baku 2024 (finance goal).

Kyoto Protocol and the Clean Development Mechanism TOP 8/22

81 Bh · 81 Ba · 80 Bh · 78 Bh · 75 Ch · 74 Ch · 70 Asa · 69 Ch2+21+21+4

The idea the whole machinery rests on. A tonne of carbon dioxide does the same damage wherever it is emitted, and it does the same good wherever it is avoided. So if a rich country must cut a tonne, and cutting it at home costs far more than preventing a tonne in a poor country, both sides gain by paying for the cheaper cut and sharing the saving. That single observation produced emission trading, joint implementation and the CDM. The danger it creates is equally simple: the project must be one that would not have happened anyway, which is the test called additionality, and every CDM project has to prove it, be validated, registered, monitored and verified before any credit is issued.

• Kyoto Protocol: extends the 1992 UNFCCC; adopted 11 December 1997, in force 16 February 2005; binding targets for 37 industrialised countries and the EC, 5 percent below 1990 over 2008 to 2012.
• Three flexible mechanisms: emission trading (buy and sell allowances), joint implementation (a project in another developed country) and the CDM (a project in a developing country).
• Two objectives: help non-Annex I parties achieve sustainable development; help Annex I parties achieve compliance.
• CDM and sustainable development and the SDGs: clean energy (SDG 7), climate action (13), health from less smoke (3), jobs and industry (8, 9), forests (15), technology transfer.
• Potential areas in Nepal: household biogas (Nepal's first registered CDM project), micro hydro, improved stoves, solar, electric transport, brick kilns, landfill methane, forestry.
• Sustainability issues: resource scarcity, visible climate change, fossil fuel dependence, pollution; governments build green industries and regulate carbon, businesses adopt corporate responsibility.

The Nepali case to quote: the Biogas Support Programme. Each household plant replaces fuelwood and kerosene, captures the methane that dung would have released, saves a woman several hours a day and the forest several trees a year, and the certified reductions were sold as carbon credits that helped subsidise the next plants. It is the chapter's clearest example of one project serving climate, health, forests and income at once.

SDGs, SDG 7 and sustainable development HOT 4/22

81 Ba · 80 Ba · 74 Ch · 73 Shr1+42+32+4

The definition has two halves, and both matter. "Needs of the present" rules out conservation that keeps people poor; "without compromising the future" rules out growth that spends the next generation's resources. Sustainability is the overlap of the two, which is why it is drawn as three pillars that have to hold together: an economic pillar that pays for itself, a social pillar that spreads the benefit fairly, and an environmental pillar that keeps the resource base intact. A project standing on only two of them falls over: a profitable, popular scheme that empties an aquifer is not sustainable, and neither is a clean scheme nobody can afford to run.

• SDGs: 17 goals and 169 targets adopted by all UN member states in 2015 as the 2030 Agenda, succeeding the Millennium Development Goals.
• SDG 7: ensure access to affordable, reliable, sustainable and modern energy for all. Targets 7.1 universal access, 7.2 more renewables, 7.3 double the rate of efficiency improvement, 7.a cooperation, 7.b infrastructure in developing countries.
• Elements: economic, social, environmental, institutional, technological, intergenerational equity.
• Energy policies for sustainable development: promote renewables, efficiency standards, electrify cooking and transport, decentralised energy, cost reflective pricing, storage and a smart grid, research and awareness.

Why SDG 7 is called the enabling goal. Read the other sixteen and count how many need energy to be met: clinics need refrigeration and light, schools need light and computers, clean water needs pumping, industry and jobs need power, and climate action is mostly a question of where energy comes from. Nepal has nearly solved the access half of SDG 7 through grid extension, and the unfinished half is clean cooking.

Conventional and non-conventional sources, fossil fuels and nuclear energy HOT 5/22

81 Bh · 81 Ba · 79 Bh · 70 Ch · 70 Asa2.5+2.51+32+3

Three different cuts through the same list. Sources are sorted by whether they run out (renewable against finite), by whether they are traded (commercial, such as petroleum and grid electricity, against traditional, such as gathered firewood), and by how long they have been used at scale (conventional against non-conventional). The three cuts do not coincide, which is why large hydro can appear as conventional and renewable at the same time, and why gathered fuelwood is renewable in principle and destructive in practice when it is cut faster than it grows.

• Conventional: long established, mostly finite: coal, petroleum and natural gas (remains of decomposed plants and animals), nuclear fission, large hydro, traditional fuelwood.
• Non-conventional (renewable): solar, wind, small hydro, geothermal, modern biomass, tidal, hydrogen and fuel cells.
• Importance of renewables: inexhaustible, clean, local (energy security), reach rural areas, no fuel cost, better health.

What a fossil fuel actually is, and why it is so hard to replace: buried plant and animal matter, cooked for millions of years under pressure until what remains is almost pure hydrocarbon. That history is why fossil fuels carry so much energy in so little mass and why they sit still in a tank until wanted. They are, in effect, ancient sunlight in a concentrated, storable form, and the hard part of the energy transition is matching that concentration and that storability with anything else.

• Nuclear energy: heat from fission, decay and fusion; fission gave about 5.7 percent of world energy and 13 percent of electricity in 2012. Fission splits a heavy nucleus such as uranium 235 and takes the energy from the mass difference, which is why one kilogram of fuel replaces thousands of tonnes of coal.
• Nuclear, for and against: huge energy from little fuel, no CO₂, base load; but long lived waste, accident risk (Chernobyl, Fukushima), high cost, proliferation, finite uranium.
• Fusion: ${}^2\text{H}+{}^3\text{H}\to {}^4\text{He}+n+17.6\text{MeV}$ at over 100 million degrees Celsius, in a tokamak; the heat raises steam for a turbine. Still experimental (ITER).

The hazards of nuclear energy are chapter 4's nuclear hazard.

Solar radiation: the definitions HOT 7/22

82 Bh · 76 Ash · 74 Ch · 72 Ch · 71 Shr · 70 Ch · 69 Ch2+431+4

Follow one ray and the five definitions fall into place. The sun delivers about 1,367 watts to every square metre held square to it above the atmosphere, and that figure barely changes through the year: it is the solar constant. The atmosphere then takes its share by absorbing and scattering, so on a clear day at noon the ground receives about 1,000 watts per square metre, which is an irradiance, a power at an instant. Add that power up through the day and the total energy is the insolation, in kWh per square metre per day. Divide the day's insolation by a convenient 1 kW per square metre and the answer is the number of peak sun hours, which is why a designer can treat a whole variable day as so many hours of full sun and size an array in one line.

Why the angle matters more than the distance. Sunlight arriving at a slant is spread over more ground, so each square metre receives less: the cosine law, and the reason a panel is tilted roughly to the latitude and faced south in Nepal. The same geometry gives the seasons and the weak morning sun. What reaches the panel comes by two routes: beam radiation straight from the disc of the sun, which casts sharp shadows and can be focused by a mirror, and diffuse radiation scattered by air, dust and cloud, which arrives from the whole sky and cannot be focused. A concentrating collector can use only the first, which is why monsoon cloud rules out concentrated solar power in Nepal while a flat PV panel keeps working.

• Insolation: solar energy received per unit area in a day, kWh/m²/day. Nepal: about 3.6 to 6.2, average about 4.7.
• Solar constant: radiation per unit area on a surface perpendicular to the rays at one astronomical unit, outside the atmosphere: about 1,367 W/m².
• Irradiance: solar power per unit area at an instant, W/m²; about 1,000 at noon on a clear day.
• Peak sun hours: hours at 1,000 W/m² that give the day's insolation: 5 kWh/m²/day is 5 peak sun hours.
• Beam (direct, casts shadows), diffuse (scattered by air, dust and cloud), and global = beam + diffuse on a horizontal surface: $G=B\cos {\theta }_z+D$.
• Factors on intensity: latitude, season, time of day, angle of incidence (cosine law), atmosphere, altitude, air mass, shading.
• Applications: PV electricity, water heating, drying, cooking, pumping, space heating, water purification.

Solar thermal energy HOT 5/22

81 Bh · 74 Ch · 71 Ch · 70 Asa · 69 Ch2×52+32+4

Heat is the easy half of solar energy. A PV cell turns 15 to 20 percent of the light that falls on it into electricity, while a black absorber under glass moves well over half of it into water as heat, because nothing has to be converted into another form of energy. So where the job really wants heat, washing, cooking, drying, warming a building, collecting it directly beats making electricity first and heating with that. The catch is the other side of the same coin: heat is hard to send anywhere and hard to keep, so solar thermal is used close to where it is collected.

Why a thermosyphon needs no pump. Water in the collector is heated, expands and becomes lighter than the cooler water in the tank above it, so it rises on its own while the cold water sinks into the collector to take its place. The loop runs itself as long as the tank sits higher than the collector, which is exactly how the rooftop heaters on Kathmandu houses work, with no electricity and nothing to break. Putting the tank below the collector kills the circulation, and then a pumped system is required.

• Collectors: flat plate (black absorber under glass, up to about 80 degrees Celsius), evacuated tube, and concentrating (trough, dish, tower) for power.
• Solar water heater: thermosyphon (tank above the collector, natural circulation, no pump, the common Nepali rooftop type) or pumped.
• Varieties of solar heating: water heater, swimming pool heating, direct gain, Trombe wall, conservatory or sunspace, air heater and dryer, cooker.
• Solar dryer: air heated in a glazed collector rises through trays of produce; faster, cleaner and safer than open sun drying.

The solar PV cell TOP 8/22

82 Bh · 79 Bh · 78 Bh · 76 Ch · 74 Ch · 72 Ka · 71 Ch · 70 Asa2+381+4

Why a cell gives 0.6 V however big it is. The voltage comes from the energy step a freed electron falls through at the junction, which is set by the band gap of silicon, about 1.1 eV, and not by the size of the wafer. Area decides how many photons are caught, so it sets the current: a cell twice the size gives twice the current at the same voltage. That is why cells are wired in series for voltage and in parallel for current, and why every module carries a fixed number of cells for its rated voltage.

Why heat is the enemy and light is not. More light frees more carriers, so I_sc rises almost in proportion to irradiance. Heat does something different: it excites carriers thermally, which lowers the barrier at the junction, so V_oc falls about 2 mV per degree per cell. Since power is the product of the two, a panel on a hot roof in the Terai can deliver noticeably less than the same panel in the cold, bright air of the mountains, even under identical sunshine. The band gap also sets what can be absorbed at all: photons below 1.1 eV pass straight through, and the surplus energy of very energetic photons is lost as heat, which is the main reason a silicon cell cannot beat about 20 percent.

• Three steps: absorption of photons above the band gap (1.1 eV) makes electron hole pairs; the junction field separates them, electrons to n, holes to p; the contacts extract them as DC through the load.
• Cell, module, array: 0.5 to 0.6 V per cell whatever its area; cells in series make a module, modules make an array.
• IV curve: short circuit current I_sc, open circuit voltage V_oc, maximum power point (V_m, I_m), fill factor $FF=V_m{I}_m/V_{oc}{I}_{sc}$.
• Temperature: a hotter cell loses V_oc (about 2 mV per degree Celsius) while I_sc barely rises, so power falls about 0.4 to 0.5 percent per degree.
• Module types: mono crystalline 15 to 20 percent, poly 13 to 15, CdTe and CIS 9 to 11, amorphous silicon 5 to 8.
• Benefits of solar electricity: free fuel, clean, modular, off grid, low running cost, strongest in the dry season when hydro is weakest.

Solar PV systems: home, server, pump, mini grid HOT 4/22

81 Ba · 79 Bh · 76 Ch · 74 Ash4+22+33

• Standalone system: array → charge controller (MPPT, protects the battery) → battery bank (nights, cloudy days) → inverter (DC to AC) → loads.
• Remote internet server: the same chain, sized for a small continuous load, with the router on DC and the server on the inverter.
• Solar water pumping: array → controller or VFD → pump → overhead tank; water is stored, not electricity. Limits: no pumping at night, less in cloud, high cost, limited head, theft, over extraction.
• Mini grid parameters: load and evening peak, insolation and peak sun hours, autonomy days, battery, array and inverter size, distribution distance and voltage drop, site, tariff and ownership.
• Design steps (the course method): load in Wh or Ah, battery size, array size, wire size, charge controller, inverter, switch.

Every system on this card is sized backwards, from the load. Start with what the user needs in a day, in watt hours; divide by the system voltage to get ampere hours; decide how many days it must run without sun, which fixes the battery; then ask how many peak sun hours the site gives and how much is lost in wires, dirt, heat and charging, which fixes the array. Only then are the controller, inverter and cables chosen to carry what has already been decided. Working the other way round, buying panels first and asking what they will run, is how systems end up too small in the monsoon and idle in spring.

Notice what the pump does differently: it stores water in a tank instead of storing electricity in a battery. Water storage is cheap, lasts for decades and needs no maintenance, so where the load can wait, as irrigation and drinking water can, the whole battery is designed out of the system. That is the cleanest example in this chapter of solving a storage problem by changing what is stored.

The school computer lab sized with this method is in Numerical solutions.

Hydropower: principle and layout TOP 8/22

82 Bh · 82 Ba · 80 Ba · 79 Bh · 75 Ch · 72 Ka · 70 Ch · 70 Asa2+381+2

Read the equation as a shopping list. The power of a site is fixed by two things the geography gives, the head H in metres and the flow Q in cubic metres per second, and one thing engineering controls, the efficiency. Because they multiply, a high waterfall with a trickle and a slow river with a large flow can produce the same power, and a site with plenty of both is rare and valuable. It also shows why Nepal is well suited: its rivers fall thousands of metres in a short distance, so head is abundant even where flow is modest. Each cubic metre per second falling one metre yields about 9.81 kW before losses, which is worth remembering as a sanity check on any calculation.

Why the layout has so many parts. Every block between the river and the turbine exists to fix one problem. The weir and intake take a share of the flow without damming the river. The settling basin drops the sand that would otherwise sandblast the runner within a season, which matters in the Himalaya where rivers carry heavy sediment. The headrace carries water along the hillside at a gentle slope, trading distance for height. The forebay keeps the penstock full and settles the last silt. The penstock is where the height finally becomes pressure. The powerhouse turns that into shaft power and then electricity, and the tailrace returns the water to the river. Remove any one and the plant either wears out or stops.

• Components: diversion weir and intake, settling basin, headrace, forebay, penstock, powerhouse (turbine, generator, governor), tailrace, transmission line.
• In the powerhouse: water from the penstock turns the turbine runner; the shaft turns the generator, whose rotating magnetic field induces voltage in the stator; the governor holds 50 Hz; a transformer steps the voltage up.
• Gross and net head: net head = gross head minus losses in the intake, canal and penstock. Only net head produces power.

Three past calculations use the power equation: see Numerical solutions.

Water turbines HOT 7/22

82 Ba · 81 Bh · 80 Bh · 76 Ash · 74 Ash · 72 Ka · 71 Shr2×2.51+21+3

The whole family splits on one question: where does the pressure drop? In an impulse turbine it drops entirely in a fixed nozzle before the water touches the runner, so what hits the buckets is a free jet at atmospheric pressure and the machine can run in open air. In a reaction turbine the pressure keeps falling as the water passes through the moving blades, so the runner must be full of water inside a sealed casing, and it needs a draft tube below it to recover the remaining energy. Every other difference in the comparison table, the casing, the draft tube, how flow is controlled and the head each one suits, follows from that single fact.

Why head chooses the machine. High head gives a fast, thin jet, which suits buckets struck one at a time: the Pelton. Low head gives a slow, fat flow, which needs a large passage the water flows through continuously: the Kaplan. The Francis sits between them and is the most common turbine in the world for that reason. Nepal's micro hydro mostly uses the cross flow, not because it is the most efficient but because a local workshop can build and repair one, which matters more in a village than two points of efficiency.

• Impulse: expansion completed in a static nozzle; the jet's momentum turns the runner at atmospheric pressure. Pelton, Turgo, cross flow.
• Reaction: water accelerated in both the guide vanes and the runner; runner full of water in a pressure casing, with a draft tube. Francis, propeller, Kaplan.
• Pelton: high head, low flow, buckets struck by a jet; 30 to 450 m, 0.92 to 0.94. Francis: medium head, inward radial and axial flow, the most common. Kaplan: low head, large flow, adjustable blades, 4 to 40 m, 0.91.
• Selection by head: high (Pelton, Turgo), medium (cross flow, Francis), low (cross flow, Kaplan, propeller).
• Turbine and capacity: $P=\eta \rho gQH$, so the turbine matched to the site's head and flow sets the installed capacity and its efficiency.

Wind energy and its availability HOT 6/22

82 Ba · 80 Ba · 74 Ch · 72 Ch · 71 Ch · 70 Asa81+32+4

Where the cube comes from, since the question is often why and not only what. The mass of air passing the rotor each second is $\rho AV$, and each kilogram of it carries $\frac{1}{2}{V}^2$ of kinetic energy. Multiply the two, one V from the flow rate and two from the energy, and the result goes as $V^3$. That is why site selection beats machine selection: a site with 20 percent more wind yields about 70 percent more energy, and no blade design makes up for slow air.

Why a turbine cannot take all the wind: the Betz limit. A turbine extracts energy by slowing the air, but the air must keep moving to make way for the air behind it. Stop it completely and nothing flows through; leave it untouched and nothing is extracted. The best compromise takes 59.3 percent of the wind's energy, and real machines reach 35 to 45 percent once blade drag, gearbox and generator losses are counted. A power coefficient above 0.593 in a question is an error, and saying so earns the mark.

Why an average wind speed is not enough. Because power goes as the cube, a few windy hours carry more energy than many calm ones, so two sites with the same mean speed can yield very differently. Wind mapping therefore records the whole distribution of speeds at hub height over a year or more, which is why the energy graph on this card peaks well above the site's 4.5 m/s average.

• Three factors: wind velocity (the major one, cubed), swept area of the rotor, air density.
• Relations: doubling V gives 8 times the power; doubling D gives 4 times.
• Betz limit: at most 59.3 percent can be extracted; real machines 35 to 45.
• Wind mapping: anemometers and vanes on masts at hub height for one to two years, Weibull distribution and wind rose, corrected against long records; satellite models and LIDAR for atlases (SWERA in Nepal).
• Scope: grid wind farms, wind solar hybrids for villages and telecom towers, battery charging, water pumping.

Wind turbines, wind parks and power control TOP 8/22

81 Bh · 81 Ba · 78 Bh · 76 Ch · 74 Ch · 74 Ash · 73 Shr · 70 Asa1+31+21+4

• A blade is a wing, not a sail. Air passing over the curved side travels further and faster than air under the flat side, so the pressure above falls and the blade is pulled round by lift, exactly as an aircraft wing is pulled up. That is why a modern turbine has two or three slender blades rather than many broad ones, and why its tips can travel several times faster than the wind itself.
• Working: wind lifts the blades and turns the rotor, slowing the wind; the low speed shaft and gearbox raise the speed; the generator makes electricity; the transformer sends it to the grid.
• Why the gearbox is there: a large rotor turns at only 10 to 60 revolutions a minute, because its tips would otherwise break the speed of sound, while a generator wants 1,000 to 1,800. The gearbox bridges that gap, and it is the part that most often fails, which is why direct drive machines with many pole generators are replacing it.
• Components: blades, hub, pitch system, low speed shaft, gearbox, high speed shaft, brake, generator, nacelle, yaw drive, anemometer and vane, controller, tower.
• Types: horizontal axis (three blades, efficient, must be yawed) and vertical axis (Darrieus, Savonius: any wind direction, ground level generator, less efficient).
• Wind park: a group of turbines at one site, spaced several diameters apart; the land between can still be farmed.
• Power control: pitch, stall, yaw and brakes; cut in about 3 to 4 m/s, rated 12 to 15 m/s, cut out about 25 m/s.

Read the power curve as four decisions. Below the cut in speed the wind cannot overcome friction, so the machine stays still. Between cut in and rated speed it takes everything it can, and output climbs steeply with the cube law. At rated speed the generator is full, so from there on the control system deliberately wastes wind, pitching or stalling the blades to hold output flat and protect the gearbox. Above the cut out speed the loads would break it, so the brake goes on and output drops to zero. A machine is therefore rated not for the strongest wind at the site but for the wind that carries most of the site's energy.

Wind energy: uses, limits and Nepal HOT 7/22

81 Ba · 78 Bh · 76 Ch · 74 Ash · 73 Shr · 72 Ch · 71 Shr81+21+3

Why wind is a smaller story in Nepal than solar or hydro. Air is about eight hundred times lighter than water, so a wind machine must sweep a huge area to gather what a small pipe of falling water delivers, and the wind must be both strong and steady to be worth the tower. Nepal's terrain gives fierce winds in a few valleys and very little elsewhere, the good sites are hard to reach with heavy equipment, and thin mountain air carries less energy for the same speed. That is why wind appears here as a hybrid partner, paired with solar and batteries at telecom towers and village mini grids, rather than as a national source.

• Advantages: renewable, free, clean, no water, quick to build, land still farmed.
• Disadvantages: intermittent, site specific, costly, noise, birds, landscape.
• Nepal: about 3,000 MW estimated, chiefly the Kali Gandaki valley of Mustang; used in wind solar hybrid mini grids, telecom towers, pumping and battery charging. Limits: terrain and transport, seasonal and local wind, thin air, little data.
• Against hydro: air is far less dense than water, output is hourly variable, capacity factor lower, but construction is quick and modular.
• Wind and geothermal for Nepal: free fuel, dry season power, remote supply; hot springs for tourism, bathing and drying; both clean.

Geothermal energy HOT 5/22

82 Ba · 75 Ch · 75 Ash · 71 Shr · 69 Ch1+21+42×4

The heat is everywhere; the water is what is rare. Temperature rises with depth everywhere on earth, so in principle any country sits on a geothermal resource. What makes a site usable is a natural plumbing system: a heat source near enough to the surface, porous rock to hold water, and a cap of tight rock to keep it in. Where all three coincide, as along plate boundaries and volcanic belts, steam or very hot water can be drilled and used. Where only the hot rock exists, the water has to be pumped down and fractured through it, which is the hot dry rock idea and is still expensive. That is the difference between the resource types on this card, and it is also why Nepal, with many hot springs but no volcanic heat, is suited to direct use rather than power generation.

Why the water is always sent back down. Every plant here re-injects the cooled water through a second well. It keeps the reservoir pressure up so the field does not run dry, it disposes of water that carries dissolved salts and gases, and it stops the ground above from sinking. A geothermal field is renewable only if it is drawn no faster than the heat flows back in, which makes re-injection part of the design rather than an afterthought.

• Resources: hydrothermal (dry steam, wet steam, hot water), hot dry rock, geopressured, magma.
• Harnessing, three ways: electricity (dry steam, flash steam, binary cycle plants, with re-injection); direct use of hot water; heat pumps using the shallow ground.
• Applications: power, space and district heating, greenhouses, drying, fish farming, industry, bathing and spas, snow melting.
• Positive attributes: available 24 hours in all weather (base load), clean, small land, no fuel.
• Nepal: about 30 low temperature hot springs (Tatopani, Jomsom, Singha), suited to direct use.

Biomass and bio-energy TOP 12/22

80 Bh · 78 Bh · 76 Ash · 74 Ch · 74 Ash · 73 Shr · 72 Ka · 71 Ch · 71 Shr · 70 Ch · 70 Asa · 69 Ch1+381.5+2.5

Biomass is stored sunlight with a short memory. Photosynthesis packs solar energy into carbohydrate; burning or digesting it releases that energy and returns the same carbon to the air. Because the carbon was taken out of the atmosphere this season rather than a hundred million years ago, biomass is counted as carbon neutral, which is the whole difference between a fuelwood fire and a coal fire. The claim holds only if the plant is regrown: cutting a forest faster than it recovers turns the same fuel into a net emission, and that, rather than the smoke alone, is why Nepal's fuelwood dependence is treated as a problem.

Why there are four conversion routes and not one. The route is chosen by what the feedstock contains. Dry, woody material with little moisture suits thermo-chemical treatment, burning it for heat, or heating it without air (pyrolysis) or with limited air (gasification) to make charcoal, oil or a combustible gas. Wet material such as dung, slurry and food waste would waste more energy drying than it yields, so it goes the bio-chemical way, where bacteria digest it into biogas or yeasts ferment sugars into ethanol. Loose, bulky residue such as rice husk is first made dense and handleable by the physical route of drying and briquetting, and oil bearing seeds are pressed and converted to biodiesel. Match the route to the moisture and the answer to any conversion question follows.

• Sources in Nepal: fuelwood, crop residue (rice husk and straw, maize stalks), cattle dung, bagasse and agro waste, municipal waste, banmara.
• Thermo-chemical: combustion; pyrolysis (no air: charcoal, oil, gas); gasification (limited air: producer gas of CO, H₂, CH₄); liquefaction.
• Bio-chemical: anaerobic digestion to biogas; alcoholic fermentation to ethanol.
• Physical (physio-chemical): size reduction, drying, briquetting; oil extraction and transesterification to biodiesel. Agro-chemical: exudates such as rubber.
• Biogas: 50 to 70 percent methane, 30 to 40 percent CO₂, about 20 MJ/m³; reduces climate change by capturing methane and saving forests.
• Synthetic fuel: syngas (CO + H₂) from gasification turned into diesel or methanol by the Fischer Tropsch process.
• Bio fuel cell: microbes or enzymes catalyse the oxidation of organic fuel to give electricity directly.
• E number: percent of ethanol in petrol, E10 = 10 percent.

Electrochemistry and fuel cells TOP 11/22

81 Bh · 81 Ba · 80 Ba · 78 Bh · 76 Ash · 75 Ch · 74 Ch · 72 Ka · 71 Shr · 70 Ch · 69 Ch41+42+1+2

Why "directly" is the whole point. Burning hydrogen in an engine turns chemical energy into heat, heat into motion and motion into electricity, and the second law taxes the heat step, capping a thermal plant near 40 percent. A fuel cell skips it: the same reaction is made to happen in two halves, at two electrodes, so the electrons are forced through the external circuit as current instead of banging about as heat. Nothing burns, nothing rotates, and the efficiency ceiling is set by chemistry rather than by Carnot, which is why fuel cells reach 40 to 60 percent and more with heat recovery.

How to keep the two cells apart in memory. Both split the reaction and use the electrolyte as a one way street for ions, but they choose different passengers and opposite directions. In the PEM cell the membrane passes H⁺ from anode to cathode, so water forms on the air side and the cell must stay cool and moist, which suits a vehicle that starts and stops. In the SOFC the ceramic passes O^2- from cathode to anode, so water forms on the fuel side and the ceramic works only when very hot, which suits a stationary plant that runs for months and can use its waste heat. Name the ion and its direction and the rest of either answer follows.

• Electrochemistry: oxidation at the anode, reduction at the cathode, ions through the electrolyte, electrons through the external circuit; $\Delta G=-nFE$.
• PEM (60 to 100 degrees Celsius): anode ${\text{H}}_2\to 2{\text{H}}^{+}+2e^{-}$; protons cross the membrane; cathode $\frac{1}{2}{\text{O}}_2+2{\text{H}}^{+}+2e^{-}\to {\text{H}}_2\text{O}$; 1.23 V. For vehicles.
• SOFC (500 to 1,000 degrees Celsius): four layers, three ceramic (cathode LSM, electrolyte YSZ, anode Ni YSZ, interconnect); O^2- ions move from cathode to anode; fuel flexible; for stationary power.
• Types: PEM, alkaline, phosphoric acid, molten carbonate, solid oxide, direct methanol.
• Fuel cell against battery: fuel supplied from outside and never runs down, against reactants stored inside and needing recharge.

Hydrogen energy TOP 10/22

82 Bh · 81 Ba · 80 Bh · 79 Bh · 74 Ch · 74 Ash · 73 Shr · 71 Ch · 71 Shr · 69 Ch2+441+3

Carrier, not source, and everything follows from that. There is no hydrogen mine. Every kilogram has to be made by spending energy, so hydrogen can never deliver more than went into it: it is a way of moving and storing energy, like a battery made of gas. That is why its colour matters. Green hydrogen is made by electrolysis on renewable electricity and carries no emissions; grey hydrogen is made by steam reforming natural gas, which is most of today's production and emits carbon dioxide while doing so. Calling hydrogen clean without saying how it was made is the commonest mistake in this topic.

The awkward physics: heavy on energy, hopeless on space. Hydrogen holds nearly three times the energy of petrol per kilogram, which is why it suits aircraft and rockets, but as a gas at ordinary pressure it holds almost nothing per litre. Everything difficult about hydrogen follows: it must be squeezed to hundreds of bar, chilled to minus 253 degrees, or locked into metal hydrides or ammonia, and each of those costs energy and equipment. The same smallness of the molecule makes it leak through joints and embrittle steel.

• Production: electrolysis (green with renewable power), steam reforming of methane (most today), coal and biomass gasification, thermolysis, biological.
• Storage: compressed (350 to 700 bar), liquid (minus 253 degrees Celsius), metal hydrides, carbon nanotubes, ammonia and organic carriers.
• Safety: lighter than air and disperses fast; invisible flame; colourless and odourless; flammable from 4 to 75 percent; low ignition energy; embrittles metals.
• For and against: clean, high energy per kg, efficient in fuel cells, stores renewables; but costly, hard to store, no infrastructure, usually made from gas today.
• Against solar: storable and on demand, but not a primary source and less efficient overall; the two work together.
• For Nepal: turn spilled wet season hydro into hydrogen, replace imported fuel and urea, store energy, clean transport.

Hazard, and the environmental impact of energy sources HOT 5/22

81 Bh · 78 Bh · 76 Ch · 76 Ash · 70 Ch41+32+3

A hazard is a potential, not an event. Risk is what that potential becomes once exposure and probability are counted in: a sealed radioactive source in a laboratory is a serious hazard at almost no risk, while a smoky kitchen stove is a smaller hazard that millions are exposed to every day, so it does far more harm. That difference is why the three hazards of this chapter are controlled in different ways.

No energy source is free of harm, because energy is never made, only moved from one store to another, and every step of that move leaves something behind. Across the whole life cycle the harm appears in four places: extraction (mining, drilling, felling), transport, conversion (burning, fission, damming) and waste (ash, flue gas, spent fuel, dead batteries). A source that looks clean at the point of use, such as an electric vehicle or a solar panel, has moved its harm to another part of that chain rather than removed it.

The three behave differently in time and in space. Emission harms continuously and everywhere, in small doses that add up. Battery harm is local and cumulative: it builds in the soil and water of the place where the batteries are dumped. Nuclear harm is rare but concentrated, and when it comes it lasts for generations and crosses borders.

• Hydropower plants in Nepal: a dry stretch below the intake, blocked fish migration, forest loss, landslides from roads and tunnels, sediment trapped and then flushed, flooding and resettlement by reservoirs. Run of river plants, which are most of Nepal's, harm far less than storage dams.
• Wind machines: noise, birds and bats killed, visual impact and shadow flicker, land for roads, interference with radio and radar, blade failure. Against this they burn nothing, emit nothing and use no water, and the land between the turbines is still farmed.
• Biomass: deforestation from fuelwood, and indoor smoke from open stoves, which is the largest environmental health problem in rural Nepal.

Emission hazard HOT 5/22

82 Bh · 82 Ba · 74 Ash · 71 Shr · 70 Asa41+32×3

Where each pollutant comes from. Perfect combustion of a hydrocarbon would give only carbon dioxide and water. Real burning is never perfect and the fuel is never pure, so four things go wrong at once. Too little oxygen or too little time leaves carbon monoxide and unburnt hydrocarbons. Sulphur that came with the coal or diesel leaves as sulphur dioxide. The nitrogen of the air itself burns at high flame temperature to give nitrogen oxides. Soot, ash and condensed vapour leave as particulate matter. Sunlight then works on the exhaust and makes a second generation of pollutants, ground level ozone and photochemical smog.

Why particle size decides the damage. A particle of 10 micrometres is caught in the nose and throat. PM2.5, a particle smaller than 2.5 micrometres, is small enough to reach the alveoli and pass into the blood, which is why it is linked not only to asthma and bronchitis but to heart attack, stroke and lung cancer. Carbon monoxide works by a different trick: it binds to haemoglobin far more readily than oxygen does, so a small concentration starves the body of oxygen and kills in a closed room.

• Three scales of impact: local, the toxic exhaust breathed near the source; regional, where pollutants travel and react to make smog, haze and acid rain; and global, where carbon dioxide and methane warm the planet and CFCs thin the ozone layer.
• Acid rain, in two steps: $\text{S}+{\text{O}}_2\to {\text{SO}}_2$, and then the sulphur dioxide oxidises and dissolves in cloud water as sulphuric acid. It acidifies lakes and soil, strips nutrients from leaves, damages crops and forests, and eats limestone buildings and monuments. Nitrogen oxides do the same through nitric acid.

Nepal's status. The Kathmandu valley is a bowl, and in winter a temperature inversion puts a lid of warm air over it, so vehicle exhaust, road dust, brick kiln smoke and rubbish fires collect instead of dispersing; the city appears near the top of world pollution rankings in those months. The vehicle fleet is large, old and diesel heavy, and every litre of that fuel is imported. Outside the cities the hazard is indoors: most rural homes still cook on fuelwood or dung in open stoves, and the smoke causes respiratory disease and thousands of deaths a year, mostly among women and young children.

• Control at the source: cleaner fuel (low sulphur, unleaded), better combustion, zigzag brick kilns, improved cook stoves, biogas and electric cooking.
• Control at the tailpipe or stack: catalytic converters, particulate filters, and scrubbers for flue gas desulphurisation.
• Control by switching: electric vehicles and electric public transport running on hydroelectricity, which moves the energy from an imported fuel to a domestic renewable one.
• Control by rule and by planning: emission standards with periodic inspection (the green sticker test), a ban on very old vehicles and on open burning, mass transit, green belts, air quality monitoring and public warning.

Battery hazard HOT 5/22

82 Ba · 80 Bh · 74 Ch · 70 Ch · 69 Ch2+22+12+3

A battery is a small sealed package of deliberately reactive chemistry: a strong acid or alkali, two reactive electrode materials, and a store of energy that is meant to come out quickly. Everything that makes it useful is what makes it dangerous once the case is opened, crushed, overheated, or left to corrode in the ground.

• Four hazards in use: battery acid, which burns skin and eyes; flammable gas, since charging a lead acid cell releases hydrogen and a damaged or overcharged lithium cell can go into thermal runaway; electric shock and short circuit, because a battery cannot be switched off; and weight, which causes lifting injuries.
• What a dumped battery releases: lead from lead acid cells, cadmium from nickel cadmium, mercury from older cells, nickel, and the lithium and cobalt compounds of modern cells, along with the acid or alkaline electrolyte.

What happens in a landfill. The casing corrodes within a few seasons, the electrolyte leaks and lowers the pH of the soil around it, and acid soil mobilises heavy metals that would otherwise stay locked in place. They travel with ground water into wells and streams, are taken up by crops and fish, and concentrate as they move up the food chain. Lead and mercury damage the brain, nerves and kidneys, and children are harmed at far lower doses than adults. Burning batteries with household rubbish, common where there is no collection, puts the same metals into the air, and lithium cells can start fires inside the waste pile.

Managing the problem, in the order that works. Reduce first, with longer lived chemistries and none containing mercury or cadmium. Then collect, through shops, service stations and municipal points, so that batteries never enter household waste. Then recycle in licensed plants that recover lead, lithium, cobalt and nickel, which also cuts the mining that damaged land in the first place. Then regulate, treating batteries as hazardous waste with extended producer responsibility or a deposit refund, so the importer takes them back. An electric vehicle battery retired from the road still holds most of its capacity, so a second life as stationary storage delays disposal by years. In use the rules are simple: goggles, gloves and apron for acid, charge in a ventilated place, and never short the terminals.

The batteries themselves, lead acid and lithium ion, are in chapter 5.

Nuclear hazard HOT 5/22

81 Ba · 80 Ba · 75 Ch · 75 Ash · 71 Shr1+33+23

What radiation actually does. Alpha, beta, gamma and neutron radiation are ionising: they carry enough energy to knock electrons out of the atoms they pass through. Inside a cell that happens mostly to water, and the fragments attack the DNA nearby. The cell then does one of three things, and those three explain every effect in this card. It repairs the damage, and nothing follows. It dies, and if enough cells in a tissue die at once the result is the acute radiation sickness of a large dose. Or it survives with the damage miscopied, which is a mutation: in a body cell that can become a cancer years later, and in a sperm or egg cell it is passed to a child. This is also why the same dose spread over years does less harm than in a minute, and why the tissues that divide fastest, bone marrow, gut lining and a growing foetus, are hit first.

• Why it matters now: about a tenth of the world's electricity is nuclear and new reactors are being built as countries cut carbon; isotopes are used everywhere in medicine and industry; one accident contaminates a region and crosses borders; the waste stays dangerous for thousands of years; and the material can be diverted to weapons or a dirty bomb, with natural disaster and cyber attack as newer risks.
• Short term (acute), after a large dose in a short time: a fall in white blood cells, then radiation sickness (nausea, vomiting, fatigue), fever, hair loss and bleeding, and at higher doses diarrhoea with about half of those exposed dying, plus skin burns.
• Long term (chronic): cancer such as leukaemia and thyroid cancer, genetic mutation and birth defects, cataract, sterility, a shortened life, and land, water and food contaminated for decades.
• Somatic against genetic: somatic effects appear in the exposed person's own body cells and stop with that person; genetic effects damage the DNA of reproductive cells and appear in children and later generations, even when the parent seems well.

Nuclear waste is what is left when the fuel can no longer sustain the reaction, plus everything the industry contaminates on the way: mining tailings, reactor parts, laboratory and hospital material. It is sorted as low, intermediate and high level. The high level part, spent fuel, is the hard one: intensely radioactive, still generating heat for years, so it must be cooled in a pool before it can be sealed in a dry cask, and it contains plutonium and fission products that stay dangerous for thousands of years. Leaks contaminate soil and ground water, transport can crash, plutonium can be stolen, and no permanent deep repository is yet in wide use, so the burden is passed to future generations.

How a plant is secured: defence in depth. No single barrier is trusted, so several independent ones are put in series, each designed to hold if the one before it fails. The ceramic fuel pellet locks most fission products in place; the metal cladding around the pellets is the second wall; the steel pressure vessel and the sealed cooling circuit are the third; and the thick concrete and steel containment building is the last. Around them sit the active systems: control rods of boron or cadmium that absorb neutrons and shut the reaction down in seconds, emergency core cooling with backup power for the heat that continues after shutdown, shielding of concrete, lead and water, filtered ventilation with continuous monitoring of whatever is released, cooled and sealed waste handling, siting away from earthquake and flood zones, licensing and inspection by the national regulator and the IAEA, emergency plans with exclusion zones, and physical and cyber security against sabotage.

• Examples to quote: Hiroshima and Nagasaki, 1945, for acute death and the cancer and genetic burden that followed; Chernobyl, 1986, for a reactor explosion that spread material across Europe, with thyroid cancers and a zone abandoned to this day; and Fukushima, 2011, for a natural disaster defeating the cooling systems and forcing a mass evacuation.

Nuclear energy as a source, fission and fusion, is in chapter 2.

Energy storage: need, forms and challenge TOP 8/22

82 Bh · 81 Bh · 76 Ch · 76 Ash · 75 Ch · 74 Ash · 73 Shr · 72 Ka2+22+34

Electricity is unusual among commodities: it is made at the instant it is used, and the grid must hold generation and demand equal second by second or the frequency drifts and the system collapses. Coal, diesel and reservoir hydro hide that problem, because their fuel is itself a store that can be burnt or released on command. Solar and wind have no such store: they deliver when the weather allows. Storage is what turns them from an interruption into a supply, and it is why a chapter on storage sits in an energy course at all.

• Why storage is necessary: match supply with demand (sunny noon against the evening peak, the monsoon against the dry season); integrate variable solar and wind; backup power for hospitals, telecom and data centres; peak shaving, so less generating capacity has to be built for a few hours a year; portable energy for vehicles, phones and tools; and grid stability, holding voltage and frequency steady.

The six forms, and what each is good for. Every store works by pushing energy uphill into some potential and letting it run back down later. The form decides how much can be stored, how fast it comes out, and how long it keeps.

Pumped hydro holds most of the world's stored electricity: water is pumped uphill with surplus power at night and released through a turbine at the peak. Nepal has no pumped storage, and Kulekhani is its one storage reservoir scheme, which is why the dry season shortage and the wet season spill are both storage problems.

• Characteristics of a storage device, the list an examiner wants: energy density (Wh/kg or Wh per litre, how much it holds), power density (W/kg, how fast it can give it back), discharge time, round trip efficiency (energy out divided by energy in), self discharge, cycle life, recharge time, cost per kWh, weight and size, safety and environmental effect.
• Energy and power are two different numbers, and mixing them up is the commonest error in this chapter. A supercapacitor has high power and low energy: it empties in seconds but holds little. A battery is the other way round, and pumped hydro further still, holding a whole night of energy. A device is chosen by how long it must run, not only by how much it holds.

Why storage is the challenge of the 21st century. Every one of these is worth a line in the answer.

• The generation mix has changed: the world is shifting to solar and wind, which produce only when the sun shines and the wind blows, so storage now decides how much of them a grid can take.
• Transport has moved onto batteries, which need high energy density, fast charging and low cost at the same time.
• Energy density is far below fuel: petrol carries thousands of watt hours per kilogram, a lithium ion cell 100 to 250, so a battery of the same range is heavy and bulky.
• Scale and cost: grid storage is counted in GWh, and every kWh of it costs.
• Materials: lithium, cobalt and nickel are limited, concentrated in few countries, and mined at an environmental and social cost.
• Life and losses: capacity fades with cycles, and round trip efficiency and self discharge take their share.
• Safety and disposal: fire and thermal runaway, and hazardous waste at the end of life.
• Demand keeps growing: data centres, electrified transport and uninterrupted supply.

• Latest technologies: lithium iron phosphate, solid state and sodium ion cells; vanadium redox flow batteries for long duration; supercapacitors for bursts; hydrogen by electrolysis (power to gas); advanced pumped storage, compressed and liquid air; flywheels and superconducting magnetic storage; molten salt for solar thermal plants.
• Complications of storage: high cost per kWh, energy lost on the round trip and to self discharge, limited life, low energy density, fire and leak hazards, disposal and recycling, and the geography that pumped hydro and compressed air need.

Batteries HOT 4/22

81 Ba · 80 Bh · 71 Shr · 70 Asa1+42+34×2

A cell has four parts: a negative electrode that gives up electrons, a positive electrode that takes them, an electrolyte that carries ions between the two, and a separator that keeps the electrodes apart while letting ions through. The electrons cannot cross the electrolyte, so they take the long way round through the external circuit, and that current is the useful output. Charging drives the same reaction backwards with an external voltage.

• Primary (use once): zinc carbon, alkaline, lithium coin cells. The reaction cannot be reversed usefully, so the cell is thrown away.
• Secondary (rechargeable): lead acid, nickel cadmium, nickel metal hydride, lithium ion and lithium polymer, and flow batteries for grid storage.

The lead acid cell, the one to write about. The positive plate is lead dioxide (PbO₂), the negative plate is spongy lead (Pb), and both sit in dilute sulphuric acid. On discharge each plate turns to lead sulphate and the acid weakens, which is why the state of charge can be read with a hydrometer: the specific gravity falls from about 1.28 charged to about 1.15 flat. Charging reverses it.

Each cell gives about 2 V whatever its size, so six in series make the familiar 12 V battery. It is heavy and holds only 30 to 40 Wh per kg, but it is cheap, gives a very high surge current for starting an engine, and is the battery in most solar home systems in Nepal.

The lithium ion cell plates no metal: lithium ions shuttle between a graphite anode and a metal oxide cathode (LiCoO₂, NMC or LiFePO₄), slipping in and out of the layers, which is called intercalation. That gives about 3.6 to 3.7 V a cell and 100 to 250 Wh per kg with little self discharge, but it needs a battery management system: overcharge, deep discharge or a puncture can start a thermal runaway fire.

• The numbers that describe a battery: capacity in ampere hours (Ah), energy in watt hours (Ah times volts), the C rate (1C empties it in an hour), depth of discharge (how much of it may be used), state of charge, cycle life, and round trip efficiency.
• Sizing, worked: a 12 V, 100 Ah lead acid battery holds 1.2 kWh, but only about half of that may be taken if it is to last, so roughly 0.6 kWh is available: a 60 W load for ten hours. The same job needs about half the weight in lithium ion, at several times the price.

What happens to used batteries is chapter 4's battery hazard.

Supercapacitors TOP 8/22

82 Bh · 82 Ba · 81 Ba · 80 Bh · 80 Ba · 78 Bh · 75 Ch · 69 Ch2+22+1+22×2.5

Where the huge capacitance comes from. An ordinary capacitor follows $C=\epsilon A/d$: more farads need more plate area and a thinner gap. A supercapacitor takes both to the limit. Its electrodes are activated carbon so porous that a gram of it carries a surface area of roughly a thousand square metres, and when voltage is applied the ions of the electrolyte line up against that surface in a double layer only a few nanometres thick. Enormous area divided by a nanometre gap gives farads where an ordinary capacitor gives microfarads. Nothing reacts chemically: the charge simply sits there, which is why the device can be filled and emptied hundreds of thousands of times without wearing out.

Each electrode carries its own double layer, so the cell behaves as two capacitors in series, $C_{total}={\displaystyle \frac{C_1{C}_2}{C_1+C_2}}$. The electrolyte breaks down above about 2.7 V, so cells are stacked in series with balancing to reach a useful voltage, and the stored energy, which goes as $V^2$, is held back by that ceiling.

• Why supercapacitors matter: very high power density, charge and discharge in seconds, hundreds of thousands of cycles with no degradation, 85 to 98 percent efficient, and they swallow energy a battery cannot absorb fast enough, such as the braking of a bus.
• Advantages over a battery: power, speed, cycle life, efficiency, a wide temperature range, little maintenance, and no chemical reaction to run away.
• Disadvantages against a battery: low energy density (5 to 10 Wh/kg), so it cannot run a load for long; the voltage falls linearly as it empties, so a converter is needed; high self discharge; low cell voltage; and a higher cost per unit of energy stored.
• Uses: regenerative braking in buses, trams and hybrids; start stop systems; memory and controller backup; smoothing solar and wind output; grid frequency regulation; and paired with a battery, so the battery is never asked for a sudden peak, which lengthens its life.

Hybrid and electric vehicles TOP 8/22

82 Ba · 80 Bh · 78 Bh · 76 Ch · 76 Ash · 75 Ash · 71 Ch · 70 Ch2+24×22+3+3

Why two power sources save fuel. A petrol engine is efficient only in a narrow band of speed and load: in traffic it idles, accelerates and brakes, and much of the fuel is wasted. An electric motor is the opposite, efficient from rest and strongest at low speed. Put both in one vehicle and each does the work it is good at: the motor pulls away and crawls in traffic, the engine runs near its best load or shuts off altogether at a stop, and in regenerative braking the motor is turned into a generator, so the kinetic energy a brake disc would have thrown away as heat goes back into the battery instead.

• Series hybrid: the engine drives only a generator, and the electric motor alone turns the wheels, so the engine can be held at one efficient speed. Used in diesel electric locomotives and range extender cars.
• Parallel hybrid: engine and motor are both coupled to the wheels and can drive them alone or together. Simple and light, and the commonest arrangement.
• Series parallel (power split): both paths through a planetary gear, switched automatically for the best efficiency at each moment.
• Plug in hybrid (PHEV): a larger battery that can also be charged from the grid, so short trips are electric and long ones are not limited by range.

• Ways to drive an electric vehicle: battery electric (BEV), a motor and a battery charged from the grid; hybrid (HEV), charged by the engine and by braking; plug in hybrid (PHEV); fuel cell (FCEV), where hydrogen makes the electricity on board; and vehicles fed from an external supply, such as trolley buses, trams and electric trains, or a solar vehicle.
• Environmental impact, both ways: less fuel per km, so less CO₂, CO, hydrocarbons and NOx, and less noise in the city; but it still burns fossil fuel, the lithium, nickel and rare earth metals must be mined, the battery costs energy to make, and the used battery is hazardous waste unless it is recycled.
• Why electric vehicles suit Nepal: they run on domestic hydroelectricity that is otherwise spilled in the wet season; they cut the petroleum import bill, the largest single item in the trade deficit; they take tailpipe emission out of the Kathmandu valley; they cost far less per kilometre and need little maintenance; they support the net zero target, because the electricity is almost all renewable; and low customs duty has already made them spread quickly. The limits are charging infrastructure outside the valley, the price of the vehicle, and the grid work needed if many charge at once.

The smart grid TOP 12/22

82 Ba · 81 Bh · 81 Ba · 80 Bh · 80 Ba · 79 Bh · 78 Bh · 76 Ash · 74 Ch · 71 Ch · 71 Shr · 69 Ch2+22×2.54×2

What is wrong with the old grid. It was built to push power one way, from a few big plants down to passive consumers. It is largely blind: the utility learns of a fault when somebody telephones, reads the meter once a month, cannot see where power is lost or stolen, and has no way to ask demand to move. Rooftop solar, batteries and electric cars now inject power at the bottom of that one way street, and the old grid has no language for them.

• Components: smart meters and the advanced metering infrastructure (AMI) behind them; sensors and phasor measurement units that watch the network in real time; a communication network; SCADA and distribution automation; distributed generation such as rooftop solar; storage and electric vehicles; and demand response systems that let load follow supply.
• A smart grid must be more reliable, more secure, more economic, more efficient, friendlier to the environment and safer than the grid it replaces.
• Principal characteristics: consumers take an active part through time of use tariffs and demand response; every form of generation and storage can join; new products, services and markets become possible; power quality is good enough for digital equipment; the grid heals itself, anticipating disturbances and isolating a fault automatically so only a few houses go dark; assets are used efficiently; and the system resists physical and cyber attack.
• Self healing, in one sentence: sensors detect the fault, automatic switches isolate the faulty section, and the control system reroutes supply around it in seconds, instead of a crew driving out to find it.

Can Nepal use a smart grid? Yes, and it is being built step by step. The Nepal Electricity Authority has begun installing smart meters and automating distribution, and it runs a load dispatch centre with SCADA. Nepal needs it to cut technical and commercial losses including theft, to balance the wet season surplus against the dry season deficit, to bring in rooftop solar and cross border trade, and to manage EV charging so it does not land on the evening peak. The obstacles are cost, weak communication in rural areas, skilled manpower, cyber security and consumer awareness.

• Socio-economic impact for Nepal: reliable power for industry, which means production and jobs; lower losses and accurate billing, so tariffs can fall; more domestic hydro used for cooking and transport instead of imported fuel; net metering income for households with rooftop solar; better health, education and communication services; and new jobs in ICT and energy services.
• Load shedding against load shifting: shedding is the emergency, cutting whole areas off in turn when demand exceeds supply, as Nepal did for up to twelve hours a day before 2018. Shifting is the plan: moving demand to off peak hours with tariffs, storage and control, so the same energy is used at a different time and nobody loses supply.