Energy Resource Engineering

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This section covers current energy trends and the pattern of demand and supply in the world and in Nepal, conventional and non-conventional or renewable energy sources, the Human Development Index and its relation to energy, and energy conservation practice.

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Classification of Energy Sources • Renewable sources are replenished on a human timescale — solar, wind, hydro, biomass, geothermal and tidal; non-renewable sources exist in a fixed stock — coal, oil, natural gas and nuclear fuel. • Conventional sources are long established and commercially dominant — coal, oil, gas, large hydro and nuclear; non-conventional sources are the newer alternatives — solar, wind, biogas, micro-hydro, tidal, geothermal and fuel cells. • The two classifications are not the same, and the overlap is examined directly: large hydro is renewable but conventional, while nuclear is non-renewable yet conventional. • Primary energy is taken directly from nature; secondary energy must be manufactured from a primary source.

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Electricity and hydrogen are energy carriers, not energy sources, which is a standard question. • Commercial energy is bought and sold — electricity, petroleum products, coal; traditional or non-commercial energy is collected and used directly — fuelwood, agricultural residue and animal dung.

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Nepal's energy balance is dominated by traditional biomass, which has historically supplied around two-thirds to three-quarters of total consumption, with petroleum products next and electricity a small though rapidly growing share.

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These proportions are shifting as electrification and cooking-fuel substitution proceed, so current figures should be checked.

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Trends, Demand and Supply • World trends: total primary energy demand continues to grow, driven by population and by rising incomes in developing economies; fossil fuels still supply about four-fifths of primary energy, but renewables — particularly solar and wind — are the fastest growing sector and their costs have fallen dramatically; energy intensity, the energy used per unit of GDP, is falling in most countries; and electrification of end uses, notably transport and cooking, is increasing the share of electricity in final consumption. • Nepal's position is distinctive and its features should be known as a list: • Very large theoretical hydropower potential, conventionally quoted as about 83,000 MW with roughly 42,000 MW regarded as technically and economically feasible, of which only a small fraction has been developed.

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These figures date from a 1960s assessment and are widely quoted but are being revised; they should be treated as indicative. • Heavy dependence on imported petroleum, which is the largest item in the trade deficit and the principal energy security vulnerability, since Nepal produces no oil or gas and has a single main import route. • A pronounced seasonal mismatch in hydropower: run-of-river plants generate at full output in the monsoon and at perhaps a third of capacity in the dry season, when demand for lighting and heating is highest.

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This is the central technical problem of Nepal's power system, and the remedies are storage projects, regional trade and demand management. • A pronounced daily peak in the evening, and a rural-urban divide in access and in the quality of supply. • Low per capita electricity consumption by international standards, though rising rapidly as access has approached universal coverage. • Biomass dominance for cooking in rural areas, with the associated indoor air pollution and drudgery of fuelwood collection, which falls chiefly on women and children.

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Energy and Human Development • The Human Development Index (HDI) is a composite measure of a long and healthy life, measured by life expectancy at birth; knowledge, measured by mean and expected years of schooling; and a decent standard of living, measured by gross national income per capita.

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It ranges from 0 to 1 and is published by UNDP. • The relationship with energy consumption is not linear, and the shape of the curve is the point examined: plotting HDI against per capita energy consumption gives a curve that rises very steeply at low consumption and then flattens into a plateau. • The implications are important and should be stated: at low levels of consumption a small increase in energy use produces a very large improvement in human development, because it buys lighting, clean cooking, refrigeration of vaccines, water pumping and the possibility of enterprise; but beyond roughly 100 GJ per capita per year the curve flattens, so further increases bring little additional development.

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It follows that increasing energy access in a poor country is developmentally far more valuable than increasing consumption in a rich one, and that a high HDI does not require the very high per capita consumption of the wealthiest countries. • Energy poverty is the lack of access to modern energy services, and it is measured by access to electricity and to clean cooking fuel.

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Sustainable Development Goal 7 is to ensure access to affordable, reliable, sustainable and modern energy for all. • The energy trilemma — the simultaneous pursuit of energy security, energy equity or affordability, and environmental sustainability — frames the policy problem, and the three objectives frequently conflict, which is why energy policy is difficult.

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Energy Conservation • Energy conservation means using less energy for the same service, through both efficiency improvement and avoidance of waste. • The governing principle, which should be stated in any answer on the subject: the cheapest unit of energy is the one not used.

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A unit saved avoids not only the cost of generation but also the transmission and distribution losses upstream of it, so a unit saved at the point of use displaces more than a unit at the power station.

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Saving is also available immediately, whereas new generation takes years to build. • Measures by sector: • Domestic — LED lighting in place of incandescent, which uses about a tenth of the energy for the same light; efficient appliances selected by star rating; improved cooking stoves and a shift to LPG, biogas or electric cooking; solar water heating; and insulation and passive solar design in buildings. • Industrial — high-efficiency motors and variable speed drives, since motors dominate industrial electricity use; power factor improvement by capacitors; waste heat recovery; insulation of steam lines; combined heat and power; energy audits and monitoring; and process integration by pinch analysis. • Transport — public transport, electric vehicles, efficient driving, vehicle maintenance and better route planning. • Utility side — reduction of technical and non-technical losses, which in Nepal have historically been high; demand side management to shift load off the evening peak by tariff structure and time-of-use metering; and improved load factor. • An energy audit is the systematic examination of energy use in a facility to identify where and how energy is used and where savings can be made.

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Its levels are the preliminary or walk-through audit, the detailed audit and the investment-grade audit, and the recommendations are ranked by simple payback period, which is the cost of the measure divided by the annual saving.

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Measures with a payback of under two years are commonly regarded as attractive, and housekeeping measures with essentially zero cost should always be implemented first. • In Nepal the Nepal Electricity Authority, the Alternative Energy Promotion Centre, the Water and Energy Commission Secretariat and the Department of Electricity Development are the principal institutions, and the Electricity Act, 2049 with the Electricity Regulatory Commission Act, 2074 form part of the legal framework.

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Institutional arrangements and legislation are under active reform, so current sources should be consulted.

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This section covers water turbines and hydroelectric plants, the classification of hydropower plants, and the engineering components and basic design of a hydropower plant and of its individual components.

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Power, Head and Classification • The fundamental relation is P = ρgQHη, where ρ is the density of water, Q the discharge in m³/s, H the net head in metres and η the overall efficiency.

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In practical units, P in kilowatts is approximately 9.81 QHη, and with a typical overall efficiency of 0.8 this gives about 8 QH.

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This is the single most useful expression in the chapter. • Gross head is the difference in level between the intake and the tailrace; net head is the gross head less all hydraulic losses in the waterway.

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It is the net head that generates power, and the distinction is examined directly. • Capacity factor or plant load factor = actual energy generated in a period / energy that would have been generated at full capacity.

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A run-of-river plant in Nepal typically has a capacity factor of 50-60 per cent because of the seasonal flow variation, while a storage plant can be operated at a much higher value.

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Note that a low capacity factor does not mean a poorly designed plant — it reflects the hydrology. • Firm power is the power that can be guaranteed to be available at all times, that is in the driest period; secondary power is the additional power available when flows are higher.

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The distinction governs the commercial value of a project, since firm power commands a far higher price.

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Basis Classification Description Capacity (Nepal) Pico, micro, mini, small, medium, large Commonly pico under 5 kW, micro up to 100 kW, mini to 1 MW, small to 25 MW, with larger classed as medium and large; national definitions vary and are periodically revised Head Low (below about 30 m), medium (30-300 m), high (above 300 m) Determines the type of turbine and the layout Hydrology and storage Run-of-river, peaking run-of-river, storage (reservoir), pumped storage Run-of-river follows the river flow; storage regulates it seasonally; pumped storage is a net consumer of energy but stores it Purpose Isolated, grid-connected, multipurpose Multipurpose schemes combine power with irrigation, water supply or flood control • Pumped storage is not a source of energy but a store: it consumes more energy pumping than it recovers generating, with a round-trip efficiency of about 70-80 per cent, but it is valuable because it absorbs surplus off-peak energy and returns it at the peak, and it provides very fast response for system stability.

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That it is a net consumer of energy is the point examined. • Nepal's projects are predominantly run-of-river, which is why the dry-season deficit persists despite a growing installed capacity; storage projects are the technical answer but are far more costly and involve resettlement and inundation.

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Components of a Hydropower Plant • The components in order of flow, which should be recited as a sequence: diversion weir and intake → gravel trap → settling basin (desander) → headrace canal or tunnel → forebay tank → penstock → anchor blocks and support piers → powerhouse containing turbine, generator and controls → draft tube (reaction turbines only) → tailrace → transmission line. • Diversion weir and intake — raise the water level and divert the required flow, with the intake placed on the outer bank of a bend where scour keeps it clear of deposition, and set above the bed to exclude bedload. • Gravel trap and settling basin — remove suspended sediment, which would otherwise abrade the turbine runner.

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Sediment abrasion is the dominant operation and maintenance problem of Himalayan hydropower, because the young, weak rocks of the mountains yield water carrying very hard quartz and feldspar particles, so the settling basin is designed to remove particles above about 0.2 mm and hard particles above 0.15 mm.

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This is examined as a distinctively Nepali design issue. • Headrace — conveys water from the intake to the forebay, as an open canal or a tunnel, at a gentle gradient with minimal loss. • Forebay — a small tank at the head of the penstock which provides a final settling opportunity, prevents air entry into the penstock, and provides a small storage to absorb rapid load changes. • Surge tank — provided on a long pressure conduit or tunnel, not on a short open headrace; it reflects the pressure wave when the turbine gates close suddenly, so that water hammer does not propagate up the tunnel, and it supplies water immediately when the load increases, before the flow in the tunnel has accelerated.

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Both functions should be given. • Penstock — the pressure pipe from the forebay to the turbine.

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Its diameter is chosen by economic analysis: a larger diameter costs more but loses less head, so the optimum is the diameter at which the annual cost of the pipe plus the annualised value of the lost energy is a minimum.

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Velocities are commonly 2-4 m/s, and the design must allow for water hammer, so an anchor block is provided at every bend and expansion joints between them. • Powerhouse — houses the turbine, generator, governor, valves, switchgear, control room and crane; it may be surface or underground. • Draft tube — fitted to a reaction turbine only.

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It converts the kinetic energy at the runner exit into pressure energy, recovering part of it, and allows the turbine to be set above the tailwater level while still using the full head down to the tailrace.

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An impulse turbine has no draft tube, because it discharges to atmosphere.

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This is a certainty in the examination. • Tailrace — returns the water to the river.

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Turbines Turbine Type Head range Characteristics Pelton Impulse High, above about 300 m (used from about 50 m upward in small schemes) Jets strike buckets at atmospheric pressure; flow controlled by a spear valve; no draft tube; good part-load efficiency; tolerant of sediment compared with reaction turbines Turgo Impulse Medium-high, 30-300 m Jet strikes the runner at an angle; handles greater flow than a Pelton of the same diameter Cross-flow (Banki-Michell) Impulse (partly) Low to medium, 2-100 m Simple, locally manufacturable, flat efficiency curve over a wide flow range; the standard turbine for Nepali micro-hydro Francis Reaction Medium, 30-300 m Mixed flow through a spiral casing and guide vanes; high efficiency at design flow but falls away at part load; draft tube required Kaplan / propeller Reaction Low, below about 30 m Axial flow; adjustable blades in the Kaplan give good part-load efficiency; suits large flow at small head Bulb / tubular Reaction Very low, under 10 m Compact axial machine for run-of-river and tidal schemes • The impulse-reaction distinction is fundamental and is examined directly: in an impulse turbine the entire pressure head is converted to velocity in the nozzle before the water strikes the runner, so the runner operates at atmospheric pressure, is not enclosed in a pressure casing and has no draft tube; in a reaction turbine the water is under pressure as it passes through the runner, the runner is fully enclosed and flooded, and a draft tube is required. • Specific speed Ns = N√P/H5/4 characterises the runner and is the systematic basis of selection: a low specific speed corresponds to a Pelton turbine for high head and low flow, an intermediate value to a Francis turbine, and a high value to a Kaplan for low head and high flow. • Cavitation in reaction turbines occurs when the pressure at the runner exit or in the draft tube falls to the vapour pressure.

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It is governed by Thoma's cavitation factor σ = (Ha − Hv − Hs)/H, where Hs is the setting of the runner above the tailwater.

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Cavitation is avoided by lowering the turbine setting, and at high altitude the reduced atmospheric pressure requires a lower setting still — a point of direct relevance in Nepal. • The governor regulates the flow to the turbine to hold the speed, and therefore the frequency, constant as the load varies.

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In micro-hydro an electronic load controller is used instead, which keeps the turbine at full flow and dumps surplus power into a ballast load, because it is far cheaper and simpler than a mechanical governor — the standard arrangement in Nepali micro-hydro and a favourite examination point.

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This section covers solar radiation, solar thermal energy, the solar cell, and the design of array and battery size for buildings and for water supply schemes.

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Solar Radiation • The solar constant is the radiation received on a unit area normal to the sun's rays outside the atmosphere, about 1367 W/m².

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At the earth's surface the value is much lower — a peak of roughly 1000 W/m² on a clear day at noon — because of absorption and scattering in the atmosphere, and it is that 1000 W/m² figure that is used as standard test conditions for rating modules. • Components of radiation: beam or direct radiation, which arrives in a straight line from the sun; diffuse radiation, which has been scattered by the atmosphere and cloud; and reflected radiation from the ground, the albedo component.

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Global radiation is the sum of all three on a horizontal surface.

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Concentrating collectors can use only the beam component, which is why they are unsuitable for hazy or cloudy climates, whereas flat plate collectors and PV modules use diffuse radiation as well — a point examined directly. • Measurement: a pyranometer measures global radiation on a horizontal surface; a pyrheliometer measures beam radiation only and must track the sun; and a sunshine recorder measures the duration of bright sunshine. • Peak sun hours is the number of hours of radiation at 1000 W/m² that would deliver the same daily energy as the actual variable radiation; it is numerically equal to the daily insolation in kWh/m².

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For Nepal it is commonly 4-5 peak sun hours a day, with a national average solar insolation usually quoted at about 4.5 kWh/m² per day and roughly 300 sunny days a year. • Tilt and orientation: in the northern hemisphere a fixed array faces south, and the optimum annual tilt is approximately equal to the latitude.

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For a system that must perform in winter the tilt is increased by about 10-15° above the latitude, and for summer performance it is reduced by the same amount.

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For a solar water pumping or standalone system in Nepal the tilt is set for the worst month, and a minimum tilt of about 10-15° is kept in any case so that rain washes the dust off.

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Tracking increases output by roughly 25-40 per cent but adds cost and maintenance, and is rarely justified for small systems. • Shading deserves particular emphasis: because the cells in a module are connected in series, shading even a small part of one module reduces the output of the whole string disproportionately, so the array must be sited clear of trees, poles and buildings throughout the year.

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Bypass diodes limit but do not eliminate the effect.

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Solar Thermal Device Description Temperature range and use Flat plate collector Absorber plate with bonded tubes in an insulated, glazed box Up to about 80 °C; domestic water heating; uses diffuse as well as beam radiation Evacuated tube collector Absorber inside an evacuated glass tube, eliminating convective loss Up to about 120-150 °C; better in cold and cloudy conditions than a flat plate Solar cooker (box type) Insulated box with a glazed lid and a reflector Up to about 100-150 °C; slow cooking Solar cooker (parabolic) Concentrating dish focusing on the pot Up to 300 °C or more; fast cooking but needs tracking and direct sun Solar dryer Direct, indirect or mixed-mode drying chamber Crop and fish drying; faster and cleaner than open sun drying Solar still Basin of water under a sloping glass cover Distillation of saline or contaminated water; output only 3-5 L/m² per day Concentrating systems Parabolic trough, dish-Stirling, central receiver 150-1000 °C; power generation and process heat; beam radiation only • The greenhouse effect in a flat plate collector is the mechanism to state: the glass cover transmits the short-wave solar radiation but is opaque to the long-wave infrared re-radiated by the hot absorber, so the heat is trapped; the cover also suppresses convective loss to the wind. • A thermosyphon system circulates water by natural convection, with the tank placed above the collector so that the warm, less dense water rises into it; a forced circulation system uses a pump and allows the tank to be placed anywhere, at the cost of the pump and its control.

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That the tank must be above the collector in a thermosyphon system is a standard question. • Solar water heating is the most economically attractive solar application in Nepal, since it displaces electricity or LPG directly and pays back in a few years.

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The Solar Cell and Photovoltaic Systems • The photovoltaic effect: light falling on a semiconductor p-n junction generates electron-hole pairs, which the built-in electric field at the junction separates, producing a potential difference and, when a circuit is connected, a direct current.

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A PV cell generates DC, so an inverter is required for AC loads — a point often asked. • Cell types: monocrystalline silicon, the most efficient at about 18-22 per cent commercially and the most expensive, recognisable by its uniform dark colour and chamfered corners; polycrystalline silicon, slightly less efficient at about 15-18 per cent and cheaper, with a mottled blue appearance; thin film (amorphous silicon, CdTe, CIGS), the least efficient at about 10-13 per cent but cheapest per watt, flexible and better in low light and at high temperature. • The effect of temperature is the counter-intuitive fact most often examined: the output of a PV module falls as its temperature rises, by roughly 0.4-0.5 per cent per °C above the 25 °C standard test condition.

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A module therefore performs better on a cold bright day than on a hot one, and arrays must be mounted with an air gap behind them for cooling.

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Irradiance principally affects the current and temperature principally affects the voltage. • Terms: open circuit voltage Voc; short circuit current Isc; the maximum power point, at which the product of voltage and current is greatest; and the fill factor, the ratio of maximum power to the product of Voc and Isc, which measures the squareness of the curve and is typically 0.7-0.8.

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A maximum power point tracker (MPPT) continuously adjusts the operating point to the maximum, gaining typically 10-30 per cent over a simple controller. • System components:

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PV array; charge controller, which prevents overcharging and deep discharge of the battery; battery bank in a standalone system; inverter for AC loads; and the loads themselves, together with mounting, cabling and protection.

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A grid-tied system has no battery and feeds surplus to the grid; a standalone system requires storage; and a hybrid system combines PV with another source such as a diesel generator or micro-hydro.

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Sizing an Array and Battery • The sizing procedure should be reproduced as a sequence: • 1.

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Estimate the daily energy demand in watt-hours, by listing each load with its power and hours of use. • 2.

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Divide by the system efficiency — allowing for inverter, battery round-trip, wiring and soiling losses, commonly giving an overall factor of about 0.6-0.7 for a battery system. • 3.

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Divide by the peak sun hours for the design month to obtain the required array wattage.

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The design month must be the worst month, not the annual average, or the system will fail in that month — the single most examined point. • 4.

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Select modules and determine the series-parallel configuration: modules in series add voltage and modules in parallel add current, and the string voltage must match the system and controller. • 5.

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Size the battery: battery capacity in ampere-hours = (daily energy demand in Wh × days of autonomy)/(system voltage × maximum depth of discharge × battery efficiency). • Days of autonomy — the number of days the system must run without sun — is commonly 2-3 for a domestic system and more for a critical load such as a telecommunications repeater. • Depth of discharge is limited to protect the battery: about 50 per cent for a lead-acid battery and up to 80 per cent for lithium-ion.

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Deep discharge is the commonest cause of premature battery failure, and the battery is almost always the shortest-lived and most troublesome component of a standalone system — which is exactly why a solar water pumping scheme stores water in a tank rather than energy in batteries. • For a water supply scheme the calculation runs from the daily water requirement and the total dynamic head to the hydraulic energy, ρgQH, which is divided by the wire-to-water efficiency — commonly 0.3-0.45 for a complete PV pumping system — to give the electrical energy, and then by the peak sun hours to give the array size.

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Storage of two to three days' water is provided in a tank. • Nepal's Alternative Energy Promotion Centre has promoted solar home systems, solar pumping and institutional solar installations extensively, and solar has become the principal means of electrification in remote areas beyond the grid.

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This section covers the availability of wind energy resources, wind turbines and wind parks, power control, the calculation of power from wind, and the sources and uses of geothermal energy.

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The Wind Resource and Power in the Wind • The power available in a wind stream is P = ½ρAV³, where ρ is air density (about 1.225 kg/m³ at sea level), A the swept area and V the wind speed. • Three consequences dominate every question on the subject: • Power varies as the cube of the wind speed, so doubling the wind speed multiplies the available power by eight, and a site with a mean speed 20 per cent higher yields about 70 per cent more energy.

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This is why site selection matters more than any other decision in a wind project, and why an accurate wind survey over at least a year is indispensable. • Power varies as the swept area and therefore as the square of the rotor diameter, so doubling the diameter quadruples the output. • Power varies with air density, which falls with altitude and rises as temperature falls, so a turbine at high altitude in Nepal produces less than the same machine at sea level in the same wind speed. • The Betz limit is the theoretical maximum fraction of the wind's kinetic energy that any turbine can extract:

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16/27, or 59.3 per cent.

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The reason is that extracting all the energy would require the air to be brought to rest behind the rotor, which would prevent any further air from passing through.

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This physical argument is what the examination asks for.

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Modern large turbines achieve a power coefficient of about 0.45-0.50, that is roughly three-quarters of the Betz limit. • Wind speed increases with height above the ground according to the power law V/Vref = (h/href)α, with α about 1/7 over open flat terrain and larger over rough or built-up ground.

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This wind shear is why hub heights have increased steadily. • The resource is described statistically by the Weibull distribution of wind speed, and by the wind rose, which shows the frequency and strength of wind from each direction. • Nepal's wind resource is limited overall but locally significant: the strong, reliable valley winds of the Kali Gandaki and Mustang, and sites in the Terai and on ridges, offer the best prospects, and studies have suggested a few thousand megawatts of potential, though very little has been developed.

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Small wind turbines and wind-solar hybrid systems have been installed for remote electrification.

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Wind Turbines and Wind Parks Feature Horizontal axis (HAWT) Vertical axis (VAWT) Axis and orientation Axis parallel to the wind; must be yawed to face it Axis vertical; accepts wind from any direction with no yaw mechanism Efficiency Higher; the standard for commercial generation Lower Generator location In the nacelle at the top of the tower At ground level, so maintenance is easier Starting Generally needs a higher wind speed to start Savonius type is self-starting at low speed Types Two- and three-blade upwind machines Darrieus (lift, egg-beater) and Savonius (drag, S-rotor) Application Utility-scale wind farms and most small turbines Niche, turbulent and urban sites, water pumping • Main components of a horizontal axis machine: rotor blades and hub; nacelle containing the low-speed shaft, gearbox, high-speed shaft, generator, brake and controller; yaw system; tower; and foundation.

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Direct-drive machines omit the gearbox, using a large multi-pole generator instead, which removes the component that historically caused the most failures. • Operating speeds, which must be known as a set: the cut-in speed, typically 3-4 m/s, at which the turbine begins to generate; the rated speed, typically 12-15 m/s, at which it reaches rated output; and the cut-out speed, typically 25 m/s, above which it is shut down to prevent damage.

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Between rated and cut-out the output is held constant at the rated value. • Power control above the rated speed: stall regulation uses blades designed so that the flow separates as the wind rises, limiting the power passively with no moving parts — simple and cheap but with less precise control; pitch regulation turns the blades about their axis to reduce the angle of attack, giving accurate control and a better-shaped power curve at the cost of an active mechanism.

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Large modern turbines are pitch regulated.

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The distinction between passive stall and active pitch control is examined directly. • Wind parks (wind farms): turbines are spaced about 3-5 rotor diameters apart across the prevailing wind and 5-10 diameters apart along it, to limit the wake effect, in which a downwind turbine receives slower and more turbulent air and therefore produces less and suffers greater fatigue loading.

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Array losses of 5-15 per cent are typical even with good layout, and this wake spacing requirement is a standard question. • Capacity factor for wind is typically 20-40 per cent onshore, higher offshore — lower than for most other technologies because of the variability of the resource.

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Geothermal Energy • Geothermal energy is heat from within the earth, derived principally from the decay of radioactive isotopes in the crust and mantle together with residual heat from the planet's formation.

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The average geothermal gradient is about 25-30 °C per kilometre of depth, but in volcanic and tectonically active regions it is far higher, which is what makes those regions exploitable. • Types of resource: hydrothermal, the only type currently exploited commercially, comprising vapour-dominated (dry steam) and liquid-dominated (hot water) fields; geopressured resources, hot pressurised brine at depth; hot dry rock or enhanced geothermal systems, where water is circulated through artificially fractured hot rock; and magma resources, which remain experimental. • Power plant types: dry steam, in which steam from the well drives the turbine directly, the simplest but rarest; flash steam, in which pressurised hot water is flashed to steam in a separator, the commonest type; and binary cycle, in which the geothermal fluid heats a secondary working fluid of low boiling point in a heat exchanger, which allows lower-temperature resources — from about 85 °C — to be used and keeps the geothermal fluid in a closed loop so that it can be reinjected without contact with the atmosphere.

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Binary cycle is the technology that has opened up moderate-temperature resources. • Direct uses, which require far lower temperatures and are of much wider application: space heating and district heating, greenhouse heating, aquaculture, crop and timber drying, industrial process heat, and balneology — the therapeutic use of hot springs.

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Ground source heat pumps are a distinct technology which uses the near-constant shallow ground temperature rather than deep geothermal heat, and they can both heat and cool. • Advantages: base load generation available continuously regardless of weather or season, which distinguishes it sharply from solar and wind; a very small land footprint; high capacity factor, often above 90 per cent; and low emissions. • Limitations: the resource is site-specific and confined to particular geology; drilling is expensive and exploration risky; the fluids are often corrosive and scaling, carrying silica, hydrogen sulphide, arsenic, boron and dissolved gases; reservoir pressure and temperature can decline with extraction unless fluid is reinjected; and induced seismicity is a recognised concern with enhanced geothermal systems.

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Reinjection of the spent fluid is standard practice for both reservoir maintenance and disposal. • Nepal's geothermal situation should be stated accurately: numerous hot springs are known along the Main Central Thrust and in the high Himalaya — Tatopani being the familiar example — and these are used for bathing and have local religious significance, but the resources identified are of low to moderate temperature, exploration has been very limited, and there is at present no geothermal electricity generation in the country.

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Direct use and ground source heat pumps are the more realistic near-term prospects.

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This section covers synthetic fuels from biomass, thermochemical, physico-chemical and biochemical conversion, bio-fuel cells, basic electrochemistry, PEM and solid oxide fuel cells, and hydrogen production and storage.

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Biomass and Conversion Routes • Biomass is organic material of recent biological origin — wood, agricultural residue, energy crops, animal dung, and the organic fraction of municipal and industrial waste.

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It is considered carbon neutral in principle, because the carbon dioxide released on combustion was absorbed from the atmosphere during growth — but this holds only if the biomass is replaced by regrowth, so unsustainable harvesting is not carbon neutral at all.

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This qualification is examined directly. • The three conversion routes must be kept clearly apart, since questions turn on the distinction:

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Route Processes Products Conditions Thermochemical Direct combustion Heat Full air supply Gasification Producer gas or syngas (CO, H₂, CH₄) Sub-stoichiometric air, oxygen or steam at 800-1000 °C Pyrolysis Bio-oil, char and gas No oxygen at all, 400-600 °C Torrefaction Dry, brittle, energy-dense solid fuel Mild heating at 200-300 °C without oxygen Physico-chemical Extraction and transesterification Biodiesel and glycerine Vegetable oil or fat plus methanol with an alkali catalyst Briquetting and pelletising Densified solid fuel Mechanical compaction Biochemical Anaerobic digestion Biogas (50-70 % CH₄) and digestate Airtight digester, mesophilic 30-40 °C or thermophilic 50-60 °C Alcoholic fermentation Ethanol Yeast acting on sugars or hydrolysed starch Composting Compost and heat Aerobic • The distinction between combustion, gasification and pyrolysis is the one most often asked, and it turns entirely on the oxygen supply: combustion has a full supply, gasification a deliberately restricted one, and pyrolysis none at all. • Biogas is the most important bioenergy technology in Nepal.

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Anaerobic digestion proceeds in four stages — hydrolysis, acidogenesis, acetogenesis and methanogenesis — the last being the slowest and the most sensitive, since methanogens are strict anaerobes that require a pH near neutral, between about 6.8 and 7.2, and a stable temperature.

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Biogas contains 50-70 per cent methane with most of the balance carbon dioxide, and has a calorific value of about 20-25 MJ/m³. • The fixed-dome digester, of which the GGC 2047 model is the Nepali standard, has no moving parts and a long life, and the gas pressure varies as gas accumulates; the floating-drum type delivers gas at constant pressure but the steel drum corrodes.

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Nepal's domestic biogas programme has installed several hundred thousand plants and is among the most successful in the world; performance falls off at low temperature, which is why plants work well in the Terai and mid-hills but poorly at high altitude. • Its benefits should be listed: clean cooking gas replacing fuelwood, with the associated reduction in indoor air pollution and in time spent collecting wood; a high-quality digested slurry fertilizer; sanitary disposal where latrines are connected; and reduced deforestation. • Improved cooking stoves raise the efficiency of fuelwood use from roughly 10 per cent for a traditional three-stone fire to 20-25 per cent, and vent the smoke outside, which is the greater health benefit, since indoor air pollution from cooking is a major cause of respiratory disease among women and children. • Biofuel generations: first from food crops, second from lignocellulosic residues and wastes, third from algae — the progression driven by the food-versus-fuel objection and by land use concerns.

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Basic Electrochemistry and Fuel Cells • In both galvanic and electrolytic cells, oxidation occurs at the anode and reduction at the cathode.

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But the sign convention reverses: the anode is negative in a galvanic cell and positive in an electrolytic cell — a classic point of confusion and a standard question. • A galvanic cell converts chemical energy to electrical energy spontaneously; an electrolytic cell consumes electrical energy to drive a non-spontaneous reaction. • A fuel cell is a galvanic cell in which the reactants are supplied continuously from outside, so it does not run down as a battery does but generates as long as fuel is supplied.

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That continuous supply is what distinguishes a fuel cell from a battery, and it is examined directly. • The defining thermodynamic advantage, which should be stated in any answer: a fuel cell converts chemical energy directly into electrical energy without an intermediate conversion to heat and mechanical work, so it is not subject to the Carnot limitation.

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Practical electrical efficiencies of 40-60 per cent are achieved, rising to 85 per cent or more in combined heat and power. • The hydrogen-oxygen cell: at the anode, H₂ → 2H⁺ + 2e⁻; at the cathode, ½O₂ + 2H⁺ + 2e⁻ → H₂O; overall 2H₂ + O₂ → 2H₂O.

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The only by-product is water, which is the environmental attraction.

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Type Electrolyte Temperature Characteristics and use PEM (proton exchange membrane) Solid polymer membrane 60-100 °C Quick start, compact, high power density; requires pure hydrogen and a platinum catalyst that is poisoned by carbon monoxide; the type used in vehicles Alkaline (AFC) Potassium hydroxide solution 60-250 °C High efficiency but intolerant of carbon dioxide; used in spacecraft Phosphoric acid (PAFC) Phosphoric acid 150-220 °C Mature and tolerant of impurities; stationary CHP Molten carbonate (MCFC) Molten carbonate salt 600-700 °C Internal reforming of natural gas; no precious metal catalyst; stationary power Solid oxide (SOFC) Solid ceramic (yttria-stabilised zirconia) 600-1000 °C Highest efficiency, tolerant of carbon monoxide, can use natural gas directly; slow to start and subject to thermal stress; stationary generation • The PEM and SOFC comparison is the one examined:

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PEM operates at low temperature, starts quickly and suits vehicles but needs pure hydrogen and a platinum catalyst that carbon monoxide poisons;

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SOFC operates at high temperature, reaches the highest efficiency, tolerates carbon monoxide and can reform fuel internally, but takes hours to start and is subject to thermal stress, so it suits continuous stationary operation. • A microbial fuel cell (bio-fuel cell) uses micro-organisms to oxidise organic matter at the anode, transferring electrons to the electrode, so that wastewater treatment and electricity generation occur together.

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Its power density is at present very low, so it remains a research and niche technology, valued more as a self-powered sensor and as a route to energy-neutral wastewater treatment than as a power source.

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Hydrogen Production and Storage • Hydrogen is an energy carrier, not a source; it must be manufactured, and the energy content of the product is always less than the energy consumed in making it. • Production routes: steam methane reforming, which supplies the great majority of world production and is cheapest but releases carbon dioxide; partial oxidation and autothermal reforming; coal gasification; electrolysis of water, which gives very pure hydrogen and is clean if the electricity is renewable but is more expensive; and biological, thermochemical and photoelectrochemical routes, which remain developmental. • The colour convention is worth knowing: green hydrogen from electrolysis using renewable electricity; blue from fossil fuel with carbon capture; grey from fossil fuel without capture; and, in some usages, pink from nuclear electricity. • Electrolysis:

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2H₂O → 2H₂ + O₂, with hydrogen at the cathode and oxygen at the anode in a 2:1 volume ratio; the theoretical decomposition voltage is 1.23 V but practical cells need 1.8-2.0 V because of overpotentials and resistance, giving an efficiency of roughly 60-80 per cent. • Storage is the central difficulty, and the reason should be stated precisely: hydrogen has by far the highest energy content per unit mass of any fuel, about 120 MJ/kg against 44 for petrol, but a very low energy content per unit volume, because its density is so low.

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It is the volumetric energy density, not the gravimetric, that makes storage hard, and this is the point most often missed. • Methods: compressed gas at 350-700 bar, the commonest, but requiring heavy tanks; liquefaction at −253 °C, which is very energy intensive, consuming some 30 per cent of the energy stored, and suffers continuous boil-off; metal hydrides, which are safe and compact but heavy and slow; and chemical carriers such as ammonia, methanol and liquid organic hydrogen carriers. • Safety: hydrogen has very wide flammability limits, from about 4 to 75 per cent in air, a very low ignition energy, and a flame that is almost invisible in daylight; against this, it is extremely buoyant and disperses upward very rapidly in the open, so outdoor leaks are far less dangerous than an equivalent leak of a heavier fuel.

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Hydrogen embrittlement of steels is a further engineering constraint. • For Nepal, the attraction of hydrogen is as a means of using surplus monsoon hydropower that would otherwise be spilled, converting it to a storable and transportable form and reducing petroleum imports; green hydrogen and its use for fertilizer manufacture and transport have been the subject of study and of a national policy framework, but development is at an early stage.

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This section covers emission hazards, battery hazards, nuclear hazards, hybrid vehicles, smart grid systems and capacitors.

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Emissions and Their Hazards Pollutant Principal energy source Effects Carbon dioxide All fossil fuel combustion Climate change; not toxic at ambient concentration Sulphur dioxide Coal and heavy fuel oil Respiratory irritation; acid deposition; damage to vegetation and buildings Oxides of nitrogen All high-temperature combustion Respiratory effects; acid deposition; precursor of ground-level ozone and photochemical smog Particulate matter Combustion of coal, biomass and diesel Cardiovascular and respiratory disease;

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PM2.5 penetrates deepest and is the most damaging Carbon monoxide Incomplete combustion Chemical asphyxiant, binding haemoglobin far more strongly than oxygen Volatile organics Fuel evaporation, incomplete combustion Ozone precursors; some toxic and carcinogenic Mercury and heavy metals Coal combustion Persistent, bioaccumulative, neurotoxic Indoor smoke Biomass cooking on open fires A leading cause of respiratory disease and premature death, falling mainly on women and children • Life cycle comparison is the fair basis, and it is examined: no energy source is entirely free of impact.

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Hydropower inundates land, displaces people, blocks fish migration, traps sediment and alters the downstream flow regime; solar and wind require land, materials and mining, and their manufacture consumes energy; nuclear produces long-lived waste; and even energy efficiency measures have an embodied impact.

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The correct comparison is therefore of impacts per unit of energy delivered over the whole life cycle, not of the operating stage alone. • Hydropower's specific impacts deserve statement for Nepal: inundation of land and displacement of communities in storage schemes; the dewatered reach between intake and powerhouse in a run-of-river scheme, requiring a minimum environmental flow release, conventionally at least 10 per cent of the minimum monthly average flow; interruption of fish migration, mitigated by fish ladders; sediment trapping behind the dam, with erosion downstream; and induced seismicity and reservoir-related effects in very large schemes.

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Battery Hazards • Lead-acid batteries: contain lead, a cumulative neurotoxin particularly damaging to children, and sulphuric acid.

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The hazards are acid burns, lead exposure during informal recycling — which is a serious occupational and public health problem in South Asia — and the evolution of hydrogen during charging, which is explosive in the 4-75 per cent range and requires that battery rooms be ventilated.

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Lead-acid batteries are, however, highly recyclable, and a formal recycling chain is the appropriate control. • Lithium-ion batteries: the hazard is thermal runaway, in which an internal short circuit, overcharging, physical damage or overheating initiates an exothermic reaction that raises the temperature further, releasing flammable and toxic gases and causing fire that is difficult to extinguish because the cell supplies its own oxidant.

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Control is by battery management systems, cell-level protection, and proper charging and storage.

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Recycling is technically more difficult than for lead-acid and the recovery rate remains low. • General principles: batteries must not be disposed of with municipal waste; the mining of lithium, cobalt and nickel carries substantial environmental and social costs, including child labour in some cobalt supply chains; and extended producer responsibility with take-back schemes is the standard policy instrument.

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Nuclear Hazards • Nuclear power generates electricity from the fission of heavy nuclei, with no combustion and therefore no carbon dioxide, sulphur dioxide or particulates from operation.

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Its energy density is enormous, and its capacity factor very high. • The hazards: radioactive waste, in particular the high-level spent fuel which remains hazardous for tens of thousands of years and for which no country has yet brought a permanent geological repository into operation; the risk of a severe accident, as at Chernobyl in 1986 and Fukushima in 2011, with consequences that cross national boundaries and persist for decades; uranium mining and milling, which leaves large volumes of radioactive tailings; the proliferation risk arising from enrichment and reprocessing technology; thermal discharge from cooling; and the technical and financial difficulty of decommissioning. • Radiation protection rests on three principles — justification, that the practice must do more good than harm; optimisation, that exposure be kept as low as reasonably achievable; and limitation, that individual dose limits not be exceeded — and on the practical measures of time, distance and shielding, distance obeying an inverse square law. • Classification of waste: low, intermediate and high level, the last comprising spent fuel and reprocessing waste, which generates heat as well as radiation and requires cooling before disposal. • Nepal has no nuclear power and no immediate prospect of it; radiation safety in the country concerns medical and industrial sources, which are regulated accordingly.

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Hybrid Vehicles, Smart Grids and Capacitors • Vehicle types should be distinguished precisely, since the examination asks for the differences: • Hybrid electric vehicle (HEV) — an internal combustion engine with an electric motor and a small battery; the battery is charged only by the engine and by regenerative braking, and the vehicle cannot be plugged in. • Plug-in hybrid (PHEV) — a larger battery that can be charged from the mains, giving a useful all-electric range with the engine available beyond it. • Battery electric vehicle (BEV) — no engine at all; charged entirely from the grid; zero tailpipe emission. • The point that must be made about electric vehicles is that their emissions are displaced rather than eliminated: they are zero at the tailpipe, but the total depends on how the electricity is generated, so an electric vehicle charged from a coal-fired grid may offer little benefit, while one charged from Nepal's hydropower is genuinely clean.

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This makes electric mobility particularly attractive for Nepal, since it substitutes domestic hydroelectricity for imported petroleum. • Regenerative braking recovers kinetic energy that would otherwise be dissipated as heat in the brakes, returning it to the battery; it is most effective in urban stop-start driving, which is why hybrid vehicles show their greatest advantage in city traffic and least on a steady motorway run — a standard question. • A smart grid is an electricity network using digital communication, sensing and control to monitor and manage the flows of electricity and information between generation and consumption. • Its features: two-way flow of both electricity and information; advanced metering infrastructure and smart meters; automated fault detection, isolation and restoration, which improves reliability; integration of distributed and variable renewable generation; demand response and time-of-use pricing; accommodation of electric vehicle charging and of storage; and reduction of technical and non-technical losses.

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The two-way flow of electricity, from consumers who also generate, is the feature that most distinguishes it from a conventional grid. • Its challenges: high capital cost, cyber security, data privacy, interoperability of standards, and the institutional capacity to operate it. • Capacitors in power systems serve two related purposes.

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Power factor correction: an inductive load such as a motor draws a lagging reactive current, so that the total current is larger than that required for the real power, increasing losses in cables and transformers and attracting a tariff penalty.

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A capacitor supplies the reactive power locally, raising the power factor towards unity and reducing the current drawn from the supply.

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Note that the capacitor does not reduce the real power consumed — it reduces the current and the losses associated with it, which is the point most often misunderstood.

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Capacitors are also used in series for voltage support on long lines, and for harmonic filtering. • Supercapacitors (ultracapacitors) store energy electrostatically rather than chemically.

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They have a very high power density and can be charged and discharged in seconds, over a million cycles, which makes them well suited to regenerative braking and to smoothing short fluctuations; but their energy density is far lower than a battery's, so they complement batteries rather than replacing them.

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The high power but low energy density contrast is the point examined.