Energy Sources and Electric Power Generation

Biomass energy is the energy stored in organic matter of plant and animal origin (wood, crop residue, animal dung, energy crops, organic waste). Plants store solar energy by photosynthesis; burning, gasifying or digesting the biomass releases this energy as heat, gas or electricity.

$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

Is biomass renewable?

Yes, conditionally. Biomass is renewable because:

• New plants grow every season or every few years using sunlight, water and CO₂.
• The CO₂ released on burning is roughly the CO₂ the plant absorbed while growing, so the carbon cycle is closed.
• Dung, crop residue and organic waste are produced continuously.

It stays renewable only if the rate of use is not greater than the rate of regrowth (sustainable harvesting). If forests are cut faster than they grow, biomass behaves like a depleting resource.

Firewood in Nepal

• Traditional biomass (firewood, dung, crop residue) supplies the largest share of Nepal's total energy, well over half (as per WECS energy synopsis reports); firewood alone is the biggest part, used mainly for cooking and space heating in rural households.
• Where it is sustainable: in mid-hills with community forests, community forest user groups manage harvest, and forest cover has been recovering. Firewood taken from such managed forests, dead branches and farm trees is renewable.
• Where it is not sustainable: near dense settlements, in the Terai and in high mountain areas (slow tree growth), harvesting is more than regrowth, causing deforestation, soil erosion and landslides.
• Use in traditional open stoves has low efficiency (about 10–15%), so more wood is burnt for the same useful heat. Indoor smoke causes respiratory disease, mainly for women and children.
• Firewood use also takes time for collection and keeps rural people away from modern energy.

Justification

Firewood in Nepal is renewable in principle, but present use is only partly sustainable. To keep it renewable:

1. Promote improved cooking stoves (ICS) and biogas (AEPC programmes).
2. Expand community forestry and plantation of fast-growing trees.
3. Replace firewood with electric cooking (induction) from hydropower where grid supply exists.
4. Use briquettes and gasifiers to raise efficiency.

Cogeneration (combined heat and power, CHP) is the simultaneous production of electricity and useful heat (steam or hot water) from a single fuel source. The heat that a normal power plant rejects to the condenser or exhaust is used for an industrial process or heating.

Principle

          +-----------+   electricity
 Fuel --> | Prime     |---------------> load / grid
          | mover +   |
          | generator |   exhaust / extracted steam
          +-----------+---------------> process heat
                                        (drying, cooking,
                                         heating)

• A conventional thermal plant converts only about 30–40% of fuel energy to electricity; the rest is lost.
• In cogeneration, overall fuel utilisation rises to about 70–85%, because the low-grade heat is also used.

Types

Where is cogeneration appropriate?

Cogeneration suits places that need both electricity and heat at the same site and at the same time:

1. Sugar mills – bagasse fired boiler; steam for juice boiling and power for the mill, surplus to grid (sugar mills in Nepal's Terai).
2. Paper and pulp, textile, chemical and food processing industries – large steam demand.
3. Cement plants – waste-heat recovery from kilns.
4. Rice mills, breweries, dairies – rice husk or biomass fired units.
5. Hospitals, hotels, campuses – electricity plus hot water / space heating.
6. District heating in cold climates.
7. Remote or high-altitude areas where both heat and power are scarce.

Benefits

• Lower fuel cost per unit of useful energy.
• Less CO₂ and pollution for the same output.
• Captive, reliable supply; less transmission loss.

Cogeneration is not appropriate where heat demand is small, far from the plant, or does not match the timing of electricity demand.

Captive power generation means a power plant set up by an industry or institution mainly for its own use, not primarily to sell to the public grid. Examples: diesel generator sets in factories, a sugar mill's bagasse-fired plant, a cement plant's waste-heat plant, or a hospital's standby generator. Surplus power may be sold to the grid under an agreement.

Needs of captive power generation

1. Unreliable grid supply – load shedding, outages and voltage dips stop production; industries need continuous power (Nepal faced long load shedding until about 2017).
2. Power quality – sensitive processes (cement, steel, pharmaceuticals, data centres, hospitals) need stable voltage and frequency.
3. Remote locations – mines, hydropower construction sites, telecom towers and hotels far from the grid.
4. Use of by-products – bagasse, rice husk, waste heat or process gas can be turned into power cheaply (often as cogeneration).
5. Cost saving – where grid tariff or demand charge is high, own generation can reduce cost.
6. Standby and emergency – hospitals, airports, banks need backup for critical loads.
7. Grid capacity limits – new large loads where transmission or substation capacity is not yet available.

Points to consider

A thermal power plant burns fuel (coal, oil, gas) to raise steam that drives a turbine, while a hydropower plant converts the potential energy of water into electricity through a water turbine. Their operating behaviour is very different.

Comparison

Remarks

• Thermal plants are best for base load where fuel is cheap; hydro storage plants are ideal for peak load and spinning reserve.
• In Nepal, hydropower is the main source because of high head and many rivers, while thermal (diesel) plants such as Duhabi and Hetauda are only for emergency backup due to imported fuel cost.
• A mix of both gives the best system: thermal for firm base load, hydro for peaking and fast regulation.

Biomass is called a renewable energy source because it is continuously replenished by nature within a human time scale, using solar energy through photosynthesis.

Reasons

1. Regrowth – trees, crops and grasses grow again in months or years after harvest; fossil fuels take millions of years to form.
2. Solar origin – biomass stores sunlight as chemical energy:

$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

As long as the sun shines, new biomass forms. 3. Closed carbon cycle – the CO₂ released when biomass is burnt was taken from the atmosphere during growth, so the net addition of CO₂ is nearly zero (carbon neutral). 4. Continuous supply of wastes – animal dung, crop residue, food and municipal organic waste are produced every day and can be used in biogas plants. 5. Can be managed – plantation, energy crops and community forestry keep the stock constant.

Condition

  Plant growth  --->  Biomass  --->  Combustion / digestion
       ^                                   |
       |          CO2 to atmosphere        |
       +-----------------------------------+

Biomass remains renewable only when harvesting does not exceed regrowth. Over-cutting of forests (deforestation) makes it non-renewable in practice and releases stored carbon permanently.

Biogas is a gas (about 55–65% methane, CH₄, and 35–45% CO₂) produced by anaerobic digestion of cattle dung and other organic waste. Nepal's household biogas programme (Biogas Support Programme, with AEPC) was among the first CDM projects registered with the UNFCCC from Nepal (2005) and earns carbon credits (CERs) for emission reduction.

How biogas plants reduce greenhouse gas emissions

1. Replacing firewood – each household plant replaces a large amount of firewood each year. Less forest is cut, so the forest keeps absorbing CO₂ (carbon sink protected). Firewood from unsustainable harvest adds net CO₂.
2. Replacing fossil fuels – biogas replaces kerosene and LPG used for cooking and lighting, avoiding CO₂ from fossil carbon.
3. Capturing methane from dung – dung left in heaps or open pits decomposes and releases methane. Methane has a global warming potential about 25–28 times that of CO₂ over 100 years. In a sealed digester this methane is captured and burnt:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Burning converts a strong GHG into CO₂, which is biogenic (carbon neutral). 4. Carbon neutral fuel – CO₂ from burning biogas came from plants eaten by cattle, so there is no net addition. 5. Bio-slurry as fertiliser – reduces use of chemical fertiliser, whose manufacture emits CO₂ and N₂O. 6. Cleaner combustion – less black carbon and smoke compared to traditional stoves.

Cycle

 Grass/fodder --> Cattle --> Dung --> Digester --> Biogas
      ^                                  |          |
      |                               Slurry      Burn
      |                              (manure)       |
      +------------- CO2 absorbed <------ CO2 <-----+

Co-benefits counted under CDM

• Reduced indoor air pollution and better health.
• Time saved in firewood collection, mainly for women.
• Revenue from CERs helps subsidise more plants.

Hence biogas plants reduce GHG emission both by avoiding methane release and by displacing firewood and fossil fuel, which justifies their CDM registration.

Energy sources are grouped by how long they have been widely used and whether they are renewable.

Conventional and non-conventional sources in Nepal

Traditional biomass is still the largest share of total energy use; imported petroleum is the second; hydropower supplies almost all grid electricity.

Technology of wind energy

Wind power converts kinetic energy of moving air into electricity.

 Wind -> Blades -> Hub -> Gearbox -> Generator
                                       |
                            Converter/Transformer
                                       |
                                Grid / battery
 (tower, yaw drive, pitch control, controller)

• Rotor: 2 or 3 blades (HAWT) or vertical-axis (Darrieus, Savonius).
• Gearbox: steps up low rotor speed to generator speed (not used in direct-drive types).
• Generator: induction generator, DFIG, or permanent magnet synchronous generator.
• Controls: pitch control, yaw control, brake, and power electronics for grid connection.
• Power: $P = \frac{1}{2}\rho A v^3 C_p$, with $C_p$ limited to 0.593 (Betz limit); practical 0.35–0.45.
• Cut-in speed about 3–4 m/s, rated about 12–15 m/s, cut-out about 25 m/s.

Possibility and scope in Nepal

• The SWERA study (Solar and Wind Energy Resource Assessment, AEPC, 2008) estimated a wind potential of about 3,000 MW in areas with good wind power density.
• Promising sites: Kagbeni and the Kali Gandaki valley (Mustang), Khumbu, Palpa and some mountain passes, where valley winds are strong in the afternoon.
• Early projects: a 20 kW turbine at Kagbeni (1989) failed soon due to poor maintenance; later small wind–solar hybrid systems (e.g. Dhaubadi, Nawalparasi) were installed by AEPC.

Scope (opportunities):

1. Electrify remote mountain villages as wind–solar–battery hybrid mini-grids.
2. Wind is often strong in the dry winter season when hydropower output is low, so it complements run-of-river hydro.
3. Telecom towers, trekking lodges and tourism areas.

Limitations:

1. Detailed long-term wind data is scarce; winds are site-specific and turbulent in valleys.
2. Transport of large blades and towers on mountain roads is difficult.
3. High cost per kW, lack of skilled maintenance, and strong competition from cheap hydropower.

Wind therefore has good scope as small and medium hybrid systems in specific windy valleys rather than large wind farms.

The power in wind is the rate of flow of kinetic energy of air passing through the area swept by the turbine rotor.

Derivation

Let $\rho$ = air density (kg/m³), $A$ = swept area (m²), $v$ = wind speed (m/s).

Kinetic energy of air of mass $m$:

$$E = \frac{1}{2} m v^2$$

Mass flow rate through area $A$:

$$\dot{m} = \rho A v$$

Power available in the wind (energy per second):

$$P_w = \frac{1}{2}\dot{m}v^2 = \frac{1}{2}\rho A v^3$$

A turbine cannot take all this power, because air must leave the rotor with some speed. The fraction extracted is the power coefficient $C_p$. By Betz's law, $C_p \le 16/27 = 0.593$.

Mechanical power on the rotor:

$$P_m = \frac{1}{2}\rho A v^3 C_p$$

Electrical output includes the gearbox and generator efficiencies:

$$P_e = \frac{1}{2}\rho A v^3 C_p\, \eta_g\, \eta_{gen}$$

For a horizontal-axis turbine with blade length (radius) $r$: $A = \pi r^2$.

Key points

• $P \propto v^3$: doubling wind speed gives 8 times the power.
• $P \propto r^2$: doubling blade length gives 4 times the power.
• $P \propto \rho$: output is less at high altitude (lower air density).
• Practical $C_p$ is about 0.35–0.45.

Both thermal (coal/oil/gas fired) and nuclear plants produce steam to run a steam turbine–generator. They differ in the heat source: combustion of fossil fuel in a boiler versus nuclear fission in a reactor.

Comparison

Remarks

• Thermal plants are flexible and cheaper to build but costly to run and polluting.
• Nuclear plants are clean in CO₂ terms and cheap to run, suited only for steady base load in large grids.
• Nuclear power is not practical for Nepal due to small grid size, cost, lack of fuel and expertise, and seismic risk.

Cogeneration (combined heat and power) is the production of electricity and useful heat from the same fuel at the same time. Heat that would be rejected in a power-only plant is used for a process.

Why it is useful in biomass-based plants

1. Low efficiency of biomass power alone – small biomass steam plants have electrical efficiency of only about 15–25% due to low steam pressure and small size. Using the exhaust/back-pressure steam for heat raises total efficiency to about 60–80%.
2. Heat is needed on site – biomass plants are normally attached to agro-industries that need steam or hot air:
   • Sugar mills burn bagasse; steam is used for juice heating and evaporation, and power for crushers.
   • Rice mills use rice husk; heat for parboiling and drying paddy.
   • Paper, timber and tea industries use residues for drying.
3. Fuel available at the site – the residue is produced at the same factory, so no transport cost; the plant is a captive plant.
4. Better economics – selling surplus electricity to the grid plus saving fuel for heat shortens payback.
5. Environment – waste disposal problem is solved and carbon-neutral fuel replaces diesel or coal.

 Bagasse -> Boiler -> HP steam -> BP turbine -> Gen
                                     |
                             LP steam to process
                            (juice heating, drying)

Example: sugar mills in the Terai of Nepal use bagasse cogeneration to meet their own demand during the crushing season.

A carbon neutral cycle means the CO₂ released when a fuel is used is equal to the CO₂ removed from the atmosphere to form that fuel, so the net change of atmospheric CO₂ is zero.

How biomass maintains it

1. Absorption – growing plants take CO₂ from air by photosynthesis and store the carbon in wood, leaves and crops.
2. Use – when biomass is burnt, gasified or digested (biogas), the stored carbon returns to the air as CO₂.
3. Regrowth – new plants grown on the same land absorb the same CO₂ again.

     CO2 in atmosphere
      ^            |
      | burning    | photosynthesis
      |            v
  Energy use <-- Biomass (wood, crops,
  (heat,          residue, dung)
   power)

Fossil fuels, by contrast, release carbon that was locked underground for millions of years, so they add new CO₂ to the air.

Conditions for being truly carbon neutral

• Harvest must not exceed regrowth (sustainable forestry, replanting).
• Use residues and wastes that would decay anyway.
• Use efficient technologies (improved stoves, gasifiers, biogas) to get more energy per kg.
• Minimise fossil fuel used in harvesting, transport and processing.

If forests are cleared without replanting, the cycle breaks and biomass becomes a net CO₂ source.

Bio-energy is the energy obtained from biomass, i.e. organic matter from plants and animals (wood, crop residue, dung, energy crops, food and municipal organic waste). It is stored solar energy, captured by photosynthesis, and is renewable if used sustainably.

Forms of bio-energy

Example: household biogas plant (Nepal)

Nepal has several hundred thousand family-size biogas plants (mainly GGC-2047 fixed dome design) promoted by AEPC and the Biogas Support Programme.

 Dung+water -> Inlet -> Digester (dome) -> Gas pipe
                           |                  |
                      Outlet tank        Stove / lamp
                           |
                   Slurry (fertiliser)

Working:

1. Fresh cattle dung is mixed with water (about 1:1) and fed through the inlet.
2. In the airtight digester, bacteria break down the organic matter without oxygen in three stages: hydrolysis, acid formation and methane formation.
3. Gas (about 60% methane) collects in the dome and flows through a pipe to the stove.
4. Digested slurry comes out of the outlet and is used as manure.

Benefits:

• Replaces firewood and kerosene; saves forest.
• Smoke-free kitchen and better health.
• Good organic fertiliser.
• Reduces GHG emission (registered under CDM).

Other examples

• Bagasse-based power in sugar mills.
• Rice husk gasifier for power in rice mills.
• Improved cook stoves burning firewood more efficiently.

Bio-energy is energy obtained from biomass (wood, crop residues, animal dung, energy crops and organic waste), either directly as heat or converted to biogas, liquid fuels or electricity. Geothermal energy is heat from the earth's interior, taken out as hot water or steam.

Advantages of biomass compared with geothermal

1. Available everywhere – biomass is found in every village; geothermal is limited to volcanic or hot-spring regions.
2. Simple, low-cost technology – stoves, biogas digesters and small gasifiers can be built locally; geothermal needs deep drilling and costly exploration.
3. Storable and transportable – wood, briquettes and biogas can be stored and moved to where they are needed; geothermal heat must be used near the well.
4. Scalable – from a single household stove to a multi-MW plant.
5. Multiple products – heat, electricity, liquid fuels, and fertiliser (slurry).
6. Waste management – turns agricultural and animal waste into energy.
7. Rural employment – collection, processing and plantation.

Disadvantages of biomass compared with geothermal

1. Low energy density – large volume and mass are needed; geothermal steam is a concentrated source.
2. Fuel supply needed – collection, transport and storage cost; geothermal fuel is free and continuous.
3. Lower efficiency – traditional burning is about 10–15% efficient.
4. Seasonal and variable supply – crop residues are seasonal; geothermal gives steady base load (capacity factor about 90%).
5. Air pollution – smoke, particulates and indoor pollution; geothermal emissions are small (mainly H₂S).
6. Land use – competes with food crops and forests; deforestation if over-used.
7. Not carbon neutral if unsustainable, whereas geothermal has very low CO₂ emission.

Summary table

Conventional energy sources are those that have been used widely for a long time and are mostly non-renewable or traditional: coal, petroleum, natural gas, large hydropower, nuclear and traditional biomass (firewood). Non-conventional sources are new, mostly renewable alternatives: solar, wind, micro hydro, biogas, modern biomass, geothermal, tidal and hydrogen.

Fossil fuel use in Nepal

Nepal has no proven oil or gas reserves, so all petroleum products (diesel, petrol, LPG, kerosene) and most coal are imported. They are used mainly in transport, cooking (LPG), industry and diesel generators. This causes a large trade deficit and pollution.

How fossil fuel can be replaced

Strategies

1. Use surplus hydropower – especially wet-season surplus for EVs and electric cooking; grid expansion and reliable distribution.
2. Promote EVs – lower import duty on EVs, charging stations; Nepal has already seen fast growth in EV imports.
3. Rural renewable programmes – AEPC subsidies for solar, biogas, micro hydro and improved cooking stoves.
4. Green hydrogen – from surplus hydro to replace fertiliser and industrial fuel in the future.
5. Policy – carbon tax on fossil fuel, net metering for rooftop solar, energy efficiency standards.

These steps reduce imports, improve energy security and lower emissions.

Conventional energy sources

Sources that have been in large-scale use for a long time, with mature technology. Most are non-renewable (except hydro and traditional biomass).

• Coal, petroleum, natural gas
• Large hydropower
• Nuclear energy
• Traditional biomass: firewood, cattle dung, crop residue burnt directly

Non-conventional energy sources

New or alternative sources, mostly renewable and environment friendly, still developing in technology and cost.

• Solar (PV, thermal)
• Wind
• Small/micro hydro
• Modern biomass: biogas, gasification, bio-fuels, briquettes
• Geothermal, tidal, wave, ocean thermal, hydrogen and fuel cells

Biomass energy

Biomass is organic matter of plant or animal origin. It stores solar energy by photosynthesis and releases it by burning or conversion. It is renewable if regrowth keeps up with use.

Conversion routes:

            +--> Direct combustion --> heat / steam power
 Biomass ---+--> Gasification -----> producer gas -> engine
            +--> Anaerobic digestion -> biogas (CH4)
            +--> Fermentation ------> ethanol
            +--> Pressing/esterify --> bio-diesel

Suitable examples:

1. Firewood in improved cooking stoves – the largest energy use in rural Nepal; improved stoves save about 30–50% of wood and reduce smoke.
2. Household biogas – cattle dung digesters supply cooking gas and slurry manure; hundreds of thousands installed in Nepal.
3. Bagasse cogeneration – sugar mills burn bagasse to make steam and electricity for the mill.
4. Rice husk gasifier – producer gas runs an engine-generator in a rice mill.
5. Briquettes – compressed sawdust, rice husk or forest litter (e.g. banmara weed) for clean household fuel.
6. Bio-ethanol – from molasses, blended with petrol.
7. Municipal waste to energy – biogas from organic waste of cities.

Advantages: renewable, carbon neutral, uses wastes, available locally, creates rural jobs. Disadvantages: low energy density, bulky to transport, smoke if burnt in open stoves, can cause deforestation if over-used.

Conventional and non-conventional energy sources

Conventional sources are long established, mostly exhaustible and polluting; non-conventional sources are mostly renewable, clean and still developing.

Electric power from solar energy

Solar energy is converted to electricity in two ways.

1. Solar photovoltaic (PV) – direct conversion

A PV cell is a p–n junction of silicon. Photons with energy greater than the band gap (about 1.1 eV for Si) create electron–hole pairs. The junction field separates them and a DC voltage (about 0.5–0.6 V per cell) appears. Cells are connected in series and parallel to form modules and arrays.

 Sunlight
   ||
   vv
 +------+     +-----------+    +---------+
 | PV   |---->| Charge    |--->| Battery |
 |array | DC  | ctrl/MPPT |    +---------+
 +------+     +-----------+         |
                  |                 |
                  v                 v
              +---------+     DC loads
              |Inverter |--> AC load / grid
              +---------+

• Off-grid (stand-alone): PV + charge controller + battery + inverter.
• Grid-tied: PV + MPPT inverter feeding directly to grid (no battery).
• Module efficiency about 15–22%.

2. Solar thermal power – indirect conversion

Mirrors concentrate sunlight to heat a fluid, which makes steam to drive a turbine–generator.

 Sun --> Concentrator (trough/dish/tower)
             |
        Hot fluid --> Heat exchanger --> Steam
                                          |
                                      Turbine --> Generator
                                          |
                                      Condenser

Types: parabolic trough, central receiver (power tower), parabolic dish (Stirling engine). Thermal storage (molten salt) allows output after sunset.

Solar in Nepal

Nepal receives about 4.5–5.5 kWh/m²/day with around 300 sunny days a year, so PV is used in solar home systems, solar pumps and grid-connected plants of several MW.

Electricity generation from solar

Solar energy is converted mainly by photovoltaic (PV) cells. A PV cell is a silicon p–n junction; sunlight creates electron–hole pairs, which the junction field separates, producing DC voltage (about 0.5–0.6 V per cell). Cells form modules and arrays.

 Sun -> PV array -> MPPT/charge ctrl -> Battery
                         |
                     Inverter -> AC loads / grid

• Off-grid systems store energy in batteries.
• Grid-tied systems feed AC directly to the grid through an inverter.
• Solar thermal plants use mirrors to make steam for a turbine (less common).

Characteristics of wind power

• Since $P = \frac{1}{2}\rho A v^3 C_p$, a site with 10% higher mean wind speed gives about 33% more energy, so site selection is critical.
• Output is variable and must be backed by storage, hydro or other plants.

Wind energy is converted to electricity by a wind turbine driving a generator. The two basic options are classified by how the turbine speed relates to the grid frequency.

Two basic options

1. Constant (fixed) speed system

• Turbine runs at nearly constant speed set by the grid frequency.
• Uses a squirrel-cage induction generator connected directly to the grid through a gearbox.
• Simple, robust, cheap.
• Cannot follow the optimum tip-speed ratio, so efficiency is lower at varying wind speeds; draws reactive power (capacitor banks needed); mechanical stress from gusts.

2. Variable speed system

• Rotor speed changes with wind speed to keep the optimum tip-speed ratio and maximum $C_p$.
• Uses a doubly fed induction generator (DFIG) with a partial converter, or a synchronous / permanent magnet generator with a full power converter (AC–DC–AC).
• Captures about 5–15% more energy, lower mechanical stress, controllable reactive power.
• Higher cost and complexity due to power electronics.

Fixed speed:
 Rotor -> Gearbox -> SCIG ------------------> Grid
                       |
                    Capacitors
Variable speed:
 Rotor -> (Gearbox) -> Generator -> AC/DC/AC -> Grid

(Some textbooks also state the two options as grid-connected versus stand-alone (with battery) systems.)

Characteristics of wind power

Power curve

 P |          ___________
   |         /           |
   |        /            |
   |      _/             |
   |_____/               |____
   +-----+-----+---------+----> v
       cut-in rated    cut-out

Wind power is clean and has low running cost, but its variability needs backup or storage.

Power varies as cube of wind velocity

Consider air of density $\rho$ moving at speed $v$ through the swept area $A$ of the rotor.

Mass of air passing per second:

$$\dot{m} = \rho A v$$

Kinetic energy carried per second (power):

$$P = \frac{1}{2}\dot{m}v^2 = \frac{1}{2}(\rho A v)v^2 = \frac{1}{2}\rho A v^3$$

The turbine extracts a fraction $C_p$ (power coefficient):

$$P_{out} = \frac{1}{2}\rho A v^3 C_p \;\Rightarrow\; P_{out} \propto v^3$$

So doubling wind speed gives $2^3 = 8$ times the power, and 10% higher speed gives $1.1^3 = 1.33$ times the power.

Other factors that guide the output of a wind project

1. Swept area / blade length – $A = \pi r^2$, so power $\propto r^2$.
2. Air density – lower at high altitude and high temperature; at about 3000 m density is roughly 25–30% less than at sea level.
3. Power coefficient – blade design, tip-speed ratio, pitch control; limited by Betz limit (0.593).
4. Hub height – wind speed increases with height above ground (wind shear).
5. Wind speed distribution – mean speed, variability and duration (Weibull distribution), turbulence.
6. Cut-in, rated and cut-out speeds of the turbine.
7. Mechanical and electrical efficiency – gearbox, generator, converter, transformer losses.
8. Availability and wake losses – downtime for maintenance; turbines in a wind farm shading each other.
9. Grid availability and curtailment.

A SWOT analysis lists the internal strengths and weaknesses and external opportunities and threats of installing wind turbines for electricity in Nepal.

Strengths

• Clean, renewable, no fuel cost, no emissions in operation.
• Good wind sites exist in some valleys and passes (Kagbeni and the Kali Gandaki valley in Mustang, Khumbu, parts of Palpa); SWERA (AEPC, 2008) estimated about 3,000 MW potential.
• Short construction time and modular sizes from small to MW scale.
• Wind is often stronger in the dry season, when run-of-river hydro output is lowest.

Weaknesses

• Wind is highly site-specific and variable in mountain terrain; turbulent valley winds.
• Lack of long-term, reliable wind data and resource maps at hub height.
• Low air density at high altitude reduces output.
• High capital cost per kW for small turbines; spare parts and technicians not available locally.
• Past failures (e.g. Kagbeni 20 kW turbine, 1989) due to poor maintenance.

Opportunities

• Wind–solar–hydro hybrid systems and mini-grids for remote villages not reached by the grid.
• Complements hydro to reduce dry-season deficit and import of power.
• Supply for telecom towers, trekking lodges and tourism.
• Climate finance, carbon credits and donor support; falling global turbine prices.
• Government and AEPC policies favouring renewable energy.

Threats

• Competition from cheaper, established hydropower and solar PV.
• Difficult transport of long blades and towers on narrow mountain roads.
• Extreme weather, icing, landslides and earthquakes damaging structures.
• Weak transmission network near windy sites.
• Policy and tariff uncertainty, low investor confidence.
• Possible impact on birds and visual/tourism concerns in protected areas.

The output of a wind turbine is the power in the wind multiplied by the efficiency (power coefficient):

$$P = \frac{1}{2}\rho A v^3 \eta$$

Given

• $\rho = 1.225$ kg/m³
• $r = 12$ m
• $v = 18$ m/s
• $\eta = 0.45$

Step 1: Swept area

$$A = \pi r^2 = \pi \times 12^2 = 452.39\ \text{m}^2$$

Step 2: Power available in the wind

$$\begin{aligned} P_w &= \frac{1}{2}\rho A v^3 \\ &= 0.5 \times 1.225 \times 452.39 \times 18^3 \\ &= 0.5 \times 1.225 \times 452.39 \times 5832 \\ &= 1{,}615{,}980\ \text{W} \approx 1.616\ \text{MW} \end{aligned}$$

Step 3: Output power

$$P = \eta P_w = 0.45 \times 1{,}615{,}980 = 727{,}191\ \text{W}$$

Answer: Output power $\approx 727.2$ kW (about 0.727 MW).

Note: 45% is below the Betz limit of 59.3%, so it is a realistic value.

Turbine output power:

$$P = \frac{1}{2}\rho A v^3 \eta, \qquad A = \pi r^2$$

Given

$\rho = 1.225$ kg/m³, $r = 6.5$ m, $\eta = 0.35$, $v_1 = 9$ m/s, $v_2 = 18$ m/s.

Swept area

$$A = \pi \times 6.5^2 = 132.73\ \text{m}^2$$

At $v_1 = 9$ m/s

$$\begin{aligned} P_{w1} &= 0.5 \times 1.225 \times 132.73 \times 9^3 \\ &= 0.5 \times 1.225 \times 132.73 \times 729 = 59{,}267\ \text{W} \\ P_1 &= 0.35 \times 59{,}267 = 20{,}743\ \text{W} \end{aligned}$$

At $v_2 = 18$ m/s

$$\begin{aligned} P_{w2} &= 0.5 \times 1.225 \times 132.73 \times 18^3 \\ &= 0.5 \times 1.225 \times 132.73 \times 5832 = 474{,}133\ \text{W} \\ P_2 &= 0.35 \times 474{,}133 = 165{,}947\ \text{W} \end{aligned}$$

Check

$$\frac{P_2}{P_1} = \left(\frac{18}{9}\right)^3 = 8, \qquad 8 \times 20.743 = 165.95\ \text{kW}$$

Answer: Output $\approx 20.74$ kW at 9 m/s and $\approx 165.95$ kW at 18 m/s. Doubling the wind speed gives 8 times the power because $P \propto v^3$.

Turbine output power:

$$P = \frac{1}{2}\rho A v^3 \eta, \qquad A = \pi r^2$$

Given

$\rho = 1.225$ kg/m³, $r = 6$ m, $\eta = 0.40$, $v_1 = 8$ m/s, $v_2 = 16$ m/s.

Swept area

$$A = \pi \times 6^2 = 113.10\ \text{m}^2$$

At $v_1 = 8$ m/s

$$\begin{aligned} P_{w1} &= 0.5 \times 1.225 \times 113.10 \times 8^3 \\ &= 0.5 \times 1.225 \times 113.10 \times 512 = 35{,}467\ \text{W} \\ P_1 &= 0.40 \times 35{,}467 = 14{,}187\ \text{W} \end{aligned}$$

At $v_2 = 16$ m/s

$$\begin{aligned} P_{w2} &= 0.5 \times 1.225 \times 113.10 \times 16^3 \\ &= 0.5 \times 1.225 \times 113.10 \times 4096 = 283{,}739\ \text{W} \\ P_2 &= 0.40 \times 283{,}739 = 113{,}495\ \text{W} \end{aligned}$$

Check

$$\frac{P_2}{P_1} = \left(\frac{16}{8}\right)^3 = 8, \qquad 8 \times 14.187 = 113.50\ \text{kW}$$

Answer: Output $\approx 14.19$ kW at 8 m/s and $\approx 113.50$ kW at 16 m/s; doubling the wind speed raises the power 8 times.

Electricity generation from a wind turbine

The moving air turns the rotor blades; the rotor converts the kinetic energy of wind into mechanical torque on a shaft. A gearbox steps up the speed, and the generator converts it to electricity, which is fed to the grid through a converter and transformer (or stored in batteries for stand-alone systems).

 Wind -> Blades/hub -> Low-speed shaft -> Gearbox
                                           |
 Grid <- Transformer <- Converter <- Generator
 (yaw drive, pitch control and brake on the nacelle)

Merits of wind energy

• Renewable, free fuel, no emissions during operation.
• Low running and maintenance cost.
• Modular; quick installation; land below can still be used for farming.
• Suitable for remote areas and hybrid systems with solar and hydro.

Power output

$$P = \frac{1}{2}\rho A v^3 C_p, \qquad A = \pi r^2$$

Given: $r = 5.5$ m (blade length), $v = 25$ m/s, $\rho = 1.23$ kg/m³, $C_p = 0.4$.

$$\begin{aligned} A &= \pi \times 5.5^2 = 95.03\ \text{m}^2 \\ P_w &= 0.5 \times 1.23 \times 95.03 \times 25^3 \\ &= 0.5 \times 1.23 \times 95.03 \times 15625 = 913{,}209\ \text{W} \\ P &= 0.4 \times 913{,}209 = 365{,}284\ \text{W} \end{aligned}$$

Answer: Power output $\approx 365.3$ kW.

Note: 25 m/s is near the usual cut-out speed of large turbines, so in practice the output would be limited to the rated value by pitch control.

Advantages of wind power

• Renewable and clean; no fuel cost and no CO₂ emission in operation.
• Low operating cost; quick to install; modular.
• Land around turbines can be used for farming.
• Good for remote and hybrid (wind–solar–hydro) systems.

Disadvantages of wind power

• Output is variable and unpredictable ($P \propto v^3$); needs storage or backup.
• Suitable sites are limited; high capital cost per kW for small units.
• Noise, visual impact, danger to birds.
• Low air density at high altitude reduces power.

Power output

$$P = \frac{1}{2}\rho A v^3 C_p, \qquad A = \pi r^2$$

Given: $r = 1.25$ m, $v = 16$ m/s, $\rho = 1.23$ kg/m³, $C_p = 0.3$.

$$\begin{aligned} A &= \pi \times 1.25^2 = 4.909\ \text{m}^2 \\ P_w &= 0.5 \times 1.23 \times 4.909 \times 16^3 \\ &= 0.5 \times 1.23 \times 4.909 \times 4096 = 12{,}365\ \text{W} \\ P &= 0.3 \times 12{,}365 = 3{,}710\ \text{W} \end{aligned}$$

Answer: Electric power output $\approx 3.71$ kW.

Advantages of wind power

• Renewable, clean, free fuel; no emissions during operation.
• Low running cost; short construction time; modular sizes.
• Land below can be used for agriculture.
• Useful in remote areas and in hybrid systems.

Disadvantages of wind power

• Intermittent and variable output; needs storage or backup supply.
• Good sites are limited and often far from load.
• High initial cost; noise, visual impact and bird deaths.
• Output falls at high altitude because of lower air density.

Power output

$$P = \frac{1}{2}\rho A v^3 C_p, \qquad A = \pi r^2$$

Given: $r = 1.5$ m, $v = 14$ m/s, $\rho = 1.23$ kg/m³, $C_p = 0.35$.

$$\begin{aligned} A &= \pi \times 1.5^2 = 7.069\ \text{m}^2 \\ P_w &= 0.5 \times 1.23 \times 7.069 \times 14^3 \\ &= 0.5 \times 1.23 \times 7.069 \times 2744 = 11{,}929\ \text{W} \\ P &= 0.35 \times 11{,}929 = 4{,}175\ \text{W} \end{aligned}$$

Answer: Electric power output $\approx 4.18$ kW.

In a tidal barrage, the basin fills at high tide and the trapped water is released through turbines when the sea falls. The potential energy of water of area $A$ and tidal range $h$ (centre of mass at $h/2$) is:

$$E = \rho g A h \cdot \frac{h}{2} = \frac{1}{2}\rho g A h^2$$

Given

• $A = 3\ \text{km} \times 5.5\ \text{km} = 3000 \times 5500 = 1.65\times 10^7\ \text{m}^2$
• $h = 10$ m, $\rho = 1023$ kg/m³, $g = 9.81$ m/s²
• 2 tides per day, $\eta = 0.60$

Step 1: Energy per tide

$$\begin{aligned} E &= 0.5 \times 1023 \times 9.81 \times 1.65\times 10^7 \times 10^2 \\ &= 8.279 \times 10^{12}\ \text{J} \\ &= \frac{8.279\times 10^{12}}{3.6\times 10^9} = 2299.8\ \text{MWh} \end{aligned}$$

Step 2: Energy per day available

$$E_{day} = 2 \times 8.279\times 10^{12} = 1.656\times 10^{13}\ \text{J}$$

Step 3: Average electrical power

$$\begin{aligned} P &= \frac{\eta E_{day}}{86400} = \frac{0.6 \times 1.656\times 10^{13}}{86400} \\ &= 1.150\times 10^{8}\ \text{W} = 115.0\ \text{MW} \end{aligned}$$

Annual energy (for reference)

$$E_{year} = 115.0\ \text{MW} \times 8760\ \text{h} \approx 1007\ \text{GWh}$$

Answer: Average power output capacity $\approx 115$ MW (energy per tide $\approx 2300$ MWh before losses).

Assumption: one emptying of the basin per tide (single-effect ebb generation), averaged over 24 hours.

Generation of electric power from a tidal project

Tides are the periodic rise and fall of sea level caused by the moon and sun's gravity; most coasts have two high and two low tides a day. A tidal power plant uses the head between the sea and a basin.

        SEA                BARRAGE            BASIN
  high tide  ~~~~~      +-----------+
                        | sluice    |     ~~~~~ stored
  low tide   ~~~~       | turbine   |           water
                        +-----------+
   <--- flow through turbine on ebb (basin -> sea)

1. A barrage (dam) is built across a bay or estuary, with sluice gates and low-head bulb (Kaplan type) turbines.
2. Filling: at rising tide the gates open and the basin fills.
3. Holding: at high tide the gates close and water is held.
4. Generation (ebb): when the sea falls, a head forms; water flows from basin to sea through the turbines, driving generators.
5. In double-effect schemes, power is also generated on the flood tide (sea to basin).

Examples: La Rance (France, 240 MW), Sihwa (South Korea, 254 MW).

Power estimation

Energy of water in the basin per tide (centre of gravity at $h/2$):

$$E = \frac{1}{2}\rho g A h^2$$

Given: $A = 3000 \times 3000 = 9\times 10^6$ m², $h = 10$ m, $\rho = 1025$ kg/m³, $g = 9.81$ m/s², 2 tides per day, $\eta = 0.30$.

$$\begin{aligned} E &= 0.5 \times 1025 \times 9.81 \times 9\times 10^6 \times 10^2 \\ &= 4.525\times 10^{12}\ \text{J per tide} \\ &= 1256.9\ \text{MWh per tide} \\ E_{day} &= 2 \times 4.525\times 10^{12} = 9.050\times 10^{12}\ \text{J} \\ P_{avg} &= \frac{\eta E_{day}}{86400} = \frac{0.30 \times 9.050\times 10^{12}}{86400} \\ &= 3.142\times 10^{7}\ \text{W} \end{aligned}$$

Answer: Average power output $\approx 31.4$ MW (about 275 GWh per year).

Assumption: one basin emptying per high tide, averaged over 24 hours.

Geothermal energy is heat from the earth's interior. In Nepal it appears as hot springs (tatopani), mostly along the Main Central Thrust and in the Himalayan region. They are of low to medium temperature (surface water typically below about 75 °C).

Locations in Nepal

Around 30 or more hot-spring areas are known, for example:

• Tatopani (Myagdi) and Singa Tatopani (Kali Gandaki valley)
• Tatopani (Sindhupalchok) and Chilime (Rasuwa)
• Jomsom area (Mustang), Darchula, Bajhang, Jumla
• Sribagar (Darchula) and Dhuseni (Bajura) areas

Appropriate uses

Because temperatures are low, direct use is more suitable than power generation:

1. Bathing, health and tourism – hot pools at Tatopani (Myagdi) attract tourists.
2. Space heating of houses, lodges and schools in cold hill areas.
3. Greenhouses – off-season vegetables and nurseries.
4. Drying of crops, herbs, fruits and timber.
5. Fish farming and hot water supply.
6. Small binary cycle power units in future, if deeper drilling finds hotter water.

Types of geothermal power plants

1. Dry steam plant – steam from the well goes straight to the turbine. Needs very hot dry steam (above 235 °C).

 Prod. well -> steam -> Turbine -> Gen
                          |
                      Condenser -> Injection well

2. Flash steam plant – high-pressure hot water (above about 180 °C) is flashed in a separator at low pressure; the steam drives the turbine and the brine is reinjected.

 Hot water -> Flash tank -> steam -> Turbine -> Gen
                  |
               brine -> Injection well

3. Binary cycle plant – moderate-temperature water (about 100–180 °C) heats a low-boiling working fluid (isobutane, pentane) through a heat exchanger; its vapour drives the turbine (Organic Rankine Cycle). Geothermal water stays in a closed loop.

 Hot water -> Heat exchanger -> back to well
                  |
         working fluid vapour -> Turbine -> Gen
                  ^                  |
                  +---- Condenser <--+

For Nepal's low-temperature resources, only the binary cycle type could be considered for electricity.

A geothermal power plant produces electricity from the heat of hot water or steam taken from underground reservoirs through wells. It works on a steam (Rankine) cycle like a thermal plant, but the boiler is replaced by the earth.

Operation (flash steam plant, most common)

 Prod. well -> Separator -> Steam -> Turbine -> Gen
                  |                    |
                Brine              Condenser <- Cooling
                  |                    |        tower
                  +-> Injection well <-+

1. Production well brings hot, pressurised water (about 180–300 °C) to the surface.
2. In the separator/flash tank the pressure drops, part of the water flashes into steam.
3. The steam drives the turbine, which turns the generator.
4. Exhaust steam is condensed in a condenser, cooled by a cooling tower.
5. Condensate and separated brine are reinjected into the reservoir to maintain pressure and avoid pollution.

Other types

• Dry steam: steam directly from the well to the turbine (e.g. The Geysers, USA).
• Binary cycle: moderate-temperature water heats an organic fluid (isobutane) in a heat exchanger; its vapour drives the turbine.

Geothermal plants run continuously as base-load plants with high capacity factor (about 90%).

A geothermal power plant uses heat from the earth (steam or hot water from wells), while a conventional thermal plant burns coal, oil or gas in a boiler. Both drive steam turbines.

Comparison

Remarks

Geothermal is cleaner and cheaper to run but is site-limited and has lower efficiency; thermal plants are flexible in location and size but costly to run and polluting.

A combined cycle gas turbine (CCGT) plant combines a gas turbine (Brayton cycle) with a steam turbine (Rankine cycle). The hot exhaust of the gas turbine (about 450–600 °C) produces steam in a heat recovery steam generator (HRSG), which drives a steam turbine.

 Air -> Compressor -> Combustor -> Gas turbine -> G1
                        ^ fuel         |
                                    exhaust
                                       |
                 Steam turbine <-- HRSG --> stack
                   |      |
                  G2   Condenser

Operational characteristics

1. High efficiency – about 50–60% (gas turbine alone 30–40%), the highest of all thermal plants.
2. Quick start – gas turbine starts in 10–30 minutes; full combined cycle in about 1–3 hours. Suitable for intermediate and peak load.
3. Good load following – can ramp fast; supports variable renewables.
4. Part-load behaviour – efficiency falls at part load; usually operated above about 40–50% load; several smaller units improve flexibility.
5. Fuel – natural gas (cleanest) or light oil; fuel cost is the main running cost.
6. Ambient sensitivity – output and efficiency drop at high air temperature and high altitude (lower air density).
7. Low emissions – less CO₂ per kWh than coal; low SO₂ and particulates.
8. Low capital cost and short construction time (2–3 years); compact; little cooling water (only steam part).
9. Phased operation – gas turbine can run in simple cycle while the steam part is built or maintained.

A combined cycle gas turbine (CCGT) plant uses a gas turbine and recovers its exhaust heat to drive a steam turbine. A hydropower plant uses the potential energy of water through a water turbine.

Comparison

Remarks

CCGT is cheap and quick to build but depends on imported fuel; hydro has high initial cost but very low running cost and long life. For Nepal, hydro is clearly preferred, while CCGT suits countries with gas supply as flexible mid-merit plants.

Energy storage stores surplus energy when generation is more than demand and returns it when demand is high. It supports variable renewables (solar, wind), peak shaving, frequency regulation and reliability.

Mechanical energy storage

1. Pumped storage hydropower (PSH)

• Two reservoirs at different levels. Off-peak power pumps water up; at peak, water flows down through the turbine.
• Energy stored: $E = \rho g V H$.
• Round-trip efficiency about 70–80%; large capacity (hundreds of MW, many hours); long life.
• Needs suitable topography; high capital cost. Nepal has good sites in the hills.

 Upper reservoir
      |   ^
 gen  v   | pump  (reversible pump-turbine)
 Lower reservoir

2. Compressed air energy storage (CAES)

• Off-peak power compresses air into underground caverns; at peak, air is heated and expanded in a turbine.
• Efficiency about 40–70%; large scale; needs suitable caverns.

3. Flywheel energy storage

• Energy stored in a rotating mass: $E = \frac{1}{2}J\omega^2$.
• Fast response (milliseconds), high power, many cycles; short duration (seconds to minutes); self-discharge by friction.
• Used for UPS, power quality and frequency regulation.

4. Gravity storage – lifting heavy blocks and lowering them to generate power (emerging).

Thermal energy storage

1. Sensible heat storage – heat stored by raising temperature of water, rock, sand, concrete or molten salt: $Q = mc\Delta T$. Example: hot water tanks, molten salt in concentrated solar power plants (allows generation after sunset).

2. Latent heat storage – uses phase change materials (PCM) such as paraffin wax or salt hydrates; stores large heat at nearly constant temperature: $Q = mL$.

3. Thermochemical storage – heat stored in reversible chemical reactions; highest energy density, still developing.

4. Cold storage – ice or chilled water made at night for daytime air conditioning, shifting load to off-peak.

Comparison

A thermal power plant converts the chemical energy of fuel into electricity through steam, while a hydropower plant converts the potential energy of water. Their ability to follow load and support the grid is very different.

Daily load curve and plant placement

 Load
  |            peak (hydro storage, gas)
  |        ___/^^^\___        /^^\
  |  _____/            \_____/    \__
  | |   intermediate (hydro / CCGT)  |
  | |--------------------------------|
  | |   base load (thermal, RoR hydro, nuclear)
  +----------------------------------------> hour
  0          6         12        18     24

• Thermal: best at constant output; takes the base load portion of the curve.
• Hydro: run-of-river plants take base load during high flow; storage and peaking RoR plants take the peak.

Comparison

Remarks

• In a mixed system, thermal plants supply base load efficiently and hydro plants follow peaks and regulate frequency.
• In Nepal, the load curve has a sharp evening peak; storage (Kulekhani) and peaking run-of-river plants meet it, while thermal plants are kept only for emergency.

A hydroelectric plant converts the potential energy of stored or flowing water into electricity, while a thermal plant burns fuel to make steam that drives a turbine.

Technical comparison

Economic comparison

Environmental and social comparison

Remarks

Hydro has high initial cost but very low running cost, long life and clean operation; thermal is cheaper and quicker to build but expensive to run and polluting. For Nepal, with large hydro potential and no fossil fuel, hydro is the main option, with thermal only as standby.

Hydropower converts the potential energy of water into electricity using a turbine and synchronous generator. Solar photovoltaic (PV) converts sunlight directly into DC electricity in semiconductor cells, converted to AC by inverters.

Comparison

Nepalese context

Hydropower:

• Nepal's grid is almost fully hydro (over 3,000 MW installed). Most plants are run-of-river; only Kulekhani is a seasonal storage project.
• Output is high in the monsoon (surplus, exported to India) and falls to about a third in the dry season (deficit, imports).
• High sediment in Himalayan rivers causes turbine erosion and maintenance cost.
• Long transmission lines from hills to load centres.

Solar PV:

• Good solar resource: about 4.5–5.5 kWh/m²/day, around 300 sunny days.
• Solar output is highest in the dry season (clear skies), when hydro output is low, so it complements run-of-river hydro.
• Widely used in remote areas (solar home systems, mini-grids, solar irrigation pumps) and in growing grid-tied plants in the Terai.
• Limited by land cost, lack of storage and inverter-based grid issues; net metering for rooftop solar has been introduced.

Conclusion

Hydro gives cheap, controllable bulk power and grid stability; solar PV is quick to install and fills the dry-season and daytime gap. For Nepal, a hydro–solar mix with hydro reservoirs acting as storage is the most reliable approach.

A solar power plant (mostly PV) converts sunlight directly into electricity; a hydropower plant converts the energy of falling water through turbines and synchronous generators.

Comparison

Remarks

• Hydro is more reliable, efficient and grid-friendly; solar is cheaper and quicker to install.
• In Nepal, solar is strongest in the dry season when river flows are low, so both complement each other.

A grid-tied solar PV plant feeds DC power from PV modules into the AC grid through inverters, without batteries. A hydropower plant converts water energy using turbines and synchronous generators.

Comparison

Remarks

• PV is cheap and fast to build but variable; hydro is costly to build but reliable and controllable.
• In Nepal, grid-tied PV complements run-of-river hydro in the dry season; hydro reservoirs can balance solar variability.

A solar PV plant converts sunlight directly to DC electricity, delivered to the grid or loads through inverters. A hydropower plant converts the energy of water through turbines and synchronous generators.

Comparison of operational characteristics

Beauty of combining solar PV and hydro

1. Seasonal complement – in Nepal, river flows are low in the dry season (winter, spring), when skies are clear and solar output is high; in the monsoon, hydro is abundant.
2. Daily complement – solar supplies daytime energy; the hydro reservoir saves water during the day and releases it for the evening peak. The reservoir acts as a large, cheap "battery".
3. Smoothing variability – fast hydro governors balance sudden changes in solar output due to clouds, keeping frequency stable.
4. Grid strength – synchronous hydro machines provide inertia and voltage support that inverter-based PV lacks.
5. Shared infrastructure – floating PV on hydro reservoirs uses the same substation and transmission line; less land needed and less evaporation.
6. Better economics – higher utilisation of transmission lines and lower need for battery storage.

 Power
  |        solar
  |       /^^^^\          hydro raised
  |      /      \        for evening peak
  |-----/--------\-----------/^^^\----
  |  hydro (held back by day)     \__
  +---------------------------------> hour
  0       6     12      18       24

Such a hybrid system gives reliable, clean and low-cost supply.

All three plants use steam turbines, but the heat source differs: thermal plants burn fossil fuel, geothermal plants use heat from the earth, and nuclear plants use fission of uranium.

Comparison

Remarks

• Thermal plants are the most flexible in location and output but polluting and fuel-dependent.
• Geothermal and nuclear are base-load plants with high availability and low fuel cost; geothermal is site-limited, nuclear needs very large investment and strict safety.

Thermal plants burn fuel to make steam for a turbine; wind plants convert the kinetic energy of air through rotor blades and a generator; PV plants convert sunlight directly into electricity in semiconductor cells.

Comparison

Remarks

Thermal plants give reliable base load but are costly to run and polluting; wind and PV are clean with low running cost but intermittent and need storage, backup or hybrid operation.

Thermal power plants burn fossil fuel to raise steam; geothermal plants take steam or hot water from the earth's heat; PV plants convert sunlight directly into DC electricity using semiconductor cells.

Working in brief

Thermal:  Fuel -> Boiler -> Steam -> Turbine -> Gen
Geotherm: Well -> Separator/HX -> Turbine -> Gen
                  (brine reinjected)
PV:       Sun -> PV array -> Inverter -> Grid

Comparison

Remarks

• Thermal: most flexible in location and output, but high fuel cost and pollution.
• Geothermal: ideal clean base load where the resource exists; not dispatchable and site-limited.
• PV: cheapest and simplest to build and maintain, but intermittent; best combined with hydro or storage.

For Nepal, thermal is limited to diesel standby, geothermal is only at low-temperature hot springs (direct use), and PV is growing as a complement to run-of-river hydro.

Thermal plants burn fuel to produce steam for turbines; hydro plants use the potential energy of water; solar PV plants convert sunlight directly into electricity.

Comparison

Remarks

• Thermal suits base load where fuel is cheap; hydro is the most efficient and flexible; PV is cheapest to build but intermittent.
• In Nepal, hydro is the backbone, PV complements it in the dry season, and thermal (diesel) is kept only for emergency.

Both tidal and wind plants convert the kinetic/potential energy of a natural moving fluid into electricity, but tidal power follows the predictable lunar cycle while wind power follows random weather. This makes their operating characteristics very different.

Daily generation curve

• Tidal: Output comes in blocks linked to the tide. A barrage with two tides a day (period about 12 h 25 min) gives 2 to 4 generating periods a day, with zero output near slack water while the head builds up. The pattern shifts by about 50 minutes each day and varies over the spring–neap cycle (about 14.8 days), but it can be predicted years ahead.
• Wind: Output depends on wind speed, $P = \frac{1}{2}\rho A C_p V^3$, so it changes from minute to minute. The daily curve is irregular; at many sites wind is stronger in the afternoon or at night, but this cannot be scheduled.

 P   Tidal (barrage)        P   Wind
 |  __      __      __      |    /\  _/\    /\_
 | |  |    |  |    |  |     |   /  \/   \__/   \
 |_|  |____|  |____|  |__   |__/              \_
 +-------------------> t    +-------------------> t
     0     12h     24h          0     12h     24h

Stability

• Tidal: Uses large, slow bulb or Kaplan turbines with synchronous generators. These have rotating inertia, governors and excitation control, so they support transient and voltage stability. The output change is gradual and known in advance.
• Wind: Modern turbines are variable-speed (DFIG or full converter). The power electronics decouple the rotor inertia from the grid, so the plant gives little natural inertia. Sudden gusts or calm periods cause power swings and frequency/voltage fluctuations. Special controls (synthetic inertia, fault ride-through) are needed.

Grid connection

• Tidal: Few, large plants (hundreds of MW) at coastal estuaries; connected to HV transmission through a step-up substation, like a hydro plant. Because output is predictable, dispatch planning is easy.
• Wind: Many small units (2–15 MW each) collected through a medium-voltage collector network and a pooling substation. Needs grid codes for reactive power, LVRT, harmonics and forecasting; high penetration needs storage or reserve.

Operating range

• Tidal: Generates only when the head across the barrage exceeds a minimum (about 1–2 m); output rises with head. Capacity factor is about 20–30%.
• Wind: Generates between cut-in speed (about 3–4 m/s) and cut-out speed (about 25 m/s), with rated output from about 12–15 m/s. Capacity factor is about 25–40% (higher offshore).

Summary

Nepal's electricity sector in this decade is moving from chronic shortage (load shedding until 2016/17) to surplus in the wet season and export, with hydropower as the backbone. The figures below are approximate, as per recent NEA annual reports and the Ministry of Energy, Water Resources and Irrigation (MoEWRI).

Generation

• Installed capacity has grown from about 1,100 MW (2017) to well over 3,000 MW in recent years, and keeps rising every year.
• Hydropower gives about 95% of the capacity; most of it is run-of-river (ROR/PROR). Solar is growing (a few hundred MW), and there is a little thermal (diesel) kept as standby.
• Independent Power Producers (IPPs) now own more capacity than NEA itself. Big recent projects include Upper Tamakoshi (456 MW), Kulekhani III, Rasuwagadhi, Solu Dudhkoshi, Upper Trishuli 3A and many IPP plants.

Demand and consumption

• Peak demand has crossed about 2,000 MW, and per-capita consumption is around 400 kWh/year (still low compared to South Asia).
• About 99% of households have access to electricity.
• The government promotes electric cooking (induction stoves), electric vehicles and industrial use to absorb the surplus.

Seasonal mismatch

• Because most plants are ROR, Nepal has surplus in the wet season (Jun–Oct) and deficit in the dry season (Dec–Apr), when power is imported from India.
• Storage projects (e.g. Tanahu 140 MW, Budhigandaki, Dudhkoshi storage) are planned to fix this.

Cross-border trade

• Since 2021 Nepal exports power to India through the Indian Energy Exchange and bilateral deals, and exports to Bangladesh through the Indian grid started in 2024.
• Nepal and India signed a long-term agreement (2024) targeting export of 10,000 MW within ten years.

Transmission and policy

• The 400 kV backbone (Dhalkebar–Muzaffarpur, Hetauda–Dhalkebar–Inaruwa, Butwal–Gorakhpur under construction) is being built to evacuate and export power.
• The Energy Development Roadmap and Action Plan (2080 BS) targets about 28,500 MW by 2035, with about 15,000 MW for export.

Challenges

Transmission delays, spill of energy in the wet season, low domestic demand, high PPA cost, dry-season import dependence, and climate risks (floods, GLOFs, sediment).

Nepal's demand for electricity is growing at about 8–10% per year, while supply has grown even faster since 2018, so the country has moved from load shedding to seasonal surplus. Figures are approximate, as per recent NEA annual reports.

Current supply

• Installed capacity: more than 3,000 MW, about 95% hydropower, mostly run-of-river.
• Sources of energy in the national grid: NEA's own plants, IPP plants (now the largest share), and imports from India in the dry season.
• Solar farms (tens of MW each) and a small diesel standby add to this.

Current demand

• Peak demand: about 2,000–2,300 MW (evening peak, mostly in winter).
• Energy consumption: around 12,000–13,000 GWh/year, per-capita about 400 kWh/year.
• Consumers: domestic users are the largest group by number; industry and commerce use the largest share of energy.
• Electricity access: about 99% of households.

Demand–supply balance

The main reason is the high share of ROR plants whose output falls to about one third in the dry season.

Trend in the near future

1. Supply: Several thousand MW of projects are under construction (mostly IPP RoR plus storage like Tanahu). The roadmap targets about 28,500 MW by 2035.
2. Demand: Expected to rise faster due to electric cooking, EVs (EV imports are rising fast), industrial corridors, data centres and electric irrigation. The NEA forecast shows peak demand rising to several thousand MW by 2030.
3. Export: Long-term Nepal–India agreement (10,000 MW in ten years) and trade with Bangladesh will make Nepal a net exporter on an annual basis.
4. Grid: 400 kV and 220 kV corridors, smart meters and better distribution will reduce losses (system loss has already fallen from about 25% to about 13%).
5. Storage and mix: More storage hydro, pumped storage and solar will be needed to fix the dry-season deficit.

Overall, Nepal is likely to stay surplus in the wet season, close the dry-season gap with storage plants, and become a regional power exporter.

In ten years Nepal has turned from a power-deficit country with long load shedding into a country with seasonal surplus and export. The figures below are approximate, as per NEA annual reports and the Energy Development Roadmap (2080 BS).

Before five years (around 2020)

• Installed capacity about 1,300–1,400 MW, almost all hydro.
• Peak demand about 1,400–1,500 MW; dry-season import from India was large.
• Load shedding had ended (2016/17), mostly through better management and imports.
• Per-capita consumption about 250–270 kWh/year; system loss about 15%.
• No regular export; surplus power was spilled in the wet season.

Today

• Installed capacity above 3,000 MW (Upper Tamakoshi 456 MW and many IPP projects added).
• Peak demand above 2,000 MW; per-capita consumption about 400 kWh/year.
• Regular export to India in the wet season and first export to Bangladesh; import still needed in winter.
• System loss reduced to about 13%; access about 99%.

After five years (expected)

• Installed capacity may reach about 8,000–10,000 MW as projects under construction finish.
• Demand will rise with electric cooking, EVs, industries and data centres; peak demand may approach 3,500–4,000 MW.
• Export will grow under the 10,000 MW Nepal–India long-term agreement, with 400 kV cross-border lines (Butwal–Gorakhpur, Dhalkebar–Sitamarhi, etc.).
• More storage (Tanahu, Dudhkoshi) and solar will reduce the dry-season import.
• Smart meters, grid automation and a competitive market (Electricity Regulatory Commission, power trading) will develop.

Comparison

The main risks are transmission delays, low domestic demand growth and climate-related damage to plants.

Electricity is still a small share (about 5–8%) of Nepal's total final energy consumption, although its share is rising quickly. Figures are approximate, as per the Water and Energy Commission Secretariat (WECS) energy synopsis and Economic Survey.

Energy mix of Nepal (approx.)

Situation

• Residential sector uses most of the energy, mainly firewood for cooking and heating in rural areas; electricity is mainly used for lighting and appliances.
• Transport depends fully on imported petroleum; electric vehicles are only starting.
• Industry uses coal and diesel along with grid electricity.
• Nepal has large hydropower potential (economic potential about 42,000 MW), but per-capita electricity use is only about 400 kWh/year.

Why the share must rise

Nepal imports all its petroleum, which creates a large trade deficit, while it now spills surplus hydropower in the wet season. Replacing LPG with induction cooking, diesel vehicles with EVs and coal boilers with electric ones would raise the share of electricity, save foreign currency and cut emissions. The government targets a much higher share of electricity in total energy use by 2030.

In the last decade renewable energy has become the fastest-growing and cheapest source of new electricity in the world. Figures below are approximate, as per IRENA, IEA and Ember reports.

Growth of capacity

• Global renewable capacity has more than doubled in ten years, from about 1,800 GW (2014) to over 4,000 GW (2024).
• Yearly additions hit new records almost every year: about 470 GW in 2023 and about 580 GW in 2024. Renewables now make up over 80% of all new power capacity added.
• Solar PV leads the growth (over 2,000 GW installed), followed by wind (over 1,000 GW). Hydropower is the largest single renewable source by energy, about 1,400 GW.

Share in electricity generation

• Renewables supplied about 22% of world electricity in 2014 and about 30–32% by 2023–2024.
• Hydro gives about 14%, wind about 8% and solar about 7% of world electricity.
• Some countries are very high: Norway, Nepal, Paraguay (almost all hydro), Denmark (wind >50%), Germany and the UK (above 40–50%).

Fall in cost

• Cost of solar PV electricity fell by about 85–90% and onshore wind by about 60–70% during the decade.
• New solar and wind are now cheaper than new coal or gas plants in most countries.
• Battery storage cost also fell by about 80–90%, making solar + storage practical.

Main drivers

1. Climate commitments: Paris Agreement (2015), net-zero targets, and the COP28 (2023) pledge to triple renewable capacity by 2030.
2. Energy security after fuel price shocks (2022).
3. Large manufacturing in China (which installs more than half of new solar and wind).
4. Policy support: auctions, feed-in tariffs, carbon pricing.

Challenges

Grid integration of variable output, need for storage and flexible plants, transmission bottlenecks, supply chains of critical minerals, and financing in developing countries.

World electricity generation from renewables has grown very fast in the last decade, led by solar and wind, and this trend is clearly affecting Nepal's power sector as well. Figures are approximate, as per IRENA, IEA and NEA reports.

World trend

• Renewable share in world electricity rose from about 22% (2014) to about 30–32% (2023–24). Over 80% of new capacity each year is renewable (about 470 GW in 2023 and about 580 GW in 2024).
• Solar PV is the fastest growing (over 2,000 GW); wind is over 1,000 GW; hydro (~1,400 GW) remains the largest renewable source by energy.
• Solar cost fell about 85–90% and wind about 60–70%; batteries became cheap enough for grid storage.
• Policies: Paris Agreement, net-zero targets, COP28 pledge to triple renewables by 2030, carbon markets.
• Old coal plants are being retired in Europe and the USA; electric vehicles and heat pumps are increasing demand for clean power.

Is it impacting Nepal? Yes

1. Solar growth: Grid-connected solar farms (Nuwakot, Devighat, Bhairahawa and many IPP plants), rooftop net-metering and solar irrigation pumps have started; NEA has called PPAs for hundreds of MW of solar.
2. Hydro as clean export: India and Bangladesh need renewable power to meet climate targets, so Nepal's hydropower has a market. Nepal now exports in the wet season and has a long-term deal for 10,000 MW with India.
3. Hybrid and storage: Falling battery prices encourage solar–hydro hybrids and battery storage to balance ROR output; pumped storage is being studied.
4. Electrification policy: Following the world trend, Nepal promotes EVs (low customs duty), electric cooking and green hydrogen studies to use clean power at home.
5. Finance: Climate funds, green bonds and carbon credits (e.g. biogas and micro-hydro CDM projects) support renewable projects.
6. Off-grid: Cheap solar home systems and mini-grids (AEPC programmes) have brought power to remote villages.

Limits

Nepal's mix is already about 95% renewable (hydro), so the main effect is diversification and export rather than replacing fossil plants. Wind is still at pilot level.

Hydropower is the main source of electricity in Nepal; its development started in 1911 and has grown fastest after private sector entry and especially after 2016. Nepal's theoretical potential is about 83,000 MW, with about 42,000 MW economically feasible.

Historical phases

1. Early period (1911–1950): Pharping (500 kW, 1911) was the first plant, followed by Sundarijal (640 kW, 1936).
2. Aid-based period (1950–1990): Panauti, Trishuli, Sunkoshi, Gandak, Kulekhani I (60 MW, the only big storage plant) built with foreign aid. Nepal Electricity Authority (NEA) formed in 1985.
3. Opening to private sector (1992–2015): Hydropower Development Policy (1992) and Electricity Act (1992) allowed IPPs. Khimti (60 MW) and Bhotekoshi (45 MW) were the first big IPP plants. Kali Gandaki A (144 MW) and Middle Marsyangdi (70 MW) built by NEA. Supply could not meet demand, so load shedding reached up to 18 hours/day (2008–2016).
4. Rapid growth (2016–present): Load shedding ended in 2016/17. Upper Tamakoshi (456 MW, 2021), Kulekhani III, Rasuwagadhi, Solu Dudhkoshi and many IPP projects added. Capacity rose above 3,000 MW. Export to India began in 2021.

Present status

• About 95% of grid capacity is hydro; most plants are run-of-river or peaking ROR, so there is a wet-season surplus and dry-season deficit.
• IPPs own more than half of installed capacity.
• Micro-hydro (through AEPC) supplies many remote villages.

Institutions and policies

NEA, Department of Electricity Development (DoED), Investment Board Nepal (IBN), Electricity Regulatory Commission (ERC), Hydropower Development Policy 2001, Energy Development Roadmap (target about 28,500 MW by 2035).

Challenges

• Few storage plants; seasonal mismatch.
• Delays in transmission lines and land acquisition.
• High sediment load damaging turbines; GLOF and landslide risks.
• Financing, PPA and market issues; local disputes.

Way forward

Storage and pumped-storage projects, 400 kV transmission and cross-border lines, power trade with India and Bangladesh, and higher domestic consumption (EVs, cooking, industries).

Hydropower brings holistic development because a project does not only produce electricity; it also builds infrastructure, creates income and supports social services in the area where it is built.

How it is possible

1. Infrastructure: Access roads, bridges, transmission and distribution lines, and telecom links built for the project are later used by local people, opening remote areas to markets.
2. Rural electrification: Local villages get reliable electricity for lighting, cooking, small industries, schools and health posts.
3. Employment and skills: Thousands of jobs during construction and permanent jobs in operation; local people gain technical skills.
4. Local economy: Hotels, shops, transport and agriculture grow around the project; new industries (cement, agro-processing) come due to cheap power.
5. Revenue sharing: As per the Electricity Act and royalty rules, a share of royalty goes to the province and local levels; "local share" schemes let affected people buy project shares.
6. Multipurpose use: Storage reservoirs give irrigation, flood control, drinking water, fisheries and tourism (boating, e.g. Kulekhani).
7. Social services under CSR: Projects build schools, health posts, drinking water schemes and scholarships.
8. Environment: Clean energy replaces firewood and diesel, reducing deforestation and indoor pollution.

Thus, with proper benefit sharing and local participation, a hydropower project becomes a centre of economic, social and environmental development of the whole region.

An isolated PV system is sized by first finding the daily energy demand, then adding the losses to get the PV array rating, the battery capacity for the required autonomy, and an inverter that can carry the connected load.

Assumptions: all loads are AC (through inverter); wiring losses are taken as 20% extra energy; peak sun hours in Nepal = 5 h/day; panel derating (temperature, dust) = 0.9.

1. Daily load energy

2. Energy to be supplied by the battery (DC side)

$$\begin{aligned} E_{inv,in} &= \frac{500}{0.95} = 526.3\ \text{Wh} \\ E_{DC} &= 1.2 \times 526.3 = 631.6\ \text{Wh/day} \end{aligned}$$

3. Battery size

$$\begin{aligned} C &= \frac{E_{DC} \times \text{days of autonomy}}{DoD \times \eta_{b} \times V} \\ &= \frac{631.6 \times 3}{0.6 \times 0.9 \times 12} = 292.4\ \text{Ah} \end{aligned}$$

Select 12 V, 300 Ah (e.g. two 12 V, 150 Ah batteries in parallel).

4. PV panel size

Energy the array must give (battery efficiency 90%):

$$\begin{aligned} E_{PV} &= \frac{631.6}{0.9} = 701.8\ \text{Wh/day} \\ P_{PV} &= \frac{701.8}{5 \times 0.9} = 155.9\ \text{W}_p \end{aligned}$$

Select 160 Wp (two 80 Wp, 12 V panels in parallel).

5. Inverter size

Connected load = 155 W. With a 25% margin for surge (fan motor, TV) and future load:

$$P_{inv} = 1.25 \times 155 = 193.75\ \text{W}$$

Select 12 V DC / 230 V AC, 250 VA pure sine wave inverter.

6. Charge controller (extra)

$I = 1.25 \times 160/12 = 16.7$ A, so a 12 V, 20 A charge controller.

Answer: PV array ≈ 156 Wp → 160 Wp; battery ≈ 292 Ah → 12 V, 300 Ah; inverter ≈ 194 W → 250 VA.

The panel is sized from the daily energy demand plus all losses divided by peak sun hours; the inverter is sized from the total connected load with a safety margin.

Assumptions: all loads run on AC through an inverter; inverter efficiency 90%; wiring/other losses 10%; battery efficiency 85%; peak sun hours 5 h/day (typical for Nepal); panel derating factor 0.9; system voltage 24 V (because the daily energy is above 1 kWh).

1. Daily energy demand

2. Energy required from the PV array

$$\begin{aligned} E_{inv,in} &= \frac{1150}{0.9} = 1277.8\ \text{Wh} \\ E_{DC} &= 1.1 \times 1277.8 = 1405.6\ \text{Wh} \\ E_{PV} &= \frac{1405.6}{0.85} = 1653.6\ \text{Wh/day} \end{aligned}$$

3. Solar panel size

$$P_{PV} = \frac{E_{PV}}{\text{PSH} \times \text{derating}} = \frac{1653.6}{5 \times 0.9} = 367.5\ \text{W}_p$$

Select 400 Wp array, e.g. four 100 Wp, 12 V panels (2 in series × 2 in parallel for 24 V).

4. Inverter size

Total connected load = 180 W. With 25% margin for starting surge and future load:

$$P_{inv} = 1.25 \times 180 = 225\ \text{W}$$

Select 24 V DC / 230 V AC, 300 VA pure sine wave inverter.

5. Battery (for completeness)

With 2 days autonomy and DoD 0.6:

$$C = \frac{1405.6 \times 2}{0.6 \times 24} = 195.2\ \text{Ah}$$

so a 24 V, 200 Ah bank (two 12 V, 200 Ah in series).

Answer: Solar panel ≈ 368 Wp → 400 Wp; inverter ≈ 225 W → 300 VA.

A stand-alone solar home system is sized by finding the daily energy demand, adding inverter and battery losses to get the energy the panel must produce, and sizing the battery to supply one day's load within an allowed depth of discharge.

Assumptions: all loads are AC through inverter; one day of autonomy; depth of discharge (DoD) = 0.6; no extra wiring loss beyond the given efficiencies.

1. Daily energy demand

2. Solar panel size

Energy passes through battery and inverter, each 80% efficient:

$$\begin{aligned} E_{PV} &= \frac{695}{0.8 \times 0.8} = 1085.9\ \text{Wh/day} \\ P_{PV} &= \frac{1085.9}{6\ \text{h}} = 181.0\ \text{W}_p \end{aligned}$$

Select 200 Wp (two 100 Wp, 12 V panels in parallel). Panel current $\approx 181/12 = 15.1$ A, so use a 12 V, 20 A charge controller.

3. Battery size

Energy the battery must deliver to the inverter:

$$\begin{aligned} E_{b} &= \frac{695}{0.8} = 868.75\ \text{Wh} \\ \text{Ah (usable)} &= \frac{868.75}{12} = 72.4\ \text{Ah} \\ C &= \frac{72.4}{DoD} = \frac{72.4}{0.6} = 120.7\ \text{Ah} \end{aligned}$$

Select 12 V, 150 Ah deep-cycle (tubular lead-acid) battery, which also gives some margin for cloudy days.

4. Inverter (extra)

Connected load 175 W × 1.25 ≈ 219 W → 12 V, 250–300 VA inverter.

Answer: Solar panel ≈ 181 Wp → 200 Wp; battery ≈ 121 Ah → 12 V, 150 Ah.

The solar panel is sized from the daily energy demand divided by the combined efficiency and the sunshine hours; the battery is sized to supply one day's load within an allowed depth of discharge.

Assumptions: all loads are AC through inverter; one day of autonomy; depth of discharge (DoD) = 0.6.

1. Daily energy demand

2. Solar panel size

$$\begin{aligned} E_{PV} &= \frac{855}{\eta_{inv}\,\eta_{b}} = \frac{855}{0.8 \times 0.8} = 1335.9\ \text{Wh/day} \\ P_{PV} &= \frac{1335.9}{7\ \text{h}} = 190.8\ \text{W}_p \end{aligned}$$

Select 200 Wp (two 100 Wp, 12 V panels in parallel). Array current $\approx 190.8/12 = 15.9$ A, so a 12 V, 20 A charge controller.

3. Battery size

$$\begin{aligned} E_{b} &= \frac{855}{0.8} = 1068.75\ \text{Wh} \\ \text{Ah (usable)} &= \frac{1068.75}{12} = 89.1\ \text{Ah} \\ C &= \frac{89.1}{0.6} = 148.4\ \text{Ah} \end{aligned}$$

Select 12 V, 150 Ah deep-cycle battery.

4. Inverter (extra)

Connected load 215 W × 1.25 ≈ 269 W → 12 V, 300 VA inverter.

Answer: Solar panel ≈ 191 Wp → 200 Wp; battery ≈ 148 Ah → 12 V, 150 Ah.