Exam practice

Almost every source of energy we use comes from the Sun. Its heat drives the water cycle, which fills the rivers used for hydroelectricity, and heats the air unevenly, which makes the wind. Its light is stored by plants in photosynthesis, which gives our food, firewood and biomass, and, over millions of years, fossil fuels. So in the end nearly all our energy is solar energy.

Non-renewable sources (coal, oil, gas) will run out, and burning them pollutes the air and causes global warming. Renewable sources (sun, water, wind, biomass) don’t run out and pollute far less. For Nepal, which imports all its petroleum, they also save money and make the country less dependent on others.

Nepal gets strong sunshine on about 300 days a year, so solar energy is plentiful and free. Vehicles running on solar electricity give no smoke, which would clean the polluted air of Kathmandu and other cities, and they need no imported petrol or diesel, so they reduce the money Nepal spends abroad. (In practice, most “solar” vehicles are electric vehicles whose batteries are charged from solar panels or from hydroelectricity.)

Nepal has many rivers fed by Himalayan snow and monsoon rain, falling steeply from great heights: ideal for hydropower (its potential is often put at over 40,000 MW that is economically usable). Hydroelectricity is renewable, makes no smoke, is cheap in the long run, and can replace imported petroleum, reducing the trade deficit. Nepal can even sell its extra electricity to neighboring countries.

Most village families keep cattle, so dung, the raw material for biogas, is available free every day. Biogas cooks food and lights lamps without smoke, so it protects health, and it replaces firewood, which saves forests and the time spent collecting wood. The spent slurry is a good fertilizer for the fields. And the Government gives help to build the plants.

Any object or substance that can give us a useful form of energy is a source of energy. Examples: the Sun, flowing water, wind, firewood, dung cake, biogas, coal, petrol, kerosene and natural gas.

Thermonuclear fusion is the joining of light nuclei (such as hydrogen) to form a heavier nucleus (such as helium) at very high temperature and pressure, releasing a huge amount of energy. In the Sun, four hydrogen nuclei fuse to form one helium nucleus:
$$4\,{}^{1}_{1}\text{H} \longrightarrow {}^{4}_{2}\text{He} + 2\,{}^{0}_{+1}\text{e} + \text{energy}$$

1. A huge amount of hydrogen, the fuel, to keep the fusion going.
2. A very high temperature (about 15 million K), which strips electrons from the atoms, leaving free protons moving very fast.
3. A very high pressure, which crowds the protons together so they collide often enough to fuse, in spite of their repulsion.

• i.Hydroelectricity: the Sun’s heat evaporates water from seas, lakes and rivers and melts snow; the vapor forms clouds and falls as rain and snow, high in the hills; that water flows down rivers and turns turbines. Without the Sun, there would be no water cycle and no river.
• ii.Fossil fuel: plants stored the Sun’s energy as chemical energy by photosynthesis; animals ate the plants. Their remains, buried for millions of years, turned into coal, petroleum and natural gas. So fossil fuels are stored solar energy.
• iii.Wind energy: the Sun heats the ground and the air above it unevenly. Warm air becomes lighter and rises, and cooler air moves in to replace it. This moving air is wind, so wind energy is made by the Sun.

The Sun’s core is at about 15 million K and under enormous pressure. There, hydrogen atoms lose their electrons, leaving protons, which move so fast and are so crowded that they fuse, in spite of repelling one another. This thermonuclear fusion happens in three steps:

1. Two protons fuse into a deuterium nucleus and a positron: ¹₁H + ¹₁H → ²₁H + ⁰₊₁e + energy.
2. Deuterium fuses with another proton to make helium-3 and a gamma ray: ²₁H + ¹₁H → ³₂He + γ.
3. Two helium-3 nuclei fuse into helium-4, freeing two protons: ³₂He + ³₂He → ⁴₂He + 2 ¹₁H + energy.

Overall, four hydrogen nuclei become one helium nucleus: 4 ¹₁H → ⁴₂He + 2 ⁰₊₁e + energy. The helium is slightly lighter than the four hydrogen nuclei; the lost mass becomes the energy that the Sun gives out as heat and light.

Solar water heater (diagram as in Figure 9.6): evacuated glass tubes slope up into an insulated tank. Sunlight heats the water in the tubes; hot water is less dense and rises into the tank, while colder, denser water sinks into the tubes to be heated in turn (convection). Hot water collects at the top of the tank, and the insulation keeps it hot at night.
Solar dryer (diagram as in Figure 9.4): an insulated box, black inside, with a sloping glass top. Sunlight passes through the glass and is absorbed by the black surface, heating the air inside. Hot air rises, takes up moisture from the food and leaves through a hole high at the back, while cool air enters through a hole low at the front. The food dries quickly and stays clean.
(Other good answers: the solar cooker, Figure 9.7, and the solar street lamp, Figure 9.3.)

A source of energy that can be used instead of the traditional sources (firewood and petroleum) is an alternative source of energy. Examples: biogas, solar energy, wind energy, briquettes, micro hydropower.

(A) Solar dryer: sunlight through the glass is absorbed by the black inner surface; the heated air rises and carries the food’s moisture out through a hole at the top, while cool air enters below.
(B) Solar cooker (dish type): the concave mirror reflects the parallel rays of sunlight to one spot, its focus, where the black pot absorbs the concentrated heat.
(C) Solar street lamp: by day the panel’s photovoltaic cells turn sunlight into electricity, which the controller stores in a battery; at night the controller connects the battery to the LED lamp.

A biofuel is a fuel made from living things or their wastes: plants, crop residue and animal dung. Its energy came from the Sun through photosynthesis, and it is renewable.

Firewood, dung cake, biogas, alcohol (ethanol), biodiesel, briquettes, husk.

Briquettes are used to heat rooms, cook food and boil water, keep a new mother and her baby warm, keep chicks warm in poultry farms, and dry vegetables, mushrooms and wool in industries.
Advantages (any two): they give less smoke than loose biomass, so less air pollution; they use up agricultural waste; they save firewood and so forests; they give more heat and are easy to store and carry; they can be made and sold for income.

Biogas is produced when bacteria decompose organic matter (dung, kitchen waste) in the absence of oxygen.
Construction (diagram as in Figure 9.10):

1. Mixing tank: dung and water are mixed into slurry.
2. Inlet pipe: carries the slurry into the digester.
3. Digester: an airtight underground tank where the slurry decomposes and makes biogas.
4. Dome: the top of the digester, where the gas collects.
5. Gas pipe with valve: takes the gas to the stove or lamp.
6. Outlet chamber: the gas pressure pushes spent slurry into it; from there it overflows into the compost pit, and is used as fertilizer.

Any two: it burns without smoke, so it reduces indoor air pollution; after the plant is built, the energy is free for years; it can be used for both cooking and lighting; it replaces firewood and saves forests; its spent slurry is a good fertilizer.

Your answer depends on where you live. For example:
Solar energy: many houses have solar water heaters on the roof, and solar panels with batteries for when the power goes off; solar street lamps light the roads.
Biogas: farming families feed dung into a biogas plant, cook on its smokeless flame, and use the slurry on their fields.
(Other possibilities: briquettes for heating in winter, micro hydropower in hill villages, improved cooking stoves, electric (induction) cooking instead of LPG.)

Cover: why Nepal needs alternatives (all petroleum is imported; air pollution; forests); each source and how much Nepal has of it (rivers, sunshine, cattle and waste, windy valleys); what has been done (AEPC, biogas plants, solar, micro hydro, electricity exports); problems (cost, transmission lines, storage, earthquakes and floods); and what you would do next.

The potential for developing alternative energy in Nepal

Nepal has no oil, coal or gas of its own worth mining. Every liter of petrol and every cylinder of cooking gas is imported, so energy costs the country huge sums every year, and the smoke from vehicles and from wood fires harms our health and our forests. Yet nature has given Nepal plenty of energy of other kinds.

Water. Rivers fed by Himalayan snow and monsoon rain fall thousands of meters in a short distance. Hydroelectricity is already Nepal’s main source of electricity, over 3,000 MW today, and the economically usable potential is many times that. Small and micro hydropower plants light villages far from the national grid.

Sun. Nepal gets strong sunshine on about 300 days a year. Solar panels, solar water heaters, dryers and cookers are already common, and solar power is cheap to install in remote places where no line can reach.

Biomass. Most families keep cattle, and every town makes tonnes of kitchen waste. Over 400,000 household biogas plants already give smokeless cooking gas and fertilizer, and large plants can turn city waste into compressed biogas to replace LPG. Briquettes turn crop waste and forest litter into clean-burning fuel.

Wind. Valleys like the Kali Gandaki in Mustang have strong, steady winds, and small wind plants already work in places like Sindhuli.

The challenges are real: building plants costs a lot at first, lines are hard to build across mountains, and floods, landslides and earthquakes can damage power stations. But with support from AEPC, local governments and private companies, and with more people switching to electric cooking and electric vehicles, Nepal can make its energy at home, clean its air, save its forests, and even earn money by selling electricity to its neighbors. Alternative energy is not just possible for Nepal; it is Nepal’s best path.