(a) The solar cells are costly and hence the energy available from them expensive.
(b) 15 km/h is the minimum wind velocity required to obtain useful energy with a windmill.
(c) Nuclear fission is the process in which a radioactive atom splits up into smaller nuclei when struck with low energy neutrons.

(a) A solar cell is a device which converts solar energy into electricity. It is a sandwich of a silicon-boron layer and a silicon-arsenic layer.
(b) Biogas is an ideal fuel for domestic use because it is a cheap and clean fuel, which does not leave behind any residue.
(c) A nuclear power plant is a device in which controlled chain reaction is used.

Advantages of Solar Cooker
a) They are environment friendly.
b) Use of a solar cooker does not involve recurring expenses on fuel.
Disadvantages of Solar Cooker.
a) Solar cooker is very slow.
b) Solar cooker can work only during summer months and is useless during winters and on cloudy days.
c) Solar cooker can be useful at those places which receive plenty of sunshine. This is useless in colder climates.

Nuclear fusion is a nuclear reaction in which two light nuclei of a heavy atom combine to form a nuclear of a heavier atom.
The conditions required for the occurrence of a fusion are:
(a) High temperature
(b) Hogh pressure

Biogas is considered as an ideal fuel because it 
Has a high calorific value
Produces no smoke on burning
Burns smoothly without any explosion
Leaves no residue on combustion

a) The construction of wind energy farm is costly.
b) The continuous whistling and whirling is irritating.
c) The location of wind energy farms affects the route of migratory birds.
d) It can be located only in vast open areas where the minimum velocity for a windmill to function is 15 km/hr.

a) using smokeless devices.
b) Afforestation.

Yes. It is true that energy can neither be created nor destroyed and one form of energy is converted into another form. But the energy in the usable form is dissipated to the surrounding in the less usable form.

a)Hydrogen is not used as a common fuel because when it is lighted, it burns with an explosion and it is very difficult to store and transport hydrogen.
b) Fuel cells are being developed that can store energy produced by burning hydrogen. Development of materials that can absorb hydrogen and then release it at a steady rate.

ii) Although wood is readily found in nature, it is still not a good source of energy because
- the energy obtained from per mass of Wood is very small.
- it produces a lot of smoke, which is very harmful for the environment and contributes to air pollution.
- using wood as a source of energy further increases the rate of deforestation.

(i) Solar constant of sun is 1.4 kW/m².
(ii) Concave mirrors are used in box type solar cookers.
(iii) It is difficult to use hydrogen as a source of energy because it has not been possible to control nuclear fusion reaction. Thus, it has not yet been possible to use energy generated from fusion reaction for anything constructive.
(iv) Fossil fuels are classified as non-renewable sources of energy because it takes a long time for them to replenish naturally. It takes millions of years for fossil fuels to replenish. With the rate at which humans are using them, they will get exhausted in the next hundred years. Thus, fossil fuels are termed as non-renewable source of energy. 

a) (i) Depletion of fossil fuels. (ii) Increase in population. (iii) Improve the quality of living
b) The energy from the sea or ocean can be obtained in any of the following three forms:
a) Wave energy
b) Tidal energy
c) Ocean thermal energy
a) Wave Energy
The energy from the sea can also b obtained in the form of sea waves When the wind blows on the surface of the sea, the very fast sea waves move on the surface possessing kinetic energy. This energy can be used generate electricity. In order to trap this energy, a wide variety of device need to be installed to turn turbine: and drive generators.
Many experimental ideas wen developed to harness this energy. 
These are as follows:
1. Setting up of floating generator: in the sea to trap the sea waves generate electricity.
2. Allowing the sea waves move and down inside large tubes. As the waves move up, the air inside the tubes is compressed and to turn a turbine to generate electricity. 
These experimental ideas many years to be installed. Thu: harnessing of sea waves energy would be possible only at those places where sea waves are very strong.
Tidal Energy The tidal waves include low tides and high tides. When the rise of sea water takes place due to gravitational pull of the moon, it is called high tide and when the fall of seawater takes place it is called low tide. The rise and fall of tidal waves takes place twice a During this period, a large movement of water takes place between the high tides and low tides resulting in the production of a very large source o energy.
The tidal energy can be harnessed only by constructing a tidal by constructing a tidal power plant across a narrow opening to the sea. During the high tides, the gates the dam are opened and the sea water is allowed to fall on the turbine t produce electricity. Thus, we can say that kinetic energy of the sea water converted into electrical energy.
During low tides, the gates of the dal are closed due to which the water level in the dam raises possessing potential energy. Now, the gates are opened and the water is allowed to fall back into the sea. This falling water is also used to rotate the turbine to generate electricity.
c) Ocean Thermal Energy
We know that a large area of sea is called ocean. The water at the surface of an ocean gets heated due to the radiation from the sun and reaches a higher temperature than the deeper levels of the ocean. This creates a temperature difference between the water at the surface of the ocean and at deeper levels of the ocean. The solar energy stored in the oceans in the form of heat is called ocean thermal energy. The heat energy produced due to the difference in the temperature Of water On the surface of the ocean and temperature of water deep into ocean. This heat energy produced can be used to produce electricity. Thus, we can define ocean thermal energy as "The heat energy due to the temperature difference between different layers of water in the Ocean' We can convert the ocean thermal energy (OTE) into electricity by ocean thermal energy conversion power plants or OTEC power plants.
The devices which are used to harness the ocean thermal energy are called ocean thermal energy conversion
[OTEC] power plants. To operate ocean thermal energy conversion [OTEC] power plant, the temperature difference between the surface water o ocean and deeper water must be 200( or more. In some ocean thermal energy conversion [OTECI power plants, the warm surface water of the ocean used to boil liquids like ammonia o chlorofluorocarbon (CFC). Then, the high-pressure vapours of the liquid an used to turn the turbine to generate electricity. The colder water from the deeper ocean is pumped up to cool the used up vapours to convert them back into liquid.

a) Solar energy can be harnessed by using solar energy devices. These devices either use solar energy directly as heat or convert it into solar energy directly as heat or convert it into electricity. One of the devices is solar cooker.
b) Limitations of solar energy:
i) It is not evenly distributed on the Earth.
ii) It requires costly equipment and infrastructure.
c) Using of solar cells.