## Tuesday, 16 August 2011

### Physics: Dailylife applications of Energy III (elective) - renewable resources and vehicles

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Renewable energy resources
Renewable energy sources are derived from a natural process and can be regenerated in a relatively short time like hydroelectric power, wind power, geothermal power, etc.
Non-renewable energy sources are regenerated in a relatively long time (e.g. fossil fuels) or even can't be regenerated (nuclear power).
Solar energy
-          Solar constant is defined as solar radiation power/area received at top of the atmospheres which varies around 1360Wm-2. Some of it is absorbed or reflected in the atmosphere so around 55% of them reach the ground.
-          Solar heating: In a solar heating device, cold water is inserted below into a tube with vacuum surrounding. The tube is black so that it absorbs radiation from Sun and heat up the water. By convection hot water flows up and collected at the top of the tank. It's used in several government buildings.
-          Solar electricity: It' s a photovoltaic cell where p and n type semiconductor is connected. Under sunlight semiconductors emit mobile electrons so that there's a positive hole and electrons keep moving to fill-up the positive hole. As a result, a steady electric field is generated. The efficiency is not so high (about 20%), it's not cost effective (low power but need a large piece of land). It's also time depending (can't be used in cloudy days and night).
Wind power
It converts k.e. of wind into electricity. Assume that:
1)       All k.e. of air converts into electricity. i.e. air passing through the turbine has zero velocity.
2)       The wind direction is perpendicular to the turbine.
Energy generated in time t = total k.e. of wind in time t passed through the turbine = mv2/2 = (ρAvt)v2/2
Then power output Pmax = ρAv3t/2t = ρAv3/2, where ρ is the density of air, A is the area of turbine and v is the velocity of the wind.
However in practical, velocity of wind passing through the turbine mustn't be zero so Pmax only denotes the theoretical maximum output power. In practical the efficiency is only about 30%. It's sustainable and cost-effective, but it needs a large space with high wind speed (probably off-shore).
Hydroelectric power
Water in river flows from high to low, so reservoir make use of their potential energy which rotates the turbine and converts into electricity according to E = mgh. The operation cost is low while it gives out quite high power, but it may affect the aquatic environment.
Nuclear power
Through nuclear reaction, binding energy is released according to the mass-energy equivalence. It's widely used in the world, but safety problems in concerned especially after the explosion of nuclear plant in Chernobyl and in Japan. Radiation leakage is dangerous so the disposal of radioactive waste is a problem as well.
Vehicles
Conventional cars are internal combustion engine vehicles (ICEV), they consumes fuels to push the engine so that the car moves. However those fossil fuels are non-renewable and consuming them gives out air pollutants which cause global warming or health hazards.
Battery electric vehicles (EV)
EV pushes the car by electricity stored in the car battery with a regenerative breaking system. In conventional cars, k.e. converts into heat by friction when the car breaks. With the regenerative breaking system, the k.e. is stored in terms of electricity and can be reused again. This type of car is more environmental-friendly since it don't need fuels and the battery is rechargeable. However the car battery is heavy and have short lifetime (6-8 hours) so it can't be used for long distance travelling.
Hybrid electric vehicles (HEV)
It uses a primary energy source of fuel and with the assistance of battery. In lower power output like city areas, it's driven by fuel tank while excessive energy is stored in the battery; in high energy output situation like highways, the battery assists the fuel tanks for higher output. The advantages include a significant cut demand in petrol (up to 50% cut), so it suits city journey a lot. The combustion engine and tank is smaller than ICEV, and no manual charging of battery can be avoided.
No matter which vehicle is used, mass transportation would be more energy efficient and more environmental friendly. In Hong Kong, the government has set up many bus interchange stations to facilitate bus services, and car parks are built along MTR stations to facilitate people to take MTR instead of private car. In order to tackle pollution problem, lead-free petrol is used, and LPG cars has been more popular.