Renewable and Non-renewable Energy Sources
Hello everyone! Welcome to one of the most important topics in Physics today: how we power our world. We use energy for everything - from charging our phones to lighting up our cities. In these notes, we'll explore the different ways we get this energy. We'll look at sources that will eventually run out (non-renewable) and those that keep on giving (renewable). Understanding this is crucial, not just for your exams, but for understanding the future of our planet. Let's get started!
The Two Big Categories: Renewable vs. Non-renewable
Think of our planet's energy resources like snacks for a very long trip. Some snacks, once you eat them, are gone forever. Others magically refill every day! This is the main difference between non-renewable and renewable energy.
Non-Renewable Energy Sources
These are the 'finite snacks'. They are natural resources that cannot be readily replaced by natural means on a level equal to our consumption. They were formed over millions of years from ancient organic matter.
Key Characteristics:
• They are finite (they will run out one day).
• They are extracted from the ground.
• Burning them often releases harmful pollutants and greenhouse gases, like carbon dioxide (CO₂).
Examples:
• Fossil Fuels: This is the main group, including coal, petroleum (oil), and natural gas.
• Nuclear Fuel: Materials like Uranium are mined from the Earth and are also finite.
Renewable Energy Sources
These are the 'magic refilling snacks'. They are sources of energy that are naturally replenished on a human timescale. The sun will keep shining, and the wind will keep blowing!
Key Characteristics:
• They are inexhaustible or can be replaced relatively quickly.
• They are generally much cleaner and have a lower impact on the environment.
Examples:
• Solar Energy (from the sun)
• Wind Energy (from moving air)
• Hydroelectric Energy (from moving water)
• Other examples include Geothermal (heat from the Earth) and Biomass (from organic matter).
Key Takeaway
The biggest difference is the rate of replenishment. Non-renewable sources are used up far faster than they are created, while renewable sources are replenished continuously or very quickly.
Our Star Player: Solar Energy
The Sun is the ultimate source of most energy on Earth. Let's see how we can harness its incredible power.
The Solar Constant
The Earth receives a huge amount of energy from the Sun every second. To measure this, scientists use a value called the solar constant.
Definition: The solar constant is the total energy from the Sun that lands on a 1 m² area per second at the Earth's average distance from the Sun, measured outside the Earth's atmosphere.
• Its value is approximately 1400 W m⁻² (or 1400 J s⁻¹ m⁻²).
• Why outside the atmosphere? Because our atmosphere reflects and absorbs some of this energy, so less power actually reaches the ground.
Solving Problems with the Solar Constant:
The total power (P) received by an object like a satellite's solar panel is the solar constant (I) multiplied by the area (A) facing the Sun.
Formula: $$P = I \times A$$
Example: A satellite has solar panels with a total area of 8 m². How much solar power do they receive in space?
Solution: P = I × A = 1400 W m⁻² × 8 m² = 11200 W.
How a Solar Cell Works
A solar cell (or photovoltaic cell) does something amazing: it converts sunlight directly into electricity!
The Process (Simplified):
1. Sunlight, which is made of tiny packets of energy called photons, hits the solar cell.
2. These photons knock electrons loose from the atoms in the semiconductor material of the cell.
3. This flow of freed electrons creates an electric current.
Energy Conversion: Light Energy → Electrical Energy
Key Takeaway
The Sun provides a constant, powerful stream of energy (measured by the solar constant). Solar cells can convert this light energy directly into useful electrical energy.
Catching the Breeze: Wind Power
Wind is just moving air, and because it has mass and is moving, it has kinetic energy. A wind turbine is designed to capture this energy.
The Power in the Wind
The amount of power a wind turbine can generate depends on a few key factors. Let's look at the formula. Don't worry, we'll break it down!
Formula for Maximum Power: $$P = \frac{1}{2} \eta \rho A v^3$$
Let's unpack the terms:
• P is the Power generated (in Watts).
• η (eta) is the efficiency of the turbine. It's a percentage (as a decimal) of how much of the wind's kinetic energy is successfully converted to electrical energy. No turbine is 100% efficient!
• ρ (rho) is the density of the air (in kg m⁻³). Usually around 1.2 kg m⁻³.
• A is the area swept by the turbine blades (in m²). If the blades have a length r, the area is $$A = \pi r^2$$.
• v is the wind speed (in m s⁻¹).
SUPER IMPORTANT POINT: Notice the v³! This means that wind speed is by far the most important factor. If you double the wind speed, the available power increases by a factor of 2³ = 8 times! This is why wind farms are built in very windy places.
Key Takeaway
Wind turbines convert the kinetic energy of wind into electricity. The power generated is extremely sensitive to wind speed (proportional to v³) and the size of the turbine blades (proportional to area A).
Go with the Flow: Hydroelectric Power
Hydroelectric power uses the energy of moving water, usually water flowing downwards from a great height.
The Energy Conversion Process
It's all about converting potential energy into electrical energy.
1. A dam is built to create a reservoir, storing a massive amount of water at a high elevation. This water has Gravitational Potential Energy (GPE).
2. The water is released and flows down through large pipes called penstocks. As it falls, its GPE is converted into Kinetic Energy (KE).
3. The fast-flowing water hits and spins the blades of a turbine, converting the water's KE into the Mechanical Energy of the spinning turbine.
4. The turbine is connected to a generator, which converts the mechanical energy into Electrical Energy.
Energy Chain: GPE → KE → Mechanical Energy → Electrical Energy
Calculating Hydroelectric Power
The power available is the rate at which the water loses GPE.
Formula: $$Power = \eta \times \frac{\Delta E_p}{t} = \eta \times \frac{mgh}{t}$$
• η is the overall efficiency of the system.
• m/t is the mass flow rate of the water (how many kg of water flow per second).
• g is the acceleration due to gravity (approx. 9.81 m s⁻²).
• h is the vertical height difference the water falls.
Key Takeaway
Hydroelectric power is generated by converting the gravitational potential energy of water stored at a height into electricity using turbines and generators.
The Heart of the Atom: Nuclear Energy
Nuclear energy is released from the nucleus of an atom. This can happen in two main ways: fission and fusion.
Binding Energy and Stability
Binding Energy is the energy that holds the nucleus together. Think of it as the 'glue' for protons and neutrons. The more stable a nucleus is, the more energy per nucleon (proton or neutron) is holding it together.
The Binding Energy Curve is a graph showing the binding energy per nucleon for different elements.
• It is low for very light elements (like Hydrogen).
• It rises to a peak at Iron (Fe-56), which is the most stable element.
• It then slowly decreases for very heavy elements (like Uranium).
Energy is released whenever the products of a reaction are more stable (higher up the curve) than the reactants.
• Nuclear Fission: A very heavy nucleus (e.g., Uranium-235) splits into two smaller, more stable nuclei. Since the products are higher up the binding energy curve, a huge amount of energy is released. This is what happens in nuclear power plants.
• Nuclear Fusion: Two very light nuclei (e.g., isotopes of Hydrogen) combine to form a heavier, more stable nucleus. Again, since the product is higher up the curve, a massive amount of energy is released. This is the process that powers the Sun.
Inside a Nuclear Fission Reactor
A nuclear power plant controls the fission process to generate heat, which then produces electricity just like a fossil fuel plant (by creating steam to turn turbines).
Key Components:
• Fuel Rods: Contain the fissionable material, typically Uranium-235.
• Moderator: A material (like water or graphite) that surrounds the fuel rods. Its job is to slow down the fast neutrons produced by fission. Slow neutrons are much more effective at causing further fission in other Uranium atoms, allowing a chain reaction to be sustained.
• Control Rods: Made of materials that absorb neutrons (like boron or cadmium). These can be raised or lowered into the reactor core. Lowering them absorbs more neutrons, slowing down the chain reaction. Raising them speeds it up. They are essential for controlling the reactor's power output and for shutting it down.
• Coolant: A fluid (usually water) that is pumped through the reactor core to absorb the immense heat generated by fission. This hot coolant is then used to boil water in a separate circuit, creating steam to drive turbines.
Key Takeaway
Nuclear energy is released when nuclei become more stable. Fission splits heavy nuclei, and fusion joins light nuclei. Fission is controlled in a reactor using fuel rods, a moderator, control rods, and a coolant to generate electricity.
Our Choices Matter: Energy's Impact on the World
No energy source is perfect. Our choice of energy has a huge impact on our environment and society.
The Environmental Impact
• Fossil Fuels: Burning them is the primary source of the greenhouse effect and global warming. The CO₂ released acts like a blanket, trapping heat in the atmosphere. They also cause air pollution and acid rain.
• Nuclear Power: It does not produce greenhouse gases. However, the major challenge is the safe disposal of radioactive waste, which remains dangerous for thousands of years. There is also the risk of accidents.
• Renewables: While much cleaner, they can have drawbacks. Hydroelectric dams can disrupt ecosystems, and large solar or wind farms require a lot of land.
Energy Consumption in Hong Kong
Hong Kong is a major city with huge energy demands.
• Most of our electricity is generated from fossil fuels (natural gas and coal) and imported nuclear power from the Daya Bay Nuclear Power Station.
• We rely heavily on imported fuels since Hong Kong has no natural energy resources of its own.
• The specific purposes for this energy are powering our buildings (lighting, air conditioning), transportation systems, and industries.
• There is a growing focus on increasing energy efficiency and exploring renewable options to reduce our environmental footprint.
Key Takeaway
Every energy source has environmental and societal consequences. The global challenge is to shift towards cleaner, more sustainable energy sources while managing the transition responsibly. In Hong Kong, this means focusing on efficiency and managing our reliance on imported fossil and nuclear fuels.