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Electrical Energy and Power Plants Notes

11.1 Electrical Energy and Power Plants

Introduction to Electrical Energy

  • We heavily rely on electricity for various essential functions.
  • Electricity is often taken for granted, yet it's invisible and difficult to define.
  • In physics, "electricity" refers to both electrical energy and the movement of charge.
  • Colloquial use of "electricity" often refers to the transfer or generation of electrical energy.
  • The law of conservation of energy applies to power plants, where different energy types convert to electrical energy.
    • Hydroelectric plants: Kinetic energy of falling water → electrical energy.
    • Natural gas/nuclear plants: Thermal energy → electrical energy.
  • Electricity is transferred via conducting wires in transmission lines.
  • Electrical devices transform electrical energy into other forms (kinetic, sound, thermal) to perform tasks.

Electrical Power

  • Power (P) is the rate at which energy is transformed.
  • Electrical power is the rate at which electrical energy is produced or consumed.
  • The terms "power" and "electrical power" are used interchangeably.
  • Equation: P = \frac{\Delta E}{\Delta t}, where:
    • P = Power (in watts, W)
    • \Delta E = Change in energy (in joules, J)
    • \Delta t = Change in time (in seconds, s)
  • Electrical devices have varying power ratings.

Sample Problem 1: Cellphone Charging

  • Problem: Calculate the power required to charge a cellphone if 740 J of energy are transferred in 1.0 min.
  • Given: \Delta E = 740 \text{ J}, \Delta t = 1.0 \text{ min}
  • Required: P
  • Analysis: P = \frac{\Delta E}{\Delta t}
  • Solution:
    • Convert minutes to seconds: \Delta t = 1.0 \text{ min} \times \frac{60 \text{ s}}{1 \text{ min}} = 60 \text{ s}
    • P = \frac{740 \text{ J}}{60 \text{ s}} = 12.33 \text{ W} \approx 12 \text{ W}
  • Statement: The power required to charge the cellphone is approximately 12 W.

Power Consumption and Generation

  • Different devices have different power needs (Table 1).
  • Power plants monitor and generate power "on demand."
  • Increased power demand (e.g., hot summer days due to air conditioning) requires increased power generation.
  • If generation capacity is reached, power may be purchased from external sources.
  • As a last resort, "brownouts" (temporary power interruptions) may occur.
  • Brownouts can damage electrical devices due to rapid power changes.
  • Electrical energy is generated on demand because storing it in large quantities is impractical.
  • Batteries are not feasible for large-scale storage due to the massive quantities needed.

Measuring Electrical Energy

  • Electrical energy is measured in kilowatt hours (kWh) because the joule (J) is too small for convenient measurement.
  • 1 kWh = 3.6 million joules.
  • A typical Ontario home uses approximately 1000 kWh of electrical energy per month.
  • Power plants use megawatt hours (MWh) to describe electrical energy generation.
  • In 2007, Ontario generated over 158 million MWh of electrical energy (approximately 12,100 kWh per person per year).
  • Canadians generally use more electrical energy compared to some other countries (e.g., Chad: 9 kWh per person per year).

Sample Problem 1: Halogen Light Bulb Energy Consumption

  • Problem: Calculate the energy needed by a 35 W halogen light bulb operating for 240 h; express the answer in both joules and kilowatt hours.
  • Given: P = 35 \text{ W}, \Delta t = 240 \text{ h}
  • Required: \Delta E
  • Analysis: P = \frac{\Delta E}{\Delta t} \implies \Delta E = P \Delta t
  • Solution:
    • Convert hours to seconds: \Delta t = 240 \text{ h} \times \frac{3600 \text{ s}}{1 \text{ h}} = 864,000 \text{ s}
    • \Delta E = (35 \text{ W}) \times (864,000 \text{ s}) = 3.024 \times 10^7 \text{ J}
    • Convert joules to kilowatt hours: 3.024 \times 10^7 \text{ J} \times \frac{1 \text{ kWh}}{3.6 \times 10^6 \text{ J}} = 8.4 \text{ kWh}
  • Alternative solution:
    • Convert watts to kilowatts: 35 \text{ W} \times \frac{1 \text{ kW}}{1000 \text{ W}} = 0.035 \text{ kW}
    • \Delta E = (0.035 \text{ kW}) \times (240 \text{ h}) = 8.4 \text{ kWh}
  • Statement: The halogen light needs 3.0 \times 10^7 \text{ J} or 8.4 kWh of energy to operate for 240 h.

Energy Efficiency and Power Plants

  • Non-renewable energy sources are depleting, and current renewable sources are insufficient.
  • Two approaches to meet electrical energy demand:
    • Conservation: Using less electrical energy.
    • Efficiency: Generating energy more efficiently.
  • Newer devices are designed to consume less energy, and there's growing awareness about energy use.
  • Power plants transform source energy into electrical energy, but the process isn't 100% efficient.
  • Efficiency is a measure of how well energy is transformed.
  • Engineers design ways to minimize energy losses during transformation.
  • Example: Improving coal-fired plant efficiency by increasing steam pressure and temperature.
  • Modern coal-fired plants can reach steam temperatures of 600°C and pressures 250 times atmospheric pressure.

Power Plant Efficiency Table

  • Table 2 shows the efficiency of different power plant technologies.
  • Hydro: 85% (e.g., Sir Adam Beck II: 1499 MW) - Energy lost as water isn't fully stopped.
  • Fossil Fuel: 45% (e.g., Nanticoke: 3640 MW) - Thermal energy not fully captured.
  • Wind: 40% (e.g., Huron Wind Farm: 9 MW) - Energy lost as not all wind is captured.
  • Nuclear: 35% (e.g., Darlington: 3500 MW) - Thermal energy not fully captured.
  • Solar: 15% (e.g., Sarnia Solar Farm: 20 MW) - Sunlight converted mostly to thermal energy instead of electrical.

Environmental and Societal Considerations

  • Evaluating energy sources and power plant technologies requires considering environmental and societal aspects.
  • Environmental aspects: Source accessibility, dependability, plant location, atmospheric emissions, and pollution.
  • Societal impacts: Financial costs, employment opportunities, safety, and effects on nearby communities.
  • Efficiency impacts resource use and operational costs of power plants.
  • Operational efficiency: Describes the amount of time something is working compared to downtime.
  • Thermal efficiency: Describes how much thermal energy is used to perform a task versus wasted thermal energy.

11.1 Summary

  • Mechanical, thermal, and radiant energy are transformed into electrical energy in power plants.
  • Electrical power is the rate at which electrical energy is generated or transformed.
  • Electrical energy is measured in kilowatt hours (kWh) for homes and megawatt hours (MWh) for power plants.
  • Power plant technologies vary in efficiency and environmental/societal impacts.
  • Improving power plant efficiency can decrease resource use.