Refigarator and heat pumps

Introduction

• Discussion on refrigerators and heat pumps as systems based on thermodynamics.

• Contrast with heat engines covered in prior content; focuses on the transfer of heat.

Thermodynamic Principles

• Refrigerators and heat pumps operate in relation to the second law of thermodynamics.

  • They extract heat from a colder area (inside the fridge) and transfer it to a warmer area (the kitchen).

  • This process defies spontaneous occurrence; it requires mechanical work.

Heat Engine vs. Heat Pump

• Heat Engine:

  • Operates by converting thermal energy from a hot reservoir into work.

  • Energy flows from hot to cold naturally, producing useful work.

• Heat Pump Diagram:

  • Has a cold temperature reservoir at the bottom, and takes heat from this cold reservoir to transport into a warmer reservoir (the home).

  • Requires work input as defined by thermodynamic laws.

Mechanics of a Refrigerator

• Compressor Operation:

  • Compresses gas into a liquid.

  • Heat loss occurs while warm liquid moves through coils in the rear, losing heat energy to the environment.

• Evaporator Process:

  • As the liquid passes through a nozzle, it evaporates back into a gas, absorbing heat from the surrounding fridge environment, thus cooling its interior.

  • This process is similar to that of aerosol spray; it demonstrates latent heat of vaporization where the evaporation results in cooling.

• Heat Cycle:

  • Cycle continues as the cold gas absorbs heat and returns to the compressor, maintaining appliance function.

Heat Pumps

• Similar in operation to refrigerators, but serve the purpose of heating spaces by extracting heat from ambient sources.

  • Sources can be air or ground-based.

  • The essential mechanics involve gas evaporation and condensation in a cycle.

• Efficiency and Applications:

  • Increasing popularity due to eco-friendly energy sources in building homes.

  • Can reverse operation for air conditioning, maintaining cool indoor conditions.

Coefficient of Performance (COP)

• Measures the efficiency of heat pumps.

  • COP = Power delivered to the house / Power used by the compressor.

  • Typical COP values for heating range between 2.5 and 2.8.

  • For cooling operations, typical COP is around 2.

Example Calculation

• Problem Statement: A heat pump has a COP of 2.5 and uses 0.9 kilowatts of power.

• Formula: Power delivered = COP × Power of the compressor.

  • Calculation: 2.5 × 0.9 = 2.25 kilowatts delivered to the house.