Refrigeration Cycle

Heat pump uses a working fluid to move thermal energy from cold to hot, the opposite direction of natural heat flow.

This is achieved through by raising the temperature of the working fluid by compression and then lowering the temperature of the working fluid by expansion. The working fluid also absorbs and releases energy from evaporation and condensation.

Coefficient of Performance

The coefficient of performance is the energy output divided by the energy output.

Unlike most things, this number is often greater than one. (Remember that we are moving thermal energy, not generating or converting it.)

The energy inputs are

  • the compression and pumping of this working fluid
  • the movement of air through the heat exchangers

Maximum COP

The limit of the coefficient of performance is given by the second law of thermodynamics which usually tells us how much work can be extracted given two different temperature sources (hot and cold).

For refrigeration COP_R = \frac{T_C}{T_H - T_C}

For a heat pump COP_{HP} = \frac{T_H}{T_H - T_C}

Note that these are lower when the temperature difference between hot and cold (inside and outside) are different. Note also that these formulas must use absolute temperature scales like Kelvin or Rankine.

SEER

  • Seasonal Energy Efficiency Rating
  • BTU/watt-hour
  • Averages the COP over a range of outdoor temperatures
  • Also includes energy for air movement

SEER2

SEER2 is an updated requirement for rating heat pumps.

HPSF

  • Heating Seasonal Performance Factor
  • BTU/watt-hour
  • Averages the COP over a range of outdoor temperatures
  • Also includes energy for air movement

What is the carbon difference for heat pump vs combustion

For high coefficients of performance and low carbon intensities of electricity, the carbon emitted to provide the same amount of heating in a home can be lower for an electric heat pump.

This is in contrast to resistive electric heating which emits more carbon than direct combustion because of the second law losses at the power plant.

Good Refrigerants

  • boil at low temperature
  • high heat of vaporization
  • low toxicity
  • low global warming effect
  • low ozone depletion

Typical Heat Pump Numbers

  • SEER of about 20 BTU/Wh
  • HSPF of about 10 BTU/Wh

Types of Heat Pumps

  • Air source pumps place the outside heat exchanger in the air
  • Ground source pumps place the outside heat exchanger underground to take advantage of low temperature swings
  • Some heat or cool water at the heat exchanger

References