Heat is the flow of energy from a body at higher temperature to one at lower temperature when they are placed in thermal contact. An everyday example of this is the natural cooling down of a hot cup of coffee when placed in contact with cooler surroundings.
What is thermal energy?
All forms of matter have internal or thermal energy. This comes about as a result of particle motion (kinetic energy) as well as the energy stored in chemical bonds present in the particles themselves (potential energy). Since particle movement increases as the temperature increases, it means that hot objects have greater particle movement than cooler objects. When placed in thermal contact, there is a natural flow of energy (called heat) from the hot object particles to the cooler object particles.
Heat pump design
It is possible to get around the natural requirement of the flow of energy from hot to cold.
If a different kind of energy in the form of work (be it electrical or mechanical) is applied, it is possible to make thermal energy transfer from a cold material to a hot material. This is what a heat pump does.
A heat pump is a device that can pump thermal energy either into (heating) or out of (cooling) an enclosed space such as a house.
The basic components of a heat pump are:
- working fluid (refrigerant)
- condenser (inside coil)
- expansion valve
- evaporator (outdoor coil)
The working fluid (refrigerant) used for most heat pumps nowadays is tetrafluoroethane. It has replaced chlorofluorocarbons (CFCs) because of the harm these chemicals were doing to the ozone layer in the upper atmosphere
Tetrafluoroethane has a relatively low latent heat of vaporisation and low boiling point (-26.3°C), but its chemical inertness and low toxicity make it an ideal working fluid.
The system is designed around the concept of latent heat of vaporisation. Thermal energy from the outside atmosphere is used to boil the working fluid (outdoor evaporator coil). The vapour produced is then allowed to condense (inside condenser coil), releasing thermal energy to the air inside the house.
Heat pump thermodynamics
Thermodynamics is the branch of science concerned with heat and its relation to energy and work. It first evolved in the 19th century as scientists and engineers were discovering how to design and operate steam engines.
Applying thermodynamic principles to the operation of a heat pump shows that to move thermal energy from the cold environment outside the house to the warm environment inside the house requires an input of work.
In this heat pump energy flow diagram:
- Qc represents the thermal energy taken from the outside air
- Qh represents the thermal energy transferred to the inside of the house
- W represents the work needed to achieve this
- Tc is the temperature of the heat pump’s working fluid (liquid)
- Th is the temperature of the heat pump’s working fluid (vapour).
The first law of thermodynamics is simply the principle of conservation of energy, which, in the case of a heat pump, states:
The increase in internal energy of a system is equal to the thermal energy added plus the work done on the system.
Using the symbols from the diagram, this reduces to:
Qh = Qc + W
For example, in the operation of a commercial heat pump, the thermal energy transferred to the inside of the building (Qh) was found to be 80 kJ, whereas the thermal energy extracted from the cold outside air was 60 kJ. To achieve this, the amount of electrical work (W) needed from a compressor was found to be 20 kJ.
These figures are in accordance with the first law of thermodynamics:
Qh = Qc + W
80 kJ = 60 kJ + 20 kJ
What this shows is that 60 kJ of ‘free’ energy from the cold outside air is being delivered to the inside air for every 20 kJ of work from the heat pump.