Common types of industrial heat pumps
A brief description of the most common types of heat pumps and their key operating principles is provided below.
Closed-Cycle Mechanical Heat Pumps use mechanical compression of a working fluid to achieve temperature lift. The working fluid is typically a common refrigerant. Most common mechanical drives are suitable for heat pump use; examples include electric motors, steam turbines, combustion engines, and combustion turbines.
Open-Cycle Mechanical Vapour Compression (MVC) Heat Pumps use a mechanical compressor to increase the pressure of waste vapour. Typically used in evaporators, the working fluid is water vapour. MVC heat pumps are considered to be open cycle because the working fluid is a process stream. Most common mechanical drives are suitable for heat pump use; examples include electric motors, steam turbines, combustion engines, and combustion turbines.
Open-Cycle Thermo compression Heat Pumps use energy in high-pressure motive steam to increase the pressure of waste vapour using a jet-ejector device. Typically used in evaporators, the working fluid is steam. As with the MVC Heat Pump, thermo-compression heat pumps are open cycle.
C
losed-Cycle
Absorption Heat Pumps use a two-component working fluid and the
principles of boiling-point elevation and heat of absorption to
achieve temperature lift and to deliver heat at higher temperatures.
The operating principle is the same as that used in steam-heated
absorption chillers that use a Lithium Bromide/water mixture as
their working fluid.
Key features of absorption systems are that they can deliver a much higher temperature lift than the other systems, their energy performance does not decline steeply at higher temperature lift, and they can be customized for combined heating and cooling applications.
Four heat exchangers—an evaporator, condenser, generator, and absorber—are found in a typical absorption heat pump (Figure 10).
Figure 10. Simplified Schematic of an Absorption Heat Pump
High-temperature prime energy (steam or fuel) is supplied to the desorber, where vapour is boiled out of the working fluid at high pressure.
The high-pressure vapour is condensed in the condenser, where the heat is recovered into a process stream. High-pressure condensate from the condenser is throttled to a lower pressure in the evaporator, where the waste heat is recovered to vaporize the low-pressure condensate. In the absorber, concentrated working fluid from the desorber contacts low-pressure vapour from the evaporator, creating heat that is recovered into a process stream. The working fluid returns to the desorber to complete the cycle.
In a typical absorption heat-pumping application, waste heat at low temperature is delivered to the evaporator, and prime heat at high temperature is delivered to the generator. An amount of heat equivalent to the sum of the high- and low-temperature heat inputs can be recovered at an intermediate temperature via the condenser and absorber. This is analogous to the thermo-compression heat pump, in which high-pressure steam is used to increase or lift low-pressure waste vapour to a higher pressure and temperature. An important variation of the Absorption Heat Pump is obtained by selecting operating parameters so that the device effects chilling at the ‘cold-end’ of the cycle while delivering hot water.
The ability to provide simultaneous cooling and heating provides additional benefits over a ‘heating-only’ heat pump and improves the economics of an installation.
An alternate configuration for an absorption heat pump allows a medium-temperature waste-heat stream to split into one higher-temperature stream and one lower-temperature stream. Adjusting the operating pressures and working-fluid concentrations accomplishes this reconfiguration.