Heat pumps comparison: What is a Heat Pump and how it works?

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1) What is a Heat Pump?
A heat pump is a device that is able to transfer heat from one fluid at a lower temperature to another at a higher temperature.
Heat pumps owe their name to the fact that they allow heat to be carried from a lower to a higher temperature level, inverting natural heat flow which - as is well known - in nature tends to be from a higher to a lower temperature.
The function of the heat pump may therefore be compared to that of a water pump positioned between two water basins that are connected to each other but which are located at different altitudes: water will naturally flow from the higher to the lower basin. It is, however, possible to return water to the higher basin by using a pump, which draws water from the lower one.

2) What it is and how it works
A heat pump consists of a closed circuit through which a special fluid (refrigerant) flows. This fluid takes on a liquid or gaseous state according to temperature and pressure conditions. This closed circuit consists of:

Electric Heat Pump Gas Absorption Heat Pump
a compressor a generator
an absorber
a condenser a condenser
an expansion valve a series of restrictors
an evaporator an evaporator







The condenser and the evaporator consist of heat exchangers, i.e. special tubes placed in contact with service fluids (which may be water or air) in which the refrigerant flows. The latter transfers heat to the condenser (the high temperature side) and takes it away from the evaporator (the low temperature side).

During operation, the refrigerant, inside the circuit, undergoes the following transformations:

Electric Heat Pump Gas Absorption Heat Pump
Condensation: refrigerant flowing from the compressor passes from a gaseous to liquid state, giving off heat to the outside Condensation: refrigerant flowing from the generator passes from a gaseous to liquid state, giving off heat to the external fluid (water or air)
Expansion: passing through the expansion valve, the liquid refrigerant cools and is partially transformed into vapour Expansion: passing through the restrictors, i.e. suitably calibrated narrowings, the refrigerant is cooled
Evaporation: the refrigerant absorbs heat and evaporates completely Evaporation: the refrigerant absorbs heat from the external fluid (water or air) and evaporates completely, returning to a gaseous state
Compression: the refrigerant, in a gaseous state and at low pressure, coming from the evaporator, is taken to a high pressure; during compression it is heated, absorbing a certain amount of heat Absorber: the refrigerant is absorbed by an absorbing fluid, making it liquid once more
Generator: the liquid solution of the refrigerant and absorbing is heated in the generator by means of a gas burner, separating the refrigerant, which evaporates, increasing in temperature and pressure
All of these transformations together make up the electric heat pump’s cycle: the compressor provides the refrigerant with energy, the refrigerant absorbs heat in the evaporator from the surrounding medium, and through the condenser, transfers it to the medium to be heated All of these transformations together make up the gas heat pump’s cycle: by supplying energy with the gas burner (methane/LPG) the refrigerant in the evaporator absorbs heat from the external fluid and, through the condenser, transfers it to the medium to be heated



















3) Heat Pump Efficiency
During its operation, a heat pump:

Electric Heat Pump Gas Absorption Heat Pump
Consumes electrical energy for the compressor Consumes natural/LPG gas in the generator
Absorbs heat in the evaporator from the surrounding medium, which may be air or water Absorbs heat in the evaporator from the surrounding medium, which may be air or water
Gives off heat to the medium to be heated in the condenser (air or water) Gives off heat to the medium to be heated (air or water) in the condenser






The advantage of using a heat pump derives from its capacity to supply more energy (i.e. heat) than that employed for its operation, insofar as it draws heat from the outside environment (air or water).

Electric Heat Pump Gas Absorption Heat Pump
The efficiency of an electric heat pump is measured by its C.O.P., or coefficient of performance, which is the ratio of the energy it supplies (i.e. heat transferred to the medium to be heated) to the electrical energy it consumes.
The C.O.P. varies according to the type of heat pump and operating conditions, and generally has a value of approximately 2.5.
This means that for each kWh of electrical energy consumed, it will supply about 2.5 kWh of heat energy to the medium to be heated.
The lower the temperature to which heat is transferred (in the condenser), and the higher the temperature of the source from which it is absorbed, the greater the pump’s efficiency will be.
It should be taken into account that the heat output yielded by the heat pump depends on the temperature at which it absorbs heat itself.
The efficiency of a gas heat pump is measured by its G.U.E. (Gas Utilization Efficiency), which is the ratio of the energy it supplies (i.e. heat transferred to the medium to be heated) to the energy consumed by the burner.
The G.U.E. varies according to the type of heat pump and operating conditions, and generally has a value of approximately 1.5.
This means that for each kWh of electrical energy consumed, it will supply about 1.5 kWh of heat energy to the medium to be heated.
When the temperature of the cold source (i.e. air) is within the range of -2ºC and 2ºC, the heat pump is deactivated, as its performance is significantly reduced. A gas heat pump can operate at air temperatures as low as -20ºC (20 degrees below zero), providing efficiency of around 1, comparable to that of a condensation boiler.

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