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TECHNOLOGY // TECHNICAL DOSSIERS // INSIDE THE ABSORPTION TECHNOLOGY // Heat Pumps Comparison: type and application

Heat Pumps Comparison: type and application

 

The cold source (from which heat is recovered)

The external medium from which heat is drawn is called a cold source. In the heat pump the refrigerant absorbs heat from the cold source by means of the evaporator.

The main cold sources are:
  • AIR: outside the space to be heated, typically air from the external environment;
  • WATER: from the water table, rivers, and lakes, when in proximity to the spaces to be heated and at a limited depth below the surface. Other reservoirs may consist of water which has accumulated in reservoirs and is subsequently heated by solar radiation. 
  • GROUND, into which specific pipes relating to the evaporator are sunk to varying depths (these pipes constitute a geothermal system).
 

The hot source (to which heat produced and recovered is transferred)

The air or water to be heated is called a hot source.
 
Electric Heat Pump Gas Absorption Heat Pump
In the condenser, the refrigerant transfers to the hot source both the heat drawn from the cold source and the heat energy supplied by the compressor. In the condenser, the refrigerant transfers to the hot source both the heat drawn from the cold source and the heat energy supplied by the burner.

Heat may be transferred to the environment through:
  • Fan coils, consisting of cabinets in which air is made to circulate through heating bodies. These may be wall-mounted, ceiling-mounted or built-in models.
  • Coils built into the floor, in which hot water circulates. Heating occurs mainly through radiation and requires a lower water temperature.
  • Canalizations, which transfer the heat produced by the heat pump to the different rooms, by means of suitable air channels and inlet vents.
 

Types of Heat Pumps


Heat pumps can be categorised on the basis of the cold source and hot source that they use.
According to the fluid used for the transfer of heat from the cold source to the heat pump, and from the heat pump to the hot source, there may be four types: 
 
Cold source   Hot source
AIR AIR-WATER the heat pump draws heat from the cold source, which consists of
air (external), and transfers it to the hot source, which consists of a water
circuit (for the heating of the rooms).
WATER
AIR AIR-AIR the heat pump draws heat from the cold source, which consists of air (external), and transfers it to the hot source, which likewise consists of air (that of the heated environment). AIR
WATER WATER-WATER the heat pump draws heat from the cold source, which consists of water (from lakes, rivers or the water table) and gives it off to the hot source, which consists of a water circuit (for the heating of the rooms). WATER
WATER WATER-AIR the heat pump draws heat from the cold source, which consists of water (from lakes, rivers or the water table) and gives it off to the hot source, which consists of air (that of the heated environment). AIR

AIR as a cold source possesses the advantage of being available everywhere; in any case, as the temperature of the cold source falls, so does the output supplied by the heat pump. 
 
 
Electric Heat Pump Gas Absorption Heat Pump
If external air is used, a defrosting system is needed at temperatures of around 0ºC, which entails additional energy consumption.
During this phase the heat pump uses the heat of the hot source in order to defrost the battery and so heating ceases for some minutes.
If external air is used, a defrosting system is needed, which is automatically activated at temperatures of around 0ºC. Heat output supplied during the defrosting phase (which lasts a few minutes) falls, but does not cease completely.

WATER as a cold source ensures optimal performance of the heat pump without it suffering from the effects of external climatic conditions; however it entails additional cost, due to the system of water adduction.
 


The GROUND as a cold source possesses the advantage of undergoing less temperature changes than the air.
 


Horizontal pipes must be installed at a depth of 1 to 1.5 metres below the ground in order not to be unduly affected by variations in external air temperature and to retain the beneficial effects of insulation. 
 
Electric Heat Pump Gas Absorption Heat Pump
Electric heat pumps require an area of land which is 2 to 3 times larger than that of the surface to be heated. They are therefore a costly solution, both in terms of the land required and the complexity of the system. Gas absorption heat pumps require an area of land which is 1,5 to 2 times larger than that of the surface to be heated.

Vertical pipes can reach a depth of several tens of metres. They do not occupy space horizontally, but they require the boring of suitable deep wells. 
 
Electric Heat Pump Gas Absorption Heat Pump
The length of the pipes depends on the type of existing land (on average from 7 to 20 metres for each kW of heat energy supplied to the hot source). The length of the pipes depends on the type of existing land (on average from 4 to 12 metres for each kW of heat energy supplied to the hot source).
 

Applications of the Heat Pumps


Possible applications of heat pumps are: 

Air conditioning
The application of heat pumps for air conditioning of environments in the residential and industrial sectors is by now a reality, as an alternative to conventional systems made up of a chiller plus boiler. The same machine, in fact, by means of a simple inversion valve, is able to swap the functions of the evaporator and the condenser, thus supplying heat in the winter and cold in the summer (reversible type). 
The application of heat pumps to the temperature regulation of internal environments (heating+cooling) is the most economical, as it entails a shorter timescale for the amortization of the initial cost of the system, thanks to greater savings in energy costs. 

Heating
Heat pumps may be used also solely for heating the internal environment. In such cases the economic aspects must be carefully weighed up, in comparison with traditional systems such as high-efficiency water and condensation boilers.

For the heating of internal environments, the system may be of two types:
  • Monovalent
  • Bivalent
 
Electric Heat Pump Gas Absorption Heat Pump
- A monovalent configuration is used when the heat pump is able to meet the heating requirements of the internal environment fully. If the heat pump uses external air as it source, this configuration may be adopted in climate zones where the external temperature rarely falls below 0ºC.
- Otherwise, a bivalent system must be built, consisting of an auxiliary heating system (i.e. a traditional boiler) which meets heating requirements when the external air temperature falls below 0º C.
- Normally, only the monovalent configuration is used, as the heat pump is able to meet heating requirements fully even when air is used as a cold source. In fact the heat pump supplies heat at air temperatures as low as -20ºC (20 degrees below zero) without the aid of other systems.
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