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Heat Pump

A guide to Heat Pumps: Types, and Costs

The heat pump is a machine that utilizes the energy of air, soil, or groundwater and converts it into thermal energy. Discover how it works.

The heat pump, using various forms of energy, can extract and transfer thermal energy. Its selection depends on the climatic characteristics of the installation location, the typological features of the building, and usage conditions.

Let’s delve into what it is, its operation, types, and costs.

What is a Heat Pump?

A heat pump is a machine capable of transferring heat from a lower temperature environment to a higher temperature one. While commonly associated with providing hot water, it has a broader function. In addition to winter heating and supplying hot water for sanitary use, it also ensures cooling of spaces by reversing the cycle.

How Does a Heat Pump Work

The operating principle is based on the Joule-Thomson expansion phenomenon of a very cold refrigerant that vaporizes at low temperatures. Through compression and expansion cycles, the heat pump transfers heat to domestic hot water.

To heat the environment, the heat pump extracts heat from a natural source (such as air, water, or soil), raises it to the desired temperature, and transports it inside the building, where it is radiated through underfloor radiant systems or terminals such as radiators or fan coils.

In essence, a similar but opposite operation to that of a refrigerator. The latter expels heat from the inside to the outside, while the heat pump captures it from the outside and, after bringing it to temperature, introduces it inside. To cool the environment, heat is extracted from the air inside the building and expelled, much like a refrigerator.

The external medium from which heat is extracted is called the cold source. In the heat pump, the refrigerant absorbs heat from the cold source through the evaporator.

The main cold sources are:

  • the air outside the room where the heat pump is installed or extracted from the room where the heat pump is installed;
  • groundwater, river water, lake water when it is present near the heated rooms and at a shallow depth.

Other sources may consist of water accumulated in tanks and heated by solar radiation and the ground, where pipes related to the evaporator are inserted.

The air or water to be heated is called the hot well. In the condenser, the refrigerant transfers both the heat taken from the cold source and the energy provided by the compressor to the hot well.

Components of a Heat Pump: Structure

The heat pump consists of a closed circuit, traversed by a special fluid (refrigerant) that, depending on the temperature and pressure conditions, assumes the liquid or vapor state.

The closed circuit consists of:

  • a compressor;
  • a condenser;
  • an expansion valve;
  • an evaporator.

Both the condenser and the evaporator consist of heat exchangers, i.e., tubes in contact with a service fluid (which can be water or air) through which the refrigerant flows.

The refrigerant releases heat to the condenser and absorbs it from the evaporator. The components of the circuit can be either grouped into a single block or divided into two parts (split systems) connected by tubes through which the refrigerant circulates.

In operation, the refrigerant undergoes the following transformations in 4 phases:

  • Phase 1 – Compression: the refrigerant gas at low pressure from the evaporator is brought to high pressure; during compression, it heats up by absorbing a certain amount of heat;
  • Phase 2 – Condensation: the refrigerant, coming from the compressor, changes from a gaseous to a liquid state, releasing heat to the outside;
  • Phase 3 – Expansion: passing through the expansion valve, the liquid refrigerant partially transforms into vapor and cools down;
  • Phase 4 – Evaporation: the refrigerant absorbs heat from the outside and evaporates completely.

The combination of these transformations constitutes the heat pump cycle: by supplying energy with the compressor, the refrigerant, in the evaporator, absorbs heat from the surrounding environment and, through the condenser, transfers it to the medium to be heated.

Operating Diagram

Below is a diagram illustrating the operation of a heat pump. The main components are depicted in the figure.

Starting from a heat source (air, soil, water), which provides heat to the refrigerant that evaporates through the evaporator. Next, it goes through the compressor, which compresses the refrigerant gas, increasing its temperature. It reaches the condenser where the hot gases release heat to the water in the heating system, condensing. The liquid then passes through the expansion valve, where it expands, reducing its temperature.

Heat Pump Diagram

Heat Pump Diagram

Types of Heat Pumps

There are various types of heat pumps whose operation is based on the same principles. Heat pumps are divided into two main categories:

  • monovalent heat pumps: independent of other heat generators;
  • bivalent heat pumps: integrated with other heat generators, particularly suitable for areas subject to a significant temperature drop.

In terms of the fluid used for heat exchange, heat pumps can be classified into 4 main types, indicating the cold source (evaporator) first and the hot well (condenser) second:

  • air-water heat pump: performs both heating and cooling functions. It generates heat through heat exchange with external air and uses it in a cycle to transfer heat from a colder fluid to a warmer one, namely sanitary water or the heating system water;
  • air-air heat pump: composed of at least two units (one of which is external) equipped with splits and an air duct system. Examples of air-air heat pumps are air conditioners;
  • water-water heat pump: uses water both as a cold source and as a hot source; in both circumstances, heat exchange occurs between the refrigerant and the water;
  • ground-water heat pump (geothermal heat pump): heats water using the heat present in the ground, capturing it through a geothermal probe. This type of machine has high energy efficiency, especially when combined with low-temperature heating systems. However, it requires careful maintenance.

Monoblock and Split Heat Pumps: Differences

For air-water heat pumps, two distinct systems have been developed: the monoblock system and the split system, depending on where the energy exchange occurs between what is recovered from external air and the fluid circulating in the heating system.

In the split system, the energy transfer occurs in a water-gas exchanger located in a housing separate from the external unit that absorbed heat from the air. This exchanger is usually placed inside the house and is contained in a module that resembles a boiler in size.

In the monoblock system, on the other hand, the exchanger is contained in the external unit itself, creating a single unit.

Therefore, the main difference between the two systems lies in the fact that in the monoblock heat pump, all components are grouped together, while in the split pump, they are distributed over two separate units, an internal and an external unit.

Monoblock and Split Heat Pumps

Monoblock and Split Heat Pumps

Efficiency and Performance of a Heat Pump

During its operation, a heat pump:

  • consumes electrical energy in the compressor;
  • absorbs heat in the evaporator from the surrounding medium, which can be air or water;
  • transfers heat to the medium to be heated in the condenser (air or water).

The advantage of using a heat pump lies in its ability to provide more energy (heat) than the electrical energy used for its operation, as it extracts heat from the external environment (air-water).

Heat pumps represent the ideal solution to reduce consumption and CO2 emissions, thereby preserving the Earth’s ecosystem. For example, they are one of the central and indispensable systems for nZEB buildings. The efficiency of a heat pump is measured by the COP (Coefficient of Performance), which is the ratio of provided energy (heat supplied to the medium to be heated) to consumed electrical energy.

The COP varies depending on the type of heat pump and operating conditions, generally having values close to 3. This means that for 1 kWh of electrical energy consumed, it will provide 3 kWh (2580 kcal) of heat to the medium to be heated.

The COP will be higher the lower the temperature at which the heat is transferred (in the condenser) and the higher the temperature of the source from which it is absorbed (in the evaporator).

Additionally, it should be noted that the thermal power delivered by the heat pump depends on the temperature at which it absorbs heat.

How Does a Heat Pump Work for Cooling?

The heat pump can also perform the cooling function. The fundamental premise is that the heat pump is designed to be reversible, meaning that the thermodynamic process of the unit can be reversed.

In this case, the heat pump extracts heat from living spaces and transfers it to the surrounding environment (air, soil, or water) through the previously described heat recovery circuit.

There are two options available for cooling:

  • active cooling: requires additional electrical energy to activate the compressor. Despite incurring additional costs, it offers higher cooling capacity compared to passive cooling. This mode is often used with air-water heat pumps;
  • passive cooling: is more energy-efficient but can only be implemented with glycol water-water or water-water heat pumps. In this case, the heat pump is not fully active, and the building’s heat is transferred to the energy source only through the circulation pump.

How Does a Heat Pump Work in Winter?

Talking about utilizing ambient heat in winter may seem unusual. However, as long as the temperature of the heat source (air, soil, or water) remains above the boiling point of the refrigerant that conveys thermal energy, it is possible to efficiently use ambient heat for heating and hot water production.

Since the boiling point of commonly used refrigerants ranges from a maximum of -57°C to a minimum of -12°C Celsius, the heating system’s operation is reliably guaranteed even during winter.

In case the system reaches its limit on exceptionally cold days, an additional electric heating element is activated to ensure safety.

The efficiency of a heat pump in winter is influenced by its design. The temperature variations of the energy sources for saline-water (geothermal) and water-water (groundwater) heat pumps are significantly lower than those of air-water heat pumps. In the ground, the temperature remains at least 10°C throughout the year from a depth of 10 meters.

Advantages of a Heat Pump

The main advantages of using a heat pump for heating and domestic hot water are related to renewable and free energy sources from which the pump extracts heat for buildings.

Other advantages include:

  • significant savings on bills;
  • low maintenance costs;
  • low environmental impact;
  • access to tax incentives.

The disadvantages of heat pumps are mainly related to installation costs, which can be recovered through tax incentives, and space requirements. Rather than disadvantages, it is essential to consider precautions in choosing models.

 

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