Heat Pump Working Principle

In this article, we discuss the working principle of a heat pump. Installing a heat pump in your home, workplace, or summer house can significantly reduce heating and cooling costs, providing a great relief to your energy bills. A relatively new technology, the heat pump will gradually replace air conditioners and central heating systems over time.

The cost of a heat pump can range from $4,000 to $20,000 depending on the size of your home or business and the type of heat pump. Generally, geothermal heat pumps, which require underground installations, will be more expensive to install than air-source heat pumps. Although the cost of the heat pump and labor fees can be a significant expense, once installed, it can substantially reduce your energy consumption. The contents of this article are as follows:

What's in this article?

• What is a heat pump?
• How does a heat pump work?
• Types of heat pumps
• Cooling cycle
• Vapor compression heat pump cycle
• Compressor
• Heat pump installation cost
• Heat pump reviews
• Applications of heat pumps

We aim to make this article the most comprehensive one about heat pumps. Over time, we may add new sections or make corrections. Let’s begin with our first question.

1- What is a heat pump?

The basic principle of the heat pump cycle, which is actually a refrigeration cycle, was introduced by Nicolas Léonard Sadi Carnot in 1824. 26 years later, in 1850, Lord Kelvin proposed that refrigeration devices could be used for heating purposes, which led to the application of heat pumps. Before World War II, many engineers and scientists conducted research and work to develop and implement the heat pump. However, during the war, the industry focused on more urgent problems, so these studies were put on hold and resumed after the war.

The potential of the heat pump industry in the 1950s diminished in the 1960s due to the high installation costs and the decreasing prices of natural gas and oil. After the 1973 energy crisis, the importance of heat pumps increased, and many studies were carried out thereafter.

Heat pumps are systems that make use of low-temperature heat from nature, making it usable for heating or cooling.

The operation of a heat pump is similar to that of the refrigerators we use in our homes. Heat pumps can be used either alone or with an additional system.

Low-temperature heat from natural sources like air, water, and soil is absorbed by a refrigerant, which turns it into vapor. This vapor is then sent to the compressor, where it is compressed to a high temperature and transferred to the distribution system.

A heat pump used for heating in the winter can also be used for cooling in the summer.

Heat pumps provide an energy saving of about 50-75% compared to fossil fuels. For more details, please click on the article titled "What is a Heat Pump?"

2- How does a heat pump work?

In this section, we want to explain how a heat pump works. A heat pump is a machine that transfers heat from a low-temperature environment to a high-temperature environment with the input of external energy. A heat pump used for heating in the winter can also be used for cooling in the summer.

The most important characteristic of a heat pump is its performance coefficient (COP). The COP value for an efficient system is typically 4, meaning that for every unit of energy input, 4 units of energy are produced. In Japan, COP values exceed 5, and the best heat pumps can achieve a COP of 6.8.

Cooling machines and heat pumps perform the same cycle, but their intended uses are different. The goal of a cooling machine is to keep a low-temperature environment by removing heat from the environment and lowering its temperature below the surrounding temperature. In contrast, the goal of a heat pump is to keep an environment warm. To achieve this, heat from a low-temperature thermal energy source is transferred to the environment that needs to be heated. The low-temperature heat source is typically cold air, well water, or soil, while the environment to be heated is the interior of a building.

In heat pump systems, the environments from which the evaporators draw heat are called "heat sources." The compatibility of these sources with the heat pump is crucial and depends on the following conditions:

• The source temperature should remain stable,
• The source temperature should be as high as possible,
• The source should be abundant and minimally affected by geographic conditions,
• The source should be clean and free from corrosion,
• The source should not cause corrosion.

The technical and economic performance of a heat pump depends on the characteristics of the heat source. For heat pumps used in buildings, the ideal heat source should have a high and stable temperature, be easily available, not carry corrosive or polluting factors, have suitable thermophysical properties, and have low investment and operating costs. In most cases, the availability of the heat source is the most important factor. The following sources can be used for heat pumps:

• Ambient air
• Soil
• Sea, river, and lake water
• Underground water
• Waste liquids
• Waste gases
• Waste heat
• Solar energy
• Rocks

Each of these has different characteristics.

• Ambient Air: Easily available and the most commonly used heat source for heat pumps. The seasonal performance factor (SPF) of air-source heat pumps is 10-30% lower than that of soil-source heat pumps. This is because the outdoor air temperature decreases, causing a large temperature difference in the evaporator, which leads to freezing and a drop in capacity and performance as energy is required to run the fans.
• Soil: A good source, but burying the heat exchanger in the soil requires high-quality materials to prevent corrosion, which increases the initial investment.
• Sea, River, Lake Water: Good sources for heat pumps, but rivers and lakes may freeze in winter, causing issues. Pollution is a concern.
• Underground Water: Has minimal temperature variation throughout the year, but using a pump to extract the water consumes additional energy.
• Waste Gases: A significant heat source for residential and commercial heat pumps. The heat pump uses heat extracted from ventilation to heat volumes and water.
• Waste Heat: May have advantages or disadvantages depending on the process.
• Solar Energy: A good source with high initial investment costs, but low maintenance and is environmentally clean.

Heat pumps can be used alone or with an additional system. Heat pumps that meet the heating needs by themselves are called "monovalent heat pumps," while those that meet the heating needs with the help of an additional source are called "bivalent heat pumps." In the bivalent setup, the heat pump covers 50-95% of the heating load. Examples of bivalent systems include solar collectors and boilers. These dual systems can operate either sequentially or together. Sequential operation means that one system switches on when the other is off. In cases where operating the heat pump is not economical, the heat pump is switched off, and the boiler is activated. Examples of systems that work together include heat pumps and solar collectors, where solar energy is used to provide the necessary temperature range for the heat pump to function.

3- Types of Heat Pumps

A heat pump can be considered as a reversed cycle of a heat machine. A heat machine transfers heat from a high-temperature environment to a low-temperature one, doing work in the process. A heat pump, on the other hand, uses external energy to transfer heat from a low-temperature heat source to a high-temperature environment.

Heat pumps used for heating in the winter can also be used for cooling in the summer. The transfer of heat from a cold heat source to a hot heat source can be done in various ways. Therefore, heat pump types include:

• Vapor Compression Heat Pump
• Absorption Heat Pump

The majority of heat pumps operate on the vapor compression cycle. The main components of a simple heat pump are the compressor, expansion valve, and two heat exchangers known as the evaporator and condenser.

The saturated vapor exiting the evaporator is compressed in the compressor to a higher pressure and temperature, turning it into superheated vapor. This vapor then enters the condenser, where it releases heat and condenses at a constant pressure. The high-pressure liquid refrigerant is then expanded through the expansion valve, bringing it to the evaporator conditions. The refrigerant enters the evaporator at a lower temperature than the heat source, where heat is transferred from the source to the refrigerant, causing it to evaporate. The cycle then repeats.

Hybrid Heat Pump: A hybrid heat pump combines a mechanical and an absorption heat pump. The operating principle is based on the fact that ammonia absorption in water occurs at much higher temperatures than ammonia condensation at a constant pressure.

More details on heat pump types can be found in our article on Heat Pump Types.

The information on the heat pump working principle on this page will be updated over time. Some of the content used in this article was sourced from Wikipedia.