- What Is a Heat Pump?
A heat pump is a technology that transfers heat in air or liquid from a lower temperature to a higher temperature. Heat pumps are used in air conditioners, refrigerators, and eco-cutes, which are indispensable in our daily lives.
Although heat pumps consume electricity to transfer heat, they are expected to be highly energy efficient because they generate more heat energy than they consume. In addition, compared to gas or oil combustion methods, CO2 emissions can be significantly reduced, making heat pumps an environmentally friendly technology that is also attracting attention.
Uses of Heat Pumps
Heat pumps used to be mainly used in refrigerators and air conditioners for cooling things. However, advances in technology have expanded the range of applications from low to high temperatures, and heat pumps are now used in a variety of fields, including heating and hot water supply.
In the home, they are used in home appliances essential to daily life, such as refrigerators, air conditioners, and washers/dryers, as well as in energy-efficient equipment such as eco-cutes and floor heating systems. They are also used in commercial air conditioners and water heaters in facilities with many users, such as offices and hospitals, and are expected to reduce utility costs significantly.
Principle of Heat Pumps
When a gas is compressed, its temperature rises; conversely, when it is expanded, its temperature falls. Heat pumps utilize this property to transfer heat. Refrigerants such as chlorofluorocarbons, which transfer heat, can effectively transfer heat by repeatedly liquefying and vaporizing through compression and expansion at temperatures close to room temperature.
The structure of a heat pump consists of a compressor, an expansion valve, two heat exchangers called an evaporator, a condenser, and piping that connects them.
Refrigerant is a medium that transfers thermal energy. CFC gas is mainly used, which repeatedly evaporates and condenses depending on pressure and temperature, changing into gas or liquid.
An expansion valve is a device that rapidly expands the high-temperature, high-pressure CFC gas to a warm, low-pressure state and then turns it into a liquid again.
A compressor is a device that compresses CFC gas to high temperatures and high pressure. Centrifugal compressors and reciprocating compressors are available.
Heat exchangers are divided into two types based on their role: evaporators and condensers. The evaporator absorbs heat from the outside and converts the CFC gas into a gas, while the condenser converts the gas into a liquid and releases the heat to the outside.
The refrigerant absorbs heat in the evaporator, converts it to gas, and is absorbed by the compressor. The gas, compressed to a high temperature and pressure, is sent to the condenser to become a liquid. It is then converted to a low temperature and pressure by an expansion valve and returned to the evaporator. Heat pumps transfer heat from the low-temperature to the high-temperature part of the air by repeating these cycles.
Types of Heat Pumps
Heat pumps are classified according to the principle of heat transfer as follows:
Heat Pumps that use heat generation and heat absorption of refrigerant
Vapor compression heat pumps, absorption heat pumps, and adsorption heat pumps use the heat of vaporization and condensation generated when the refrigerant vaporizes. Heat pumps that utilize the heat of vaporization of ammonia are mainly used in refrigerators and freezers.
Heat Pumps utilizing heat other than air heat
Geothermal, water-source, and solar heat are used in heat pumps. In all cases, the heat source must be nearby, but the heat can be propagated more efficiently than air heat.
Heat Pump using lattice vibration
This heat pump uses semiconductors. By passing an electric current through a thermoelectric element, lattice motion is generated in the element. This lattice motion enables heat transfer and fine temperature control. For this reason, they are mainly used in medical devices and experimental equipment that require precise temperature control. However, the high performance of this heat pump makes it an expensive heat pump.
Other Heat Pumps
Recently, heat pumps that utilize both thermoelectricity and vaporization heat have appeared. As you can see, new heat pump technologies are being developed every year, making it possible to capture and store heat more efficiently.
Performance Indicators for Heat Pumps
The performance of a heat pump is expressed as the ratio of the cooling or heating capacity (kW) it can produce to the power consumption (kW). This is called the energy consumption efficiency COP (Coefficient Of Performance). The higher this value is, the more energy-saving effects can be expected. In air conditioners in particular, COP is used as an indicator of the energy-saving capability of air conditioners as cooling COP and heating COP.
However, COP indicates energy consumption efficiency under a certain temperature environment, and when air conditioners are used, performance varies depending on the room and outside temperature. Therefore, APF (Annual Performance Factor) is now the mainstream energy-saving standard, and unlike COP, APF is defined as an indicator of energy saving instead of COP in the “Energy Conservation Law” revised in September 2006. Unlike COP, APF indicates the operating efficiency after one year of operation. Therefore, it can be said that APF shows an operating efficiency closer to actual operation.
Advantages and Disadvantages of Heat Pumps
The following are the merits and demerits of heat pumps, a technology that efficiently collects heat from the outside and uses it as a large heat source.
Advantages
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Saves Electricity
By gathering heat from the outside, there is no need to create new heat, so electronic devices equipped with heat pumps have relatively low electricity bills.
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High Safety
Since no combustion is involved in the generation of heat, it is safer.
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Reduced Carbon Dioxide Emissions
Since there is no combustion process, carbon dioxide emissions are relatively low.
Disadvantages
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Easily Affected by the External Environment
Since heat pumps collect heat from the outside, they are less efficient when the outside temperature is low.
Effective Use of Heat Pumps
Heat pumps are excellent for energy and cost savings because they can produce cooling and heating capacities that exceed their power consumption, but they have the disadvantage of being easily affected by the external environment. However, heat pumps can extract large amounts of energy from small temperature differences. More energy savings can be expected by using them more effectively.
In a heat pump, one side is heated and the other side is cooled by heat transfer. Normally, either one or the other is used, but if a system that can use both heating and cooling at the same time is constructed, it is possible to create a greater energy-saving effect.
Another effective way to use heat pumps is to use underground heat as a heat source. Compared to the outside air, the temperature of underground heat changes little throughout the year, so the temperature of the ground and groundwater is cooler in summer and warmer in winter. By using this method, unused underground heat can be effectively utilized and CO2 emissions can be greatly reduced.
In addition, the greatest feature of heat pumps is that they can maximize the use of thermal energy that is difficult to use as it is, such as exhaust heat from factories and hot spring water.