Heat Pumps Classification

Naturally, heat flows from high-temperature to lower-temperature area. Natural heat transfer can be either conduction, convection, or radiation. Whereas, to be able to move heat from low-temperature to the higher, it requires special effort so as to combat natural heat transfer. To be able to do this effort required a special tool called heat pump.

Heat pump is a tool for moving heat energy from a heat source, to a destination medium termed “heat sink.” Heat pumps are designed to absorb heat energy from cold-temperature chambers, and throw them into hotter spaces. This tool will require an external power supply so it can move energy from a heat source to a heat sink.

One of the heat pumps that are familiar to us is air conditioning (AirCond). However, AirCond is only one example of heat pump applications that are used quite often we meet. In addition to air conditioning there are several other types of known heat pumps. The classification of heat pumps is based on different working principles. Here are the following:

Mechanical Refrigerator

The mechanical refrigerator or also known as a mechanical heat pump, uses the properties of a volatile and condensable special fluid. The special fluid as we know as refrigerant, is made to perform a cycle so that it can absorb heat in cold air, and throw it in hot air. Refrigerant is compressed to make it hotter in hot air areas, and refrigerant pressure is released to absorb heat in cold air environments.

The picture above is a simple cycle of the refrigerator system. A refrigerator system is composed of four main components: compressor, condenser, expansion valve (also called check valve, or metering device), and an evaporator. The refrigerator cycle begins with the inclusion of saturated vapor refrigerant-phase to the inlet of the compressor. Through the compressor, the refrigerant compressed to increase the pressure, while also make the temperature increased.

Thermodynamically, these hot and compressed refrigerants enter its superheater phase. This condition is a good time to remove the heat contained in the refrigerant to the cooling medium such as air or water. The heat dissipation from the refrigerant can occur not only because of fan, also because the superheater refrigerant temperature is hotter than the temperature of the cooling medium. This heat dissipation process occurs in the condenser.

One of the unique properties of the refrigerant is volatile, which means it has a low boiling point, as well as its high dew point. his property makes the refrigerant easy to reach the superheater phase when compressed, and immediately turns its phase to liquid after experiencing cooling process in the condenser. Thermodynamically, the liquid phase of this refrigerant is called liquid saturation phase. Furthermore, this liquid refrigerant will pass through an expansion valve so that the pressure drops suddenly. Decreasing the pressure of the refrigerant fluid will lead to change its phase adiabatically to steam again. This pressure drop will be followed by temperature decrease  so that it reaches a cooler temperature than the air to be cooled.

The cold refrigerant fluid, then enter the evaporator. A fan will circulate the hot air so it can pass through the evaporator grille. In this component there is heat transfer from hot air to cold refrigerant. This process will make the air circulation to cool, this air is needed in the refrigerator process to cool the room. While the fluid refrigerant will become hotter so that it returns to saturated vapor. To meet the refrigerator engine cycle, the saturated vapor of this refrigerant will re-enter the compressor and continue to cycle the system.

The mechanical refrigerator cooling system became the most common and most widely used. Starting from the refrigerator, air conditioning, air conditioning in the vehicle, or even if the cycle is used upside down, it can also be used to warm the room at the winter.

Magnetic Refrigerator

Magnetic refrigerator is a cooling technology use magnetocaloric effects. The magnetocaloric effect is a magneto-thermodynamic phenomenon in which a temperature change in a particular material occurs as a result of being exposed by changes in magnetic field values. Simply put, a material will rise in temperature when it is placed in higher magnetic fields. Similarly, the temperature of the material will also decrease when the magnetic field strength is lowered. But one thing to note is that these two concepts must occur in an isolated chamber so there is no absorption or heat release, an adiabatic process is a must. So that the increase or decrease of material temperature due to the magnetic field change is happening naturally.

Similar to a mechanical refrigerator, the magnetic refrigerator process also occurs cyclically. There are four cycles in the magnetic refrigerator process: adiabatic magnetization, isomagnetic enthalpies transfer, adiabatic demagnetization, and isomagnetic entropy transfer.

  1. Adiabatic Magnetization
    A magnetocaloric material, which acts as a refrigerant, is placed in an isolated chamber. Furthermore, the chamber is subjected to a magnetic field so that the atoms refrigerant material repositioned. This condition results in magnetic entropy and the material’s heat capacity being reduced. Because the system is isolated so that there is no heat transfer (adiabatic), and the total value of the entropy is not reduced, the temperature of the refrigerant material will naturally increase.
  2. Isomagnetic Enthalpy Transfer
    The second condition, with a fixed system induced by an unchanged magnetic field (isomagnetic), the heat contained by the refrigerant is cooled by the cooling fluid. The cooling fluid can be either gas or liquid. The cooling medium commonly used in this system is helium gas. The magnitude of the fixed magnetic field remains to prevent the refrigerant atoms from being re-positioned to their original position when the heat is absorbed by the cooling fluid. After the heat of the refrigerant is fully absorbed by the cooling fluid, the coolant fluid is subsequently removed from the system.
  3. Adiabatic Demagnetization
    Furthermore, the refrigerant material is re-insulated to keep no heat coming out or entering the system (adiabatic). In such conditions, the magnetic field value is lowered so that the thermal energy of the material creates a magnetic moment to overcome the change of magnetic field. This process is what makes the refrigerant temperature down. The energy transfer (and entropy) occurs from the thermal entropy to the magnetic entropy, indicating a change in the magnetic field of the system.
  4. Isomagnetic Entropy Transfer
    The next cycle process is to keep the magnetic field of the system unchanged (isomagnetic), to keep the refrigerant material unheated. Then made thermal contact between the environment or air to be cooled, with refrigerant material that is in cold conditions. Since the refrigerant temperature is cooler than the air the environment wants to cool, there is heat transfer from the hot air to the refrigerant. This process will lower the air temperature so that there is a temperature equilibrium between the two. Next, the cycle process starts at the beginning.The magnetic refrigerator technique is well suited to produce very cold environmental temperatures. Compared with conventional mechanical refrigerators, magnetic refrigerators tend to be safer, less noisy, simpler, higher cooling efficiency, and certainly environmentally friendly because they do not use harmful refrigerant materials such as CFCs in the process of mechanical refrigerators that damage ozone.

    An example of a material that has a high magnetocaloric effect and is suitable for use as a refrigerant, is gadolinium (Gd) material and its compound mixtures. The gadolinium temperature rises as it enters a certain magnetic field, and the temperature will drop too if it leaves the magnetic field. The gadolinium compounds commonly used for refrigerants include Gd5Si2Ge2, Gd5 (SixGe1-x) 4, as well as a mixture of Gd-Bi-Sb (Gadolinium-Bismuth-Antimony) composition.

Thermo-electric Refrigerator

The thermo-electric cooling system is incorporated into a solid material heat engine using the Peltier effect (also known as thermo-electric effect) to create a heat flow between two different materials. This thermo-electric effect transfers heat from one side of the material to the other, by consuming electrical energy.

Two semiconductor materials with different electron densities are used as the main components of the thermo-electric heat engine. Two materials called the n-type and p-type are arranged in parallel, but are interconnected in series in an electrical circuit. Both of them then flanked by two material heat conductor on each side. When the voltage is applied to the free end of each semiconductor, there will be DC current flow across the system, and result in a temperature difference. One side of the plate will be cooler than the surrounding area so it will absorb the heat. The heat is absorbed for the next flow to another side plate that is hotter than the surrounding room, so that heat can be discharged to the heat sink.

The thermoelectric heat has a lower thermal efficiency than mechanical refrigerator. The thermo-electric system is only capable of achieving thermal efficiency of 10-15% only. Far enough when compared with conventional mechanical coolers that can achieve efficiencies up to 40-60%. Thermo-electric refrigerator is only capable of producing a temperature differential of 70 degrees only. Materials commonly used as a thermo-electric semiconductors include bismuth telluride, lead telluride, silicon germanium, and a mixture of bismuth-antimony.

Termo-acoustic Refrigerator

The last known model of heat pump is to use sound. Slightly odd indeed, but the sound that comes out of a speaker can be used to cool the room temperature if treated with the right way and tools. Thermo-acoustic refrigerator uses sound properties to be able to compress and stretch the air. Through this principle, the compressed air will rise in temperature, while the pressure dropped air will also fall in temperature. So if a proper system is made by using this phenomenon, an inverse Brayton cycle can be made.

The main components of thermo-acoustic refrigerators include sound sources (speakers), hot and cold heat exchangers flanking a regenerator or stack (a section composed of small parallel spaces), as well as a resonator chamber. The components are arranged in a long tube room with the speakers at one end. When the stack is placed at the correct distance inside the resonator, a temperature difference will be created on both sides of the stack. If both sides of the stack are paired with heat exchangers, then heat transfer will be created.

The inverse thermodynamic cycle of Brayton from this cooling machine is:

Adiabatic compressed gas. The air will rise in pressure at the same time the temperature along with the sound frequency resonance that occurs. The temperature in this room is now higher than the temperature of the heat exchanger metal pile.

Isobaric heat transfer. The heat will move from the compressed air to the heat exchanger plate. The process of heat transfer occurs under no pressure changes.

Adiabatic expansion. Stack will expand the air so that the pressure back down. This pressure drop will simultaneously lower the air temperature so that it is lower than the plate temperature.

Isobaric heat transfer. After the air temperature drops, heat exchanger heat will move into the air without any change of pressure, so that the air temperature will return to room temperature.

References: Wikipedia: Heat Pump, Wikipedia: Magnetic Refrigeration, Wikipedia: Thermoelectric Cooling, Wikipedia: Thermoacoustic Hot Air Engine