Thermoelectric heat pumps use the thermoelectric effect, specifically the Peltier effect, to heat or cool materials by applying an electrical current across them. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler (TEC) and occasionally a thermoelectric battery. It can be used either for heating or for cooling, although in practice the main application is cooling since heating can be achieved with simpler devices (with Joule heating).
Thermoelectric temperature control heats or cools materials by applying an electrical current across them. A typical Peltier cell absorbs heat on one side and produces heat on the other.L. E. (2008). Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science, 321(5895), 1457-1461. http://engin1000.pbworks.com/f/TE_rev.pdf Because of this, Peltier cells can be used for temperature control. However, the use of this effect for air conditioning on a large scale (for homes or commercial buildings) is rare due to its low efficiency and high cost relative to other options.
Thermoelectric cooling This technology is far less commonly applied to refrigeration than vapor-compression refrigeration is. The primary advantages of a Peltier cooler compared to a vapor-compression refrigerator are its lack of moving parts or circulating liquid, very long life, invulnerability to leaks, small size, and flexible shape. Its main disadvantages are high cost for a given cooling capacity and poor power efficiency (a low coefficient of performance or COP). Many researchers and companies are trying to develop Peltier coolers that are cheap and efficient.
Operating principle
Thermoelectric coolers operate by the Peltier effect (one of three phenomena that make up the thermoelectric effect). A thermoelectric module is made from three components: the conductors, the legs, and the substrate. Many of these modules are connected electrically in series, but thermally in parallel. When a DC electric current flows through the device, it brings heat from one side to the other, so that one side gets colder while the other gets hotter.
The hot side is attached to a heat sink to limit its temperature increase, while the cold side goes below the ambient temperature. In special applications, multiple coolers can be cascaded or staged together for lower temperature, but overall efficiency (COP) drops significantly. The maximum COP of any refrigeration cycle is ultimately limited by the difference in temperature between the hot and cold side. The higher the temperature difference, the lower the maximum theoretical COP. Both temperatures depend on the rate of heat transfer to or away from the device as well as heat movement driven within.
A typical Peltier cell based heat pump can be used by coupling the thermoelectric generators with photovoltaic air cooled panels.Alexandra Thedeby, Heating and Cooling with Solar Powered Peltier Elements, Ms. Thesis, Department of Energy Planning, Division of Efficient Energy Systems, Faculty of Engineering, Lund University
http://www.ees.energy.lth.se/fileadmin/ees/Publikationer/2014/Ex5308-AlexandraThedeby-HeatingAndCoolingWithSolarPoweredPeltierElements....pdf Considering the system with an air plant that ensures the possibility of heating on one side and cooling on the other.Martín-Gómez, C., Ibáñez-Puy, M., Bermejo-Busto, J., Sacristán Fernández, J. A., Ramos, J. C., & Rivas, A. (2016). Thermoelectric cooling heating unit prototype. Building Services Engineering Research and Technology, 37(4), 431-449. By changing the configuration it allows both winter and summer acclimatization.Yilmazoglu, M. Z. (2016). Experimental and numerical investigation of a prototype thermoelectric heating and cooling unit. Energy and Buildings, 113, 51-60. These elements are expected to be an effective element for zero-energy buildings, if coupled with solar thermal energy and photovoltaicLiu, Z., Zhang, L., Gong, G., Li, H., & Tang, G. (2015). Review of solar thermoelectric cooling technologies for use in zero energy buildings. Energy and Buildings, 102, 207-216. with particular reference to create radiant heat pumps on the walls of a building.Khire, R. A., Messac, A., & Van Dessel, S. (2005). Design of thermoelectric heat pump unit for active building envelope systems. International Journal of Heat and Mass Transfer, 48(19-20), 4028-4040. https://messac.expressions.syr.edu/wp-content/uploads/2012/05/Messac_2005_JHMT_ABE.pdf
This acclimatization method ensures the ideal efficiency during summer cooling if coupled with a photovoltaic (PV) generator. The air circulation could be also used to cool PV modules.
The most important engineering requirement is the accurate design of heat sinksTrancossi, M., Pascoa, J. (2020). Design of ventilated cross flow heat sinks. Modelling, Measurement and Control C, Vol. 79, No. 3, pp. 90-97. https://doi.org/10.18280/mmc_c.790305 to optimize the heat exchange and minimize the fluiddynamic losses.
Construction
Design Two unique semiconductors, one n-type and one p-type, are used because they need to have different electron densities. The alternating p & n-type semiconductor pillars are placed thermally in parallel to each other and electrically in series and then joined with a thermally conducting plate on each side, usually ceramic, removing the need for a separate insulator. When a voltage is applied to the free ends of the two semiconductors there is a flow of DC current across the junction of the semiconductors, causing a temperature difference. The side with the cooling plate absorbs heat which is then transported by the semiconductor to the other side of the device.
The cooling ability of the total unit is then proportional to the total cross section of all the pillars, which are often connected in series electrically to reduce the current needed to practical levels. The length of the pillars is a balance between longer pillars, which will have a greater thermal resistance between the sides and allow a lower temperature to be reached but produce more resistive heating, and shorter pillars, which will have a greater electrical efficiency but let more heat leak from the hot to cold side by thermal conduction. For large temperature differences, longer pillars are far less efficient than stacking separate, progressively larger modules; the modules get larger as each layer must remove both the heat moved by the above layer and the waste heat of the layer.
Materials
Requirements for thermoelectric materials: Narrow band-gap semiconductors because of room-temperature operation; High electrical conductivity (to reduce electrical resistance, a source of waste heat); Low thermal conductivity (so that heat doesn't come back from the hot side to the cool side); this usually translates to heavy elements Large unit cell, complex structure; Highly anisotropic or highly symmetric; Complex compositions.
Materials suitable for high efficiency TEC systems must have a combination of low thermal conductivity and high electrical conductivity. The combined effect of different material combinations is commonly compared using a figure of merit known as ZT, a measure of the system's efficiency. The equation for ZT is given below, where is the Seebeck coefficient, is the electrical conductivity and is the thermal conductivity.
There are few materials that are suitable for TEC applications since the relationship between thermal and electrical conductivity is usually a positive correlation. Improvements in reduced thermal transport with increased electrical conductivity are an active area of material science research. Common thermoelectric materials used as semiconductors include bismuth telluride, lead telluride, silicon–germanium, and bismuth antimonide alloys. Of these, bismuth telluride is the most commonly used. New high-performance materials for thermoelectric cooling are being actively researched.
For decades, narrow bandgap semiconductors, such as bismuth, tellurium and their compounds, have been used as materials of thermocouples.
Identification and characteristics
The vast majority of thermoelectric coolers have an ID printed on the cooled side. These universal IDs indicate the size, number of stages, number of couples, and current rating in amps, as seen in the adjacent diagram.
For example, the common thermoelectric element TEC1-12706 has a square shape of 40 mm size and 3–4 mm high, and are found for a few dollars. It is able to move around 60 W or generate a 60 °C temperature difference with a 6 A current. Their electrical resistance is about 1–2 ohm.
Strengths and weaknesses
There are many factors motivating further research on TEC including lower carbon emissions and ease of manufacturing. However, several challenges have arisen.
Benefits A significant benefit of TEC systems is that they have no moving parts. This lack of mechanical wear and reduced instances of failure due to fatigue and fracture from mechanical vibration and stress increases the lifespan of the system and lowers the maintenance requirements. Current technologies show the mean time between failures (MTBF) to exceed 100,000 hours at ambient temperatures. The condition of a thermoelectric electric can be measured by the change of their AC resistance (ACR); as it wears out, the ACR will increase.
The fact that TEC systems are current-controlled leads to another series of benefits. Because the flow of heat is directly proportional to the applied DC current, heat may be added or removed with accurate control of the direction and amount of electric current. In contrast to methods that use resistive heating or cooling methods that involve gases, TEC allows for an equal degree of control over the flow of heat (both in and out of a system under control). Because of this precise bidirectional heat flow control, temperatures of controlled systems can be precise to fractions of a degree, often reaching precision of milli Kelvin (mK) in laboratory settings.
TEC devices are also more flexible in shape than their more traditional counterparts. They can be used in environments with less space or more severe conditions than a conventional refrigerator. The ability to tailor their geometry allows for the delivery of precise cooling to very small areas. These factors make them a common choice in scientific and engineering applications with demanding requirements where cost and absolute energy efficiency are not primary concerns.
Another benefit of TEC is that it does not use refrigerants in its operation. Prior to their phaseout some early refrigerants, such as chlorofluorocarbons (CFCs), contributed significantly to ozone depletion. Many refrigerants used today also have significant environmental impact with global warming potential or carry other safety risks with them.
Disadvantages TEC systems have a number of notable disadvantages. Foremost is their limited energy efficiency compared to conventional vapor-compression systems and the constraints on both the temperature difference and the total heat flux (heat flow) that they are able to generate per unit area. This topic is further discussed in the performance section below. In practice, the mean working life of a low cost thermoelectric module is less than 10 years typically because gasses diffuse through the elastomeric perimeter seal of the module, or the seal bond line fails at the plate.
Performance Peltier (thermoelectric) performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameters.