3. Methods to enhance the TEC performance in refrigeration units

Some methods to enhance the TEC performance are [35]:


current, there is insufficient current to obtain ΔTmax. Working above the maximum current, the power dissipation inside the TEC starts to rise the device temperature and to decrease ΔT. The

<sup>I</sup>max <sup>¼</sup> <sup>α</sup>NP∙ð Þ Th � <sup>Δ</sup>Tmax <sup>∙</sup>R�<sup>1</sup> (40)

cmax for a TEC is the maximum thermal load obtained when

<sup>∙</sup>ð Þ <sup>2</sup><sup>R</sup> �<sup>1</sup> (41)

Vmax ¼ αNP∙Th (42)

(43)

maximum current is almost constant which is the operating range of the device:

<sup>Q</sup>\_ <sup>c</sup>max <sup>¼</sup> <sup>α</sup>NP

cooling capacity, divided by the input electrical power:

<sup>2</sup> T<sup>2</sup>

<sup>h</sup> � <sup>Δ</sup>T<sup>2</sup> max

The maximum voltage represents the DC voltage which gives ΔTmax at I = Imax. In this case COP has a minimum value. At maximum voltage the power dissipation inside the TEC starts to rise the device temperature and to decrease ΔT. The maximum voltage depends on the

The coefficient of performance (COP) represents the heat absorbed at the cold junction or

COP <sup>¼</sup> <sup>Q</sup>\_ <sup>c</sup> Pel

Various papers explain the COP dependence of the characteristics of the materials on the Thomson effect and on temperature. The TEC performance is improved by raising the figure of merit of the thermoelectric elements and considering the Thomson effect [29]. The validity of the Thomson effect is taken into account in the relationships of the cooling capacity and input electrical power and implicitly in the COP relationship, if the dependence on temperature of Seebeck coefficient is considered [30]. In this case the Thomson effect gives a reduction with about 7.1% for the input electrical power and with about 7% for the cooling capacity considering the positive values of the Thomson coefficient; instead, an improvement in both the input electrical power and the cooling capacity is observed for negative values of the Thomson

The COP is also influenced by the thermal and electrical contact resistances. The COP of the thermoelectric module can be improved up to 60% by decreasing the electrical contact resistances [27]. Furthermore, the COP depends on the thermoelement length. The COP rises with the increment of the thermoelement length. For a thermoelement with a shorter length, the contact resistance becomes closer to the resistance of the thermoelement, notably affecting this

The maximum COP (indicated as COPmax) of a TEC is used for its sizing [9, 20]. The COPmax has the benefit of minimum input electrical power, therefore, minimum total heat to be rejected

The maximum cooling capacity Q\_

ΔT = 0 and I=Imax:

234 Bringing Thermoelectricity into Reality

temperature:

coefficient [31].

indicator [27].

by the heat sink: Q\_

<sup>h</sup> <sup>¼</sup> <sup>Q</sup>\_

<sup>c</sup> þ Pel:


#### 3.1. Development of thermoelectric materials with high performance

A thermoelectric refrigerator unit operates with COP typically less 0.5 due to the limited cooling temperature to ΔTmax ffi 20 K under the ambient temperature [20]. Figure 6 shows a comparison of the theoretical COP of a TEC with respect to household refrigerators [36]. Refrigerators with thermoelectric modules with materials based on alloys of Bi2Te3 have a COP about 1 [9] which is low enough to be competitive to the vapour-compression systems with COP = 2 ÷ 4 [37–39]. The low COP values of TECs are not considered a drawback. These systems are more suitable for a niche market sector (below 25 W) such as military and medical industries, in applications such as temperature stabilization of semiconductor lasers and vaccine cooling. Furthermore, they are also suitable for the civil market (e.g., portable refrigeration, car-seat cooler, high-quality beverage conservators). For these applications, the thermoelectric elements have the advantages that do not suffer vibrations and shocks [21, 40–42].

3.3. Thermal design

temperature T<sup>c</sup> of the TEC.

during the phase change, so that Q\_

Thermal design involves the determination of the heat sink geometry considering the thermal

The heat sink located on the hot side is useful to dissipate heat from the TEC system to the environment and is considered an important factor affecting the TEC performance. Therefore, to enhance the TEC performance, the heat sink must have a low thermal resistance (to be minimized). Important TEC efficiency improvements are obtained by optimization of the different types of heat exchangers at the hot side (water-air system with a cold plate, pump

A normal heat sink uses fins to increase heat transfer surface. When the thermal resistance of the heat sink is computed, it is necessary to take into account an additional heat thermal resistance of the thermal grease applied to provide a good thermal contact between TEC and heat sink [45]. The design of the heat sinks is presented in [4], where some aspects useful to

Sometimes at the hot side of TEC, thermal storage using phase change materials (PCMs) or a heat pipe heat exchanger may take the place of a heat sink with fins in order to reduce the

For thermal storage, the heat sink is designed to have a high storage capacity to keep the sink temperature less than the junction temperature. In this case, PCMs are useful to improve the performance of the thermoelectric refrigerator. These high-energy-density materials have the advantage that the heat is transferred at constant temperature. In this case, T<sup>c</sup> is constant

refrigerator, the utilization of a heat sink with fins at the cold side of the TEC supposes a fast T<sup>c</sup> reduction until ΔTmax of the cooler is obtained, while using PCMs a slow reduction of the cold side temperature till ΔTmax is obtained. Furthermore, PCMs operate in a wide range of phase change temperatures, providing different alternatives to be used at the hot side and at the cold side of the TEC [46]. These materials are very suitable for different types of thermoelectric refrigerators for food (domestic refrigerators; refrigerators/freezers; hotel room minibar refrigerators preferred for their silent operation; refrigerators for mobile homes, trucks, recreational vehicles, and cars; food service refrigerators for airborne application; and portable picnic coolers) as well as for medicine storage which need precise temperature control [8, 46]. Refrigerators based on PCMs exhibit useful storage capacity behaviour in case of blackout, as they are able to limit the temperature variation during a blackout much more than other materials. The heat pipes are heat exchangers with very high thermal conductivity using ethanol or methanol as refrigerant water. They are used on the both sides of the TEC to dissipate both the cooling and waste heat to the heat sinks. If heat pipes with a thermosiphon are used at the hot side of the TEC, the waste heat is rejected to the environment by natural or forced convection. These systems have a low thermal resistance, leading to reduction of the temperature differential between the hot side temperature of the TEC and the environmental temperature. The heat pipes are used at the cold side of TEC to keep T<sup>c</sup> constant during peak and

<sup>c</sup> and COP remain constant as well. For a conventional

Thermoelectric Refrigeration Principles http://dx.doi.org/10.5772/intechopen.75439 237

resistances and the optimization of the heat sink characteristic.

and fan coil, finned heat sink with fan, heat pipe with fan) [44].

find the optimal heat sink geometry (fin thickness and position) are detailed.

Figure 6. Chart of COP vs. temperature ratio for different refrigerators.

#### 3.2. TEC design

TEC design involves the choice of number of thermocouples and thermoelement length and is carried out by looking at the module parameters. For example, for domestic refrigeration the thermoelement length optimization is strongly linked to COP, cooling capacity and material consumption. To improve COP and cooling capacity, the contact resistances, especially thermal contact resistance, must be reduced.

In devices with long thermoelements, the COP is high, and the contact resistances have a little effect on the cooling capacity, while in devices with short thermoelements, the contact resistances have an increased influence on the cooling capacity; in particular, starting from a long thermoelement and reducing its length, the cooling capacity increases up to a maximum value; then it decreases sharply [20].

As mentioned above, the thermal performance of a TEC depends on the thermoelectric material properties which change with the TEC operating temperature. Some manufacturers use the maximum design parameters and the performance chart [43]. The performance chart takes ΔT and Q\_ <sup>c</sup> as inputs and determines the current and thus the voltage needed to produce the cooling effect. Another possibility is to consider voltage, current and ΔT as known quantities (e.g. measured) in order to find Q\_ c. A key aspect of the use of the TEC performance chart is that the module parameters are considered to be known and unchanged for different devices, while actually these parameters change because of the outcomes of the manufacturing process. The design procedure illustrated in [43] is simplified by considering the thermal resistance of the heat sink as one of the key parameters, avoiding the heat transfer analysis of the heat sink.

#### 3.3. Thermal design

3.2. TEC design

236 Bringing Thermoelectricity into Reality

contact resistance, must be reduced.

Figure 6. Chart of COP vs. temperature ratio for different refrigerators.

then it decreases sharply [20].

(e.g. measured) in order to find Q\_

ΔT and Q\_

TEC design involves the choice of number of thermocouples and thermoelement length and is carried out by looking at the module parameters. For example, for domestic refrigeration the thermoelement length optimization is strongly linked to COP, cooling capacity and material consumption. To improve COP and cooling capacity, the contact resistances, especially thermal

In devices with long thermoelements, the COP is high, and the contact resistances have a little effect on the cooling capacity, while in devices with short thermoelements, the contact resistances have an increased influence on the cooling capacity; in particular, starting from a long thermoelement and reducing its length, the cooling capacity increases up to a maximum value;

As mentioned above, the thermal performance of a TEC depends on the thermoelectric material properties which change with the TEC operating temperature. Some manufacturers use the maximum design parameters and the performance chart [43]. The performance chart takes

cooling effect. Another possibility is to consider voltage, current and ΔT as known quantities

the module parameters are considered to be known and unchanged for different devices, while actually these parameters change because of the outcomes of the manufacturing process. The design procedure illustrated in [43] is simplified by considering the thermal resistance of the heat sink as one of the key parameters, avoiding the heat transfer analysis of the heat sink.

<sup>c</sup> as inputs and determines the current and thus the voltage needed to produce the

c. A key aspect of the use of the TEC performance chart is that

Thermal design involves the determination of the heat sink geometry considering the thermal resistances and the optimization of the heat sink characteristic.

The heat sink located on the hot side is useful to dissipate heat from the TEC system to the environment and is considered an important factor affecting the TEC performance. Therefore, to enhance the TEC performance, the heat sink must have a low thermal resistance (to be minimized). Important TEC efficiency improvements are obtained by optimization of the different types of heat exchangers at the hot side (water-air system with a cold plate, pump and fan coil, finned heat sink with fan, heat pipe with fan) [44].

A normal heat sink uses fins to increase heat transfer surface. When the thermal resistance of the heat sink is computed, it is necessary to take into account an additional heat thermal resistance of the thermal grease applied to provide a good thermal contact between TEC and heat sink [45]. The design of the heat sinks is presented in [4], where some aspects useful to find the optimal heat sink geometry (fin thickness and position) are detailed.

Sometimes at the hot side of TEC, thermal storage using phase change materials (PCMs) or a heat pipe heat exchanger may take the place of a heat sink with fins in order to reduce the temperature T<sup>c</sup> of the TEC.

For thermal storage, the heat sink is designed to have a high storage capacity to keep the sink temperature less than the junction temperature. In this case, PCMs are useful to improve the performance of the thermoelectric refrigerator. These high-energy-density materials have the advantage that the heat is transferred at constant temperature. In this case, T<sup>c</sup> is constant during the phase change, so that Q\_ <sup>c</sup> and COP remain constant as well. For a conventional refrigerator, the utilization of a heat sink with fins at the cold side of the TEC supposes a fast T<sup>c</sup> reduction until ΔTmax of the cooler is obtained, while using PCMs a slow reduction of the cold side temperature till ΔTmax is obtained. Furthermore, PCMs operate in a wide range of phase change temperatures, providing different alternatives to be used at the hot side and at the cold side of the TEC [46]. These materials are very suitable for different types of thermoelectric refrigerators for food (domestic refrigerators; refrigerators/freezers; hotel room minibar refrigerators preferred for their silent operation; refrigerators for mobile homes, trucks, recreational vehicles, and cars; food service refrigerators for airborne application; and portable picnic coolers) as well as for medicine storage which need precise temperature control [8, 46]. Refrigerators based on PCMs exhibit useful storage capacity behaviour in case of blackout, as they are able to limit the temperature variation during a blackout much more than other materials.

The heat pipes are heat exchangers with very high thermal conductivity using ethanol or methanol as refrigerant water. They are used on the both sides of the TEC to dissipate both the cooling and waste heat to the heat sinks. If heat pipes with a thermosiphon are used at the hot side of the TEC, the waste heat is rejected to the environment by natural or forced convection. These systems have a low thermal resistance, leading to reduction of the temperature differential between the hot side temperature of the TEC and the environmental temperature. The heat pipes are used at the cold side of TEC to keep T<sup>c</sup> constant during peak and off-electricity times [46, 47]. Two prototypes of thermoelectric refrigerator are described in [48], one with finned heat sink and the other one with a finned heat sink integrated in an aluminum thermosiphon in which phase change occurs. The thermosiphon depends on the specific latent heat at the phase change from vapour state to liquid state, useful to disperse the heat efficiently to the environment. The results of the experimental heat sink optimization demonstrated that the thermal resistance between the hot side of the TEC and the environment reduced with about 23.8% at 293 K environment temperature and 51.4% at 308 K, with respect to a commercial finned heat sink, and between 13.8% and 45% with respect to an optimized finned heat sink. Much more, the COP of this prototype is 26% at ambient temperature of 293 K, achieving 36.5% improvement at 303 K.

control systems at a maximum voltage decreases the electric power consumption about 40% and raises the COP near the maximum value obtained with the control system [11]. In addition, in [49] a reduction of electric power consumption was obtained by experimental optimization of the temperature controller for a thermoelectric refrigerator in stationary state. Their work experimentally demonstrated that the on/off temperature control system used in commercial thermoelectric refrigerators is not so efficient, but is used due to its simplicity and low cost. More efficient temperature control systems applied by the manufacturers to increase the performance of the thermoelectric refrigerator include the use of different voltage supply levels for the modules or the exploitation of proportional-integral-differential control systems. However, these control systems have a higher cost with respect to the on/off control system [11]. A more elaborated idling voltage control system was proposed from experimental optimization in [49], obtaining a 32% reduction of the electric power consumption and a COP growth of 64% compared with the normal on/off temperature control system. In this way, in the long term, the savings due to lower consumption compensate for the higher cost of the idling voltage control

Thermoelectric Refrigeration Principles http://dx.doi.org/10.5772/intechopen.75439 239

In spite of their relatively low efficiency with respect to other refrigeration technologies, the TEC technologies are experiencing a period of development, with subsequent efficiency improvement and reduction of the manufacturing costs [52]. One of the drivers that have increased the interest in the development and use of TECs as refrigerators is the absence of environmental pollution in the TEC operation, in particular, the absence of chlorofluorocarbon (CFC) issues. The current trends towards replacement of CFCs consider good solutions with low global warming potential (GWP) using natural refrigerants like CO2 used at pressures much higher than traditional refrigerants [6, 53]. Further drivers to increase the TEC applications depend on positive aspects of the TECs such as low noise, possibility of operation in different positions, absence of mechanical vibrations, ease of transportation and possibility to

Today, thermoelectric refrigerators are the most significant applications at the commercial level [17, 40]. In addition to domestic refrigerators [10, 54, 60], other applications have been developed for food-related services, such as portable refrigerators [55–57], food expositors, refrigerators mounted on vehicles for perishable food transportation as well as low-power refrigerators for minibar, hotel room, offices, boats and aircraft services [8]. Further applications are available for the medical sector (vaccine transportation and instruments for blood coagulators, dew point sensors and others), for the military sector and for scientific devices subject to precise temperature control [58]. In addition, thermoelectric systems are found in the automobile industry for air conditioning or car-seat coolers [59] and in different applications to

Besides the applications mentioned above, the present trend towards the use of green energy raises the attention on the possibility of supplying the thermoelectric refrigerator through

system with respect to the cost of the on/off control system.

4. Thermoelectric refrigeration unit applications

obtain accurate temperature control.

the microelectronics sector [60, 61].

#### 3.4. Optimization of the internal temperature controller of the insulated compartment

The operating conditions of a thermoelectric refrigerator depend on parameters as environment temperature, humidity, lower setpoint of the internal temperature and difference between higher and lower setpoints of the internal temperature [49].

In vapour-compression refrigerators, the internal temperature control inside the insulated compartment is generally inaccurate due to the multitude of start and stop cycles made by the compressor, leading to a temperature variation bigger than 8C, with a negative effect on the quality of the food and on the conservation of the perishable food [50, 51]. This represents a drawback for these refrigerators compared with the thermoelectric refrigerators in which there are no start and stop cycles and the supply voltage gradually increases. However, overall the thermoelectric refrigerators are not competitive with vapour-compression refrigerators in terms of COP [10, 11, 48].

Most of the thermoelectric refrigerators have on/off control systems for the internal temperature. This control is critical in the period in which the TEC is switched off, because in this period, the heat stored in the heat sink connected to the hot terminal returns into the refrigerator compartment; in this way, the power consumption of the refrigerator increases and the COP decreases [11].

Vian and Astrain [50] carried out a study of the total power consumption on a hybrid thermoelectric system (with vapour freezer and refrigerator compartments and a thermoelectric compartment) at ambient temperature of 25C. To optimize the system, a thermal bridge (aluminium slab) was used between the freezer compartment and the thermoelectric compartment. This thermal bridge was useful to transfer the heat flow rate from the thermoelectric compartment to the freezer in order to maintain a constant temperature of 0C inside the thermoelectric compartment. The power consumption in these environmental conditions for each compartment was 0.67 kWh/day for the refrigerator, 0.58 kWh/day for the freezer and 0.2 kWh/day for the thermoelectric system. The results demonstrated that the total electric power consumption reduced from 63.3 W to 49.9 W (20% improvement) due to the thermal bridge. If the environmental conditions are modified (e.g., a rise of the temperature at 30C), the total power consumption for this unit rises by 30%. Further studies showed that the thermoelectric refrigerator works in any operating condition, but the utilization of on/off control systems at a maximum voltage decreases the electric power consumption about 40% and raises the COP near the maximum value obtained with the control system [11]. In addition, in [49] a reduction of electric power consumption was obtained by experimental optimization of the temperature controller for a thermoelectric refrigerator in stationary state. Their work experimentally demonstrated that the on/off temperature control system used in commercial thermoelectric refrigerators is not so efficient, but is used due to its simplicity and low cost. More efficient temperature control systems applied by the manufacturers to increase the performance of the thermoelectric refrigerator include the use of different voltage supply levels for the modules or the exploitation of proportional-integral-differential control systems. However, these control systems have a higher cost with respect to the on/off control system [11]. A more elaborated idling voltage control system was proposed from experimental optimization in [49], obtaining a 32% reduction of the electric power consumption and a COP growth of 64% compared with the normal on/off temperature control system. In this way, in the long term, the savings due to lower consumption compensate for the higher cost of the idling voltage control system with respect to the cost of the on/off control system.
