3.2.2.2 Spacecraft applications

The radioisotope thermoelectric generators (RTGs) are a solid and highly reliable source of electrical energy to power space vehicles being capable of operating in vacuum and to resist at high vibrations [98, 99]. RTGs are used to power space vehicles for distant NASA space expeditions (e.g., several years or several decades) where sunlight is not enough to supply solar panels [29]. The natural radioactive decay of plutonium-238 releases huge amounts of heat, which is suitable for utilisation in RTGs to convert it into electricity. The thermoelectric materials used the thermocouples of the RTGs are adequate for high temperature considering that the heat source temperature is about 1000°C [100]. These semiconductor materials can be silicon germanium (Si Ge), lead tin telluride (PbSnTe), tellurides of antimony, germanium, and silver (TAGS) and lead telluride (PbTe).

### 3.2.3 Marine applications

Up to now, just a few surveys have been performed in the marine industry due to the lack of clear and stringent international rules at the global level. The marine transport has a significant influence on climate change because is a large amount of the greenhouse gas emissions [29]. The naval transport generates a wide amount of waste heat, used to provide thermal energy onboard and seldom electrical energy.

#### Thermoelectric Energy Harvesting: Basic Principles and Applications DOI: http://dx.doi.org/10.5772/intechopen.83495

The heat sources on the marine vessels are the main engine, lubrication oil cooler, an electrical generating unit, generator and incinerators. The utilisation of waste heat onboard is for heating heavy fuel oil and accommodation places, and for freshwater production. The main engine represents the principal source of waste heat. Board incinerators are used for burning the onboard waste instead to be thrown overboard to pollute the sea water. The incinerators are the most favourable TEG systems due to the availability of their high-temperature differences [101, 102].

The specialists' attention is focused on the future design and optimisation of the high-power density TEGs for the marine environment, as well as on the development of hybrid thermoelectric ships considered as green platforms for assessing the efficiency of TEGs [103].

### 3.2.4 Industrial applications

phase-change material (PCM) heat storage unit to obtain a temperature difference during take-off and landing (Figure 10). PCM is considered an essential element for the heat storage unit because it can maximise the ΔT of the TEG system to solve the low TEG conversion efficiency [93]. In this case, the electrical energy is generated [94]. Water is an adequate PCM for heat storage. The temperature difference across the TEG is obtained from the slow changing temperature of the heat storage unit and the rapidly changing temperature of the aircraft fuselage. A lot of energy is

An application of Bi2Te3 modules on turbine nozzles has been addressed in [97]. Even though the electric power that can be harvested may be significant, the weight

Future applications in aircraft may be envisioned in locations in which there are hot and cold heat flows, especially with the use of light thermoelectric materials. However, one of the main issues remains the weight of the heat exchangers [29].

The radioisotope thermoelectric generators (RTGs) are a solid and highly reliable source of electrical energy to power space vehicles being capable of operating in vacuum and to resist at high vibrations [98, 99]. RTGs are used to power space vehicles for distant NASA space expeditions (e.g., several years or several decades) where sunlight is not enough to supply solar panels [29]. The natural radioactive decay of plutonium-238 releases huge amounts of heat, which is suitable for utilisation in RTGs to convert it into electricity. The thermoelectric materials used the thermocouples of the RTGs are adequate for high temperature considering that the heat source temperature is about 1000°C [100]. These semiconductor materials can be silicon germanium (Si Ge), lead tin telluride (PbSnTe), tellurides of anti-

Up to now, just a few surveys have been performed in the marine industry due to the lack of clear and stringent international rules at the global level. The marine transport has a significant influence on climate change because is a large amount of the greenhouse gas emissions [29]. The naval transport generates a wide amount of waste heat, used to provide thermal energy onboard and seldom electrical energy.

produced during the PC, through latent heat [95, 96].

3.2.2.2 Spacecraft applications

Green Energy Advances

3.2.3 Marine applications

Figure 10.

22

Schematic of the thermoelectric harvesting system in an aircraft.

of the cold exchanger is still excessive for the specific application.

mony, germanium, and silver (TAGS) and lead telluride (PbTe).

The industry is the field where most amounts of heat are emitted and released into the atmosphere in the form of flue gases and radiant heat energy with a negative impact to the environmental pollution (emissions of CO2). For this reason, thermoelectric harvesters are good candidates to recover waste heat from industries and convert it into useful power (e.g., to supply small sensing electronic device in a plant).

Utilisation of TEGs in the industrial field is beneficial from two points of view:


The results of a test carried out on a TEG system attached at a carburising furnace (made of 16 Bi2Te3 modules and a heat exchanger) are indicated in [104]. The system harvested about 20% of the heat (P = 4 kW). The maximum electrical output power generated by TEG has been approximately 214 W, leading to thermoelectric conversion efficiency 5%. Aranguren et al. [105] built a TEG prototype. The TEG has been attached at the exhaust of a combustion chamber, with 48 modules connected in series and two different kinds of finned heat sinks, heat exchangers and heat pipes. This TEG was used to recover waste heat from the combustion chamber. In this case, the main objective has been to maximise the electric output power generated by the TEG.

For this reason, the dissipation systems have been used on both sides of TEG. This prototype has obtained a 21.56 W of net power using about 100 W/m<sup>2</sup> from the exhaust gases of the combustion chamber. To recover the radiant heat from melted metal from the steelmaking industry, the TEG systems are also considered good candidates [29].

Furthermore, TEGs are useful for recovery of waste heat from the cement rotary kiln to generate electricity, considering that the rotary kiln is the main equipment used for large-scale industrial cement production [106]. The performance of this hybrid Bi2Te3 and PbTe thermoelectric heat recovery system is obtained by developing a mathematical model. In this case, about 211 kW electrical output power and 3283 kW heat loss are saved by using a thermoelectric waste heat energy recovery system. The contribution of TEG is about 2%.

The electric output power evaluation of a TEG system attached to an industrial thermal oil heater is presented in Barma et al. [107]. The impact of different design and flow parameters has been assessed to maximise the electrical output power. The estimated annual electrical power generation from the proposed system was about 181,209 kWh. The thermal efficiency of the TEG based on recently developed thermoelectric materials (N-type hot forged Bi2Te3 and P-type (Bi,Sb)2Te3 used for the temperature range of 300–573 K) was enhanced up to 8.18%.

Considering that most of the stoves are used in the rural areas in remote locations away from the grid, where the income of the population is very low, the solutions with such hybrid systems (TEGs attached to stoves) must be as inexpensive as possible. These hybrid systems must be very reliable and durable, taking into account this vital problem [111]. In this case, a TEG device attached to the stove equipment used for heating and food preparation could be an attractive option. The biomass stoves-powered TEG uses heat exchangers on both sides of TEG, as well as a power management system. The internal temperatures of a stove are bigger than 600°C, while most commercial TEGs can operate continuously at temperatures higher than 250°C, so that an appropriate hot side temperature could be obtained. To obtain a maximum electric output power, the cold side temperature must be very low. In this case, the cold sink dissipates a big amount of heat maintaining its

Thermoelectric Energy Harvesting: Basic Principles and Applications

DOI: http://dx.doi.org/10.5772/intechopen.83495

According to the literature survey, various extensive reviews about stoveattached TEG systems have been presented [27, 29, 111–114]. A good option is the utilisation of the stoves using water as the cooling medium of the TEG to produce the maximum electric output power. The electrical output power decreases if the number of the TEGs is increased on the same heat sink. Such a hybrid system is suitable for complete combustion. The overall efficiency could be substantially

Solar TEG (STEG) systems, PV systems and concentrating solar power plants can generate electricity by using the solar heat. A STEG is composed of a TEG system sandwiched between a solar absorber and a heat sink as shown in Figure 11. The solar flux is absorbed by the solar absorber and concentrated into one point. Then, the heat is transferred through TEG by using a pipeline, and is partially

low temperature [112].

improved [111].

Figure 11.

25

Components of STEG system.

3.2.5.3 TEGs attached to the solar systems

### 3.2.5 Residential applications

In residential applications, TEGs can be feasible where the heat is transferred from high temperatures to a reduced temperature heat source, and then is released into the environment. In addition, TEG can be also feasible when thermal energy is accessible in high amounts without additional costs. Types of residential applications where TEG systems could be mounted are with TEGs attached to domestic boilers, TEGs attached to stoves, as well as TEGs attached to solar systems [27].

## 3.2.5.1 TEGs connected to the local heating boiler

The heating boilers for residential applications provide central heating and hot water. These heating boilers are highly used in the places where the winter season with temperatures under 0°C has a long duration and the heating is necessary for all this period. The fuels used by these heating boilers can be biomass (e.g., firewood, wood pellets, wood chips) or renewable resources that provide a lower carbon footprint compared to fossil fuels [108, 109]. In spite of higher pollution, some residential applications use fossil fuel-fired boilers (boilers supplied with natural gas) due to their low maintenance [27]. Furthermore, the fuel combustion is high, providing combustion temperatures bigger than 1000°C.

Instead, the heating boilers contain enhanced heat transfer surfaces due to the fins, and inside them combustion takes place at over 500°C. The heat obtained is used for water heating at temperatures fewer than 80°C. By considering that some thermoelectric materials are available for high temperatures, TEGs are very suitable for this type of equipment. In this case, high-temperature differences are obtained if the TEG cold side is mounted to the water heating side and the TEG hot side is mounted to the combustion chamber of the heat exchanger. The TEG system attached to the heating boiler must provide an electric output power P ¼ 30÷70W, as this boiler has to generate the power necessary to supply the auxiliary devices and the pumps of the heating system. These boilers are widely reviewed in [27].

#### 3.2.5.2 TEGs attached to the stove

At the global level, over than 14% of the population is still living without electricity access according to the Energy Access Outlook 2017 [110]. To deliver a small amount of electricity by using power plants to this population could be very costly. Grid connection of villages in remote areas supposes to take into account the cost of the connection of the new power lines to the grid and the distribution cost on long distances [29]. Also, traditional biomass stoves ('threestone fires', 'built-in stoves' or 'mud-stoves') have a reduced thermal efficiency due to incomplete fuel burning, and a lot of the heat generated is wasted through the exhausts. Furthermore, the indoor air quality is very poor related to the utilisation of biomass fuels with a negative impact on the users' health (e.g., lung diseases, respiratory tract infections, cardiac problems, stroke, eye diseases, tuberculosis and cancer) [111].

Thermoelectric Energy Harvesting: Basic Principles and Applications DOI: http://dx.doi.org/10.5772/intechopen.83495

The electric output power evaluation of a TEG system attached to an industrial thermal oil heater is presented in Barma et al. [107]. The impact of different design and flow parameters has been assessed to maximise the electrical output power. The estimated annual electrical power generation from the proposed system was about 181,209 kWh. The thermal efficiency of the TEG based on recently developed thermoelectric materials (N-type hot forged Bi2Te3 and P-type (Bi,Sb)2Te3 used for

In residential applications, TEGs can be feasible where the heat is transferred from high temperatures to a reduced temperature heat source, and then is released into the environment. In addition, TEG can be also feasible when thermal energy is accessible in high amounts without additional costs. Types of residential applications where TEG systems could be mounted are with TEGs attached to domestic boilers, TEGs attached to stoves, as well as TEGs attached to solar systems [27].

The heating boilers for residential applications provide central heating and hot water. These heating boilers are highly used in the places where the winter season with temperatures under 0°C has a long duration and the heating is necessary for all this period. The fuels used by these heating boilers can be biomass (e.g., firewood, wood pellets, wood chips) or renewable resources that provide a lower carbon footprint compared to fossil fuels [108, 109]. In spite of higher pollution, some residential applications use fossil fuel-fired boilers (boilers supplied with natural gas) due to their low maintenance [27]. Furthermore, the fuel combustion is high,

Instead, the heating boilers contain enhanced heat transfer surfaces due to the fins, and inside them combustion takes place at over 500°C. The heat obtained is used for water heating at temperatures fewer than 80°C. By considering that some thermoelectric materials are available for high temperatures, TEGs are very suitable for this type of equipment. In this case, high-temperature differences are obtained if the TEG cold side is mounted to the water heating side and the TEG hot side is mounted to the combustion chamber of the heat exchanger. The TEG system attached to the heating boiler must provide an electric output power P ¼ 30÷70W, as this boiler has to generate the power necessary to supply the auxiliary devices and

the pumps of the heating system. These boilers are widely reviewed in [27].

At the global level, over than 14% of the population is still living without electricity access according to the Energy Access Outlook 2017 [110]. To deliver a small amount of electricity by using power plants to this population could be very costly. Grid connection of villages in remote areas supposes to take into account the cost of the connection of the new power lines to the grid and the distribution cost on long distances [29]. Also, traditional biomass stoves ('threestone fires', 'built-in stoves' or 'mud-stoves') have a reduced thermal efficiency due to incomplete fuel burning, and a lot of the heat generated is wasted through the exhausts. Furthermore, the indoor air quality is very poor related to the utilisation of biomass fuels with a negative impact on the users' health (e.g., lung diseases, respiratory tract infections, cardiac problems, stroke, eye diseases, tuberculosis and cancer) [111].

the temperature range of 300–573 K) was enhanced up to 8.18%.

3.2.5 Residential applications

Green Energy Advances

3.2.5.1 TEGs connected to the local heating boiler

providing combustion temperatures bigger than 1000°C.

3.2.5.2 TEGs attached to the stove

24

Considering that most of the stoves are used in the rural areas in remote locations away from the grid, where the income of the population is very low, the solutions with such hybrid systems (TEGs attached to stoves) must be as inexpensive as possible. These hybrid systems must be very reliable and durable, taking into account this vital problem [111]. In this case, a TEG device attached to the stove equipment used for heating and food preparation could be an attractive option. The biomass stoves-powered TEG uses heat exchangers on both sides of TEG, as well as a power management system. The internal temperatures of a stove are bigger than 600°C, while most commercial TEGs can operate continuously at temperatures higher than 250°C, so that an appropriate hot side temperature could be obtained. To obtain a maximum electric output power, the cold side temperature must be very low. In this case, the cold sink dissipates a big amount of heat maintaining its low temperature [112].

According to the literature survey, various extensive reviews about stoveattached TEG systems have been presented [27, 29, 111–114]. A good option is the utilisation of the stoves using water as the cooling medium of the TEG to produce the maximum electric output power. The electrical output power decreases if the number of the TEGs is increased on the same heat sink. Such a hybrid system is suitable for complete combustion. The overall efficiency could be substantially improved [111].

#### 3.2.5.3 TEGs attached to the solar systems

Solar TEG (STEG) systems, PV systems and concentrating solar power plants can generate electricity by using the solar heat. A STEG is composed of a TEG system sandwiched between a solar absorber and a heat sink as shown in Figure 11. The solar flux is absorbed by the solar absorber and concentrated into one point. Then, the heat is transferred through TEG by using a pipeline, and is partially

Figure 11. Components of STEG system.

converted into electrical power by the TEG. A heat sink rejects the excess heat at the cold junction of the TEG to keep a proper ΔT across the TEG [115].

applications to enhance the efficiency of the systems for energy production with a higher power, the efficiency increase is still somewhat limited to consider that an investment in TEG integration may be profitable. Nevertheless, there is a growing interest in the potential of thermoelectric applications. For the future, faster development of TEG solutions can be expected in a broader range of green energy applications. This development depends on improvements in the TEG technology, better information on the TEG characteristics, and the testing of new solutions aimed at promoting better integration in the energy production systems.

Thermoelectric Energy Harvesting: Basic Principles and Applications

DOI: http://dx.doi.org/10.5772/intechopen.83495

Nomenclature

COP coefficient of performance

HMS health monitoring systems

MPPT maximum power point tracking

RTG radioisotope thermoelectric generator

STEG solar thermoelectric generator TEG thermoelectric generator WSN wireless sensor node

<sup>J</sup> relative current density (A � <sup>m</sup>�2) <sup>k</sup> thermal conductivity (W � <sup>m</sup>�<sup>1</sup> � <sup>K</sup>�<sup>1</sup>

n number of thermoelectric couples

C)

<sup>K</sup> thermal conductance (W � <sup>K</sup>�<sup>1</sup>

)

)

MPP maximum power point

PCM phase-change material

<sup>E</sup> electric field (V � <sup>m</sup>�1) I electrical current (AÞ <sup>J</sup> current density (A � <sup>m</sup>�2)

L length (m) m resistance ratio

P electric power (W) Q\_ heat flow rate (W) R resistance (Ω) S cross-section (m2) t temperature (°

T absolute temperature (K) ΔT temperature difference (K)

DC direct current ECG electrocardiography EEG electroencephalography EMG electromyography

IR infrared sensor

PANI polyaniline PA polyamide

PC phase-change

RF radio-frequency

PEDOT poly

Symbols

27

Acronyms

Due to the development of the thermoelectric materials, a solar TEG with an incident flux of 100 kW/m<sup>2</sup> and a hot side temperature of 1000°C could obtain 15.9% conversion efficiency. The solar TEG is very attractive for standalone power conversion. The efficiency of a solar TEG depends on both the efficiency with which sunlight is absorbed and converted into heat, and the TEG efficiency η TEG. Furthermore, the total efficiency of a solar TEG is also influenced by the heat lost from the surface. The efficiency of solar TEG systems is relatively small due to the low Carnot efficiency provoked by the reduced temperature difference across the TEG and the reduced ZT [116]. Its improvement needs to rise temperature differences and to develop new materials with high ZT like nanostructured and complex bulk materials (e.g., a device with ZT = 2 and a temperature of 1500°C would lead to obtain a conversion efficiency about 30.6%) [117]. According to the literature survey, both residential and commercial applications gain much more interest in the regions of incident solar radiation of solar TEGs. This can be explained by the fact that most of the heat released at the cold side of the TEG can be used for domestic hot water and space heating [115].

#### 3.2.6 Grid integration of TEG

Most TEG applications have been designed for autonomous operation within a local system. Of course, the TEG output may be connected to different types of loads. In general, a TEG can be seen as a renewable energy power generation source that supplies an autonomous system or a grid-connected system. To be suitable for grid connection, the TEG needs an appropriate power conditioning system. This power conditioning system has to be a power electronic system, with specific regulation capabilities, different with respect to the ones used for solar photovoltaic and wind power systems [114], because the TEG operating conditions are different with respect to the other renewable energy sources. Molina et al. [118] proposed a control strategy to perform energy conversion from DC to AC output voltage, which maintains the operation of the thermoelectric device at the MPP. In the same proposal, active and reactive power controls are addressed by using a dedicated power conditioning system.

### 4. Conclusions

This chapter has addressed the structures and applications of TEGs in various contexts. It has emerged that the TEG is a viable solution for energy harvesting, able to supply electrical loads in relatively low-power applications. The TEG efficiency is also typically low. Thereby, the advantages of using TEG have to be found in the characteristics of specific applications in which there is a significantly hightemperature difference across the TEG system, and other solutions with higher efficiency cannot be applied because of various limitations. These limitations may be the relatively high temperatures for the materials adopted, the strict requirements on the system to be used (regarding the type of operation, emissions of pollutants, the position of the device during operation or noise). In these cases, TEGs may be fully competitive with the other solutions.

In particular, the use of TEGs is entirely consistent with the provision of green energy through energy harvesting from even small temperature differences. Some low-power applications have been identified on electronic circuits, sensors, waste heat recovery, residential energy harvesting and automotive systems. In other

Thermoelectric Energy Harvesting: Basic Principles and Applications DOI: http://dx.doi.org/10.5772/intechopen.83495

applications to enhance the efficiency of the systems for energy production with a higher power, the efficiency increase is still somewhat limited to consider that an investment in TEG integration may be profitable. Nevertheless, there is a growing interest in the potential of thermoelectric applications. For the future, faster development of TEG solutions can be expected in a broader range of green energy applications. This development depends on improvements in the TEG technology, better information on the TEG characteristics, and the testing of new solutions aimed at promoting better integration in the energy production systems.
