In this case, the generated power and the thermoelectric conversion efficiency are measured with high accuracy [ 80 ]. The electrical signal generated by the heat-flux sensor is proportional to the heat flow rate applied to the sensor surface. The convective heat flow rate is measured from the temperature difference between two sides of a thermal resistive element plate placed across the flow of heat.
The heat radiated from the mass is absorbed by the infrared sensor IR and the temperature increase leads to the generation of the Seebeck voltage.
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The waste heat energy released into the environment is one of the most significant sources of clean, fuel-free and cheap energy available. The unfavourable effects of global warming can be diminished using the TEG system by harvesting waste heat from residential, industrial and commercial fields [ 36 ]. Different waste heat sources and temperature ranges for thermoelectric energy harvesting are shown in Table 2 [ 69 ]. Different waste heat sources and temperature ranges for thermoelectric harvesting technology.
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The automotive industry is considered as the most attractive sector in which TEGs are used to recover the lost heat. It has been demonstrated that vehicles the gasoline vehicle and hybrid electric vehicles have inefficient internal combustion engines. This can be observed in the Sankey diagram depicted in [ 83 ], which presents the energy flow direction of an internal combustion engine. In this case, the TEG technology could be an option to recuperate the waste heat energy for gasoline vehicles and hybrid electric vehicles.
A significant power conversion could be achieved by combining cooling system losses with the heat recovery from automobile exhausts.
The components where TEGs could be attached in a vehicle are the exhaust system and the radiators. In this case, the amount of waste heat is decreased and exhaust temperatures are reduced. These aspects require more efficiency from the TEG device. Furthermore, the design of such power conversion system takes into account various heat exchangers mounted on the TEG device. Important testing is helpful to confirm the reliability of TEG systems in automotive applications.
Furthermore, the design requires knowing the maximum electric output power and conversion efficiency from TEG systems [ 37 ]. The main components of the automotive TEG that considers waste heat like their energy source are one heat exchanger which takes heat from engine coolant and the exhaust gases and release it to the hot side of the TEG; the TEG system; one heat exchanger which takes the heat from the TEG and releases it to the coolant or to the air; the electrical power conditioning and the interface unit to supply the electric output power of the TEG system to the automobile electric system Figure 9.
Supplementary at these components, there are secondary components e.
Thacher et al. The experimental results showed good performance of the system at high speeds. Hsiao et al. The results showed better performance by attaching TEGs to the exhaust pipe than to the radiators.
Hsu et al. An application to recover waste heat has been developed by Hsu et al. Tian et al. They found that a segmented TEG is suitable for large temperature difference and a high-temperature heat source, and has a higher potential for waste heat recovery compared to the traditional device.
Meng et al. The results showed that the effects of the different properties and the heat loss to the environmental gas on performance are considerable. To recover waste heat from the exhaust gas of engines, the research efforts of manufacturers focused on different solutions to compete in the production of ever-cleaner cars. Even if the cost of the bismuth telluride is relatively high, the technical feasibility of TEGs for the automobile industry is widely demonstrated, making it very attractive.
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The goal of the manufacturers is to develop TEG systems with automated production and low-cost thermoelectric materials [ 29 ]. A considerable amount of heat is released into the atmosphere from space vehicles turbine engines from helicopters and aircraft jet engines [ 29 ]. To obtain a significant reduction of the gas pollutant into the environment, it is necessary a remarkable reduction of electricity consumption and utilisation of the available energy in these types of vehicles. Implicitly, their operating costs are reduced [ 25 ].
To power these space vehicles, TEG systems are used e. TEG for energy harvesting uses the available temperature gradient and collects sufficient energy to power up an energy wireless sensor node WSN to be autonomous. The main components of a WSN are the energy source and the wireless sensor unit. An in-depth review of WSN mechanisms and applications is presented in [ 10 ]. A TEG energy harvesting captures enough energy for a wireless sensor. One side of the TEG is fixed directly to the fuselage and the other side is attached to a phase-change material PCM heat storage unit to obtain a temperature difference during take-off and landing Figure In this case, the electrical energy is generated [ 94 ].edutoursport.com/libraries/2020-03-30/249.php
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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 produced during the PC, through latent heat [ 95 , 96 ]. An application of Bi 2 Te 3 modules on turbine nozzles has been addressed in [ 97 ]. Even though the electric power that can be harvested may be significant, the weight of the cold exchanger is still excessive for the specific application.
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 ]. The natural radioactive decay of plutonium releases huge amounts of heat, which is suitable for utilisation in RTGs to convert it into electricity.
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. The heat sources on the marine vessels are the main engine, lubrication oil cooler, an electrical generating unit, generator and incinerators.
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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 [ , ].
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 CO 2. For this reason, thermoelectric harvesters are good candidates to recover waste heat from industries and convert it into useful power e. Utilisation of TEGs in the industrial field is beneficial from two points of view: in the industrial applications where recoverability of the waste heat by the conventional system radiated heat energy is very difficult to be done;.
The results of a test carried out on a TEG system attached at a carburising furnace made of 16 Bi 2 Te 3 modules and a heat exchanger are indicated in [ ]. Aranguren et al. 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 To recover the radiant heat from melted metal from the steelmaking industry, the TEG systems are also considered good candidates [ 29 ].