Thermoelectric energy conversion in buildings

consume almost one-third of the total global energy and emit nearly 15% of the direct CO 2 generated on the planet. However, intelligent buildings taking advantage of modern energy ef ﬁ cient technologies have attracted large interest in recent years. In this regard, the application of various energy harvesters to convert different forms of energy present inside and around buildings into electrical energy has been widely investigated. These include photovoltaic, piezoelectric, pyroelectric, electromagnetic, and thermoelectric devices. Among them, thermoelectric generators (TEGs), capable of producing electricity directly from a temperature gradient, have demonstrated great potential. This review paper categorizes and explains plausible applications of thermoelectric materials and devices in buildings. In particular, state-of-the-art cement- and concrete-based thermoelectric composites and the potential applications of TEGs in windows, walls, pipes


Introduction
Economic and social growth require massive energy consumption, which is primarily supplied by non-renewable fossil fuels [1]. However, over half of the industrial energy utilization is wasted as heat [2]. Buildings also have a large share in global energy consumption and produce almost one third of the waste heat in the world [3]. A building's energy consumption is mainly related to its thermal management systems and electronic devices. Different types of sensors, actuators, and Internet-of-Things (IoT) devices are increasingly incorporated into various research areas and applications including smart and green buildings [4e6]. The principle of IoT is to provide communication between devices (things) without a need for human interference. A traditional IoT system consists of numerous communicating nodes that monitor specific parameters, such as temperature, presence, light intensity, and traffic density. Further development of IoT technology is envisaged to lead to billions of devices being installed in smart buildings over the next few decades. A typical IoT node uses 0.86 kW/h of energy annually and, considering the globally predicted number of IoT nodes for the next few decades (tens of billions), the IoT's overall energy consumption could exceed 8.6 TW.h/year, putting it on a par with other largescale energy uses [7,8]. But how this new technology will be powered remains an open question.
Most such electronic devices require a low electric voltage and rely on batteries as their main power source. Batteries are currently the only feasible solution for powering independent embedded systems. However, they have limitations in term of miniaturization and well-known environmental downsides if proper disposal is not available. As an alternative to batteries, self-powered integrated systems can produce their own electrical energy from the surrounding environment, such as the ambient waste heat harvested by thermoelectric generators (TEGs). This could bring multiple advantages such as low cost, easy installation, topological flexibility, and capability of gathering and storing data [8,9].
Many technologies have been proposed recently for generating electricity from renewable energy sources, and unused or waste energy sources in buildings and integrating them with selfpowered smart devices and sensors [10e14]. For instance, extensive research has been performed on using photovoltaic devices in buildings for generating electricity from solar energy [15,16]. On the other hand, using TE cooling and heating devices in buildings that work based on the Peltier effect have also attracted more attention in recent years. Several review papers have been published to explain the integration of TE cooling and heating systems in the building envelope, the use of individual TE cooling and heating devices in buildings, the design parameters of these systems, the use of photovoltaic devices for powering TE cooling and heating systems, and the effect of these systems on occupant thermal comfort [17e19]. Another effective solution is electricity generation from waste thermal energy, as buildings often contain numerous heat sources and stores in different temperature ranges. TEGs can convert a temperature gradient into electrical power and are highly reliable and durable, eco-friendly, and require no maintenance [20].
Many review papers have been published explaining different aspects of using photovoltaic devices as well as the use of TE cooling and heating systems in building. In recent years, research on TE energy harvesting in buildings, mostly based on advanced cement and concrete, has become a topic of considerable interest. However, very few publications have presented a thorough analysis of the fundamentals, materials, devices, and applications of the TEGs in buildings [21]. In this regard, the present article provides a comprehensive review on the application of TEGs in building stock, covering materials, devices, and application points of view. Major advances in TE energy harvesting research in buildings are presented, starting with an introduction to the fundamentals of thermoelectricity and different types of TE materials. Then, recently emerged TEGs for application in various building parts, such as windows, walls, chimneys and pipes, are discussed, analyzing materials, fabrication methods, and the advantages and challenges in each case. Next, case studies of using TEGs in building for powering electronic device are demonstrated. Finally, practical solutions for current challenges in TE energy harvesting in buildings are outlined and a roadmap for future research to advance this field is provided.

TE materials and devices
The TE effect directly converts a temperature difference into electrical voltage and vice versa [22], as shown in Fig. 1a. Two materials, A and B, are orientated such that one of their sides is kept at a low-temperature, T 1 , and the other at a high-temperature, T 2 . The materials are connected electrically on the hot side and the voltage across them is monitored on the cold side. The temperature difference will cause the heat to flow by charge carriers (electrons and holes) and by lattice vibrations (phonons) [22]. In each material, the motion of charged particles will cause space charges at the contacts of both materials, which will, in turn, create an electric field opposing the motion, until equilibrium is reached.
The heat-to-electricity conversion efficiency of a TE material is closely related to a dimensionless figure of merit (ZT) as presented by Eq. (1) [23]: where ZT is typically assessed at the mean temperature of the hot side temperature (T H ) and the cold side temperature (T C ). From this equation, it is obvious that a rise in ZT contributes to an improvement in energy conversion capacity. Depending on how the TE legs are arranged on the substrate with regard to the direction of heat flow, there are three different design approaches comprising planar, vertical, and mixed configurations for TEGs [27]. In planar TEGs, as shown in Fig. 1b, there is a lateral heat flow from the hot side to the cold side [28]. In vertical TEGs, there is a vertical heat flow along the TEGs (Fig. 1c) [29]. In the mixed TEGs, both lateral and vertical heat flows are present (Fig. 1d). Table 1 shows the features of these TEGs configurations. TE materials can be classified into organic and inorganic groups. Inorganic materials include chalcogenides, skutterudite, half-Heusler, Zintl, alloy, and oxides. Organic TE materials include conducting polymers [30], coordination polymers, carbon-based materials, small molecules, and single molecules These materials have temperature-dependent properties, therefore, there is a specific temperature range where each materials shows the best conversion efficiency [31,32]. Fig. 2a, b shows a summary of the ZT of different TE material categories vs. temperature. For near-roomtemperature applications, refrigeration and waste heat recovery up to 500 K, Bi 2 Te 3 alloys have been proved to possess the greatest ZT for both n-and p-type TE materials. In addition to reducing lattice thermal conductivity, alloying Bi 2 Te 3 with Sb 2 Te 3 and Bi 2 Se 3 allows for optimum electrical conductivity and Seebeck coefficient levels. The most commonly studied p-type composition is Bi 0.5 Sb 1.5 Te 3 [33], whereas for n-type composition, it is Bi 2.0 Te 2.7 Se 0.3 [34,35]. Peak ZT values for these alloys are typically in the range of 1.5e2.3. Bi 2 Te 3 alloys are the best room-temperature TE materials that have been largely explored since the 1950s. However, one of the main reasons limiting their wide applications is the scarcity of the tellurium (Te) element. In the last decade, some new Te-free TE candidates with high ZT, such as Mg 3 Sb 2 /MgAgSb [36] and Mg 3 Bi 2 [37], have shown potential for replacing Bi 2 Te 3 . In addition to these materials, quenched GeTe-based alloys have recently shown an average ZT value of 1.1 around room temperature, which is even superior to the state-of-the-art commercial Bi 0.5 Sb 1.5 Te 3 ingots [38].
Additionally, transparent TE materials are excellent candidates to use on standalone energy sources placed on glass windows of building and cars or on the displays of PCs or smart phones [48e50]. TE materials for these applications have to satisfy the following conditions: first, they must be producible as films with high ZT values; second, these films must have a high optical transmittance in the visible range; third, the constituent materials should be non-toxic and inexpensive. For instance, conventional heavy metal TE materials, such as Bi 2 Te 3 and PbTe, are not suitable for the global energy harvesting applications because they are expensive and toxic. Conversely, copper iodide is an environment friendly material, composed of non-toxic and naturally abundant elements which have a ZT ¼ 0.21 at room temperature [51]. Transparent oxide TE materials such as SnO 2 [52], InGaZnO [49], poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS)/indium tin oxide-PEDOT: PSS [48], and GZO [50] are other candidates that have transmittance over 80% in the visible range and show a good TE performance.
TEGs are used for energy harvesting in various applications such as wearable devices, health monitoring and tracking systems, automobile engines, industrial electronic devices, self-powered wireless platforms, and aerospace [53,54]. According to the  United Nations Environmental Program, buildings are responsible for 40% of the total global energy consumption [55]. Applying TEGs in buildings emerges as a significant alternative energy source to fossil fuels, with many advantages. For instance, TEGs have a long operative life without considerable changes in their performance over time. Therefore, they are highly reliable and have low maintenance requirements, reducing the whole life cost of the buildings. In the following sections, the application of TEGs and TE materials in different parts of buildings are discussed. For instance, TEGs have a long operative life without considerable changes in their performance over time. Therefore, they are highly reliable and have low maintenance requirements, reducing the whole life cost of the buildings. In the following sections, the application of TEGs and TE materials in different parts of buildings are discussed in detail.

TE energy harvesting in buildings
As mentioned in the previous sections, a TEG is an effective device to harvest the waste thermal energy in buildings. In this section, the implementation methods of TEGs and TE materials in different parts of a building will be discussed, including in windows, walls, pipes, chimneys, as well as cement and concrete.

TEGs in windows
Integration of energy harvesting systems onto windows can provide renewable on-site energy supply without altering the building aesthetics or imposing further design constraints. In this section, embedding TEGs onto windows to generate electricity from a temperature difference between the outdoor and indoor environments is discussed. Window glass serves as the interface between the hot (or cold) side on the exterior and the cold (or hot) side on the interior side. As a result, it can be used in both hot and cold climate conditions. In the former case, the exterior and interior sides of the window act only as the hot and cold sides, respectively. Here, the TEGs act as a solar thermal energy harvester, as shown in Fig. 3. The latter case normally happens in Winter time, when the temperature inside of the building is higher than that of outside. The TEGs used in these cases should preferably be transparent or at least semi-transparent.
Transparent TEGs for integration into building windows are normally in the form of thin films because the substrate material and the TE materials need to be transparent to retain an almost complete transparency of the windows. Traditional TEGs cannot be simply applied to window glass in this case due to their nontransparent nature. In this regard, Chen et al. [63] prepared an innovative transparent TEG on a glass substrate with many suspending bridge-type polysilicon TE elements using surface micromachining of microelectromechanical systems technology. Fig. 4aed shows a schematic structure of a transparent TEG device. A glass substrate with conductive indium tin oxide for bottom and top electrodes and p-and n-type suspending bridge-type polysilicon TE elements between the electrodes were used in this TEG. p-and n-type TE elements showed a high Seebeck coefficient of 460.2 mV/K and À87.7 mV/K, respectively. The simulation of the transparent TEG by ANSYS software with 3 Â 5 TE couplings is shown in Fig. 4eef. The temperature difference between the hot and cold sides is 10 K, indicating that when the suspending spacing is 3 mm, the voltage difference is around 4.56 mV. The theoretical calculations showed that each 60 cm Â 90 cm glass window with a polysilicon-based transparent mTEG array can generate an electrical power of 0.05e0.1 W at a temperature difference of 5 Ke10 K between the two window surfaces [70].   [47] Brian et al. [71] installed 12 TEGs, a CE8301 DC-DC boost converter for amplifying and regulating output voltage, a type-A USB connector, and a power bank inside an aluminum frame of the window, as shown in Fig. 4gej. Between the hours of 1:00 p.m. and 3:00 p.m., the hot side received heat from the sun's light while the air from the fully air-conditioned room cooled the cold side. The generated voltage was fed into the 5 V DC-DC booster with a type-A USB connector, which produced a more consistent and improved voltage output. Their results showed that the highest temperature difference of 17 K delivered the highest voltage output of 2e2.5 V and a maximum output power of 0.35 W [71].
Transparent TEGs are promising systems for thermal energy harvesting from building windows, but achieving high transparency comes at the cost of low power-conversion efficiency. Since high performance TEGs are normally based on nontransparent inorganic materials, a tradeoff between the TEG's transparency and performance should be considered for applications in building windows [72e74]. Semi-transparent TEG configurations use bulk TE materials with high figure-of-merits (ZT) around room temperature [75]. Inayat et al. [69] integrated nanomanufactured pellets of TE nanomaterials comprising Bi 1.75 Te 3.25 (n-type legs) and Sb 2 Te 3 (p-type legs) alloyed with sulfur into window glasses, as shown in Fig. 5a, b. The fabricated TEG with 4 pellets delivered 0.112 mW of power at a temperature gradient of 23.5 K. Fig. 5c, d illustrates the fabricated TEG integrated into a glass substrate and corresponding output power. In another work [59], they fabricated a 12 Â 6 array of TEGs made of Bi 0.4 Sb 1.6 Te 3 (p-type legs) and Bi 1.75 Te 3.25 (n-type legs) on a 132.25 cm 2 plexiglass panel. The TE materials were ball-milled followed by hot pressing to make a cylindrical shape with a diameter of 5 mm. The panel was 6 mm thick, which is similar to single-pane glass used in normal windows. The fabricated panel generated an electrical power of 0.18 mW at a temperature difference of 25 K. Fig. 5eeh shows a scanning electron microscopy (SEM) of p-and n-type TE materials, the fabricated TEGS, and the output power. Recently, Zhang et al. [76] introduced a structure with 12 Bi 2 Te 3 -based TEGs that could be embedded on a window and the output voltage reached up to $ 4 V within an area of 0.01 m 2 exposed to sunshine (Fig. 5i, l). A film built from Cs 0.33 WO 3 powders and resin with wavelength-selective absorption was merged onto a totally transparent glass, allowing visible light to pass through while absorbing and converting as much ultraviolet and near infrared light as possible (Fig. 5j, k).
Transparent organic TEGs have also been mounted onto glass substrates and used in building windows. Shakeel et al. [77] employed PEDOT: PSS and a highly conductive silver paste onto a glass substrate for printing a TEG. Fig. 5m,n shows a schematic of the printing process and the printed TEG. The fabricated TEG had an output power of 5 nW at 393 K. Metal oxides are also a good class of semiconductor material that can be used for transparent TEGs. In this context, Klochko et al. [78] used electrodeposited arrays of vertically aligned ZnO nanorods on transparent fluorine-doped tin oxide substrates to form a semi-transparent TEG. The SEM micrograph of the ZnO film, the device structure schematic, and the fabricated film are illustrated in Fig. 5o,p.
Another approach to develop TEGs for windows is based on radiative cooling, which can use the temperature difference caused by passive cooling via an atmospheric window [79]. This approach uses the maximized emission of infrared thermal radiation by the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation [80]. As shown in Fig. 6a, b, Xia et al. [81] described a type of TEG based on a space cold source that can continually output electricity 24 h a day, regardless of the availability of any natural energy resource. The radiative cooler, the heat sink, and the TE legs are the three primary components of the designed TEG. When the TEG is placed outside, the radiative cooler constantly transfers heat to outer space, and its temperature drops below ambient. At the same time, the copper heat sink maintains a temperature that is similar to the ambient temperature. The TE legs are connected at both ends to the cooler and the heat sink, resulting in a temperature difference between them. Bi 2 Te 3 and Sb 2 Te 3 TE films were screen-printed on the surface of kraft papers. A maximum power factor of 2.5 mW/m/K 2 and 1.6 mW/m/K 2 were obtained for Bi 2 Te 3 and Sb 2 Te 3 , respectively.
Furthermore, Inayat and Hussein [82] reported a rational design of TEGs into the side wall of Plexiglas strips. A larger Plexiglas panel was made out of smaller strips of the Plexiglas joined and cured together and the complimentary TE materials (Bi 2 Te 3 and Sb 2 Te 3 ) were sputtered on the inner sidewalls of the individual strips. This technique was successfully implemented for two different types of layouts including checkerboard (Fig. 6c) and stripe (Fig. 6d) based on the layout type used for TE material deposition. The maximum output power for checkerboard and strip devices is around 10 nW (DT ¼ 21 K) and 15 nW (DT ¼ 38 K), respectively.
In short, building windows have a huge potential for embedding TEGs in the near future to harvest the temperature difference across them and produce electricity for buildings.

TEGs in walls
The use of TE devices in walls was demonstrated for the first time in 1995 in a US patent entitled 'Superinsulation Panel with Thermoelectric Device and Method' [85]. These devices can be TE coolers (TECs), TE heaters (TEHs), or TEGs. When TEC and TEH devices are embedded onto a wall, they can cool down and heat up the wall surface, respectively. This allows electrical energy management in the building during summers and winters by effectively controlling the heat gain and loss. It has been reported that the energy efficiency of the system can be 4.8 times higher than that of a normal wall [86]. The conceptual configuration is made up of a large-scale TEC or TEH and a heat sink, as depicted in Fig. 7a [87]. The inside surface of the wall comprises a TEC or TEH. The outside surface of the wall acts as a heat sink to improve the heat dissipation. Fig. 7b, c shows one of the experimental platforms created in a laboratory. This kind of wall can not only decrease heat gain or loss in comparison with traditional walls but can also supply a certain amount of indoor cooling or heating capacity by altering the applied current, that is, active control of the wall temperature. In the case of TEGs, a conceptual design of a TE wall that can generate electricity for electric gadgets inside the building is shown in Fig. 7d. Byon and Jeong [88] reported a TEG-based passive energy harvesting system integrated with a phase change material that is designed to harvest energy from waste heat on the exterior walls of buildings. The TEG used in this study was a commercial HMN-6055 unit. Also, a phase change material was used to cool the cold side of the TEG made of paraffin wax (Parafol 17e97). Parafol 17e97 was used as the heat sink because it has a large heat capacity (200e220 kJ/kg) and can be used for longer periods than other candidates such as fatty acids [89]. As shown in Fig. 7e, f, this structure can be used both in day and night time. Experimental results indicated that 2.1 kWh/m 2 of electricity could be generated via this system over a year.
In another approach, Yun et al. [90] investigated the viability of employing TEGs to generate electricity from temperature gradients across vacuum insulation panels (VIPs) to power sensors. VIPs have recently shown promise in improving residential thermal efficiency and lowering the heating and cooling energy expenditures, but at high initial financial outlay. A vacuum is supported by a porous core that is surrounded by an impermeable barrier material in VIPs. Fig. 7g, h shows an experimental setup of a TEG inside an airconditioned apartment window to monitor electricity generation. Open circuit voltage, interior and exterior temperatures, and power generated at the highest power point were monitored throughout each test. An average output power of 28.7 mW was achieved over a summer day, which was enough to charge a capacitor and power a sensor node.

Cement-and concrete-based TEGs
Concrete is one of the most important components of civil infrastructure. Cement-based materials have been used as the binder in concrete for a variety of applications including building,  [70], (g) TEGs setup integrated on a residential window frame and frame with booster connected to power bank, (h) schematic of the front view of the window frame (unit of dimension in millimeter), (i) front view of complete TEG window frame, and (j) installed window frame [71]. TEG, thermoelectric energy generator. roads, dams, and bridges [91]. The concrete-based structures in buildings are subjected to various energy sources, such as solar, thermal, and mechanical forces, as shown in Fig. 8a [92]. As a result, the use of energy harvesting systems based on cement and concrete has attracted extensive attention to provide electrical energy for buildings recently. As shown in Fig. 8b, TEGs can be integrated into the concrete for converting the temperature difference between the outside and internal surfaces of concrete structures into electricity [93]. Cement-based TE materials are prepared by two methods including dry-mixing and wet-mixing. In the dry-mixing approach, the cement powder and additives are mixed without water and dispersant while in the wet-mixing approach, the additives are first dissolved into a solution, as shown in Fig. 8c, d. The TE properties of these composites strongly depend on the type of additives and the mixing process. In addition, it should be noted that resistivity measurements in cement-based composites are sensitive to the shape and position of electrodes used for this purpose. Metal electrodes exist in the form of rods, meshes, and sheets [94], and the resistance measured using a metal mesh can be expected to be lower than that of a rod, because the mesh has a larger contact area which increases the possibility of effective conduction paths (Fig. 8e) [95]. Substantial work has been done to improve the TE properties of cement-based composites that are reviewed in the following sections [93].

Carbon fiber-concrete composites
The addition of carbon fibers (CFs) to concrete was originally intended to improve the composite's mechanical properties. However, in 1998, Sun et al. [97] measured the TE properties of CFreinforced concrete containing short polyacrylonitrile-based CFs, and they observed hole and ionic conduction in their samples. Later, Sun et al. [98] further investigated the TE properties of CFreinforced materials and reported a positive Seebeck coefficient in their samples. The maximum Seebeck coefficient was achieved in the 1 wt % CF-reinforced concrete sample. CFs are made in a variety of grades having different electrical resistivity, which affects their interfacial contact with the cement matrix and the electrical conductivity of the composite [98]. Three types of CFs comprising crystalline, amorphous, and crystalline intercalated fibers were used and their effect on the TE properties of the composites were investigated [99]. The Seebeck coefficient of cement pastes without fibers were positive, and the addition of the crystalline intercalated fibers altered the Seebeck coefficient to negative values.
Wen et al. [100] reported that the use of bromine as an intercalator in CF-reinforced concrete drastically improved the Seebeck coefficient from À0.8 mV/K to À17 mV/K. The addition of some ceramics such as Ca 3 Co 4 O 9 [101], Bi 2 O 3 , and Fe 2 O 3 [102] into CF can also boost the Seebeck coefficient. Wei et al. [103] used a thin pyrolytic carbon layer in CF-reinforced concrete and obtained a ZT value of 3.11 Â 10 À3 at 328.5 K [103].
It has been reported that manganese dioxide in the form of bulk, film, and powder exhibits good TE properties [93]. For instance, a MnO 2 thin film with a thickness of 160 nm showed a very high Seebeck coefficient of 2500 mV/K [104]. In this regard, Ji et al. [105] investigated a new CF-reinforced concrete composite, in which nano a-MnO 2 crystals were deposited on the surface of the CFs before addition to the cement paste. The MnO 2 /CF-reinforced concrete composite was prepared based on a redox reaction between potassium permanganate (KMnO 4 ) and CFs in an acidic environment. The Seebeck coefficient of the concrete composites with different MnO 2 /CF content is shown in Fig. 9d, indicating an enhancement of the Seebeck coefficient with CF and MnO 2 content. This enhancement was attributed to the improved efficiency of charge-carrying networks between the CFs and cement paste. Also, the maximum ZT value of the MnO 2 /CF-reinforced cement paste was 2.12 Â 10 À3 , which was about 10 4 times that of the pure CFreinforced cement paste. Wei et al. [106] reported an expanded graphite/CF-reinforced cement composite (EGCFRC) whose TE properties significantly increased by introducing the ionic liquid (IL) 1-butyl-3-methylimidazolium bromide (IL [Bmim]Br) into the composite interfaces. A typical SEM image of [Bmim]Br-EGCFRC, photograph of the measurement setup, and a microstructural schematic of the composite are shown in Fig. 9aec. While the Seebeck coefficient of the EGCFRC increased greatly with increasing the [Bmim]Br content, the electrical conductivity did not change much. This enhancement was due to the acceleration of migration and rearrangement of the anions and cations of [Bmim]Br on the electrode surface followed by raising the TE potential at both ends  approach for investigating the TE properties of a slab sample of CFreinforced cement-based concrete under simulated solar irradiation. For a 1 wt % CF sample, the highest ZT of 1.334 Â 10 À7 was attained at 300 K [107].

Steel fiber-concrete composites
Short steel fibers have been added to concrete for improving its mechanical properties. However, they can boost the p-type Seebeck effect in concrete composites. The Seebeck coefficient of steel fiberconcrete composites is even higher than that of CF-concrete composites. This is due to an increase in the linearity and reversibility of Seebeck voltage with the temperature difference between the hot and cold ends in heating and cooling cycles [108]. The steel fibers in general have a larger diameter (>0.6 mm) than CFs (~0.15 mm) [109]. It has been reported that the addition of steel fibers into concrete can change the Seebeck coefficient's sign in the composite. For instance, the original concrete and steel fibers had positive Seebeck coefficients of þ3 mV/K and þ8 mV/K, respectively. However, the composite's Seebeck coefficient value was negative (À63 ± 5 mV/K) when the fiber volume fraction was 0.20%. Then, as the fiber content increased beyond 0.20%, the value became positive (31 ± 3 mV/K) [110]. In these samples, a junction was made between steel fibers and concrete that scatters the charge carriers as well as the lattice vibration mode. The authors concluded that the change of scattering mechanism might have altered the Seebeck coefficient's sign.  [93], (c) the schematic diagram of the procedures for sample fabrication and property measurement: (c) dry-mixing, (d) wet-mixing [96], and (e) the schematic of electrical resistance measurement using copper mesh [95]. TE, thermoelectric.

Carbon nanotube-cement composites
Carbon nanostructures in general, and carbon nanotubes (CNTs) in particular, are one of the most important elements of modern materials research and development. Fundamental characteristics of CNTs include their light weight, ultrasmall size, high strength, and excellent electrical conductivity [111]. CNTs have roughly 100 times the strength of steel fibers and have been shown to effectively improve the mechanical strength of concrete composites. Zuo et al. [112] were the first to report that combining CNTs and CF in a cement-based composite can affect its TE performance through hole conduction. Later, Wei et al. [113] added CNTs with different weight percentages into the cement matrix for producing CNT reinforced cement composites (CNTs/CC) through mixing and dry compression using a shearing technique. Fig. 10a shows the fabrication method of raw CNTs and CNTs/CC with 10 wt % of CNT. A uniform dispersion of CNTs in the cement matrix resulted in a relatively high electrical conductivity of 0.818 S/cm and a Seebeck coefficient of 58.0 mV/K at the CNT content of 15 wt %. The highest ZT value of 9.33 Â 10 À5 was obtained at 348 K for composites with 15.0 wt % CNTs [113]. Furthermore, as compared to pure CNT-filled CCs, lithium carbonate-filled multiwalled CNTs showed an improved Seebeck effect and a power factor that led to a ZT of 1.75 Â 10 À5 at 343 K. This Seebeck coefficient improvement was attributed to alteration of the electronic band structure and Fermi level of CNTs when filled with lithium acetate [114]. Wei et al. [115] introduced a new method to enhance the TE properties of cementitious materials at low carbon content. As shown in Fig. 10b, CNTs were pretreated with acid to create surface defects. As a result, higher carrier scattering at their interface with the cementitious material occurred, which reduced the carrier mobility and increased the effective carrier mass, thus improved the Seebeck coefficient of these cementitious composites (Fig. 10c). The power factor and ZT values were 0.12 mW/m/K 2 and 0.38 Â 10 À4 , respectively, which were both three times higher than those before modification. Vareli et al. [116] fabricated single walled CNT (SWCNT)-cement nanocomposites using SWCNTs in the form of bucky papers dispersed in deionized (DI) water and stabilized by an anionic surfactant (Fig. 10d). The schematic illustration of the SWCNT conductive network development into the cementitious matrix is shown in Fig. 10e. Fig. 10feh schematically illustrate the equivalent circuit, real TEG with the corresponding dimensions and infrared thermography image of the TEG device upon exposure to a temperature difference of 25 K. This structure shows the highest TE performance with a very high Seebeck coefficient of 1348.8 mV/K and a power factor of 2.89 mW/m/K 2 .

Oxide-CCs
Oxide-CCs are based on two families of oxides as filler materials: metal oxide and graphene oxide semiconductors [117]. Metal oxide semiconductors have high Seebeck coefficients and thermal stability, and their earth abundancy makes them suitable candidates for high-temperature TE applications [118]. Several metal oxides have been investigated as a TE material such as SrTiO 3 [119], ZnO [120], MnO 2 [121], and BiCuSeO [122]. MnO 2 films with~2 mm thickness exhibit a Seebeck coefficient of À460 mV/K at room temperature and À1900 mV/K at 623 K [123]. MnO 2 nanopowder was synthesized and added to cement to boost its TE properties [124]. Around room temperature, compacted MnO 2 powder had a Seebeck coefficient of À5490 mV/K. The incorporation of MnO 2 nanopowder into the cement matrix enhanced the TE effect and a high Seebeck coefficient of about À3085 mV/K was achieved at 5 wt % of oxide powder. This value was more than 1000 times higher than that of the cement without MnO 2 powder [124].
ZnO is another good candidate to boost the TE properties of CCs. Ghosh et al. [125] reported a new method to achieve a high ZT for cement-based composites using graphene and different metal oxide nanoparticles. In their research, cement was hybridized with graphene nanoplatelets and metallic oxide particles (ZnO, MnO 2 , Fe 2 O 3 ) by dry mixing and pressing. The maximum ZT of 1.01 Â 10 À2 at 343 K was achieved with ZnO inclusions. Similarly, the addition of ZnO and Fe 2 O 3 to CFRC can boost the Seebeck coefficient to over 1000 mV/K, about ten times higher than CFRC [126].
Also, the incorporation of aluminum-doped ZnO nanoparticles into cement can improve the TE properties of cement due to the creation of Zn(OH) 2 and increasing pore solution volume. Because of a decrease in hydration reactions, adding ZnO to the cement mixture raised the Seebeck coefficient by 17%, while thermal conductivity declined by 9% due to lower crystallinity and density of materials, while electrical conductivity increased by 37% due to increased ionic movements [127].
Another strategy to improve the TE properties of a CFRC matrix is the addition of metal oxide microparticles [102]. The Seebeck coefficient values of the composites with 5.0 wt % Bi 2 O 3 and 5.0 wt % Fe 2 O 3 microparticles were 100.3 mV/K and 92.6 mV/K, respectively.
As the cement matrix was combined with 2.5e5.0 wt % Bi 2 O 3 microparticles, the Seebeck coefficient increased four to five times its Reduced graphene oxide (RGO), which has the advantages of strong mechanical characteristics, high thermal stability, and high electrical conductivity, is the most common nano-filler for cementbased composites [128]. Phonon scattering at the interfaces of RGO and cement paste can reduce the thermal conductivity of the composites considerably [129]. On the other hand, a good dispersion of RGO in cement-based composites enhances the electrical conductivity. Recently, researchers have used dispersing agents [130], ultrasonication [131], and ball milling [132] to add RGO into CCs; however, improving the TE properties using RGO is still challenging due to agglomeration and non-uniform morphology of the cement-RGO composites. To improve RGO distribution in cement and reduce agglomeration, Wei et al. [133] used a liquid-phase mixing process to enhance cement particles coherence to RGO flakes. The composite with 5 wt % RGO had an electrical conductivity of 2.01 S/cm, a maximum ZT of 0.23 Â 10 À4 , and a load power of 8.27 Â 10 À3 mW/m 2 at 338 K. Cui et al. [134] prepared a mixed 'ionic-electronic' cement-based composite through leaching treatment of the RGO to enhance its TE properties. The cement pastes with different RGO content were fabricated via mechanical stirring, ultrasonication, mixing, and casting as shown in Fig. 11a. The TE properties were measured under different conditions: after sealed curing, after drying, and after leaching. As shown in Fig. 11b, when the temperature difference increases, the holes and the metal cations both move from the hot side to the cold side. When the temperature difference is stable, the Seebeck voltage will decrease rapidly due to creation of an opposite hole drift current towards the hot side through an internal electric field caused by the metal cations. After the holes completely compensate for the ionic potential difference, the Seebeck voltage reaches a stable value. The maximum power factor of 6.55 Â 10 À2 mW/m/K 2 was obtained for this composite after leaching. The power factor in the stable-state decreased to 3.72 Â 10 À3 mW/m/K 2 which was still 3e4 times higher than that of the composites after drying. Fig. 10. Carbon nanotube-cement composites for TEGs application in buildings. (a) Schematic of mixing and dry compression shear methods, SEM images of (b) preparation of pretreated CNTs cement-based composites, (c) Schematic diagram of the structure of carbon nanotubes with different processing methods (mixed acids or HCl gas), (d) the preparation of a SWCNT bucky paper film, and the fabrication of cement/SWCNT nanocomposites i.e. mechanical mixing of the p-type SWCNT dispersion, perchloroethylene (PCE), and cement followed by casting resulting into 10 Â 10 Â 60 mm 3 samples, (e) schematic illustration of the SWCNT conductive network development into the cementitious matrix during cement hydration, (f) schematic illustration of the TEG module equivalent circuit consisting of 10 p-type cement/SWCNT(0.5) nanocomposite blocks electrically connected in series, (g) the real TEG device upon being exposed to DT ¼ 25 K, and (h) the corresponding IR-T image showing thermal distribution [116]. CNT, carbon nanotube; IR-T, infrared thermography; SEM, scanning electron microscopy; SWCNT, single walled carbon nanotube; TEG, thermoelectric energy generator.

TEGs on chimneys
A chimney's job in houses is to take hot combustion gases directly outside, therefore, waste heat recovery from chimneys using TEGs is a viable application. Fig. 12a depicts a physical model that consists of several TEGs installed on the exterior surface of a vertical chimney wall using heat spreaders that are cooled by natural convection. Using ANSYS software to analyze the effect of Fig. 11. Reduced graphene oxide-cement composites for TEGs application in buildings. (a) Schematic representation of sample preparations and (b) cooperative working mechanism of mixed "ionic-electronic" thermoelectric cement-based composite (M-TECC) [134]. TEG, thermoelectric energy generator. spreaders showed that they can enhance the total output power by 17% and 42% at spreader lengths of 40 and 140 mm, respectively, at 140 mm pitch. Fig. 12b shows the simulated temperature distribution on the heat spreader and heat sink at different spreader lengths. The results indicate that the TEG's cold side temperature decreases by increasing the spreader length [135]. In addition, the numerical results were validated by developing an experimental setup through measuring the electrical output of a TEG module at different temperatures. The generated voltage in experimental and simulation results was in good agreement with~5% discrepancy for similar operating conditions. Also, several simulation and experimental works have been performed to investigate the effect of spacing and spreader thickness of TEG modules [136], tilt angle of TEGs from the chimney wall [137], installing a flap on the heat sink of TEGs [138], and various cooling methods for TEGs when employed on chimneys [139,140]. For instance, Eldesoukey and Hassan [141] investigated the theoretical impact of the flow regime on the performance of TEGs cooled by forced convection from a chimney, as shown in Fig. 12c. The maximum output power was achieved when the TEGs were located at the inlet of chimneys as well as spots of turbulent flow inside and outside the chimney. It should be noted that very few studies have been conducted on waste heat recovery from chimneys using TEGs.

TEGs on heat pipes
A heat pipe can transfer heat over several meters through a dynamic cycle of phase change heat transfer of a liquid alkali metal sealed in the metal pipe [142]. Research articles related to waste heat recovery from heat pipes using TEGs are rare. As an example, as shown in Fig. 13a, an annular unileg TEG was applied to an exhaust pipe to reduce the thermal stress of the TE materials [143].  [143], (b) schematic diagram of NUSTER-100 reactor system [144], (c) schematic diagram of the heat pipe reactor with TEGs [145], (d) schematic diagram illustrating integration of a conventional rigid planar TE module through an additional heat exchanger indirectly and a conformal TE module directly integrated on a heat pipe with a diameter of 40 mm, (e) demonstration of the conformal 72-couple hH TE module integrated directly on the cylindrical heat pipe [146], (f) schematic illustration showing a sequential 3D printing of Pb 1-x M x Te TE inks with various doping contents (M ¼ Na or Sb, 0.5% x 2%). Photograph showing the 3D-printed PbTe structures with various shapes such as cuboid, cylinder, plate, disk, perforated-plate, and tube, (g) scheme showing the TEG tube made of the 3D-printed p-type and n-type PbTe tubes at the front view, and (h) photograph of the fabricated TEG tube chipping unipair of p-type and n-type PbTe legs and schematic model of a TEG tube chipping ten pairs of TE legs assembled from the fabricated unit module [147]. TE, thermoelectric; TEG, thermoelectric energy generator.  [ 146,147] This raised the TE output power over two fold in comparison with the flat commercial TEGs. Moreover, as shown in Fig. 13b, c, TEGs have been integrated into reactors' heat pipes, with efficiency of such generators reaching over 8%, which is a significant value in the field of thermoelectricity. Due to the high heat capacity in reactor heat pipes, high-temperature TE materials have been used to manufacture suitable TEGs [143e145]. As pipes generally have a cylindrical shape, the flexibility of the TEGs is crucial for their successful application. Li et al. [146] reported a flexible TEG based on half-Heusler alloys (hHs) that are leading candidates for medium-to high-temperature power generation applications. This TEG could be directly integrated on flue gas platforms, e.g. cylindrical tubes, to form large area flexible TEGs. The n-and plegs of the TEG were made of (Hf 0.6 Zr 0.4 )NiSn 0.99 Sb 0.1 and Nb 0.95 Ti 0.05 FeSb, respectively (Fig. 13d). Fig. 13e shows the conformal 72-couple hH TEG integrated directly on a cylindrical heat pipe. A high power density of 3.13 W/cm 2 and a maximum output power of 56.6 W was achieved under a DT ¼ 570 K from a single module of dimension 4.2 cm Â 4.3 cm. Lee et al. [147] developed a 3D printing method to manufacture high performance TEG tubes. Using atomic doping to induce charge imbalances in PbTe particles, 3D-printable viscoelastic TE inks were prepared. In Fig. 13f, extrusion-based 3D printing of Na-and Sb-doped PbTe colloidal inks is demonstrated to create PbTe-based TEGs with defined shapes and to analyze retention of the 3D-structure after ink's deposition and drying out. The best achieved ZT for this structure was 1.4 for the p-type and 1.2 for the n-type legs. Furthermore, they fabricated TEG tubes with 3D-printed n-and ptype PbTe legs with an output power of~150/mW/cm 2 at DT ¼ 300 K (Fig. 13g,h). There is no doubt about the huge potential of waste thermal energy harvesting in building pipes, especially in engine houses; however, TEGs have largely not yet been implemented into heat pipes in buildings.
In summary, the output electrical power of TEGs used in different building parts and on a small-scale varies between nW and W levels. The output power strongly depends on the available surface area of the building part where thermal energy is harvested from. Walls and other concrete elements of buildings have huge surface areas for thermal energy conversion. Also, currently, glass is largely incorporated in building windows, and it can be effectively used for thermal energy harvesting. TEGs used in windows are generally in the form transparent thin films as earlier discussed; however, TEGs used in walls, chimney, and heat pipe are normally bulky, giving much more freedom in terms of materials and design.
As the pipes have a cylindrical geometric shape, it is more efficient to use conformal TEGs around them to maximize the conversion of thermal energy into electrical energy. However, the conventional flat TEGs can be used in other parts of the buildings including walls, and chimneys. Table 4 shows a comparison of TEG applications in different parts of a building. While the operation temperatures of TEGs in most cases are in similar ranges, their output power can be very different.

Case study
In addition to the presented TEGs in previous sections, there are also more energy efficient TEGs in R&D that have not been used in building applications yet. It is reported that an IoT node consumes about 0.86 kW h of energy annually [7], and this energy can be provided by TEGs implemented in different parts of a building. In this section, some practical case studies will be introduced that utilized TEGs in buildings for powering multiple appliances, as schematically shown in Fig. 14 [148].
The implementation of IoT has created requirements and possibilities for novel wireless sensing concepts [149,150]. Small, low cost, and low power electronic components, such as energy harvesting circuits and systems-on-chip (SoC) integrating microcontrollers with radio transceivers, are becoming more widely available, making it easier to build wireless sensor networks (WSNs) with a variety of sensor nodes [151]. One of the most difficult aspects of using wireless sensor nodes is to provide sustainable power sources. The majority of the current small sensor nodes are powered by batteries, which require replacement from time to time. However, energy harvesting technologies, such as thermoelectricity, have been developed to self-power WSN systems by scavenging ambient energy [152,153].
Wang et al. [154] fabricated a Bi 2 Te 3 TEG as a TE energy harvester that powered WSNs for building energy management applications. Fig. 15a shows the schematic and fabricated TEGs. Most TEGs have a voltage output of less than 500 mV. Also, generally, energy harvesting systems, such as TEGs, cannot be directly connected to devices that they are powering due to their power fluctuations that greatly depend on environmental conditions. In this regard, additional circuitry for power management is needed to deliver the required voltage to a load. Fig. 15bee shows a detailed circuit design, electronic components, and a complete prototype module. For TE energy harvesting, a DC/DC converter with a lower minimum start-up voltage, supercapacitors as an energy storage unit, and an output voltage regulator are required in order to maintain a constant voltage to the WSN. Jaakkola et al. [155] designed a selfpowered sensor node for a WSN using flexible TEGs made of aluminum-doped zinc oxide on substrates such as polyimide and glass. These TEGs generated electricity for the sensor nodes from temperature gradients across a building window. The topology of the sensor node is schematically illustrated in Fig. 15f. The conductor and TEG patterns, and principle of the 3D structure of the folded TEG, are shown in Fig. 15g, h. The heat flux and current flow in the folded TEG are parallel to the film surface, whereas the temperature gradient is perpendicular to the TEG plane. Fig. 15i demonstrates the opened structure with the TEG foil under test between the halves of the device. A prototype of a flexible transparent TEG with a folded area of 67 cm 2 produced an output power of 1.6 mW at a temperature difference of 43 K. With this temperature difference, a normal window glass of 0.5 m 2 can be supplied with 74 of these TEGs, producing roughly 118 mW.

Summary and outlook
In this review, the potential of TEGs for use in different building parts, such as windows, walls, chimney and heating pipes, and cement-based materials, were analyzed and their advantages for thermal energy harvesting were identified. Among them, much investigation has been carried out on cement-based TEGs and their composites. However, the best ZT achieved from cementitious materials is a thousand times less than the values reported for typical TE materials used in commercial TEGs [22,25]. In addition, using TEGs on pipes was identified as a promising way to harvest considerable thermal energy, but there remains a significant  [154], (f) topology of the sensor node, (g) the conductor and TEG patterns, (h) the principle of the 3D structure of the folded TEG, and (i) the fabricated TE test rig opened with the TEG foil under test in place [155]. BEM, building energy management; TE, thermoelectric; TEG, thermoelectric energy generator; WSN, wireless sensor network. challenge regarding the required cylindrical shape and flexibility of the TEGs.
The outlook for future research on TEGs in the buildings is summarized below.
(1) Developing transparent TEGs. The total area of building glass in China is estimated to be greater than 15 billion m 2 [156]. Recovering thermal energy from windows, as the interface between the interior and exterior surfaces of a building are exposed to different temperatures, will open enormous opportunities for novel applications such as powering smart electronics and indoor lighting. Transparent TEGs have shown great potential, but the increased transparency in TEGs comes at the expense of reduced efficiency. As a result, future research could be focused on developing new advanced materials with high transparency and high TE efficiency for use in building windows. Special attention needs to be paid to the thermal and mechanical stability of these materials exposed to cyclic low/high temperatures. (2) Developing flexible and conformal TEGs. Although common TEGs have a flat and bulky shape, many heat sources in buildings have a convex or concave shape, such as heating pipes. In order to optimize thermal heat recovery from them, different TEG structures have been suggested up till now. Nevertheless, these structures suffer from low-temperature differences along the TEGs, effective interfacial heat transfer designs, scalable manufacturing, and reliable mechanical durability for long-term use. In recent years, investigations on conformal materials such as Parylene AF4, 3D printing methods [157], and ultra-thin glass substrates [158] have significantly improved both durability and efficiency of flexible electronic devices. Inspired by these results, developing a novel 3D printing method using high conformal TE materials and ultra-thin substrates could be the future research pathway to fabricate high performance and flexible TEGs for exploiting energy harvesting applications in buildings. It is reasonable to believe that in the next few decades, heating pipe energy harvesting in buildings using TEGs can be one of the most accessible and predominantly used methods of generating clean energy. (3) Developing novel cement-based composites with high performance TE materials. Cement-based composites have low electrical conductivity due to the insulating properties of concrete, thus their ZT is down to impractical levels. An effective approach to improve the electrical conductivity of cement/concrete is to hybridize it with highly conductive materials such as graphene and metal particles. The Seebeck coefficient of cement paste is not low, but it can be even further improved by the addition of TE materials such as calcium cobalt oxide, bismuth telluride, and copper selenide. There is huge potential for research on cement and concretebased TEGs, in terms of improvement of their electrical conductivity and Seebeck coefficient. (4) Developing power management unit. Energy generation in TEGs is directly proportional to the temperature gradient across their hot and cold sides. Therefore, the performance of TEGs in buildings depends on the location, time of the day, and the season. For example, the amount of electrical energy generated by TEGs on windows in a warm summer day decreases significantly at night-time due to the reduction of the temperature difference between indoor and outdoor. Therefore, it is necessary to develop a power management unit that can boost the TEGs output, regulate the generated voltage for powering electronic devices, and store the produced power to supply it when needed.
(5) Market prospects for integrating TEGs in buildings. According to Technavio's latest analysis [159], the value of the smart buildings market is expected to increase by 64 billion dollars between 2020 and 2025. Also, the energy harvesting market [160] and waste heat recovery market [161] will be growing by 1.5 billion dollars (from 2024 to 2026) and 11.3 billion dollars (from 2020 to 2024), respectively. For these sectors, the mildly developed TE energy harvesting from buildings is a promising solution towards more sustainable buildings. In addition, the number of academic works in the field of TE energy harvesting in buildings has been steadily increasing in the past decade. Thus, it can be envisaged that research on TEGs in buildings will lead to more commercialization and large-scale applications in the near future.

Credit author statement
Milad Jabri: conceptualization, Methodology, Writing e Original Draft.
Roger P. West: writing e Review & Editing.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability
Data will be made available on request.