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Review

Investigation of Integrated and Non-Integrated Thermoelectric Systems for Buildings—A Review

by
Ramakrishnan Iyer
and
Aritra Ghosh
*
Faculty of Environment, Science and Economy (ESE), Renewable Energy Engineering, University of Exeter, Penryn TR10 9FE, UK
*
Author to whom correspondence should be addressed.
Energies 2023, 16(19), 6979; https://doi.org/10.3390/en16196979
Submission received: 11 July 2023 / Revised: 12 September 2023 / Accepted: 27 September 2023 / Published: 7 October 2023
(This article belongs to the Section G: Energy and Buildings)
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Abstract

:
Countless years have been spent researching the strategies necessary for improving the energy consumption of buildings globally. There have been numerous attempts at achieving both passive solutions and enhancing and optimising the existing active systems. This paper seeks to review, analyse and summarise the possibilities of using thermoelectricity in two different contexts to the integration with buildings, integrated thermoelectric systems, and non-integrated thermoelectric systems. The utilisation of thermoelectricity in cohorts with existing renewable technologies and the utilisation of thermoelectric systems that operate individually, both have the potential to provide the occupants of a building with conditions pertinent to thermal and visual comfort. The results in this paper are classified according to the integration types of thermoelectric systems within different parts of the fabric of a building while maintaining an active role in enhancing the building envelope and self-contained thermoelectric systems that sustain a passive role for the same. The introduction to this paper also gives a very broad and surface-level insight into categorisation of different kinds of thermoelectric systems that are being studied and researched across the world.

1. Introduction

Gone are the days of brutalist architectural designs for houses that involve small and traditional windowpanes which was designed in an era before our society was dependent on electricity for basic needs. Modern built structures maximise the space used for but not limited to windows, they also include glass-facades and cleverly designed skylights to add to the aesthetics as well allowing more natural light for visual comfort. This evolution in house design, brings in new challenges for the desired quantities of fenestrations. One of the key challenges is the behaviour of glass, when left untreated, acts as a very poor insulator [1]. This attribute of glass leads to a huge thermal loss observed from within the building [2].
This is where thermoelectric devices and their applications become paramount in heating or cooling buildings. The working principle of thermoelectric applications is bidirectional in nature as it benefits from both the Seebeck effect and the Peltier effect. The Peltier effect not only enables the conversion of electrical energy into thermal energy, but also subtracts a limited amount of energy on one side while releasing a higher amount of energy on the other side [3]. This imbalance of energy aids in either cooling or heating a building depending on the geography and use case. Conversely, the Seebeck effect allows a micro-voltage to be generated due to two dissimilar metals being connected to each other at different temperatures [4]. Compared to other conventional methods of controlling a building’s thermal envelope, integration of thermoelectric heating or cooling would be substantially quieter and less hazardous [5]. Figure 1, Figure 2 and Figure 3 show the various thermo-electric system and its operation.
The application of thermoelectric principles is not a novel concept, ever since the 1820s there have been several efforts that have been made in order to secure the electrical energy which could potentially be harvested from heat or temperature gradients. A notable effort was made during the early space race throughout the 1950s and 60s where both NASA and the former Soviet Union made efforts to effectively convert the heat radiated from radionuclides which typically operate in a temperature range of 600 to 1200 degree Celsius either in argon controlled chambers or in space into electricity that can power devices that other typical renewable energies at the time like solar energy could not reach [8,9]. Certain examples like Radioisotope Thermoelectric Generators (RTGs) used by NASA are very robust example of such application which incorporated this application in a spacecraft that operated for 10 years without any intervention. One of the main benefits of using this was the noiseless composition of the system that consisted of no moving parts [10].
This paper advocates and highlights the integration of thermoelectric applications in buildings as opposed to them being an independent entity because buildings, more than any sector account for more than half of all the energy loss that occur due to a lack of efficiency and poor implementation in the UK and several other countries globally. This energy loss occurs in two different ways, one of them being the heat loss observed due to the fabric of the building and the other one being the heat loss due to ventilation [11]. Thermal load is not the only factor responsible for energy loss because heating pertaining to space heating as well as water takes up a significant portion of electrical load supplied to the building as well. One crucial application that has helped built structures meet their load demands is photovoltaic systems, specifically integrated into buildings, aptly named as building integrated photovoltaic (BIPV) [12] systems and building integrated photovoltaic thermal systems (BIPV/T) [13]. There have been several approaches that have been investigated for this purpose. Ref. [14] highlights a few key techniques that have amassed a huge interest in the built environment industry. This integration is not limited to fenestrations and facades that only cover a portion of a building, but also expands its use case catering to photovoltaic materials that could be incorporated in claddings, roof as well as ventilated facades. This paper aims to highlight and review the importance of thermoelectric (TE) technologies that could be integrated within built structures, which can also be supplemented by existing promising renewable technologies like BIPV and BIPV/T. Studies like [15] show us that energy saving potential can be improved significantly using TEs integrated with PV systems [16].
However, this paper also seeks to compare other aspects of TEs that include contrasting integrated and non-integrated TE systems in buildings which operate independently in order to utilise thermoelectricity for ventilation, cooling and heating purposes [17]. Moreover, a comparison is drawn between two different approaches for the utilisation of thermoelectric principles which include the conversion of electricity into heat and the other approach suggesting the opposite, by exploiting the temperature gradient between the interior and the exterior of a built structure to generate a secondary source of electricity for buildings [18,19]. Another theme that this paper has taken into account is the comparison between glazed and unglazed photovoltaic thermoelectric (PV-TE) hybrid system that could be potentially implemented across any façade or a glass window; for the sake of drawing this comparison, Ref. [20] incorporates nanofluids to be able to serve as a heat sink in order to enhance the removal of heat. This paper also aims to study and review the evolution of independent TE systems into building integrated photovoltaic thermoelectric (BIPVTE) that could potentially create an active building envelope which is capable of adapting to any ambient thermal environment and not just limited to one climactic zone [21].
The integration of thermoelectric systems into building envelopes has garnered significant attention in recent years due to its potential to revolutionize the way buildings manage their thermal comfort and energy efficiency. This paper presents an expanded and enriched review, delving into the intricate dynamics of both integrated and non-integrated thermoelectric systems for building applications. In our increasingly energy-conscious world, the quest for sustainable and energy-efficient building solutions has driven researchers to explore innovative technologies. Thermoelectric systems offer a promising avenue, capitalizing on the Seebeck effect to convert temperature differences into electrical energy, and vice versa. While previous studies have examined various aspects of these systems, we aim to provide a more comprehensive understanding by incorporating recent engineering insights and comparative analyses.

1.1. Review Scope and Objectives

The review focuses on two primary categories of thermoelectric systems for buildings: Integrated Thermoelectric Cooling-Heating Units (TCHUs) and Ventilated Active Thermoelectric Envelopes (VATEs). These systems represent distinct approaches to harnessing the potential of thermoelectric technology within building environments.
  • Thermoelectric Cooling-Heating Units (TCHUs)
TCHUs are characterized by their integration into a building’s heating, cooling, and ventilation systems. Our review scrutinizes the latest advancements in TCHUs, shedding light on their performance in both heating and cooling modes. Detailed engineering insights reveal the influence of temperature differences, the impact of solar radiation, and the benefits of ventilation. Notably, we examine the trade-offs associated with the integration of TCHUs, such as the creation of thermal bridges and the necessity of optimizing control systems for enhanced efficiency.
  • Ventilated Active Thermoelectric Envelopes (VATEs)
In contrast, VATEs rely on active ventilation to enhance thermal control within building envelopes. Our exploration of VATEs encompasses their response to varying external conditions, emphasizing the significant influence of solar radiation on system performance. We delve into the intricate balance between conduction and convection within VATEs, elucidating how the integration of thermoelectric systems can introduce thermal bridge effects. These insights provide a clearer picture of the challenges associated with achieving consistent comfort conditions.

1.2. Comparative Analysis

One of the key contributions of this review is a comparative analysis of TCHUs and VATEs. By synthesizing engineering insights, we highlight the differences in their temperature control capabilities, response to solar radiation, ventilation benefits, thermal bridge effects, heat transfer mechanisms, and overall efficiency. This comparative assessment aims to assist researchers and practitioners in selecting the most suitable thermoelectric system for specific building applications.
In conclusion, this review paper presents an extensive exploration of integrated and non-integrated thermoelectric systems for buildings. By incorporating recent engineering insights and conducting comparative analyses, offering a nuanced perspective on the advantages, limitations, and challenges associated with these innovative technologies. The findings provide valuable guidance for researchers, designers, and policymakers seeking to harness the full potential of thermoelectric systems to create sustainable and energy-efficient buildings.

1.3. Literature Review

This paper uses Köppen–Geiger Photovoltaic climate classifications in order to understand the performance better that varies region to region [22]. Consequently, this paper also reviews the performance of a self-adaptive BIPVTE wall system for both summer and cold winter zones in China, to acquire a better insight into the differences between two weather patterns [23]. This paper looks into a novel approach of using the orientation of a built structure in order to maximise the solar gain with a combination of south facing PV wall and a north facing TE air duct system for a tropical climate, this study has been reviewed in order to consider thermal comfort as a parameter as well, along with thermal efficiency and energy performance [24]. Ultimately, any novel TE system is as good as the efficiency of the materials that comprises the TE module. Thus, it is paramount to select and improve materials of high quality in order to obtain a high level of performance from TE materials [25]. So, as a result, this paper also examines the need of high quality, high-performance materials as well as making sure that the materials are deemed sustainable in order to promote the sustainable development goals (SDG) set by the United Nations (UN). The development of non-toxic and partially transparent Zinc Oxide with Fluorine-doped Tin Oxide (ZnO/FTO) solar based thermoelectric nanogenerator (TENG) in order to achieve glazing that is energy efficient [1,26] is crucial for the industry to progress into a commercial stage from the development phase.
This technology is only as good as the material synthesis and the manufacturing implications that come with it. The final factor that must be taken into consideration for developing the TE materials further and making them feasible is scalability. From Figure 4, it is easy to understand that not all materials, in fact most TE materials are not suitable for large scale fabrication which severely limits the commercialisation aspect of TE systems and TGZs subsequently. Some fabrication techniques like sputtering, spin coating and absorption by spray are simply not feasible to manufacture on a large scale yet [27,28,29,30]. However, some technologies like Embedded TGZs in windows may need multiple drill holes with n and p type nanomaterials, then pile them using a hot press to connect them, this technology is simple in complexity, however, poses challenges during manufacture. Another interesting development in this avenue could be seen in [31], which advocates for manufacturing a higher yield of electricity for small surface areas using unique methods that focuses on designing superlattice structures as desired and other aspects like specific doping and composition. This paper advocates for the use of thermoelectric generators in small wearable devices that can harness the heat released from the body, this is made possible because wearable devices use a very meagre amount of electricity compared to larger devices like a mobile phone. The paper in discussion focuses mainly on using transverse Seebeck effect and how to obtain high-spin Seebeck signals in different TE materials.
Upon careful inspection of properties various materials, the scale of difficulty in manufacturing is depicted below:
Energies 16 06979 g004
Figure 4. An illustration of various thermoelectric materials based on three scales of fabrication-fabrications possible in large scale, possibly fabricated on a large scale but with multiple steps and the ones that are difficult to fabricate on a large scale.
Figure 4. An illustration of various thermoelectric materials based on three scales of fabrication-fabrications possible in large scale, possibly fabricated on a large scale but with multiple steps and the ones that are difficult to fabricate on a large scale.
Energies 16 06979 g004

2. Materials and Methods

A comprehensive study requires an extensive pool of literature that has the ability to supplement the review article. Most of the literature is derived from major publication journals like Energy and Buildings, Building and Environment, Applied Energy, Building Services Engineering Research and Technology, Journal of Cleaner Production, Renewable Energy, Energies, Solar Energy, Energy Conversion and Management, Engineering Science and Technology, International Journal of Hydrogen Energy and Energy Procedia. A methodical snowball search was set up using important keywords that revolve around the periphery of the topic chosen, this was performed in order to obtain relevant sources of literature that did not show up in major search databases like Google Scholar, Scopus and ScienceDirect. The keyword search comprised of words related to TEs (thermoelectric, glazing, thermoelectric air-cooling system, material property, thermoelectric cooling, thermoelectric heating, air conditioning, building application, thermal comfort, thermos-electric air duct, thermoelectric module, thermoelectric nano-generators, nanofluid, cooling performance, heating performance, thermoelectric wall, coupling effect, Peltier), while some keywords involved in the search involved terms associated with TE integrated and non-integrated PV systems (PV-TE system, multi-objective optimisation, photovoltaic, BIPV, solar energy, PV cells, Photovoltaic module, shading effects, Heat exchange network analysis, BAPV, hot-humid tropics, BIPVT, BIPVTE, BIPVT Performance Assessment, Building integrated photovoltaics, PV, Renewable Energy, Hybrid PV/T, PV-TEG, Solar thermal-TEG) and ultimately, the other keywords were terms pertaining to built-structures, building envelopes and other relevant parts of a building for TE implementation (Glazing, building application, Integration, monitoring, Ventilation and air-conditioning (HVAC), Active building envelope, Passive building envelope, Venetian blind, Façade, solar façade, building, energy, environment, energy recovery, zero energy building, building simulation, Fabric heat loss, Ventilation heat loss, airflow rate, U-value, heat energy loss, glass cover, building services). It was observed that using Boolean operators like ‘AND’ and ‘OR’ during keyword search in order to obtain the literature specific to two areas, for example, in order to obtain the literature that contains both TE and building envelope related terms, searches like ‘thermoelectric OR glazing’ led to the desired literature more accurately.
Furthermore, records related to PVs, but irrelevant to BIPVs and TEs were excluded from the literature selection. Similarly, the literature specific to TEs yet irrelevant to application of TEs in built structures like enhanced electrochemical developments within the industry were removed from the selection of the literature. Moreover, those not in English or irrelevant to application with or without the integration of BIPVs were also excluded. Most of the retained literature dated beyond 2016, with only a few articles chosen from before the said year. A word cloud that depicts the frequency of the highest used words in larger fonts and the least used words in smaller fonts was used to derive a more accurate representation of keywords pertaining to TEs and BIPVTEs in general as mentioned in Figure 5. When categorised by region, a large portion of the selected literature are from Europe, Middle East region, and North America. Some works are also found from South America and South-east Asia and China. But the literature from China, Japan, Saudi Arabia and Turkey, due to them being irrelevant to TE and BIPVTE applications or them being non-English literature that, could not be used in the review.

3. Results and Discussion

This section thoroughly discusses the evolution of TEs that lead to the amalgamation of TEs into BIPV and BIPV/T systems. Resulting in a solution with a multi-pronged approach toward the implementation of TE in buildings known as BIPVTE. However, it is worth noting that this sector has not been matured enough to be rolled out on a large-scale commercial basis and is strictly limited to a few prototypes and experimental setups. A testament to this is the most relevant literature being published after 2016. This section reviews all the themes explored exhaustively to understand the scope of TE’s application alongside BIPVs or otherwise.

3.1. Integrated and Non-Integrated TE Systems

3.1.1. Integrated TE Systems

According to Building Research Establishment Environmental Assessment Method (BREEAM), buildings are increasingly becoming, both a consumer and a producer of energy in the coming years, so incorporating renewable technologies into buildings could help fulfil the demands of the grid as well as ease the load of consumption within the building by the means of improving the quality of building fabric and minimising energy losses observed in the building [32]. According to [33], the layer that distinguishes between the interior of a building to the exterior, can be labelled as a building envelope, essentially forming a fabric. Any integration that can be used to improve the said fabric of the building, can be passed as a building integrated renewable system (BIRS). In this context, TEs that can be integrated with existing solar PVs in order to draw a nominal amount of energy that can be then supplied to embedded TE systems in various parts of the fabric. This integration causes the TEs to be labelled as BIPVTEs shown in Figure 6. This integration can be seen in multiple parts of the building.
Integrating TE based cooling and heating using power from PV systems has already been investigated using an adaptable and scalable PV-TE battery wall system, according to [34], which can help the system realise 72–92% energy savings, around 88 to 100% energy savings and 100% energy savings in cold, mixed and cooling dominant zones, respectively.
This design has a radiant wall that is powered by BIPV systems and has both cooling and heating modes incorporated within the walls. However, a rigorous amount of modelling and calculations have been performed to realise this design philosophy into implementation as well. The mathematical model was achieved by accounting for the iterative temperature while also simultaneously considering heat losses and weather conditions by [35].
Depending on the use case of the integrated systems, in order to achieve net zero energy building (NZEB), Ref. [36] has come up with five different scenarios and have presented an economic, environmental and energy assessment of a BIPVTE system. These five scenarios include:
PV-TE, Grid-TE, PV-Grid-TE, PV-Battery-TE and PV-Grid-Battery-TE systems.
The final scenario of PV-Grid-Battery-TE system has the highest CO₂ reduction of 3.04 tonnes per square metres.
Furthermore, studies like [37] have ventured into solutions pertaining to single and double façades that building integrated solar systems (BISS) like BIPV and BIPV/T can offer. Ref. [38] investigates the use of double-skin semi-transparent photovoltaic (DS-STPV) window that has a thermoelectric effect on windows in cold regions in China, shown in Figure 7. This study was substantiated by simulations obtained from reputed software programmes like EnergyPlus in order to confirm the reliability of the model.
Similarly, ref. [39] uses a TE air-cooling system in order to cool the glazing surfaces’ airflow in hot climates during summer, this reduces the thermal discomfort of the occupants significantly and reduced the cooling loads of the window as well. The result of the study shows that after a certain number of modules, increasing TEM count on the windows do not contribute to reduction in energy, in fact, in some cases it was also responsible for increase in the consumption of energy and had no considerable impact on energy savings.
A novel and unique approach by [40] entails the use of an operating strategy for TEs by using active thermoelectric (ATE) windows. This approach utilises an active heat management system that encourages the use of thermostats to actively control some additional auxiliary setups like fans and cooling units to achieve a high level of energy efficiency and optimise the heat transfer happening through the windows making it adaptable to any climatic condition. Another approach for windows and fenestrations include a solar thermoelectric generator (STEG) which convert solar thermal energy into electricity, using STEGs, Ref. [41] exploits some very unique properties of aerogels which are high solar light transmission and thermal insulation at the same time, which in turn, become absolutely crucial for the solar receiving parts of STEGs. This is implemented in buildings using a highly transparent aerogel window that shows low thermal conductivity and a high solar transmittance of 96.5% which is deemed higher than the solar transmittance of soda-lime glass. The working of the glass is illustrated below in Figure 8.
Finally, a very innovative and an imaginative solution for integrated TE systems is the integration of TEs in blinds, specifically venetian blinds that are integrated with PV modules. A study by [42] analyses the electrical harvesting capabilities of venetian blinds using PV modules. The size of the PV modules attached to blinds vary depending on the angle of the blinds with respect to the frames to maximise on the solar gain. This was analysed using a heat exchange network analysis between the integrated blinds and PV module.

3.1.2. Non-Integrated TE Systems

Some TE systems do not embed well with the building envelopes and instead act as their own independent systems that supplement the rest of the built structure. Ref. [26] suggests that TEs on their own have not evolved to be a primary source of heating or electrical demand, but can rather act as a supportive, secondary source of power with other renewable systems, namely solar PVs and solar thermal systems. This is why independent non-integrated TE systems are usually only broadly classified into two segments, TEs in ventilation systems and TE systems used in Phase Change Materials (PCM).
Applications of TEs in ventilations consist of, but are not limited to, the utilisation of flow and return air that runs through the system. According to [43], (shown in Figure 9), TEs have evolved from being used in conventional single refrigeration or generation and have progressed into tough energy consumption. Which means that the novel prototype mentioned in this study can recover the heat from the exhaust of air conditioners in China using an experimental setup made in Hunan University thermoelectric lab.
In 2014, another model for using TEs in ventilation systems was being investigated by [7], which incorporated the use of a heat pipe exchanger in order to optimise a thermoelectric generator (TEG). After analysing the energy and exergy models for both summer and winter conditions in Changsha, China, it was concluded that a TEG can provide the required energy to handle the fresh air intake as well as recovering the heat from exhausted air in a ventilation system.
As a final set of comparison, an open-type TE system was setup with multiple channels. Ref. [44] established a mathematical model to understand heat transfer using a one-dimensional treatment of electrical and thermal power was carried out. This proved that the closely looped air circulation enabled a simultaneous recycling of heat and enhancement of heater system performance. The results of the study conducted above show that TE based multiple channelled ventilation system trumped the heating coefficients of a conventional electric heater.
A detailed and exhaustive survey of PCMs and thermoelectric cooler (TEC) based battery thermal management system (BTMS) was presented by [45]. This technology is typically only limited to EVs but could easily be implemented across a battery management system in a household or industrial building. This system oversees a plethora of auxiliary systems like forced air cooling and liquid cooling systems, fins and heat pipes. However, due to the low cost, abundance, high sensible and latent heat, PCMs have gained a lot of attention in the last two decades. Using a TEC in the said BTMS would turn this into a semi-passive system with a separate cooling system that further enhances the performance of existing battery or storage management systems in buildings, despite being a non-integrated system [46,47,48,49].
Refs. [7,9,33] advocate for a comparison between conventional compressor-based heating systems and claim that TE based heating solutions can be more impactful than compressor-based systems. Furthermore, ref. [50] investigates non-compressor based PCM-integrated thermoelectric cooling system (PCM-TEC) for buildings in specific. The utilisation of PCM is reserved for but not limited to space cooling during the night which allows the PCM to store cold thermal energy, consequently, acting as a heat sink to reduce the hot temperatures from the sides of thermoelectric modules (TEMs) during daytime. This process increases the overall efficiency of the PCM-TEC system significantly as the experimental tests show that the coefficient of performance (COP) compared to a conventional system increases by 56%.

3.2. Engineering Insights into Real Life Use Case of Thermoelectric Heating and Cooling Unit (TCHU) as Studied in [19]:

This comparative insight between cooling and heating revolves around the following equations:
Q h = S m · I · T h + 1 2 · R m · I 2 − K m T h − T c
Q c = S m · I · T c + 1 2 · R m · I 2 − K m T h − T c
where, Q c is power absorbed in cooling in watts (W) and Q h is power absorbed in heating in watts (W). K m is thermal conductance of thermoelectric generation module (W/K) and R m is electrical resistance of thermoelectric module (ohm). I is the current intensity in amperes (A), T h and T c are temperatures of hot and cold faces, respectively and S m is the Seebeck coefficient of thermoelectric module (V/K).
  • Cooling Insights:
    • Power Consumption Difference: in the cooling experiments, two different voltages were applied: 7.2 V and 12 V. Interestingly, even though the voltage was higher in the 12 V case, the heat absorbed (Qc) showed only a small difference of about 100 W compared to the 7.2 V case. This can be explained by the temperature difference between the faces of the Peltier cell. In the 7.2 V case, the temperature difference was about 9 °C, while in the 12 V case, it increased to about 17 °C. As explained by Equation (2), temperature difference plays a significant role in power absorption, and a smaller temperature difference results in higher power absorption for the same voltage.
    • Joule Effect Impact: however, it’s essential to consider the negative Joule effect included in Equation (2). The Joule effect becomes more pronounced as current intensity increases. This leads to an intriguing finding. It may be more efficient to install more cells with a lower voltage and, consequently, lower current intensity than to use fewer cells at higher intensity. With this approach, the cooling and heating capacities (Qc and Qh) can remain the same, but the coefficient of performance (COP) would improve due to lower electrical power consumption.
    • COP Values: the COP obtained in these cooling tests ranged between 0.75 and 0.78 at 7.2 V and between 0.66 and 0.62 at 12 V. It’s worth noting that these values were lower than those reported in some other articles due to the inclusion of fan consumption.
    • Qc Variation: the relationship between Qc and different voltage and temperature differences follows a pattern similar to an inverse hyperbolic cosine function. This suggests that increasing the voltage beyond a certain point does not significantly increase Qc. Therefore, it’s crucial to optimize the system based on these findings.
  • Heating Insights:
    • Higher Heat Generation: in heating mode, the experiments showed that the power generated (Qh) had higher values compared to cooling, as predicted by Equations (1) and (2). In heating mode, the Joule effect was beneficial, meaning that higher current intensity led to higher heat generation (Qh).
    • Temperature Difference Impact: similar to cooling, the temperature difference between faces (Th − Tc) also influenced the results. Lower temperature differences resulted in better performance for both absorbed and generated heat. Reducing this temperature difference requires careful design of the façade in which the thermoelectric unit is embedded.
    • COP Values in Heating: the COP values obtained at 7.2 V ranged between 0.80 and 0.86, while at 12 V, they ranged between 1.40 and 1.30. These values included the impact of fan consumption, and further tests without fans were suggested to assess their effect on system performance.
    • Qh Variation: the relationship between Qh and different voltage and temperature differences follows a profile similar to an exponential function. This indicates that power generation rapidly increases with voltage in heating mode.

3.3. General Insights of TCHU

  • Optimal Design: the findings emphasize the importance of carefully designing the thermoelectric cooling and heating unit (TCHU) and its embedded façade to minimize the temperature difference between faces, thereby enhancing performance.
  • Control System Optimization: a more precise control system could further improve system efficiency. By regulating the system based on temperature differences, it’s possible to work within a more efficient range.
  • Cell Configuration: installing more cells and operating them at lower intensity and voltage levels appears to be more efficient than using fewer cells at higher intensity and voltage. However, a cost–benefit analysis is recommended to determine if the additional investment is justified.
  • System Emphasis: depending on the climate and application, it may be more advantageous to design the TCHU as a cooling machine, as indicated by higher COP values in cooling mode.
Table 1 showed the difference between the engineering insights into cooling and heating design units.

3.4. Engineering Insights into the Real Life Use Case of Ventilated Active Thermoelectric Envelope (VATE) as Studied in [51]

  • Seasonal Temperature Variation: the system effectively maintains a constant interior temperature of 21 °C during winter (Test 1.1). However, during summer (Test 1.2), even with a set point of 23 °C, the interior temperature can rise to 32 °C due to the influence of solar radiation.
  • Solar Radiation Impact: solar radiation has a pronounced impact on system performance, particularly during summer. This leads to temperature peaks during the late afternoon, with differences of up to 25 °C between the cavity and exterior temperatures (Test 3.2).
  • Ventilation Benefit in Winter: during winter, the ventilated façade facilitates heat loss, as evidenced by similar cavity and exterior temperatures during the night (Test 2.1)
  • Thermal Bridge Effect: integrating the thermoelectric system creates a thermal bridge, causing a drop in thermal resistance. This effect is particularly significant during maximum solar radiation, resulting in temperature peaks inside the prototype, especially during summer (Tests 2.2 and 3.2).
  • Heat Transfer Mechanisms: convection and conduction play a role in heat transfer. Conduction becomes more relevant with the thermoelectric system, leading to higher inside temperatures when solar radiation is intense (Test 2.2). In non-ventilated façades, the conduction effect is less pronounced (Test 3.2).
These insights provide a quantitative understanding of the system’s behaviour and underline the importance of addressing solar radiation effects, optimizing ventilation, and managing thermal bridges for better thermoelectric system performance in building envelopes.

3.5. Difference between TCHU and VATE Based on Their Respective Engineering Insights

Table 2 listed the difference between thermoelectric cooling-heating unit and ventilated active thermoelectric envelope.

4. Perspective

Examining the results of both integrated and non-integrated TE systems, it is safe to say that the solutions can be classified into active and passive TE models for a building. Systems that integrate well within the fabric of the building, ranging from radiant walls, aerogel windows, thermoelectric wall or cooling and heating or novel systems like blind integrated systems, they actively play a role in maintain the insulated envelope of the building intact and additionally, also supply the buildings with either heating or electrical demand based on the system requirements and initial setup. These systems can be labelled as active TE systems (ATES). Whereas non-integrated ventilation-based TE systems as well as PCM-infused TEC systems that act independently to the fabric of the building could be coined as passive TE systems (PTES). This classification helps us understand the scope and application types of a nascent technology like TE, whether it’s used for glazing, opaque facades, embedding into window frames, ventilation inlets and outlets or as a replacement for standard HVAC systems [26,52,53].

5. Conclusions

In conclusion, thermoelectric (TE) systems have demonstrated immense potential in the realm of building-integrated energy solutions. While it is evident that TE technology is still in its nascent stages of development, our comprehensive review, enriched with recent engineering insights and comparative analyses, provides a clearer perspective on its current status and future prospects. The integration of TE systems into building environments, both as Integrated Thermoelectric Climate Handling Units (TCHUs) and Ventilated Active Thermoelectric Envelopes (VATEs), presents an exciting avenue for enhancing energy efficiency and thermal comfort. Our examination of TCHUs has unveiled the intricate dynamics of temperature control, revealing the trade-offs associated with their integration. Likewise, our exploration of VATEs highlights the influence of external factors, such as solar radiation, on their performance, shedding light on the challenges posed by thermal bridges.
Comparatively, TCHUs and VATEs exhibit distinct temperature control capabilities, response mechanisms to solar radiation, and potential benefits in different building scenarios. While TEs are not yet positioned to serve as primary sources of energy, they undeniably offer significant value as secondary or supplementary sources of heat and electricity. TEs can generate limited amounts of electricity, which can power auxiliary systems like mechanical ventilation and heat recovery units or support tertiary systems within renewable energy setups.
The realm where TEs shine most brightly is in their capacity to serve as efficient sources of heating. Leveraging the Peltier and Seebeck effects, TEs can generate heat, potentially replacing conventional air conditioning systems. Their lack of moving parts and non-toxic nature, exemplified by modern TEs like ZnO/FTO, make them an attractive addition to Building-Integrated Photovoltaic (BIPV) systems. This evolution toward Building-Integrated Photovoltaic-Thermoelectric (BIPV-TE) systems holds promise for stable and sustainable energy solutions. While TE systems are indeed in the early stages of their journey, they represent a vital component of the ongoing paradigm shift toward more energy-efficient and environmentally conscious buildings. The road ahead requires further research, planning, and investment, as well as increased interest from the domestic and industrial building sectors. With continued dedication, TE systems have the potential to become integral contributors to the future of sustainable building technology.
In summary, the potential for TE systems to play a significant role in building energy solutions cannot be discounted. As we move forward, it is essential to recognize their role as secondary and supplementary sources of heat and electricity, with a particular emphasis on their prowess in heating applications. The incorporation of TEs into BIPV systems signifies a promising step toward energy-efficient, stable, and sustainable BIPV-TE systems, which hold the key to greener and smarter building practices.

Author Contributions

Conceptualization, R.I. and A.G.; methodology, R.I.; software, R.I. and A.G.; validation, R.I. and A.G., formal analysis, R.I.; investigation, R.I. and A.G.; resources, R.I. and A.G.; data curation, R.I. and A.G.; writing—original draft preparation, R.I.; writing—review and editing, R.I. and A.G.; visualization, A.G.; supervision, A.G.; project administration, A.G.; funding acquisition, A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Micro-voltage generated as a result of exposure of dissimilar metals with a thermal gradient. (a) Illustrates electricity generation with the help of a heat input and heat sink; (b) Illustrates electricity generation with the help of cooling and heating with PN diodes in opposite configuration [6].
Figure 1. Micro-voltage generated as a result of exposure of dissimilar metals with a thermal gradient. (a) Illustrates electricity generation with the help of a heat input and heat sink; (b) Illustrates electricity generation with the help of cooling and heating with PN diodes in opposite configuration [6].
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Figure 2. This figure illustrates both the electricity generation and reversal of thermoelectric application showcasing heating and cooling capacities due to the Peltier effect (a) Illustrates electricity generated; (b) Depicts heating due to reversed current flow; (c) Depicts cooling due to reversed current flow [6].
Figure 2. This figure illustrates both the electricity generation and reversal of thermoelectric application showcasing heating and cooling capacities due to the Peltier effect (a) Illustrates electricity generated; (b) Depicts heating due to reversed current flow; (c) Depicts cooling due to reversed current flow [6].
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Figure 3. Schematic diagram of the thermoelectric module as mentioned in [7].
Figure 3. Schematic diagram of the thermoelectric module as mentioned in [7].
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Figure 5. Word cloud with all the keywords arranged according to the frequency of words directly corresponding to the increase in font size.
Figure 5. Word cloud with all the keywords arranged according to the frequency of words directly corresponding to the increase in font size.
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Figure 6. Side view and internal front view of BIPVTE system circuits [23].
Figure 6. Side view and internal front view of BIPVTE system circuits [23].
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Figure 7. Photograph of the experimental setup by [38].
Figure 7. Photograph of the experimental setup by [38].
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Figure 8. (a) Depicts a cross sectional view of aerogel windows as illustrated in [41]; (b) Represents uncovered, 5 mm and 10 mm thick aerogel windows.
Figure 8. (a) Depicts a cross sectional view of aerogel windows as illustrated in [41]; (b) Represents uncovered, 5 mm and 10 mm thick aerogel windows.
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Figure 9. Schematics of a prototype thermoelectric domestic ventilation system as mentioned in [43].
Figure 9. Schematics of a prototype thermoelectric domestic ventilation system as mentioned in [43].
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Table 1. Differences between the engineering insights into cooling and heating design units.
Table 1. Differences between the engineering insights into cooling and heating design units.
AspectCooling InsightsHeating Insights
Power Consumption Difference:Voltage: 7.2 VVoltage: 7.2 V
Qc Difference: ~100 W Qh Range: 0.80–0.86
Joule Effect ImpactNegative in Cooling Lower temperature difference improves
COP ValuesRange: 0.75–0.78 at 7.2 VRange: 1.40–1.30
Range: 0.66–0.62 at 12 V
Qc and Qh VariationSimilar to inverse hyperbolic cosine function Follows an exponential profile
Control System OptimizationConsider more precise control system for optimal performanceSuggests a more precise control system for optimal performance
Cell ConfigurationInstalling more cells at lower intensity and voltage levels for more efficiencyEmphasizes installing more cells at lower intensity and voltage levels
System EmphasisEmphasizes designing as a cooling machine and a higher COP in cooling modeDepending on the climate, designing as a heating machine may prove to be disadvantageous
Table 2. Difference between Thermoelectric Cooling-Heating Unit (TCHU) and Ventilated Active Thermoelectric Envelope (VATE).
Table 2. Difference between Thermoelectric Cooling-Heating Unit (TCHU) and Ventilated Active Thermoelectric Envelope (VATE).
AspectTCHU SystemVATE System
Temperature ControlEffective control for both heating and cooling with COP between 0.62–1.40 (12 V) and 0.75–0.78 (7.2 V)Significant temperature variations observed, with peaks of up to 32 °C during summer due to solar radiation
Influence of Solar RadiationLimited influence observed, mainly due to variations in temperature differencesSolar radiation has a pronounced impact, leading to temperature peaks during summer
Ventilation BenefitsVentilation can improve heat loss in winterVentilation effect on temperature is less pronounced
Thermal Bridge EffectIntegrating the TCHU creates a thermal bridge, leading to significant temperature peaksThermal bridge effect noted due to integration of thermoelectric system in the façade
Heat Transfer MechanismsConduction and convection play a role, with conduction becoming more relevantConduction effect more pronounced in the presence of the thermoelectric system
System EfficiencyOP values of 0.62–1.40 achieved with a voltage range of 7.2–12 VDifficulty in achieving comfort temperature due to solar radiation effects
Overall PerformanceEffective system operation but with consideration for fans’ energy consumptionPerformance not as expected, especially during summer
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Iyer, R.; Ghosh, A. Investigation of Integrated and Non-Integrated Thermoelectric Systems for Buildings—A Review. Energies 2023, 16, 6979. https://doi.org/10.3390/en16196979

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Iyer R, Ghosh A. Investigation of Integrated and Non-Integrated Thermoelectric Systems for Buildings—A Review. Energies. 2023; 16(19):6979. https://doi.org/10.3390/en16196979

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Iyer, Ramakrishnan, and Aritra Ghosh. 2023. "Investigation of Integrated and Non-Integrated Thermoelectric Systems for Buildings—A Review" Energies 16, no. 19: 6979. https://doi.org/10.3390/en16196979

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