A review of Thermally Activated Building Systems (TABS) for improving the thermal behavior of buildings

: In recent years, several alternatives for improving the thermal comfort conditions inside 1 buildings have been proposed. Among these alternatives, Thermally Activated Building Systems 2 (TABS) have become of interest due to the benefits this technology brings to the building sector. 3 The TABS are embedded in different building components and exchange heat with building 4 envelope to improve the indoor air temperature. This review presents relevant results presented in 5 the literature on the thermal behavior of TABS, the different types of TABS configurations, and the 6 main parameters of TABS studied such as pipe separation, fluid inlet temperature, fluid velocity 7 and volumetric flow rate. The potential of TABS to improve the thermal comfort conditions and 8 provide energy savings is also discussed. Further, this study presents the different modes of 9 application. 10 with the slab during the summer in Australia. They carried out experimental tests with the integration of BioPCM in a rectangular aluminum air duct 2 m long, 0.30 m wide, and 0.30 m high to validate the model. The authors found that if only PCM was placed in the ventilation duct, they obtained a reduction in the average peak indoor air temperature of 1.21°C, and with an alveolar slab roof, this reduction was 3.62°C. The proposed system reduced the peak temperature during the day up to 4.7°C. Another study of roofs with PCM and TABS is also available. Yu et al., [25] studied a roof with embedded tubes through which air circulates. They validated and compared through a CFD numerical simulation the thermal properties of the system with a phase change material as insulation. The authors proposed a concrete roof with a thickness of 0.19 m and a layer of 0.03 m of paraffin as PCM. The results showed that the optimum phase transition temperature increases linearly by approximately 2°C when the average temperature of the outdoor air rises. Compared to a roof without PCM, they found that the interior surface temperature decreases between 3.7 and 4.0°C in different regions of China. In a more recent study, the same authors Yu et al., [26] proposed a ventilated roof model with embedded pipes and a stabilized layer of PCM (VRSP). The authors developed a steady-state three-dimensional heat transfer model of the VRSP system in ANSYS FLUENT. The convective heat transfer coefficient in the interior surface of the roof was 8.72 W m − 2 and 23.26 W m − 2 in the exterior surface, and the indoor air temperature was set at 26°C. The effect of the phase transition temperature, the thickness of the PCM layer, and the air flow rate in the tubes were studied. The researchers found that the optimal design of the roof had values for the phase transition temperature of 29 - 31°C, the thickness of the PCM layer 0.02-0.35 m, and an airflow rate of 1.4 - 2.5 m s − 1 . It was shown that the optimum design reduced the average temperatures of 17.5, 19, and 20°C, while the water inlet velocity was set at 0.5 m s − 1 . They found that by supplying the water in the tubes at 17.5°C, a heat exchange with the wall internal surface of 25.5 W m − 2 could be obtained. The results showed that the finite difference model could predict the behavior of construction with embedded pipes. The relative errors were 6.5% and 4.4% between the measurement and the prediction by the FDFD model for the external surface heat flux and the pipe-embedded building envelope internal pipe surface heat flux. In other research of the same group, Zhu et al., [31] developed a semi-dynamic thermal model of an active pipe-embedded building. The model consists of a construction with embedded pipes in a wall 3 m high and 2 m wide. This model was coupled with an RC model that predicts heat transfer along the width of the structure and an NTU model to evaluate heat transfer in the pipes. To assess the behavior of the semi-dynamic model, they developed a CFD model in FORTRAN that functioned as an experimental virtual test for comparison. They tested and verified three case studies where the water inlet temperature was set at 20°C, varying the water inlet velocity from 0.5 to 0.7 m s − 1 and the thermophysical properties of the wall, meanwhile was set the pipe spacing at 0.02 m. The authors observed that the changes in the heat fluxes taken away by the water are not obvious with different velocities in the water. Meanwhile, the average difference of about 0.5°C on the outlet temperature of the fluid was found throughout the day. The results demonstrated that the semi-dynamic model predicts the thermal behavior of a TABS with a relative error of 5%. Later, Zhu et al., [32] validated a simplified semi-dynamic model of a chamber with tubes embedded in the envelope. They built two chambers with a controlled environment to perform the validation, one with pipes embedded in the envelope and the other without embedded pipes as a reference. The walls of the chambers were made of alveolar brick, with a layer of cement mortar covering both surfaces of the walls, with polybutylene tubes of 0.020 internal diameters, using ethanol as working fluid. The authors varied the fluid temperature from 25 - 65°C and the fluid fill ratio from 60 to 144%. The authors found that the fill ratio between the volume of the working fluid and the evaporator volume has a critical impact on the thermal resistances and the starting behavior of the TPTL. They found that the optimal fill ratio is around 116%. Qu et al., [35] investigated the relationship between the design and the operating parameters of a thermally activated wall system (TAW) using a mathematical model developed in COMSOL and validated with experimental data from a test chamber. The variables analyzed were the separation between each tube, the area of the thermally activated wall, the flow rate, and the inlet temperature of the water. The authors proposed optimal design graphics for a thermally activated wall system for China’s climatic zones. The test chamber was constructed of 2 m × 2 m × 2 m, thermally activated on the south wall with embedded tubes, where tested three separations between tubes (0.01, 0.02, and 0.03 m). The water flow circulating through the TAW had a velocity of 0.2 m s − 1 , and a heat pump supplied three different temperatures (15, 17, and 19°C). The results indicated that the water inlet temperature and the indoor air temperature affect the heat transfer of the TAW. They found that the maximum inner wall surface temperature occurs for a separation between tubes of 0.02 m, a water velocity of 0.2 m s − 1 , the maximum and minimum values reach 1.78°C and 1.80°C during the cooling and heating mode. The authors used COMSOL Multiphysics to analyze the influence of temperature and inlet velocity of water and the number of cooling surfaces (area). The CFD model was validated using experimental data from a room with a 3.46 m x 3.46 m x 3.15 m. The room was built with 0.23 m thick brick walls, a 0.15 m thick concrete roof, and a floor with cross-linked polyethylene pipes of 0.013 m in diameter. The researchers found that the parameter that had the most significant effect on thermal comfort was the number of cooling surfaces. They showed that if all the room surfaces are cooled, with a flow of 19 L h − 1 of water, it reduced the average indoor temperature of up to 5.7°C. The same authors, Leo Samuel et [52] carried out an experimental study of a scale enclosure with a thermally activated construction system, using water pipes embedded in concrete used in the roof, floor, and walls, with separate water flow controls. The experimental prototype measures 3.5 × 3.5 × 3.15 m with a 15 cm thick reinforced concrete slab, surrounded by trees and structures that provide partial shade. They used ½” schedule 40 PVC pipes, with a 10 cm separation between pipes. They studied temperature, relative humidity, air speed, and water flow through the pipes. The authors found that if only the cooling is activated on the roof, the indoor temperature remained around 33.1°C. However, when the cooling is activated on the walls, floor, and roof, the temperature decreases to 29.2°C. The authors conclude that this system, coupled with a


TABS embedded in building roofs
Building roofs are usually the building components with the most significant temperature fluctuations and receive 28 solar energy for more hours than any other component. Thus, in zones with a warm climate, building roofs are sources of 29 unwanted heat that affect indoor thermal comfort conditions. This section focuses on the research works related to TABS 30 systems integrated into building roofs. Several studies were developed to determine the influence of roofs with TABS on the thermal comfort conditions of 33 buildings. For instance, Gwerder et al., [13] proposed a control algorithm for TABS to comply with comfort requirements. 34 The proposed method incorporates the change between heating and cooling modes of the TABS to satisfy thermal comfort. 35 The researchers used the algorithm in a simulation example. They considered a construction 6 m long, 6 m wide, and 3 m  The heat gain variations also can be handled with the consideration of intermittent operation.
[14] Experimental Temperate Heating, Cooling Water - The TABS maintained the indoor air temperature two buildings within 80-90% of comfort satisfaction zone.
[15] Theoretical-Experimental -Cooling Water The TABS caused the indoor air temperature to be within 80-90% of the satisfaction zone.
[16] Experimental -Cooling Water -TABS maintained the operating temperature in the range 23-25°C, with the CO 2 levels at 850 ppm.
[17] Experimental -Cooling Water The TABS improved the indoor air temperature by maintaining it stable within 21.1 and 21.8°C [18] Theoretical -Heating, Cooling Water The thermal comfort improved by 5% with the TABS installed in the roof.
[19] Theoretical-Experimental Hot and humid Cooling Water The TABS decreased up to 5.1°C the roof interior surface temperature and 6.7°C the indoor air of the test chamber.
[20] Theoretical-Experimental Tropical Cooling Methanol An CLPHP coupled to a metal roof reduced the indoor air temperature up to 13%.   [24] Theoretical-Experimental -Cooling Air

Combination of TABS with other technologies for roofs
When the PCM is placed in the ventilation duct, it reduced the average peak indoor air temperature of 1.21°C, and with an alveolar slab roof, this reduction was 3.62°C. The proposed system can reduce up to 4.7°C the peak temperature during the day.
[25] Theoretical Cool, winter, hot summer and mild regions -Air The interior surface temperature decreases between 3.7 and 4.0°C in China's different regions than a roof without PCM.
[26] Theoretical --Air The optimal values for the phase transition temperature are 29-31°C, the thickness of the PCM layer 0.02-0.35 m, and an airflow rate of 1.4-2.5 m s −1 .
[27] Theoretical -Heating Water The potential for energy storage capacity in wood is 53% greater than a concrete structure  The heating load was reduced by 10%, the cooling load was reduced 36%, and total energy consumption decreased 13% with the TABS.

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The walls of a building are another building envelope components that exchange energy with the outdoor environ- temperature of the panels, using a test contact, thermistors, and a thermal imaging camera. The differences between the 162 average temperatures of the panel surfaces were 1.8 to 4.5%, when measured using a non-contact and contact method.

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The authors concluded that the difference between the analytically calculated average temperature and the experimental  Faxen-Rydberg-Huber expression can contribute to determination of the average surface temperature of the wall heating panel. The uncertainty of geometric characteristics and thermophysical properties of individual wall panel layers has substantial impact on the accuracy.
[30] Theoretical-Experimental Heating, Cooling Water The system can exchange to 25.5 W m −2 with the internal surface of the wall. The finite-difference model can predict the behavior of a TABS.
[31] Theoretical -Water The semi-dynamic models still consume much less computation time than the CFD model. The semi-dynamic model can be integrated with conventional software for building performance evaluation. The semi-dynamic model can predict the thermal behavior of a TABS with a relative error of 5%.
[32] Theoretical-Experimental Heating, Cooling Water The difference between the model and the experimental validation was minimal with 11% of average relative error.
[33] Theoretical-Experimental Heating Water and antifreeze The system performance is affected by the weather, the indoor temperature, the solar absorptivity, and the mass flow rate.
[34] Theoretical-Experimental Heating Ethanol The fill ratio between the volume of the working fluid and the evaporator volume has a critical impact on the thermal resistances and the system's starting behavior. The optimal fill ratio is around 116%.
[35] Theoretical-Experimental Heating, Cooling Water The water inlet temperature and indoor air temperature affect the thermally activated wall system's heat transfer. The maximum temperature difference occurs with a configuration of separation between tubes of 0.02 m, a water velocity of 0.2 m s −1 .
[36] Theoretical Heating, Cooling Water Inlet water temperature has more a significant effect than the sol-air temperature.
[37] Theoretical-Experimental -Water The separation and the depth at which the tubes are placed significantly influence the walls thermal behavior. The system has better performance when placing the tubes in the wall at 0.045 and 0.065 m, with a separation of 0.0125 and 0.015 m. concluded that the WIPH system has a greater heat dissipation effect in the summer. Its heat transfer capacity was 50.7 k 240 W m −2 , and the average temperature of the WIPH was 2°C lower than the conventional wall.     [39] Theoretical Heating Water When the TABS system is adapted for heating in a home, it can provide energy savings up to 75%.

Heat losses and heat dissipation of walls integrated with TABS
[40] Theoretical Heating Water The efficiency increases when the inlet temperature increased. The system design depends on the meteorological conditions.
[41] Theoretical Heating -A separation between tubes of 0.01 m could be used for the thermal barrier function and separation between pipes of 0.075 m for the heating function. The thermo-activated PCM composite wall oriented to the north was more effective because it had an interior temperature increase of up to 1.8°C and reduced energy consumption by 65%.
[42] Theoretical Heating Water The TABS system provided energy savings of up to 40% for heating.

Heating, Cooling Water
Precooling a room overnight and reducing the water supply temperature improved thermal comfort and reduce the unit capacity by over 35%.
[44] Theoretical-Experimental Heating, Cooling - The thickness of thermal insulation, the thickness of load-bearing part of walls and their material, the axial distance of pipes, pipe dimensions, mean heat transfer medium temperature, heat storage capacity/cold material of building structures affect the thermal and cooling performance of thermal insulation panels.

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TABS has been studied to decrease or increase the indoor temperature of buildings and coupled with other technolo-290 gies. Authors around the world have analyzed different parameters and scenarios with TABS.  Table 7 summarizes the works that analyzed the installation of TABS in different building envelope components 359 at the same time. In addition, the TABS with an acoustic insulation system in the roof and its influence on the thermal 360 comfort of the occupants was studied. [48] Theoretical-Experimental Hot semi-arid Cooling Water The system maintained indoor air temperature between 23.5°C and 28°C.
[49] Theoretical-Experimental --- The operating temperature increased 0.8 K when the roof is covered with vertical sound absorbers and increase 1.6 K with horizontal acoustic. TABS requires a well-balanced acoustic design to provide the occupants an optimal comfort level.
[50] Theoretical-Experimental  Increasing the thermal conductivity of the pipes from 0.14 to 1.4 W m −1 K −1 considerably improves the system's cooling performance. The best combination of the parameters was internal diameter of the pipe of 0.0017 m, thermal conductivity of the tube of 0.14 W m −1 K −1 and a thickness in the roof and floor of 0.2 m. This combination reduced the indoor operating temperature by 4.7°C.
[54] Experimental -Heating, Cooling Water -Implementing a TABS with mechanical ventilation systems improves the thermal comfort conditions in an enclosure.

TABS capacity to lost and storage heat 362
To minimize the energy losses on the buildings, some authors have integrated some materials to insulate construc-

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[58] compared six radiant heating systems to make a guide that allows choosing a system according to its application.

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The authors compared six heating systems with PE-Xa pipes with different diameters, embedded in Floor, Floor with  Table 8 summarizes the characteristics of the studies presented in this section. In this section, the authors analyzed 403 the effect the TABS behavior connected to a ground heat exchanger and including insulating materials and the heat loss 404 capacity. Through the thermal barrier system, the control method was efficient to maintain a comfortable temperature inside, finding that the temperature variations in the exterior and interior wall of construction were smaller than 1°C.
[57] Theoretical-Experimental -Cooling Water - Heat removal in an enclosure increases when tube spacing and tube depth are decreased. The potential of a roof is higher (20-30%) compared to a floor TABS, with the same characteristics.
[58] Theoretical -Heating Water The thermal performance depends on the location of the tubes with respect to the indoor environment.
[59] Theoretical-Experimental -Heating Water Implementing aluminum fins on the heat exchanger tubes improves the thermal behavior of a floor. Storage capacity increases with fin material embedded in exchanger tubes.

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Regarding the improvements of thermal comfort provided by TABS when installed in building roofs, the results were Other studies show that when TABS are installed in more than one building envelope component, they provide The studies analyzed indicated that most TABS systems were developed for TABS embedded in roofs and walls.