Study on the Performance of Trombe Wall in Hot Summer and Cold Winter Climate under Non-air Conditioning Condition

. Trombe walls with phase change materials (PCMs) are well known as passive energy saving technologies, but the interaction between PCMs and Trombe walls was unclear. To explore the effect on indoor thermal environment of various operating strategies, eight cases were simulated under hot summer and cold winter climate. Integrated indoor discomfort duration (I D ), integrated discomfort degree-hour (I DH ), PCM liquid fraction, wall interior surface temperature and heat flux were used to assess the thermal performance throughout the year, compared with a traditional building. The results show that the effects of various operating strategies to indoors was closely related to ventilation, PCM type and position and massive wall. Ventilation was the most effective way to supply heating in winter and lower indoor temperature in summer; while its efficiency might be weakened if PCMs was installed, even though the overall indoor environment was improved. Without ventilation, massive wall with double PCM layers got better thermal performance than single layer, but the result would be contrary with ventilation. The optimal case for thermal performance is the Trombe wall with ventilation and single PCM layer, whose I DH were 2653.8℃·h in summer and 10204.3℃·h in winter.


Introduction
With the increasing requirements for thermal comfort of buildings, 60%-70% of building energy consumption is used for heating in winter and cooling in summer [1]. Trombe wall had been widely concerned as a passive energy saving technology [2] because of its many advantages, such as simple structure, energy conservation and environmental protection [3]. Meanwhile, phase change materials (PCMs) have the characteristics of high latent heat and low thermal conductivity, which enhance the thermal performance of Trombe wall [4]. It was a smart choice to combine the Trombe wall with PCMs, which can effectively improve indoor comfort and reduce building energy consumption [5].
Trombe wall with PCMs has a smaller volume to store energy [6] to effectively improves the traditional Trombe wall's shortcomings of low thermal resistance [7] and unstable heat transfer [8]. Duan et al. [9] chose the mixture of 55% decanoic acid and 45% lauric acid as a kind of PCM to integrate with Trombe wall. It was observed that the integration increased indoor air temperature by 0.82 -1.88 ℃ for low heat input mode and 1.75 -3.27 ℃ for high heat input mode. Abbassi et al. [10] found that an 8 m 2 Trombe wall saved heating auxiliary energy by about 77% annually in a simple classical Tunisian building. Rabani et al. [11] simulated and calculated the Trombe wall room with different * Corresponding author: xiaoqinsun@csust.edu.cn PCMs in cloudy days by using a two-dimensional simplified model. It was found that the paraffin Trombe wall could keep the room warmer than other PCMs, and the time reached 9 h. Zhou et al. [12] conducted a collector-storage wall using PCMs. The indoor temperature was above 22 ℃ during the whole discharging period since the PCM in collector-storage wall released latent heat. Delta winglet vortex generators were used to enhance heat transfer [13]. Liu et al. [14] studied effects of external insulation component on thermal performance of a Trombe wall with PCMs. The results indicated that it had a significant impact of the thermal conduction resistance of the insulation component and the thermal resistance of the closed air cavity. Zalewski et al. [15] proposed a small scale Trombe wall by experiment and simulation in France. The results indicated that the hydrated salt delayed solar peak by 2h and 40min. A similar conclusion was that a 4cm PCM Trombe wall showed almost four times delay shorter than a 15 cm concrete wall [16].
There have been many related studies on Trombe walls with PCMs, but most of them focused on the structure and composition of the wall, and less on the operating strategies of the Trombe wall. Therefore, in this paper, eight cases, representing different operating conditions for traditional building and Trombe wall, were simulated. The PCMs with a melting temperature of 25°C was used for single layer and the two layers of PCMs with a melting temperature of 18℃ and 28℃ were used for double layers. Integrated indoor https://doi.org/10.1051/e3sconf/202235603055 E3S Web of Conferences 356, 03055 (2022) ROOMVENT 2022 discomfort duration (ID), integrated discomfort degreehour (IDH), PCM liquid fraction, wall interior surface temperature and heat flux were used to assess the thermal performance throughout the year, compared with a traditional building.

Architectural buildings and verification
The building model designed by Design Building software in this paper is shown in Figure 1   As shown in Figure 3, the simulated data were compared with the measured data of the control group. The average errors of the indoor air temperature is 6.59% and the interior surface temperature of the south wall is 4.05%,indicating that the simulation experiment has good accuracy.
(b) South wall inner wall temperature.

Simulation scenario
Besides the traditional building and the PCM building model (phase change layer is arranged on the south wall), six types of Trombe wall structures were designed respectively, as shown in Table 1. Among them, the total amount of PCM in single-layer PCM wall and doublelayer PCM wall was the same, and the volume fraction of PCM in the PCM layer was 30%.

Summer condition
The cumulative indoor uncomfortable duration ID and uncomfortable degree-hour IDH of each model in summer are shown in Figure 4. Compared with Model 1, the ID and IDH of Model 2 were reduced by -15 hours and 372℃·h in the whole summer period, respectively. Compared with Model 1, Model 3 showed obvious overheating phenomenon, with ID and IDH increasing by 43 hours and 346℃·h, respectively. Model 4 eliminated the overheating problem, compared with Model 1. ID and IDH were reduced by 7 hours and 158℃·h, respectively. And Models 5 and 7 maintained indoor thermal environment as in Model 1, but ID increased by 105 and 129 hours, respectively. The minimum IDH value appeared in Model 6 in summer, that is, the single layer PCM Trombe wall model under ventilation condition, which was 2653℃·h.

Winter condition
The cumulative indoor uncomfortable duration ID and uncomfortable degree-hour IDH of each model in winter are shown in Figure 5. Compared with Model 1, the ID and IDH of Model 2 were reduced by -10 hours and 7℃·h in the whole winter period, respectively. Compared with Model 1, the indoor thermal environment of Model 3 was also significantly improved, with 47 hours of ID and 816℃·h less IDH. Compared with Model 1, Model 4 improved the thermal comfort of indoor environment effectively, and ID and IDH were 202 hours and 1447℃·h less, respectively. Models 5 and 7 reduced 8% of the indoor comfort, but ID compared with Model 1 only reduced 14 and 7 hours, respectively. In winter, the lowest value of IDH appeared in Model 6, which was 10204℃·h.

Entire year condition
According to Figure 6, Model 1 had the worst indoor thermal comfort, with ID and IDH of 3013 hours and 14901℃·h, respectively. Among them, the lowest value of ID in the whole year appeared in Model 4, which were 2804 hours, while the lowest value of IDH was 12857℃·h in Model 6.
In conclusion, Trombe wall structure optimized the indoor thermal environment of buildings throughout the year, and the Trombe wall with PCMs combined with natural ventilation can meet the demand of indoor heating and cooling, which has an obvious improvement on the indoor thermal environment throughout the year.

Conclusion
This paper compared the thermal performance of a traditional lightweight building, a PCM building, a Trombe wall building and a Trombe wall building with PCMs. The indoor thermal environment of each building model under different operation strategies was analyzed. The following conclusions were drawn: (1) Adding PCMs into the building wall improved the thermal performance of the building in summer effectively, but lead to an increase in indoor uncomfortable time. It cannot play a good role in winter when radiation was weak, which can apply to all regions.
(2) Trombe wall structure will aggravate the indoor overheating problem in summer in any regions, which cannot be eliminated completely by using single or double layers of PCMs.
(3) Natural ventilation was used to solve the indoor overheating problem. In winter, both natural ventilation and PCMs promoted the heating effect further. In summer, the Trombe wall with PCMs combined with ventilation improved its temperature regulation effect effectively.