1. Introduction
Site artifacts, as records of ancient human activities, are valuable cultural and historical heritages [
1]. With urban development, numerous urban-based archaeological sites must coexist with urban production and living spaces. The challenge in heritage conservation and urban planning is how to effectively display site artifacts as public cultural resources while ensuring their protection [
2]. Given their immovable nature, museums built at the original sites of artifacts transform their environment from outdoor to indoor, providing physical protection and optimized preservation conditions. Hence, establishing site museums is key to protecting and displaying these precious artifacts [
3,
4]. While site museums benefit artifact preservation, factors like soil salinity and air pollution (see
Figure 1a) mean that museums alone cannot fully resolve artifact diseases, leaving many artifacts at risk even within museums [
5,
6,
7].
To ensure the integrity of collections and facilitate public display, museums should design lighting and environmental control systems to preemptively protect artifacts and create a comfortable viewing environment [
8,
9]. Different types of museums have varying requirements for interior environmental design and control, especially regarding lighting, which affects both artifact preservation and visitor experience [
10]. Natural lighting reduces energy consumption and improves illumination, but direct sunlight can cause overheating and color degradation of artifacts [
11]. Although direct sunlight has a limited direct aging effect on inorganic materials, its photothermal impact can lead to various environmental changes, causing damage to artifacts. This includes the following: (1) direct damage to artifacts made of specific materials [
12]; (2) accelerated evaporation of moisture and rapid accumulation of salts on soil site surfaces [
13]; (3) excessive heat introduction, resulting in significant diurnal temperature fluctuations on the soil site surface [
14]; (4) increased energy consumption for environmental control systems [
15]; and (5) in humid climates, the combined effects of light and moisture can promote moss growth on the soil site surface, leading to diseases [
16]. Balancing natural lighting to optimize display effects while minimizing solar radiation on artifacts is a significant challenge for museum designers. Designers and managers must strike a balance between artifact preservation and visitor experience [
17,
18].
Research has been conducted on optimizing the design of museum skylights to reduce their impact on artifacts. Many museums worldwide are transformed from buildings initially designed for other purposes, often featuring numerous skylights and side windows for natural lighting before renovation. To assess the risks of natural lighting post-renovation and develop appropriate strategies, Hoyo-Meléndez et al. [
19] proposed a method for assessing natural lighting in museum galleries, focusing on the preventive conservation of artifacts, exemplified by the Donald W. Reynolds Center for American Art and Portraiture. Khaled A. Al-Sallal et al. [
10] studied the impact of various sky models, building orientations, and window-to-wall ratios on artifacts and visitor comfort in museum galleries converted from traditional buildings in the UAE, providing analysis methods and guidelines for lighting design in such transformations. Addressing the direct sunlight issue from the pyramid-shaped skylight of the Seoul Art Museum, Chang-Sung Kim et al. [
20] proposed replacing it with monitor-shaped and saw-tooth-shaped skylights, using RADIANCE software (version 1.02) for simulation and design optimization to effectively block direct light and stabilize illumination.
The impact of lighting on artifacts is closely related to the intensity of light and the sensitivity of the material. The International Commission on Illumination categorizes artifacts into four types based on their light sensitivity and sets standards for light intensity and annual exposure, as shown in
Table 1 [
11]. These standards, based on the aging characteristics of materials and the direct effect of light, are crucial for museum lighting design.
Table 1 indicates that for inorganic artifacts like earthen sites and pottery, which are insensitive to light, there are no restrictions on light intensity and cumulative exposure in their preservation environment according to the standards. Most unearthed site artifacts are inorganic materials insensitive to light, leading many galleries to adopt large spaces with skylights, as shown in
Figure 1b, to enhance display effects [
21,
22]. However, such large spaces with skylights, as in the Terracotta Army Museum, which is part of China’s first emperor’s mausoleum, can negatively impact the preservation environment of artifacts, with temperature fluctuations exceeding 11 °C observed in the exhibit area during the summer [
23]. Hu et al. [
24] conducted seasonal environmental sampling in five site museums along the Yangtze River and found fluctuations and seasonal variations in temperature and humidity in the exhibition halls. In archaeological sites where museums are built in situ, the storage environment of artifacts changes from solely soil to a soil–air coupled system (see
Figure 1c). Although direct sunlight has limited direct aging effects on inorganic materials, its photothermal effect can cause various environmental change-induced damages to the artifacts. Luo et al. [
25], taking the Tang (618–907 AD) and Song (960–1279 AD) dynasty site at Chengdu’s Jiangnan Guan Street as an example, analyzed the impact of direct sunlight through skylights on the degradation of artifacts in a soil–air coupled environment. They found that direct sunlight is a primary cause of salt accumulation, moss growth, and short-term fluctuations in temperature and humidity, suggesting the use of shading measures for improvement.
Building on Luo et al. [
25], this paper focuses on the improvement needs of a site museum in the city center, selecting the Tang and Song dynasty site at Chengdu’s Jiangnan Guan Street for study, located in Chengdu, Sichuan Province, China. It designs a shading structure to improve the artifact environment, considering the relationship between artifact diseases and the environment and the characteristics of the site and surrounding urban architecture. Computational fluid dynamics and lighting simulation were employed to assess the artifact conservation and display environment, laying the scientific groundwork for future enhancement and conservation strategies.
4. Analysis of Museum Exhibition Hall Artifact Environmental Monitoring Results
Figure 9 demonstrates that the daytime illuminance in the relic area peaks at 100,000 lx, exhibiting a dual peak pattern at noon and in the afternoon. This occurs primarily because a south-side high-rise building obstructs the exhibition hall lighting between these peak periods. Weather conditions significantly influence the variation in light intensity. During the measurement period, peak illuminance on sunny days neared 1.0 × 10
5 lx, whereas it remained around 2.0 × 10
4 lx on cloudy and rainy days, notably on the 10th, 16th, 18th, and 20th. Illuminance and cumulative light intensity values substantially surpass the recommended levels for museum exhibit lighting.
Figure 10 shows the distribution of temperature and relative humidity at points T1 and T2 within the exhibition hall, while
Table 6 presents the statistical values of these measurements. During the test period, the average temperature at point T1 under the illuminated area was 25.7 °C, with an average diurnal temperature fluctuation of 7.4 °C, both significantly exceeding the recommended average value of 20 °C and the maximum fluctuation limit of 5 °C as per the “Museum Building Design Standards” (JGJ 66-2015) [
27]. In contrast, at the non-illuminated area under the concrete roof, the average temperature at point T2 was 23.3 °C, with an average diurnal fluctuation of 2.7 °C. Although these values are slightly above the recommended 20 °C, the excess is minor. This is mainly due to the influence of solar radiative heat from the large, illuminated area, while the temperature fluctuation meets the standard’s maximum limit of 5 °C. The distribution of relative humidity mirrors that of temperature. The average relative humidity at points T1 and T2 was 58.5% and 68.7%, respectively, with diurnal fluctuations of 16.1% and 6.6%. The fluctuation in the illuminated area at T1 significantly exceeds the recommended maximum limit of 5% [
27]. The statistical data reveal that rooftop skylights significantly impact the environmental conditions in heritage conservation areas. Under direct sunlight, the temperature and humidity fluctuations in illuminated areas are pronounced, making it challenging to maintain the stable environmental conditions required for preserving artifacts.
Figure 11 shows the temperature distribution on the surface of the earthen site at measurement points S1 and S2, with
Table 7 presenting the corresponding temperature statistics. The charts indicate significant temperature fluctuations at the earthen site in illuminated areas. During the monitoring period, the highest temperature at point S1 reached 44.5 °C, with an average temperature of 28.8 °C and an average diurnal temperature variation of 9.8 °C. In contrast, the average temperature at the non-illuminated area’s measurement point S2 was significantly lower, at only 24.0 °C, with an average diurnal temperature variation of just 1.5 °C. The test results indicate that solar radiation introduced through transparent glass causes unstable temperature fluctuations in the earthen site itself, and the environmental control system has not effectively regulated temperatures in the illuminated areas. Therefore, implementing shading measures is essential to improve the effectiveness of the environmental control system.
5. Protective Shading Structure Plan for the Site
Comprehensive analysis and numerical simulation results indicate that direct sunlight through the glass roof of the exhibition hall has accelerated the occurrence of environmental diseases affecting artifacts at the Chengdu Jiangnan Pavilion Street site. Therefore, implementing shading measures to block direct sunlight and rainwater will help prevent the occurrence of these environmental diseases. Given that the rooftop plaza above the Chengdu Jiangnan Pavilion Street site serves as an entrance and emergency access for the IFS Center, it is inappropriate to use permanent architectural structures for shading. This study draws on the tensioned membrane structure of the new Arc de Triomphe entrance in France [
28], designing a tensioned membrane shading structure above the Chengdu Jiangnan Pavilion Street site that does not interfere with tourist observation or plaza functionality (see
Figure 12). Additionally, the effects of this shading solution were predicted using the aforementioned lighting simulation software (Ecotect Analysis 2011) in conjunction with computational fluid dynamics for the indoor environment.
Figure 13 illustrates the relationship between the height and width of the shading structure and the solar altitude angle. Given that Chengdu’s Jiangnan Museum Street is in the Northern Hemisphere, the design of the shading structure primarily focuses on the southward extension length
L (as shown in
Figure 13), which can be estimated using Equation (2) based on the projection relationship.
In the formula, h represents the height of the shading structure, and denotes the solar altitude angle. Considering the solar altitude angle , in winter, a longer L is required for effective shading due to the lower solar altitude. Additionally, the higher the shading structure height h, the longer L is needed to shade the glass roof location. As the site is located in the city center and constrained by the main municipal road to the south of the site plaza, the maximum value of L is set to not exceed 2 m. Considering that summer has the strongest outdoor solar radiation, the solar altitude angle at 16:00 on the summer solstice should be chosen to determine the projection length in the southward direction of the building. Additionally, due to the height restrictions of surrounding buildings, the height h of the exhibition hall is set to not exceed 8 m.
This study finds that the effectiveness of a sunshade in a museum is influenced by the solar altitude angle, with taller sunshades casting larger shadows and proving more effective in winter due to the lower solar angle. Adjusting the sunshade’s height changes the area it covers, requiring a proportional increase in its extension length to maintain consistent shadow coverage. Moreover, taller and longer sunshades enhance thermal comfort and energy efficiency by reducing heat gain. However, the museum’s location in a busy commercial district with high-rise buildings imposes spatial constraints on sunshade construction. Therefore, the design must consider these limitations to achieve optimal shading while being mindful of the surrounding urban landscape and cityscape impact, balancing shade effectiveness with space constraints. To further this study, an evaluation of the impact of different exhibition hall heights
h (4 m, 6 m, and 8 m) and extension lengths
L (0 m, 1 m, and 2 m) on shading effectiveness was conducted, as detailed in
Table 8. This evaluation also includes a comparison with scenarios where no shading structure is present, providing a comprehensive understanding of the sunshade’s impact.
7. Conclusions
This research, undertaken at the Jiangnan Guan Street site museum in Chengdu, focused on tackling the issues posed by skylight illumination in exhibition halls. Our aim was to harmonize the needs of artifact conservation with the requirements of tourist observation and urban functions. Our findings reveal that despite its aesthetic appeal, skylight illumination may not be ideal for artifacts in soil–air conservation settings, as inorganic materials often do not react to light. A major result of this study is the suggestion to incorporate tensioned membrane shades into the hall’s roof, effectively improving the conservation conditions for artifacts impacted by the skylight.
Our analysis indicates that direct sunlight penetrating the exposed roof greatly exacerbates the instability and degradation of soil artifacts. Implementing the sunshade significantly enhanced artifact storage by moderating temperature fluctuations and reducing solar exposure, offering a sustainable preservation method for artifacts in urban areas. Employing computational fluid dynamics and daylighting simulations, we gathered concrete proof of the sunshade’s role in stabilizing the artifact preservation environment.
These results are especially pertinent to the protection of the Chengdu Jiangnan Pavilion Street site, indicating novel environmental control strategies for urban site museums. Additionally, this study emphasizes the importance for policy makers to contemplate the renovation and enhancement of such exhibition halls. The comparative outcomes of our CFD and daylighting simulations clearly advocate the advantages of sunshade structures, aiding in persuading policy makers to endorse environmental upgrades.
However, a challenge exists in the urban placement of many site museums, where the size of sunshade structures is limited by the surrounding buildings. As shown in our case study, this limitation means full site coverage is not always achievable. Nevertheless, CFD and daylighting simulations prove invaluable in predicting design scheme performance and guiding optimization efforts.