Keywords

28.1 Introduction

Temperature plays an important role in the structural performance of bridges. Bridges undergo deformations such as displacement, strain, tilt, crack etc. because of their operational loads such as traffic and temperature. Such deformations should be minimised to ensure public safety, bridge serviceability, and lower life-cycle maintenance cost. Several case studies in real-life bridges demonstrated the impact of temperature load on bridges [1]. For example, temperature load caused strains of magnitude equal to or larger than static and traffic load on the Yangtze River Bridge in Jiangsu, China [2], it induced a 4–8% change in natural frequency in Dowling Hall Footbridge in the USA [3]. Besides the normal ambient temperature, extreme temperature events pose threats to bridge safety as well. For instance, the Hammersmith Bridge in London was closed to traffic in August 2020 due to tiny cracks in cast-iron pedestals caused by hot temperatures [4]. The same bridge, which was opened to pedestrians and cyclists after the 2020 event, was wrapped in silver insulation foil in July 2022 heatwave to prevent it from overheating [5]. While wrapping a bridge in insulation foil is a temporary solution to extreme events, there is a need for a long-term sustainable solution to minimise temperature-induced deformations in bridges.

This research investigates if incorporating small-scale photovoltaic (PV) solar panels on the bridge surface can reduce temperature-induced deformations. Solar cells have been incorporated into road infrastructure for a long time. For example, the SolaRoad pilot, a bike lane in Krommenie, Netherlands [6] and the Wattway project, a 1-km vehicular road in Normandy village, France [7] integrated solar cells in the road pavement. However, these innovative projects did not succeed due to excessive wear and tear and unfavourable inclination angle of solar panels [8, 9]. The South Korean Bike Highway eliminates these issues by integrating solar panels on the elevated shade of a 20-km bike lane running parallel to the highway [10]. The elevated panels provide a flexible inclination angle and reduce wear and tear due to traffic load. Solar panels have been successfully integrated to power several bridges such as Blackfriars Bridge, UK and Kennedy Bridge, Germany.

Higher public acceptance of solar panels compared to other renewable alternatives such as wind turbines, due to reduced human hazards, also encourages the implementation of solar panels in bridges. This paper evaluates the structural health benefit of integrating solar panels on bridges in addition to generating renewable energy. The solar panels attached to the bridge surface are expected to utilise solar radiation to generate electricity and reduce temperature load on the bridge elements underneath the panels. The bridge will undergo smaller deformations and thereby increasing its lifespan. The panels are proposed to be installed on exposed bridge surfaces such as truss, deck periphery, roof etc. excluding pavement to avoid direct traffic load. This hypothesis is tested in the laboratory on a bridge truss. An Aluminium truss is equipped with small solar panels and is exposed to 1-h heating and cooling cycles of two infrared heaters. The deformation of the truss before and after the installation of solar panels is monitored with Linear Variable Differential Transformer (LVDT) and strain gauges. Variation of deformations such as displacement and strain before and after solar panel installation reflects the effect of the panels. A reduced deformation will indicate that solar panel installation in strategic locations can improve the structural performance of bridges. The next sections describe the methodology and results of the laboratory experiments.

28.2 Methodology

The solar panels attached to the bridge surface will utilise solar energy to generate electricity, creating a shading effect for the bridge underneath. The bridge will undergo less deformation due to reduced temperature load, thereby enhancing the serviceability and life of the bridge. This hypothesis is tested using experimental methods. Figure 28.1 demonstrates the conceptual framework of the experiments. Bridges deform due to ambient temperature load throughout their lifetime. These bridges are monitored using suitable structural health monitoring (SHM) system under two operational conditions: (i) before installation of solar panels and (ii) after installation of solar panels. The SHM system measures responses such as displacement, strain, surface temperature, etc. The SHM measurements can be analysed to create two outputs: (A) bridge deformation without solar panel installation and (B) bridge deformation with solar panel installation. The magnitude of A being greater than B will indicate that bridge deformation is reduced after solar panel installation.

Fig. 28.1
A flow diagram. Illustrations, A and B, of the bridge without solar panels and bridge with solar panels lead to measuring bridge deformations. It gives 2 outputs of bridge deformation. The results for comparison of A and B under A greater than B, A = B and A less than B are present.

Conceptual framework of the experiment

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28.3 Case Study

The proposed concept is examined in the laboratory with a series of experiments. A test rig is designed to test the effect of solar panels on bridge deformations due to temperature load. This section briefs the experimental setup and enumerates the results.

28.3.1 Description of the Test

Figure 28.2 shows an Aluminium truss equipped with an SHM system and solar panels. The SHM system consists of Linear Variable Differential Transformers (LVDT) and thermocouples to measure displacement and temperature. The truss is painted black. Details of the both end pinned truss are provided elsewhere [11]. The truss is exposed to cyclic heating and cooling cycles using two infrared heaters. Figure 28.3 shows the location of the solar panels and sensors (L: LVDT and T: Thermocouple) on the truss.

Fig. 28.2
A photo of a setup with 2 heaters at 2 ends. It has a horizontal frame with solar panels. It has an enlarged photo of a solar panel on the frame.

Test rig set-up

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Fig. 28.3
2 illustrations of a trapezoidal frame with alternating short and long vertical bars with respective short and long diagonals. The bottom and top chords are labeled. Dots for sensor location are on the top and bottom of some bars. Small bars for solar panels are over the frame.

Layout of the SHM sensors and solar panels, (a) in E2, limited solar panels and (b) in E3, all solar panels. (Symbol T# thermocouple number and L# LVDT number)

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The solar panels are 55 mm × 70 mm in size and have a typical power output of 0.5W. Experiments are carried out in three stages: E1: truss is not equipped with solar panels, E2: truss is equipped with a limited number of solar panels (Fig. 28.3a) and E3: truss is equipped with more solar panels (Fig. 28.3b). The number of panels is increased in stages (E2 and E3) to see the effect of increased panelling on bridge deformations. E1, E2 and E3 were carried out on three different days with heaters placed approximately 1 m away from the truss.

28.3.2 Results and Discussion

The three events, i.e., E1, E2 and E3 are carried out with 1 h of heating and an average 1 h of cooling cycle, except in E3 where the intermediate cooling cycle was 2 h long. Constant supervision is needed when heaters are turned ON to ensure health and safety requirements. The cooling cycles are performed without constant supervision as the heaters are turned OFF. The longer cooling cycle in E3 is caused by the unavailability of the supervisor to monitor the heating cycle after 1 h. Such irregular durations are also seen in the real-world where daily daylight and night durations are not identical. The LVDTs and thermocouples measured displacement and temperature without much noise (see Fig. 28.4). A summary of the temperature and displacement range due to the temperature cycles is listed in Table 28.1. The peaks reported here are the difference between the maximum temperature or displacement and the minimum temperature or displacement during the entire experiment duration.

Fig. 28.4
A line graph of temperature versus displacement versus time elapsed plots 3 sets of 2 fluctuating lines for E 1 without solar panel, E 2 with solar panel, and E 3 with solar panel. The line for temperature has the highest peak in all 3 graphs.

Temperature (in blue) and displacement (in red) time history before and after installation of solar panels

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Table 28.1 Summary of temperature and displacement change due to solar panel installation
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Figure 28.4 shows the displacement time history of L6 and the temperature history of T7 (refer to Fig. 28.3 for sensor location). L6 is towards the left of the bottom chord while T7 is at the centre of the bottom chord. The PV panels are concentrated around L6 in both E2 and E3. However, the T7 location has a denser network of PV panels in E3 than in E2. The base temperature of each location is its mean temperature during the hour before the heating is first turned ON. The temperatures reported in Fig. 28.4 are normalized by deducting the base temperature from the thermocouple readings. A clear reduction in surface temperature and displacement of the truss is evident because of the shading effect of solar panels. Displacement was reduced by 33.9% for a 25.6% reduction in temperature in E2. While displacement was reduced by 34.3% for a 15.0% reduction in temperature in E3.

It is interesting to note a similar reduction in displacement in E3 although temperature reduction due to PV panel mounting is less by 10.6% compared to E2. The higher temperature gradient in E3 (by 2.56 ˚C) can be attributed to the combined effect of (i) human error in the placement of heaters in the exact location and at a constant angle in every event, (ii) a slightly longer first heating cycle in E3 (by 3 min) than E2, (iii) a longer cooling period before the second heating cycle in E3 than E2, and (iv) due to the reflective surfaces of the solar panels. Nevertheless, the displacement range of E2 and E3 are similar despite the higher temperature gradient in E3. Structural deformations are caused by the combined effect of all loads in a structure rather than a local load at a particular location. Thus, the combined shading effect of the extra solar panels in E3 may have caused a similar displacement range in E2 and E3. Further, the peak temperature load above the base temperature in E2 and E3 are comparable (17.66 ˚C and 20.01 ˚C respectively).

The thermal response of real bridges with mounted PV is expected to be similar although not as straightforward as the simplified experiment on the aluminium truss. Steel and concrete bridges will have lower temperature gradients than the aluminium truss due to the lower coefficient of thermal expansion. Further, the temperature rise will be slow in the longer heating and cooling cycles in a day. Yet, the shading of the mounted PV panels will reduce the temperature of the surface underneath it and thereby reduce the deformation.

28.4 Conclusion

This research has investigated the effect of solar panel installation to reduce temperature-induced deformations in bridges. Small size solar panels are suggested to install as tiling on bridge surfaces excluding the pavement. The shading of panels is expected to reduce the bridge surface temperature and thereby reduce temperature-induced deformations such as displacement. The hypothesis was tested in a laboratory aluminium truss equipped with solar panels, LVDT and thermocouples. The truss was subjected to artificial heating and cooling cycles. The following conclusions can be drawn from the results:

  • The temperature of the truss was reduced by 15–25.6% near midspan after the installation of the solar panels.

  • The displacement of the truss was reduced by around 34.0% in experimental events E2 and E3 after solar panels installation.

  • Variation in displacement was found to be similar when the number of solar panels is increased on the surface of the truss. Whereas the temperature gradient increased by 2.56 ˚C when the number of panels are increased. This oddness could be related to experimental error, non-identical setup of the experiment events or caused by the reflective surfaces of the solar panels. This will be investigated further in future work.

The results show the promising potential of solar panel installation to improve the structural health of bridges. This is encouraging to promote more solar panel integration in civil infrastructures. Installing solar panels will enhance the lifespan of bridges and infrastructures due to the reduction in thermal stresses due to solar heating and at the same time provide renewable energy to be utilised in the urban environment.