How to Maximize Urban Cooling By Covering Car Parks with Solar Panels

Car parks without shade present multiple issues that affect both the environment and human comfort. One of the primary problems is the contribution to the urban heat island effect. Traditional car parks are usually paved with dark materials like asphalt, which absorb and retain heat throughout the day.

What are the Effects of Sunlight on a Car Park

When car park surfaces are left exposed to direct sunlight, they can reach extremely high temperatures, sometimes surpassing 60°C (140°F). This intense heat not only makes the car park itself uncomfortable but also raises the ambient temperature in surrounding areas. This leads to increased energy consumption as nearby buildings require more air conditioning, contributing to higher electricity costs and greater carbon emissions due to the elevated demand for cooling.

Moreover, vehicles parked in unshaded areas can suffer from quite some heat build-up inside. The temperature inside a car exposed to direct sunlight can become much hotter than the outside air temperature, creating an uncomfortable and sometimes dangerous environment, particularly for children, pets, or the elderly who may be left inside. The high interior temperatures can also cause damage to the vehicle’s interior, such as fading upholstery and cracking dashboards. Additionally, surfaces like metal seatbelt buckles, steering wheels, and other parts of vehicles can become dangerously hot when left in unshaded areas, posing a burn hazard to anyone entering the vehicle. On top vehicles that have been sitting in the sun often require more fuel to cool down the interior once the engine is started, leading to higher fuel consumption and increased greenhouse gas emissions.

And that’s not all. The infrastructure of car parks also suffers from the lack of shade. Continuous exposure to high temperatures accelerates the deterioration of asphalt and other paving materials, leading to more frequent maintenance needs and higher costs. The hot surfaces can also exacerbate stormwater runoff issues, as the heat causes pollutants like oils and chemicals to be more readily washed off into stormwater systems during rain, contributing to water pollution.

As far as health is concerned, prolonged exposure to high temperatures while walking across a hot car park can lead to heat exhaustion or heat stroke, posing a serious risk to public health.

A Solution: Covering Car Parks with Solar Panels

Covering car parks with solar panels is a smart choice. Car parks are already impervious surfaces, often sitting idle under the sun, making them perfect for solar energy capture without further disrupting the environment.

In the below tweet on X, it’s even mentioned that by contrast, placing panels on farmland can reduce agricultural productivity and impact local ecosystems. A reason more to use solar panels on non-productive or idle land such as a carpark.

There are several studies that have examined the temperature effects of installing solar panels on car parks and other urban surfaces. These studies often focus on the balance between the shading benefits provided by solar panels and the potential heat generated by the panels themselves.

Here’s a summary of some key findings:

1. Shading and Surface Temperature Reduction

  • Study on Solar Canopies in Car Parks: A study conducted in California analyzed the impact of solar canopies installed over car parks. The study found that the shaded areas beneath the solar panels experienced a significant reduction in surface temperature, often by as much as 20-30°C (36-54°F) compared to nearby unshaded asphalt surfaces. This temperature reduction was primarily due to the panels blocking direct sunlight from reaching the ground, which is a major contributor to the urban heat island effect.
  • Cooling Effect on Surrounding Areas: The same study noted that the air temperature in the immediate vicinity of the solar canopies was also cooler, though the reduction in air temperature was less pronounced than the reduction in surface temperature. The cooling effect is generally more noticeable closer to the surface.

2. Thermal Behavior of Solar Panels

  • Impact on Surface Temperatures: Research has shown that solar panels can reach surface temperatures of 40-50°C (104-122°F) or more, depending on the ambient conditions and the efficiency of the panels. However, because these panels are often mounted above the ground with air circulation between the panels and the surface, the heat is dissipated, reducing the warming effect on the ground below.
  • Net Temperature Effect: Another study from Arizona State University looked at solar panels on both rooftops and parking lots. The study concluded that while solar panels do get hot, the net effect on urban temperatures is complex. The panels reduce the amount of sunlight hitting the ground, thereby lowering surface temperatures, while the panels themselves radiate some heat. The net impact was found to be generally neutral to slightly cooling, especially when panels are placed over highly absorptive surfaces like dark asphalt.

3. Long-Term Temperature Impacts

  • Urban Heat Island Mitigation: A broader study on urban heat island effects and solar energy installations suggests that widespread use of solar panels, particularly over car parks and other impervious surfaces, can contribute to the mitigation of urban heat islands. This effect is more pronounced when solar panels replace traditional dark surfaces, which are major contributors to urban heating.

4. Comparative Studies on Different Surfaces

  • Solar Panels vs. Vegetated Areas: Comparative studies have shown that while vegetated areas like green roofs offer greater cooling benefits due to evapotranspiration, solar panels still offer a net cooling effect compared to bare asphalt or concrete surfaces. When combined with green infrastructure, the cooling benefits are maximized.

And We Can Go Way Further

There are ways to mitigate the urban heat island effect by using different underground surfaces in conjunction with solar panels installed on car parks. Here’s how you can approach this:

1. Cool Pavements

  • High-Albedo Materials: Replacing traditional asphalt with cool pavements that have higher reflectivity (albedo) can significantly reduce surface temperatures. Materials such as light-colored concrete, permeable pavers, or specially coated asphalt can reflect more sunlight, thereby reducing heat absorption and lowering the surrounding air temperature.
  • Permeable Pavements: These allow water to pass through the surface, reducing runoff and enhancing groundwater recharge. The water stored in the underlying layers can also cool the surface as it evaporates, further mitigating heat.

2. Vegetated Surfaces

  • Green Parking Lots: Incorporating green spaces or vegetated areas into car parks can reduce the heat absorbed by surfaces. Techniques such as planting grass between parking spots, using ground covers, or integrating bioswales and rain gardens can help cool the area through evapotranspiration.
  • Tree Canopy Integration: In addition to solar canopies, planting trees around the car park can provide additional shading and reduce temperatures. Trees absorb sunlight and cool the environment through the process of transpiration, where water vapor is released into the air.

3. Underground Cooling Systems

  • Geothermal Heat Exchange: Implementing a geothermal system beneath the car park can help dissipate heat. These systems use the stable temperatures found underground to cool the surface above. During the summer, excess heat from the surface can be transferred below ground, reducing surface temperatures.
  • Subsurface Water Reservoirs: Installing subsurface water reservoirs or channels can help cool the area. Water stored underground can absorb heat from the surface, and as it evaporates or is released, it cools the surrounding area.

4. Reflective or Cool Roofs on Parking Structures

  • Cool Roofing Materials: If the car park includes a parking structure, using cool roofing materials on the top of the structure can reflect more sunlight and reduce the overall heat absorbed by the building. This complements the cooling effect of solar panels installed on the roof.

5. Thermal Energy Storage

  • Phase Change Materials (PCMs): Integrating phase change materials beneath the car park surface can store excess heat during the day and release it at night. PCMs absorb and release thermal energy during the process of melting and solidifying, helping to regulate temperature fluctuations.

6. Combination Approaches

  • Integrated Design: The most effective solution may involve combining several of these approaches. For instance, using cool pavements in conjunction with subsurface water reservoirs and vegetated areas can create a multi-layered cooling effect. This holistic design approach maximizes the reduction of the urban heat island effect while also enhancing the aesthetic and environmental value of the space.

7. Advanced Paving Technologies

  • Photocatalytic Pavements: These surfaces contain compounds like titanium dioxide, which not only reflect sunlight but also help break down pollutants in the air, improving air quality in addition to reducing heat.
  • Thermochromic Surfaces: These surfaces change color based on temperature, reflecting more light when it’s hot and absorbing more when it’s cooler, thus providing a dynamic response to environmental conditions.

Shading is King

The majority of studies indicate that solar panels, when installed over car parks, tend to reduce the surface temperature beneath them due to shading. While the panels themselves do heat up, the overall impact on the surrounding environment can be neutral or even slightly cooling, especially when compared to traditional urban surfaces like asphalt or concrete. These findings suggest that using solar panels in urban environments, including car parks, can be a viable strategy for mitigating some of the urban heat island effect.