Impact of urbanization: Urban or urban heat island - avoided through solar roofing while generating electricity

An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than the surrounding rural areas due to human activity. The temperature difference is usually greater at night than during the day and most pronounced when winds are weak. UHI is particularly noticeable in summer and winter. The primary cause of the UHI effect lies in changes to the land surface. One study has shown that heat islands can be influenced by proximity to different types of land cover, such that proximity to barren land leads to warming of the urban soil, while proximity to vegetation makes it cooler. Waste heat generated by energy use is another factor. As a population center grows, its area increases, and the average temperature rises. The term "heat island" is also used; it can refer to any area that is relatively hotter than its surroundings but generally refers to areas disturbed by human activity.

Monthly rainfall is higher in the rain shadow of cities, partly due to the UHI. The increasing heat in urban centers lengthens growing seasons and reduces the occurrence of weak tornadoes. The UHI worsens air quality by increasing the production of pollutants such as ozone, and it degrades water quality as warmer water flows into the region's rivers, stressing their ecosystems.

Not all cities exhibit a pronounced urban heat island effect, and its characteristics depend heavily on the background climate of the area in which the city is located. The urban heat island effect can be mitigated by green roofs, passive radiative cooling during the day, and the use of light-colored surfaces in urban areas, which reflect more sunlight and absorb less heat. Urbanization has exacerbated the impacts of climate change in cities.

The phenomenon was first studied and described by Luke Howard in the 1810s, although he was not the one who named it. Research into the urban atmosphere continued into the nineteenth century. Between the 1920s and 1940s, researchers in Europe, Mexico, India, Japan, and the United States, working within the emerging fields of local climatology or microscale meteorology, sought new methods to understand the phenomenon. In 1929, Albert Peppler used the term "urban heat island," which is considered the first example of an urban heat island. Between 1990 and 2000, approximately 30 studies were published annually; by 2010, this number had risen to 100, and by 2015, it had exceeded 300.

There are several causes of the urban heat island effect. Dark surfaces absorb significantly more solar radiation, causing streets and buildings in urban areas to heat up more during the day than in suburban and rural areas. Materials commonly used for road surfaces and roofs in urban areas, such as concrete and asphalt, have significantly different thermal volume properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than those of the surrounding rural areas. This alters the energy balance of the urban area, often resulting in higher temperatures than in the surrounding rural areas. Another important reason is the lack of evapotranspiration (e.g., due to a lack of vegetation) in urban areas. The U.S. Forest Service found in 2018 that cities in the United States lose 36 million trees every year. With the decline in vegetation, cities also lose the shade and cooling effect of trees through evaporation.

Other causes of urban heat islands (UHI) are due to geometric effects. The tall buildings in many urban areas offer multiple surfaces for the reflection and absorption of sunlight, thus increasing the efficiency of urban heat islands. This is known as the "urban canyon effect." Another effect of buildings is the blocking of wind, which also prevents cooling by convection and the removal of pollutants. Waste heat from cars, air conditioners, industry, and other sources also contributes to the UHI effect. High levels of pollution in urban areas can also exacerbate UHI, as many forms of pollution alter the radiative properties of the atmosphere. UHI not only increases temperatures in cities but also ozone concentrations, since ozone is a greenhouse gas whose formation accelerates with rising temperatures.

In most cities, the temperature difference between the urban and surrounding rural areas is greatest at night. While the temperature difference is considerable throughout the year, it is generally greater in winter. The typical temperature difference between the city center and the surrounding fields is several degrees. The temperature difference between a city center and the surrounding suburbs is sometimes mentioned in weather reports, e.g., 20°C in the city center, 18°C ​​in the suburbs. The average annual air temperature of a city with 1 million inhabitants or more can be 1.0–3.0°C warmer than the surrounding area. In the evening, the difference can be as much as 12°C.

The urban heat island effect (UHI) can be defined either as the air temperature difference (the canopy UHI) or as the surface temperature difference (surface UHI) between urban and rural areas. Both exhibit slightly different diurnal and seasonal variability and have different causes.

The IPCC noted that “urban heat islands are known to increase nighttime temperatures more than daytime temperatures compared to non-urban areas.” In Barcelona, ​​Spain, for example, daytime maximum temperatures are 0.2°C cooler and minimum temperatures 2.9°C warmer than at a nearby rural station. A description of the very first UHI report by Luke Howard from the late 1810s states that central London is 2.1°C warmer at night than the surrounding countryside. Although the warmer air temperature within the UHI is generally most noticeable at night, urban heat islands exhibit significant and somewhat paradoxical daytime behavior. The air temperature difference between the UHI and the surrounding area is large at night and small during the day. The opposite is true for the surface temperatures of the urban landscape within the UHI.

During the day, especially under clear skies, urban surfaces heat up through the absorption of solar radiation. Surfaces in urban areas tend to heat up faster than those in the surrounding rural areas. Due to their high heat capacity, urban surfaces act as a vast reservoir of thermal energy. For example, concrete can store approximately 2,000 times more heat than a comparable volume of air. Therefore, the high daytime surface temperatures within the urban heat island (UHI) are easily detected by thermal remote sensing. As is often the case with daytime warming, this warming also leads to convection winds within the urban boundary layer. It is thought that, due to the resulting atmospheric mixing, the disturbance of the air temperature within the UHI is generally minimal or nonexistent during the day, even though surface temperatures can reach extremely high levels.

At night, the situation reverses. The absence of solar heating leads to a decrease in atmospheric convection and a stabilization of the urban boundary layer. If the stabilization is sufficient, an inversion layer forms. This traps the urban air near the surface, keeping it warm by the still-warm urban surfaces, resulting in warmer nighttime air temperatures within the urban heat island (UHI). Aside from the heat-retaining properties of urban areas, the nighttime maximum in street canyons could also be due to obstructed views of the sky during cooling: surfaces lose heat at night primarily through radiation to the comparatively cool sky, and this is blocked by buildings in an urban area. Radiative cooling is more dominant when wind speeds are low and the sky is clear, and indeed, the UHI is greatest at night under these conditions.

The Intergovernmental Panel on Climate Change (IPCC) – The Intergovernmental Panel on Climate Change is an intergovernmental body of the United Nations responsible for advancing our knowledge of human-induced climate change. It was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) and subsequently endorsed by the United Nations General Assembly. Based in Geneva, Switzerland, it comprises 195 member states. The IPCC is governed by its member states, which elect a Board of Scientists to serve for the duration of an assessment cycle (usually six to seven years). The IPCC is supported by a Secretariat and various Technical Support Units, consisting of specialized working groups and task forces.

The IPCC provides objective and comprehensive scientific information on human-induced climate change, including its natural, political, and economic impacts and risks, as well as possible responses. The IPCC does not conduct its own research or monitor climate change; instead, it undertakes a regular, systematic review of all relevant published literature. Thousands of scientists and other experts volunteer to review the data and compile the key findings in assessment reports for policymakers and the public.

The IPCC is an internationally recognized authority on climate change, and its work enjoys broad support among leading climate scientists and governments. Its reports play a key role in the United Nations Framework Convention on Climate Change (UNFCCC), with the Fifth Assessment Report significantly influencing the landmark 2015 Paris Agreement. The IPCC, together with Al Gore, was awarded the Nobel Peace Prize in 2007 for its contribution to our understanding of climate change.

In 2015, the IPCC began its sixth assessment cycle, which is scheduled to conclude in 2023. In August 2021, the IPCC published its Working Group I contribution to the Sixth Assessment Report (IPCC AR6) on the physical basis of climate change, which The Guardian described as the most serious warning yet of major, unavoidable, and irreversible climate change—a topic picked up by many newspapers worldwide. On February 28, 2022, the IPCC published its Working Group II report on impacts and adaptation. The Working Group III contribution to the Sixth Assessment Report, on climate change mitigation, was published on April 4, 2022. The Sixth Assessment Report is scheduled to conclude with a synthesis report in March 2023.

During the period of the Sixth Assessment Report, the IPCC published three special reports: the Special Report on Global Warming of 1.5 °C in 2018, and the Special Report on Climate Change and Land (SRCCL) and the Special Report on Ocean and Cryosphere in a Changing Climate (SROCC), both in 2019. It also updated its methodologies in 2019. Therefore, the sixth assessment cycle has been described as the most ambitious in the IPCC's history.

The temperature difference of the urban heat island effect is not only greater at night than during the day, but also greater in winter than in summer. This is especially true in snowy regions, as snow typically lingers for a shorter time in cities than in the surrounding rural areas (this is due to the greater insulating capacity of cities as well as human activities such as plowing). This reduces the albedo (a measure of the brightness of a body) in the city, thus amplifying the warming effect. Higher wind speeds in rural areas, particularly in winter, can also contribute to cooler temperatures compared to urban areas. In regions with distinct wet and dry seasons, the urban heat island effect is more pronounced during the dry season. The thermal time constant of moist soil is much higher than that of dry soil. Consequently, moist soils in rural areas cool down more slowly than dry soils, helping to minimize the nighttime temperature difference between urban and rural areas.

If a city or municipality has a good weather monitoring system, the urban heat island effect (UHI) can be measured directly. Alternatively, a complex simulation of the location can be used to calculate the UHI, or an empirical approximation method can be employed. Such models make it possible to incorporate the UHI into estimates of future temperature increases in cities due to climate change.

In 1969, Leonard O. Myrup published the first comprehensive numerical treatment for predicting the effects of the urban heat island (UHI). In his work, he provides an overview of the UHI and criticizes the existing theories as being too qualitative. A general numerical energy budget model is described and applied to the urban atmosphere. Calculations for several special cases, as well as a sensitivity analysis, are presented. The model is found to correctly predict the magnitude of the urban temperature surplus. The heat island effect is the net result of several competing physical processes. In general, reduced evaporation in the city center and the thermal properties of urban building and paving materials are the dominant parameters. It is proposed that such a model could be used in engineering calculations to improve the climate of existing and future cities.

Asphalt +
Asphalt parking & solar carport power generation
= Functionality expansion & densification
= Measure against urban heat islands

Asphalt has become increasingly popular for covering cities in recent years. This is due to the fact that asphalt is a very durable and inexpensive surface. However, asphalt also has some disadvantages, especially when used in large quantities in urban areas.

One of the biggest disadvantages of asphalt is its significant heat absorption. This is a problem because cities are already very hot during the summer months, and the numerous asphalt surfaces exacerbate the heat. As a result, city residents suffer greatly from the heat, and it can even lead to health problems.

Overheating in cities is a major problem caused by the use of asphalt. Several options exist to counteract this problem. One is to create more green spaces in cities, as trees and plants can absorb heat. The use of solar carports or solar parking facilities can also help reduce urban heat. These facilities are equipped with photovoltaic modules that harness solar energy to generate electricity. At the same time, they provide shade, thus reducing the heating of the surrounding area.

Solar carports and solar parking facilities are therefore a good way to reduce urban heat island effects. They are not only sustainable, as they do not burn fossil fuels and thus produce no CO2 emissions, but also help to make urban temperatures more comfortable.

A study by DeLorean Power in Switzerland found that employee parking behavior ideally corresponds to the amount of solar power generated. The electric vehicle's daily mileage can be covered in almost any weather, and any surplus energy can be fed into the grid. The annual solar power generation in the parking lot matches the vehicle's energy needs. Solar parking lots have the greatest potential for electricity generation of all infrastructure sectors. In Switzerland, there are approximately two parking spaces available for every registered car. In suitable regions, this could generate over 10 terawatt-hours of solar power per year (15% of current electricity consumption). "It's astonishing how few pilot plants there are," the study authors stated. Furthermore, such a roof protects the car from the elements and reduces heat buildup in the summer.

According to an analysis by the Federal Statistical Office (FSO), Switzerland has at least 5 million above-ground parking spaces (6,400 hectares) with approximately 4.7 million registered passenger cars. These parking areas were recorded using a digital method that only identifies larger adjacent areas and not individual parking spaces. Traffic experts therefore estimate that there are between 8 and 10 million parking spaces. That's about two per car.

According to another study, “Solar Power Generation for Infrastructure Facilities and Conversion Areas,” above-ground or open parking areas have the greatest PV potential of all infrastructure areas. These areas can supply up to 10 terawatt-hours (TWh) of PV electricity per year. This brings the total electricity production in Switzerland to 65.5 TWh.

The average parking area is 12.5 square meters (2.5 meters x 5 meters). This is also the area that a solar roof must cover. The energy yield of a PV system depends on many factors, including solar irradiance, component efficiency, and module orientation. In Thurgau, approximately 1000 kWh of electricity per year can be generated with 1 kW of installed PV capacity (1000 kWh per 1 kWp).

Depending on the PV modules used, 1 kWp requires an installed capacity of 4 to 8 square meters. This study assumes 5 m² per kWp. Therefore, a 12.5 m² parking space with a 2.5 kWp system can be installed, generating 2,500 kWh of solar power per year. The average Swiss household consumption is around 4,500 kWh/year (excluding heating, ventilation, and electric vehicles).

The modular design of a carport system is advantageous, allowing the roof to be adapted to almost any parking space, thus ensuring continued good utilization of the parking area and guaranteeing expandability.

Bifacial modules allow for increased light transmission through the carport. This is visually appealing and leads to higher solar yields, as these PV modules can also utilize light entering from below, thus delivering 10-20% more energy. Currently, bifacial technology is not widely used because its economic viability is not guaranteed due to higher module prices. However, it is expected that this technology will become more established in the coming years.

In our 4+2+ modular and scalable solar carport system, which uses semi-transparent and bifacial modules, these points apply and are already an additional price-competitive alternative :

Solar roofing options specifically for vehicles – Image: Xpert.Digital

More about it here:

• Numbers, data, facts: solar carport, carport with solar roof, roofing variants in comparison and model parking space with electricity yield

Limitless: Modular and scalable solar carport system for cars and trucks

Limitless: Modular and scalable solar carport system for cars and trucks

Technical specifications: Modular and scalable solar carport system for cars and trucks

Technical specifications: Modular and scalable solar carport system for cars and trucks

Advantages at a glance:

• Flexible and modular (scalable) design
• Clearance height for cars from 2.66 m (extendable to 4.5 m or more for trucks)
• Parking space depth for cars up to 6.1 m, opposite side up to 12.5 m possible.
  The depth depends on the dimensions of the solar modules used.
• The solar carport system is optimally designed for semi-transparent solar modules with
  12%/40% light transmission (!) – and is certified for overhead mounting.
• Optionally available with powerful LED lighting, dimmable and with motion control
• Also suitable for parking spaces with inclined positioning
• No hidden costs regarding foundations.
  Use of point foundations (most economical option, no extensive excavation for concrete slabs etc. is necessary for structural stability) or installation with base plates, depending on the existing soil conditions/asphalt.

Further sources:

• Cost factor of ground foundation for solar carports
• Solar carports where standard no longer applies – The optimal solution for every challenge with solar roofing for open parking spaces
• Solar carport systems: Which is the better and/or more cost-effective option?
• The solar carport strategy for open parking spaces
• The modular solar carport system for all applications and situations

Truck solar carport system

Due to the fact that the 4+2+ column technology offers the most flexible solution (both technically and in terms of price) for a parking space roofing system, it can also be easily extended and applied to larger vehicles such as trucks with appropriate modifications.

Ant colonies in urban heat islands have an increased tolerance for heat without this coming at the expense of their tolerance for cold.

Species that are able to adapt well can take advantage of the conditions created by urban heat islands to thrive in regions outside their normal range. Examples include the grey-headed flying fox (Pteropus poliocephalus) and the house gecko (Hemidactylus frenatus). Grey-headed flying foxes found in Melbourne, Australia, colonized urban habitats after temperatures there rose. The temperature increase and the resulting warmer winters make the urban climate more similar to the species' more northerly habitat in the wild.

Attempts to mitigate and manage urban heat islands reduce temperature fluctuations and the availability of food and water. In temperate climates, urban heat islands extend the growing season, thereby altering the reproductive strategies of the species living there. This is best observed in the effects that urban heat islands have on water temperature. Because the temperature of nearby buildings sometimes differs from the surface air temperature by more than 28°C, precipitation warms rapidly, causing runoff into nearby streams, lakes, and rivers (or other bodies of water) to experience excessive thermal pollution. This increased thermal pollution has the potential to raise water temperature by 11 to 17°C (20 to 30°F). This increase causes thermal stress and shock for fish species living in these waters due to the rapid temperature change in their habitat.

Urban heat islands, caused by cities, have altered the natural selection process. Selection pressures such as temporal variations in food, predators, and water are reduced, allowing a range of new selection forces to come into play. For example, there are more insects in urban habitats than in rural areas. Insects are ectothermic, meaning they rely on the ambient temperature to regulate their body temperature, so the warmer urban climate is ideal for their survival. A study of Parthenolecanium quercifex (oak scale insects) conducted in Raleigh, North Carolina, showed that this particular species prefers warmer climates and is therefore more numerous in urban habitats than on oak trees in rural areas. Over time, they have adapted to thrive in warmer climates rather than cooler ones.

The presence of non-native species is heavily dependent on human activity. A prime example is the populations of crag martins that nest under the eaves of buildings in urban areas. They take advantage of the protection humans provide in the upper reaches of buildings, leading to an increase in their populations due to the added shelter and reduced predator pressure.

Beyond their effects on temperature, ultra-high temperatures (UHIs) can have secondary effects on local meteorology, including changes in local wind patterns, cloud and fog development, humidity, and precipitation. The additional heat generated by UHIs leads to stronger upward motion, which can trigger additional shower and thunderstorm activity. Furthermore, UHIs create a local low-pressure area during the day, drawing in relatively moist air from the surrounding rural areas, which can lead to more favorable conditions for cloud formation. Precipitation amounts in the rain shadow of cities are increased by 48% to 116%. Partly as a consequence of this warming, monthly precipitation within a radius of 20 miles (32 km) to 40 miles (64 km) downwind of cities is about 28% higher than upwind. In some cities, total precipitation has increased by 51%.

In a few areas, studies have suggested that metropolitan areas are less prone to weak tornadoes due to turbulent mixing caused by the urban heat island effect. Using satellite imagery, researchers discovered that urban climates have a noticeable impact on growing seasons up to 10 kilometers (6.2 miles) from the city limits. In 70 cities in eastern North America, the growing season in urban areas was about 15 days longer than in rural areas outside the city's influence.

Studies in China have shown that the urban heat island effect contributes to global warming by about 30%. On the other hand, a 1999 comparison of urban and rural areas suggested that the urban heat island effect has only a minor influence on the development of the global average temperature. One study concluded that cities alter the climate in an area two to four times larger than their own surface area. Another states that urban heat islands influence the global climate by affecting the jet stream. Several studies have shown that the effects of heat islands are becoming increasingly pronounced as climate change progresses.

Urban heat islands (UHIs) can directly impact the health and well-being of city dwellers. In the United States alone, an average of 1,000 people die each year as a result of extreme heat. Because UHIs are characterized by elevated temperatures, they can potentially increase the intensity and duration of heat waves in cities. Research has shown that the mortality rate during a heat wave increases exponentially with the peak temperature, an effect amplified by UHIs. The number of people exposed to extreme temperatures is increased by UHI-induced warming. The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives city dwellers of the nighttime cooling found in rural areas.

Research in the United States suggests that the link between extreme temperatures and mortality varies by location. Heat tends to increase the risk of death in northern cities rather than in southern regions. For example, when Chicago, Denver, or New York experience unusually hot summer temperatures, an increase in illness and death is to be expected. Conversely, parts of the country that are mild to hot year-round face less of a public health risk from excessive heat. Research indicates that residents of southern cities like Miami, Tampa, Los Angeles, and Phoenix are more accustomed to hot weather and therefore less vulnerable to heat-related deaths. Overall, however, people in the United States appear to be becoming more accustomed to hotter temperatures with each passing decade, although this could be due to better infrastructure, more modern buildings, and greater public awareness.

It has been reported that higher temperatures can lead to heatstroke, heat exhaustion, heat syncope, and heat cramps. Some studies have also investigated how severe heatstroke can lead to permanent damage to organ systems. This damage can increase the risk of premature death because it can result in severe impairment of organ function. Other complications of heatstroke include respiratory distress syndrome in adults and disseminated intravascular coagulation (DIC). Some researchers have found that any impairment of the human body's ability to thermoregulate theoretically increases the risk of death. This includes conditions that can affect a person's mobility, consciousness, or behavior. Researchers have found that people with cognitive problems (e.g., depression, dementia, Parkinson's disease) are more vulnerable in high temperatures and need to be especially careful, as heat has been shown to affect cognitive performance to varying degrees. People with diabetes, obesity, sleep deprivation, or cardiovascular/cerebrovascular disease should avoid excessive heat exposure. Some common medications that affect thermoregulation can also increase the risk of death. These include anticholinergics, diuretics, phenothiazines, and barbiturates. Heat can affect not only health but also behavior. A US study suggests that heat can make people more irritable and aggressive, noting that the number of violent crimes increased by 4.58 per 100,000 for every degree Celsius increase in temperature.

A researcher found that high UHI intensity correlates with elevated concentrations of air pollutants, which accumulate at night and can affect the air quality the following day. These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. The production of these pollutants, combined with the higher temperatures in UHIs, can accelerate ozone formation. Surface ozone is considered a harmful pollutant. Studies suggest that higher temperatures in UHIs can increase the number of polluted days, but also indicate that other factors (e.g., air pressure, cloud cover, wind speed) can also influence pollution. Studies from Hong Kong have found that neighborhoods with poorer ventilation of the urban outdoor air tend to experience stronger effects of the urban heat island effect and have significantly higher overall mortality compared to areas with better ventilation.

The Centers for Disease Control and Prevention note that “it is difficult to make valid predictions about heat-related illnesses and deaths under different climate change scenarios” and that “heat-related deaths are preventable, as demonstrated by the decline in overall mortality during heat events over the past 35 years.” However, some studies suggest that the health impacts of UHI may be disproportionate, as the effects can be unevenly distributed based on age, ethnicity, and socioeconomic status. This raises the possibility that the health impacts of UHI are an environmental justice issue.