More than two-thirds of the world’s population lives in urban areas, and the problem of heat islands has been exacerbated by increasing urbanization. Urban heat islands are areas where air temperatures are much hotter than in surrounding rural regions. In a study published by NASA, scientists found that older cities sitting on top of solar energy collectors can help reduce local temperature differences between daytime and nighttime by up to 5 degrees Fahrenheit.
Impact of Solar Energy on Microclimates:
1. Temperature Difference Between Day and Night
The amount of solar radiation facing a city depends on the position of the Sun relative to the Earth’s axis, which is tilted at an angle to the plane of its orbit. Differences can be substantial over short distances, particularly in low and flat cities. When the Sun is near the horizon at noon in an urban area, it receives less direct sunlight than higher above the horizon. But even during midday, with no direct sunlight, an urban area receives more diffuse light than open countryside. Therefore in a city, temperatures are typically higher when there’s no direct sunlight and lower when it’s sunny.
2. Quality of Direct Solar Radiation
The intensity of the Sun’s radiation at the Earth’s surface is greater in an urban area than in a rural one. This is because cities tend to be located in higher latitudes, closer to the equator.
The intensity of solar radiation decreases with distance from the equator toward both poles, so it becomes greater as you move from the equator to the pole. In addition, wavelengths of solar radiation affect temperature differently: shorter waves have a larger impact than longer ones. The Sun produces more intense radiation at noon than at dawn or dusk for a given wavelength.
3. Altitude
The altitude at which a solar collector is located influences its ability to deliver energy. Many cities are built on higher ground because they require more infrastructure to compress the air and provide drainage, so the air is cooler at lower altitudes. A city’s altitude can affect heat islands in several ways:
This effect varies depending on where in the world you’re located. In Africa, while many cities and towns sit at higher elevations than surrounding rural lands when looked at above, there isn’t a sizeable difference between urban areas and rural areas. However, the difference between urban and rural areas is appreciably more marked for cities in northern Europe and North America.
4. Population Density
In a city, the amount of air moving through a solar collector increases as the population density increases. The effect of air movement on solar energy production is known as the cool breeze effect: an increase in air movement causes an increase in heat transfer to the collector surface resulting in higher energy production. This effect varies with latitude: at higher latitudes, where there are fewer days with clear skies, average temperature differences between day and night are reduced, and this effect is less significant because of less turbulence caused by high population density.
5. Wind Speed and Latitude Variation
The wind speed varies with latitude. At higher latitudes, strong winds occur fewer times each year and less often during the day than at lower latitudes, where tropical cyclone-type disturbances occur more frequently from June to September than from October to May (when polar lows are more frequent). This results in a lack of continuity of solar radiation over the year, leading to much smaller temperature differences between day and night.
6. Solar Collector Efficiency
The efficiency of a solar collector is affected by many factors, including the design; orientation; shape, and size of the collector surface area; the efficiency and quality of its components, such as its mirrors; and even air pressure and humidity in the vicinity of the collector.
7. Distance From The City Center to Collectors
In cities, they are often located further away from their population centers than rural areas because they need larger infrastructure, such as road and drainage systems.
8. Day and Night Side Orientation of Collectors
Solar collectors on the street level are more efficient at heating daylights than those at a higher level. It’s been found that the orientation of a collector is also critical, as well as its location on the ground, with greater effectiveness achieved when the collector is oriented at roughly 45 degrees to horizontal.
9. Urban Heat Island Effect
As cities grow, so do the number of buildings and their surfaces—they cover more land area, and their buildings are taller and denser than rural areas. Collectively all these factors lead to a phenomenon known as the urban heat island effect or the difference in temperature between urban and rural areas. UHI is already well documented, but how it impacts solar collectors has received little attention.
During the day, as temperatures rise higher, convective heating of buildings decreases, and air conditioning increases. This extra cooling is essential for comfort but causes a small decrease in solar energy production.
10. Day vs. Night Solar Energy Production
Initially, more energy is produced at night than during daylight hours because there’s less air movement, reducing the collector surface’s convective cooling. This difference disappears as the solar collector age and becomes more efficient.
11. Collection Orientation and Aerodynamics
A solar collector’s orientation doesn’t significantly impact its radiation properties either day or night. As the collector ages, it gradually becomes more effective at producing solar energy because of increased surface area; therefore, it is less important to orient it correctly. However, if the orientation is at right angles to horizontal, the collector can be more efficient in the summer but less efficient in winter when there’s less daylight.
12. Average Air Temperature and Solar Collector Efficiency
A strong correlation exists between average air temperature, solar energy production, and collector efficiency. The impact of average temperature must be considered when assessing the efficiency of a solar collector to ensure that it has been installed correctly—it should not be assessed using ideal laboratory conditions such as those seen in Europe, where average temperatures are around 15°C.
What this means for your project is that if the site you plan to install your solar collectors in is below 50° latitude, you will likely see a reduced energy output during winter months.
Solar energy is a cheap, clean, and reliable form of energy, and there are now suitable technologies to convert sunlight into electricity. However, the process is energy-intensive, which means that the cost of getting power from solar is one of the highest. Concentrating solar power has been around less than other technologies, so we must look to the future to see how solar technology will develop.
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