Throughout history, a strong connection has been established between mankind and the sun. Many prehistoric cultures revered the sun as a god, and modern science has confirmed that the sun and its energy are vital to life on earth. Solar power is usually the subject of science fiction: Isaac Asimov’s short story “The Last Question” (1956). Imagine a small solar collector with a width of 1 mile (=1609.344m) around the earth and supply all of the earth. Electricity, launched in 2061, and some novels explore the application of solar power in long-distance space travel. The advancement of modern solar technology has brought this kind of science fiction story closer to reality, and future technology may only be limited by imagination.
The sun is the source of almost all energy on the earth. Sunlight is a direct form, as well as indirect forms such as wind and waves. Even the coal resources we mine today were once living plants. The way they obtain energy is photosynthesis: the process of converting sunlight, carbon dioxide and water into carbohydrates. The sun’s seemingly endless energy supply is driven by a process called nuclear fusion, in which multiple hydrogen atoms combine to form a helium atom and release huge amounts of energy. Helium atoms can also combine with other helium or hydrogen atoms to release more energy.
The energy produced by the heliocentric is released in the form of electromagnetic radiation. There are many forms of electromagnetic radiation, including microwaves (used in microwave ovens), radio waves (used in telecommunications), and visible light. The design of solar cells focuses on capturing visible light energy.
Quantification of solar radiation
Although the radiation emitted by the sun is quite uniform, the radiation received on the surface of the earth varies greatly due to the earth’s orbit (the reason for the formation of seasons), rotation (the reason for the formation of day and night), and the reflectivity of a specific area (introduced later) . The designer of the photovoltaic array needs to quantify the amount of solar radiation received throughout the year at a specific site. The amount of solar energy received per unit area per day is called exposure, and the unit can be kWh/m2/day or solar peak hours (PSH, later introduction). Note that most photovoltaic arrays are placed on a horizontal surface at a fixed angle, and the exposure of the horizontal plane is different from that of the inclined surface of the photovoltaic array. Usually, the data of the horizontal plane and different inclination angles are required.
The influence of the earth’s atmosphere on solar radiation
The earth’s atmosphere reflects a lot of solar radiation – without this layer of protection, life cannot survive on earth. When solar radiation reaches the top of the earth’s atmosphere, its peak irradiance is 1367W/m2 (called the solar constant). When solar radiation reaches the surface of the earth, the peak irradiance is about 1000W/m2. The reason for the difference between the solar constant and the peak irradiance on the earth’s surface is the earth’s reflectivity-the amount of solar energy reflected off a surface at a specific location on the earth. There are several ways in which the earth reflects light:
●The atmosphere reflects radiation back to space.
●The clouds reflect radiation in the stratosphere.
●The surface of the earth reflects sunlight.
The average rate of the earth reflecting sunlight (earth reflectivity) is 30%. The polar regions have the highest reflectivity because ice and snow reflect most of the sunlight: ocean areas have lower reflectivity because the dark sea water absorbs a lot of sunlight.
Irradiance consists of direct radiation and scattered radiation, and depends on the reflectivity (reflected solar radiation) of a specific location. Scattered radiation is the part of solar radiation that is scattered, absorbed or reflected by the atmosphere. It can be understood that the scattered radiation only accounts for 10% of visible light on a sunny day, but more scattered radiation will reach the surface of the earth on a cloudy day, which means that there is more scattered radiation. Air quality also affects the irradiance of a location. The greater the mass of the atmosphere, the higher the probability of sunlight being reflected or scattered, which means that less solar radiation reaches the surface of the earth.
Air quality 1.5 (AM1.5) is the standard condition for photovoltaic module rating. AM0 refers to the air quality of space; AM1 corresponds to the conditions when the sun is directly above the sky. AM1 will not appear in areas outside the equator, because the sun there will not appear directly above.
Solar radiation varies greatly with location and date. When designing a system, it is very important to consider the solar radiation characteristics of the photovoltaic array installation location. For example, in a German city, a photovoltaic array is used to supply power to a house, and its capacity is significantly greater than that of a photovoltaic array that supplies power to houses of the same energy consumption in the desert area of Australia.
Places such as Poland are far from the equator and receive a lot of solar radiation during the long summer day, but in winter when the day is short, they receive very little solar radiation.
The figure shows that the area near the equator (Kenya) receives significantly more solar radiation than the area near the poles (Belan and Australia). Areas far from the equator also receive greater changes in the amount of solar radiation received
Usually the National Weather Service or PV module suppliers can provide solar radiation data. The NASA website provides data for most parts of the world; the European Union Joint Research Center provides a free web tool-Photovoltaic Geographic Information System (PVGIS), which can estimate the daily output of photovoltaic arrays anywhere in Europe or Africa.