 The solar radiation energy projected vertically onto a unit area in a unit time is called solar radiation intensity. The intensity of solar radiation is a physical quantity that represents the strength of solar radiation, and the unit is J/(cm2·min). Infrared radiation affects the intensity of solar radiation at the upper boundary of the atmosphere. The intensity of solar radiant energy depends on the altitude of the sun.

Figure 1-1 The relationship between solar radiation intensity and solar altitude

(Figure 1-1 shows the relationship between solar radiation intensity and solar altitude), the distance between the sun and the ground, and the sunshine time. The greater the solar elevation angle, the greater the intensity of solar radiation. Because the same beam of light, when it is directed, the irradiation area is the smallest, and the solar radiation obtained per unit area is more; on the contrary, when the ray is slanted, the irradiation area is large, and the solar radiation obtained per unit area is less. The sun’s altitude angle varies from time to time and from place to place. During a day, the solar altitude angle is greater than morning and evening at noon; summer is greater than winter; low latitudes are greater than high latitudes. When the earth is at the perihelion, the solar radiation is greater than the aphelion. When the earth passed the perihelion in early January, the solar radiation per unit area of ​​the earth’s surface was 7% more than when it passed the aphelion in early July. The intensity of solar radiation is proportional to the duration of sunshine. The length of sunshine varies with latitude and season.

1.2 Conversion relationship of solar radiation energy

In unit time, the energy emitted by the sun in the form of radiation is called solar radiant power or radiant flux, in watts (W); the radiant power (radiant flux) projected by the sun onto a unit area is called radiance or irradiance Degree, the unit is watts per square meter (W/m2). This physical quantity usually characterizes the instantaneous intensity of solar radiation; and within a period of time, the radiation energy projected by the sun onto a unit area is called radiation or irradiance, and the unit is kWh/(m²·year (or month, day)]. This physical quantity characterizes the total amount of radiation, and is usually measured as a cumulative value.

(2) Conversion relationship of solar radiation energy

There are many units of solar radiation energy, such as calories, kilocalories, joules, megajoules and so on. In order to bring convenience to the design of solar photovoltaic power generation system, it is necessary to know the conversion relationship between them.

1cal(cal)=4.1868J(joule)=1.1628mWh(milliwatt hour)

lkWh (kilowatt hour) = 1000000mWh (milliwatt hour) = 3.6MJ (megajoule) = 859.845kcal (kcal)

1MJ/m2=23.889cal/cm2=27,78mW·h/cm2=0.2778kW·h/m2

1kW·h/m2=3.6MJ/m2

85.98cal/cm2=100m W. h/cm2

In the measurement and calculation of solar power generation, the most convenient units should be kilowatts (kW) and kilowatt-hours (kWh), and the unit of area should be unified as square meter (m2) as much as possible, and data from different sources should be converted first. This can avoid many calculation errors and errors.

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