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Perfect Artificial Sunlight - Guidance for Broad Applications of Solar Simulators

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What is the solar spectrum?

  Sunlight is a portion of the electromagnetic radiation given off by the Sun. The spectrum of the Sun’s solar radiation is close to that of a black body with a temperature of about 5,800 K. 99.9% of the energy is concentrated in the infrared, visible and ultraviolet regions.

  The solar spectrum is divided into five regions in order of wavelength (Reference: Naylor, Mark; Kevin C. Farmer. Sun damage and prevention; http://telemedicine.org/sundam/sundam2.4.1.html):

  1. UVC (Ultraviolet C): which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth’s surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
  2. UVB (Ultraviolet B): UVB range spans 280 to 315 nm. It is also greatly absorbed by the Earth’s atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn. In addition to this short-term effect, it enhances skin ageing and significantly promotes the development of skin cancer but is also required for vitamin D synthesis in the skin of mammals.
  3. UVA (Ultraviolet A): UVA spans 315 to 400 nm. This band has less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis.
  4. Visible: Visible light spans 400 to 760 nm. This range is visible to the naked eye. It is also the strongest output range of the Sun’s total irradiance spectrum.
  5. Infrared: Infrared spans 700 nm to 1 mm. It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types based on wavelength:
    Infrared – A:760 nm to 1,400 nm
    Infrared – B:1,400 nm to 3,000 nm
    Infrared – C:3,000 nm to 1 mm

  The solar spectrum varies along with the factors of time, atmospheric thickness, and cloud thickness. Scattering from the sky and reflecting from the surrounding will also produce a lot of diffuse light.

Solar irradiance spectrum

Figure 1. Solar irradiance spectrum above atmosphere (yellow) and at earth surface (red) (from Wikipedia: https://en.wikipedia.org/wiki/Sunlight).

What is Air Mass? How does Air Mass affect sunlight?

  Air Mass (AM):Air mass is defined as the degree of influence of the atmosphere on the amount of sunlight received by the Earth’s surface. Different Air Mass also represents different solar spectrum. The elevation angle of the sun’s light changes with time. When the sun is at different elevation angles, the light will pass through the atmosphere of different thicknesses, which is Air Mass.

  Formula of Air Mass (AM): AM= 1/cosθ, θ is the angle between the sun and the ground.

Air Mass AM definition AM0 AM1 AM1.5

Figure 2. Different air mass represents different solar spectrums.

AM0: The spectrum outside the atmosphere, approximated by the 5,800 K black body, is referred to as “AM0”, meaning “zero atmospheres”. Solar cells used for space power applications, like those on communications satellites, are generally characterized using AM0.

AM1: The spectrum after travelling through the atmosphere to sea level with the sun directly overhead is referred to, by definition, as “AM1.” This means “one atmosphere.”

AM1.5: 1.5 atmosphere thickness, corresponds to a solar zenith angle of θ=48.2°. The illuminance for daylight under A.M.1.5 is given as 109,870 lux (corresponding with the A.M. 1.5 spectrum to 1000.4 W/m2).

AM 1.5D: The direct radiation of the test plane when sunlight passes through 1.5 times thickness of the atmosphere at an angle of θ=48.2°.

AM 1.5G: The global radiation of the test plane when sunlight passes through 1.5 times thickness of the atmosphere at an angle of θ=48.2°. The sum of the direct and diffuse radiation.

The difference between AM 1.5G and AM1.5D: Solar radiation refers to the electromagnetic waves and particle streams emitted by the sun into space. The solar radiation energy received by earth is only one-twentieth billionth of the total solar radiation. Common solar radiation nouns are:

  • Direct: When sunlight passes through the atmosphere, part of it reaches the ground without changing its radiation direction.
  • Diffuse: Another portion of solar radiation that changes direction after being reflected and scattered by the atmosphere.
  • Global: The sum of the direct and diffuse solar radiation reaching the ground after the weakening of the atmosphere.

The relationship between Direct/Diffuse/Global is as the formula:Global= Direct * cos(θ)+Diffuse

  Since solar cells are mainly affected by light, the power generation of solar cells are significantly affected in different regions and at different times. In order to consistently evaluate the power generation of solar cells, the international standard of test conditions are defined: AM 1.5, 1000W/m2, 25 °C (AM=1/cosθ).

Solar Constant: The solar constant is a flux density measuring mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays.

It is measured by satellite as being 1366 watts per square meter (AM 0 =1366 W/m2)

The cross-sectional area of the earth is 127,400,000 square kilometers, so the power received by the entire earth is 1.74×1017 watts.

Because there are often sunspots and other solar activities on the surface of the sun, the solar constant is not a physical constant; the variation range is about 1% in a year.

International standards for evaluating solar simulators and related parameters for calculating formula

  AM Spectral Irradiance: When sunlight passes through the atmosphere, it will be affected by air particles and generate a lot of diffuse light. When the incident angle is different, the distance of sunlight passing through the atmosphere is different accordingly, and the degree of scattering is also different. ASTM (American Society for Testing and Materials) and IEC (International Electrotechnical Commission) have formulated the standard values of AM spectral irradiance as follows:

AMInternational StandardIrradiance (W/m2)
AM 0ASTM E4901366.1
AM 1.5 GASTM G1731000.4
AM 1.5 DASTM G173900.1
AM 1.5 GIEC 60904-31000

  IEC 60904-3 defines AM1.5G spectral irradiance data. It is commonly used for the measurement of ground–mounted PV modules or solar cells. There are two types of irradiance wavelength range: Restricted and Extended. Shown as below (Table 1 & Table 2):

  Table 1 – Global reference solar spectral irradiance distribution given in IEC 60904-3 contribution of wavelength intervals to total irradiance in the restricted wavelength range 400 nm to 1100 nm:

 Wavelength range
(nm)
Percentage of total irradiance in the wavelength 400 nm to 1100 nm
(%)
Cumulative integrated irradiance
(%)
1400 to 50018.418.4
2500 to 60019.938.3
3600 to 70018.456.7
4700 to 80014.971.6
5800 to 90012.584.1
6900 to 110015.9100.0

  Table 2 – Global reference solar spectral irradiance distribution given in IEC 60904-3 contribution of wavelength intervals to total irradiance in the extended wavelength range 300 nm to 1200 nm:

 Wavelength range
(nm)
Percentage of total irradiance in the wavelength 400 nm to 1100 nm
(%)
Cumulative integrated irradiance
(%)
1300 to 47016.6116.61
2470 to 56116.7433.35
3561 to 65716.6750.02
4657 to 77216.6366.65
5772 to 91916.6683.31
6919 to 120016.69100.00

  In addition to irradiance, IEC (International Electrotechnical Commission) has published IEC 60904-9, which defines the regulations for evaluating the solar simulators. Important parameters and descriptions are as follows:

  • Solar simulator: equipment employing a light source with a spectral distribution similar to the natural sunlight used to evaluate characteristics of PV devices.
  • Spatial non-uniformity of irradiance: Spatial non-uniformity of irradiance over the designed test area. Use an irradiance detector (ex: photodiode) to measure the irradiance at different positions on the specified irradiation surface (the measuring value is photocurrent), and then calculate it through the following formula:
Non-uniformity Solar Simulator

Note: During the measurement, the step width of each movement of the detector should not be greater than one-fifth of the minimum dimension of the designed test area.

  • Spectral match: spectral match of a solar simulator defined by the deviation from AM 1.5 reference spectral irradiance as laid down in IEC 60904-3. There are two types of the wavelength range: 400~1100 nm (Table 1) & 300~1200 nm (Table 2).
  • Temporal instability: this is a stability index. To ensure the accuracy of solar cell efficiency measurement, the output beam of the solar simulator is required to maintain a stable illuminance. The calculation formula is as follows:
Instability Solar Simulator

According to the different IV measurement systems, the instability can be divided into two categories:

  • Short-term Instability (STI): When there are three separate data input lines that simultaneously store vales of irradiance, current and voltage, the temporal instability is Class A (if spectral classification is performed in the restricted wavelength range 400 nm to 1100 nm) or Class A+ (if spectral classification is performed in the extended wavelength range 300 nm to 1200 nm) for STI. *Note: The delay in simultaneous triggering of the three multiple channels is typically less than 10 ns.
  • Long-term Instability (LTI): For a three channel I-V measurement (irradiance, current, voltage) with a pulsed or stead-state solar simulator, the value is the time for acquiring the I-V characteristic.

  According to IEC 60904-9:2020, the solar simulators are classified below:

ClassificationSpectral match to all intervals specified in Table 1 or Table 2Spatial non-uniformity of irradiance
(%)
Temporal Instability: Short term instability of irradiance, STI
(%)
Temporal Instability: Long term instability of irradiance, LTI
(%)
A+ (*)0.875 to 1.12510.251
A0.75 to 1.2520.52
B0.6 to 1.4525
C0.4 to 2.0101010

Note*:Class A+ is defined for the wavelength range of 300~1200 nm only.

Fields of application of solar simulators

  Solar simulators can be applied in a wide range of fields, including energy science, biotechnology, environmental engineering, etc.:

FieldApplication
Energy SciencePerformance testing of solar cells or PV modules
New energy development (e.g.: water splitting)
BiotechnologyMedical/Cosmetic research and application
Material ScienceNew material development and testing
Photocatalyst
ArchitectureWeathering Life Testing of Building Materials
Color study of coatings/paint
AgricultureExperiments and Tests of Agriculture, Forestry, Fisheries and Animal Husbandry
Environmental EngineeringResearch of the interaction between humans and the environment

How to choose a solar simulator?

SpectrumApplicationRecommended Products
AM 0Space application, SatelliteAM 0 Solar Simulator: SS-ZXR
AM 1.5Photovoltaic Device ResearchAM1.5G Solar Simulator: SS-X
Novel Photovoltaic Devices or Tandem Photovoltaic Device ResearchAM1.5G Solar Simulator (Spectrum Adjustable): SS-PST100R
New energy development (e.g.: water splitting)
Photocatalyst
Xe Light Source: ALS-300-G2
Environmentally sensitive material development (Glovebox integration)Customized Light Source: SS-XRC, SS-FZ5
IndoorResearch on Indoor Photovoltaic DevicesAmbient Light Simulator: ILS-30

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