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Solar Cells and Arrays: Principles, Analysis, and Design

Solar Cells and Arrays: Principles, Analysis, and Design

INTRODUCTION #

The main energy source in our earth is the sun radiation. The solar radiation amounts to 1.7  1017 W; 34% will be reflected back, 42% will be converted to heat directly, 23% is stored in water vapor, wind water waves consumes about 1%, and plants consume 0.023%

The human consumption comes from fossil fuel, nuclear energy from uranium, and geo heat. Any forms of energy are converted to heat and ultimately to radiation. Till now, there is no appreciable direct conversion path from the solar radiation to the human consumption. This is because:

  • There is no efficient conversion machine till now,
  • The solar radiation has a low density,
  • The solar power is not constant. It varies daily, from season to season, and also from place to place on the earth.

GENERAL PHOTOVOLTAIC SYSTEM #

The photovoltaic (PV) system converts the solar radiation into electricity directly. The block diagram of a general PV system is shown in Fig. 1.1.

FIGURE 1.1
The general photovoltaic system.

It consists of the following building blocks:

  1. The PV array: Its function is the conversion of solar radiation into electricity. It is the major unit in the system.
  2. Battery storage: To be available at the absence of the solar radiation, the electric energy produced by the array must be partly stored, normally using batteries. So, the second main unit is the battery storage.
  3. Power conditioning circuits: According to the nature of the load, the generated electric power must be conditioned using DC/DC converters and DC/AC inverters.

The PV array is composed of solar modules. Each module contains a matrix of solar cells connected in series and parallel to satisfy the terminal properties of the whole generator. Accordingly, the solar cell is the basic element in the PV generator. This element is the basic solar radiation converter into electricity.

THE SOLAR RADIATION #

One must be able to describe the solar radiation as it is the input power source to the PV generators. The sun emits electromagnetic radiations as a black body having a surface temperature of about 6000 K. This is because of the nuclear reaction running in it, where the sun is converting hydrogen into helium. The radius of the sun amounts to 1.39  109 m. The total radiation power received from the sun on a unit area perpendicular to the sun rays at the mean earthesun distance, termed an astronomical unit, is called the solar constant (SC), where 1 astronomical unit ¼ 1 AU ¼ 1.496  1011 m. The solar radiation intensity at other distances is expressed in terms of SC with SC ¼ 1.353 kW/m2. Like a black body radiation, the sun’s radiation covers a wide spectrum of wavelengths from deep ultraviolet to far infrared. The power spectral distribution of the sunlight is shown in Fig. 1.2.

The vertical axis represents the spectral irradiance I(l) while the horizontal axis represents the wavelength in mm. The irradiance I(l) is equal to the incident solar power/m2/dl ¼ [W/m2/mm], where dl is the respective wavelength range in mm. It is clear from this figure that the maximum spectral irradiance lies at l ¼ 0.5 mm. The spectral irradiance decreases because of the presence of air in the atmosphere. The air molecules scatter and absorb the solar radiation. There are multiple absorption bands for O2, H2O, and CO2. It is important to notice that the solar irradiance resembles the black body radiation at w6000 K represented by the dashed line.

FIGURE 1.2
The power spectral distribution of the sunlight.

The solar power intensity without the effect of the atmosphere or ground surface is called the Air Mass Zero where one AM0 ¼ 1 SC ¼ 135.3 mW/cm2. The solar power intensity after crossing one air mass perpendicular to the earth is called the air mass 1, AM1. It represents the area under the spectral irradiance curve and amounts to 92.5 mW/cm2. If q is the angle of incidence with normal to the earth surface, then the optical path in units of the air mass will be larger. The air mass AM ¼ 1/(cos q).

THE INCIDENT SOLAR RADIATION “INSOLATION” #

The actual incident solar radiation, the insolation is the incident solar power per unit area at certain location and it varies according to:

  1. the position on the earth, because of the earth’s spin axes inclination
  2. the time of the day because of earth spinning
  3. the month of the year because of the earth rotation around the sun
  4. the angle of incidence
  5. the presence of clouds in the air mass and environmental effects

As an illustration, Fig. 1.3A shows the insolation within the hours of the day in watts per square meter, with the insolation reaching its peak at noon. Moreover, Fig. 1.3B shows the insolation among the months of the year, for Kansas City at latitude 39.1 N, where January has the lowest insolation while July has the highest. Complete measured INSOLATION DATA in a specific place on the earth can be obtained from the nearest Regional Climate Centers or from national insolation databases such as the National Solar Radiation Database in the United States. In addition, insolation could be calculated according to the method developed in the Web site.

FIGURE 1.3
(A) INSOLATION within the hours of the day and (B) insolation within the months of the year

Design considerations

The PV generator must be designed to supply the load safely in smallest Insolation days in winter, and the climatic variations must be taken into consideration in array reliability.

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