CHALLENGES INVOLVED IN SOLAR POWER GENERATION

PV sources inherently have a unique operating point called the maximum power point (MPP) at which maximum power may be extracted from them. However, this point is never constant with respect to time; it changes with solar irradiance, temperature, and load profile.5 In order to extract the maximum possible power from the PV system under varying conditions, a maximum power point tracking (MPPT) scheme is generally adopted6, which requires power electronic interfaces such as DC/DC converters or DC/AC inverters.

Depending on the current and voltage required by the system load, series-parallel connections of individual PV modules are usually adopted. A “string” refers to a set of series-connected PV modules. Under normal operating conditions, each module in a string receives the same amount of irradiance, and the PV characteristics of the string resembles that of the single module, with the voltage increased by as many times as the number of series modules and the current being the same. In such cases, the P-V curve of the string exhibits a single peak.

However, as PV modules are installed in outdoor environments, it is practically possible that the modules receive unequal levels of irradi- ance, and this phenomenon is termed as partial shading. Partial shading is usually caused by the shadow of buildings or other structures, trees, clouds, or dirt.7 Under partial shading conditions, the hot spot problem occurs wherein the shaded module becomes reverse biased and hence behaves as a load. To eliminate this problem, bypass diodes are connected in each module to safely bypass the reverse current. Due to the presence of these diodes, the string P-V characteristic is drastically affected on the occurrence of partial shading.8 Under partial shading conditions, multiple peaks are developed, with several local MPPs (LMPPs), and the location of the global MPP (GMPP) depends on the degree of mismatch in the modules' irradiance. Conventional MPPT algorithms are not designed to handle local peaks efficiently. When the MPPT algorithm is locked on a local MPP, the PV system efficiency falls dramatically.

In addition to partial shading, PV systems are subjected to various failures or faults among the PV arrays, power conditioning units, batteries, wiring, and utility interconnections.9 It is difficult to shut down PV modules completely during arrays faults, since they are energized by sunlight in the daytime. In general, PV array faults can be classified as line-ground faults, line-line faults and open-circuit faults. Among these faults, line-line faults and ground faults are the most common faults in solar PV arrays, which potentially involve large fault currents. Without proper fault detection or protection, they could cause severe problems, such as dc arcs and even fire hazards.10 For example, a multipoint ground fault reportedly14 caused a fire in a PV power plant in Bakersfield, California and in another instance; a fire was caused by a double-ground fault in a large PV power plant in Mount Holly, North California.15 These fire incidents not only underscore the weaknesses in conventional fault detection and protection schemes for PV arrays, but also reveal the urgent need for a better protection system. Among these two abnormal conditions, viz. partial shading and faults, the former will exist for a short period of tune, whereas the latter would persist over time. In a PV installation, there is a need to differentiate partial shading fr om faulty conditions in order to avoid inadvertent shutdowns.

MITIGATION TECHNIQUES FOR OVERCOMING THE CHALLENGES INVOLVED IN SOLAR POWER GENERATION

Several efficient control strategies are reported to detect partial shading and faults in PV array for the enhanced extraction of PV power output to improve the system efficiency and reliability.

Without proper fault detection and protection, they could cause severe problems in the PV array, such as dc arcs and even fire hazards10. Conventional protection systems for PV arrays consist of overcurrent protection devices (OCPD), Arc-fault circuit interrupters (AJFCI) and ground fault protection devices (GFPD) as stated in US NEC16. The challenges involved in the protection of PV arrays have been discussed in17, including the negative impact of environmental conditions, MPPT, and blocking diodes in fault detection, along with the inability of OCPD to detect faults under certain conditions.

Faults are detected by comparing the actual electrical array quantities with expected array quantities.18 A similar approach is adopted in19, but the measured considered here are the inadiances in the horizontal plane and in the PV module plane, the ambient temperature, as well as electrical quantities at the DC and AC side of the PV system, which are fed into a simulation to calculate the normalized capture losses. A fault detection method has been proposed20, in which all PV string currents are measured and tested under outlier detection rules. Even the best performing outlier detection rule for short-circuit faults produces false alarms during normal conditions before and after the occurrence of partial shading. An overview of different fault detection schemes and recommendations for improvement are reported in21. A method to detect faults and partial shading using the measured array voltage, current, and irradiance is proposed22 for operation under normal operating condition, partial shading, and faults.

GREEN CHEMISTRY FOR GREEN ENERGY

Owing to the increasing demand of electric power, were materials plays a major role in sustainable power extraction, green chemistry opens new doors for a green future. There is a symbiotic relationship between green chemistry and renewable energy. The usage of unsafe substances or materials in the design, synthesis, and application of chemical products brings new environmental problems like global wanning, acid rain, ozone layer depletion, and many other harmful side effects, which necessitates that there is requirement for practicing green chemistry and materials. This facilitates, minimizing the consumption of materials and energy, produces least or zero waste materials, economically strong and safe, making it the best solution.

There are twelve basic principles in green chemistiy: (1) prevent waste, (2) maximize atom economy, (2) less hazardous chemical synthesis, (4) safer chemicals and products, (5) safer solvents and reaction conditions, (6) increase energy efficiency, (7) use renewable feed stocks, (8) avoid chemical derivatives, (9) use catalysts, (10) design chemicals and products to degrade after use, (11) analyze in real time to prevent pollution, and (12) minimize potential for accidents.11-13 Applying a few of these principles during synthesis of products is known as green chemistiy approach. The next section of this chapter discusses about the scope of using green chemistiy for fabricating solar cells with the help of organic materials.

GREEN CHEMISTRY FOR ORGANIC SOLAR CELLS

(a) Conducting Polymer

The strategies for producing conjugated polymers using green chemistiy are discussed in this section. Polymers are usually used as insulators. However, in the mid 1970s, polyacetylene was accidentally fabricated by the scientist Shirakawa, the fu st polymer capable of conducting electricity. It is an organic polymer with the repeating unit (C,H,)n. Later, in 2000, the chemistiy Nobel prize was awarded to Alan J. Heeger, Alan G MacDi- annid and Hideki Shirakawa for the discoveiy and study of conducting polymers.

Usually the synthesized conducting polymers exhibit veiy low conductivities. However, conductivity can be increased by addmg dopants (p-type dopants or electron acceptor and n-type dopant or electron donar dopants). Synthetic or conducting polymers are used as conductors over metals owing to the following advantages like lighter in weight, easy, and less energy consumption during processing, corrosion resistant, cheapest materials, easy transportation, easy to handle, etc. Due to this capability, they are used in the fabrication of electronic devices, solar energy conversion, rechargeable batteries, sensors, etc.

For a polymer to behave as a conducting polymer, the polymer chain should contain pi-electrons/lone pair of electrons or vacant p-orbitals. Few examples of conducting polymers that can be used for fabricating solar cells are given in Table 5.4.

TABLE 5.4 List of Conducting Polymers, their Energy Gap and Conductivity.

Polymer

Discovery

Structure

Energy

baudgap

(eV)

Conductivity

(S/cm)

Polythiophene

1981

2.1

10-103

Polyacetylene

1977

1.5

103-1.7 x 105

Polyaniline

1980

3.2

30-200

Polypynole

1979

3.1

10:-7.5 x Ю3

Poly (3,4-ethylene- dioxythiophene)

1980

1.1

300

Poly (p-phenylene vinylene)

1979

2.5

3-5 x Ю3

Polyphenylene and Polyp araphenylene

1979

3.0

10--10-3

Mechanism of conduction in polymers and electron transfer from valence baud (VB) to conduction band (CB)

FIGURE 5.10 Mechanism of conduction in polymers and electron transfer from valence baud (VB) to conduction band (CB).

KEYWORDS

  • renewable power generation
  • solar photovoltaic (PV) module
  • solar PV module modeling
  • aging of PV modules
  • wide bandgap semiconductor materials
  • challenges in solar power generation
  • organic solar cells

REFERENCES

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